Genetics for Pediatricians

Remedica genetics series

PediatriciansGenetics for

Mohnish SuriIan D Young

Series EditorEli Hatchwell

Genetics for PediatriciansM

ohnish

Suri, Ia

n D

Young

Genetic testing plays an important role in the investigation of almostevery child who presents with one of the many common inheriteddisorders. It can be difficult for even the most conscientiouspractitioner to keep abreast of developments and to appreciate both the significance and the relevance of some of the majordiscoveries of recent years. So, it is with the busy generalpediatrician in mind that this contemporary account of the molecular aspects of pediatric disorders has been prepared.

“This text is designed to be readily accessible, and effectivelyblends clinical features with molecular and clinical genetics. It will provide a valuable bridge between standard pediatric sources and Internet-provided databases. Suri and Young are highly respected clinical geneticists with vast experience in the pediatric applications of their speciality. They are alsoaccomplished communicators – they recognize the challenges of clinical syndrome identification, and the necessity to balancediagnostic enthusiasm with restraint when it comes to selectingfrom an ever-expanding repertoire of investigations, many of which generate both personal and financial pressures.”

Derek JohnstonChildren’s Department, University Hospital, Queen’s Medical Centre, Nottingham, UK

“Pediatricians will find this easy-to-read book a major step forwardin their clinical practice. It should be of interest to workingpediatricians who need help in diagnosing syndromes and inunderstanding the molecular tests that are needed for diagnosis. It wisely does not attempt to discuss every single genetic conditionthat exists, but confines itself to the important conditions.”

Jo SibertHead of Department and Professor of Child Health, Department ofChild Health, University of Wales School of Medicine, Cardiff, UK

9 781901 346633

ISBN 1-901346-63-3

Rem

ed

ica

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Genetics for Pediatricians

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The Remedica Genetics for… SeriesGenetics for CardiologistsGenetics for DermatologistsGenetics for EndocrinologistsGenetics for HematologistsGenetics for OncologistsGenetics for OphthalmologistsGenetics for Orthopedic SurgeonsGenetics for PediatriciansGenetics for PulmonologistsGenetics for Rheumatologists

Published by Remedica Publishing

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E-mail: [email protected]

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Publisher: Andrew Ward

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© 2004 Remedica Publishing

While every effort is made by the publishers and authors to see that no inaccurate or misleading data, opinions, or

statements appear in this book, they wish to make it clear that the material contained in the publication represents

a summary of the independent evaluations and opinions of the authors and contributors. As a consequence, the authors,

publishers, and any sponsoring company accept no responsibility for the consequences of any such inaccurate or

misleading data, opinions, or statements. Neither do they endorse the content of the publication or the use of any

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All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or

by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publisher.

ISBN 1 901346 63 3

ISSN 1472 4618

British Library Cataloguing-in Publication Data

A catalogue record for this book is available from the British Library.

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Mohnish Suri Department of Clinical GeneticsCity HospitalNottinghamUK

Ian D YoungDepartment of Clinical GeneticsLeicester Royal InfirmaryLeicesterUK

Series EditorEli HatchwellInvestigatorCold Spring Harbor LaboratoryUSA

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To our wives and parents.

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Introduction to the Genetics for… series

Medicine is changing. The revolution in molecular genetics hasfundamentally altered our notions of disease etiology and classification,and promises novel therapeutic interventions. Standard diagnosticapproaches to disease focused entirely on clinical features and relativelycrude clinical diagnostic tests. Little account was traditionally taken of possible familial influences in disease.

The rapidity of the genetics revolution has left many physicians behind,particularly those whose medical education largely preceded its birth.Even for those who might have been aware of molecular genetics and its possible impact, the field was often viewed as highly specialist andnot necessarily relevant to everyday clinical practice. Furthermore, whilegenetic disorders were viewed as representing a small minority of thetotal clinical load, it is now becoming clear that the opposite is true: few clinical conditions are totally without some genetic influence.

The physician will soon need to be as familiar with genetic testing ashe/she is with routine hematology and biochemistry analysis. Whilerapid and routine testing in molecular genetics is still an evolving field,in many situations such tests are already routine and represent essentialadjuncts to clinical diagnosis (a good example is cystic fibrosis).

This series of monographs is intended to bring specialists up to date inmolecular genetics, both generally and also in very specific ways thatare relevant to the given specialty. The aims are generally two-fold:

(i) to set the relevant specialty in the context of the new genetics in general and more specifically

(ii) to allow the specialist, with little experience of genetics or its nomenclature, an entry into the world of genetic testing as it pertains to his/her specialty

These monographs are not intended as comprehensive accounts ofeach specialty — such reference texts are already available. Emphasishas been placed on those disorders with a strong genetic etiology and, in particular, those for which diagnostic testing is available.

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Introduction

The glossary is designed as a general introduction to molecular geneticsand its language.

The revolution in genetics has been paralleled in recent years bythe information revolution. The two complement each other, and theWorld Wide Web is a rich source of information about genetics. Thefollowing sites are highly recommended as sources of information:

1. PubMed. Free on-line database of medical literature.http://www.ncbi.nlm.nih.gov/PubMed/

2. NCBI. Main entry to genome databases and other information about the human genome project.http://www.ncbi.nlm.nih.gov/

3. OMIM. Online Mendelian Inheritance in Man. The Online version of McKusick’s catalogue of Mendelian disorders.Excellent links to PubMed and other databases.http://www.ncbi.nlm.nih.gov/omim/

4. Mutation database, Cardiff.http://www.uwcm.ac.uk/uwcm/mg/hgmd0.html

5. National Coalition for Health Professional Education in Genetics.An organization designed to prepare health professionals for thegenomics revolution. http://www.nchpeg.org/

Finally, a series of articles from the New England Journal of Medicine,entitled Genomic Medicine, has been made available free of charge athttp://www.nejm.org.

Eli HatchwellCold Spring Harbor Laboratory

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Preface

There can be very few areas of medicine in which progress has beenachieved at such a rapid pace as molecular genetics. Almost everycommon single-gene disorder has succumbed to the march of scientificprogress to the extent that genetic testing now plays an important role inthe investigation of almost every child who presents with one of the manycommon inherited disorders which make a major contribution to pediatricmorbidity and mortality throughout the world. The rate of progress issuch that it can be difficult for even the most conscientious practitioner to keep abreast of developments and to appreciate both the significanceand the relevance of some of the major discoveries of recent years.

It is with the busy general pediatrician in mind that this contemporaryaccount of the molecular aspects of pediatric disorders has beenprepared. The number of conditions which have been mapped or inwhich the causative gene has been isolated is vast. Thus in order toensure that this text is of manageable proportions, coverage has beenrestricted to the more common single-gene disorders which are likely tobe encountered in general pediatric practice. “Small print” obscuritiesand the many inborn errors for which comprehensive biochemicaltesting is available have generally been omitted. Instead attention has been focused on the more common conditions in which molecularanalysis can play an important role in diagnosis or in the management of a child and his or her family. In some instances, notably with eye and skin disorders, we have also omitted rare disorders which fall within the remit of other specialties, particularly when these havereceived detailed coverage in other books in this series.

In addition to providing a unique insight into the cause of so manypreviously unexplained disorders, recent advances in moleculargenetics have also demonstrated that, far from being straightforward,Mendelian inheritance and its contribution to genetic disease can beremarkably complex. Thus a “simple” disorder such as cystic fibrosishas proved to be extremely heterogeneous both clinically and at themolecular level, with over 1,000 different mutations reported at themain disease locus. Indeed, for many conditions such as cystic fibrosisand β-thalassemia, mutational heterogeneity has proved to be thenorm. Entities such as nonsyndromal sensorineural hearing loss illustrate

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Preface

that locus heterogeneity can also be extremely important. Furtherexamples of etiologic complexity are provided by the Bardet–Biedlsyndrome, which shows not only locus heterogeneity but also thecurious phenomenon of triallelic inheritance, and by Hirschsprungdisease, for which the concept of synergistic heterozygosity has startedto shed light on how genes at several loci can interact to contribute towhat is commonly referred to as oligogenic or polygenic inheritance.And if this was not enough, research on pediatric disorders such as the fragile X syndrome and the Angelman/Prader–Willi syndromes hasidentified “new” genetic mechanisms such as triplet repeat instabilitywith anticipation, and imprinting/uniparental disomy, respectively. So as well as providing a useful up-to-date account of molecularpathogenesis, we hope that this text will also help readers becomebetter acquainted with some of the new and exciting developments thathave characterized molecular genetic research over the last few years.

In writing this book we would like to offer our thanks to colleagues whohave provided photographs, and to Mrs Diane Castledine for secretarialassistance. Above all we would like to express our gratitude to, andadmiration for, the many children and families who, over the years, have taught us so much more than they could possibly have learnedfrom us.

Mohnish SuriIan D Young

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Contents

1. Progressive Ataxias and Neurologic Disorders 1Ataxia–Telangiectasia 2Duchenne Muscular Dystrophy 4Facioscapulohumeral Muscular Dystrophy 7Friedreich Ataxia 8Hereditary Motor and Sensory Neuropathy 10Limb-girdle Muscular Dystrophy 18Myotonic Dystrophy 23Spinal Muscular Atrophy 27

2. Cerebral Malformations and Mental Retardation Syndromes 29Angelman Syndrome 30Fragile X Syndrome 34Holoprosencephaly 36Hunter Syndrome 40Huntington Disease 41Lesch–Nyhan Syndrome 43Lissencephaly 45Lowe Syndrome 52Neuronal Ceroid Lipofuscinosis 53Pelizaeus–Merzbacher Syndrome 57Prader–Willi Syndrome 59Rett Syndrome 61X-linked Adrenoleukodystrophy 62X-linked α-Thalassemia and Mental Retardation Syndrome 64X-linked Hydrocephalus 66

3. Disorders of Vision 69Aniridia 70Bardet–Biedl Syndrome 72Juvenile Retinoschisis 74Leber Congenital Amaurosis 75Norrie Disease 79Rieger Syndrome 80

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4. Hearing Disorders 83Nonsyndromal Hearing Loss 84Hearing Loss due to Connexin 26 Gene Defect 85Pendred Syndrome 86Usher Syndrome 87Waardenburg Syndrome 90

5. Neurocutaneous Disorders and Childhood Cancer 93Multiple Endocrine Neoplasia Type 2 94Neurofibromatosis Type 1 96Retinoblastoma 98Tuberous Sclerosis 101von Hippel–Lindau Disease 103

6. Connective Tissue and Skeletal Disorders 107Achondroplasia 108Ehlers–Danlos Syndrome 110Hereditary Multiple Exostoses 115Marfan Syndrome 117Osteogenesis Imperfecta 119Pseudoachondroplasia 124Stickler Syndrome 125

7. Cardio-respiratory Disorders 129Barth Syndrome 130Cystic Fibrosis 131DiGeorge/Shprintzen Syndrome 133Holt–Oram Syndrome 135Laterality Defects 137Noonan Syndrome 138Primary Ciliary Dyskinesia 139Williams Syndrome 141

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8. Craniofacial Disorders 143Apert Syndrome 144Crouzon Syndrome 146Greig Syndrome 148Pfeiffer Syndrome 149Rubinstein–Taybi Syndrome 151Saethre–Chotzen Syndrome 152Sotos Syndrome 153Treacher Collins Syndrome 154Van der Woude Syndrome 155

9. Endocrine Disorders 157Androgen Insensitivity Syndrome 158Congenital Adrenal Hyperplasia 160Diabetes Insipidus 163Growth Hormone Deficiency 164Growth Hormone Receptor Defects 166Panhypopituitarism 167Pseudohypoparathyroidism 169

10. Gastrointestinal and Hepatic Diseases 173Alagille Syndrome 174α1-Antitrypsin Deficiency 175Hirschsprung Disease 177

11. Hematologic Disorders 181Fanconi Anemia 182Glucose-6-Phosphate Dehydrogenase Deficiency 183Hemophilia A 185Hemophilia B 187Hereditary Elliptocytosis 189Hereditary Spherocytosis 190Sickle Cell Anemia 193α-Thalassemia 194β-Thalassemia 197von Willebrand Disease 198

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12. Immunologic Disorders 201Bruton Agammaglobulinemia 202Chronic Granulomatous Disease 203Severe Combined Immunodeficiency 205Wiskott–Aldrich Syndrome 207

13. Metabolic Disorders 209Medium Chain Acyl-CoA Dehydrogenase Deficiency 210Menkes Disease 211Ornithine Transcarbamylase Deficiency 212Phenylketonuria 214Wilson Disease 215

14. Renal Disorders 217Alport Syndrome 218Beckwith–Wiedemann Syndrome 220Cystinosis 224Orofaciodigital Syndrome Type I 225Polycystic Kidney Disease 226

15. Abbreviations 229

16. Glossary 235

17. Index 285

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1. Progressive Ataxias and Neurologic Disorders

Ataxia–Telangiectasia 2

Duchenne Muscular Dystrophy 4

Facioscapulohumeral Muscular Dystrophy 7

Friedreich Ataxia 8

Hereditary Motor and Sensory Neuropathy 10

Limb-girdle Muscular Dystrophy 18

Myotonic Dystrophy 23

Spinal Muscular Atrophy 27

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Ataxia–Telangiectasia2

Ataxia–Telangiectasia(also known as: AT; Louis-Bar syndrome)

MIM 208900

Clinical features AT is a neurocutaneous syndrome. Patients present with progressivetruncal and gait ataxia, unusual head movements, choreoathetosis, andoculomotor apraxia in both horizontal and vertical gaze. Other featuresinclude motor developmental delay, dysarthria, and mask-like facies.Telangiectasia appears over the bulbar conjunctiva, face, and ears fromthe age of 3–4 years (see Figure 1). Many children have a history ofrecurrent respiratory infections, and 30%–40% of patients develop a malignancy. These include T-cell leukemias and B-cell lymphomas in children and epithelial tumors (such as breast and ovarian cancer) in adults. Patients with AT usually survive into their twenties, althoughlonger survival periods have been documented. Investigations showelevated levels of α-fetoprotein and carcinoembryonic antigen and reduced levels of immunoglobulin (Ig)G2, IgA, and IgE.

Chromosome analysis can show reciprocal balanced translocationsinvolving the short arm of chromosome 7 and the long arm ofchromosome 14, or the short arm of chromosome 2 and the long arm of chromosome 22.

Age of onset Most children present with ataxia between the ages of 1 and 3 years.

Epidemiology The population incidence is estimated to be about 1 in 40,000 to 1 in 100,000 live births. About 1% of the general population arebelieved to be carriers (heterozygotes).

Inheritance Autosomal recessive

Chromosomal 11q22.3location

Gene ATM (ataxia–telangiectasia mutated)

Mutational Over 400 mutations have been described. These include small and largespectrum deletions and insertions, as well as nonsense, missense, and splice-site

mutations. About 65%–70% of mutations result in protein truncation,and these mutations produce no detectable protein. The remaining

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Progressive Ataxias and Neurologic Disorders 3

mutations result in the production of a normal-sized protein that isnonfunctional. Almost all nonconsanguineous patients are compoundheterozygotes, ie, they have different mutations in their two ATM alleles.

Molecular ATM has 66 exons and encodes a protein with 3,056 amino acids.pathogenesis The ATM protein is ubiquitously expressed and has homology to yeast

and mammalian phosphatidylinositol-3 kinases, which are involved insignal transduction, cell cycle control, and DNA repair. It is believed thatthe ATM protein phosphorylates several other proteins, including p53,ABL, BRCA1, TERF1, RAD9, and nibrin (the protein product of the genefor Nijmegen breakage syndrome, MIM 251260), after cell exposure toionizing radiation. This delays the progression of the cell through the cellcycle at the G1/S checkpoint, allowing the cell to repair DNA damagebefore entering the S phase. Without ATM protein the cell would be ableto progress to the S phase without repairing the DNA damage sustainedby radiation exposure, which could predispose to the development ofcancer. The molecular pathogenesis of the neurocutaneous phenotype of AT is unknown.

Genetic diagnosis The diagnosis can be confirmed by demonstrating increased and counseling chromosomal breakage in cultured lymphocytes after X-irradiation

and reduced or absent expression of ATM protein in lymphocytes.Genetic testing is only available on a research basis.

Figure 1. Telangiectasia over thebulbar conjunctiva in a child withataxia–telangiectasia.


Duchenne Muscular Dystrophy4

Counseling is on the basis of autosomal recessive inheritance. Carrier females (particularly those who carry a missense mutation) are at increased risk of developing breast cancer. Missense mutations in ATM are believed to be associated with an increased cancer risk as a result of a dominant-negative effect.

Prenatal diagnosis is possible by linkage analysis or by ATM mutationanalysis if mutations have been identified previously in an affected child from the family. Prenatal diagnosis has been attempted byamniocentesis followed by X-irradiation of cultured amniocytes to look for chromosomal breakage, but this method of prenatal diagnosis is unreliable.

Duchenne Muscular Dystrophy(also known as: DMD)

MIM 310200

Clinical features This condition mainly affects males, who present with delayedmotor-developmental milestones, proximal muscle weakness withpseudohypertrophy of some muscles, particularly the calves (seeFigure 2), and cardiomyopathy. The muscle weakness is progressive. In classical cases, loss of ambulation occurs before the age of 12 yearsand death occurs in the twenties. Intermediate forms of DMD exist in which progression is slower, with loss of ambulation occurringbetween 11 and 16 years of age. Learning difficulties are seen inapproximately 60% of patients. Death is usually due to respiratoryinfection or cardiomyopathy. About 2.5% of female carriers aresymptomatic (manifesting carriers).

Figure 2. Calf hypertrophy in a boy with Duchenne muscular dystrophy.



Age of onset Usually in the first year of life, although diagnosis is often delayed.

Epidemiology This is the most common form of muscular dystrophy, affecting 1 in 3,500 live-born males. The prevalence of symptomatic carriers in the female population is estimated to be 1 in 100,000.

Inheritance X-linked recessive

Chromosomal Xp21.2location

Gene DMD (dystrophin)

Mutational An intragenic deletion that involves one or more exons is present inspectrum 65%–70% of patients. There are two deletion hotspots, one between

exons 2 and 20 and the other between exons 44 and 53. Intragenicduplications are seen in 5%–6% of cases. The remainder of casesinvolve point mutations (nonsense, missense, and splice-sitemutations), which are distributed across the whole gene.

Molecular DMD is the largest known gene in the human genome. It is 2.4 Mb pathogenesis in size and composed of 79 exons. It encodes a large, rod-shaped

cytoskeletal protein made up of 3,685 amino acids. The dystrophinprotein has an actin-binding domain, two calpain-homology domains,22 spectrin repeats, one WW domain (a short domain of about 40 aminoacids that contains two tryptophan residues that are spaced 20–23 aminoacids apart – the term WW derives from the two tryptophan residues, as the single letter code for tryptophan is W) and one ZZ-type zinc fingerdomain. The gene is subject to alternative splicing, and there are at least four isoforms of dystrophin. These include a muscle (M) isoform, a brain (B) isoform, and a cerebellar Purkinje (CP) isoform.

Dystrophin is expressed in several tissues and plays an important role in anchoring the cytoskeleton to the plasma membrane. In muscle,dystrophin links the sarcolemmal cytoskeleton to the extracellular matrix.It is thought to protect the sarcolemma during muscular contractions.Mutations that result in the DMD phenotype are associated with proteintruncation or loss of the translational reading frame. These mutationsresult in the absence of dystrophin.

Mutations that maintain the translational reading frame result in thephenotype of Becker muscular dystrophy (MIM 300376). These


Duchenne Muscular Dystrophy6

mutations result in the production of a shortened and only partiallyfunctional protein. Patients with Becker muscular dystrophy haveclinical features similar to those of DMD, but the condition is milder,progression is slower, and survival is prolonged.

Mutations in the 5 end of DMD and in-frame deletions in exons 48 and 49 can also cause X-linked dilated cardiomyopathy (MIM 302045).Mutations in the 5 end of DMD result in failure to transcribe the Misoform in skeletal and cardiac muscle. However, the absence of thisisoform in skeletal muscle can be compensated for by up-regulation ofthe B and CP isoforms. This does not appear to be the case in cardiacmuscle, where the lack of dystrophin expression results in cardiomyopathy.The mechanism by which in-frame deletions in exons 48 and 49 cause X-linked dilated cardiomyopathy is not understood, but it has been suggested that intron 48 might contain sequences that are necessary for the expression of dystrophin in cardiac muscle.

Genetic diagnosis The diagnosis of DMD is based on clinical features, markedly and counseling elevated plasma creatine kinase (CK) levels, muscle biopsy (with

immunohistochemistry using monoclonal antibodies to dystrophin), and mutation testing. Routine genetic testing can only detect intragenicdeletions and duplications. Testing for point mutations in DMD is onlyundertaken in a few specialized research laboratories and is bestperformed on dystrophin mRNA extracted from a fresh or frozen muscle biopsy.

Genetic counseling is on an X-linked recessive basis. Female relatives of affected males who have an intragenic deletion or duplication can beoffered carrier testing. Carrier females have a 50% chance of having anaffected son, and can be offered a reliable genetic prenatal test for thiscondition by chorionic villus sampling.

There is a two-thirds chance that the mother of a sporadic case (single affected male with no family history) is a carrier. The mother of a sporadic case can also have somatic or gonadal mosaicism for theDMD mutation. Therefore, there is a 10%–15% recurrence risk of DMDin a subsequent pregnancy for the mother of a sporadic case. In DMDfamilies in which the DMD mutation cannot be identified, carrier testinginvolves linkage analysis and serial plasma CK assays. Linkage analysis using intragenic and flanking markers can also be used for prenataldiagnosis in these families.



Facioscapulohumeral Muscular Dystrophy(also known as: FSHMD)

MIM 158900 (type 1A)158901 (type 1B)

Clinical features This is a slowly progressive muscular dystrophy. The affected patientusually presents with facial weakness, shoulder-girdle weakness andwasting, and scapular winging. Later, there is involvement of feet and hip-girdle dorsiflexors. There is often striking wasting of the neckmuscles and the muscles of the upper arm. Retinal vasculopathy andhigh-frequency sensorineural hearing loss are also recognized features.

Age of onset Late childhood or adolescence.

Epidemiology The incidence of FSHMD is approximately 1 in 20,000.

Inheritance Autosomal dominant

Chromosomal Type 1A: 4q35location Type 1B: unknown

Gene Unknown (both types)

Mutational Most cases of FSHMD are type 1A. Although the gene for thisspectrum condition has not yet been identified, almost all patients have a

chromosomal rearrangement in the subtelomeric region of the long arm of chromosome 4 (4q35). This region contains a polymorphic3.3-kb repeat element termed D4Z4. In the general population, thenumber of D4Z4 repeats varies from 10 to more than 100. Affectedindividuals have a deletion in this region that reduces the number of D4Z4 repeats to less than 10. This reduction is the basis of a diagnostic molecular genetic test for FSHMD type 1A.

Molecular Unknown. It has been suggested that deletion of the D4Z4 repeat pathogenesis sequences could interfere with the expression of a gene located some

distance away on the long arm of chromosome 4 by a “position effect”.Recent work suggests that an element within the D4Z4 repeat sequencespecifically binds a multiprotein complex that mediates transcriptionalrepression of genes at 4q35. Deletion of D4Z4 sequences below a certainnumber could result in a reduction in the number of repressor complexes.


Friedreich Ataxia8

This could decrease or abolish the transcriptional repression of 4q35genes, with overexpression of one or more of these genes resulting in the FSHMD phenotype.

Genetic diagnosis Genetic testing is routinely available and enables a diagnosis to and counseling be made in most cases. Counseling is on the basis of autosomal

dominant inheritance. About 30% of patients represent new mutations.The condition demonstrates 95% penetrance by the age of 20 years,although penetrance is lower in females. Anticipation has beendescribed in some families. Prenatal testing can be done by genetic testing or, in suitable families, by linkage analysis.

Friedreich Ataxia

MIM 229300 (Friedreich ataxia 1)601992 (Friedreich ataxia 2)

Clinical features This is the most common cause of cerebellar ataxia in childhood.Affected children present with dysarthria and progressive ataxia of their gait. Neurologic examination demonstrates weakness of the lower limbs, absent knee and ankle jerks, extensor plantar reflexes,decreased position and vibration sense in legs, positive Romberg sign, and pes cavus. Other features include scoliosis, diabetes mellitus, optic atrophy, and deafness. Nerve conduction studies show reduced or absent sensory action potentials, but normalmotor-nerve conduction velocities. Echocardiograms show features of hypertrophic cardiomyopathy in 70% of patients.

Age of onset Usually between 5 and 15 years of age. Almost all cases present before the age of 25 years, although onset after this age has also been described (late-onset form).

Epidemiology The estimated population prevalence is 1–2 per 50,000. The carrier(heterozygote) frequency is between 1 in 60 and 1 in 110.


Chromosomal Friedreich ataxia 1: 9q13location Friedreich ataxia 2: 9p11–p23



Gene Friedreich ataxia 1: FRDA (frataxin)Friedreich ataxia 2: unknown

Mutational FRDA has seven exons that are subject to alternative splicing. The majorspectrum protein product of this gene is the 210-amino-acid protein frataxin.

This is encoded by exons 1–4 spliced to exon 5A. The majority ofpatients (~96%) are homozygous for an expansion of a GAA tripletrepeat motif in the first intron of the gene. The normal number of GAA triplet repeats is 9–22. In affected individuals the size range is66–1,700 repeats, with most patients having 600–1,200 repeats. The remaining patients are compound heterozygotes for a pathogenicGAA repeat expansion in one FRDA allele and an inactivating mutation(nonsense or frame-shift) in the other allele.

Molecular Frataxin is located in the inner mitochondrial membrane, where it playspathogenesis an important role in oxidative phosphorylation and iron homeostasis.

The GAA repeat expansion interferes with the transcription of FRDA,resulting in frataxin deficiency. This is associated with a defect ofmitochondrial oxidative phosphorylation and accumulation of ironwithin the mitochondria. Thus, Friedreich ataxia is essentially amitochondrial disorder and this is reflected in its clinical features.

Genetic diagnosis Genetic testing is available from diagnostic laboratories. However,and counseling it is limited to testing for the pathogenic GAA repeat expansion.

Counseling is on the basis of autosomal recessive inheritance. The sibling recurrence risk is 25%, but there can be marked variability in the expression of the condition in members of the same family. This can manifest as a different age of onset and/or a difference in the rate of progression.

Carrier testing and prenatal diagnosis are available in families wheremolecular genetic analysis has confirmed that the affected individual is homozygous for a GAA repeat expansion.


Hereditary Motor and Sensory Neuropathy10

Hereditary Motor and Sensory Neuropathy(also known as: HMSN; Charcot–Marie–Tooth disease; peroneal muscular atrophy)

The hereditary motor and sensory neuropathies are a clinically and genetically heterogeneousgroup of disorders. Four main clinical phenotypes can be recognized: classical HMSN,Dejerine–Sottas syndrome, congenital hypomyelinating neuropathy (CHN), and hereditaryneuropathy with liability to pressure palsies (HNPP). Each of these phenotypes is discussed in turn. Table 1 summarizes the classification, distinguishing clinical features, inheritancepatterns, and molecular genetics of the various forms of HMSN.

MIM See Table 1.

Clinical Features Classical HMSN/HMSN I & IIPatients with classical HMSN present with distal weakness and wasting ofthe legs, often associated with pes cavus and loss of ankle jerks. Sensorysymptoms are usually mild and include paresthesia of the hands and feet.The condition progresses at a variable rate to involve the small muscles ofthe hands and proximal parts of the lower limbs. Classical HMSN patientscan be divided into two groups based on their nerve conduction velocities(NCVs). Patients with HMSN I have a demyelinating neuropathy withreduced NCVs (patients over the age of 2 years have a motor NCV of<38 m/s in the median nerve). Patients with HMSN II have an axonal formof neuropathy, with normal or only slightly reduced NCVs (patients overthe age of 2 years have a motor NCV of >38 m/s in the median nerve).

Dejerine–Sottas syndrome/HMSN IIIThe Dejerine–Sottas syndrome phenotype is more severe than that ofclassical HMSN, and patients with this condition present with hypotonia,generalized muscle weakness, motor developmental delay, ataxia, andareflexia. They often have palpable peripheral nerves. Muscle weaknesstends to progress more rapidly than in classical HMSN, and patients areoften nonambulatory by adolescence. However, the condition is quitevariable in its severity and progression. Nerve conduction studies showvery low NCVs (often <10 m/s), in association with demyelination with onion-bulb formation or hypomyelination on sural nerve biopsy.

HMSN IVAutosomal recessive forms of HMSN I are designated HMSN IV. The phenotype is similar to that of HMSN I, but HMSN IV tends topresent earlier and progress more rapidly. NCVs are usually <20 m/s.



Congenital hypomyelinating neuropathyThis is the most severe form of HMSN. Affected children present ininfancy with severe hypotonia, generalized weakness, and areflexia. The condition can mimic spinal muscular atrophy, though nerveconduction studies show very slow or unrecordable NCVs and suralnerve biopsy shows amyelination or hypomyelination of nerve fibers.Affected children can have respiratory and swallowing difficulties, and an arthrogryposis-like presentation has also been described. CHN can take a lethal course, causing early death, though improvement has been described in some children.

Hereditary neuropathy with liability to pressure palsiesThis is the mildest form of HMSN. Affected individuals present withrecurrent peroneal- and ulnar-nerve pressure palsies, from which theyoften make a complete recovery. Nerve conduction studies show slightlyreduced NCVs, with prolonged distal motor latencies of median andperoneal nerves. Sural nerve biopsy shows sausage-shaped swellings ofthe myelin sheath of nerve fibers. These swellings are called tomaculae.

Age of onset HMSN I and II usually present in the first decade of life, but onset canalso occur in adult life.

HMSN III usually presents in the first 2 years of life.

HMSN IV usually presents in the first decade of life.

CHN usually presents at birth or during early infancy.

HNPP usually presents in adult life.

Epidemiology The population prevalence of all forms of HMSN is about 1 in 2,500.

Inheritance, See Table 1.chromosomal location, gene, and mutational spectrum

Molecular PMP22 is composed of four exons. It is expressed in Schwann cellspathogenesis and encodes a 160-amino-acid integral membrane protein called

peripheral myelin protein 22. This protein is involved in the formationand compaction of myelin in peripheral nerves. Mutations in PMP22cause HMSN I, HMSN III, and HNPP. Duplication of PMP22 is believed to cause HMSN IA by a dosage effect. It has been suggested thatoverexpression of PMP22 could cause the protein to accumulate



in the late Golgi-cell membrane compartment of Schwann cells, whichcould interfere with normal myelin assembly. Deletions of PMP22 causeHNPP as a result of haploinsufficiency. Point mutations in PMP22are associated with a more severe phenotype than duplications of this gene, and are believed to cause HMSN by a dominant-negative effect(ie, the mutant protein interferes with the function of the normal protein produced by the normal allele of this gene).

MPZ has seven exons and encodes a 248-amino-acid integralmembrane protein called myelin protein zero. This protein has a large extracellular domain containing an immunoglobulin V-type fold, a single transmembrane segment, and a short cytoplasmicC-terminal end. Myelin protein zero is a major structural component of the myelin of peripheral nerves and is involved in the formation and compaction of myelin. Mutations in this gene can cause HMSN I, II, III, and CHN by interfering with the function of the protein in themyelin sheath of peripheral nerves.

LITAF is a widely expressed gene with four exons that encodes a161-amino-acid protein called lipopolysaccharide-induced tumornecrosis factor-α factor. This protein plays an important role in theregulation of tumor necrosis factor-α and could play a role in proteindegradation pathways. Missense mutations in this gene cause HMSN IC, but the precise molecular mechanism is unknown.

EGR2 has two exons and encodes a transcription factor protein (earlygrowth response 2) with 476 amino acids. This protein is involved in the differentiation and maintenance of Schwann cells by regulatingtranscription of MPZ and PRX. Mutations in EGR2 can cause HMSN I,HMSN III, and CHN.

NEFL contains four exons. It encodes the neurofilament protein (light polypeptide) that has 543 amino acids. This protein is one of three components of neurofilaments. Neurofilaments are cytoplasmicintermediate filaments of neurons. They are believed to play a role in the maturation of regenerating myelinated nerve fibers, and mutationsin NEFL could lead to HMSN IF and IIE by interfering with this function.

CX32 (or GJB1) is a small gene with only two exons. It is expressed inmyelinated peripheral nerves and codes for connexin 32, a gap-junctionprotein with 283 amino acids. Gap junctions are involved in cell–cellcommunication. Mutations in CX32 result in an X-linked dominant form



Cla

ssifi

catio

n M

IMD

istin

guis

hing

Inhe

ritan

ceC

hrom

osom

al

Gen

eP

rodu

ctM

utat

iona

l spe

ctru

mcl

inic

al fe

atur

eslo

catio

n

HM

SN IA

118220

Slow

NC

Vs

Aut

osom

al

17p1

1.2

PM

P2

2Pe

riphe

ral m

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A 1

.5-M

b du

plic

atio

n of

do

min

ant

prot

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22

17p1

1.2

incl

udin

g P

MP

22

is th

e m

ost c

omm

onca

use

of H

MSN

IA. P

oint

mut

atio

ns in

this

gen

eha

ve a

lso

been

iden

tifie

d,in

clud

ing

mis

sens

e an

dfram

e-sh

ift m

utat

ions

HM

SN IB

118200

Slow

NC

Vs

Aut

osom

al

1q2

2

MP

ZM

yelin

pro

tein

zer

oM

isse

nse

mut

atio

nsdo

min

ant

HM

SN IC

601098

Slow

NC

VsAut

osom

al

16p1

2–p

13.3

LITA

FLi

popo

lysa

ccha

ride-

Mis

sens

e m

utat

ions

dom

inan

tin

duce

d tu

mor

necr

osis

fact

or-α

fact

orH

MSN

ID

607678

Very

slo

w N

CVs

Aut

osom

al10q2

1.1

–q22.1

EGR

2Ea

rly g

row

th

Mis

sens

e m

utat

ion

dom

inan

tre

spon

se 2

(Arg

409Tr

p)H

MSN

IF

607734

Slow

NC

Vs, o

nset

Aut

osom

al8p2

1N

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Neu

rofil

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or

in in

fanc

y or

ear

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ant

prot

ein,

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isse

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mut

atio

n ch

ildho

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lype

ptid

e(P

ro8Arg

)H

MSN

X

302800

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es a

re m

ore

X-lin

ked

Xq

13.1

CX3

2C

onne

xin

32

Mis

sens

e m

utat

ions

se

vere

ly a

ffect

ed

dom

inan

tac

coun

t for

~75%

of a

llth

an fe

mal

es. M

ales

:m

utat

ions

. Non

sens

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dsl

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ales

: fram

e-sh

ift m

utat

ions

, as

norm

al o

r slo

w N

CVs

wel

l as

in-f

ram

e de

letio

ns

and

inse

rtio

ns h

ave

also

been

iden

tifie

dH

MSN

IIA

118210

Nor

mal

or s

light

ly

Aut

osom

al1p3

6.2

KIF

1B

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fam

ily

Mis

sens

e m

utat

ions

redu

ced

NC

Vsdo

min

ant

mem

ber 1

BH

MSN

IIB

600882

Ulc

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mut

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g Aut

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al3q2

1R

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7R

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nse

mut

atio

nsfe

atur

es, n

orm

al o

r do

min

ant

bind

ing

prot

ein

7sl

ight

ly re

duce

d N

CVs

HM

SN II

B1

605588

Slow

NC

VsAut

osom

al1q2

1.2

LMN

ALa

min

A/C

Mis

sens

e m

utat

ion

rece

ssiv

e(A

rg298C

ys)

HM

SN II

C

606071

Wea

knes

s of

voc

al c

ord

Aut

osom

alU

nkno

wn

Unk

now

nU

nkno

wn

Unk

now

nan

d in

terc

osta

l mus

cles

,do

min

ant

norm

al N

CVs

HM

SN II

D

601472

Wea

knes

s an

d w

astin

gAut

osom

al7p1

5G

AR

SG

lycy

l-tR

NA

Mis

sens

e m

utat

ions

of h

and

at o

nset

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min

ant

synt

heta

seno

rmal

NC

Vs



Cla

ssifi

catio

n M

IMD

istin

guis

hing

Inhe

ritan

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hrom

osom

al

Gen

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utat

iona

l spe

ctru

mcl

inic

al fe

atur

eslo

catio

n

HM

SN II

E 607684

Nor

mal

or s

light

ly

Aut

osom

al8p2

1N

EFL

Neu

rofil

amen

t pro

tein

,M

isse

nse

mut

atio

nsre

duce

d N

CVs

dom

inan

tlig

ht p

olyp

eptid

e

MSN

IIF

606595

Nor

mal

NC

Vs

Aut

osom

al7q1

1–q

21

Unk

now

nU

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Unk

now

ndo

min

ant

HM

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607706

Nor

mal

or s

light

lyAut

osom

al8q1

3–q

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1G

angl

iosi

de-in

duce

d N

onse

nse

mut

atio

nsre

duce

d N

CVs

with

re

cess

ive

diffe

rent

iatio

n-vo

cal c

ord

pare

sis

asso

ciat

ed p

rote

in 1

HM

SN II

H

607731

–Aut

osom

al8q2

1.3

Unk

now

nU

nkno

wn

Unk

now

nre

cess

ive

HM

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I 607677

Nor

mal

or s

light

ly

Aut

osom

al1q2

2M

PZ

Mye

lin p

rote

in z

ero

Mis

sens

e m

utat

ions

(tw

ore

duce

d N

CVs

dom

inan

tpa

tient

s ha

d th

ree

diffe

rent

mis

sens

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e sa

me

alle

le)

HM

SN II

J 607736

Nor

mal

or s

light

ly

Aut

osom

al1q2

2M

PZ

Mye

lin p

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in z

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Mis

sens

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redu

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Vs w

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min

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(Thr

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et o

r Asp

75Va

l)pa

pilla

ry a

bnor

mal

ities

and

deaf

ness

HM

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607831

Slig

htly

redu

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osom

al8q1

3–q

21.1

GD

AP

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angl

iosi

de-in

duce

dH

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ity fo

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ith o

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in e

arly

re

cess

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rent

iatio

n-Se

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opch

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soci

ated

pro

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I/145900

See

text

Aut

osom

al17p1

1.2

PM

P2

2Pe

riphe

ral m

yelin

Mis

sens

e an

dD

ejer

ine–

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min

ant

prot

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22

fram

e-sh

ift m

utat

ion

synd

rom

e

Aut

osom

al1q2

2

MP

ZM

yelin

pro

tein

zer

o M

isse

nse

mut

atio

ns

dom

inan

t

Aut

osom

al10q2

1.1

–q22.1

EGR

2Ea

rly g

row

th

Mis

sens

e m

utat

ion

dom

inan

tre

spon

se 2

(Arg

359Tr

p)

Aut

osom

al17p1

1.2

PM

P2

2Pe

riphe

ral m

yelin

1.5

-Mb

dupl

icat

ion

ofre

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ive

prot

ein

22

17p1

1.2

, inc

ludi

ng

PM

P2

2in

bot

h al

lele

s or

dupl

icat

ion

of o

ne a

llele

an

d a

poin

t mut

atio

n(u

sual

ly a

mis

sens

e m

utat

ion)

in th

e ot

her a

llele

Aut

osom

al19q1

3.1

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PR

XPe

riaxi

n N

onse

nse

and

rece

ssiv

e fram

e-sh

ift m

utat

ions


Cla

ssifi

catio

n M

IMD

istin

guis

hing

Inhe

ritan

ceC

hrom

osom

al

Gen

eP

rodu

ctM

utat

iona

l spe

ctru

mcl

inic

al fe

atur

eslo

catio

n

HM

SN IV

A

214400

Bot

h a

dem

yelin

atin

g Aut

osom

al8q1

3–q

21.1

GD

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linat

ing

type

:an

d an

axo

nal f

orm

rece

ssiv

edi

ffere

ntia

tion-

nons

ense

and

are

reco

gniz

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asso

ciat

edm

isse

nse

mut

atio

nsAxo

nal t

ype:

pr

otei

n 1

Axo

nal t

ype:

patie

nts

can

have

no

nsen

se, m

isse

nse,

and

voca

l cor

d pa

resi

sfram

e-sh

ift m

utat

ions

HM

SN IV

B1

601382

Slow

NC

VsAut

osom

al11q2

2M

TMR

2M

yotu

bula

rin-r

elat

edN

onse

nse,

fram

e-sh

ift,

rece

ssiv

epr

otei

n 2

and

splic

e-si

te m

utat

ions

HM

SN IV

B2

604563

Slow

NC

VsAut

osom

al11p1

5SB

F2SE

T-bi

ndin

g fa

ctor

2

Non

sens

e m

utat

ions

rece

ssiv

ean

d in

-fra

me

dele

tion

HM

SN IV

C

601596

Slow

NC

VsAut

osom

al5q3

2U

nkno

wn

Unk

now

nU

nkno

wn

rece

ssiv

e

HM

SN IV

D/

601455

Ons

et in

firs

t dec

ade,

Aut

osom

al8q2

4.3

ND

RG

1N

-myc

dow

nstr

eam

-N

onse

nse

mut

atio

nH

MSN

L

early

-ons

et d

eafn

ess,

re

cess

ive

regu

late

d ge

ne 1

(Arg

148St

op)

(see

text

)sl

ow N

CVs

prot

ein

HN

PP

162500

See

text

Aut

osom

al17p1

1.2

PM

P2

2Pe

riphe

ral m

yelin

O

ver 8

5%

of p

atie

nts

dom

inan

tpr

otei

n 22

have

a 1

.5-M

b de

letio

n of

17p1

1.2

, inc

ludi

ngP

MP

22

. The

rem

aind

erha

ve fr

ame-

shift

or s

plic

e-si

te m

utat

ions

that

resu

lt in

loss

of f

unct

ion

of th

e ge

ne

CH

N

605253

See

text

Aut

osom

al1q2

2M

PZ

Mye

lin p

rote

in z

ero

Non

sens

e m

utat

ion

dom

inan

t

Aut

osom

al10q2

1.1

–q22.1

EGR

2Ea

rly g

row

thD

oubl

e m

isse

nse

mut

atio

ndo

min

ant

resp

onse

2

(on

sam

e al

lele

)

Aut

osom

al

10q2

1.1

–q22.1

EGR

2

Early

gro

wth

M

isse

nse

mut

atio

nre

cess

ive

resp

onse

2

Tabl

e 1

.Her

edita

ry m

otor

and

sen

sory

neu

ropa

thie

s (H

MSN

s): c

lass

ifica

tions

, inh

erita

nce

patt

erns

, and

mol

ecul

ar g

enet

ics.

CH

N: c

onge

nita

l hyp

omye

linat

ing

neur

opat

hy; H

NPP:

her

edita

ry n

euro

path

y w

ith li

abili

ty to

pre

ssur

e pa

lsie

s; N

CV:

ner

ve c

ondu

ctio

n ve

loci

ty.




of HMSN. Mutant protein may have an increased tendency to formconducting hemichannels compared with normal protein. This couldprevent the normal functioning of Schwann cells and neurons byincreasing their membrane permeability.

KIF1B has 47 exons and encodes an N-terminal-type motor protein with 1,816 amino acids. It acts as a motor for the anterograde transportof mitochondria. A mutation in this gene could result in the production of a mutant protein without any motor activity. The precise mechanismby which a mutation in this gene causes HMSN IIA is unknown.

RAB7 has six exons and encodes a ubiquitously expressed protein with 207 amino acids called RAS-associated protein 7. This is a smallGTPase, which is a member of the RAS-related GTP-binding proteinfamily. It is believed to be involved in vesicular transport of proteins. It is not understood how mutations in this gene result in HMSN IIB.

LMNA has 10 exons and codes for two proteins by alternative splicing of its exons. The gene products include lamin A and lamin C. Bothproteins are components of the nuclear lamina. The mechanisms bywhich mutations in this gene cause HMSN IIB1 are not understood.Mutations in LMNA can cause several other conditions (see LGMD entry, p.18).

GARS has 17 exons and encodes glycyl-tRNA synthetase. This is an enzyme with 685 amino acids that catalyses the esterification of glycine to its cognate tRNA during protein synthesis. Missensemutations in GARS cause HMSN IID and an autosomal dominant form of distal spinal muscular atrophy (type V, MIM 600794) by an unknown mechanism.

GDAP1 has six exons and codes for ganglioside-induced differentiation-associated protein. This has 358 amino acids and may be involved inthe signal transduction pathway in neuronal development. The precisemechanism by which mutations in GDAP1 cause HMSN IIG, IIK, andIVA is not understood.

PRX contains six exons and produces two mRNA transcripts. Onetranscript produces L-periaxin and the other S-periaxin. Both proteinsare expressed in Schwann cells and interact with the C-termini ofplasma membrane proteins and with cytoskeletal proteins, and arerequired for the maintenance of peripheral nerve myelin. Mutations in this gene cause a form of HMSN III.



MTMR2 is composed of 15 exons. Its protein product, myotubularin-related protein 2, has 643 amino acids and is ubiquitously expressed.It is a dual-specificity phosphatase with homology to myotubularin. It is not understood how mutations in this gene cause HMSN IVB1.

SBF2 is a large gene with 43 exons. Its protein product, SET-bindingfactor 2, has 1,849 amino acids and is a member of the pseudophosphatasebranch of myotubularin-related proteins. It is expressed in fetal brain,spinal cord, and peripheral nerves and is involved in the differentiationof Schwann cells during myelination. Mutations in SBF2 cause HMSN IVB2.

NDRG1 has 16 exons. Its protein product has 394 amino acids and is ubiquitously expressed. It appears to be expressed at particularly high levels in Schwann cells. NDRG1 protein is involved in growth arrest and cell differentiation, and it appears to have a role in Schwanncell signaling that is necessary for axonal survival. Mutations in NDRG1cause HMSN IVD (this condition is also called HMSN, Lom type, orHMSN L because it only affects members of the Gypsy community of Lom in Bulgaria).

Genetic diagnosis PMP22, MPZ, and CX32 mutation analysis is available from several

and counseling diagnostic laboratories. However, testing for mutations in the othergenes is not routinely available at this time. All autosomal dominant and sporadic cases of HMSN I should be tested for mutations in PMP22 and MPZ. Patients from X-linked dominant HMSN families,sporadic male cases with HMSN I, and sporadic female cases withHMSN II should also be tested for mutations in CX32.

Detailed pedigree analysis can often establish the mode of inheritance of HMSN in a family and allow accurate genetic advice to be given toother family members. HMSN I can show remarkable interfamilial andintrafamilial variability of expression. Therefore, parents of an apparentlysporadic case should be carefully examined and offered nerve conductionstudies to determine whether one parent is mildly affected. Predictivetesting can be offered to at-risk members of families in which a mutationhas been identified.

Counseling in HNPP families is carried out on an autosomal-dominant basis.


Limb-girdle Muscular Dystrophy18

Limb-girdle Muscular Dystrophy(also known as: LGMD)

The LGMDs are a group of hereditary muscle disorders that predominantly affect the shoulderand pelvic girdles. There are several autosomal dominant (LGMD1) and autosomal recessive(LGMD2) forms with remarkable locus heterogeneity. Table 2 summarizes the classification,distinguishing clinical features, inheritance pattern, and molecular genetics of the variousforms of LGMD.

MIM See Table 2.

Clinical features The LGMDs are a clinically and genetically heterogeneous group of disorders. Affected individuals present with proximal weakness of the upper and lower limbs.

Age of onset See Table 2.

Epidemiology LGMDs affect all populations, but their incidence varies in differentpopulations. Autosomal dominant forms only account for about 10% ofcases. Mutations in one of the sarcoglycan genes (sarcoglycanopathies)can be seen in 8%–25% of patients with LGMD. In most populations the most frequently seen form of LGMD is LGMD2A, which accounts for 40%–45% of cases. However, LGMD2I is probably the mostcommon form of LGMD in the UK.

Inheritance, See Table 2.chromosomallocation,and gene

Molecular TTID is composed of 10 exons and codes for a structural muscle protein pathogenesis with 498 amino acids called titin immunoglobulin domain protein or

myotilin. This is a thin, filament-associated, Z-disc protein that binds toα-actinin, F-actin, and filamin c. It cross-links actin filaments and controlssarcomere assembly, and is believed to play an important role in thestabilization and anchorage of thin filaments. Mutations in TTID probablycause LGMD by interfering with the proper organization of Z-discs.

LMNA contains 10 exons and encodes two proteins as a result ofalternative splicing of its exons. These proteins include lamin A (664 amino acids) and lamin C (572 amino acids). Both lamins



are nuclear envelope proteins. How mutations in LMNA cause LGMD is not known. Mutations in LMNA have also been described in severalother conditions, including an autosomal dominant form of dilatedcardiomyopathy (type 1A, MIM 115200), an autosomal dominant formof Emery–Dreifuss muscular dystrophy (MIM 181350), Dunnigan typepartial lipodystrophy (MIM 151660), an autosomal recessive form ofHMSN (HMSN IIB1, MIM 605588 – see p.13), and two dysmorphicsyndromes associated with premature ageing: Hutchinson-Gilfordsyndrome or progeria (MIM 176670) and mandibuloacral dysplasia(MIM 248370).

CAV3 contains three exons and encodes caveolin 3, which has131 amino acids. Caveolin 3 is the muscle-specific form of the caveolinprotein family. Caveolins are the main protein components of caveolae(50–100 nm invaginations of plasma membranes). Mutations in CAV3act in a dominant-negative manner by interfering with oligomerization of caveolin 3. This disrupts caveolae formation in the sarcolemmalmembrane. Caveolin 3 interacts with dysferlin at the surface of thesarcolemmal membrane, and is also involved in normal expression of α-dystroglycan at the sarcolemmal surface. Caveolin 3 deficiencycould therefore result in muscular dystrophy by interfering with thenormal expression of dysferlin and α-dystroglycan.

CAPN3 has 24 exons and encodes an 821-amino-acid protein calledcalpain 3. This is a muscle-specific, calcium-dependent protease.It appears to have a role in controlling the levels of muscle-specifictranscription factors, though the precise role of calpain 3 in muscle and the mechanism by which a deficiency of this protein causesmuscular dystrophy are unknown.

DYSF is a large gene with 55 exons. It encodes dysferlin, a 2,080 amino-acid protein that localizes to the sarcolemmal membrane andco-immunoprecipitates with caveolin 3 in skeletal muscle. It is expressedvery early in human development. Studies in mice have shown thatdysferlin has an essential role in the resealing of the sarcolemma inresponse to injury. Therefore, mutations in DYSF probably causemuscular degeneration by disrupting the muscle membrane repairmachinery. Mutations in DYSF have also been identified in Miyoshimyopathy, an autosomal recessive distal myopathy (MIM 254130).

SGCA has eight exons and encodes α-sarcoglycan (also called 50-kDadystrophin-associated glycoprotein [DAG]), which has 387 amino acids.


Limb-girdle Muscular Dystrophy

Cla

ssifi

catio

n M

IMD

istin

ctiv

eA

ge o

fIn

herit

ance

Chr

omos

omal

Gen

eP

rote

in p

rodu

ct

Mut

atio

nal s

pect

rum

cl

inic

alon

set

loca

tion

feat

ures

LGM

D1A

159000

Dys

arth

ria18–3

5 y

ears

Aut

osom

al5q3

1TT

IDTi

tin im

mun

oglo

bulin

-Mis

sens

e m

utat

ions

dom

inan

tdo

mai

n pr

otei

n or

myo

tilin

LGM

D1B

159001

Car

diac

invo

lvem

ent,

4–3

8 y

ears

Aut

osom

al1q2

1.2

LMN

ALa

min

A/C

M

isse

nse

and

splic

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at re

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pre

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prot

ein

trun

catio

n

Tabl

e 2

.Lim

b-gi

rdle

mus

cula

r dys

trop

hies

(LG

MD

s): c

lass

ifica

tion,

clin

ical

feat

ures

, and

mol

ecul

ar g

enet

ics.

CK

: cre

atin

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nase

.


Limb-girdle Muscular Dystrophy22

SGCB has only six exons and encodes a protein with 318 amino acids,which is called β-sarcoglycan (43-kDa DAG). SGCD has nine exons and encodes δ-sarcoglycan (35-kDa DAG), which has 290 amino acids.SGCG is composed of eight exons and codes for γ-sarcoglycan (35-kDaDAG). This protein also has 290 amino acids. The sarcoglycans aretransmembrane proteins that are an important component of thedystrophin–glycoprotein complex at the sarcolemmal membrane. The components of this complex link dystrophin inside the sarcolemmato the laminin α2 chain of merosin and other proteins in the extracellularmatrix. The dystrophin–glycoprotein complex is believed to play acritical role in maintaining the integrity of the sarcolemmal membrane,particularly during muscle contraction. Therefore, absence or deficiencyof the critical components of this complex can result in the phenotype of muscular dystrophy. Heterozygous mutations in SGCD can also cause one form of dilated cardiomyopathy type 1L (MIM 606685).

TCAP is a small gene with only two exons. It encodes a structuralsarcomeric protein called titin cap or telethonin. This protein has 167 amino acids and localizes to the Z-disc of adult skeletal muscle.

TRIM32 has two exons and encodes a protein with 653 amino acids. Its protein product, TRIM 32 protein, is thought to be an E3 ubiquitinligase. The mechanism by which mutations in this gene result in theLGMD phenotype is unknown.

FKRP is composed of four exons and encodes fukutin-related protein,which has 495 amino acids. Fukutin-related protein is probably aGolgi-resident glycosyltransferase that is involved in the glycosylation of α-dystroglycan. This protein links the dystrophin–glycoproteincomplex to various extracellular proteins, including the laminin α2 chain of merosin, neurexin, and agrin. Deficiency of fukutin-relatedprotein probably results in muscular dystrophy due to aberrantglycosylation of α-dystroglycan.

Genetic diagnosis The diagnosis of LGMD is made by the combination of clinical features,and counseling immunohistochemistry on a muscle biopsy sample, and molecular genetic

analysis. Immunohistochemistry and genetic testing are only availablefrom a few specialized laboratories. Interpretation of the results of muscleimmunohistochemistry is difficult and should only be carried out bylaboratories experienced in the use of this technique. It is important torule out facioscapulohumeral muscular dystropy and Emery–Dreifuss



muscular dystrophy in families that appear to have an autosomaldominant form of LGMD. In large families with an autosomal recessiveform of LGMD, linkage analysis might allow the precise form of LGMD to be identified. However, this should always be confirmed by muscleimmunohistochemistry or mutation analysis of the relevant gene.

Because of the genetic heterogeneity of LGMD, genetic counseling isdifficult. Accurate genetic counseling and prenatal diagnosis are onlypossible in families where a definitive diagnosis can be made by muscleimmunohistochemistry and genetic testing. Accurate counseling is also possible in large families with several affected members, where it is possible to determine the precise mode of inheritance (autosomalrecessive or dominant). Isolated cases of LGMD could represent anautosomal recessive form of LGMD or a new autosomal dominant mutation.

Myotonic Dystrophy(also known as: MD; dystrophia myotonica; Steinert disease. Includes proximal myotonic myopathy [PROMM])

MIM 160900 (MD1)602668 (MD2)600109 (PROMM)

Clinical features Four forms of MD1 can be recognized based on age of onset and clinicalfeatures. These include a mild form, an adult or classical form, a congenitalform, and a childhood or juvenile form. Patients with mild MD usuallypresent with presenile cataracts. The classical form of MD is a multisystemdisorder. Symptoms include: muscle weakness and wasting, grip andpercussion myotonia, cardiac arrhythmias that can present as syncope or sudden death, gastrointestinal problems, cataracts, an increasedincidence of diabetes mellitus, and testicular atrophy in males.

The distribution of muscle weakness and wasting is characteristic and is responsible for the well-recognized facial features of this condition.These include frontal balding, ptosis, facial weakness, bitemporalnarrowing (due to wasting of the temporalis muscles), wasting of the jawmuscles, and a slender neck due to wasting of the sternomastoids. Earlyappearance and progression of male pattern baldness is also a feature.Distal limb muscles tend to be affected earlier than proximal muscles.


Myotonic Dystrophy24

Congenital MD is the most severe form of this disease and is the result of anticipation. It can present antenatally as reduced fetal movementsand polyhydramnios, or in the neonatal period as severe hypotonia,respiratory distress (often requiring ventilation), feeding difficulties,facial weakness, cardiac problems (cardiomyopathy or arrhythmias),and talipes or arthrogryposis. A chest X-ray will often show thin ribs.Many patients with congenital MD die in childhood. Survivors showdelayed development and have learning difficulties and characteristicfacies (facial diplegia, an open-mouthed appearance with a tented upper lip, and a prominent, everted lower lip).

A childhood or juvenile form of this condition has also been described.These patients usually present between 1 and 10 years of age withspeech and language delay and learning difficulties, although somepatients present with muscle weakness and myotonia at school age.

MD2 and PROMM are probably a single entity as they have similarclinical features and are allelic or have very closely linked genes.Patients with these conditions present with slowly progressive proximalmuscle weakness, mild myotonia, cardiac arrhythmias, and late-onsetcataracts. White matter changes have been described in some families.Features that help to distinguish these conditions from the classical formof MD1 include the absence of facial weakness and the characteristicfacial features that are seen in patients with classical MD, absent or minimal distal limb weakness, and the presence of myalgia.

Age of onset The mild form of MD1 presents in late adult life, the classical formpresents in late adolescence or early adult life, the congenital formpresents antenatally or in the neonatal period, and the childhood or juvenile form presents in early childhood.

MD2 and PROMM present in adulthood.

Epidemiology MD1 has an estimated incidence of 1 in 8,000. It appears to beparticularly prevalent in the Saguenay-Lac-St-Jean region of Canada,where its prevalence is 1 in 475.

MD2 and PROMM are rare disorders. Their population incidence and prevalence are unknown.

Inheritance MD1: autosomal dominantMD2/PROMM: autosomal dominant



Chromosome MD1: 19q13.3location MD2/PROMM: 3q13.3–q24

Genes MD1: DMPK (dystrophia myotonica protein kinase)MD2/PROMM: ZNF9 (zinc finger protein 9)

Mutational MD1 is caused by the expansion of a CTG triplet repeat motif inspectrum the 3 -untranslated region of the last exon of DMPK. In the general

population, the number of CTG repeats varies from 5 to 37. Affectedindividuals have more than 50 repeats and there appears to be acorrelation between the number of repeats and the severity of thephenotype. Repeat sizes of 50–100 are associated with the mild form of MD, whereas repeat sizes of between 500 and 1,500 result in thecongenital MD phenotype. Expansions of between 100 and 500 areusually associated with classical MD, but it is not possible to predict the age of onset or the severity of disease in this group of patients.

The CTG repeat shows meiotic instability and its size tends to increase in successive generations. This is responsible for the phenomenon ofanticipation, in which the phenotype of a disease increases in severity in successive generations. Maternal transmission can be associated with a large expansion in the CTG repeat number, whereas paternaltransmission is usually associated with a modest expansion of the repeat or, in some cases, a decrease in the number of repeats. Thus,congenital MD, which is caused by very large CTG repeat expansions, is almost always maternally transmitted. In contrast, the childhood or juvenile form of MD is more frequently paternally transmitted.

Patients with MD2/PROMM have an expansion of a CCTG repeat motif in the first intron of ZNF9. Affected individuals have between 75 and 11,000 repeats, with an average of 5,000.

Molecular DMPK has 15 exons and produces two main protein isoforms of 71 kDapathogenesis and 80 kDa as a result of alternative splicing. Both of these isoforms are

predominantly expressed in skeletal and cardiac muscle. The precisemechanism by which the CTG repeat expansion causes MD1 is unknown.Interest has focused on the possibility that the allele with the CTG repeatexpansion produces mRNA that inappropriately binds to proteins viaCUG repeats (thymidine is replaced by uridine in RNA). One particularprotein (CUG-binding protein) is involved in processing mRNA from


Myotonic Dystrophy26

several other genes, including cardiac troponin T. Binding of CUG-bindingprotein to the mRNA product of the DMPK allele with the CTG repeatexpansion could interfere with its ability to process the mRNAs ofseveral other genes, and altered expression of these genes could result in the MD1 phenotype. Thus, the CTG repeat expansion in DMPK would appear to be a gain-of-function mutation.

ZNF9 has five exons and encodes a protein with 177 amino acids. This protein has seven zinc-finger domains and is believed to be anRNA-binding protein. Mutant ZNF9 mRNA accumulates in the nucleus and probably results in the MD2/PROMM phenotype in a manneranalogous to the expansions in the DMPK gene that cause MD1.

Genetic diagnosis Genetic testing for MD1 is widely available, so all patients with MD

and counseling should have genetic testing to confirm the diagnosis. Patients whoseclinical features are suggestive of MD but who test negative for the CTG repeat expansion in DMPK are likely to have MD2/PROMM or analternative myotonic disorder. Counseling is on the basis of autosomaldominant inheritance. Women with MD1 should be told that theirchildren could be affected with congenital MD as a result of anticipation.Women who have neuromuscular disease or who have previously had an affected child with congenital MD are particularly at risk of having a baby with congenital MD. Patients with MD should be told that they are at risk of developing cardiac arrhythmias, cataracts, and diabetesmellitus, and they should be under the care of a physician. They shouldalso be told about the complications of general anesthesia, includingmalignant hyperthermia and postanesthetic apnea. Patients should be asked to carry an alert card or bracelet. Presymptomatic/predictivegenetic testing can be offered to those from families where a CTG repeatexpansion has been previously documented in an affected individual (and who therefore have a 50% risk of being affected). Prenatal diagnosisis also available by testing DNA extracted from either a chorionic villussample or cultured amniocytes for the CTG repeat expansion.

Genetic testing for MD2/PROMM is only available on a research basis.Counseling is as for autosomal dominant inheritance. Anticipation does occur, but is milder than that seen in MD1.



Spinal Muscular Atrophy(also known as: SMA)

MIM 253300 (SMA type I/Werdnig–Hoffmann disease)253550 (SMA type II)253400 (SMA type III/Kugelberg–Welander syndrome)

Clinical features Children with SMA present with generalized muscle weakness and wasting,hypotonia, and areflexia. The muscle weakness begins proximally, and characteristically involves the intercostal muscles and later thediaphragm, but spares the extraocular muscles and facial muscles.Fasciculation of the tongue and other muscles is a helpful diagnosticclue. Childhood SMA is classified into three types based on age of onset,extent of motor development, and prognosis. The most severe form isSMA type I. Children with this condition never learn to sit and usuallydie by the age of 2 years. Patients with SMA type II learn to sit withoutsupport, but never learn to walk unaided. The prognosis is variable, withsome patients dying in childhood and others surviving to adulthood.Patients with SMA type III are able to walk independently. They haveslowly progressive muscle weakness, and survive into adulthood.

Diagnosis can be confirmed by electromyography (which shows aneurogenic pattern) and by muscle biopsy (which shows groupedatrophy of both type I and II fibers, with hypertrophy of type I fibers).

Age of onset SMA type I: before 3 monthsSMA type II: 3–18 monthsSMA type III: after 18 months

Epidemiology The incidence of all forms of SMA is about 1 in 10,000 live births. SMA has been described in all ethnic groups. The heterozygote (carrier)frequency is about 1 in 50.

Inheritance All forms of childhood SMA are inherited in an autosomal recessivemanner. However, a small proportion (~2%) of those with type II or IIImay have a form inherited in an autosomal dominant manner.

Chromosomal 5q12.2–q13.3location

Gene SMN1 (survival of motor neuron 1)


Spinal Muscular Atrophy28

Mutational There are two copies of the SMN gene: a telomeric copy (SMN1) spectrum and a centromeric copy (SMN2). There are only minor differences in

the coding sequence of these two genes. Both genes are expressed, but SMN1 produces much higher levels of the functional full-lengthtranscript than SMN2. Because of the homology between these twogenes, gene conversion events are frequent, resulting in the conversionof SMN1 to SMN2.

The vast majority of patients with SMA (~96%) are homozygous for adeletion or gene conversion of SMN1. A small number of patients (~4%)are compound heterozygotes with a deletion or gene conversion affectingone SMN1 allele and a different mutation in the other allele. Othermutations in SMN1 are rare, but can include point mutations (mostlymissense mutations and splice-site mutations) and frame-shift mutations.

The presence of multiple copies of SMN2 in patients homozygous for a deletion or gene conversion of SMN1 can modify the phenotype andlead to less severe disease (SMA types II or III). Other genetic modifiersof the phenotype have also been described (eg, splicing mechanisms of the SMN2 gene and deletion of the H4F5 gene that lies upstream of SMN1).

Molecular The protein product of the SMN1 and SMN2 genes is expressed in pathogenesis several areas, including the central nervous system, skeletal muscle,

heart, liver, kidneys, lungs, thymus, and pancreas. Within the centralnervous system it is expressed in the anterior horn cells of the spinalcord. The SMN protein is localized to the cytoplasm and nucleus. In the nucleus it is localized in small, discrete, dot-like structures called“gems”. It interacts with several small nuclear ribonucleoproteins andappears to have an important role in the generation of the pre-mRNAsplicing machinery, and, therefore, in mRNA processing in the cell.Although SMN1 is ubiquitously expressed, loss of function of this gene only results in degeneration of spinal motor neurons because these cells are believed to need high levels of SMN protein to survive.

Genetic diagnosis Genetic testing for SMA is routinely available. Carrier testing for and counseling SMA is also available from diagnostic laboratories. Counseling is

on an autosomal recessive basis. Prenatal diagnosis can be offered to parents of children with SMA in whom the diagnosis has beenconfirmed by genetic testing.


2. Cerebral Malformations and MentalRetardation Syndromes

Angelman Syndrome 30

Fragile X Syndrome 34

Holoprosencephaly 36

Hunter Syndrome 40

Huntington Disease 41

Lesch–Nyhan Syndrome 43

Lissencephaly 45

Lowe Syndrome 52

Neuronal Ceroid Lipofuscinosis 53

Pelizaeus–Merzbacher Syndrome 57

Prader–Willi Syndrome 59

Rett Syndrome 61

X-linked Adrenoleukodystrophy 62

X-linked α-Thalassemia and Mental Retardation Syndrome 64

X-linked Hydrocephalus 66


Angelman Syndrome30

Angelman Syndrome(also known as: AS; happy puppet syndrome)

MIM 105830

Clinical features Affected children show severe developmental delay with very limitedspeech, ataxia, and easily provoked laughter; they have a happydemeanor and excitable personality. Convulsions occur in 80%, usually with onset in early childhood. Other common features includemicrocephaly, drooling, prognathism, hypopigmentation, and a scoliosis(this can progress and require surgical correction). Life expectancy is relatively normal.

Age of onset The diagnosis is usually made in early childhood. With the benefit of hindsight, it often becomes apparent that problems were first evident in infancy.

Epidemiology The incidence is estimated to be between 1 in 10,000 and 1 in 40,000.

Inheritance The mode of inheritance is complex, as four different causes of AS are recognized: class I (maternally derived chromosome 15q11–q13interstitial deletion), class II (paternal uniparental disomy [UPD] forchromosome 15), class III (imprinting defect involving the Prader–Willisyndrome [PWS]/AS critical region), and class IV (mutation in UBE3A).

Chromosomal 15q11–q13 location

Gene UBE3A (ubiquitin protein ligase E3A)

Mutational Missense, nonsense, splice-site, and frame-shift mutations.spectrum

Molecular AS is caused by abnormal expression of the maternally imprintedpathogenesis UBE3A gene, which contains 16 exons and encodes a ubiquitin protein

ligase that is thought to play a role in the localization of proteins in thebrain. UBE3A is chiefly expressed in the hippocampus and cerebellumand is expressed only from the maternal allele in brain (ie, it showstissue-specific imprinting).


Cerebral Malformations and Mental Retardation Syndromes 31

Class I. There is an interstitial deletion involving 15q11–q13 in thematernally derived chromosome 15. This accounts for approximately70% of all cases. Most such deletions occur de novo. Rarely, a morecomplex chromosome rearrangement involving deletion of 15q11–q13is identified, possibly as a de novo finding or as a result of malsegregationof a maternally balanced rearrangement.

Class II. Paternal UPD for chromosome 15 accounts for around 2% of all cases. In this situation, both number 15 chromosomes are of paternalorigin. This may be due to nondisjunction in paternal meiosis, resulting in a disomic sperm. The disomic sperm may then have fertilized a monosomicovum, resulting in transient trisomy 15 in the zygote with subsequent lossof the maternally derived chromosome 15 (“trisomy rescue”).

Class III. Imprinting defects account for approximately 4% of all cases.Roughly half of these are caused by very small deletions involving thePWS/AS imprinting box/center (see Figure 1). The cause of the remaining50% is uncertain.

Class IV. Mutations in UBE3A are found in 5%–10% of cases. Most of these arise de novo, but the mother is a carrier in up to 20% of casesthrough either inheriting a “silent” mutation from her father or showinggerm-line mosaicism.

In around 10%–15% of cases no chromosomal or molecular abnormalitycan be identified. In these situations there may be an alternative diagnosis,such as Rett syndrome (p.61–2), or there may be a mutation in an asyet unidentified UBE3A-related gene.

Figure 1. Simplified diagram of the PWS/AS (Prader–Willi syndrome/Angelman syndrome) critical regionon chromosome 15. In the maternal chromosome the AS imprinting center (IC) is active and methylates(silences) the SNRPN promoter. UBE3A is expressed. In the paternal chromosome the PWS IC activatesSNRPN and other adjacent genes, including antisense UBE3A, which silences UBE3A. M and P refer tomaternally and paternally derived patterns of expression, respectively.

CentromereM

ASIC

P P

PWSIC

SN

RP

N

P

antis

ense

UB

E3A

UB

E3A

qter

M


Angelman Syndrome32

Genotype–phenotype Children with class I microdeletions are the most severely affected,correlation with the highest incidence of microcephaly and seizures and the

most severe learning disability, with absent speech. They alone showhypopigmentation, almost certainly because of haploinsufficiency for the P gene, which is located at 15q11–q13 and deficiency of whichcauses type II oculocutaneous albinism (MIM 203200). The next mostseverely affected children are those with class IV UBE3A mutations.These children often have microcephaly and seizures, but demonstratebetter developmental progress than children in class I. Children withUPD (class II) and imprinting mutations (class III) are the least severelyaffected, with a low incidence of microcephaly and seizures, but theystill show severe learning disability with only very limited speech.

Genetic diagnosis Diagnosis in classes I–III can be made by methylation analysis using and counseling a methylation-sensitive restriction enzyme and a probe from the PWS/AS

critical region (see Figure 2). This reveals the presence of only a paternalband. Microdeletion analysis by fluorescence in situ hybridization (FISH)identifies all class I cases (see Figure 3). If a chromosomal abnormalityis identified, parental chromosome studies should be undertaken.Paternal UPD (class II) is identified by restriction fragment lengthpolymorphism or microsatellite analysis. Class IV cases (UBE3Amutations) can only be detected by mutation analysis (usually single-strand conformation polymorphism screening followed by directsequencing). Most microdeletions identified by FISH are de novoand have a low risk of recurrence of less than 1%, which is attributable to maternal germ-line mosaicism. The recurrence risk for paternal UPDcases (class II) is negligible. If an imprinting center microdeletion orUBE3A mutation is present in the affected child’s mother then therecurrence risk is 50%; otherwise, the recurrence risk is low but not negligible because of possible maternal germ-line mosaicism.



Figure 2. The principle underlying the methylation test for the Angelman and Prader–Willi syndromes.“A” represents sites cleaved by a nonmethylation-sensitive restriction enzyme. “B” represents a restriction sitecleaved by a methylation-sensitive restriction enzyme such as HpaII. Normally, the maternal chromosome 15is imprinted (methylated) so that cleavage at site “B” does not occur. In Angelman syndrome there is absenceof a normally imprinted maternal chromosome. The opposite applies in Prader–Willi syndrome.

Figure 3. Deletion of the proximal long arm of one number 15 chromosome (15q11–q13) in a child with Angelman syndrome demonstrated by fluorescence in situ hybridization analysis. Image courtesy of Mrs Karen Marshall, Cytogenetics Laboratory, Leicester Royal Infirmary, UK.

Paternal chromosome 15

A B AProbe

Maternal chromosome 15

Maternalchromosome

Paternalchromosome

Normal Prader—Willisyndrome

Angelmansyndrome

Southernblot


Fragile X Syndrome34

Fragile X Syndrome

MIM 309550

Clinical features This is the most common inherited cause of mental retardation. Both sexes can be affected, but facial dysmorphism is usually seen in males. Presenting features include global developmental delay,moderate to severe learning difficulties, autistic features, attentiondeficit hyperactivity disorder, and seizures. Dysmorphic features include macrocephaly, a long face with broad forehead, anteverted ears, and prominent chin. Additional features include hyperextensiblemetacarpophalangeal joints, pes planus, hypotonia, and macro-orchidism in postpubertal males.

Age of onset The condition can be diagnosed in infancy, but diagnosis is oftendelayed until childhood.

Epidemiology Prevalence is about 1 in 5,000 in males and 1 in 8,000 in females.Studies indicate that about 0.6% of patients (male and female) withmental retardation have fragile X syndrome.

Inheritance X-linked dominant

Chromosomal Xq27.3location

Gene FMR1 (fragile X mental retardation 1)

Mutational The FMR1 gene has a polymorphic CGG repeat motif in the 5 spectrum untranslated region of the first exon. In the vast majority of patients

(~99%), this motif is expanded to more than 200 repeats. Anexpansion of this size is called a “full mutation”. Normal individualshave 6–54 CGG repeats. Fragile X carriers have 55–200 CGG repeats.An expansion of this size is called a “premutation”. Premutations areunstable in meiosis and can expand to a full mutation on maternaltransmission. Individuals with 45–54 repeats are said to have an“intermediate” allele. These alleles can demonstrate instability, eitherincreasing or decreasing in size during meiosis. Intermediate alleles do not usually expand to a full mutation in a single generation. About1% of patients with fragile X syndrome have other FMR1 mutations.



These include partial or complete gene deletions as well as missense,frame-shift, and splice-site mutations.

Molecular FMR1 has 17 exons and encodes FMR protein (FMRP), which haspathogenesis 632 amino acids. Expression levels of FMRP are highest in the brain,

testes, lymphocytes, and placenta. Expansion of CGG repeat numbers to more than 200 results in hypermethylation of the repeats andsurrounding sequences. This results in transcriptional silencing of theFMR1 gene, which is then unable to produce FMRP. Other mutations in this gene result in the production of a truncated protein that is rapidlydegraded or the production of nonfunctional protein. Alleles of FMR1with normal or premutation-sized CGG repeats produce normal amountsof FMRP. There is good evidence to suggest that FMRP is a translationalregulator. It is able to bind RNA, including its own mRNA and about 4% of neuronal mRNA transcripts. The absence of FMRP could alter the transcriptional profile of many of these mRNAs, some of whichprobably encode important neuronal proteins.

Genetic diagnosis Molecular genetic testing for fragile X syndrome is widely available.and counseling However, this is confined to testing for the CGG expansion mutation.

Almost all patients with nonspecific developmental delay will undergogenetic testing for fragile X syndrome, but the diagnostic yield of suchtesting is poor. More focused testing (such as testing of males withmoderate to severe developmental delay and autistic features or some of the facial features of fragile X syndrome, or testing males or femaleswith a family history of mental retardation) improves the diagnostic yield.

Chromosome analysis to look for the fragile site at Xq27.3 (FRAXA) isnot a reliable test for fragile X syndrome because it does not detect allfemales. Also, it is not a reliable method of carrier detection. In addition,other fragile sites located close by (FRAXE and FRAXF) can causediagnostic confusion.

Genetic counseling in fragile X syndrome is difficult. Males with apremutation are called “normal transmitting males”. They will transmittheir X chromosome, with the premutation, to all of their daughters and their Y chromosome to all of their sons. The premutation usuallyundergoes slight expansion when paternally transmitted, but expansionof a premutation to a full mutation is not seen. Therefore, all daughtersof a normal transmitting male are fragile X carriers.


Holoprosencephaly36

Females with a premutation are called carriers. They will transmit their X chromosome with the CGG expansion to half their sons and half theirdaughters. A premutation has a high risk of expanding to a full mutationwhen maternally transmitted, and the risk of expansion depends on thesize of the premutation. Premutations with more than 70 CGG repeatshave an 80% chance of expanding to a full mutation. The smallestknown premutation that has expanded to a full mutation in a singlegeneration had 59 CGG repeats. All males who inherit a full mutationfrom their mother will be affected with fragile X syndrome. Between50% and 80% of females with a full mutation have some degree oflearning difficulties. Prenatal diagnosis can be offered to female carriers.

Holoprosencephaly (also known as: HPE)

MIM See Table 1.

Table 1. Holoprosencephaly (HPE): MIM numbers, chromosomal locations, and genes.

Clinical features HPE has a very variable phenotype. Severe forms (alobar or semilobarHPE) present with characteristic facial dysmorphism (such as cebocephalyor premaxillary agenesis), microcephaly, profound developmental delay,spastic quadriparesis, seizures, and failure to thrive. Mild forms (lobarHPE) can present with normal facial features or mild facial dysmorphism(such as ocular hypotelorism, abnormal superior labial frenulum, single median maxillary incisor, and high-arched palate), together with microcephaly, developmental delay, and subtle neurologicabnormalities such as anosmia.

Type MIM Chromosomal location Gene

HPE1 236100 21q22.3 Unknown

HPE2 157170 2p21 SIX3 (sine oculis homeobox,Drosophila homolog 3)

HPE3 142945 7q36 SHH (Sonic hedgehog)

HPE4 142946 18p11.3 TGIF (transforming growth factorβ-induced factor)

HPE5 603073 13q32 ZIC2 (zinc finger protein of cerebellum 2)

HPE6 605934 2q37.1–q37.3 Unknown

HPE7 601309 9q22.3 PTCH (homolog of Drosophilasegment polarity gene patched)



HPE can be an isolated problem or part of a multiple malformationsyndrome. Isolated HPE is usually caused by a single gene mutation.Syndromic forms of HPE can be caused by chromosomal aberrations or single gene mutations. Trisomy 13 is an important cause of HPE.Mutations in the genes that are discussed here result in isolated,nonsyndromic HPE.

Age of onset Severe forms are usually apparent at birth or in the neonatal period.Diagnosis of the milder forms is often delayed, sometimes untillate childhood.

Epidemiology HPE has been reported in all populations and has an incidence of 1 in 16,000 live births.

Inheritance Usually autosomal dominant with reduced penetrance. Most cases are sporadic.

Chromosomal See Table 1.location and gene

Mutational SIX3 mutations (causing HPE2) are a rare cause of familial andspectrum sporadic cases of isolated HPE. Most reported mutations have

been missense mutations that are predicted to result in functionalinactivation of the gene. One family had a frame-shift mutation that resulted in protein truncation.

SHH mutations (causing HPE3) are seen in 6%–7% of sporadic cases with isolated HPE. However, mutations in this gene can be seen in 35%–40% of families with autosomal dominant HPE. The most frequently seen mutations are nonsense and missensemutations, but deletions and insertions have also been identified.Mutations are predicted to have a loss of function effect. There is no genotype–phenotype correlation.

TGIF mutations (causing HPE4) are seen in only 1%–2% of patients with isolated HPE. All mutations identified so far have been missense.These mutations are predicted to have a loss of function effect.

ZIC2 mutations (causing HPE5) account for only 2%–3% of isolatedHPE cases. Patients with ZIC2 mutations have severe or mild HPE with relatively normal facial features. Most mutations in this gene have been frame-shift mutations, usually short insertions that result


Holoprosencephaly 38

in loss of function. In one family, a 30-bp insertion was identified in exon 3. This resulted in expansion of a polyalanine tract from itsnormal length of 15–25 residues by 10 residues.

PTCH mutations (causing HPE7) are a rare cause of sporadic and familialHPE. Mutations in PTCH also cause nevoid basal cell carcinoma syndrome(Gorlin syndrome, MIM 109400). All HPE-causing mutations in PTCHhave been missense mutations, which can show nonpenetrance.

Molecular SIX3 is a homologue of the Drosophila “sine oculis” gene, which encodespathogenesis a nuclear protein that is involved in eye development. SIX3 has two exons

and is expressed in fetal and adult retinal tissue. Its protein product has332 amino acids and is believed to be a transcription factor essential for the development of the eyes and anterior neural plate in humans.Mutations in this gene are a rare cause of autosomal dominant andsporadic HPE. Mutations are most likely to cause HPE by interferingwith normal ventral induction.

SHH is a small gene with only three exons that encodes a signalingprotein called Sonic hedgehog (SHH), which has 462 amino acids. SHH has an N-terminal signaling domain and a C-terminal catalyticdomain and is believed to have an important role in patterning of the ventral neural tube. Therefore, mutations in SHH cause HPE by disrupting ventral induction in early embryogenesis.

TGIF has three exons and encodes a protein with 272 amino acids calledtransforming growth factor β-induced factor. This has several putativefunctions. It competitively inhibits binding of the retinoic acid receptor toa retinoid-responsive promoter. Reduced TGIF levels could down-regulateSHH expression by enhancing the binding of retinoic acid receptors. Italso interacts with a SMAD2/SMAD4 complex in the nucleus forming a transcriptional repressor. Finally, it is also believed to link the NODALsignaling pathway to normal development of the human forebrain.

The ZIC2 gene is a homolog of the Drosophila “odd-paired” gene.It has three exons and is only expressed in the fetal brain in humans.Unlike SHH (which is expressed in the ventral neural tube), ZIC2 isexpressed in the dorsal neural tube. The protein product of this gene has 533 amino acids and is known to be involved in normal neuraldevelopment. The mechanism by which mutations in this gene causeHPE is not understood.



PTCH has 23 exons and encodes a 1,447-amino-acid protein, with 12 transmembrane spanning segments, that is a transmembranereceptor for SHH. PTCH normally represses SHH signaling by binding to another transmembrane protein called Smoothened. When SHHbinds to PTCH it releases Smoothened from repression allowingintracellular signaling to proceed. Mutations in PTCH probably causeHPE by enhancing the repressive activity of PTCH on SHH signaling.

Genetic diagnosis The diagnosis of HPE can be confirmed by neuroimaging. All affectedand counseling patients should be examined carefully for other congenital abnormalities

and chromosome analysis should be performed to rule out a syndromicform of HPE.

Parents of patients with isolated nonsyndromic HPE should be carefullyexamined for microcephaly, iris coloboma, anosmia or hyposmia, absent or abnormal superior labial frenulum, single median maxillaryincisor, and high-arched palate. These clinical findings are called HPE“microforms”. They can be seen in individuals who carry a mutation inone of the HPE genes, but have no evidence of HPE on neuroimaging. If HPE microforms are seen in the parent of an affected child then thatparent is likely to be a carrier of a mutation in one of the HPE genes and the family should be counseled on an autosomal dominant basis. It is important to remember that autosomal dominant HPE can showremarkable intrafamilial variability of expression and another affectedchild could be severely affected with alobar HPE or could present withonly HPE microforms. If neither parent has any HPE microforms butthere is parental consanguinity, the family should be counseled on anautosomal recessive basis. If there is no parental consanguinity then the affected child is likely to have a sporadic form of HPE. The siblingrecurrence risk of HPE in this situation is ~6%.

Genetic testing is only available on a research basis. It is not very helpfulin sporadic cases because of the degree of genetic heterogeneity andbecause mutations can only be identified in a very small number ofpatients. It is helpful to look for mutations in SHH in families in whichHPE is clearly segregating in an autosomal dominant manner.


Hunter Syndrome40

Hunter Syndrome(also known as: HS; mucopolysaccharidosis type II [MPS II])

MIM 309900

Clinical features This condition occurs in a severe (MPSIIA) and mild form (MPSIIB). The severe form is three times more frequent. Affected individualspresent with coarsening of the facial features, short stature withdysostosis multiplex on skeletal survey, hepatosplenomegaly,progressive sensorineural hearing loss, cardiac valvular disease, and retinitis pigmentosa. The severe form is associated with progressivemental retardation; death occurs in most cases by the age of 15 years.In contrast, patients with the mild form of the disease are notintellectually impaired and have prolonged survival.

Age of onset In severe cases onset is in the first year of life. In mild cases onset can be delayed until mid-childhood or even later.

Epidemiology The condition affects all races, but is particularly prevalent in IsraeliJews, where its incidence is 1 in 34,000 male births. In the Britishpopulation the incidence is estimated to be 1 in 132,000 male births.


Chromosomal Xq28location

Gene IDS (iduronate 2-sulfatase)

Mutational Over 150 different mutations have been identified in IDS. Approximatelyspectrum 20% of patients have complete deletion of or a gross structural alteration

in IDS. These patients usually present with the severe form of thiscondition. Another 20% of patients have small intragenic deletions and the remainder have point mutations in IDS. The latter includenonsense, missense, frame-shift, and splice-site mutations. Missensemutations can result in a severe or intermediate phenotype.

Molecular IDS has nine exons and encodes the 550-amino-acid lysosomal enzymepathogenesis iduronate 2-sulfatase. Mutations in IDS result in deficiency of this

enzyme. This enzyme acts on dermatan and heparan sulfate and


catalyzes the first step in degradation of these glycosaminoglycans in the lysosome. Deficiency of iduronate 2-sulfatase therefore results inaccumulation of dermatan and heparan sulfate in tissues and excretionof these glycosaminoglycans in urine. The clinical features of HS are the result of tissue accumulation of these glycosaminoglycans.

Genetic diagnosis The diagnosis of HS can be made by iduronate 2-sulfatase assay in and counseling white cells or plasma. Counseling is on an X-linked recessive basis.

Plasma assay of iduronate 2-sulfatase can be used for carrier detection(levels of this enzyme are approximately 50% lower than normal incarriers). However, IDS mutation analysis is the most reliable method of carrier detection. Prenatal diagnosis is possible by measuringiduronate 2-sulfatase levels in uncultured or cultured chorionic villi or cultured amniocytes.

Huntington Disease (also known as: HD)

MIM 143100

Clinical features This is an adult-onset neurodegenerative disorder that presents with thetriad of personality change, chorea, and dementia. Psychiatric problemscan occur, including depression and social withdrawal. Dementia is a late feature of the disease, but social functioning may be impaired at an early stage. The condition is slowly progressive with a typicalduration of about 15 years. A more severe, juvenile-onset form alsoexists. This presents with rigidity, dystonia, seizures, ataxia, andcognitive decline. This form tends to progress more rapidly than theadult-onset form, with death in the 20s. The juvenile form is usuallypaternally inherited and is the result of anticipation.

Epidemiology The population prevalence of HD is 4–7 per 100,000. The conditionaffects all races although it appears to have a lower incidence inJapanese, Chinese, Finnish, and African–American populations.

Age of onset The usual age of onset is 35–40 years. However, onset has beendescribed in the mid-70s and in childhood. The juvenile form presentsin late childhood or adolescence.



Huntington Disease42


Chromosomal 4p16.3location

Gene IT15 (important transcript 15)

Mutational All patients with HD have a pathogenic expansion of a CAG repeat motifspectrum in the first exon of IT15. In the normal population, the number of these

repeats varies between nine and 35. Patients with HD have ≥36 CAGrepeats. Individuals with 36–39 repeats usually develop HD, but mayremain asymptomatic or develop HD late in life. Individuals with≥40 repeats will almost invariably develop HD. There appears to besome correlation between increasing size of the CAG repeat expansionand earlier age of onset, but there are not enough data presentlyavailable to use this information for counseling purposes.

Molecular IT15 is a large gene with 67 exons. It encodes a widely expressed protein pathogenesis with 3,144 amino acids called huntingtin. The CAG-repeat expansion

in IT15 is incorporated into this protein and results in the production of mutant huntingtin with an expanded polyglutamine tract, whichaccumulates in the nucleus. In transgenic mice, cells expressingintranuclear huntingtin undergo apoptosis. Expression of caspase 1triggers apoptosis by activating caspase 3. This may also be themechanism of neuronal injury in patients with HD. The CAG repeatexpansion therefore appears to be a toxic gain of function mutation.

Genetic diagnosis Genetic testing for HD is available as a diagnostic service from mostand counseling molecular genetic laboratories. A clinical diagnosis of HD should

always be confirmed by genetic testing.

Counseling is on an autosomal dominant basis. Adults at 50% (and 25%) risk of inheriting this condition can be offered predictive(presymptomatic) testing after appropriate counseling. Predictive testingfor children is not recommended because it confers no medical benefitand removes the opportunity for that child to make the decision aboutpredictive testing in adulthood. This is important because almost half ofall adults who seek predictive testing for HD elect not to proceed aftergenetic counseling. Predictive testing of children also results in loss ofconfidentiality as the result of the test will be disclosed to their parentsand often their general practitioner. Children who are shown to have



inherited a pathogenic CAG repeat expansion on testing could be treateddifferently from their siblings. Other important reasons for not offeringpredictive testing to children are the adverse implications that a “bad”result could have for future employment and life insurance.

Juvenile HD is almost always paternally inherited. If this diagnosis is suspected in a child or adolescent then genetic testing can beundertaken to confirm the diagnosis.

Prenatal diagnosis of HD by direct mutation testing can be offered toindividuals who have been shown to carry a pathogenic CAG repeatexpansion. Individuals at 50% risk of inheriting HD who have not hadpredictive testing and who do not want this test can be offered prenataldiagnosis by exclusion testing (a form of linkage analysis that is used to determine whether the fetus has inherited a marker linked to the IT15 gene from the affected or unaffected grandparent).

Lesch–Nyhan Syndrome(also known as: HGPRT [hypoxanthine-guanine phosphoribosyl transferase] deficiency)

MIM 300322

Clinical features This condition, which almost exclusively affects males, presents withdevelopmental delay, learning difficulties, self-injurious behavior (self-mutilation and biting of lips, cheeks, hands, and fingers), and involuntarymovements (chorea, athetosis, and dystonia). Affected children arehypotonic initially, but later develop spasticity. Other features include the formation of renal urate calculi, and gout can develop late in thecourse of the disease. The diagnosis is suggested by elevated plasmaurate levels and an elevated urinary urate to creatinine ratio in males with developmental delay, and can be confirmed by demonstration of reduced or undetectable levels of HGPRT in red blood cells.

Age of onset First year of life.

Epidemiology Lesch–Nyhan syndrome is seen in all ethnic groups. It is a rare disorderwith a population prevalence of about 1 in 380,000.


Chromosomal Xq26–q27.2location


Lesch–Nyhan Syndrome44

Gene HPRT1 (hypoxanthine-guanine phosphoribosyl transferase 1)

Mutational Over 200 mutations have been identified in HPRT1. Most mutations spectrum are unique and there are no mutational hot-spots in the gene. Missense,

nonsense, and frame-shift mutations are seen in about 70% of patients.A large deletion or insertion is seen in 10%–12% of patients. Splice-sitemutations are seen in about 12%–13% of patients.

Molecular HPRT1 has nine exons and encodes the HGPRT enzyme, which has pathogenesis 657 amino acids. This enzyme converts hypoxanthine to inosine

monophosphate and guanine to guanine monophosphate. Mostmutations in HPRT1 result in reduced production of HGPRT. Deficiencyof HGPRT interferes with the normal reutilization of hypoxanthine,which is converted to xanthine and uric acid. The precise mechanism by which HGPRT deficiency causes neurologic problems is notunderstood. Patients with Lesch–Nyhan syndrome are thought to have very few dopaminergic neurons in their brain, and this could contribute to the neurologic features of this condition.

Genetic diagnosis HPRT1 mutation analysis is not routinely available. Counseling is onand counseling the basis of X-linked recessive inheritance. Reliable carrier testing is

available for the female relatives of affected males in whom a mutationhas been identified in HPRT1.

Prenatal diagnosis is available by HGPRT assay on cultured chorionicvilli or cultured amniocytes, or by mutation analysis in families in whichan HPRT1 mutation has been identified in an affected male. Prenataldiagnosis should be offered to all women who have had an affected child (whether or not they are carriers) because of the possibility of gonadal mosaicism.



Lissencephaly(also known as: agyria spectrum; pachygyria spectrum)

Lissencephaly is a congenital malformation of the brain that is characterized by a completely or relatively smooth brain surface. It includes a spectrum of abnormalities of cortical sulcationranging from agyria (completely smooth brain due to absent gyri) to pachygyria (areas of brainwith reduced number of broad gyri). Histologically there are two main types of lissencephaly. In classical or type I lissencephaly the cerebral cortex is thick with only four layers (normal cerebralcortex has six layers) due to undermigration of neuronal precursors. In cobblestone or type IIlissencephaly the cerebral cortex is completely disorganized with clusters of cortical neuronsseparated by glio-mesenchymal tissue. The surface of the cerebral cortex has a warty appearancedue to the presence of clusters of neurons that have overmigrated during cortical development.

Lissencephaly can be an isolated abnormality or part of a wider syndrome. Several syndromicforms of lissencephaly have been described. Table 2 lists the distinguishing clinical features,inheritance pattern, and molecular genetics of isolated and syndromic forms of classical andcobblestone lissencephaly.

MIM See Table 2.

Clinical features Classical (Type I) LissencephalyThis is usually an isolated malformation (isolated lissencephalysequence). Children with this condition usually present withglobal developmental delay, microcephaly, seizures, and spasticity.Neuroimaging shows agyria or pachygyria, a thick cortical plate with shallow Sylvian fissures, a hypoplastic corpus callosum, dilatedposterior horns of the lateral ventricles, and a normal cerebellum.

There is also an X-linked form of isolated classical lissencephaly. Thispresents as classical lissencephaly in males, but as subcortical bandheterotopia in females, who present with mild to moderate learningdifficulties and epilepsy. The condition can be diagnosed by an MRI scan which shows the characteristic “double cortex” sign. This consistsof a band of heterotopic gray matter lying parallel to and just beneath the cerebral cortex and separated from it by a band of white matter.

Miller–Dieker syndrome is a syndromic form of classical lissencephaly.Children with this syndrome have characteristic facial features with a high, square forehead, bitemporal narrowing, vertical furrowing of the forehead, epicanthic folds, small nose with anteverted nares, thin upper lip, and micrognathia.


Lissencephaly46

Another rare syndromic form of classical lissencephaly is X-linkedlissencephaly with ambiguous genitalia. This condition affects karyotypicmales (ie, children with the chromosome pattern 46,XY) who presentwith ambiguous genitalia, profound developmental delay, seizures withonset soon after birth, hypotonia or spasticity with brisk deep tendonreflexes, feeding difficulties, and lissencephaly that is more severe in the posterior regions of the cerebral cortex. Cortical thickness in thesechildren is only moderately increased (5–7 mm compared with thenormal cortical thickness of 2–3 mm) and all children also haveagenesis of the corpus callosum. The prognosis is poor; most affected children die in the first year of life.

Cobblestone (Type II) LissencephalyThis is usually seen as a component of several rare syndromes. Isolatedcobblestone lissencephaly has been described, but is extremely rare.Patients with cobblestone lissencephaly usually present with congenitalhydrocephalus or microcephaly, seizures, hypotonia, severe developmentaldelay, muscular weakness, and hypotonia. Ocular abnormalities andencephalocele are important presenting features of Walker–Warburgsyndrome. Neuroimaging shows agyria or pachygyria, thick cortex witha granular surface, hydrocephalus, and a small cerebellum with vermisaplasia or hypoplasia. The distinguishing clinical features, inheritancepattern, and molecular genetics of syndromic forms of cobblestonelissencephaly are listed in Table 2.

Age of onset Children with classical and cobblestone lissencephaly are usuallydiagnosed in the first year of life. The diagnosis of SCBH is often delayed until late childhood or adolescence.

Epidemiology The prevalence of classical lissencephaly is 11.7 per 1,000,000 births.The prevalence of cobblestone lissencephaly is unknown.

Genes and molecular LIS1 (PAFAH1B1) has 11 exons and encodes the 409-amino-acidpathogenesis containing α subunit of the platelet-activating factor acetylhydrolase

isoform 1B. This protein interacts with several microtubule-associatedproteins, including doublecortin, dynein, and dynactin. Therefore, the protein product of LIS1 is probably involved in cellular division ofneuronal progenitor cells as well as neuronal migration. Deletions of LIS1 result in haploinsufficiency of this gene and ~90% of intragenicmutations in this gene result in the production of a truncated protein.



These mutations are associated with a severe phenotype. Missensemutations in LIS1 are associated with a milder phenotype (such aspachygyria with more severe involvement of the posterior parietal andoccipital lobes or SCBH). Somatic mosaicism for LIS1 mutations canresult in the phenotype of SCBH in males and females. Deletions andintragenic mutations in LIS1 most probably cause lissencephaly bypreventing normal neuronal migration.

14-3-3ε is composed of six exons and codes for a 255-amino-acidprotein that is the epsilon subunit of tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein. The protein product of 14-3-3ε is required for cytoplasmic dynein function and neuronalmigration. It lies 40 kb telomeric to LIS1 at 17p13.3, and deletion ofboth 14-3-3ε and LIS1 appears to cause the more severe lissencephalyphenotype of Miller–Dieker syndrome.

DCX is made up of seven exons. It encodes doublecortin, which has 402 amino acids. Doublecortin is almost exclusively expressed in thefrontal lobes of the fetal brain. It is believed to direct normal neuronalmigration by regulating the organization and stability of microtubules in fetal neurons by interacting with the protein product of LIS1. Mutationsin DCX cause lissencephaly in males by preventing normal neuronalmigration. Females with DCX mutations have two populations of corticalneurons due to X inactivation. In one group of neurons, the X chromosomethat carries the mutated DCX gene is inactivated. These neurons migratenormally to form a cerebral cortex of normal appearance. In the othergroup of neurons, the X chromosome with the normal DCX gene isinactivated. These neurons undergo migrational arrest. This results in theformation of a second cortical layer that lies deep in the normal cortex(subcortical band heterotopia or double cortex). Rarely, males with DCXmutations can present with SCBH. Some of these males exhibit somaticmosaicism for DCX mutations and the others are thought to have a mildmutation that results in some residual function of doublecortin.

ARX is homologous to the Drosophila aristaless gene. It has five exonsand encodes a protein with 562 amino acids. It is expressed in theembryonic and fetal forebrain and testes. The ARX protein is involved in the differentiation, radial and tangential migration and maintenanceof specific neuronal subtypes in the cerebral cortex. It is also involved in differentiation of the testes. Loss of function mutations in ARX causeXLAG by interfering with normal neuronal migration (particularly


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Lissencephaly50

tangential migration in the developing cortex) and loss of normaltesticular differentiation. Mutations in ARX can also be seen in patients with X-linked infantile spasms (MIM 308350), X-linked mentalretardation (MIM 300419 and 300430), X-linked myoclonic epilepsywith mental retardation and spasticity (MIM 300432), and X-linkedmental retardation with dystonic movements, ataxia, and seizures(Partington syndrome, MIM 309510).

POMT1 contains 20 exons and encodes the proteinO-mannosyltransferase 1. This is a ubiquitously expressed protein with 725 amino acids that is thought to catalyze the first step inO-mannosylation (a form of glycosylation) of target proteins in the brain, nerves, and skeletal muscle. An important protein that undergoes O-mannosylation is α-dystroglycan. Mutations in POMT1result in reduced or absent O-mannosylation of α-dystroglycan.Hypoglycosylation of α-dystroglycan, which links the sarcolemmaldystrophin–glycoprotein complex to various extracellular proteins (such as the laminin α2 chain of merosin, neurexin, and agrin), is believed to be an important factor in the pathogenesis of thecongenital muscular dystrophy of Walker–Warburg syndrome.Glycosylated α-dystroglycan is also believed to play an important role in normal neuronal migration. Therefore, absent or reducedglycosylation of α-dystroglycan could cause cobblestone lissencephaly by allowing overmigration of neurons in the developing brain.

POMGnT1 has 22 exons and encodes the protein O-mannose β-1,2-N-acetylglucosaminyltransferase-1. This enzyme is also involved in the O-mannosylation of target proteins (such asα-dystroglycan). Mutations in this gene result in hypoglycosylation of target proteins, particularly α-dystroglycan. This is thought to beresponsible for the brain abnormalities and congenital musculardystrophy of muscle–eye–brain disease.

FCMD is composed of 10 exons and encodes fukutin, which has 461 amino acids. The precise function of this protein is unknown, but it is probably a glycosyl transferase that is involved in the glycosylation of cell surface molecules, such as α-dystroglycan, like the proteinproducts of POMT1 and POMGnT1 (see above). Hypoglycosylation ofα-dystroglycan in patients with fukutin deficiency is probably responsiblefor the muscle and brain abnormalities seen in Fukuyama congenitalmuscular dystrophy. Compound heterozygosity for the common 3-kb



retrotransposon insertion in one allele and a point mutation in the otherallele is associated with a more severe phenotype compared withhomozygosity for the common 3-kb retrotransposon insertion.

RELN is a very large gene with 64 exons. It encodes reelin, which has3,641 amino acids. Reelin is expressed in fetal and postnatal brain andliver. Reelin is thought to play an important role in normal lamination ofthe cerebral and cerebellar cortex by arresting normal neuronal migration.It probably does this by modulating integrin-mediated cell–cell adhesion.

Genetic diagnosis All children with classical lissencephaly should have chromosome and counseling analysis to look for a rearrangement involving 17p13.3 and fluorescence

in situ hybridization (FISH) analysis to look for a deletion of this region.Genetic testing for LIS1 mutations (large deletions and intragenicmutations) is only available on a research basis. Parents of children with classical lissencephaly in whom a 17p13.3 rearrangement ordeletion is identified should also be tested for a rearrangement of 17p13by chromosome and FISH analysis. If parental karyotypes are normalthe recurrence risk of classical lissencephaly in another child is likely to be low.

Families with lissencephaly in males and SCBH in females, or with morethan one affected male with lissencephaly, should be tested for mutationsin DCX, which is available from specialized laboratories. Mutation testingshould be offered to mothers of male patients in whom a DCX mutation isidentified. Prenatal testing should be offered to the mothers of all malepatients with classical lissencephaly in whom a DCX mutation is identified,even if the mother has tested negative for the mutation. There is likely tobe a significant recurrence risk for classical lissencephaly in a son or forSCBH in a daughter due to the possibility of low-level somatic or gonadalmosaicism for this mutation in the mother. Mothers of male patients withclassical lissencephaly should be investigated for SCBH by cranial MRIscan if they have learning difficulties or epilepsy. If SCBH is identified inthe mother, counseling should be on an X-linked basis: sons have a 50%risk of being affected with classical lissencephaly and daughters have a 50% risk of being affected with SCBH.

Genetic testing for syndromic forms of cobblestone lissencephaly is onlyavailable on a research basis. All forms of cobblestone lissencephaly areinherited in an autosomal recessive manner and this is the basis for geneticcounseling of these families. Prenatal diagnosis of Walker–Warburgsyndrome may be possible by antenatal ultrasound scanning.


Lowe Syndrome52

Lowe Syndrome(also known as: oculo-cerebro-renal syndrome)

MIM 309000

Clinical features This condition almost always affects males, who present with ocularanomalies, mental retardation, and renal Fanconi syndrome. The ocularanomalies include congenital or early-onset cataracts and early-onsetglaucoma that results in visual impairment or blindness. Other featuresinclude feeding difficulties, failure to thrive, epilepsy, hypotonia, scoliosis,behavioral problems, and short stature. Fanconi syndrome can progressto renal failure. Teenagers and adults with Lowe syndrome can developarthropathy. Most affected males die by the age of 40 years.

Age of onset The ocular features are present at birth or develop in early infancy.Fanconi syndrome is usually present by the age of 1 year.

Epidemiology A rare disorder involving all ethnic groups.


Chromosomal Xq26.1location

Gene OCRL1 (oculo-cerebro-renal syndrome, Lowe 1)

Mutational Mutations can be identified in OCRL1 in approximately 95% of affectedspectrum males. They include nonsense, missense, frame-shift, and splice-site

mutations. Most mutations are unique and involve exons 10, 12–15, 18,19, 21, and 22. OCRL1 is partially or wholly deleted in ~7% of patients.

Molecular OCRL1 is a large gene with 24 exons. One small exon (18a) is subjectpathogenesis subject to alternative splicing. The isoform of OCRL1 that includes

this exon is expressed primarily in neural tissue. The gene codes forphosphatidylinositol-4,5-bisphosphate-5-phosphatase, which is presentin the trans-Golgi network of several cell types. This enzyme is thoughtto regulate intracellular levels of phosphatidylinositol-4,5-bisphosphate.Mutations in OCRL1 result in reduced or absent activity ofphosphatidylinositol-4,5-bisphosphate-5-phosphatase. This results inelevated intracellular levels of phosphatidylinositol-4,5-bisphosphatewhich is thought to interfere with the normal function of the Golgi network.



This could result in abnormal cell migration and cell differentiation in the brain, lens, and kidneys (by a mechanism that is not understood), or could involve changes in the composition of the cell membrane.

Genetic diagnosis OCRL1 mutation analysis is available from a limited number of and counseling diagnostic laboratories. The diagnosis can be confirmed in affected males

by demonstrating reduced activity of the enzyme phosphatidylinositol-4,5-bisphosphate-5-phosphatase in cultured skin fibroblasts.

Counseling is on the basis of X-linked recessive inheritance. There is a 70% chance that the mother of an isolated case of Lowe syndrome is a carrier. Virtually all female carriers have cortical lenticular opacitiesor posterior lenticonus on slit-lamp examination. Prenatal diagnosis hasbeen established by identifying cataracts on antenatal scans in the latesecond and third trimester. Earlier prenatal diagnosis is available byphosphatidylinositol-4,5-bisphosphate-5-phosphatase assay oncultured chorionic villi or cultured amniocytes or by mutation analysis in families in which an OCRL1 mutation has been identified in anaffected male. Prenatal diagnosis should be offered to all women who have had an affected son (whether or not they are carriers) because of the possibility of gonadal mosaicism.

Neuronal Ceroid Lipofuscinosis(also known as: NCL; Batten disease)

MIM See Table 3

Clinical features The NCLs are a group of neurodegenerative disorders that are characterizedby the deposition of autofluorescent material (with similarities to ceroidand lipofuscin) in various cells, including neurons. All the NCLs presentwith psychomotor deterioration, epilepsy, and visual impairment, andaffected patients die prematurely. The various types are differentiated by the age of onset, the presenting clinical features, and the appearance of the accumulated material on electron microscopic analysis.

Infantile NCLThis condition presents with early psychomotor deterioration, ataxia,autistic features, and repetitive hand movements reminiscent of Rettsyndrome. Myoclonic jerks, optic atrophy, and acquired microcephaly are evident later in the course of the condition. The characteristic


Neuronal Ceroid Lipofuscinosis54

Table 3. Neuronal ceroid lipofuscinosis (NCL): types, MIM numbers, ages of onset, chromosomal locations, and genes.CLN: ceroid lipofuscinosis, neuronal.

electrophysiologic findings include an extinguished electroretinogram (ERG)and visual evoked potential (VEP) and “vanishing” electroencephalograph(EEG). Ultrastructural examination of a conjunctival, skin, or rectal biopsyshows granular osmiophilic deposits (GRODs) in several different cell types.Most children die before the age of 5 years.

Late infantile NCLThis form of NCL presents with psychomotor deterioration and seizures.Visual failure occurs late in the course of the illness. Electrophysiologicfindings include an abnormal EEG with multifocal spikes and slowwaves, and a characteristic response to photic stimulation at a slow rate with each flash producing a spike over the occipital region. The ERG is extinguished, but the VEP shows giant waves. Ultrastructuralexamination of conjunctival, skin, or rectal biopsy shows inclusions withcurvilinear profiles. Most children die between 6 and 15 years of age.

Juvenile NCLThis condition presents with progressive visual impairment leading toblindness by the age of 8 years. Retinal examination shows an absentmacular reflex and sometimes bull’s-eye maculopathy. Behavioralproblems and loss of scholastic skills are other early features, butneurologic problems are only evident late in the course of the disease.Electrophysiologic investigations show an absent ERG and VEP early in the course of the disease. Vacuolated lymphocytes on peripheralsmear examination are a characteristic finding. Electron microscopicexamination of peripheral blood lymphocytes, skin, conjunctival, orrectal biopsy shows fingerprint inclusions. The condition is slowlyprogressive, with death occurring between the ages of 15 and 30 years.

Type MIM Age of onset Chromosomal Genelocation

Infantile NCL or 256730 6–18 months 1p32 CLN1Santavuori–Haltia–Hagberg disease

Late infantile NCL or 204500 18 months to 4 years 11p15.5 CLN2Jansky–Bielschowski disease

Juvenile NCL or 204200 5–10 years 16p12 CLN3Spielmeyer–Vogt–Sjögren disease

Adult NCL or Kufs disease 204300 Late 20s or early 30s Unknown Unknown

Late infantile NCL, Finnish variant 256731 4–7 years 13q21.1–q32 CLN5



Adult NCLThis condition presents with cognitive problems and behavioraldisturbances. Extrapyramidal signs and myoclonic seizures can be seenlater, but visual impairment is not a feature of this form of NCL. EEG mayshow spike-wave complexes and a positive response to photic stimulationin some cases, but ERG and VEP are normal. Ultrastructural examinationof rectal biopsy is usually required to make the diagnosis. This can showGRODs or deposits with fingerprint or rectilinear profiles predominantly in cells of neural origin. The condition is very slowly progressive, withprolonged survival (death occurs more than 20 years after onset).

Late infantile NCL, Finnish variantThis condition has a similar presentation to classical late infantile NCL, but the age of onset is similar to that of juvenile NCL. The electrophysiologicfindings are also similar to classic late infantile NCL. However, electronmicroscopic examination of tissue biopsies shows deposits with fingerprintor rectilinear profiles. Disease progress can be slower than in classical lateinfantile NCL, with death occurring between the ages of 13 and 30 years.

Epidemiology The NCLs affect all races with a population incidence of 1 per100,000–1,000,000. Infantile NCL is seen most frequently in Finland.Juvenile NCL has an incidence of about 1 in 25,000 live births and is one of the most common neurodegenerative diseases of childhood.The Finnish variant of late infantile NCL is only seen in familiesoriginating from the west coast of Finland.

Age of onset, See Table 3.chromosomal location, and gene

Inheritance All forms of NCL are inherited in an autosomal recessive manner.Autosomal dominant inheritance has been documented in one familywith adult NCL.

Mutational spectrum CLN1: approximately 60% of mutations identified in this gene aremissense mutations. One missense mutation, Arg122Trp, accounts for 98% of all mutations in the Finnish population. Other mutationsinclude nonsense, frame-shift, and splice-site mutations. Two nonsensemutations (Arg151Stop and Leu10Stop) account for ~40% of mutationsin non-Finnish patients.


Neuronal Ceroid Lipofuscinosis56

CLN2: half of all mutations identified in this gene are missense. Othermutations include splice-site and nonsense mutations as well as deletions.There are two common mutations in CLN2, an intronic splice-sitemutation (IVS5-1G>C), and a nonsense mutation (Arg208Stop), which together account for almost 60% of all mutations.

CLN3: mutations identified in this gene include missense, nonsense,and splice-site mutations, as well as small and large deletions and oneintronic mutation. A 1.02-kb deletion that removes exons 7 and 8 is the most frequently identified mutation. About 80% of Finnish patientswith juvenile NCL are homozygous for this mutation. This is also themost common mutation in other populations. Those patients who are not homozygous for the 1.02-kb deletion are often compoundheterozygotes, with this deletion in one allele and a different mutation in their other allele.

CLN5: only three mutations have been identified in this gene: a 2-bp deletion in exon 4, one nonsense, and one missense mutation.Approximately 90% of patients are homozygous for the 2-bp deletion.

Molecular CLN1 has 7 exons and encodes a 306-amino-acid protein calledpathogenesis palmitoyl-protein thioesterase (PPT). This is a lysosomal enzyme that

removes palmitate groups from cysteine residues in S-acylated proteins.Mutations in CLN1 result in loss of activity of PPT, which presumablyresults in the accumulation of S-acylated proteins in lysosomes.

CLN2 has 13 exons. It encodes a novel 562-amino-acid lysosomalprotease. This lysosomal protease is identical to tripeptidyl peptidase I,an enzyme that removes three amino acids from the N-terminal regionsof proteins undergoing lysosomal degradation. Mutations in CLN2result in a deficiency of this enzyme and the accumulation of proteins in lysosomes.

CLN3 encodes a novel transmembrane protein of unknown function. The CLN3 protein has been localized to lysosomes, mitochondria, and Golgi. It may be responsible for regulating lysosomal pH by anunknown mechanism. Nearly all mutations in CLN3 are predicted toresult in loss of function of the CLN3 protein, which could result in anabnormal lysosomal pH. This could interfere with protein degradation in lysosomes, resulting in the abnormal accumulation of these proteins.

CLN5 contains four exons and is believed to encode a novel protein with407 amino acids. This protein has no homology to any other protein.



Recent work suggests it is a soluble lysosomal glycoprotein. The commonmutation in CLN5 blocks the lysosomal targeting of this protein. Thisimplies that the pathogenesis of the Finnish variant of late infantile NCLcould be the result of defective lysosomal trafficking of the proteinproduct of CLN5 interfering with its normal biologic function.

Genetic diagnosis Counseling for all types of NCL is on an autosomal recessive basis. and counseling Genetic testing for infantile, late infantile, and juvenile NCL is available

from only a few specialized laboratories. Prenatal diagnosis is possibleby mutation analysis if the precise mutations have been identified in an affected child, or by linkage analysis if the mutations have not beenidentified. However, before undertaking linkage analysis it is importantto confirm the exact type of NCL in the affected child. In infantile NCL,prenatal diagnosis by genetic testing can be combined with PPT assayon chorionic villi. In late infantile NCL, prenatal testing has beenperformed by electron microscopic examination of unculturedamniocytes for deposits with the typical curvilinear profile.

Pelizaeus–Merzbacher Syndrome

MIM See Table 4.

Clinical features Affected children present with rotary or roving nystagmus,developmental delay, seizures, and optic atrophy. Hypotonia is initiallypresent; spasticity develops later, mainly in the lower limbs. Otherclinical features include laryngeal stridor, extrapyramidal signs, andneuroregression. Brainstem-evoked potentials are abnormal and MRIscans of the brain show delayed or poor myelination. The classical typemainly affects males, but the connatal variant affects both sexes. Theclinical picture is more severe in the connatal variant, with acquisition of very few developmental milestones, very rapid progression, and earlydeath. Compared with the classical type, there is almost no myelinationseen on an MRI scan of the brain in the connatal variant.

Age of onset Both forms present in early infancy, although the connatal type canpresent in the neonatal period.

Epidemiology Extremely rare


Pelizaeus–Merzbacher Syndrome58

See Table 4.

Table 4. Pelizaeus–Merzbacher syndrome: MIM numbers, inheritances, chromosomallocations, genes, and mutational spectra.

Molecular Classical type: PLP1 has seven exons and encodes proteolipid protein 1.pathogenesis There are two isoforms of this protein. One isoform has 276 amino

acids while the other isoform (called DM20) has only 241 amino acids. Proteolipid protein 1 is an integral membrane protein with fourtransmembrane domains. It is one of the main components of myelin in the central nervous system. Mutations in the PL1P gene result inreduced production of protein, which results in absent or delayedmyelination in the brain. Missense mutations in the PLP1 gene(Gly220Cys, Ala242Gln, and Ala242Val) can be associated with a very early presentation of Pelizaeus–Merzbacher syndrome in males with similarities to the connatal form of this condition. Mutations in PLP1 also cause one form of X-linked spastic paraplegia (SPG2, MIM 312920).

Connatal variant: unknown

Genetic diagnosis Classical type: the diagnosis can be made by an MRI scan of the brainand counseling and abnormal brainstem-evoked responses. PLP1 mutation analysis

is not available as a routine service. Counseling is on the basis of X-linked recessive inheritance. Carrier females can show white matterabnormalities on an MRI brain scan. Reliable carrier testing and prenataldiagnosis is available if the PLP1 mutation is identified in the affectedmale. Prenatal diagnosis is possible using linkage analysis, or mutationanalysis in families in which a PLP1 mutation has been identified in an affected male.

Type MIM Inheritance Chromosomal Gene Mutationallocation spectrum

Classical 312080 X-linked Xq22 PLP1 Deletions, type recessive (proteolipid duplications,

protein-1) and point mutations. Duplications are seen in 50% of familial cases

Connatal 260600 Autosomal Unknown Unknown Unknownvariant recessive

Inheritance,chromosomal location, gene, and mutational spectrum



Connatal type: counseling is on the basis of autosomal recessiveinheritance. Some affected males and females have been shown to have mutations in PLP. In these families, counseling would be on the basis of X-linked inheritance.

Prader–Willi Syndrome(also known as: PWS; Prader–Labhardt–Willi syndrome)

MIM 176270

Clinical features Presentation is usually in the neonatal period with hypotonia and poorfeeding, followed (in early childhood) by the onset of life-long hyperphagialeading to obesity. Other features include mild mental retardation, shortstature, small hands and feet, hypogonadotrophic hypogonadism,hypopigmentation, and challenging behavior (particularly in laterchildhood and adolescence). High pain threshold, poor temperaturecontrol, skin picking, and sleep disturbance can also occur.

Age of onset Hypotonia can manifest before birth as reduced fetal movement.

Epidemiology The incidence is between 1 in 10,000 and 1 in 15,000. All races are affected.

Inheritance This is complex. Four different mechanisms have been identified:paternally derived chromosome 15q11–q13 interstitial deletion/microdeletion, maternal uniparental disomy (UPD) for chromosome 15,chromosome abnormality (such as an unbalanced translocationinvolving loss of 15q11–q13 on the paternally derived chromosome), an imprinting defect (involving the PWS/Angelman syndrome [see p.31]critical region).

Chromosomal 15q11–q13location

Gene SNRPN (small nucleoribonucleoprotein N) ZNF127 (zinc finger protein 127)NDN (necdin)

Mutational spectrum Deletions in SNRPN, involving the promoter region and exon 1


Prader–Willi Syndrome60

Molecular PWS represents the opposing phenotype to AS in that it is caused by pathogenesis the absence of paternally expressed genes at chromosome 15q11–q13,

where there is a cluster of genes that show parent-specific imprinting (see Figure 1, p.31). SNRPN is primarily implicated. This consists of at least 10 exons and is abundantly expressed (but only from thepaternally derived allele) in the brain, heart, and striated muscle, where it encodes a protein involved in pre-mRNA splicing andprocessing. Other paternally expressed genes in the PWS critical regioninclude ZNF127 (which encodes a factor involved in protein–proteininteraction) and NDN (which encodes the protein necdin, which isexpressed in neurons in the developing nervous system). Deficiency of all of these genes, and probably of others that are as yet unidentified,contributes to the PWS phenotype.

In approximately 70%–75% of cases there is a 4-Mb deletion in thepaternally derived chromosome 15, which results from misalignmentwith unequal recombination between regions of flanking homologousEND repeats. These repeats are derived from large genomic duplicationsof a gene called HERC2. Around 25% of cases result from maternal UPDfor chromosome 15. In approximately 1%–3% of cases an “imprintingmutation” impairs the setting of the normal paternal imprint (whichnormally arises close to the SNRPN promoter). In many cases, thisinvolves a tiny deletion of 6–200 kb extending into the promoter andfirst exon of SNRPN.

Genotype–phenotype Children with maternal UPD15 show a lower incidence of skin pickingcorrelation and hypopigmentation than deletion cases. The lower incidence of

hypopigmentation is explained by the presence of the nonimprintedP or OCA2 (type 2 oculo-cutaneous albinism) gene in the deletion region.

Genetic diagnosis Diagnosis is best made by methylation analysis using a methylation- and counseling sensitive restriction enzyme and a probe such as SNRPN (see Figure 2,

p.33). A microdeletion can sometimes be seen on conventionalchromosome analysis but is much more reliably identified by fluorescencein situ hybridization (see Figure 3, p.33). UPD can be detected usinginformative microsatellite markers. The recurrence risk for the commonde novo microdeletion is <1% (attributable to paternal germ linemosaicism) and is negligible for UPD. Imprinting errors can arise de novo (recurrence risk <1%) or be silently transmitted (eg, from a paternal grandmother), with a potential recurrence risk of 50%.



Rett Syndrome

MIM 312750

Clinical features Almost all affected cases are female. Patients show normal developmentand head growth until 6 months of age. After this age they showpsychomotor regression and deceleration of head growth. An earlyfeature is loss of acquired hand skills, associated with the developmentof repetitive hand movements or hand stereotypies (wringing, clapping,and mouthing movements). Patients develop autistic features withsevere mental retardation and little or no useful speech. Additionalfeatures include episodic hyperventilation or apnea, epilepsy, spasticity,scoliosis, poor growth, and small feet with peripheral vasomotordisturbances. Older patients can develop cardiac arrhythmias.

Age of onset From 6 months to 3 years

Epidemiology 1 in 10,000 to 1 in 15,000 births

Inheritance X-linked dominant with male lethality. Most cases represent new mutations.


Gene MECP2 (methyl-CpG-binding protein 2)

Mutational Over 170 mutations have been described. These include missense,spectrum nonsense, and splice-site mutations, as well as gross deletions,

small insertions and deletions, and complex rearrangements.

Molecular MECP2 has four exons and encodes a ubiquitously expressed protein pathogenesis called methyl-CpG-binding protein 2. This protein has 486 amino acids

and two structural domains: the methyl-CpG binding domain (MBD) and the transcriptional repression domain (TRD). MBD recognizes amethylated CpG dinucleotide while TRD interacts with other proteinssuch as Sin3A and histone deacetylases to selectively repress genetranscription. MECP2 mutations may allow transcription of genes that are not normally transcribed in early embryonic development.Normal brain development could be particularly sensitive to thetranscription of these genes.


X-linked Adrenoleukodystrophy62

Mutations in MECP2 have been identified in males with an X-linkedform of severe mental retardation associated with progressive spasticity(MIM 300279), X-linked mental retardation with psychosis, pyramidalsigns, and macro-orchidism (MIM 300055), X-linked nonspecificmental retardation, and neonatal-onset nonprogressive encephalopathy.Mutations in this gene have also been identified in females with featuressuggestive of Angelman syndrome (see p.30–3).

Genetic diagnosis Although the majority of cases of Rett syndrome are sporadic in origin,and counseling there have been a few instances of recurrence in another female child,

probably as a result of gonadal mosaicism. If a girl with Rett syndromehas no male or female sibling or maternal relative with mental retardationthen the risk of recurrence is likely to be very small. If there is a male or female sibling or a maternal relative with mental retardation then the recurrence risk could be as high as 50%. If a MECP2 mutation is identified in the affected child then the parents should be offered a prenatal test for Rett syndrome.

X-linked Adrenoleukodystrophy (also known as: X-ALD)

MIM 300100

Clinical features The childhood form of X-ALD usually presents with gradual intellectualdecline and progressive gait abnormalities. Additional problems includefocal seizures, cortical blindness, extrapyramidal signs, and cerebellarataxia. Affected children develop dementia and severe, terminal, spasticquadriplegia. Some children also develop features of adrenal insufficiency.

About 25% of patients present with progressive spastic paraparesis and features of adrenal insufficiency. This phenotype is calledadrenomyeloneuropathy (AMN). X-ALD presents as isolated Addison disease in 10% of patients.

Age of onset The childhood form of X-ALD usually presents between the ages of 5 and 10 years. AMN usually presents in adult life.



Epidemiology This condition affects all ethnic groups. Its incidence is estimated to bebetween 1 in 20,000 and 1 in 100,000. A recent study from the USAsuggested a minimum incidence of 1 in 42,000.



Gene ABCD1 (ATP-binding cassette, subfamily D, member 1)

Mutational Over 400 mutations have been identified in ABCD1, of which most spectrum are unique to a particular family. Missense mutations account for

approximately 50% of the total. Other mutations include frame-shift,splice-site, and nonsense mutations. Large deletions are seen in about5%–6% of patients. Most mutations result in complete absence of theprotein product. There appears to be no correlation between genotypeand phenotype. There is interfamilial phenotypic variability.

Molecular X-ALD is a peroxisomal disorder that is associated with elevated pathogenesis levels of saturated very long-chain fatty acids (VLCFAs), particularly

hexacosanoate (C26:0), in all body tissues. This is due to the inability of VLCFAs to be degraded in peroxisomes. The first step in the degradationof VLCFAs is catalyzed by the enzyme VLCFA-CoA synthetase (alsocalled lignoceroyl-CoA ligase). ABCD1 is composed of 10 exons and its protein product (ALD protein) has 745 amino acids. ALD protein is a peroxisomal membrane protein that is involved in the import oranchoring of VLCFA-CoA synthetase into the peroxisomal membrane.Deficiency of ALD protein results in a deficiency of VLCFA-CoAsynthetase in the peroxisomal membrane and decreased peroxisomaldegradation of VLCFAs. Accumulation of VLCFAs and their disruptiveeffects on the cell membrane structure and function could explain the neurologic manifestations of X-ALD. Adrenal dysfunction is due to accumulation of cholesterol esters of VLCFAs in the cells of the zona fasciculata and reticularis.

Genetic diagnosis The diagnosis of X-ALD can be made by neuroimaging (contrast-and counseling enhanced CT or MRI scans) and by measuring the levels of VLCFAs

(C26:0 and C26:C22 ratio) in plasma, red cells, or cultured fibroblasts.All affected patients should be tested for adrenal insufficiency as this


X-linked α-Thalassemia and Mental Retardation Syndrome64

is often subclinical. Molecular genetic analysis is only available from a few specialized laboratories and is helpful in carrier detection andprenatal diagnosis. Counseling is on the basis of X-linked recessiveinheritance. Only 5% of patients are affected as the result of a newmutation. Carriers can be identified by elevated levels of VLCFAs inplasma or red cells, but false negative results can be obtained in about15% of cases. Therefore, genetic testing is the most reliable method of carrier detection. Almost 20% of carriers develop a mild AMN-likephenotype between the ages of 25 and 50 years. Prenatal diagnosis is possible by measuring levels of VLCFAs in cultured chorionic villi or cultured amniocytes. However, this should only be undertaken by a laboratory familiar with this analysis in order to reduce the possibility of a false negative result. Where possible, biochemical analysis shouldbe complemented by genetic testing (mutation or linkage analysis).

X-linked αα-Thalassemia and Mental Retardation Syndrome(also known as: ATR-X syndrome)

MIM 301040

Clinical features This condition mainly affects males, who present with characteristicfacial features, severe to profound mental retardation, genital abnormalities,and α-thalassemia. The facial features include short palpebral fissureswith telecanthus, epicanthic folds, a small, triangular, and antevertednose with short columella (with alae nasi extending below the level ofthe columella), inverted V-shaped upper lip, and a full, everted lower lip. Some patients are never able to walk and most have no speech.Genital abnormalities can range from cryptorchidism to complete sexreversal in a patient with a 46,XY karyotype. The α-thalassemia is mildand difficult to detect by hemoglobin (Hb) electrophoresis. It is bestidentified by looking for HbH inclusion bodies in red cells in a freshblood sample. Additional features include neonatal hypotonia, epilepsy, microcephaly, and short stature.

Age of onset Neonates present with hypotonia and feeding difficulties. Developmentaldelay is usually evident in infancy, and the characteristic facial featurescan be recognized in early childhood.



Epidemiology Rare



Gene XNP (X-linked nuclear protein)

Mutational Over 50 mutations have been identified. These include missense, spectrum nonsense, and frame-shift mutations, as well as intragenic deletions.

An XNP mutation has also been described in a large family withJuberg–Marsidi syndrome (MIM 309590). This is an X-linked recessive form of mental retardation. Affected males have severe mental retardation, failure to thrive, short stature, deafness, genitalabnormalities (small penis, poorly formed scrotum, and cryptorchidism),and delayed bone age. These patients do not have HbH inclusions in their red cells, while patients with ATR-X syndrome do.

Molecular XNP contains 36 exons and codes for ATRX protein, which has pathogenesis 2,375 amino acids. This is thought to be a chromatin-mediated

transcription regulator. It is associated with pericentric heterochromatinduring interphase and with centromeres of many chromosomes and the stalks of acrocentric chromosomes in metaphase. This suggests thatATRX protein is also involved in establishing or maintaining the patternof methylation in the genome. Mutations in XNP are associated with a decrease in the levels of ATRX protein. This down-regulates expressionof the α-globin gene, which results in the α-thalassemia of ATR-Xsyndrome. The other features of ATR-X syndrome are thought to be theresult of reduced levels of ATRX protein interfering with the expression of other genes. These genes have yet to be identified. Mutations thatresult in a truncated protein product that lacks the C-terminal conserveddomains are associated with the most severe genital abnormalities.

Genetic diagnosis The diagnosis can be established by demonstrating HbH inclusions in redand counseling cells from a fresh blood sample in a male patient with the characteristic

clinical features. However, HbH inclusions are not seen in all cases. If this diagnosis is strongly suspected, then additional tests should be performed, including methylation studies and sequencing of exons8–10 of XNP (these code for the zinc finger motif at the N-terminal


X-linked Hydrocephalus66

region of the ATRX protein). The latter test is able to identify about 60% of mutations in the gene. These tests are only available fromspecialized laboratories.

Counseling is on the basis of X-linked recessive inheritance. There is an85% chance that the mother of a sporadic case is a carrier. Carriers areasymptomatic and do not have HbH inclusions in their red cells. This isdue to skewed X-chromosome inactivation in which the X chromosomethat carries the mutated XNP gene is preferentially inactivated. Carrierscan only be reliably identified by XNP mutation analysis. Gonadal andgonosomal mosaicism for XNP mutations has been demonstrated infemales. Therefore, prenatal testing should be offered to all families who have an affected son with ATR-X syndrome, even if the mother hasbeen shown not to carry the XNP mutation identified in the affectedchild. Prenatal diagnosis is possible by XNP mutation analysis or bylinkage analysis in families where an XNP mutation has not beenidentified in the proband.

X-linked Hydrocephalus(also known as: X-linked aqueduct stenosis)

MIM 307000

Clinical features This condition almost exclusively affects males, who present withmacrocephaly, severe developmental delay, adducted thumbs, and spasticparaplegia. An MRI scan of the brain may show an absent or dysplasticcorpus callosum, aqueduct stenosis, and an absence of pyramids.

It is important to remember that most cases of hydrocephalus arenongenetic in origin. Hydrocephalus can be seen in children witha chromosomal abnormality (mosaic trisomy 8 or diploid/triploidmosaicism) and it can be a feature of a multiple malformation syndrome(eg, hydrolethalus syndrome or Walker–Warburg syndrome).

Age of onset At or soon after birth. Hydrocephalus is often identified on antenatal scans.



Epidemiology This is the most common inherited cause of hydrocephalus. Its incidenceis estimated to be 1 in 30,000 live male births. Between 10% and 30%of males with congenital hydrocephalus may have the X-linked form.



Gene L1CAM (L1 cell-adhesion molecule)

Mutational Over 90 mutations have been identified in L1CAM. These include spectrum nonsense, missense, splice-site, and frame-shift mutations. Intragenic

deletions and duplications have also been described. Mutations inL1CAM can cause: X-linked hydrocephalus; the syndrome of mentalretardation, aphasia, shuffling gait, and adducted thumbs (MASAsyndrome; MIM 303350); X-linked complicated spastic paraparesis(MIM 303350); and X-linked agenesis of the corpus callosum. All ofthese phenotypes can be seen in the same family. Truncating mutationsin the extracellular region of the gene result in a severe phenotype,whereas missense mutations in the extracellular domain and mutationsin the cytoplasm domain are associated with a milder phenotype.

Molecular L1CAM is a large gene with 28 exons. It encodes the L1-cell adhesion pathogenesis molecule, which has 1,257 amino acids. This is a surface glycoprotein

belonging to the immunoglobulin superfamily that is expressed inneurons and Schwann cells. It has six immunoglobulin-like C2-typedomains and five fibronectin type III-like domains. In the developingbrain it is involved in cell–cell and cell–substrate adhesion, neuronalmigration, growth, and development, and myelination of axons. It is also involved in the establishment of long-term memory. The precisemechanisms by which mutations in L1CAM cause disease are unknown.

Genetic diagnosis L1CAM mutation analysis should be performed in males with and counseling congenital or early-onset hydrocephalus and the clinical features

described above. It should also be performed in patients with MASAsyndrome, families with complicated spastic paraparesis affectingmales, and X-linked agenesis of the corpus callosum. Genetic testing is available from diagnostic laboratories. Counseling is on the basis ofX-linked recessive inheritance. Carriers can only be reliably identified


X-linked Hydrocephalus68

by L1CAM mutation analysis. The condition can show markedinterfamilial and intrafamilial variability of expression. Prenataldiagnosis is possible by direct mutation analysis (in families where an L1CAM mutation has been identified in an affected male) or by linkage analysis. Prenatal diagnosis is also possible by serialantenatal ultrasonography to look for ventriculomegaly.


3. Disorders of Vision

Aniridia 70

Bardet–Biedl Syndrome 72

Juvenile Retinoschisis 74

Leber Congenital Amaurosis 75

Norrie Disease 79

Rieger Syndrome 80


Aniridia70

Aniridia(including: WAGR [Wilms’ tumor, aniridia, genitourinary anomalies, mental retardation] syndrome)

MIM 106210

Clinical features Children with this condition present with complete or partial absence of the iris. Associated findings include photophobia, impaired vision, nystagmus, corneal dystrophy, glaucoma, cataracts, and dislocated lenses.

Age of onset The diagnosis is usually obvious at birth.

Epidemiology Aniridia is a rare ocular malformation with an incidence of 1 in 56,000 live births.

Inheritance Autosomal dominant. About 25%–30% of cases are sporadicin origin.

Chromosomal 11p13location

Gene PAX6 (paired box gene 6)

Mutational Cytogenetically visible interstitial deletions of 11p13 or a cryptic spectrum deletion of this region can be identified in a significant proportion of

patients with both familial and sporadic aniridia. Some patients with a cryptic 11p13 deletion have been found to be mosaic for the deletion.Chromosomal rearrangements (translocations, inversions, and insertions)involving 11p13 have also been seen in familial and sporadic cases ofaniridia. In most of these cases the 11p13 breakpoint was a considerabledistance from PAX6. These rearrangements are believed to interfere with the expression of PAX6 by a “position effect”. Interstitial deletionsof 11p13 can be associated with the WAGR syndrome phenotype (MIM 194070). Patients with familial and sporadic aniridia who do nothave a deletion or rearrangement of 11p13 have intragenic mutations in PAX6. These include nonsense, missense, and splice-site mutationsas well as small deletions and insertions. The paired homeodomain of the gene is a mutational hotspot. Almost all mutations are believed to result in a loss of function effect.


Disorders of Vision 71

Mutations in PAX6 can also give rise to ocular phenotypes other thananiridia. These include Peters’ anomaly (MIM 604229), autosomaldominant keratitis (MIM 148190), isolated foveal hypoplasia (MIM136520), congenital cataracts, ectopic pupils, and multiple ocularanomalies (including Peters’ anomaly, Axenfeld anomaly, congenitalcataract, and foveal hypoplasia). Mutations in PAX6 have also beenassociated with subtle central nervous system (CNS) malformations,such as agenesis or hypoplasia of the anterior commissure andcerebellar anomalies in patients with aniridia.

Molecular PAX6 contains 14 exons and is expressed in the eye, forebrain,pathogenesis cerebellum, and olfactory bulbs of the fetus. It encodes a 422-amino-

acid transcriptional regulator protein involved in ocular, CNS, pituitary,and pancreatic development. Aniridia is believed to be the result of haploinsufficiency or a loss of function mutation of PAX6.

The WAGR syndrome phenotype is a contiguous gene syndrome. The interstitial deletion of 11p13 in these patients includes the PAX6gene and the adjacent WT1 gene. Haploinsufficiency of PAX6 causesaniridia while haploinsufficiency of the WT1 gene causes genitourinaryabnormalities (such as small penis, hypospadias, cryptorchidism, and ambiguous genitalia) in patients with a male karyotype andpredisposition to Wilms’ tumor. Deletion of other, as yet unidentified,genes is believed to cause mental retardation.

Genetic diagnosis All patients with aniridia should have chromosome analysis andand counseling fluorescence in situ hybridization (FISH) analysis with probes for PAX6,

WT1, and flanking markers. Patients with aniridia who have a deletionof WT1 on FISH analysis are at high risk of developing Wilms’ tumor andshould be offered regular screening with renal ultrasound scans. PAX6mutation analysis is only available on a research basis. In familial formsof aniridia, counseling is on an autosomal dominant basis. Parents ofsporadic cases in whom a cytogenetically visible or cryptic 11p13deletion has been identified should be tested for a rearrangement of11p13 by chromosome and FISH analysis. Sibling recurrence risk woulddepend on whether one of the parents carries an 11p13 rearrangement.Parents of a sporadic case in whom no 11p13 deletion or rearrangementcan be identified should be carefully examined for anterior chamber and iris abnormalities and early-onset cataracts. If no ocular abnormalityis identified in either parent then the sibling recurrence risk is low.


Bardet–Biedl Syndrome72

Prenatal diagnosis can be offered to the parents of patients with an11p13 deletion or rearrangement and to parents of patients in whom a PAX6 mutation has been identified.

Bardet–Biedl Syndrome(also known as: BBS)

MIM See Table 1.

Clinical features Typically, the clinical features consist of obesity, postaxial polydactyly,hypogonadism, learning disabilities, rod–cone retinal dystrophy, andstructural renal abnormalities (including hypoplasia and cystic dysplasia).Obesity and retinal dystrophy tend to be progressive. Renal function canalso deteriorate leading to renal failure in adult life. Other less commonfeatures include cataracts, brachydactyly and/or syndactyly, cardiacdefects, and diabetes mellitus. Learning disabilities are usually relativelymild, and in the absence of severe renal involvement life expectancy is usually normal.

Age of onset Polydactyly is apparent at birth. Obesity becomes apparent in childhood.Retinal dystrophy usually presents in childhood or the teenage years.

Epidemiology The prevalence in populations of European origin is approximately 1 in 150,000. Much higher incidences have been observed inNewfoundland and in parts of the Middle East, notably in Kuwait.

Inheritance Autosomal recessive (although molecular analysis has demonstratedtriallelic inheritance in some families).

Table 1. Bardet–Biedl syndrome (BBS): types, MIM numbers, chromosomal locations,

and genes.


BBS1 209901 11q13 BBS1

BBS2 606151 16q21 BBS2

BBS3 600151 3p13–p12 Unknown

BBS4 600374 15q22.3–q23 BBS4

BBS5 603650 2q31 Unknown

BBS6 604896 20p12 MKKS (Mckusick–Kaufman syndrome)




Mutational BBS1: missense, nonsense, and splice-site mutations with a presumed spectrum loss of function effect. One particular mutation (Met390Arg) appears

to be a common cause of BBS.

BBS2: missense, nonsense, and frame-shift mutations with a loss of function effect.

BBS4: deletions, insertions, and splice-site mutations with a loss of function effect.

BBS6: missense and nonsense point mutations and frame-shiftdeletions with a loss of function effect.

Molecular BBS1 is composed of 17 exons and spans approximately 23 kb.pathogenesis It shows ubiquitous expression and, like BBS2, encodes a protein

of unknown function.

BBS2 contains 17 exons and shows strong evolutionary conservationwith widespread tissue expression. The structure of the protein productdoes not resemble that of any other known protein and its function is unknown.

BBS4 contains 16 exons with several Alu repeat sequences.Theserepeat sequences predispose to the formation of deletion mutations by unequal homologous recombination. It shows ubiquitous expressionwith highest levels in the kidneys. The predicted protein sequencesuggests that BBS4 encodes a protein that mediates protein–proteininteractions and plays a role in regulating cell signaling.

BBS6 is caused by mutations in MKKS. Mutations in this gene alsocause the McKusick–Kaufman syndrome (MIM 236700), which is characterized by hydrometrocolpos, postaxial polydactyly, andcongenital cardiac defects. MKKS contains six exons and encodes a protein that shows similarity to type II chaperonins. These facilitate ATP-dependent protein folding. The relationship between MKKSgenotype and phenotype is not clear-cut. There is some evidence thatmutations with a milder effect on protein structure and function (ie,missense as opposed to nonsense) give rise to the McKusick–Kaufmansyndrome. Alternatively the Bardet–Biedl phenotype may require the presence of a third BBS mutation as discussed below.


Juvenile Retinoschisis74

Genetic diagnosis Mutation analysis for BBS1, BBS2, BBS4, and MKKS is available inand counseling a few specialist centers on a research basis only. Mutations in BBS1 are

the most frequent cause of BBS. Traditionally, BBS has been assumed toshow straightforward autosomal recessive inheritance. However, recentstudies have shown that some affected individuals have mutations innot two but three BBS genes (eg, two BBS2 mutations and one BBS6mutation). This has been described as “triallelic inheritance” andsuggests that disease expression requires both a dominantly inheritedsusceptibility mutation at one BBS locus and recessive homologousmutations at another BBS locus. Such a mechanism would imply a recurrence risk for siblings of 1 in 8. In practice, a recurrence risk of 1 in 4 is usually quoted.

Juvenile Retinoschisis(also known as: X-linked retinoschisis)

MIM 312700

Clinical features This condition presents with reduced visual acuity in males. There is no history of preceding night blindness and color vision is usuallyunaffected. Most affected individuals have moderate visual impairment in childhood and teenage years. Slowly progressive macular dystrophydevelops in adulthood and can progress to blindness. The condition canbe diagnosed by retinal examination and confirmed by dark-adapted flashelectroretinography (ERG). The characteristic retinal abnormality is thepresence of intraretinal cysts that extend from the fovea in a spoke-wheelpattern. These cysts can also involve the peripheral retina. The ERGshows an electronegative pattern with normal amplitude of the a-waveand reduced amplitude of the b-wave. Female carriers are usuallyasymptomatic and only rarely show abnormalities on retinal examination.

Age of onset Visual loss presents in early childhood, but the age of onset and the rate of progression can show great variability in affected members of the same family.

Epidemiology This is a rare cause of visual impairment in males. It is particularlyprevalent in Finland.

Chromosomal Xp22.1–p22.2location



Gene RS1 (retinoschisis 1)

Mutational Missense mutations account for approximately 75% of mutations inspectrum the RS1 gene. Most of these mutations localize to exons 4–6, which

code for the highly conserved discoidin domain of the gene. Oneparticular missense mutation (Glu72Lys) has been identified in almost15% of affected patients. Other mutations include nonsense, splice-site,and frame-shift mutations. There is no genotype–phenotype correlation.

Molecular RS1 has six exons and encodes a 224-amino-acid protein called pathogenesis retinoschisin. This contains a highly conserved discoidin domain that is

believed to be involved in cell–cell adhesion and phospholipid binding.Retinoschisin is a secreted protein and is predicted to have a globularconformation. It appears to be released by the photoreceptor cells of the retina and has functions within the inner retinal layers. The precisefunction of retinoschisin is unknown, but it may be involved in celladhesion processes during retinal development. There is evidence tosuggest that missense mutations interfere with normal protein foldingand result in the production of a protein with abnormal conformationthat cannot be secreted.

Genetic diagnosis Genetic testing is only available on a research basis. Counseling isand counseling on an X-linked recessive basis. Genetic testing is the only reliable

method of identifying female carriers. Prenatal diagnosis can beundertaken by linkage analysis or mutation analysis (if the familialmutation is known).

Leber Congenital Amaurosis(also known as: LCA)

MIM See Table 2.

Clinical features All forms of LCA present with congenital blindness or early onset visualloss, roving eye movements, and a severely attenuated or extinguishedelectroretinogram. Some children demonstrate the oculodigital(Franceschetti’s) sign (affected children poke or put pressure on theireyes) and others have severe photophobia. Other ocular findings caninclude refractive error (high myopia or hypermetropia), cataracts, andkeratoconus. The retina appears normal initially. Later retinal findings


Leber Congenital Amaurosis76

can resemble retinitis pigmentosa, although other retinal appearanceshave also been reported (eg, chorioretinal atrophy or macular colobomas).LCA can be an isolated problem (uncomplicated LCA) or part of a syndrome(syndromic or complicated LCA). Patients with syndromic forms of LCAcan have developmental delay, neurologic abnormalities (such as hypotoniaand cerebellar vermis hypoplasia), cardiomyopathy, hepatic fibrosis, andrenal abnormalities (cystic renal dysplasia, juvenile nephronophthisis).LCA is a recognized feature of several well-known syndromes such as infantile Refsum disease, Joubert syndrome, Zellweger syndrome,and Senior–Loken syndrome (the combination of LCA and juvenilenephronophthisis). Only the molecular genetics of the isolated forms of LCA are discussed here as these forms comprise 80%–90% of allpatients with LCA.

Age of onset At birth or in early infancy (before the age of 6 months)

Epidemiology LCA affects all ethnic groups, with a population prevalence ofapproximately 3 per 100,000. Almost 70% of patients with LCA1 are of Mediterranean origin.

Inheritance Almost all forms of LCA are inherited in an autosomal recessive manner.Mutations in the CRX gene are thought to result in an autosomaldominant form of LCA.


Table 2. Leber congenital amaurosis (LCA): types, MIM numbers, chromosomal locations, and genes.


LCA1 204000 17p13.1 GUCY2D (guanylate cyclase 2D)

LCA2 204100 1p31 RPE65 (retinal pigment epithelium-specific protein 65 kDa)

LCA3 604232 14q24 Unknown

LCA4 604393 17p13.1 AIPL1 (aryl hydrocarbon-interacting receptor protein-like 1)

LCA5 604537 6q11–q16 Unknown

LCA6 605446 14q11 RPGRIP1 (retinitis pigmentosa GTPase regulator-interactingprotein 1)

LCA due 602225 19q13.3 CRX1 (cone–rod homeobox-containing gene)to mutationsin CRX gene

LCA due to 604210 1q31–q32.1 CRB1 (crumbs homolog 1)mutations inCRB1 gene



Mutational Mutations in the six genes identified for isolated LCA account for only spectrum 50% of all cases. Mutations in most of these genes can also result

in other types of retinal dystrophies such as retinitis pigmentosa,cone–rod dystrophy, and juvenile retinal dystrophy.

GUCY2DMutations in this gene have included frame-shift, nonsense, splice-site,and missense mutations. Several patients carry protein-truncatingmutations in both alleles. Mutations in this gene can be seen in6%–20% of patients with LCA.

RPE65Nonsense, missense, and splice-site mutations have been identified in this gene. Between 3% and 16% of patients with LCA have mutations in RPE65.

AIPL1About 70% of mutations in this gene are null (nonsense, frame-shift,and splice-site) mutations. One nonsense mutation (Trp278Stop)accounts for approximately 50% of mutations in this gene. Missensemutations have also been identified. Mutations in this gene are seen in ~6% of patients with LCA.

RPGRIP1Most mutations in this gene are predicted to result in protein truncation.These include nonsense and splice-site mutations. Two missensemutations and one in-frame deletion have also been identified.Mutations in RPGRIP1 are seen in 5%–6% of LCA patients.

CRXMutations in only a single allele of CRX can cause LCA. Frame-shift and missense mutations have been identified in this gene. Only onefamily has been reported in which LCA resulted from a homozygousmissense CRX mutation. Most LCA patients with CRX mutations havebeen sporadic cases. Therefore, their LCA could be the result of a newautosomal dominant mutation in this gene or there could be anunidentified mutation in the other allele of this gene or in another gene (digenic inheritance) in these patients. Mutations in CRXare seen in only 2%–3% of LCA patients.

CRB1Nonsense, frame-shift, and missense mutations have been identified inthis gene. One missense mutation (C948Y) accounts for approximately


Leber Congenital Amaurosis78

25% of all mutations. About 9%–14% of patients with LCA havemutations in CRB1.

Molecular GUCY2D has 20 exons. Its protein product is a 1,103- amino-acid pathogenesis photoreceptor guanylate cyclase involved in the phototransduction

cascade. In the dark state cGMP levels are restored by guanylatecyclase. Loss of function mutations in GUCY2D interfere with therestoration of the basal cGMP levels in photoreceptors. The effects of this are similar to constant light exposure which results in impairment of photoreceptor function.

RPE65 is composed of 14 exons. It encodes a 533-amino-acid retinal-pigment-specific protein that is involved in the metabolism of all trans-retinyl esters to 11-cis-retinol. Mutations in this gene probably result in retinal dystrophy by interfering with the production of 11-cis-retinol.

AIPL1 has six exons and encodes a 384-amino-acid protein that isexpressed in the rod photoreceptor cells of the peripheral and centralretina. Its precise function is unknown, but it may be essential for themaintenance of rod photoreceptor function.

RPGRIP1 contains 24 exons and encodes a protein that interacts withthe protein product of the RPGR (retinitis pigmentosa GTPase regulator)gene. The RPGRIP protein has 1,267 amino acids. It is localized in theconnecting cilia of rod and cone photoreceptors and is believed to be a structural component of the ciliary axoneme.

CRX is a small gene. It has only three exons and it encodes a299-amino-acid photoreceptor-specific transcription factor that controlsthe expression of several photoreceptor-specific genes. It is believed to play an important role in the differentiation of photoreceptor cells.

CRB1 contains 11 exons. It encodes a 1,376-amino-acid extracellularprotein that has homology to a Drosophila protein called “crumbs”. This protein is probably involved in cell–cell interaction and maintainingcell polarity in the retina.

Genetic diagnosis Molecular genetic analysis in children with LCA is difficult becauseand counseling of the extent of genetic heterogeneity. Genetic testing is only available

on a research basis from a few specialized laboratories.

Genetic counseling is on an autosomal recessive basis. The siblingrecurrence risk is 25% and offspring recurrence risks for affectedindividuals are likely to be relatively low. The only exception is LCA



patients with heterozygous CRX mutations – in these cases the siblingrecurrence risk is probably less than 25% while the offspring recurrencerisk could be as high as 50%.

Norrie Disease

MIM 310600

Clinical features The condition shows both interfamilial and intrafamilial variability ofexpression. Affected males present with bilateral pseudoglioma andblindness. The characteristic ocular findings include iris atrophy andsynechiae, retrolental mass, and retinal folds or detachment. Cataractsand phthisis bulbi can develop later. Mental retardation is seen in two thirds of patients. Behavioral or psychiatric problems are seen in about 25% of patients. One third of affected males have late-onset,progressive, high-frequency sensorineural hearing loss. Female carriersare usually asymptomatic, although retinal abnormalities such as retinaldetachment have been identified in a small number of carriers.

Age of onset The ocular features are apparent at birth in almost all affected males.

Epidemiology No reliable epidemiologic data are available.



Gene NDP (Norrie disease protein)

Mutational Whole gene deletions, intragenic deletions, and point mutationsspectrum have all been described. Point mutations include both missense

and nonsense mutations. All mutations are believed to result in a loss of function effect.

Mutations in NDP have also been identified in Coat’s disease (MIM 300216), X-linked exudative vitreoretinopathy (MIM 305390),and in a small proportion (~3%) of patients with advanced retinopathyof prematurity.

Molecular NDP has three exons and encodes a 133-amino-acid protein knownpathogenesis as norrin. Norrin is a member of a superfamily of growth factors


Rieger Syndrome80

containing a cysteine knot motif. It is expressed in the outer nuclear, innernuclear, and ganglion cell layers of the retina, the cerebellar granularlayer, hippocampus, olfactory bulb, and cerebral cortex. The exactfunction of this protein is unknown, but it could be involved in cell–cellinteraction and neurodevelopment. It is likely to play a critical role in differentiation of the retina and retinal vasculogenesis.

Genetic diagnosis Mutation analysis is available from a few diagnostic laboratories.and counseling This is very useful for confirmation of diagnosis, identification of carriers,

and prenatal diagnosis. Counseling is on an X-linked recessive basis.

Rieger Syndrome(also known as: iridogoniodysgenesis type II)

MIM See Table 3.

Clinical features Rieger syndrome is characterized by the combination of anteriorsegment dysgenesis, facial dysmorphism, dental anomalies, andredundant umbilicus. Anterior chamber abnormalities can includeposterior embryotoxon, Axenfeld anomaly, Rieger anomaly, pupillaryabnormalities (such as dyscoria and polycoria), and iris strandsextending to Schwalbe’s line. Glaucoma develops later in most patients. Dental anomalies include conical crowns of anterior teeth and hypodontia of primary and permanent dentition. Other findingsinclude cleft palate, hypospadias, and anal stenosis. Umbilicalanomalies have not been described in Rieger syndrome type II.

Age of onset The ocular and umbilical anomalies are apparent at birth.

Epidemiology Rieger syndrome is rare; it has a population prevalence of 1 in 200,000.



Table 3. Rieger syndrome: types, MIM numbers, chromosomal locations, and genes.


Type I 180500 4q25–q26 PITX2 (paired-like homeodomain transcription factor 2)

Type II 601499 13q14 Unknown



Mutational Missense, splice-site, and nonsense mutations have all been identified spectrum in PITX2. In-frame and frame-shift duplications have also been identified.

Mutations in PITX2 can also cause autosomal dominant iris hypoplasia(MIM 137600) and Peters’ anomaly (MIM 604229).

Molecular PITX2 has five exons. It encodes a 317-amino-acid transcription factor pathogenesis that is involved in development of the anterior pituitary and teeth.

Mutations in PITX2 are thought to result in functional haploinsufficiencyof this gene.

Genetic diagnosis PITX2 mutation analysis is only available on a research basis.and counseling Counseling is on an autosomal dominant basis. The condition can

show variability of expression both between and within families. All affected individuals require life-long screening for glaucoma.


82


4. Hearing Disorders

Nonsyndromal Hearing Loss 84

Hearing Loss due to Connexin 26 Gene Defect 85

Pendred Syndrome 86

Usher Syndrome 87

Waardenburg Syndrome 90

4

83


Nonsyndromal Hearing Loss84

Nonsyndromal Hearing Loss

At the time of writing over 70 loci for nonsyndromal hearing loss have been identified andapproximately 20 of the relevant genes have been isolated. These are summarized in Table 1.Mutations in each of these genes make only a small contribution to inherited hearing loss.The exception is the connexin 26 gene (CX26 or GJB2), which accounts for up to 40% of all childhood hearing loss in some populations.

Table 1. Nonsyndromal sensorineural hearing loss: disorders, MIM numbers, loci, chromosomal locations, and genes. Loci are indicated as DFNA (autosomal dominant), DFNB (autosomal recessive), and DFN (X-linked).A full list of hearing loss loci is maintained at the hereditary hearing loss homepage (http://dnalab-www.uia.ac.be/dnalab/hhh/).

Disorder MIM Locus Chromosomal location Gene

DFNA 602121 1 5q31 HDIA8 (diaphanous)603324 2 1p34 CX31 (connexin 31)121011 3 13q12 CX26/30 (connexin 26/30)600994 5 7p15 DFNA5 (deafness, autosomal dominant nonsyndromic

sensorineural 5)606201 6 (=14) 4p16.3 WFS1 (Wolfram syndrome)602574 8 (=12) 11q22–q24 TECTA (tectorin)603196 9 14q12–q13 COCH (cochlin)603550 10 6q22–q23 EYA4 (eyes absent)276903 11 11q13.5 MYO7A Myosin VIIA)120290 13 6p21 COL11A2 (collagen 11α2)

602460 15 5q31 POU4F3 (POU domain 4F3)160775 17 22q MYH9 (myosin heavy chain 9) 600970 22 6q13 MYO6 (myosin VI)

28 8q22 TFCP2L3 (transcription factor CP2L3)DFNB 220290 1 13q12 CX26/30 (connexin 26/30)

276903 2 11q13.5 MYO7A (myosin VIIA)602666 3 17p11.2 MYO15 (myosin XV) 605646 4 7q31 PDS (pendrin)607237 6 3p14–p21 TMIE (transmembrane inner-ear-expressed gene)606706 7 (=11) 9q13–q21 TMCI (transmembrane cochlear-expressed gene)605511 8 (=10) 21q22 TMPRSS3 (transmembrane protease)603681 9 2p22–p23 OTOF (otoferlin)605516 12 10q21–q22 CDH23 (cadherin 23)606440 16 15q21–q22 STRC (stereocilin) 276904 18 11p14–p15.1 USHIC (usher IC)602574 21 11q22–q24 TECTA (tectorin) 607038 22 16p12.2 OTOA (otoancorin) 605608 29 21q22 CLDN14 (claudin 14) 606808 30 10p11.1 MYO3A (myosin IIIA)

DFN 300356 1 Xq22 DDP (deafness/dystonia peptide)300039 3 Xq21.1 POU3F4 (POU domain 3F4)


Hearing Disorders 85

Hearing Loss due to Connexin 26 Gene Defect

MIM 220290

Clinical features Hearing loss caused by homozygous or compound heterozygousmutations in the connexin 26 gene is usually severe to profound.Audiology indicates that the hearing loss is sensorineural in origin with a sloping or flat audiometric curve. Radiologic studies of the inner ear are normal. Variation in severity within families and sibships is well recognized. Heterozygous mutations in connexin 26 can cause a rarer autosomal dominant nonsyndromal form of hearing loss as well as a very rare syndromal form in association with palmo-plantarkeratoderma (MIM 148350).

Age on onset Usually the hearing loss is congenital and nonprogressive.

Epidemiology The most common mutation (see “35delG”, below) has an estimatedcarrier frequency of approximately 1 in 50 in most European, Mediterranean,and North American populations. Another mutation (167delT) shows a carrier frequency of 3%–4% in the Ashkenazi Jewish population.

Inheritance Usually autosomal recessive. Some mutations are manifest in theheterozygous state (ie, autosomal-dominant inheritance).


Gene CX26 (connexin 26), also known as GJB2 (gap junction β-2)

Mutational spectrum The most common mutation is a deletion of one of a series of six guanineresidues (35delG). Other common mutations include 167delT in theAshkenazi Jewish population and 235delC in the Japanese. The fullmutational spectrum includes missense and nonsense point mutations,splice-site mutations, and frame-shift deletions and insertions. Most of these mutations are believed to exert a loss of function effect.

Molecular Connexin genes encode the subunits of gap junction proteins which formpathogenesis intercellular channels. These facilitate the transport of small molecules

and ions between adjacent cells. Each gap junction consists of twoconnexons, or hemichannels, made up of six connexin subunits. Normal gap junction formation and function fails in individuals who


Pendred Syndrome86

are homozygous for the 35delG mutation. This leads to an inability torecycle the potassium ions needed for the initiation of action potentialsin the cochlear hair cells.

Mutations in CX26 account for up to 50% of all autosomal recessivenonsyndromal sensorineural hearing loss in populations of Europeanand Mediterranean origin, with 35delG representing around 70% of all CX26 mutations. (The 35delG mutation is sometimes denoted as30delG, ie deletion of the first rather than the last of the six contiguousguanine residues.)

In many individuals only a single CX26 mutation can be identified, a finding which has been difficult to interpret. Recent studies haveshown that many such individuals have a second mutation consisting of a deletion in CX30, which encodes connexin 30 and is contiguouswith CX26 at chromosome 13q11–12. This combined CX26–CX30locus constitutes the DFNB1 (deafness-recessive 1) locus and has been cited as an example of digenic inheritance.

Genetic diagnosis Mutation analysis for the common 35delG mutation is readily available.and counseling When homozygous mutations are identified a sibling recurrence risk of

1 in 4 can be given with confidence. In theory, prenatal diagnosis couldbe offered, but this raises very difficult and contentious ethical issues.Analysis for other mutations in CX26 and in other nonsyndromal hearingloss genes is available on a limited research basis.

Pendred Syndrome(also known as: PDS; deafness with goiter)

MIM 274600

Clinical features These consist chiefly of progressive hearing loss and goiter. The hearingloss is sensorineural in origin and may be present at birth or becomeapparent in early childhood. Disturbance of vestibular function is variable.Radiology of the inner ear often reveals the presence of a Mondinideformity (deficiency of the interscalar septum in the inner coils of the cochlea) and all patients show enlargement of the endolymphaticsac and duct in association with a dilated vestibular aqueduct. Goiterdevelops in approximately 80% of cases, usually in early adult life, and hypothyroidism occurs in around 50% of all cases.



Age of onset The hearing loss is often congenital in onset or develops in earlychildhood. Rarely a goiter can be present at birth. Hypothyroidism has been detected on neonatal screening.

Epidemiology The estimated incidence is 7.5–10 per 100,000. PDS is the mostcommon cause of syndromal hearing loss and accounts for around5%–8% of all childhood-onset hearing loss.


Chromosomal 7q31location

Gene SLC26A4 (solute carrier family 26, member 4; also known as pendrin)

Mutational Missense point mutations, splice-site mutations, and single base spectrum deletions. All have a loss of function effect.

Molecular The PDS gene encodes a 780-amino-acid protein, known as pendrin,pathogenesis which is expressed in the inner ear, the thyroid gland, and renal cortical

collecting ducts. Pendrin acts as a transporter of chloride and iodideanions. Its expression pattern in the cochlea suggests an important rolein maintaining homeostasis of the endolymphatic fluid. Its precise role in the organification of iodide in the thyroid gland remains unclear, but is thought to involve the transport of iodide across the apical membraneof the thyrocyte into the colloid space.

Genetic diagnosis Mutation analysis is available only on a limited research basis. Thus theand counseling diagnosis is usually based on the combined clinical, biochemical, and

radiologic findings. Counseling is as for autosomal recessive inheritance.

Usher Syndrome

MIM See Table 2.

Clinical features These consist essentially of variable sensorineural hearing loss andretinitis pigmentosa. In type I hearing loss is severe to profound, visualloss begins in early childhood, and vestibular function is impaired. Intype II hearing loss is moderate to severe, vestibular function is normal,and the retinopathy manifests in late childhood or early teens. Type III is


Usher Syndrome88

the mildest form with childhood onset, slowly progressive hearing loss,variable vestibular involvement, and teenage-onset night blindness. This progresses slowly and is of variable severity.

Age of onset In types I and II the hearing loss is congenital. In type III onsetof hearing loss is in childhood, after the acquisition of speech (ie, “postlingual”).

Epidemiology The overall incidence has been estimated to be around 3–4 per 100,000.Usher syndrome is believed to account for 3%–6% of serious hearing lossin children. Types IC and IIIA show increased incidences in the Acadianpopulation of Louisiana and in the Finnish population, respectively.


Chromosomal See Table 2.location, gene, and mutational spectrum

Molecular Type IB is caused by mutations in MYO7A and accounts for 75% pathogenesis of all type I cases. MYO7A contains 49 exons. It encodes myosin VIIa,

a member of the myosin family of proteins that interact with actinfilaments to convert energy from ATP into mechanical force. MYO7Ais expressed in the apical stereocilia and cytoplasm of hair cells, thecochlea and vestibular system, both the apical processes of pigmentepithelium cells, and the connecting cilia of rod and cone photoreceptorcells in the retina. Normally, stereocilia bend in response to vibrationsresulting in the opening of ion channels which leads to the conversion of mechanical to electrical energy in hair cells. Mutant myosin VIIaimpedes this process and also prevents the normal distribution ofmelanosomes in the retinal pigment epithelium. MYO7A mutations also account for the human nonsyndromal DFNB2 and DFNA11 forms of hearing loss (see Table 2). Mutations in the mouse orthologcause the deafness syndrome known as “shaker”.

Type IC results from mutations in the USHIC gene, which encodes a PDZ-domain-containing protein known as harmonin. PDZ proteinsorganize multiprotein complexes in areas such as synaptic junctions and anchor transmembrane proteins, such as receptors and ionchannels. USHIC and MYO7A have similar expression patterns in the USHIC and MYO7A have similar expression patterns in the inner



ear and are components of the same multiprotein complex. A contiguousgene deletion syndrome that includes the USHIC locus has beendescribed resulting in hearing loss, hyperinsulinism, renal tubulardysfunction, and enteropathy.

Type ID is caused by mutations in CDH23, which encodes a largetransmembrane protein entitled otocadherin. This is an importantcomponent of hair bundle formation. Mutations in the mouse orthologcause the condition known as “waltzer” in which the organization ofstereocilia is disrupted. Mutations in CDH23 also cause the DFNB12form of nonsyndromal hearing loss (see Table 1, p.84).

Table 2. Usher syndrome: types, MIM numbers, chromosomal locations, genes, and mutational spectra. VNTR: variable number of tandem repeats.

Type MIM Chromosomal Gene Mutational spectrumlocation

Type IA 276900 14q32 Unknown Unknown

Type IB 276903 11q13.5 MYO7A Missense, nonsense,(myosin VIIA) and splice-site mutations.

Also frame-shift deletions. All with a loss of function effect

Type IC 276904 11p15.1 USH1C Splice-site mutations and (harmonin) deletions. Also expansion

of an intronic VNTR. All with a loss of function effect

Type ID 601067 10q21–q22 CDH23 Missense, nonsense, and(cadherin 23) splice-site mutations. Also

deletions and insertions

Type IE 602097 21q21 Unknown Unknown

Type IF 602083 10q21–q22 PCDH15 Nonsense mutations and (protocadherin insertions with a probable15) loss of function effect

Type IG 606943 17q24–q25 Unknown Unknown

Type IIA 276901 1q41 USH2A A single mutation, (usherin) 2299delG, accounts for

16%–44% of all mutantalleles. Also missense,nonsense, splice-site, andframe-shift mutations

Type IIB 276905 3p24.2–p23 Unknown Unknown

Type IIC 605472 5q14–q21 Unknown Unknown

Type IIIA 276902 3q21–q25 USH3A Missense, nonsense, (Usher syndrome and deletion mutations type III)


Waardenburg Syndrome90

Type IF is caused by mutations in PCDH15, which encodes a protocadherin protein expressed in the inner ear and retina.

Type IIA results from mutations in USH2A, which contains 21 exons and encodes a protein given the name of usherin. This contains a singlelaminin type VI domain, 10 laminin-like epidermal growth factor domainsand four fibronectin type III domains. Mutations are scattered throughoutthe gene; to date, no clear genotype–phenotype correlation has emerged.

The gene that causes type IIIA is known as USH3A and containsfour exons that encode a 120-amino-acid transmembrane proteinof unknown function.

Genetic diagnosis Specific mutation analysis is only available at specialized research-and counseling based laboratories. Counseling is as for autosomal recessive inheritance.

Waardenburg Syndrome(also known as: WS. Includes Klein–Waardenburg and Waardenburg–Shah syndromes)

MIM See Table 3.

Clinical features Hearing loss and variable depigmentation occur in all forms of WS. The hearing loss is sensorineural, shows congenital onset, and is usuallynonprogressive. It can be unilateral or bilateral and varies from mild to profound. The incidence of significant hearing loss in types I and II is approximately 70% and 90%, respectively. Absence of normalpigmentation manifests in the eyes with hypoplastic blue irides orcomplete or partial iris heterochromia, in the hair with a white forelock,and in the skin with areas of hypopigmentation.

Types I and II are distinguished by the presence of dystopia canthorum(lateral displacement of the inner canthi) in type I, but not in type II.Types III and IV are both rare and are characterized by the presence of upper limb abnormalities such as contractures in type III andHirschsprung disease in type IV.

Age of onset Hearing loss is congenital. Areas of hypopigmentation and irisheterochromia become apparent in infancy or early childhood.

Epidemiology The overall incidence has been estimated to be approximately 1 in 40,000. WS has been reported in all ethnic groups.



Inheritance, See Table 3.chromosomal location, gene, and mutational spectrum

Molecular WS types I and III represent neurocristopathies in that they result from pathogenesis abnormal development, migration, or differentiation of neural crest cells

that originate in the neural groove and neural tube. Studies in micesuggest that PAX3 is expressed in neural crest-derived cells duringembryogenesis as well as in segmental mesoderm and the developinglimb buds. PAX3 encodes a DNA binding transcription factor thatcontains a highly conserved 130-amino-acid paired-box (hence “PAX”)domain. Recognized PAX3 mutations act either as null alleles or impairnormal DNA binding. Thus at a simplistic level, mutations in PAX3that exert a loss of function effect appear to result in dose-dependent

Table 3. Waardenburg syndrome (WS): types, MIM numbers, inheritances, chromosomal locations, genes, andmutational spectra.

Type MIM Inheritance Chromosomal Gene Mutational location spectrum

Type I 193500 Autosomal 2q35 PAX3 Missense, nonsense, dominant (paired-box 3) and splice-site mutations.

Also frame-shift insertionsand deletions. All witha loss of function effect

Type IIA 193510 Autosomal 3p14.1–p12.3 MITF Missense, nonsense, anddominant (microphthalmia- splice-site mutations

associated transcription factor)

Type IIB 600193 Autosomal 1p21–p13.3 Unknown Unknowndominant

Type IIC 606662 Autosomal 8p23 Unknown Unknowndominant

Type III 148820 Autosomal 2q35 PAX3 Missense and nonsense(Klein–Waardenburg dominant mutationssyndrome) or autosomal

recessive

Type IV 277580 Autosomal 13q22 EDNRB Nonsense mutations (Waardenburg–Shah recessive (endothelin with a loss of function effectsyndrome) or autosomal receptor, type B)

dominant

20q13.2–q13.3 EDN3(endothelin 3)

22q13 SOX10 (SRY-box 10)


Waardenburg Syndrome92

impairment of normal neural crest development. The very rare WS typeIII represents the severe end of the PAX3 neurocristopathy phenotypeand has been described in association with both heterozygous andhomozygous mutations.

MITF also encodes a transcription factor. This transactivates the gene for tyrosinase, an enzyme that is essential for normal melanocytedifferentiation. Thus the clinical features in WS type IIA are attributableto an abnormality of melanocytes rather than neural crest cells.Interestingly, it has recently emerged that MITF expression is regulatedby PAX3 and SOX10 acting synergistically, indicating that the clinicalfeatures of WS types I and III result from a combination of both neuralcrest cell and melanocyte dysfunction.

WS type IV is an extremely rare form of neurocristopathy. It is associatedwith absence of melanocytes and inner ear cells, giving rise to the featuresof WS, together with absence of parasympathetic enteric neurons of the terminal hindgut, resulting in Hirschsprung disease. The type IVphenotype can be caused by homozygous mutations in either EDNRB or in the gene that encodes its ligand, EDN3. Heterozygous mutations in either of these genes can result in Hirschsprung disease. Type IV can also be caused by heterozygous loss of function mutations in SOX10,a member of the SOX family of transcription regulators. A few type IVpatients with SOX10 mutations also show progressive central nervoussystem involvement with mental retardation, nystagmus, cerebellarataxia, and spasticity – findings consistent with abnormal SOX10expression in glial cells.

Genetic diagnosis Mutation analysis for PAX3 and MITF is available at a few specialist and counseling laboratories. Mutation analysis for other WS genes is only offered on

a research basis. Counseling in most cases of WS is on the basis ofautosomal dominant inheritance with variable expression, but cautionshould be exercised when counseling for types III and IV, both of whichcan show either autosomal dominant or autosomal recessive inheritance.


5. Neurocutaneous Disorders and Childhood Cancer

Multiple Endocrine Neoplasia Type 2 94

Neurofibromatosis Type 1 96

Retinoblastoma 98

Tuberous Sclerosis 101

von Hippel–Lindau Disease 103


Multiple Endocrine Neoplasia Type 294

Multiple Endocrine Neoplasia Type 2(also known as: MEN2)

MIM 171400 (MEN2A or Sipple syndrome)162300 (MEN2B or Wagenmann–Froboese syndrome)155240 (familial medullary thyroid carcinoma [FMTC])

Clinical features MEN2AThis is the most common form of MEN2. It presents with medullarythyroid carcinoma (MTC). About 50% of all patients developpheochromocytoma and 15%–30% develop hyperparathyroidism (due to parathyroid hyperplasia or adenoma). Hirschsprung disease has also been described in affected patients.

MEN2BThis is a rare form of MEN2. Affected individuals have a Marfanoidhabitus, thickened everted margins of the upper eyelid, neuromas of thelip and tongue, and proximal muscle weakness with wasting. Medullatedcorneal nerve fibers can be seen on slit-lamp examination. Almost 90%of patients develop MTC and 50% develop pheochromocytoma.

FMTCThis diagnosis is made in families with four or more cases of MTCwithout any of the features of MEN2A or MEN2B.

Age of onset Variable presentation in childhood has been described, particularly in MEN2B.

Epidemiology This is a rare cancer predisposition syndrome.


Chromosomal 10q11.2 location

Gene RET (rearranged during transfection) proto-oncogene

Mutational MEN2Aspectrum Almost all cases of this condition are caused by missense mutations

involving six codons in exons 10 and 11. These are codons 609, 611,618, 620, 630, and 634, all of which code for cysteine. Mutations in


Neurocutaneous Disorders and Childhood Cancer 95

codon 634 are seen in 87% of patients. Small frame-shift insertions in exon 11 have also been described in two families.

MEN2BTwo missense mutations (Met918Thr and Ala883Phe) in exon 15 accountfor ~99% of cases. The former mutation is seen in 95% of patients andthe latter accounts for 4% of cases.

FMTCMutations that cause MEN2A can also cause FMTC. Missense mutationsinvolving codons 618, 620, and 634 account for 77% of cases.Mutations that have only been identified in patients with FMTC includeAsp768Asn, Tyr791Phe, Ser891Ala, and a 2-bp insertion in exon 8.

Molecular RET spans over 55 kb of genomic DNA and contains 21 exons. It encodespathogenesis a receptor tyrosine kinase that is primarily expressed in neural crest and

urogenital precursor cells. It is involved in cell survival, proliferation, anddifferentiation. The receptor is activated by a complex comprising its ligandand a cell-surface bound coreceptor for the ligand. The ligand first binds tothe coreceptor, which in turn presents the ligand to RET. The binding of theligand to RET results in its dimerization. This causes autophosphorylation of tyrosine residues in its intracellular tyrosine kinase domain. Interaction of adaptor proteins with sequences adjacent to the phosphorylated tyrosineresidues causes activation of several downstream pathways, including theRAS–MAP (mitogen-activated protein) kinase pathway that is needed forneuronal growth and differentiation. Activation of phosphatidylinositol-3phosphate kinase (PI3-K) by RET is associated with cell proliferation andcellular motility. Activation of other downstream pathways can result in aneoplastic phenotype. Ligands for RET are members of the glial cell-linederived neurotrophic factor (GDNF) family and include GDNF, neurturin,persephin, and artemin. Mutations in the extracellular domain that causeMEN2A and FMTC result in activation of the receptor by ligand-independentdimerization. Mutations in the intracellular tyrosine kinase domain thatcause MEN2B result in activation of the kinase activity of this domain in the absence of ligand binding and dimerization of the receptor. Activatingmutations in RET cause a neoplastic phenotype by persistent downstreamsignaling. Inactivating mutations of RET are associated with Hirschsprungdisease (see p.177–9) and Haddad syndrome (MIM 209880). The lattercondition is the combination of Hirschsprung disease and congenital centralhypoventilation syndrome, also known as Ondine’s curse.


Neurofibromatosis Type 196

Genetic diagnosis Counseling in MEN2 is on an autosomal dominant basis. About 5%and counseling of cases of MEN2A and almost 50% of cases of MEN2B are the result of

a new mutation in RET. Genetic testing should be performed in all casesand is available from diagnostic laboratories. Identification of a mutation in RET allows confirmation of diagnosis and enables predictive testing tobe offered to at-risk family members. Because MTC can occur at an earlyage, predictive testing should be offered to at-risk individuals by the age of 5 years in MEN2A and FMTC families and before the age of 5 years in MEN2B families. Children and older individuals in whom a RETmutation is identified should be offered prophylactic total thyroidectomy.This should be carried out by 5 years of age in MEN2A and FMTC familiesand before the age of 5 years in MEN2B families (some experts recommendprophylactic thyroidectomy before 6 months of age). The presence of anunsuspected pheochromocytoma should always be ruled out in thesepatients prior to surgery as this tumor can cause sudden death as a resultof an anesthesia-induced hypertensive crisis. Affected and at-risk individualsfrom MEN2A and FMTC families should be screened for pheochromocytomaand hyperparathyroidism by measuring catecholamine levels in a 24-hoururine sample and plasma calcium levels on an annual basis. At-riskindividuals from MEN2 families (all types) who cannot be offeredpredictive testing should also be screened for MTC by an annualpentagastrin stimulation test until the age of 35 years.

Neurofibromatosis Type 1(also known as: NF1; Von Recklinghausen’s disease)

MIM 162200

Clinical features Multiple café-au-lait (CAL) patches are the usual presenting feature.Affected children have six or more CAL patches greater than 5 mm indiameter (in postpubertal children these patches should be more than15 mm in diameter). Other cutaneous manifestations of NF1 includeaxillary (see Figure 1) and inguinal freckling and cutaneous neurofibromas(see Figure 2). Other diagnostic features include plexiform neurofibromas,Lisch nodules (iris hamartomas), pseudoarthrosis of the tibia, sphenoidwing dysplasia, and optic nerve glioma. Affected children often havemacrocephaly and short stature. Complications include learningdifficulties, epilepsy, scoliosis, hypertension, and plexiformneurofibromas of the head and neck.



Figure 1. Axillary Figure 2. Multiple café-au-lait patches freckling. and cutaneous neurofibromas.

Age of onset First year of life

Epidemiology The condition affects individuals of all races. It is one of the mostcommon autosomal dominant disorders with a population prevalence of 1 in 3,000.



Gene NF1

Mutational The NF1 gene is very large, spanning over 350 kb of genomic DNA withspectrum 60 exons. Over 500 mutations have been identified. Most mutations

are “private” (ie, restricted to a particular family). All types of mutationshave been described (including nonsense, missense, frame-shift, andsplice-site mutations) as well as small and large intragenic deletions and other rearrangements. Around 80% of mutations result in theproduction of a truncated protein and these mutations are evenlydistributed over the entire coding sequence of the gene.

The entire NF1 gene is deleted in about 5% of patients. These patients have a distinct phenotype with severe learning difficulties, facial dysmorphism, relatively large hands and feet, overgrowth, and numerous neurofibromas. Whole gene deletions can be detected by fluorescence in situ hybridization analysis.


Retinoblastoma98

Molecular The NF1 gene encodes a protein called neurofibromin, which is pathogenesis widely expressed in several tissues. Neurofibromin has a domain

that demonstrates homology to GTPase-activating protein (GAP). The GAP-related domain down-regulates the activity of RAS, which is a major regulator of cellular growth and differentiation. The precisecellular functions of neurofibromin are unknown. There is evidence tosuggest that NF1 is a tumor suppressor gene. NF1 is thought to resultfrom haploinsufficiency.

Genetic diagnosis NF1 is a clinical diagnosis. Genetic testing is difficult because of theand counseling large size of the gene, the vast number of mutations that have been

identified, and the high proportion of “novel” mutations. Mutationtesting is only available on a research basis at the present time.

Counseling is on an autosomal dominant basis. About 50% of casesrepresent new mutations. NF1 shows a lot of variability in expression in affected members of the same family. Parents of such cases should be carefully examined clinically to look for the cutaneous features of NF1and by slit-lamp examination to look for Lisch nodules. If neither parentfulfils the diagnostic criteria for NF1 there is a 1%–2% recurrence riskfor this condition in their next pregnancy because of the possibility of gonadal mosaicism.

In large families with affected individuals in two or more generations,prenatal diagnosis can usually be offered by linkage analysis usingintragenic and flanking markers. However, many families elect not to have a prenatal test as this does not provide any information about the severity or complications of NF1 in an affected child.

Retinoblastoma

MIM 180200

Clinical features The usual presenting features of retinoblastoma are leucocoria (white-eye or cat’s-eye reflex) or strabismus. Atypical presentations includeglaucoma, uveitis, hyphema, and vitreous hemorrhage.

Age of onset Most retinoblastomas present before the age of 5 years. Bilateral diseasepresents earlier (mean age at diagnosis 12 months) than unilateraldisease (mean age at diagnosis 24 months).



Epidemiology Retinoblastoma is the most common ocular tumor of childhood with a global incidence of about 1 in 15,000–20,000 live births.



Gene RB1 (retinoblastoma 1)

Mutational A small number of patients with retinoblastoma (2%–3%) have anspectrum interstitial deletion or a translocation involving 13q14. Patients with

an interstitial deletion of this region usually have other clinical featuressuch as microcephaly, developmental delay, and dysmorphic features.Over 350 mutations have been identified in RB1. These include largerearrangements, small intragenic deletions and duplications, and pointmutations. Point mutations include nonsense, missense, splice-site, and frame-shift mutations. Most mutations result in protein truncation.

Molecular RB1 is a tumor suppressor gene. It contains 27 exons and it encodespathogenesis a 724-amino-acid protein. Its protein product (Rb) is a nuclear

phosphoprotein that inhibits cellular proliferation by inhibiting progress ofcells from the G1 to the S phase of the cell cycle. It does this by interactingwith the E2F family of transcription factors. The Rb–E2F complex arrestscells in the G1 phase of the cell cycle by transcriptional repression of othergenes such as TGFB and CDKN2A. Both alleles of RB1 have to beinactivated before uncontrolled cellular proliferation can occur.

Genetic diagnosis Only 10% of patients with retinoblastoma have a family history ofand counseling this tumor. These patients have an inherited germ line RB1 mutation and

develop bilateral disease. About 30% of patients have bilateral diseasebut no family history of retinoblastoma. These patients also have germline RB1 mutations, but these are assumed to be “new” mutations. The remaining patients have unilateral disease and no family history of retinoblastoma. Many patients with unilateral multifocal tumors arealso likely to have a “new” germ line RB1 mutation, although somaticmosaicism for an RB1 mutation is also possible. Approximately 85% of patients with a unilateral unifocal tumor have sporadic disease as aresult of chance inactivation of both alleles of RB1 in the tumor. About10% of patients with a unilateral unifocal tumor have a “new” germ lineRB1 mutation and 5% are somatic mosaic for an RB1 mutation.


Retinoblastoma100

Table 1. Family history, tumor-type, probability of germline mutation, and risks

to offspring and siblings.

All patients with retinoblastoma (familial and sporadic) should beoffered genetic testing, which is available from a few specializedlaboratories. In cases with unilateral disease both blood and fresh or archived tumor tissue will be needed for RB1 mutation analysis.

Genetic counseling is guided by family history, the number anddistribution of tumors, and the results of genetic testing (see Table 1).Siblings and offspring of patients should be offered regular screening for retinoblastoma by retinal examination (under anesthesia until the age of 3 years) from birth to the age of 11 years. This can be stopped if genetic testing shows that the individual being screened has notinherited the RB1 mutation identified in the affected proband.

Long-term survivors of retinoblastoma with a germ line RB1 mutationare at increased risk of developing second non-ocular malignant tumors. These include osteosarcoma, soft tissue sarcomas (such as fibrosarcoma), malignant melanoma, and brain tumors. The risk of a second non-ocular tumor is much higher in patients who receivedradiotherapy for the treatment of their retinoblastoma.

Family Tumor type Probability Risk to offspring Risk to siblingshistory of germline

mutation

Positive Bilateral 100% 50% –retinoblastoma

Negative Bilateral 95% Assumed to Around 3%–5%retinoblastoma be 50% (due to germline

mosaicism)

Negative Multifocal, Uncertain Difficult to Difficult tounilateral determine determineretinoblastoma

Negative Unifocal, 5%–10% 2%–5% 1%unilateralretinoblastoma



Tuberous Sclerosis(also known as: TS; TS complex; epiloia; Bourneville–Pringle syndrome)

MIM 191100

Clinical features TS is a neurocutaneous disorder. The neurologic features includeepilepsy, developmental delay, learning difficulties, and behavioralproblems (including autistic features). Only 40%–50% of affectedindividuals have mental retardation. The most commonly seen seizuresare infantile spasms. Cutaneous manifestations include fibrous foreheadplaques, hypomelanotic or ash-leaf macules (see Figures 3 and 4), café-au-lait patches, confetti hypopigmentation, shagreen patch, facialangiofibromas, and periungual fibromas (see Figure 5). Renal lesionsinclude cysts and angiomyolipomas. Other manifestations includecardiac rhabdomyomas, retinal hamartomas (phakomas), dental pits,phalangeal cysts, and pulmonary lymphangioleiomyomatosis (in adultfemales). Characteristic neuroradiologic findings include calcifiedsubependymal nodules, cortical tubers or hamartomas, and giant cell astrocytomas (see Figure 6).

Age of onset First year of life

Epidemiology TS affects all populations with a prevalence of 1 in 6,000–10,000.


Figure 3. Hypomelanotic Figure 4. Hypomelanotic macule seen (ash-leaf) macule in under Wood’s light.patient with tuberous sclerosis.


Tuberous Sclerosis102

Figure 5. Multiple periungual Figure 6. Calcified subependymalfibromas in tuberous sclerosis. nodules and calcified cortical tuber

on computed tomography scan in a patient with tuberous sclerosis.

Chromosomal 9q34.3, 16p13.3

location

Gene TSC1 (tuberous sclerosis complex 1), TSC2 (tuberous sclerosis

complex 2)

Mutational spectrum TSC1 has 23 exons and codes for a 1,164-amino-acid protein called

hamartin. Mutations in TSC1 are seen in about 50% of familial cases,

but are only identified in about 10%–20% of sporadic cases. These include

small deletions, nonsense mutations, frame-shift mutations, and splice-

site mutations. About 50% of mutations are single base substitutions,

82% of which are nonsense mutations. Virtually all TSC1 mutations

are inactivating as they result in the production of a truncated protein.

TSC2 contains 41 exons and encodes a 1,807-amino-acid protein

called tuberin. Mutations in TSC2 can be identified in 50% of familial

cases and about 80% of sporadic cases. All types of mutations have

been described. Deletions, insertions, nonsense, frame-shift, and

splice-site mutations result in protein truncation. Missense mutations

have also been described. These usually involve the GTPase-activating

protein (GAP)-related domain of the protein and result in the production

of a protein with reduced activity.

Molecular Both hamartin and tuberin are believed to be GAPs. Both proteins have

pathogenesis been shown to associate in vivo and probably act through the same



pathway. Hamartin exhibits GAP activity towards RAP1 and RAB5,

whereas tuberin only exhibits GAP activity towards RAB5. RAP1 is

a p21 protein that is capable of suppressing cellular transformation.

RAB5 is involved in the regulation of the endocytic pathway. Precisely

how mutations in TSC1 and TSC2 give rise to the phenotype of TS is

unknown. There is evidence to suggest that both TSC1 and TSC2 are

tumor suppressor genes. Loss of both alleles of either TSC1 or TSC2

can result in the formation of TS-related hamartomas.

Genetic diagnosis The diagnosis of TS requires both clinical and radiologic examination.

and counseling Genetic testing is available from a small number of diagnostic

laboratories. Counseling is on the basis of autosomal dominant

inheritance. The condition shows high penetrance, but instances of

nonpenetrance and somatic mosaicism have been described. About

60% of cases are the result of a new mutation. Both parents of an

affected child should be carefully examined for the cutaneous and retinal

findings of TS. Some experts also advocate testing both parents of an

affected child with cranial CT or MRI scans and renal ultrasound scans.

If the parents do not have TS then the risk of recurrence in another child

is small (2%–3%) and probably due to gonadal mosaicism. Prenatal

diagnosis can be offered to families in which a mutation in TSC1 or

TSC2 has been identified in an index case.

von Hippel–Lindau Disease(also known as: VHL)

MIM 193300

Clinical features VHL disease is a tumor predisposition syndrome. Affected individuals are at risk of developing: cerebellar, brain-stem, and spinal hemangioblastomas; retinal angiomas; clear cell renal cellcarcinoma; and pheochromocytomas. Central nervous system (CNS)hemangioblastomas, retinal angiomas, and pheochromocytomas areusually benign, but clear cell renal cell carcinoma is a malignant tumorand is the most frequent cause of death in patients with VHL. Lesscommon tumors include pancreatic islet cell tumors, endolymphatic sactumors, epididymal tumors, and paragangliomas (pheochromocytomas


von Hippel–Lindau Disease104

located in extra-adrenal locations). Renal and pancreatic cysts can alsobe seen in this condition.

Age of onset Onset before the age of 5 years is exceptional. Children usually presentwith either retinal angioma or CNS hemangioblastoma.

Epidemiology This condition affects all ethnic groups, with an incidence of 1 in 36,000 live births.

Inheritance Autosomal dominant with age-dependent penetrance: the condition has 2% penetrance by the age of 5 years, 19% penetrance by the age of 15 years, and 99% penetrance by the age of 65 years.

Chromosomal 3p25–p26location

Gene VHL

Mutational More than 300 mutations have been described in VHL. Approximatelyspectrum 70% of patients have point mutations whereas the remainder have

partial or complete deletions of the gene. Point mutations includenonsense, missense, and splice-site mutations. Mutations are spreadacross all three exons of the gene, but codon 167 appears to be a mutational hotspot. There appears to be genotype–phenotypecorrelation. Most patients with truncating or null mutations in VHLhave VHL without pheochromocytoma (VHL type 1). Most patients with missense mutations have VHL with pheochromocytoma or isolated pheochromocytoma (VHL type 2).

Molecular VHL spans about 10 kb of genomic DNA. It has three exons andpathogenesis produces two transcripts as a result of alternative splicing of exon 2.

One transcript codes for a 213-amino-acid protein (isoform 1) and the other for a 172-amino-acid protein (isoform 2).

The protein product of VHL (pVHL) forms a complex with several cellularproteins including elongin C, elongin B, and cullin 2. This complex hasE3 ubiquitin ligase activity and is involved in degradation of hypoxia-inducible factor 1α (HIF-1α). Loss of function of pVHL leads to thestabilization of HIF-1α, which activates transcription of several hypoxia-inducible genes such as vascular endothelial growth factor (VEGF),glucose transporter 1 (GLUT-1), and platelet-derived growth factor-B



(PDGF-B). Overexpression of these genes could result in the productionof the vascular tumors of VHL. There is evidence to suggest that pVHLalso interacts with other proteins such as fibronectin, protein kinase C,and probably other (as yet unidentified) proteins. Through theseinteractions it could promote correct formation of the fibronectinextracellular matrix and could also inhibit cell signaling.

VHL is a tumor suppressor gene. The normal allele of this gene isinactivated in VHL-related tumors in individuals who inherit a mutationin one allele of this gene (ie, individuals with a germ line mutation). The reason for the genotype–phenotype correlation is unknown. Germ line mutations that result in the production of pVHL with someresidual function (some missense mutations) are associated with a predisposition to pheochromocytoma, whereas mutations thatcompletely inactivate the gene (deletions and protein-truncatingmutations) are not associated with predisposition to these tumors.

Genetic diagnosis Counseling is on an autosomal dominant basis. Genetic testing is and counseling available on a routine basis. All patients with VHL should be offered

genetic testing so that predictive testing can be offered by directmutation analysis to other family members who are at risk of inheritingthis condition. Relatives of affected individuals in whom a VHL mutationcannot be identified can be offered predictive testing by linkage analysis(using markers closely linked to VHL). Affected patients should beoffered regular screening for other VHL-related tumors. Therecommended screening protocol for affected individuals includesphysical examination, retinal examination (by direct and indirectophthalmoscopy and fluorescein angiography), abdominal ultrasoundscan, and 24-hour urine collection for catecholamines on an annualbasis. In addition, affected individuals should be offered 3-yearly CT orMRI brain scans until the age of 50 years and 5-yearly scans thereafter.

Individuals at 50% risk of inheriting VHL should also be offered screeninguntil their risk status can be clarified by predictive testing (by mutation or linkage analysis). The recommended screening protocol for at-riskrelatives includes annual retinal examination by direct and indirectophthalmoscopy from the age of 5 years, annual 24-hour urine collectionfor catecholamines from the age of 10 years, 3-yearly CT or MRI scan of the brain, and annual abdominal scan from the age of 15 years. If theresults of predictive testing show that an at-risk relative has not inherited


von Hippel–Lindau Disease106

the familial VHL mutation then screening can be stopped. An at-riskrelative shown to have inherited the VHL mutation (or shown, by linkageanalysis, to have inherited the high-risk VHL allele) should be screenedusing the same protocol that is used for affected individuals.


6. Connective Tissue and Skeletal Disorders

Achondroplasia 108

Ehlers–Danlos Syndrome 110

Hereditary Multiple Exostoses 115

Marfan Syndrome 117

Osteogenesis Imperfecta 119

Pseudoachondroplasia 124

Stickler Syndrome 125


Achondroplasia108

AchondroplasiaMIM 100800

Clinical features The clinical features are characteristic and distinctive (see Figure 1).Facially, there is macrocephaly with frontal bossing and a flat nasalbridge. The trunk is relatively normal in length with a lumbar lordosis.The limbs show rhizomelic shortening with short fingers giving a“trident” appearance to the hand. Average adult height is 130 cm in males and 125 cm in females. In the absence of central nervoussystem complications, both intelligence and life expectancy are normal.Complications can include hydrocephalus (rare, seen in <5%), cervicalcord compression due to a small foramen magnum (seen in 5%–10% ofcases), spinal stenosis (seen in >50% of cases by the age of 60 years),and premature osteoarthritis, which is common in middle age.

Age of onset The diagnosis is usually evident at birth and can be suspected on thebasis of limb shortening during the third trimester of pregnancy.

Epidemiology The incidence in neonates is approximately 1 in 10,000–20,000. The condition shows no ethnic predilection.

Inheritance Autosomal dominant with full penetrance and consistent expression

Figure 1. A young child with achondroplasia.


Connective Tissue and Skeletal Disorders 109


Gene FGFR3 (fibroblast growth factor receptor 3)

Mutational Point mutations with a gain of function effectspectrum

Molecular FGFR3 encodes a receptor with an extracellular region containing three pathogenesis immunoglobulin-like domains, a single membrane-spanning domain,

and an intracellular split tyrosine kinase domain (see Figure 2). Onbinding with the specific growth factor ligand, the receptor moleculesdimerize with subsequent phosphorylation of the tyrosine residues in theintracellular domain. This activation of the receptor leads to suppressionof endochondral growth through the process of signal transduction.

Almost all cases of achondroplasia are caused by either a G→A or G→Cmutation (Gly380Arg) at nucleotide 1138 in the transmembrane domainof FGFR3. Other mutations in FGFR3 result in a spectrum of skeletalinvolvement causing short stature ranging from the severe and invariablylethal forms of thanatophoric dysplasia to the much milder condition ofhypochondroplasia (see Table 1). The mutations that cause achondroplasia

Figure 2. Diagrammatic representation of FGFR3 (fibroblast growth factor receptor 3) with arrows showingthe sites of common mutations. ACH: achondroplasia; HCH: hypochondroplasia; Ig: immunoglobulin loop;TDI/TDII: thanatophoric dysplasia types 1 and 2; TK1/2: tyrosine kinase domains 1 and 2; TM:transmembrane domain.

TDI TDI ACH HCH TDII

742C→T

1138G→A

orG→C

1620C→A

1948A→G

Arg248Cys Tyr373Cys

Gly380Arg

Asn540Lys Lys650Glu

Ig I Ig II Ig III

TM TK1 TK2


Ehlers–Danlos Syndrome110

Table 1. Skeletal disorders caused by mutations in FGFR3 (fibroblast growth factor receptor3). SADDAN: severe achondroplasia with developmental delay and acanthosis nigricans.

result in ligand-dependent receptor activity. In contrast, the mutationsthat are associated with thanatophoric dysplasia result in receptoractivation that is independent of ligand binding.

Genetic diagnosis Detection of the common 1,138 G→A and 1,138 G→C mutations and counseling is readily available. Counseling is as for autosomal dominant inheritance.

Approximately 80% of all cases result from new mutations almost all ofwhich have occurred in the paternally derived allele, as suggested by thelong-observed paternal age effect. If both parents have achondroplasia,each of their children has a 1 in 4 chance of being homozygous affected,which almost always results in death in the perinatal period.

Ehlers–Danlos Syndrome

MIM See Table 2.

Clinical features The term “Ehlers–Danlos syndrome” embraces a group of disorders that share the common features of increased joint laxity and skinhyperextensibility (see Figures 3 and 4) with other variablemanifestations, such as skin friability and easy bruising, periodontaldisease, mitral valve prolapse, and vascular rupture. Clinical featurescharacteristic of the different traditional subtypes are listed in Table 2.Marked clinical and genetic heterogeneity exist and many patients are difficult to classify.

In the classic case the skin is soft and velvety in texture with increasedextensibility but normal recoil elasticity. The latter feature distinguishesEhlers–Danlos syndrome from cutis laxa. In severe cases the skin tearseasily and heals with thin “cigarette paper” scars. Joint involvement can

Disorder MIM Amino acid residue affected

Achondroplasia 100800 Gly380Arg

Hypochondroplasia 146000 Asn540Lys

SADDAN Not listed Lys650Met

Thanatophoric dysplasia• type I (curved femora) 187600 Arg248Cys,Tyr373Cys

• type II (straight femora with 187601 Lys650Glucloverleaf skull)



Figure 3. Skin laxity in Figure 4. Joint laxity inEhlers–Danlos syndrome. Ehlers–Danlos syndrome.

vary from mildly increased laxity to severe instability with recurrentdislocation. Vascular manifestations are also variable and can rangefrom a mild bruising tendency to catastrophic arterial rupture.

Age of onset Prematurity, possibly the result of deficient collagen in the amnioticmembranes, has been noted in some cases. More often, presentation is in early childhood with bruising, skin fragility, and mild delay inachieving motor milestones because of increased joint laxity.

Epidemiology Severe types are rare, but mild forms are relatively common.

Inheritance, See Table 2.chromosomal location, and gene

Mutational spectrum Types I and IIand molecular Most cases are caused by mutations in either COL5A1 or COL5A2.pathogenesis In COL5A1 there are usually splice-site mutations or small deletions

or insertions resulting in frame-shifts, all with a loss of function effect as manifested by “nonsense-mediated” decay of the mRNA. In COL5A2both splicing and missense mutations have been identified. Thesemutations are thought to interfere with posttranslational modification of type V collagen and thereby prevent normal processing of collagen in skin and connective tissue.



Table 2. Ehlers–Danlos syndrome: clinical classification. AD: autosomal dominant; AR: autosomal recessive; ATP7A: ATP-ase, Cu(2+)-transporting, α polypeptide; XR: X-linked recessive. *Only a single family reported. Existence of this form has not been confirmed. †Has been reclassified as a disorder of copper transport. ADAMTS2 is a disintegrin-like and metalloproteinase with thrombospondin type 1 motif 2 (ie, procollagen I N-proteinase).

Type Name MIM Inheritance Clinical features Chromosomal Genelocation

I Classical or gravis 130000 AD Moderate to severe 2q31 COL5A2 skin involvement, (collagen, type V, α-2)mild joint laxity 9q34.2–q34.3 COL5A1,

(collagen, type V, α-1) 17q21.32–q22 COL1A1

(collagen, type I, α-I)

II Classical or mitis 130010 AD Mild skin and joint 2q31 COL5A2involvement (collagen, type V, α-2)

9q34.2–q34.3 COL5A1(collagen, type V, α-1)

III Familial 130020 AD Marked joint laxity 2q31 COL3A1hypermobility with recurrent (collagen, type III, α-1)

dislocation

IV Arterial or vascular 130050 AD Bruising; “acrogeric” 2q31 COL3A1extremities; rupture (collagen, type III, α-1)of arteries, bowel, and uterus

V* “X-linked” 305200 XR Mild skin and joint Unknown Unknowninvolvement

VI Ocular-scoliotic 225400 AR Moderate skin and 1p36.3–p36.2 PLOD1joint involvement, (procollagen-lysine,ocular fragility and 2-oxoglutaraterupture, progressive 5-dioxygenase)scoliosis

VIIA Arthrochalasis 130060 AD Congenital hip 17q21.31–q22 COL1A1multiplex dislocation and (collagen, type I, α-1)congenita severe generalized

joint laxity

VIIB Arthrochalasis 130060 AD Congenital hip 7q22.1 COL1A2 multiplex dislocation and (collagen, type I, α-2)congenita severe generalized

joint laxity

VIIC Dermatosparaxis 225410 AR Severe skin and 5q23 ADAMTS2joint involvement

VIII Periodontal 130080 AD Mild skin Unknown Unknowninvolvement withgeneralizedperiodontitis

IX† Occipital horn 304150 XR Soft skin, bladder Xq12–q13 ATP7A (ATPase,syndrome diverticula, occipital Cu[2+]-transporting,

exostoses (“horns”) α polypeptide)



Mutations in COL1A1 have been identified in a few patients withclassical type I features. These are missense point mutations resulting in substitution of a conserved arginine by cysteine in the triple helicaldomain, which in turn leads to the synthesis of a structurally deficientcollagen molecule.

Type IIIA missense point mutation in COL3A1 resulting in a glycine to serinesubstitution has been identified in a single family. In most families thebasic defect is unknown.

Type IVMost if not all cases are caused by mutations in COL3A1 leading toabnormal synthesis and secretion of type III procollagen. These consistmainly of missense point mutations resulting in substitution of glycineresidues in the triple helical domain of the pro-α1(III) chain (ie, adominant-negative effect). Splice-site mutations (resulting in exon-skipping) and small deletions have also been observed. On electronmicroscopy, skin fibroblasts show intracellular storage of the abnormaltype III procollagen molecules in the dilated rough endoplasmic reticulum.

Type VUnknown

Type VIThis is caused by loss of function mutations in PLOD1, which encodeslysyl hydroxylase, the enzyme that converts lysine to hydroxylysine in collagen types I and III. Reduced hydroxylation results in reducedintermolecular cross-linkage with loss of tissue integrity. Approximately20 mutations have been identified, most of which are unique. However,two recurrent mutations, a duplication of exons 10–16 and a nonsenseTyr511Stop mutation in exon 14, account for approximately half of allreported cases.

Type VIITypes VIIA and VIIB are caused by mutations that result in loss of exon 6 of COL1A1 or COL1A2, respectively. These can involve splice-sitemutations or deletions. Exon 6 in COL1A1 and COL1A2 encodes theN-proteinase site which enables cleavage of the secreted procollagen by procollagen I N-proteinase to form the mature collagen molecule.Exon 6 also encodes a lysine residue which, after hydroxylation, isinvolved in cross-linking. The integration of immature procollagen



with mature collagen chains (together with defective cross-linking)results in reduced tensile strength in ligaments and occasionally in bone.

In type VIIC the basic defect lies in reduced activity of the procollagen IN-proteinase which normally cleaves pro-α1(I) and pro-α2(I) chains to form the mature type I collagen molecule. Most of the small numberof reported cases have been caused by homozygosity for a nonsensepoint mutation resulting in a premature termination codon.

Type VIIIUnknown.

Type IXThis can be considered a mild allelic variant of Menkes disease (seep.211–2). In seven of eight reported cases, a splice-site mutation wasidentified in ATP7A (which encodes a Golgi-membrane-bound coppertransport protein). These represent mild loss of function mutationsresulting in relatively normal mRNA processing with some residualfunctional protein activity.

Genetic diagnosis In most cases, the diagnosis and counseling are based on clinicaland counseling features and family history. Specific mutation analysis for most forms of

Ehlers–Danlos syndrome is available at only a few research laboratories.Electron microscopy of collagen fibrils in skin can reveal characteristicfindings such as increased diameter, irregular borders, and “cauliflower”fibers in types I/II and reduced diameter in type IV, in association withintracellular storage of abnormal type III procollagen. The diagnosis of type IV can be confirmed by the demonstration of reduced type IIIprocollagen production using cultured fibroblasts. Type VI can bediagnosed biochemically by an assay of lysyl hydroxylase activity incultured cells or by demonstrating an altered ratio of hydroxylated tounhydroxylated cross-links in urine. Cultured fibroblasts from childrenwith type VII show failure of cleavage of type I procollagen to collagen.In type VIIC, “hieroglyphic” fibers are seen on electron microscopy.



Hereditary Multiple Exostoses(also known as: HME; diaphyseal aclasis; multiple cartilaginous exostoses;multiple osteochondromatosis)

Clinical features These consist primarily of bony lumps, which first appear in earlychildhood and then increase in size and number until adult life whengrowth ceases. The lumps are usually not painful but can causeproblems by pressing on adjacent nerves or tendons. They can also lead to deformity by causing disproportionate growth, particularly in the forearm with bowing of the radius and ulna.

The incidence of malignant change to chondrosarcoma or osteosarcomawas thought to be high. However, several contemporary studies haveconcluded that the true incidence is of the order of 0.5%–2.0%.Malignancy shows a mean age of onset of 31 years; it is rare before10 years and after 50 years.

The exostoses occur most commonly at the ends of the long bonespointing away from the epiphyses (see Figure 5). This location gives the impression that they move down the diaphyses with the passage of time. This can result in a Madelung deformity at the wrist as well as bowing of the radius and ulna. Different sized exostoses also occur on the ribs and on both the pectoral and pelvic girdles.

Age of onset The bony lumps are usually first noted in early childhood.

Epidemiology The overall incidence has been estimated to be around 1 in 50,000. All ethnic groups are affected.

Figure 5. Typical appearance of exostoses in hereditary multiple exostoses.


Hereditary Multiple Exostoses116


MIM, chromosomal See Table 3.location, gene, and mutational spectrum

Table 3. Hereditary multiple exostoses (HME): types, MIM numbers, chromosomal locations, genes, and mutational spectra.

Molecular EXT1 and EXT2 consist of 11 and 15 exons, respectively. They encode pathogenesis transmembrane glycoproteins localized to the endoplasmic reticulum.

Together these proteins form an oligomeric complex which acts as aglycosyltransferase in the polymerization of heparan sulfate. Heparansulfate is an important constituent of glycosaminoglycans (GAGs – formerlyknown as mucopolysaccharides). GAGs are known to function as cofactorsin signal transduction (the process whereby cells receive instructions[“signals”], such as to grow, differentiate, or migrate). The EXT1/EXT2heterocomplex product has much higher glycosyltransferase activity than EXT1 or EXT2 homocomplexes alone.

EXT1 and EXT2 thus act as tumor suppressors in that their combinedprotein product has a negative regulatory role on cell turnover. Studies in osteochondromas (exostoses) from patients with HME have revealedloss of heterozygosity for EXT1. This implies loss of the wild-type alleleso that the cell line giving rise to the clonal osteochondroma contains


Type I 133700 8q24.11–q24.13 EXT1 Missense, nonsense,(exostoses, frame-shift, and splice-multiple, site point mutations, type I) deletions, insertions,

and in-frame deletions. All with a loss of functioneffect. These account for around 70% of all cases of HME.

Type II 133701 11p11.2–p12 EXT2 Missense, nonsense, and(exostoses, splice-site point mutations,multiple, and frame-shift deletionstype II) and insertions. All with a

loss of function effect.

Type III 600209 19p EXT3 Unknown(exostoses,multiple, type III)(unknown)



only the inherited mutant allele. Further malignant transformation to an osteochondrosarcoma almost certainly involves additionalsomatically acquired mutational events consistent with the multistepconcept of carcinogenesis.

As well as being responsible for HME types I and II, EXT1 and EXT2are also involved in two contiguous gene deletion syndromes involving8q24.11–q24.33 and 11p11.2–p12, respectively. Deletion of8q24.11–q24.33 results in Langer–Giedion syndrome, in whichmultiple exostoses are associated with a characteristic facies, sparsescalp hair, short angulated digits, short stature, and variable mentalretardation. Deletion of 11p11.2–p12 also leads to the development of multiple exostoses and mental retardation: additional findings include subtle facial anomalies and biparietal foramina.

Genetic diagnosis Specific mutation analysis is not routinely available. Chromosome and counseling analysis looking for a deletion is indicated when multiple exostoses are

associated with other findings such as mental retardation. Counseling ison the basis of autosomal dominant inheritance with variable expression.

Marfan Syndrome

MIM 154700

Clinical features Primarily involving the musculoskeletal, cardiovascular, and ocularsystems. Musculoskeletal involvement is typified by the Marfanoidhabitus (tall stature with long limbs, together with pectus deformity,scoliosis, pes planus, and hyperextensible joints). Cardiovascularfeatures include mitral valve prolapse, aortic enlargement, and aorticdissection. The main ocular features consist of myopia and ectopia lentis(see Figure 6). Other findings and complications can include spontaneouspneumothorax with apical blebs on chest X-ray, lumbosacral duralectasia (as revealed by computed tomography and magnetic resonanceimaging scans), herniae, and skin striae.

Contemporary photographs indicate that Abraham Lincoln had aMarfanoid habitus, prompting suggestions that he may have had Marfan syndrome. To date, efforts to analyze his DNA extracted fromarchival tissues have been resisted, chiefly because of the complexity of mutation detection in FBN1.


Marfan Syndrome118

Figure 6. Lens subluxation in Marfan syndrome.

Age of onset In the severe neonatal form, aortic dilatation and valvular insufficiencyare present at birth. More commonly, the diagnosis becomes apparent in mid childhood.

Epidemiology Marfan syndrome occurs in all ethnic groups, with an estimatedprevalence of at least 1 in 14,000.

Inheritance Autosomal dominant with very variable expression and a few reports of nonpenetrance (ie, complete failure of expression).Approximately 25% of cases result from new mutations.


Gene FBN1 (fibrillin 1)

Mutational Mainly missense mutations with a dominant-negative effect. spectrum Also small frame-shift deletions and insertions.

Molecular Fibrillin 1 is an important constituent of microfibrils in the extracellularpathogenesis matrix. Microfibrils act as a template for the deposition of elastin and

facilitate the linkage of elastin fibers both to each other and to adjacentcomponents of the extracellular matrix. They also play an importantanchoring role in skin and in the ocular zonule where they hold the lens in place.

FBN1 is a large gene with 65 exons and encodes a protein with fivedistinct structural and functional domains (A–E). Domains B and Dcontain motifs that show homology to epidermal growth factor (EGF)and contain a consensus sequence for calcium-binding. Over 100mutations have been identified in FBN1 in Marfan syndrome patients,



most of which are unique (or “private”) to each individual family or sporadic case. In around 20%–25% of cases that fulfill the strictdiagnostic criteria no mutation can be identified. These cases areprobably caused by mutations in regulatory regions of FBN1 as there is no firm evidence for a second disease locus at present.

Approximately 70% of all mutations are missense and most of theseinvolve the calcium-binding EGF-like motifs. Some genotype–phenotypecorrelations have emerged. The severe neonatal form of Marfan syndromeis associated with missense mutations in exons 24–27 and exon-skippingmutations in exons 31 and 32. A few specific mutations have beenfound in families segregating either isolated aortic aneurysms or ectopia lentis (dislocation or subluxation of the lens). Most mutationsare believed to exert a dominant-negative effect by encoding abnormalfibrillin 1 monomers that interact with wild-type monomers to preventthe formation of normal microfibrillar aggregates.

Mutations in exons 24–34 in FBN2 on chromosome 5q23–31 causethe rare condition, congenital contractural arachnodactyly, also knownas Beals’ syndrome (MIM 121050). This condition shows phenotypicoverlap with the musculoskeletal features of Marfan syndrome butwithout the ocular or cardiovascular complications.

Genetic diagnosis Specific mutation analysis is difficult because of the large size of and counseling FBN1. Linkage analysis can be offered in informative families for

diagnostic purposes. Fibrillin immunofluorescence in skin samples orfibroblast cultures is not sufficiently reliable to be used for diagnosticpurposes. Counseling is on the basis of autosomal dominant inheritancewith marked interfamilial and intrafamilial variation in severity.

Osteogenesis Imperfecta(also known as: OI. Includes: osteogenesis imperfecta congenita [OIC]; osteogenesis imperfecta tarda [OIT])

MIM 166200 (type I)166210 (type II)166220 (type IV)259420 (type III)


Osteogenesis Imperfecta120

Figure 7. Appearance of the teeth in a child with osteogenesis imperfecta.

Clinical features Increased bone fragility occurs in all forms of OI. Variable findingsinclude blue sclerae, deafness, joint laxity, and dentinogenesisimperfecta (see Figure 7). Clinical involvement can vary from mild (with a history of only a few fractures and no deformity) to profoundlysevere with death in utero. Four clinical and radiologic groupings arerecognized (see Table 4). Not all patients fall precisely within thisclassification, but the clinical, radiologic, and molecular findings within these four categories are generally consistent.

Age of onset This is very variable and can range from before birth (with limbshortening as a result of multiple fractures) to adult life.

Epidemiology The combined incidence of all forms of OI is approximately1 in 5,000. Type I is by far the most common.

Inheritance Most cases show autosomal dominant inheritance. Autosomal recessiveinheritance has been confirmed biochemically and by molecular analysisin only a few families.

Chromosomal 17q21.31–q22 (COL1A1 [collagen type I α1 chain])location and gene 7q22.1 (COL1A2 [collagen type I α2 chain])

Mutational spectrum Point mutations and deletions with a loss of function effect (null alleles).Missense mutations, exon-skipping mutations, insertions and deletionswith a dominant-negative effect.

Molecular Almost all cases of OI are caused by heterozygous mutations in one pathogenesis of the type I collagen genes. Type I collagen is a trimer made up of two

pro-α1(I) chains and one pro-α2(I) chain encoded by COL1A1 andCOL1A2, respectively. Mutations that result in failure of synthesis of a chain or in its inability to be incorporated into a type I collagen trimer



Table 4. Osteogenesis imperfecta: clinical and radiologic features.

result in mild OI in which type I collagen is reduced in quantity but isrelatively normal qualitatively. Such mutations generally result in nullalleles with a loss of function effect.

In contrast, mutations that lead to the production of a mutant chain that is processed into the mature collagen molecule cause severe OI byacting in a dominant-negative manner. These mutations, which includeinsertions, deletions, and exon-skipping mutations, lead to malalignmentof the pro-α1 and pro-α2 chains. Similarly, missense point mutationsthat cause substitution of one of the glycine residues in the repetitiveGly–X–Y structure of a collagen chain lead to abnormal configuration of any triple helix into which the mutant collagen chain is incorporated.As each mature triple helical collagen molecule contains two pro-α1chains, a mutation with a dominant-negative effect will result in 75% of all mature type I collagen molecules being abnormal (see Figure 8). In general, glycine substitutions in carboxy-terminal sites produce lethaloutcomes (as in type II OIC, see Figure 9), whereas glycine substitutionsnearer the amino-terminal result in milder phenotypes.

Mutations in the large family of collagen genes account for a diversegroup of inherited disorders, many of which are extremely rare. These are summarized in Table 5.

Type Clinical Radiologic

I Osseous fragility (mild to moderate) Osteoporosis (mild)Blue sclerae Platyspondyly (mild)Mixed hearing loss in adult life (50%) Wormian bones Mitral valve prolapse (15%)

II Osseous fragility (very severe) Gross underossificationSoft calvarium Crumpled long bonesShort trunk and limbs Beaded ribsSmall chest and protuberant abdomen Platyspondyly (severe)Early lethality

III Osseous fragility (severe) Osteoporosis (severe)Progressive deformity Thin bowed long bonesShort stature Biconcave vertebraeNormal sclerae Wormian bones

IV Osseous fragility (variable) OsteoporosisNormal scleraeDentinogenesis imperfectaOccasional severe deformity


Osteogenesis Imperfecta122

Table 5. Collagen gene disorders. Collagen types are indicated by Roman numerals. The constituent chains are designated using Arabic numerals followed in brackets by the collagen type. Collagen genes are identified by the collagen type, written in Arabic numerals, followed by a capital A, followed by the number of the pro-α chain they encode; for example, COL2A1 encodes the α1(II) chain of type II collagen.EDS: Ehlers–Danlos syndrome; MED: multiple epiphyseal dysplasia; SEDC:spondyloepiphyseal dysplasia congenita; SEMD: spondyloepimetaphyseal dysplasia.

MIM Gene Locus Gene Disorderproduct

120150 COL1A1 17q21.31–q22 α1(I) Osteogenesis imperfectaEDSVIIIOsteoporosis

120160 COL1A2 7q22.1 α2(I) Osteogenesis imperfectaEDSVIIOsteoporosis

120140 COL2A1 12q13.11–q13.2 α1(II) Achondrogenesis IIHypochondrogenesisSEDCSEMDKniest dysplasiaStickler syndrome IOsteoarthritis

120180 COL3A1 2q31 α1(III) EDSIV

120070 COL4A3 2q36–q37 α3(IV) Alport syndrome

120131 COL4A4 2q36–q37 α4(IV) Alport syndrome

303630 COL4A5 Xq22.3 α5(IV) Alport syndrome

303631 COL4A6 Xq22.3 α6(IV) Leiomyomatosis

120215 COL5A1 9q34.2–q34.3 α1(V) EDSI and EDSII

120190 COL5A2 2q31 α2(V) EDSI and EDSII

120220 COL6A1 21q22.3 α1(VI) Bethlem myopathy

120240 COL6A2 21q22.3 α2(VI) Bethlem myopathy

120250 COL6A3 2q37 α3(VI) Bethlem myopathy

120120 COL7A1 3p21.3 α1(VII) Epidermolysis bullosa(dystrophic types)

120210 COL9A1 6q13 α1(IX) MED1

120260 COL9A2 1p33–p32.2 α2(IX) MED2

120270 COL9A3 20q13.3 α3(IX) MED3

120110 COL10A1 6q21–q22.3 α1(X) Metaphysealchondrodysplasia (Schmid)

120280 COL11A1 1p21 α1(XI) Stickler syndrome II

120290 COL11A2 6p21.3 α2(XI) Stickler syndrome IIIWeissenbacher–Zweymullersyndrome

113811 COL17A1 10q24.3 α1(XVII) Junctional epidermolysis bullosa

120328 COL18A1 21q22.3 α1(XVIII) Knobloch syndrome(myopia and encephalocele)



Figure 8. Different mutational mechanisms in osteogenesis imperfecta (OI).

Figure 9. X-ray findings in a baby with type II osteogenesis imperfecta.

Normal synthesis

Chains Collagen trimers Outcome

pro-α1 (A)

pro-α1 (B)

pro-α2 (C)} }

AAC

ABC

BAC

BBC

Normal type Icollagen fiber

Mutation with a loss of function effect


*pro-α1 (A*)

pro-α1 (B)

pro-α2 (C)} }BBC

BBC

Reduced amount oftype I collagencausing mild OI

}Mutation with a dominant-negative effect


A*A*C

A*BC

BA*C

BBC

*pro-α1 (A*)

pro-α1 (B)

proα2 (C)}

Degraded or abnormallybranched type Icollagen fiberscausing severe OI


Pseudoachondroplasia124

Genetic diagnosis In theory, the diagnosis of type I OI could be established by demonstratingand counseling reduced synthesis of procollagen I by dermal fibroblasts, but in practice

this is rarely available. Similarly, collagen gene mutation analysis is notreadily available, since it would be a very expensive exercise becausecollagen type I genes are very large and each family harbors a uniquemutation. Thus in almost all instances the diagnosis is based on clinicaland radiologic features.

Counseling is as for autosomal dominant inheritance. For type II OI the observed recurrence risk in siblings is around 5%–6%, reflecting the fact that many apparently new mutations stem from parental germ-line mosaicism.

Pseudoachondroplasia(also known as: pseudoachondroplastic spondyloepiphyseal dysplasia)

MIM 177170

Clinical features Both trunk and limb length are reduced, sometimes severely. Olderchildren develop lordosis/scoliosis with a waddling gait. The lower limbscan show genu valgum or genu varum. Ligamentous laxity is particularlysevere in the hands with short soft hypermobile fingers. Adult height isbetween 80 and 130 cm. Early osteoarthritis is a frequent complicationand hip replacement is required in approximately 33% of affectedindividuals by the age of 33 years. The tubular bones show shorteningwith irregular expanded metaphyses and small fragmented epiphyses.The vertebral bodies are flattened with anterior tonguing.

Age of onset Usually presents in the second year of life with short stature.

Epidemiology Pseudoachondroplasia is rare. No precise incidence figures are available.

Inheritance Autosomal dominant. A very rare autosomal recessive form may exist.


Gene COMP (cartilage oligomeric matrix protein)

Mutational Point mutations and deletions with a dominant-negative effect. spectrum Also expansion of a short GAC repeat.



Molecular COMP is an extracellular calcium binding protein involved in chondrocytepathogenesis migration and proliferation. It is expressed at high levels in chondrocytes

in developing bone and tendon. Reported mutations occur in one of thecalcium binding domains. These have an adverse effect, as calcium bindingto COMP is a cooperative process involving all of the seven calciumbinding regions. COMP is a pentamer, so incorporation of a single mutantchain can disrupt protein function (hence the dominant-negative effect).In pseudoachondroplasia, chondrocytes show abnormal inclusions inthe rough endoplasmic reticulum. These probably represent proteoglycanaccumulation resulting from defective calcium-dependent proteoglycanbinding. Mutations in COMP result in a phenotypic spectrum varyingfrom pseudoachondroplasia (at the severe end) to various forms ofmultiple epiphyseal dysplasia (at the mild end).

Genetic diagnosis Limited mutation detection is available in a few specialist laboratories.

and counseling Counseling is as for autosomal dominant inheritance. One case of

possible autosomal recessive inheritance has been shown to be due

to dominant transmission of a parental mosaic germline mutation.

Stickler Syndrome(also known as: hereditary arthro-ophthalmopathy. Includes Marshall syndrome and Weissenbacher–Zweymuller syndrome)

MIM See Table 6.

Clinical features The different forms of Stickler syndrome share a common phenotype whichdiffers mainly in the degree of ocular involvement. Patients typically havea flat facial profile with depressed nasal bridge, anteverted nares, andmicrognathia. Approximately 25% have a cleft palate which togetherwith severe micrognathia can contribute to a diagnosis of Pierre Robinanomaly. In childhood there may be generalized joint hypermobilitywhereas a degenerative arthropathy often develops in middle age. Mitral valve prolapse has been reported in some studies but not in others. Early onset hearing loss of mixed conductive and sensorineuralorigin occurs in around 40% of cases and is reported as being morecommon in Stickler syndrome type II than in type I.

Most patients with Stickler syndrome show congenital onsetnonprogressive severe high myopia with a high risk of subsequent retinal


Stickler Syndrome126

Table 6. Stickler syndrome: types, MIM numbers, chromosomal locations, genes, and mutational spectra.

detachment. Cataracts can also be present and may be congenital. Intype I (also known as the membranous vitreous type) vestigial vitreousgel occupies the immediate retrolental space surrounded by a distinctfolded membrane. In type II (also known as the beaded vitreous type),there are irregularly thickened bundles of fibers sparsely distributedthroughout the vitreous cavity. There is no ocular involvement in type III.

Age of onset The facial features and myopia are present from birth.

Epidemiology Stickler syndrome is relatively common, although many cases are undiagnosed.


Chromosome See Table 6.location, gene, andmutational spectrum

Molecular Type Ipathogenesis Patients with this type of Stickler syndrome fall at the mild end of the

phenotypic spectrum caused by mutations in COL2A1 (see Table 5,p.122). Type II collagen is present in hyaline cartilage, in the ocularvitreous, and in the nucleus pulposus of the intervertebral discs. Thenature of the identified mutations, which almost invariably result in the generation of a stop codon and a severely truncated type II collagenchain, indicates that this phenotype results from a quantitative defect in type II collagen synthesis in contrast to the other skeletal disordersassociated with mutations in COL2A1 (see Table 5, p.122).


Type I 108300 12q13.11–q13.2 COL2A1 (collagen, Nonsense point mutationstype II, α1) introducing a premature stop codon

with a probable loss of function(“null allele”) effect

Type II 604841 1p21 COL11A1 (collagen, Point mutations affecting splicingtype XI, α1) consensus sequences of 54-bp exons

or deletions causing loss of 54-bp exons in COL11A1

Type III 184840 6p21.3 COL11A2 (collagen, Missense and splice site pointtype XI, α2) mutations. Also a 27-bp deletion.

All with a probable dominant-negative effect



Type IIType XI collagen is a heterotrimer made up of three distinct chains:α1(XI), α2(XI), and α3(XI). The α1(XI) and α2(XI) chains are encodedby COL11A1 and COL11A2, respectively, whereas the α3(XI) chain is a posttranslationally modified variant of the COL2A1 gene product. In vivo, type XI collagen associates with type II collagen. Thus it ispredictable that mutations involving either type II or type XI collagenresult in a similar phenotype.

Almost all of the mutations identified in COL11A1 result in deletion of 54-bp exons in the major triple-helical domain. This is consistent with a dominant-negative effect resulting from integration of shortenedchains into the type XI collagen helical heterotrimer.

Similar mutations have been identified in patients with Marshallsyndrome, a condition known to show clinical overlap with Sticklersyndrome but with more pronounced facial features, shorter stature, and a lower incidence of retinal detachment. Studies at the molecularlevel have confirmed the clinical suspicion that Marshall syndrome and Stickler syndrome represent slightly different manifestations of a single syndrome.

Type IIIThis non-ocular form of Stickler syndrome is caused by loss of functionmutations in COL11A2, which encodes the α2(XI) chain of type XIcollagen. Unlike COL11A1, COL11A2 is not expressed in the vitreous,which accounts for the absence of ocular involvement. Mutations inCOL11A1 have also been identified in patients with Weissenbacher–Zweymuller syndrome which is characterized at birth by the Pierre Robin anomaly, nasal hypoplasia, short “dumb-bell” shaped humeri and femora, and coronal vertebral clefting. Affected children go on to develop deafness and large epiphyses (giving rise to the termotospondylomegaepiphyseal dysplasia). Thus, as with type II Sticklersyndrome, molecular studies have provided support for nosologic“lumping” rather than splitting of these overlapping clinical entities.

Genetic diagnosis Specific mutation analysis is not routinely available. The diagnosisand counseling is based on clinical, ocular, and radiologic findings and can be very

difficult in mild cases. Counseling is on the basis of autosomal dominant inheritance with very variable expression.


128


7. Cardio-respiratory Disorders

Barth Syndrome 130

Cystic Fibrosis 131

DiGeorge/Shprintzen Syndrome 133

Holt–Oram Syndrome 135

Laterality Defects 137

Noonan Syndrome 138

Primary Ciliary Dyskinesia 139

Williams Syndrome 141


Barth Syndrome130

Barth Syndrome(also known as: 3αα-methylglutaconic aciduria II)

MIM 302060

Clinical features Dilated cardiomyopathy, skeletal myopathy, neutropenia, short stature,and abnormal mitochondria with tightly packed cristae and inclusionbodies. Death often occurs in infancy or early childhood as a result ofheart failure and/or sepsis associated with agranulocytosis. The heartmay show features of endocardial fibroelastosis and there may beincreased levels of 3α-methylglutaconic acid in urine.

Age of onset Either congenital or in early infancy

Epidemiology The condition appears to be rare, but may be under diagnosed.



Gene G4.5 or TAZ (tafazzin)

Mutational Heterogeneous with missense, nonsense, and splice-site mutations spectrum plus deletion/insertion frame-shifts.

Molecular G4.5 contains 11 exons and produces several different mRNA moleculespathogenesis due to alternative splicing of exons 5–7. Variable hydrophobic and

hydrophilic regions exist in the protein at the N-terminal and centralregions, respectively. The precise role of the protein, named tafazzin(after an Italian television personality), is unknown although a possiblehomology to acyltransferases has been suggested. No cleargenotype–phenotype correlation has emerged.

Genetic diagnosis Mutation analysis is available on a limited basis. This should be and counseling considered in families showing X-linked recessive inheritance of isolated

dilated cardiomyopathy as this can also be due to mutations in G4.5.Counseling is as for X-linked recessive inheritance.


Cardio-respiratory Disorders 131

Cystic Fibrosis (also known as: CF; mucoviscidosis)

MIM 219700

Clinical features Classic CF is characterized by recurrent pulmonary and/or gastrointestinaldisease commencing in infancy or early childhood. Pulmonary involvementmanifests as recurrent infection and inflammation leading to chronicbronchitis, bronchiectasis, fibrosis, and respiratory failure, and culminatingeventually in cor pulmonale and death. Gastrointestinal problems caninclude meconium ileus (10%–15%), malabsorption due to pancreaticinsufficiency (90%), recurrent distal intestinal obstruction (20%), andrectal prolapse (20%). Other complications include insulin-dependentdiabetes mellitus (5%), cirrhosis (2%–5%), gallstones (10%), and maleinfertility (96%). Life expectancy has improved dramatically from 5 yearsin 1955 to around 30 years at present. In nonclassic CF, the features aremuch less severe with the mildest presentation being that of congenitalbilateral absence of the vas deferens (CBAVD, MIM 277180) in otherwisehealthy adult males.

Age of onset Gastrointestinal involvement may present in the second trimester asincreased bowel echogenicity on ultrasound or as meconium ileus soonafter birth. Sixty percent of all cases are diagnosed by the age of 1 year.

Epidemiology The incidence of CF shows marked variation amongst different racesranging from 1 in 2,500–3,000 in individuals of European origin to 1 in 15,000 in African Americans.



Gene CFTR (cystic fibrosis transmembrane conductance regulator)

Mutational Over 1,000 mutations have been reported to the CF mutational spectrum database (www.genet.sickkids.on.ca). These include missense (40%),

nonsense (20%), and splice-site (10%) point mutations, as well asframe-shift deletions and insertions, promoter deletions, and intronicpoint mutations (which activate cryptic splice donor sites). One specific


Cystic Fibrosis 132

mutation, ∆F508 (deletion of phenylalanine residue at position 508),accounts for 60%–80% of all CF mutant alleles in Europe.

Molecular CFTR encodes a 1,480-amino-acid protein known as the CF pathogenesis transmembrane conductance regulator (CFTR). This contains two

transmembrane domains (which anchor it to the cell membrane), two nucleotide-binding folds (NBF) (which interact with ATP), and a single regulatory (R) domain (which is phosphorylated by protein kinase A). CFTR functions as a chloride channel which is activated by phosphorylation of the R domain and ATP interaction with the NBF domains. Activation also results in opening of adjacent outwardlyrectifying chloride channels (ORCC) and closure of epithelial sodiumchannels. Defective CFTR activity in epithelial cells lining the airways of the lungs results in low volumes of airway-surface liquid with increased viscosity of pulmonary secretions.

Mutations in CFTR can be classified either on the basis of their effect on CFTR function or on their phenotypic outcome. Five classes offunctional mutation are recognized. These are:

1) reduced synthesis (eg, nonsense, frame-shift, and splice-sitemutations resulting in reduced mRNA)

2) defective maturation (eg, missense mutations and ∆F508).These mutations prevent normal processing of CFTR to the cell membrane

3) abnormal activation (eg, missense mutations, such as Gly551Asp,involving an ATP-binding domain)

4) altered conductance (eg, missense mutations, such as Arg117His,involving the CFTR chloride channel)

5) defective regulation (eg, missense mutations which impair regulationof the ORCC and epithelial sodium channel)

Phenotypically, CFTR mutations are classified on the basis of whetherthey cause classic CF with or without pancreatic insufficiency, or muchmilder nonclassic forms of CF such as CBAVD. Homozygosity for ∆F508 results in classic CF with chronic lung disease and pancreaticinsufficiency. Most nonsense mutations also result in classic CF. Somemissense mutations, such as Arg117His, result in a milder phenotypewith pancreatic sufficiency. Isolated CBAVD is associated with “mild”mutations such as Arg117His (which allows partial CFTR function).



The complexity of the genotype–phenotype relationship is illustrated by the observation that when Arg117His is in cis (ie, in the same allele)with a 5T polythymidine polymorphism in intron 8 (which influences the splicing efficiency of exon 9) it causes severe CF when another CFmutation is present on the other allele. However, when Arg117His is incis with a 7T polymorphism it causes only CBAVD in combination withanother CF allele. The difficulty of predicting phenotype from genotype is increased by the demonstration of at least one CF modifier locus(CFM1, MIM 603855) on chromosome 19.

Genetic diagnosis CFTR analysis for common mutations is widely available and usuallyand counseling involves testing for ∆F508 and up to 30 other known mutations,

which account for 85%–90% of all CF alleles in a particular population.Rare mutations can be investigated at specific reference laboratories.Counseling is as for autosomal recessive inheritance with carrierfrequencies in Caucasian populations of 1 in 20–30 depending on thedisease incidence. Carrier detection by “cascade screening” is offered in families with an affected child. Prenatal diagnosis can be offeredeither directly by mutation analysis or indirectly by linkage analysisusing intragenic polymorphisms in informative families.

DiGeorge/Shprintzen Syndrome(includes: CATCH 22; conotruncal anomaly face syndrome; DiGeorge syndrome; Shprintzen syndrome; velocardiofacial syndrome [VCFS])

MIM 188400

Clinical features These are extremely variable and embrace a wide spectrum of clinicalinvolvement, ranging from the classical severe DiGeorge syndrome to themuch milder Shprintzen/VCFS phenotype. The DiGeorge syndrome ischaracterized by thymic aplasia or hypoplasia, hypoparathyroidism, andcardiac malformations (notably interrupted aortic arch with a ventricularseptal defect and persistent truncus arteriosus). Central nervous systemabnormalities occur in one third of cases and many affected children die in early infancy as a consequence of the severe cardiac defect. In the much milder Shprintzen syndrome/VCFS, affected children usually have a characteristic facies (long face with short palpebral fissures and a broadbulbous nasal tip) with short stature, cleft palate (possibly submucous),


DiGeorge/Shprintzen Syndrome134

and a cardiac anomaly (most commonly Fallot’s tetralogy or a ventricularseptal defect). Average IQ is approximately 80 and many children haveparticular problems with language and speech. Some also manifestbehavioral problems and affected adults show an increased incidence of schizophrenia and bipolar depression.

Age of onset The abnormalities are present at birth. Presentation can be in infancywith hypocalcemia, failure to thrive, and cardiac failure or with learningdifficulties and velopharyngeal incompetence (in later childhood).

Epidemiology All populations are affected with an estimated incidence of 1 in 4,000.

Inheritance Autosomal dominant/chromosomal microdeletion


Gene TBX1 (T-BOX1) (although this is proven in mice, it has not been provenin humans; this is discussed below)

Mutational Unknownspectrum

Molecular Approximately 90% of all cases of the DiGeorge/Shprintzen syndromepathogenesis have a microdeletion involving the proximal region of the long arm of

one chromosome 22. This is readily identifiable using fluorescence in situ hybridization (FISH) (see Figure 1). In 90% of these cases the deletion is 3 Mb in size and encompasses an estimated 30 genes. In the remaining 10%, the deletion is 1.5 Mb in size and contains 24 genes. The 22q11 region contains eight low-copy repeat sequenceswhich flank the deletions and probably account for their generationthrough unequal crossing over.

Despite extensive research, no specific gene has been found to accountfor the associated clinical abnormalities. In mice, however, it has beenestablished that haploinsufficiency for Tbx1 (the human homolog of which is located in the 22q11 deletion region) contributes to or causes the cardiovascular abnormalities. Understanding of thegenotype–phenotype relationship is complicated by reports of theDiGeorge/Shprintzen phenotype in individuals with non-overlappingdeletions, implying either that no specific gene is causal or that the



Figure 1. Chromosome 22 microdeletion in a child with DiGeorge/Shprintzensyndrome demonstrated by fluorescence in situ hybridization. Image courtesy of Mrs Karen Marshall, Cytogenetics Laboratory, Leicester Royal Infirmary, UK.

various deletions have long-range negative effects on the expression of neighboring genes. TBX1 is a particularly attractive candidate for the causal gene as it is a putative transcription regulatory gene and defects in the closely related TBX5 account for the Holt–Oram syndrome in which cardiac defects are common (see next entry).

Genetic diagnosis Detection of a microdeletion on chromosome 22q11 is widely and counseling available by FISH. More specific molecular analysis is only undertaken

on a research basis. De novo mutations account for 80%–90% of casesand convey a low (<1%) recurrence risk for siblings. The parents of all cases should be offered microdeletion analysis since 10%–20% ofcases will have an “asymptomatic” deletion carrier parent. Intrafamilialvariation can be marked and can extend to affected monozygotic twinswho have been reported to show discordant phenotypes.

Holt–Oram Syndrome(also known as: HOS; heart–hand syndrome)

MIM 142900

Clinical features These involve primarily the upper limbs and the heart. Findings in the upper limbs are variable, usually asymmetrical, and mostcommonly present as absence or hypoplasia of the thumbs and radii.Other findings can vary from minimal involvement (such as clinodactyly or limitation of supination) to hypoplasia of the ulnae and humeri, or even phocomelia. Cardiac defects occur in over 90% of cases with atrial septal defect (35%) and ventricular septal defect (25%) being the most common abnormalities.


Holt–Oram Syndrome136

Age of onset The limb and heart abnormalities are present at birth and in severecases can be detected on ultrasound during the second trimester.

Epidemiology HOS is rare, with an estimated incidence of approximately 1 in 100,000.



Gene TBX5 (T-BOX 5)

Mutational Missense, nonsense, and frame-shift deletion mutations all result spectrum in haploinsufficiency.

Molecular TBX5 is one of a series of developmental regulatory genes which sharepathogenesis a common conserved motif known as the T-BOX. T-BOX genes act as

transcription factors through binding of the T-BOX domain to DNA.TBX5 is expressed not only in embryonic forelimbs and heart, but also in the lungs, pharynx, and retina, which are not involved in the HOS phenotype. Expression studies have shown that TBX5 associateswith other regulatory genes to promote cardiomyocyte differentiation.Truncating mutations tend to result in severe heart and limbmalformations, whereas missense mutations result in severe heart or limb abnormalities with only mild involvement of the other system.Correlation of the phenotype associated with specific mutationsindicates that organ-specific gene activation by TBX5 is determined by binding to different target DNA sequences.

Genetic diagnosis TBX5 mutation analysis is undertaken at a few specializedand counseling laboratories. Mutations are identified in only ~50% of familial

cases. Counseling is as for autosomal dominant inheritance with close to complete penetrance, but marked intrafamilial variabilityin expression.



Laterality Defects(also known as: asplenia with cardiovascular anomalies; heterotaxy; isomerism;Ivemark syndrome; polysplenia syndrome; situs ambiguus)

MIM 208530, 306955, 601086

Clinical features Situs solitus refers to normal orientation of the heart and abdominalorgans. Complete reversal of normal lateralization is referred to as situsinversus, as seen in Kartagener syndrome (see p.139–41). Any otherdisturbance of lateralization is known as situs ambiguus or heterotaxy.Isomerism is a form of situs ambiguus in which organs such as the lungs and heart, which normally have distinguishable right and leftforms, develop so that their left and right sides are mirror images of one other. Thus both lungs may be trilobed in right isomerism or bilobed in left isomerism. Asplenia and polysplenia occur in right and left isomerism, respectively.

Severe cardiac abnormalities are common in both forms of isomerism,notably atrioventricular septal defect, single ventricle, double outlet rightventricle, and pulmonary stenosis in right isomerism; and atrioventricularseptal defect, interruption of the inferior vena cava, and pulmonarystenosis/atresia in left isomerism. Intestinal malrotation and urogenitaldefects can also occur.

Age of onset The malformations are present at birth and can often be detected in pregnancy by ultrasonography.

Epidemiology The incidence has been estimated at around 1 in 24,000.

Inheritance Most cases are sporadic. Families showing autosomal dominant,autosomal recessive, and X-linked inheritance have been reported.

Chromosomal 6p21 (autosomal-dominant form), unknown (autosomal-recessivelocation form), and Xq26.2 (X-linked form), respectively.

Gene Unknown (autosomal-dominant and autosomal-recessive forms); ZIC3 (zinc finger protein of cerebellum, 3) (X-linked form).

Mutational ZIC3: missense, nonsense, and frame-shift mutations with a loss spectrum of function effect.


Noonan Syndrome138

Molecular ZIC3 encodes a zinc finger transcription regulator which is thoughtpathogenesis to be expressed at an early stage in the determination of left–right

asymmetry in the human embryo. Mutations in ZIC3 have beenidentified in a small number of families in which hemizygous males showed situs ambiguus with variable cardiac, splenic, andgastrointestinal abnormalities. Heterozygous females in one of thesefamilies showed situs inversus. Females in the other families werenormal. Affected males in these families also showed an increasedincidence of midline defects such as cerebellar hypoplasia and sacralagenesis. The explanation for these observations is not known.

Genetic diagnosis Mutation analysis is not available. Most cases are sporadic withand counseling an empiric recurrence risk for siblings of around 5%.

Noonan Syndrome

MIM 163950

Clinical features Typically, these consist of mild short stature, neck webbing, congenitalheart disease, characteristic facies, and undescended testes in boys.The most common cardiac anomalies are pulmonary stenosis, atrial and ventricular septal defects, patent ductus arteriosus, and hypertrophiccardiomyopathy. Facial features include hypertelorism, downwardsloping palpebral fissures, ptosis, and low-set posteriorly rotated ears.Approximately one third of all cases have mild learning difficulties and around 50% have a bleeding diathesis with prolonged activatedpartial thromboplastin time. Other features can include hydrops and polyhydramnios in pregnancy, and major feeding difficulties in early childhood.

Age of onset The clinical features are often apparent at birth although the diagnosis is usually not made until mid-childhood.

Epidemiology The incidence has been estimated to be between 1 in 1,000–2,500.



Gene PTPN11 (protein-tyrosine phosphatase, nonreceptor-type, 11)



Mutational Missense mutations with a gain of function effect. Mutations in exons 3spectrum and 7 account for approximately 80% of all identified mutations.

Molecular PTPN11 encodes a nonreceptor type protein tyrosine phosphatase pathogenesis known as SHP-2, which is ubiquitously expressed and is involved in

mesodermal patterning, limb development, and hematopoietic celldifferentiation. SHP-2 plays a key role in cell signaling, therebymediating the cellular response to growth factors, hormones, andcytokines. Activation of SHP-2 results from interaction of an amino-terminal switch domain with a tyrosine phosphatase domain resulting in modulation of phosphatase activity. Almost all identified mutationsinvolve either the switch or the tyrosine phosphatase domain and resultin stabilization of phosphatase activity by a gain of function effect. One particular mutation, Asn308Asp, accounts for approximately onethird of all reported cases. It is not clear exactly how increased tyrosinephosphatase activity results in the clinical phenotype, although SHP-2 is known to be expressed in the developing pulmonary and aortic valves.This probably accounts for the observation that pulmonary stenosis ismore common in cases caused by PTPN11 mutations than in casesfrom families which are not linked to this locus.

Genetic diagnosis Mutation analysis for PTPN11 is only available on a limited basis.and counseling Only 50% of families are linked to this locus. Genetic counseling is

as for autosomal dominant inheritance with variable expression andalmost complete penetrance. Note that features of Noonan syndromehave been observed in a few individuals with neurofibromatosis type I.In some of these individuals, rearrangements have been identified inNF1 (see p.96–8).

Primary Ciliary Dyskinesia(also known as: PCD; immotile cilia syndrome. Includes Kartagener syndrome.)

MIM 242650 (primary ciliary dyskinesia)244400 (Kartagener syndrome)

Clinical features PCD is a clinically and genetically heterogeneous group of disorderscharacterized by ciliary dysfunction leading to chronic bronchiectasis,otitis, and sinusitis. Most affected males are infertile due to spermimmotility. Approximately 50% of patients show situs inversus


Primary Ciliary Dyskinesia140

Table 1. Primary ciliary dyskinesia: chromosomal locations, genes, and mutational spectra.

(ie, complete reversal of normal left–right asymmetry, with dextrocardia).The combination of PCD with situs inversus constitutes the conditionknown as Kartagener syndrome.

Age of onset Presentation is usually in early childhood with recurrent respiratory infection.

Epidemiology The estimated incidence worldwide is approximately 1 in 16,000. An increased incidence has been noted in Polynesians in New Zealandand Samoa.


Chromosomal See Table 1.location, gene, andmutational spectrum

Molecular The basic defect in PCD lies in abnormal cilia motility with loss of the pathogenesis dynein arms being the most common finding on electron microscopy.

Dyneins consist of a large family of proteins involved in microtubule-dependent cell motility. Inner and outer dynein arms are bound to eachmicrotubule doublet of the ciliary axoneme where they generate motilitythrough ATP-dependent cycles of attachment and detachment. Dyneinarms are composed of a mixture of heavy, intermediate, and light chainseach encoded by a different gene. Thus PCD can be expected to showmarked locus heterogeneity.

To date, mutations have been identified in two dynein assembly genes,DNAH5 and DNAI1 (encoding heavy chain 5 and intermediate chain 1,respectively). These mutations are predicted to result in truncatednonfunctional proteins which prevent normal dynein arm formationresulting in immotile cilia and subsequent chronic respiratory disease.

Chromosomal location Gene Mutational spectrum

5p15–p14 DNAH5 (dynein, Missense, nonsense, splice-site,axonemal, heavy chain 5) and frame-shift mutations

9p21–p13 DNAI1 (dynein, Missense mutations, axonemal, intermediate insertions, and deletions.chain 1) A common 1-bp splice-site

insertion mutation in intron 1has been reported



Genetic diagnosis Specific mutation analysis is not generally available. Counseling is

and counseling on the basis of autosomal recessive inheritance with recurrence risks

of 1 in 4 for PCD and 1 in 8 for Kartagener syndrome. This risk of

1 in 8 is a consequence of the randomization of left–right axis

asymmetry which ensues from abnormal cilia motility.

Williams Syndrome(also known as: WS; infantile hypercalcemia; Williams–Beuren syndrome)

MIM 194050

Clinical features These consist primarily of a characteristic facial appearance, mild mentalretardation with an engaging extrovert (“cocktail party”) personality,cardiac anomalies, and variable renal involvement.The facial featuresconsist of blue stellate irides, anteverted nostrils, long philtrum, full lips,and wide mouth. Supravalvular aortic stenosis (SVAS) and supravalvularpulmonary stenosis occur in around 65% and 25% of cases, respectively.Renal involvement can include renal artery stenosis, nephrocalcinosissecondary to hypercalcemia, aplasia, hypoplasia, and duplication.Average IQ is approximately 50 with relatively good language and verbal skills in contrast to poor mathematical abilities.

Age of onset The clinical features are apparent at birth.

Epidemiology The estimated incidence is 1 in 10,000–20,000.

Inheritance Chromosomal microdeletion.


Gene ELN (elastin) and up to 16 other genes.

Mutational Microdeletion in 95% of cases. A broad spectrum of mutationsspectrum (including missense, nonsense, frame-shift, and splice-site) have

been identified in ELN in isolated nonsyndromal SVAS.

Molecular The common microdeletion in WS patients is approximately 1.5 Mb andpathogenesis contains at least 17 genes. This microdeletion is generated by unequal

meiotic recombination mediated by regions of flanking homologous DNA.Smaller microdeletions have been noted in a few patients. An inversion


Williams Syndrome142

involving the WS region has been noted in some nondeleted patients and curiously this inversion has also been found in asymptomatic parentsof some children with a deletion, implying that it may predispose tomicrodeletion formation. Mosaicism for the deletion has been observed in a small proportion of children. In some WS patients no molecularabnormality can be identified. There is no evidence for a significant parent-of-origin effect.

Haploinsufficiency for elastin (encoded by ELN within the deleted region)accounts for the SVAS seen in WS patients and for nonsyndromal SVAS(MIM 185500). Insoluble elastin is believed to be an important regulatorof cellular proliferation in arterial smooth muscle. Hence reduced levelsof elastin lead to increased proliferation of arterial smooth muscle cells.Hemizygosity for LIMK1 (lim domain kinase 1) is thought to contribute toimpaired visuospatial cognition in WS children. Other genotype–phenotypecorrelations have not been established.

Genetic diagnosis The common microdeletion can be readily identified using fluorescence and counseling in situ hybridization. Molecular (cytogenetic) diagnosis in nondeletion

cases is only undertaken on a research basis. The recurrence risk for a de novo microdeletion is low (<1%).


8. Craniofacial Disorders

Apert Syndrome 144

Crouzon Syndrome 146

Greig Syndrome 148

Pfeiffer Syndrome 149

Rubinstein–Taybi Syndrome 151

Saethre–Chotzen Syndrome 152

Sotos Syndrome 153

Treacher Collins Syndrome 154

Van der Woude Syndrome 155


Apert Syndrome144

Apert Syndrome(also known as: acrocephalosyndactyly type I)

MIM 101200

Clinical features These comprise a combination of acrocephaly and brachycephaly due to multiple suture synostosis, particularly involving the coronal sutures,together with osseous and/or cutaneous syndactyly involving the secondto fifth fingers and all toes (see Figure 1). The facies is usually flat with a high broad forehead, shallow orbits, and hypertelorism. Approximately50% of cases show mild to moderate mental retardation possiblybecause of associated cerebral malformations. Some studies suggestthat early craniectomy improves the intellectual outcome but thisremains uncertain.

Age of onset The features are obvious at birth and can be suspected on ultrasoundexamination in the second trimester.

Epidemiology The incidence at birth varies between 1 in 50,000–100,000 in different populations.



Gene FGFR2 (fibroblast growth factor receptor 2)

Mutational Two specific missense substitutions, which have a gain spectrum of function effect.

Figure 1. Appearance of a hand and foot in a child with Apert syndrome.


Craniofacial Disorders 145

Molecular Almost all cases of Apert syndrome are caused by one of two missensepathogenesis point mutations, Ser252Phe (934 C→G) or Pro253Arg (937 C→G) in

FGFR2. This encodes a membrane-bound receptor with an extracellularregion containing three immunoglobulin (Ig)-like domains, a singlemembrane-spanning domain and an intracellular split tyrosine kinasedomain (see Figure 2). On binding with the specific FGF ligand (22distinct FGFs have been identified), the receptor molecules dimerizewith subsequent phosphorylation and activation of the tyrosine residuesin the intracellular domain. An alternative splicing event generates twoforms of the IgIII domain with different binding characteristics. Thealternative forms consist of IgIIIa with either IgIIIb or IgIIIc. IgIIIa/IIIb is known as FGFR2b and encodes a keratinocyte growth factor receptorwhich is expressed in epithelium, whereas IgIIIa/IIIc, also known asFGFR2c, encodes a receptor known as BEK (bacterially expressedkinase) which is expressed in mesenchyme. Thus FGFR2 expressionaccounts for two distinct receptor activities which helps explain thediverse phenotypic features seen in affected children – see below.

Figure 2. Diagrammatic representation of fibroblast growth factor receptors (FGFRs) with arrows showinglocations of the common craniosynostosis syndrome mutations. A: Apert syndrome; C: Crouzon syndrome; C*: Crouzon syndrome with acanthosis nigricans; IgI, IgII, and IgIII: immunoglobulin like-domains;M: Muenke syndrome (isolated coronal synostosis); P: Pfeiffer syndrome; TK1 and TK2: tyrosine kinase domains; TM: transmembrane domains.

PPro252Arg

IgI IgII IgIII

TM TK1 TK2

ASer252PhePro253Arg

C C+P C

MPro250Arg

C*Ala391Glu

FGFR1

FGFR2

FGFR3


Crouzon Syndrome146

In Apert syndrome, the two common mutations occur in the ligand-bindinglinker region between the second and third immunoglobulin-like domains,resulting in loss of ligand specificity with ectopic ligand-dependentactivation. The Pro253Arg mutation is thought to be associated withmore severe syndactyly because of inappropriate FGF7/10 binding byFGFR2c in epithelium, whereas the Ser252Phe mutation is associatedwith a higher incidence of cleft palate because of enhanced FGF2/FGFR2csignaling in mesenchyme. Both of these mutations show a positiveassociation with advanced paternal age and occur almost exclusively in spermatogenesis as opposed to oogenesis.

Genetic diagnosis Analysis for the two common recurring mutations is readily available.and counseling Most cases result from new dominant mutations. The risk to each

offspring of an affected individual is 1 in 2.

Crouzon Syndrome(also known as: craniofacial dysostosis)

MIM 123500

Clinical features These consist of variable craniosynostosis, maxillary hypoplasia,shallow orbits, and proptosis. Craniosynostosis usually develops ininfancy and can involve any or all of the coronal, sagittal, and lambdoidsutures. Inconsistent features include optic atrophy, cleft lip/palate, iris coloboma, and acanthosis nigricans. Progressive hydrocephalus and conductive hearing loss are potential complications. Intelligence is usually normal and involvement of the extremities is minimal.

Age of onset The facial features are usually apparent by the age of 1 year.

Epidemiology The birth prevalence is estimated to be approximately 1 in 60,000. All races are affected.

Inheritance Autosomal dominant. Approximately one third to one half of all casesresult from new mutations.




Table 1. Crouzon syndrome: chromosomal locations, genes, and mutational spectra.

Molecular Most cases of Crouzon syndrome are caused by missense mutations pathogenesis in exons IIIa or IIIc of FGFR2, which encodes the third extracellular

immunoglobulin domain of the membrane spanning FGFR2 tyrosinekinase receptor (see p.145). New mutations have been shown to arise almost exclusively in the paternal germ line. FGFR2 encodes twoalternative products, keratinocyte growth factor receptor (KGFR) andBEK (bacterially expressed kinase), which have different ligand-bindingcharacteristics and different patterns of expression. KGFR is involved in skin development whereas BEK is active in osteogenesis, with BEKtranscripts concentrated in the bones of the skull including the ossiclesin the middle ear. The gain of function mutations in the BEK form ofFGFR2 that cause Crouzon syndrome result in cross-linking of unpairedcysteine residues leading to covalent dimerization and activation ofreceptor subunits.

Note that mutations in FGFR2 also cause Apert syndrome (see previousentry) and many cases of Pfeiffer syndrome (see p.149–50). They alsoaccount for other much rarer craniosynostosis syndromes, including the Antley–Bixler syndrome (MIM 207410 – main clinical features:craniosynostosis, midface hypoplasia, humeroradial synostosis, bowedfemora), the Beare–Stevenson cutis gyrata syndrome (MIM 123790 –main clinical features: craniosynostosis, cutis gyrata, acanthosisnigricans, anogenital anomalies) and the Jackson–Weiss syndrome(MIM 123150 – main clinical features: craniosynostosis with variablefoot abnormalities). Rather than discrete pathologic entities, thesedisorders are now viewed as representing overlapping and somewhatvariable clinical phenotypes resulting from the effects of particularmutations on ligand-binding, splice-form expression, and FGFR2b/2creceptor activation (see p.145).

The occurrence of Crouzon syndrome with acanthosis nigricans iscaused by a specific Ala391Glu (1172 C→A) missense mutation in thetransmembrane domain of FGFR3. This mutation occurs very close to

Chromosomal Gene Mutational spectrumlocation

10q26 FGFR2 (fibroblast growth Mainly missense, also factor receptor 2) splice-site mutations

4p16.3 FGFR3 (fibroblast growth Missense mutationfactor receptor 3)


Greig Syndrome148

the site of the recurrent mutation which accounts for most cases of achondroplasia (nucleotide 1138 – see p.108–10).

Genetic diagnosis Screening for the common mutations is undertaken on a limited and counseling service basis. Counseling is as for autosomal dominant inheritance

with variable expression.

Greig Syndrome(also known as: Greig cephalopolysyndactyly syndrome)

MIM 175700

Clinical features The head shows frontal bossing with a broad forehead, hypertelorism,and occasional craniosynostosis (craniosynostosis is seen in 5% ofcases). The thumbs and big toes are broad with postaxial polydactyly in the hands and preaxial polydactyly or polysyndactyly in the feet (see Figure 3). Variable soft tissue syndactyly occurs in the fingers and toes. Intelligence is normal and there are few significant medical problems.

Figure 3. The feet of a child with Greig syndrome.

Age of onset The features are apparent at birth.

Epidemiology The condition is rare. Precise incidence figures are not available.



Gene GLI3 (glioma-associated oncogene 3 or GLI–Kruppel family member 3)



Mutational Missense, nonsense, and frame-shift mutations; all have a loss spectrum of function (haploinsufficiency) effect.

Molecular GLI3 encodes a transcription factor with a central DNA-binding domainpathogenesis composed of five zinc finger motifs together with transcription and

repression domains. In Drosophila, and probably also in humans, theGLI3 homolog (known as cubitus interruptus) is actively involved inpositive and negative regulation of the Sonic hedgehog developmentalpathway in limb development. In humans with Greig syndrome,mutations map not only to the zinc finger domain but also throughoutthe coding regions of GLI3. Curiously, mutations in GLI3 also account for preaxial polydactyly type IV (MIM 174700), postaxial polydactylytype A1 (MIM 174200), and the Pallister–Hall syndrome (MIM 146510)in which meso-axial or postaxial polydactyly is associated with a hypothalamic hamartoma and an imperforate anus.

Genetic counseling Mutation analysis of GLI3 is available on a limited research basis.and diagnosis Counseling is as for autosomal dominant inheritance with very

variable expression.

Pfeiffer Syndrome(also known as: acrocephalosyndactyly, type V)

MIM 101600

Clinical features These consist of variable craniosynostosis with broad thumbs and bigtoes in valgus position and mild soft-tissue syndactyly. In severe casesthere is a cloverleaf skull with ocular proptosis and hydrocephalus. Mildcases may only show turricephaly and/or brachycephaly with maxillaryhypoplasia and down-slanting palpebral fissures. Occasional findingsinclude brachydactyly, radioulnar synostosis, cerebellar herniation, and hearing loss.

Age of onset The clinical features are usually apparent at birth.

Epidemiology Pfeiffer syndrome is less common than Apert and Crouzon syndromes.Precise figures are not available.



Pfeiffer Syndrome150


Table 2. Pfeiffer syndrome: chromosomal locations, genes, and mutational spectra.

Molecular Pfeiffer syndrome can be caused by a specific single mutation (involvingpathogenesis the linker region between the second and third immunoglobulin loops in

FGFR1) or by multiple mutations (involving the same region or the thirdimmunoglobulin domain of FGFR2). The recurrent mutation in FGFR1involves a C→G transversion resulting in a Pro252Arg substitution.Mutations at the same region in FGFR2 (ie, Ser252Phe and Pro253Arg)result in Apert syndrome (see p.145). An identical mutation in FGFR3(Pro250Arg) results in nonsyndromal coronal craniosynostosis or thecombination of coronal craniosynostosis, with carpal and tarsalsynostosis and cone-shaped epiphyses of the phalanges, a conditionalso known as Muenke syndrome (MIM 602849).

The spectrum of mutations in FGFR2 which causes Pfeiffer syndrome is very similar to that which causes Crouzon syndrome and there havebeen several reports of identical mutations causing both syndromes.These observations support the hypothesis that Pfeiffer and Crouzonsyndromes represent overlapping phenotypes. New mutations whichcause these syndromes show a paternal age effect and have beendemonstrated to arise almost exclusively in the paternal germ line.

Genetic diagnosis Screening for the common mutations is available on a limited serviceand counseling basis. Counseling is as for autosomal dominant inheritance with full

penetrance but very variable expression. The parents of a child with an apparent new mutation should be examined carefully to exclude mild involvement.


8p11.2–p11.2 FGFR1 (fibroblast growth A single common missense factor receptor 1) mutation

10q26 FGFR2 (fibroblast growth Mainly missense, alsofactor receptor 2) splice-site mutations



Rubinstein–Taybi Syndrome

MIM 180849

Clinical features Typically, these consist of a characteristic facial appearance withmicrocephaly, down-slanting palpebral fissures, beaked nose, long nasalseptum, broad angulated thumbs, and big toes. Intelligence is impaired,with an average IQ of 50. Other findings and complications includecongenital heart defects (33%), patellar dislocation, retinal dystrophy,and keloid scar formation.

Age of onset The features are usually apparent in infancy or early childhood.

Epidemiology The estimated incidence at birth is 1 in 275,000–300,000.



Gene CBP or CREBBP (cyclic AMP regulated enhancer binding protein)

Mutational Mainly microdeletions and truncating nonsense mutations; thesespectrum probably have a loss of function effect.

Molecular CBP encodes a large nuclear protein that is involved in transcriptionpathogenesis regulation and the integration of several different transduction pathways.

Studies in Drosophila indicate that CBP may interact with the GLI3 andTWIST pathways, which would account for the degree of phenotypicoverlap seen in the Rubinstein–Taybi, Greig (see p.148–9), andSaethre–Chotzen (see p.152–3) syndromes. CBP probably acts byremodeling the structure of chromatin thereby allowing transcriptionfactors to the nuclear DNA.

Genetic diagnosis Mutation analysis is not readily available and even in a research settingand counseling mutations can be found in only ~45%–50% of cases. The 10% of cases

caused by microdeletions can be detected by fluorescence in situhybridization. Most cases are sporadic, probably because of a veryreduced reproductive capacity. A few examples of affected parent andchild have been reported with marked intrafamilial variation. Thus, the parents of apparent isolated cases should be examined carefully.


Saethre–Chotzen Syndrome152

Saethre–Chotzen Syndrome(also known as: SCS; acrocephalosyndactyly type III)

MIM 101400

Clinical features SCS is characterized by variable craniosynostosis with subtle facialdysmorphism and digital anomalies. Coronal suture synostosis results in brachycephaly and, if asymmetrical, plagiocephaly. Facial featuresinclude asymmetry, a broad sloping forehead, ptosis, and small, low-setears with a prominent crus. The hands and feet show brachydactylywith syndactyly, most commonly between the second and third fingersand between the second and third toes, and fifth finger clinodactyly. The big toes may be broad and bifid. Intelligence is usually normal or, occasionally, mildly impaired.

Age of onset Craniosynostosis may be present at birth or develop in early childhood.

Epidemiology SCS is one of the most common craniosynostosis syndromes with an estimated incidence of 1 in 25,000–50,000 live births.



Gene TWIST

Mutational Missense and nonsense mutations, insertions, and both smallspectrum and large deletions; they all have a probable loss of function effect.

Molecular Approximately 50% of all cases have a mutation in, or deletion of, TWIST,pathogenesis which encodes a transcription factor with a basic helix–loop–helix motif

consisting of a DNA-binding domain followed by two helices and anintervening loop domain. The gene is so named because of the twistedappearance of the body seen in Drosophila with a recessive lethalmutation. Mutations in SCS patients are evenly distributed amongst the DNA-binding, helix, and loop domains. Large megabase deletionsembracing the TWIST locus have been identified in a few patients who have significant learning difficulties.

Studies in Drosophila indicate that TWIST regulates expression of theFGFR gene family. This would be consistent with the observation that



some patients with SCS-like features have been shown to have either a mutation in FGFR2 (located at 10q26) or, more often, the commonPro250Arg mutation in FGFR3 (see p.145). In one study of 32 unrelatedSCS patients, 12 were found to have a mutation in TWIST, seven had the Pro250Arg mutation in FGFR3 and one had a 6-bp deletion in FGFR2. No mutation could be identified in the remaining 12 cases.

Genetic diagnosis Mutation screening for TWIST is undertaken at a small number ofand counseling specialist laboratories as is detection of the common Pro250Arg FGFR3

mutation. Counseling is on the basis of autosomal dominant inheritancewith marked intrafamilial variation.

Sotos Syndrome(also known as: cerebral gigantism)

MIM 117550

Clinical features Overgrowth predates delivery (with mean birth weights of 4.2 kg in boys and 4.0 kg in girls) and is marked for the first 4 years, followed by a gradual fall to the 97th centile during later childhood. Affectedchildren show macrocephaly with frontal bossing, hypertelorism, and a prominent jaw. The primary dentition erupts prematurely and bone age is advanced. Neurodevelopmental findings include hypotonia in infancy, nonprogressive ataxia, and mild developmental delay.Increased susceptibility to develop childhood neoplasm has beenreported but the overall risk is thought to be low (<5%).

Age of onset The clinical features are apparent at birth.

Epidemiology All ethnic groups are affected and several hundred cases have beenreported although no accurate incidence figures are available.



Gene NSD1 (nuclear receptor SET-domain protein 1)

Mutational Missense, nonsense, and frame-shift mutations, there is also spectrum a common 2.2-Mb microdeletion.


Treacher Collins Syndrome154

Molecular NSD1 contains 23 exons and encodes a multiple-domain protein thatpathogenesis shows homology to a family of proteins which act as coregulators of

androgen/steroid receptors. The mechanism by which mutations inNSD1 cause Sotos syndrome is unknown. The fact that mutationsidentified to date are predicted to result in haploinsufficiency suggests a regulatory role in growth suppression. Mutations in NSD1 also cause another overgrowth disorder known as Weaver syndrome(MIM 277590).

Genetic diagnosis Mutation analysis is available only on a very restricted research basis.and counseling Thus the diagnosis is made on the clinical features and radiologic

evidence of advanced bone age, which may be disharmonic withphalangeal age in advance of carpal age. Counseling is as for autosomal dominant inheritance with very variable expression.

Treacher Collins Syndrome(also known as: mandibulofacial dysostosis; Treacher Collins–Franceschetti syndrome)

MIM 154500

Clinical features These are limited to abnormalities of craniofacial development.Maxillary and mandibular hypoplasia result in down-slanting palpebralfissures with underdevelopment of the lateral third of the lower eyelids,small cheekbones, and microretrognathia. The pinnae are small and/ormisplaced in 80% of cases. Bilateral conductive hearing loss and cleftpalate each occur in approximately 30% of cases. Intelligence is normal.

Age of onset The features are evident at birth.

Epidemiology The incidence is approximately 1 in 50,000 live births. All ethnic groupsare affected.


Chromosomal 5q32–q33.1location

Gene TCOF1 (treacle)

Mutational Mainly nonsense and frame-shift deletion/insertion mutations resulting spectrum in premature termination and haploinsufficiency.



Molecular TCOF1 contains 25 exons and shows peak expression at the edges pathogenesis of the developing neural folds and in the developing branchial arches.

The encoded protein, known as treacle, shows homology to a family of nucleolar phosphoproteins with nuclear and nucleolar localizationsignals. A role as a chaperone in nuclear–cytoplasmic transport throughnuclear–nucleolar shuttling is predicted. Specific genotype–phenotypecorrelations have not been identified.

Genetic diagnosis Mutation analysis is available on a limited research basis. Counselingand counseling is as for autosomal dominant inheritance with marked inter- and

intrafamilial variation.

Van der Woude Syndrome(also known as: lip-pit syndrome)

MIM 119500

Clinical features This is the most common syndromal form of cleft lip/palate. Featuresconsist of cleft lip and/or cleft palate in association with characteristicpits closely adjacent to the midline in the lower lip. The orofacial cleftingshows marked variation between and within families so that both cleftlip and cleft palate can occur in affected members of the same kindred.This is in contrast to nonsyndromal cleft lip/palate, which showspolygenic/multifactorial inheritance.

Age of onset The features are apparent at birth.

Epidemiology The estimated incidence is 1 in 35,000–100,000.



Gene IRF6 (interferon regulatory factor 6)

Mutational Missense, nonsense, and frame-shift insertions and deletions spectrum with either a loss of function or dominant-negative effect.

Molecular IRF6 is one of a family of nine transcription factors which sharepathogenesis conserved DNA-binding and protein-binding domains. In mice,


Van der Woude Syndrome156

it is expressed along the medial edge of the fusing palate, as well as in tooth buds, genitalia, and skin. The precise function of IRF6in humans is unknown although a role in the transforming growth factor-β signaling pathway is suspected.

Van der Woude syndrome is associated with mutations in IRF6 whichhave a loss of function effect. Truncating mutations are distributedthroughout the gene whereas missense mutations occur mainly in one of the conserved DNA-binding or protein-binding domains. Missensemutations that have a dominant-negative effect (by allowing a mutatedIRF6 protein to bind to other proteins) result in a different condition, the popliteal pterygium syndrome (MIM 119500). In the poplitealpterygium syndrome, orofacial clefting and lip pits occur in associationwith webbing of the skin (particularly in the popliteal fossa) and genital abnormalities.

Genetic diagnosis Mutation analysis is available only on a research basis. Counseling and counseling is as for autosomal dominant inheritance with slightly reduced

penetrance (95%) and variable expression.


9. Endocrine Disorders

Androgen Insensitivity Syndrome 158

Congenital Adrenal Hyperplasia 160

Diabetes Insipidus 163

Growth Hormone Deficiency 164

Growth Hormone Receptor Defects 166

Panhypopituitarism 167

Pseudohypoparathyroidism 169


Androgen Insensitivity Syndrome158

Androgen Insensitivity Syndrome(also known as: AIS; androgen receptor deficiency; testicular feminization syndrome.Includes Reifenstein syndrome)

MIM 300068, 312300

Clinical findings Complete and partial androgen insensitivity are amongst the more commoncauses of male pseudohermaphroditism, in which individuals have a malekaryotype (46,XY) but ambiguous or female external genitalia. Other rarersingle-gene causes of male pseudohermaphroditism are summarized inTable 1. In complete AIS there is full sex-reversal, with female externalgenitalia, female breast development, a blind vagina, absent uterus, andabdominal or inguinal testes. In partial or incomplete AIS (also known as Reifenstein syndrome) there is variable genital ambiguity ranging fromhypospadias and oligospermia (with gynecomastia) to an almost completefemale phenotype with partial labio-scrotal fusion. It has been suggestedthat Joan of Arc may have been affected by this syndrome.

Age of onset The findings are present at birth and date from late embryogenesis whensex development is complete.

Epidemiology The estimated incidence is 1 in 20,000 to 1 in 50,000.

Inheritance X-linked recessive, only chromosomal males are affected

Chromosomal Xq11–q12location

Gene AR (androgen receptor)

Mutational Mainly missense and nonsense point mutations. Also splice-site spectrum mutations, deletions, and insertions, all with a loss of function effect.

Molecular The androgen receptor gene contains eight exons and encodes a proteinpathogenesis with regulatory, DNA-binding, nuclear localization, and androgen-binding

domains. Mutations are found throughout the gene, with clustering inthe DNA- and androgen-binding domains. There is a weak genotype–phenotype correlation, with large deletions and truncating (nonsense)mutations resulting in complete absence of functional receptor. Impairedreceptor activity results in insensitivity to circulating androgen in lateembryogenesis and early fetal life (when genital development is determined).


Endocrine Disorders 159

Table 1. Single-gene causes of male pseudohermaphroditism. AR: autosomal recessive; DHEA: dehydroepiandrosterone;XR: X-linked recessive.

In complete AIS, failure of Wolffian duct stimulation results in theabsence of internal male genitalia. Normal production of anti-Müllerianhormone by the testes results in failure of uterine development. Thetestes continue to produce both testosterone and small quantities ofestrogen, which result in breast development and pubertal feminizationin the complete syndrome.

Note that expansion of a CAG triplet repeat in the first exon of AR causesan X-linked form of spino-bulbar muscular atrophy known as Kennedydisease (MIM 313200). This progressive neurologic disorder presents in adult life and is associated with oligospermia and gynecomastia.

Genetic diagnosis Diagnosis is made on the basis of clinical and cytogenetic findings,and counseling supported by androgen receptor assay using fibroblasts cultured from

genital skin. Specific mutation analysis is available at a small number of specialized laboratories. Counseling is as for X-linked recessiveinheritance. The testes should be removed, usually after puberty toallow for spontaneous pubertal feminization, in view of the associated5% risk of gonadal neoplasia in adult life.

Disorder MIM Inheritance Basic defect Clinical features in males

Androgen 300068 XR Absence of functional See textinsensitivity androgen receptors

5α-Reductase 264600 AR Conversion of Ambiguous genitalia with deficiency testosterone to masculinization at puberty

dihydrotestosterone

Leydig cell 152790 AR Absence of functional Female external genitalia, hypoplasia luteinizing hormone absent uterus, testes with

receptors no Leydig cells

Testosterone 1) Lipoid 201710 AR Conversion of Ambiguous or female biosynthesis 1) congenital cholesterol to external genitalia with defects 1) adrenal pregnenolone severe salt loss

1) hyperplasia

2) 17α-Hydroxylase/ 202110 AR Conversion of Ambiguous external 1) 17,20-desmolase pregnenolone to genitalia with incomplete1) deficiency DHEA Wolffian duct development

3) 3β-Hydroxysteroid 201810 AR Conversion of DHEA Ambiguous external 1) dehydrogenase to androstenedione genitalia with normal1) deficiency Wolffian ducts and

severe salt loss

4) 17β-Hydroxysteroid 264300 AR Conversion of Ambiguous genitalia1) dehydrogenase androstenedione to with Wolffian ducts and1) deficiency testosterone pubertal virilization


Congenital Adrenal Hyperplasia160

Congenital Adrenal Hyperplasia(also known as: CAH; 21-hydroxylase deficiency; CAH type 1; CYP21 deficiency)

MIM 201910

Clinical features The clinical features are a direct consequence of impaired cortisol andaldosterone synthesis, leading to increased pituitary adrenocorticotrophichormone secretion, which in turn leads to elevated adrenal production of cortisol precursors and androgens. Three forms of presentation arerecognized: the classic virilizing and salt-losing forms, and a nonclassicattenuated form.

Children with the classic virilizing form show progressive virilizationwhich can manifest in female neonates as genital ambiguity withclitoromegaly and variable fusion of the labio-scrotal folds. In olderchildren, excess androgen production results in precocious sexual hairwith accelerated skeletal maturation, and ultimately short stature. In the salt-losing form, affected females are virilized and infants of both sexes develop hyponatremic circulatory collapse soon after birth due to deficient aldosterone synthesis, which (if untreated) islife-threatening. Presentation is variable in the nonclassic late-onsetform, and can include premature development of pubic hair, severeacne, delayed menarche, hirsutism, and oligomenorrhea.

Age of onset In the classic forms, presentation is at birth or in early infancy.

Epidemiology CAH type I accounts for 90%–95% of all forms of CAH, with an estimatedworldwide frequency of 1 in 15,000 live births. A particularly highincidence has been noted in the Eskimo population of southwest Alaska.



Gene CYP21 (cytochrome P450, subfamily XXIA)

Mutational Mainly deletions and gene conversions resulting from nonreciprocal spectrum transfer of inactivating mutations from the closely adjacent pseudogene

(see below). Also, a wide range of missense, nonsense, splice-site, and frame-shift mutations.



Figure 1. Adrenal steroidogenesis. The major enzymatic steps are shown. Steroids with mineralocorticoidactivity are shown with a gray background. Androgenic steroids are shown with a purple background.Cortisol is the major glucocorticoid produced in man.

Molecular CYP21 is situated within the HLA complex. It contains 10 exons that pathogenesis encode the adrenal steroid 21-hydroxylase, which is a microsomal

cytochrome P450. This enzyme is responsible for the conversion of progesterone to deoxycorticosterone and of 17-OH progesterone to 11-deoxycortisol (see Figure 1). CYP21 lies in a tandem pairedarrangement with its pseudogene CYP21P and two isoforms of the C4complement gene (ie, C4A–CYP21P–C4B–CYP21). This arrangementprobably arose as a result of a recombination or duplication event inearly evolution. The nature of this arrangement predisposes to two forms of mutation: deletions (due to unequal crossing-over in meiosisfollowing misalignment) and conversions (due to transfer of silencingDNA sequences from CYP21P to CYP21). These mutations account for most cases of classic CAH type I. Missense mutations with a mildereffect on enzyme activity tend to be associated with the nonclassicphenotype, although the relationship between genotype and phenotypeis not consistent, as illustrated by reports of discordant diseasepresentations in individuals with apparently identical mutations.

17

18-OH-Corticosterone

Aldosterone

11-Deoxycorticosterone 11-Deoxycortisol

Corticosterone Cortisol

DHEAPregnenolone -OH-Pregnenolone DHEA

Progesterone 17-OH-Progesterone Androstenedione

Cholesterol DHEA-2


Congenital Adrenal Hyperplasia162

Table 2. Rare causes of congenital adrenal hyperplasia (CAH) (all show autosomal recessive inheritance).

Genetic diagnosis Diagnosis is based on the combination of clinical and biochemicaland counseling findings, notably elevated 17-OH progesterone levels, as measured

in serum by radioimmunoassay. Mutation detection is undertaken atspecialist designated laboratories, but only about 70% of all mutationscan be identified. Linkage analysis, formerly based on human leukocyteantigen haplotype studies and now using intragenic markers, can be utilized for carrier detection and prenatal diagnosis when specificmutations cannot be identified. Prenatal treatment with maternaldexamethasone commencing before 6–7 weeks gestation preventsvirilization in a high proportion of affected female fetuses. For this to be considered it is necessary that informative molecular analysis has been undertaken in an older affected sibling, so that appropriatemutation detection or linkage analysis can be carried out on chorionicvilli from the new pregnancy, with a view to continuing thedexamethasone only if the fetus is both female and affected.

Other, much rarer, causes of congenital adrenal hyperplasia aresummarized in Table 2.

Disorder MIM Locus Gene Mutational spectrum Clinical features

11β-Hydroxylase 202010 8q21 CYP11B1 Missense, nonsense, As in CAH type I, plus hypertensiondeficiency splicing, and frame-shift with hypokalemic alkalosis

deletions/insertions

Aldosterone synthase 203400 8q21 CYP11B2 Missense, nonsense, Salt loss leading to dehydration deficiency and deletions

3β-Hydroxysteroid 201810 1p13.1 HSD3B2 Mainly missense point Ambiguous external genitalia indehydrogenase mutations males and females, plus salt loss deficiency in some cases

17α-Hyroxylase/ 202110 10q24.3 CYP17 Missense, nonsense, Ambiguous genitalia in males,17,20-lyase splice-site, and frame- primary amenorrhea in females, deficiency shift deletions/insertions hypertension

Congenital lipoid 201710 8p11.2 StAR Mainly nonsense and Ambiguous or female external adrenal hyperplasia frame-shift deletions/ genitalia, with severe salt loss

insertions and susceptibility to infection



Diabetes Insipidus(also known as: DI. Includes neurohypophyseal and nephrogenic DI)

MIM See Table 3.

Clinical findings Excessive thirst and polyuria occur in all forms of DI. In the nephrogenicforms, severe fluid loss in infancy can result in hypernatremic dehydration,leading to convulsions and death or residual mental retardation. Bladderdistension and chronic renal disease are possible later complications.Some women with neurohypophyseal DI encounter problems with laborand lactation due to associated oxytocin deficiency.

Age of onset Soon after birth or in early infancy

Epidemiology Many families have been reported with both neurohypophyseal andnephrogenic forms, but accurate incidence figures are not available.

Inheritance, See Table 3.chromosomal location, gene, andmutational spectrum

Molecular Neurohypophyseal DIpathogenesis This is caused by mutations in the antidiuretic gene AVP, which

contains three exons and encodes a complex protein consisting of a signal peptide, the active hormone vasopressin (from exon 1), and its carrier protein neurophysin (from exons 2 and 3). Synthesisoccurs in the neurons of the supraoptic and paraventricular nuclei

Table 3. Diabetes insipidus: MIM numbers, types, inheritance, chromosomal locations, genes, and mutational spectra.

Type MIM Inheritance Chromosomal Gene Mutational location spectrum

Neurohypophyseal DI 125700 Autosomal 20p13 AVP (arginine Mainly missense pointdominant (rarely vasopressin) mutations. Also nonsenseautosomal mutations and deletionsrecessive)

Nephrogenic DI 304800 X-linked Xq28 AVPR2 (arginine Mainly missense point type I recessive vasopressin mutations. Also nonsense

receptor 2) and deletion or insertion frame-shift mutations

Nephrogenic DI 125800 Autosomal 12q13 AQP2 Mainly missense pointtype II dominant (aquaporin-2) mutations


Growth Hormone Deficiency164

of the hypothalamus, followed by transport down neuronal axons to the posterior pituitary for storage and subsequent release. A largenumber of mainly missense mutations have been reported across thegene. These disturb various functions, including cleavage of the signal peptide, which results in accumulation of precursors in the endoplasmicreticulum, and failure of neurophysin transport.

Nephrogenic DIMost cases are caused by mutations in AVPR2, which encodes theantidiuretic hormone (ADH) V2 receptor, a member of the superfamily of G protein-coupled receptors with seven membrane-spanning domains.Normally, the V2 receptor activates adenyl cyclase. Patients with X-linked nephrogenic DI do not show an increase in urinary cyclic AMPexcretion following ADH administration and are therefore described as being ADH insensitive. Mutations have been reported across the gene in evolutionarily conserved positions of the receptor with no cleargenotype–phenotype correlation. Approximately 5% of cases are causedby mutations in the aquaporin-2 gene, which encodes the water channelof the collecting duct. Individuals with this autosomal dominant form of DI are therapeutically unresponsive to ADH but show a normal urinary cyclic AMP response to ADH administration.

Genetic diagnosis Diagnosis is based on biochemical studies of plasma and urine and counseling osmolality, and the patient’s response to water deprivation and to

exogenous vasopressin administration. Mutation analysis is not readilyavailable, so genetic counseling should be based on the family historyand biochemical findings.

Growth Hormone Deficiency(also known as: GH deficiency; pituitary dwarfism; primordial dwarfism)

MIM See Table 4.

Clinical features Type I patients have proportionate short stature with increasedsubcutaneous fat, wrinkled skin, and a high-pitched voice. Pubertystarts spontaneously, but may be delayed. Similar features are seen insome individuals with type II, whereas others simply show short stature.Bone age is retarded in both groups. Types I and II can sometimes bedistinguished on the basis of a decreased (type I) or increased (type II)



Table 4. Growth hormone deficiency: types, MIM numbers, inheritance, chromosomal locations, and genes.

insulin response to glucose ingestion. In type III there is proportionateshort stature, retarded bone age, delayed onset of puberty, andassociated hypogammaglobulinemia.

Age on onset Growth deceleration usually becomes apparent in early childhood. In severe type I cases (designated type IA), short stature may beapparent at birth.

Epidemiology GH deficiency affects approximately 1 in 5,000 to 1 in 10,000 children.Estimates of the proportion of familial cases range from 3% to 30%.

Inheritance, See Table 4.chromosomal location, gene, andmutational spectrum

Molecular GH1 is located within a cluster of five GH genes thought to have arisenpathogenesis from a series of ancestral gene duplication events. It contains five exons

plus control sequences that bind to the product of PIT1, a pituitary-specific transcription factor, which regulates pituitary development andhormone expression (see the entry on panhypopituitarism p.167–9).GH1 encodes circulating growth hormone which, at the cellular level,binds with two GH receptor molecules to initiate signal transduction.

Mutations in GH1 account for both types I and II GH deficiency. In typeIA, the most severe form, homozygosity or compound heterozygosity forlarge deletions or truncating mutations results in complete absence of GH secretion. Many of these patients develop antibodies to exogenousGH. In type IB, the most common form, small quantities of GH areproduced and the production of antibodies to GH is not a problem. Some of these patients are homozygous for intronic donor splice-site

Type MIM Inheritance Chromosomal location Gene Mutational spectrum

I 262400 Autosomal 17q22–q24 GH1 Mainly large deletions (type IA) recessive (growth hormone) and splice-site frame-shift

mutations (type IB), all with a loss of function effect

II 173100 Autosomal 17q22–q24 GH1 Splice-site mutations with dominant (growth hormone) a dominant-negative effect

III 307200 X-linked Xq21.3–q22 BTK Splice-site mutation leading recessive (Bruton tyrosine kinase) to exon skipping


Growth Hormone Receptor Defects166

mutations, whereas in other type IB familial cases, no GH mutations have been identified. In these patients the defects are suspected to lie in the GH-releasing hormone gene or in the gene that encodes itsreceptor (MIM 139191).

Mutations in type II GH deficiency involve splicing transcripts for intron3, leading to skipping of exon 3 and loss of 40 amino acids from the GHprotein. It has been proposed that this shortened GH protein inactivatesthe normal GH protein through disruption of normal intracellular proteintransport or by the formation of abnormal heterodimers.

Genetic diagnosis GH1 mutation analysis is available on a limited basis and should be and counseling pursued if possible, particularly when the family history points to a

genetic cause (eg, consanguinity or the existence of an affected relative).Counseling of isolated cases is difficult as many cases are caused byfactors that convey a low recurrence risk (eg, trauma, neoplasia, andinfection) and there is no reliable method for identifying those cases with a Mendelian etiology.

Growth Hormone Receptor Defects(also known as: GHR defects; growth hormone insensitivity syndrome; Laron dwarfism)

MIM 262500

Clinical features The clinical features are similar to those seen in isolated severe growthhormone (GH) deficiency, but with the important distinction that plasmalevels of GH are high. Affected individuals show marked proportionateshort stature with small facies, blue sclerae, obesity, high-pitched voice,and delayed bone age. The onset of puberty is also delayed.

Biochemically, these individuals have high levels of circulating GH andlow levels of insulin-like growth factor 1 (IGF1). Exogenous GH does not induce an IGF1 response. Hypoglycemia is common in infancy and there is a poor insulin response to glucose ingestion.

Age of onset Severe growth retardation becomes apparent in early childhood.

Epidemiology The condition is very rare. Accurate incidence figures are not available.




Chromosomal 5p13–p12location

Gene GHR (growth hormone receptor)

Mutational Large deletions plus missense, nonsense, and splice-site mutations,spectrum all with a loss of function effect.

Molecular GHR contains nine exons (numbered 2–10) that encode a receptor pathogenesis protein with extracellular, transmembrane, and cytoplasmic domains.

The extracellular domain is encoded by exons 2–7 and occurs freely inplasma as GHBP (GH-binding protein) due to either alternative splicingof GHR mRNA or proteolysis of the mature peptide. Thus assay of GHBPin plasma can assist in diagnosis. Activation of the cytoplasmic domainby ligand (GH) binding promotes signal transduction, leading to theproduction of IGF1.

Mutations in GHR are scattered throughout the gene. Most of the reporteddeletions are in the extracellular domain, resulting in undetectable levelsof GHBP. There is evidence that point mutations tend to be associatedwith less severe short stature than deletions and splicing abnormalities.

Note that some individuals with Laron dwarfism (Type II, MIM 245590)have postreceptor defects involving IGF1 or its receptor. There is someevidence that nonresponsiveness to IGF1 may account for the shortstature seen in African pygmies.

Genetic diagnosis Diagnosis is based on the combination of clinical and hormonal and counseling findings. Specific mutation analysis is available only on a limited

research basis. Counseling is as for autosomal recessive inheritance.

Panhypopituitarism(also known as: combined pituitary hormone deficiency)

MIM 262600

Clinical features These are variable and depend on which of the pituitary hormones aredeficient. Deficiency of growth hormone results in the clinical features of isolated growth hormone deficiency (p.164–6). Thyroid-stimulatinghormone (TSH) deficiency results in variable hypothyroidism. Lack ofgonadotropins, the most common tropic hormone deficiency, results


Panhypopituitarism168

in sexual immaturity and primary amenorrhea in the female andhypogonadism in the male. Adrenocorticotrophic hormone deficiencymay cause severe hypoglycemia.

Age of onset Infancy to late childhood

Epidemiology Genetic forms of panhypopituitarism are rare. Accurate figures are not available.

Inheritance Autosomal recessive and autosomal dominant


Table 5. Panhypopituitarism: chromosomal locations, genes, and mutational spectra.

Molecular PROP1, PIT1, and LHX3 all encode transcription factors involved pathogenesis in pituitary development and hormonal synthesis. PROP1 regulates

expression of PIT1, which in turn binds to and activates the promoters of the growth hormone and prolactin genes. Inactivating mutations inPROP1 result in deficiencies of luteinizing hormone, follicle-stimulatinghormone, growth hormone, TSH, and prolactin, and account for a largeproportion of familial panhypopituitarism cases. Mutations in LHX3 alsoresult in multiple hormone deficiency and rigidity of the cervical spine.


3p11 PIT1 (pituitary-specific Inactivating missense and nonsensetranscription factor 1) point mutations and deletions.

Also missense point mutations with a dominant-negative effect

5q PROP1 (prophet of PIT1) A common 2-bp frame-shift deletion.Also missense point mutations and other frame-shift deletions

9q34.4 LHX3 (lim homeobox 3) Missense mutation and intragenic deletion



Genetic diagnosis Mutation analysis is undertaken only on a very limited research basis.and counseling Most cases of multiple pituitary hormone deficiency are not genetic

in origin, but no method other than mutation analysis is available fordistinguishing these from the rare familial forms. Thus, accurate geneticcounseling for an isolated/sporadic case cannot be achieved.

Pseudohypoparathyroidism(also known as: PHP; Albright’s hereditary osteodystrophy [AHO];pseudopseudohypoparathyroidism [PPHP])

MIM See Table 6.

Clinical features The clinical phenotype associated with PHPIA was described by Albrightand colleagues in 1942 and is referred to as Albright’s hereditaryosteodystrophy (AHO). Typically, patients have a round face, shortstature, obesity, brachydactyly, and ectopic calcification. Mentalretardation is present in over 50% of cases and can vary from mild to severe. Other occasional findings include cataracts and abnormaldentition, with thin enamel and small crowns. Biochemical changesusually include hypocalcemia and hyperphosphatemia in the presenceof a raised level of parathyroid hormone (hence the designation “PHP”).The diagnosis can be confirmed by demonstrating absence of the normalrise in plasma and urinary cAMP in response to an intravenous injectionof parathyroid hormone. Hypothyroidism occurs in over 50% of cases.Patients with PPHP have the phenotype of AHO with hypocalcemia,hyperphosphatemia, and raised parathyroid hormone levels. However,they show a normal increase in urinary cAMP excretion in response to parathyroid hormone administration.

The physical features of AHO are not present in patients with PHP types IB and II. These patients exhibit parathyroid-hormone resistanthypocalcaemia and hyperphosphatemia, with an abnormal cAMPresponse in type IB and a relatively normal response in type II.

Age of onset Ectopic calcification may be noted at birth. Other features becomeapparent in early childhood.

Epidemiology All forms of PHP are rare, although no precise incidence figures are available.


Pseudohypoparathyroidism170

Table 6. Pseudohypoparathyroidism: MIM numbers, types, inheritance, chromosomallocations, and genes.

Inheritance, See Table 6.chromosomal location, and gene

Mutational spectrum Missense and nonsense point mutations, and deletions with a loss of function effect.

Molecular GNAS1encodes the alpha subunit of the guanine nucleotide-binding pathogenesis (Gs) protein. This constitutes a component of the signaling system

whereby parathyroid hormone and other hormones stimulate adenylatecyclase to produce intracellular cAMP. In PHP type IA and PPHP, Gsαactivity is reduced by 50% in erythrocyte membranes. Thus, theseconditions are both caused by heterozygous loss of function mutations in GNAS1. Curiously, both phenotypes can be found within the samekindred, but not within the same sibship.

One possible explanation for this unusual example of an identicalmutation causing diverse phenotypes is that the GNAS1 gene showsdifferential tissue expression depending on whether it was inherited from the father or the mother. Support for this hypothesis of parentalimprinting comes from animal studies indicating that the homologousregion in mice is imprinted, and from the observation in humans thatmaternal transmission of an inactivating mutation usually results in PHP type IA whereas paternal transmission results in PPHP. It has beenpostulated that there is paternal silencing of GNAS1 in the renal cortex.

Type MIM Inheritance Chromosomal Genelocation

PHP 103580 Autosomal dominant 20q13.2 GNAS1 (guanine type IA with a possible nucleotide-binding protein

parent-of-origin alpha-stimulating(imprinting) effect activity polypeptide 1)

PHP 603233 Autosomal dominant 20q13.3 Unknowntype IB

PHP 203330 Autosomal recessive Unknown Unknowntype II

PPHP 300800 Autosomal dominant 20q13.2 GNAS1with a possibleparent-of-origin(imprinting) effect



The precise cause of PHP type IB is not known, although the locus hasbeen mapped to a region of chromosome 20 close to the imprintedGNAS1 locus. However, Gsα levels are normal in PHP type IB. Thebasic defect is likely to involve regulatory regions of the maternalGNAS1, which are involved in establishing the parent-specific pattern of imprinting. Support for this hypothesis comes from the report of a boy with paternal uniparental disomy for chromosome 20q. This boy showed parathyroid hormone-resistant hypocalcemia andhyperphosphatemia with normal levels of Gsα protein, but absence of the normal maternal imprinting pattern of GNAS1.

Note that the other condition associated eponymously with Albright,McCune–Albright syndrome, is caused by somatic mosaicism foractivating mutations in GNAS1. This is in contrast to PHP type IA,which results from inactivating mutations.

Genetic diagnosis Specific mutation analysis is available at only a few specialist and counseling laboratories. Counseling for types 1A, IB, and PPHP is on the basis

of autosomal dominant inheritance, with variable expression possiblydue to tissue-specific imprinting. PHP type II is extremely rare and has been described in only a few families.


172


10. Gastrointestinal and Hepatic Diseases

Alagille Syndrome 174

α1-Antitrypsin Deficiency 175

Hirschsprung Disease 177


Alagille Syndrome174

Alagille Syndrome(also known as: arterio-hepatic dysplasia)

MIM 118450

Clinical features These are variable in both degree and age of onset. Patients with the full syndrome have characteristic facies with a broad forehead, deep-seteyes, and pointed chin (see Figure 1). Typically, they also have neonataljaundice due to paucity of intralobular bile ducts (90%), posteriorembryotoxon in the eyes (80%), valvular or peripheral pulmonarystenosis (70%), and butterfly vertebrae (50%).

Growth retardation is common and 40% of affected children have renalabnormalities. Approximately 25% develop potentially life-threateningliver failure.

Age of onset Usually neonatal

Epidemiology The estimated incidence is 1 in 100,000 live births.



Gene JAG1 (jagged 1)

Figure 1. A young child with Alagille syndrome.


Gastrointestinal and Hepatic Diseases 175

Mutational This is very heterogeneous. Small frame-shift deletions and insertionsspectrum account for over 50% of mutations. The remainder consist of missense,

nonsense, and splice-site mutations, with large deletions constitutingapproximately 5%.

Molecular JAG1 contains 26 exons and produces a 5.9-kb mRNA transcript pathogenesis that encodes a ligand for NOTCH1, a member of the NOTCH family of

transmembrane receptors. These play an important role in determiningcell fate. The protein product contains a ligand domain known as DSL(delta/serrate/lag2), a NOTCH region, and a transmembrane domain. It shows an expression pattern consistent with the clinical phenotype.Mutations predominate in the extracellular region of the protein andmost are predicted to have a truncating effect. Mutations that result indeletion of the DSL domain show an association with early liver failure.

Genetic diagnosis Mutation analysis is not widely available, so diagnosis is usuallyand counseling based on a combination of clinical and histologic findings. Expression

is very variable. The parents of an apparently isolated case should beoffered full assessment (including liver function tests, ocular examination,spinal X-rays, and echocardiography). Both somatic and germ linemosaicism have been reported in apparently unaffected parents of severely affected children.

αα1-Antitrypsin Deficiency

MIM 107400

Clinical features These include liver disease (which can present in the neonatal period)and chronic progressive obstructive lung disease (which usuallydevelops in middle age). Approximately 10% of neonates with severe α1-antitrypsin (α1AT) deficiency develop neonatal hepatitis syndrome,with prolonged conjugated hyperbilirubinemia, disturbed liver functiontests, and hepatosplenomegaly. Over two thirds of these children showspontaneous recovery, but 10%–15% go on to develop irreversiblecirrhosis. Other childhood problems are rare. Most affected adultsdevelop obstructive lung disease in middle or old age, with an earlier age of onset in cigarette smokers.

Age of onset Liver disease can present in the neonatal period.


α1-Antitrypsin Deficiency176

Epidemiology The severe PI*Z deficiency allele (see below) shows the highestincidence in Scandinavia, with an allele frequency of close to 1 in 40,giving a population prevalence of homozygotes of just over 1 in 1,600.The prevalence in other Caucasian populations is approximately 1 in2,500. The mild PI*S deficiency allele shows an allele frequency of 1 in 30 to 1 in 40 in Europeans. α1AT deficiency is very rare in Asianand Afro-Caribbean populations.



Gene PI (protease inhibitor 1)

Mutational Very heterogeneous, but mainly missense point mutations. spectrum Also frame-shift insertions and deletions.

PI variants and Over 75 PI variants have been identified, initially by using starch gel nomenclature electrophoresis and more recently by techniques such as isoelectric

focusing and agarose electrophoresis. These are named on the basis of their mobility (F [fast], M [medium], S [slow], and Z [very slow]);subsequently, alleles were designated as PI*F, PI*M, PI*S, PI*Z.Phenotypes are described as PI MZ, PI SZ, etc. Alleles that produce no detectable α1AT in serum are known as null alleles and aredesignated as PI*Q0. The common mild PI*S and severe PI*Zdeficiency alleles have been shown to be due to Glu264Val andGlu342Lys substitutions respectively.

Molecular PI contains six introns and seven exons that encode the 394-amino-acidpathogenesis α1AT protein, a member of the serpin family of protease inhibitors. α1AT

is the major plasma inhibitor of leukocyte elastase and also has inhibitoryactivity against proteinase 3, cathepsin G, chymotrypsin, trypsin, plasmin,and thrombin. Protease inhibition is mediated by a reactive loop of 16-amino-acid residues, which protrude from the molecule and bind with the target protease to prevent further enzyme activity.

Mutant alleles are classified into three groups, depending on their effectson α1AT activity. Deficiency alleles, such as PI*S and PI*Z, result inreduced levels of serum α1AT activity. Null alleles result in completeabsence of serum α1AT activity. Finally, dysfunctional alleles, such



as PI*Pittsburgh, result in altered protein function. In the case ofPI*Pittsburgh, this manifests as a bleeding disorder because ofantithrombin 3 activity. Pulmonary disease is a direct consequence of reduced antielastase activity. Hepatic disease ensues from theaccumulation of mutant protein aggregates in hepatocytes, which, in the case of the Z protein, occurs because the mobile reactive center loop of one molecule can become inserted into that of anothermolecule. Susceptibility to the development of hepatic disease isprobably determined by polymorphic variation in the endoplasmicreticulum pathways responsible for the degradation of the mutantZ-protein aggregates.

Genetic diagnosis Diagnosis is based on conventional assay of serum α1AT activityand counseling and the establishment of the PI phenotype by isoelectric focusing.

This is increasingly being supplemented by direct mutation analysis.Therapeutic approaches include the regular infusion of purified humanα1AT to prevent progressive lung disease, and liver transplantation for advanced liver disease.

Hirschsprung Disease(also known as: HSCR; aganglionic megacolon; congenital intestinal aganglionosis)

MIM 142623

Clinical features HSCR is characterized histopathologically by congenital absence ofganglion cells in the myenteric and submucosal plexuses of the colonand rectum. It is divided into long and short segment types (L-HSCR and S-HSCR), depending on the presence or absence of diseaseinvolvement proximally beyond the sigmoid colon. Around 70% of cases are isolated, or “nonsyndromal”; the remaining 30% areassociated with other abnormalities or conditions such as Down’ssyndrome and Waardenburg syndrome type IV (see p.90–2).

Affected children present with either acute intestinal obstruction and abdominal distension or with chronic constipation and failure to thrive. The diagnosis is made by anorectal manometry andrectal biopsy.

Age of onset Presentation is usually at birth or in early infancy.


Hirschsprung Disease178

Table 1. Hirschsprung disease: chromosomal locations, genes, and mutational spectra.

Epidemiology The estimated incidence is 1 in 4,000. S-HSCR accounts for around80% of cases. The male to female ratio is 4:1.

Inheritance Oligogenic


Molecular The underlying genetic contribution to HSCR is complex. The prevailingpathogenesis view is that RET is the major susceptibility gene, with several other

genes making a smaller contribution. This has given rise to the conceptof “synergistic heterozygosity”. Heterozygous mutations in RET havebeen noted in around 50% of all familial cases and in up to 75% ofchildren with L-HSCR. L-HSCR is known to convey greater familial riskthan S-HSCR (see below) and this is attributed to the consequences of adverse interaction between the products of different genes involvedin the RET (ie, RET and its ligand GDNF), endothelin (ie, EDNRBand its ligand EDN3), and SOX-10 mediated pathways.

RET encodes a transmembrane tyrosine kinase receptor that mediatescell signaling in the embryonic enteric nervous system. Similarly, the


1p36.1 ECE1 (endothelin-converting Missense mutation enzyme 1)

2q22 SIP1 (survival of motor-neurons Large deletions andinteracting protein 1) truncating mutations

3p21 Unknown Unknown

5p13.1–p12 GDNF (glial cell line-derived Missense mutationsneurotropic factor)

10q11.2 RET (receptor tyrosine kinase) Mainly missense mutations.Also nonsense, splice-site, andframe-shift mutations. All haveprobable loss of function effect

13q22 EDNRB (endothelin receptor, Mainly missense and type B) nonsense mutations

19q12 Unknown Unknown

20q13.2–q13.3 EDN3 (endothelin 3) Missense and nonsense mutations

22q13 SOX-10 (sex-determining- Nonsense mutationsfactor-related box 10)



endothelin-induced signaling pathway is active in the development and migration of colonic neural crest cells. Mutations in one or more of these genes are sufficient to cause disease. The relationship betweengenotype and phenotypic expression remains unclear.

RET is also implicated in S-HSCR but with a lower incidence of detectedgerm line mutations. As yet unknown genes at the other loci (3p21 and 19q12) appear to act as modifiers of RET expression in causingS-HSCR. Similarly, an intragenic polymorphism (c135G/A) in RET alsoinfluences disease expression. Note that mutations in RET are alsoresponsible for multiple endocrine adenomatosis type 2 (multipleendocrine neoplasia [MEN]2; see p.94–6). Both HSCR and MEN2 have been described in the same individual or the same family.

HSCR is a common feature in children with an interstitial deletion ofchromosome 2q22. This is due to haploinsufficiency for SIP1, whichencodes a transcriptional repressor involved in the patterning of neuralcrest cells and the central nervous system. Children with this condition,sometimes referred to as Mowat–Wilson syndrome, usually show mentalretardation with microcephaly and facial dysmorphism.

Genetic diagnosis Specific mutation analysis is available on a very limited basis, mainlyand counseling in a research setting. Counseling still relies heavily on empiric risk

figures, which are higher for L-HSCR than for S-HSCR and areinfluenced by the sex of both the proband and the relative at risk. For example, the recurrence risk for the brother of a boy with L-HSCR is 16%, whereas the risk for the sister of a girl with S-HSCR is 3%. Note that L-HSCR and S-HSCR can occur within the same family.


180


11. Hematologic disorders

Fanconi Anemia 182

Glucose-6-Phosphate Dehydrogenase Deficiency 183

Hemophilia A 185

Hemophilia B 187

Hereditary Elliptocytosis 189

Hereditary Spherocytosis 190

Sickle Cell Anemia 193

α-Thalassemia 194

β-Thalassemia 197

von Willebrand Disease 198


Fanconi Anemia182

Fanconi Anemia(also known as: FA; Fanconi pancytopenia)

MIM See Table 1.

Clinical features The most consistent and characteristic feature is progressive irreversiblebone marrow failure, usually with childhood onset, resulting in anemia,leucopenia, and thrombocytopenia. Affected individuals also show anincreased incidence of malignancy, notably acute myeloid leukemia and squamous cell carcinoma. Approximately two thirds of cases haveone or more congenital malformations including radial-ray and cardiacdefects, microcephaly, microphthalmia, and genital anomalies. Otherfindings can include short stature and café-au-lait pigmentation.Average life expectancy is approximately 20 years.

Age of onset Malformations may be apparent at birth. Pancytopenia usually developsby 10 years of age.

Epidemiology The estimated worldwide incidence is approximately 1–5 in 1,000,000.There is a particularly high incidence (1 in 22,000) in white Afrikaans-speaking South Africans.



Molecular The letters A to G represent separate complementation groups pathogenesis identified on the basis of cell fusion studies looking at the correction

of hypersensitivity to cross-linking agents. Each group corresponds to a distinct FA disease gene, which produces a specific FA protein. Theseproteins form a nuclear complex comprised of proteins A, B, C, E, F, and G, which is activated by DNA damage. This in turn activates the D2 protein, which is targeted to nuclear foci where it interacts with theBRCA1 protein, among others, to repair DNA damage and particularlyinterstrand cross-links. Group A accounts for around 60% of all cases; it may be more common than the other forms because the high incidenceof homopolymeric tracts in FANCA predisposes to microdeletion formationthrough misalignment. A specific mutation in FANCC shows a carrier


Hematologic Disorders 183

Table 1. Fanconi anemia: MIM numbers, groups, chromosomal locations, genes, and mutational spectra.

frequency of approximately 1 in 150 in Ashkenazi Jews and is associatedin homozygous form with a high incidence of malformations and earlyonset of anemia.

Genetic diagnosis Specific mutation analysis is available on a very limited basis and and counseling usually in a research setting. The diagnosis is usually made by

demonstrating hypersensitivity to agents such as mitomycin C anddiepoxybutane, which induce cross-links, resulting in increasedbreakage in metaphase chromosomes. Counseling is as for autosomal recessive inheritance.

Glucose-6-Phosphate Dehydrogenase Deficiency(also known as: G6PD deficiency; favism)

MIM 305900

Clinical features These are variable and show a close correlation with the underlyingmutation and its effect on G6PD activity and stability (see Table 2).Generally, hemolysis only occurs after exposure to drugs with a directoxidant action (eg, primaquine, dapsone, nitrofurantoin) or duringsevere intercurrent illness, such as infection or diabetic ketoacidosis.

MIM Group Chromosomal Gene Mutationlocation

227650 A 16q24.3 FANCA (Fanconi anemia Mainly microdeletions and insertionscomplementation group A)

227660 B 13q12.3 BRCA2 (breast cancer 2, early onset) Nonsense and frame-shift mutations

227645 C 9q22.3 FANCC (Fanconi anemia Missense, nonsense, and complementation group C) splice-site mutations. Also deletions

and insertions

605724 D1 13q12.3 BRCA2 (breast cancer 2, early onset) Missense and frame-shift mutations

227646 D2 3p25.3 FANCD2 (Fanconi anemia Missense and nonsense mutationscomplementation group D2) Also deletions and insertions

600901 E 6p22–p21 FANCE (Fanconi anemia Missense, nonsense, and complementation group E) splice-site mutations

603467 F 11p15 FANCF (Fanconi anemia Deletions and nonsense mutationscomplementation group F)

602956 G 9p13 FANCG = XRCC9 (X-ray repair Nonsense and splice-site mutationscomplementing defective, in Chinese and deletionshamster, 9)


Glucose-6-Phosphate Dehydrogenase Deficiency184

Table 2. Functional classification of glucose-6-phosphate dehydrogenase (G6PD) deficiency.

Neonatal jaundice leading to kernicterus can occur with class II variants.Ingestion of the fava bean causes hemolysis (favism) in G6PD-deficientCaucasians and Asians, but only rarely in those of African origin.

Age of onset From birth onwards in severe cases

Epidemiology The worldwide distribution is similar to that of malaria with incidencesof 10%–20% in Africans and African Americans, 2%–20% inMediterranean regions, and 10%–15% in the Middle East andSoutheast Asia.



Gene G6PD

Mutational Very heterogeneous. Almost all mutations are missense, spectrum with a few small in-frame deletions.

Molecular G6PD is 18-kb in length and contains 13 exons that encode thepathogenesis 515-amino-acid G6PD enzyme. G6PD oxidizes glucose-6-phosphate

to 6-phosphogluconolactone to generate the only source of NADPHavailable for red blood cells to prevent damage by oxidation. Normally, as red blood cells age the intracellular activity of G6PD decreases. This decline in activity is exacerbated by reduced stability of the G6PDA– variant, which is common in African Americans, and resultsfrom the presence of two substitutions: Asn126Asp and either Val68Metor Arg227Leu. (Presence of only the Asn126Asp substitution converts the normal G6PDB to G6PDA+ and is of no clinical significance.) This decline in G6PDA– stability, and hence activity, in older red blood

Class G6PD activity Clinical features Examples

I Very low Chronic nonspherocytic G6PDChicago

hemolytic anemia

II <10% Some prolonged, intermittent hemolysis G6PDMediterranean

III 10%–60% Self-limited, moderate, G6PDA–

intermittent hemolysis

IV Normal None G6PDB, G6PDA+



cells results in a population of cells susceptible to oxidant damage andexplains why attacks of hemolysis are relatively mild and self-limiting.

In contrast, the defect in G6PDMediterranean (caused by a Ser188Phesubstitution) results in G6PD instability in red blood cells of all ages, so that spontaneous recovery is much slower. The fact that mutationsthat would result in complete loss of function (such as frame-shifts) are never observed, indicates that zero enzyme activity is not compatible with survival.

Genetic diagnosis Specific mutation analysis is not routinely available. Diagnosis is usuallyand counseling made on the basis of the clinical and family history, the presence of

Heinz bodies during hemolytic episodes, and specific enzyme assayusing commercially available kits. Carrier detection based on enzymeassay is unreliable because of random X-chromosome inactivation.Microscopic examination of individual cells on a blood film is preferable.Counseling is on the basis of X-linked recessive inheritance, withemphasis on the importance of avoiding drugs and other agents that can induce hemolysis.

Hemophilia A(also known as: classic hemophilia; factor VIII deficiency)

MIM 306700

Clinical features There is a wide spectrum of severity depending on the level of factor VIIIactivity in plasma. Factor VIII levels less than 1% of normal are seen inthe 50% of patients with severe disease who present with spontaneoushemorrhage into joints, muscles, and internal organs. Levels of between1% and 5% result in moderate disease, with excessive bleeding afterminor trauma. Levels of between 5% and 25% cause mild disease with abnormal bleeding only after major trauma or surgery.

Age of onset Children with severe disease present soon after birth with bleeding from the umbilicus or in infancy with spontaneous hemarthrosis.

Epidemiology The incidence in males is between 1 in 5,000 and 1 in 10,000. All ethnic groups are affected.



Hemophilia A186

Figure 1. How the common hemophilia A mutation is generated by a crossoverwithin a loop or “flip” inversion. Gene A represents homologous sequences within the factor VIII gene and at a subtelomeric position on the same arm of the X chromosome.


Gene F8 (also known as F8C [factor VIII])

Mutational A common “flip inversion” accounts for 40%–50% of all serious casesspectrum (see Figure 1). The remaining cases show marked heterogeneity, with

missense, nonsense, and splice-site point mutations, together with large and small deletions and insertions. Missense mutations are more common in patients with mild disease and in those who arecross-reacting material (CRM) positive (ie, those who produce normallevels of a nonfunctional protein).

Molecular F8C is a large gene with 26 exons and a 9-kb mRNA transcript. Factorpathogenesis VIII is synthesized in the liver and circulates in the plasma bound to

von Willebrand factor for stability. The factor VIII protein contains sixstructural domains (A1–A2–B–A3–C1–C2), the largest of which, the B domain, is excised during activation to form heavy and light chains.Cleavage at specific sites (Arg372 and Arg1689) by thrombin is necessary

Centromere Gene ATelomere

Factor VIIIgene

Normal X chromosome

Centromere

Crossoversite

CentromereTelomere

Recombinant X chromosome

“Flip” inversion in meiosis I



for activation. This allows factor VIII to bind to a phospholipid surfaceand interact with factor IXa to form the X-ase complex necessary for the activation of factor X.

All mutations interfere with the production, stability, or function of factorVIII coagulation activity. Approximately 5% of patients are CRM positive,and around half of these have a CpG missense point mutation. Mostpatients with severe disease are CRM negative, in that they do notproduce a protein with either antigenic or coagulation activity. Suchindividuals are at risk of developing antibodies to factor VIII followingreplacement therapy.

Genetic diagnosis Diagnosis of the common “flip” inversion and general mutation and counseling screening are widely offered as a service. Linkage analysis using

intragenic polymorphisms can be used when a specific mutation cannotbe identified. Carrier detection by molecular analysis is more reliablethan older methods based on assay of antigenic and coagulation activity.

Around 30% of cases are isolated. As most point mutations and almostall “flip” inversions originate in a male meiosis, it is prudent to assumethat the mother of an isolated case with one of these mutations is a carrier until proven otherwise.

Hemophilia B(also known as: Christmas disease; factor IX deficiency)

MIM 306900

Clinical features The clinical features are indistinguishable from those seen in hemophiliaA, with levels of factor IX activity less than 1% causing severe diseaseand levels of 1%–5% causing moderate disease.

Age of onset Severe disease can present soon after birth or later in infancy.

Epidemiology The incidence in males is approximately 1 in 30,000 to 1 in 50,000 in all ethnic groups.


Chromosomal Xq27.1–q27.2location


Hemophilia B188

Gene F9 (factor IX)

Mutational Numerous point mutations (>1,500) include missense, nonsense,spectrum splice-site, and CpG dinucleotide changes. There are also deletions

and insertions.

Molecular The factor IX gene spans 34 kb and contains eight exons that encode pathogenesis a 461-amino-acid precursor factor IX protein. This contains a signal

peptide followed by a propeptide domain, which undergoes vitamin K-dependent posttranslational modification. The mature factor IXglycoprotein circulates in blood as a zymogen and contains an amino-terminal domain with α-carboxyglutamic acid residues and two epidermal growth factor domains. Factor IX is activated to factor IXa by cleavage at Arg145/Arg146. Factor IXa assembles with activated factor VIII to form the X-ase complex responsible for the cleavage of factor X to its active form, factor Xa.

Most missense mutations interfere with an important step in factor IXprotein formation or function such as posttranslational modification,zymogen activation, or protein assembly. Point mutations in the promoterregion close to the major transcription start site are associated with theunique factor IX Leyden form of hemophilia B. This presents as severedisease in childhood with spontaneous remission at puberty. Patientswith deletions resulting in severe disease with no factor IX proteinproduction are at increased risk of developing “inhibitor” antibodiesfollowing replacement therapy.

Genetic diagnosis Mutation detection by direct DNA sequence analysis is offered at severaland counseling clinical reference laboratories. When a specific mutation cannot be

identified, linkage analysis using intragenic polymorphisms can be usedfor carrier detection and, if appropriate, prenatal diagnosis. Molecularanalysis for carrier detection is much more reliable than assay of antigento coagulation ratio or coagulation activity as the latter is influenced by random X-chromosome inactivation.



Hereditary Elliptocytosis (also known as: HE)

MIM 130500

Clinical features HE constitutes a group of inherited disorders characterized byoval-shaped red blood cells (elliptocytes) in peripheral blood. In “common HE”, many affected individuals are asymptomatic, while a minority have a mild hemolytic anemia with splenomegaly and occasional gall stones. In “homozygous HE”, there is moderate to severe hemolytic anemia with elliptocytes and poikilocytes in theblood film. In “hereditary pyropoikilocytosis” (HPP, MIM 266140), there is severe hemolytic anemia with splenomegaly and increasedosmotic fragility. Individuals with homozygous HE and HPP usuallybenefit from splenectomy.

Age of onset Hematologic abnormalities are present at birth.

Epidemiology Common HE affects approximately 1 in 200 individuals in parts ofAfrica and Southeast Asia where malaria is endemic. The incidencein Caucasians is 1 in 2,000 to 1 in 4,000. Homozygous HE and HPPare rare.

Inheritance Autosomal dominant (common HE), autosomal recessive (homozygous HE and HPP)


Table 3. Hereditary elliptocytosis: chromosomal locations, genes, and mutational spectra.


1p36.2–p34 EPB41 (erythrocyte membrane Missense mutations, deletions,protein band 4.1 = protein 4.1) and duplications

1q21 SPTA1 (α-spectrin) Mainly missense mutations

14q22–q23.2 SPTB (β-spectrin) Missense mutations and deletions

17q21–q22 AE1 (anion exchanger Common 27-bp deletionmember 1 = band 3)


Hereditary Spherocytosis190

Molecular All forms of HE are caused by defects in the red blood cell cytoskeleton pathogenesis (see next entry on hereditary spherocytosis), in which actin and protein

4.1 interact with spectrin at the junction of the spectrin heterotetramersto maintain both the shape and the deformability of the cell. CommonHE is caused by mutations in EPB41 (40%) or SPTA1 (60%). EPB41 islinked to the Rh locus on chromosome 1, and HE used to be subdividedinto types 1 and 2 on the basis of linkage or nonlinkage to this locus.Codon 28 is a hotspot for mutations in SPTA1, with Arg28 being criticalfor spectrin heterodimer stability. Individuals with homozygous HE areusually homozygotes or compound heterozygotes for missense mutationsin SPTA1, resulting in severely impaired spectrin assembly. HPP canalso be caused by homozygosity or compound heterozygosity for SPTA1mutations or by a single mutation in SPTA1, together with anothermutation influencing spectrin stability.

A specific form of elliptocytosis (known as Southeast Asian ovalocytosis)occurs at a high frequency in Malaysia, the Philippines, and Papua NewGuinea. This is caused by a deletion of nine amino acids in the boundaryof cytoplasmic and membrane domains of the band 3 protein, whichbecomes entangled in the cytoskeleton, leading to increased cell rigidity.The relatively high incidence of this particular deletion is explained by the observation that it conveys protection to cerebral malaria.

Genetic diagnosis Mutation analysis is available only in a research setting. The diagnosisand counseling is made on the presence of elliptocytes with or without poikilocytes in a

peripheral blood film, supplemented by hematologic indices and osmoticfragility studies. Counseling is on the basis of autosomal dominant (HE)or autosomal recessive (homozygous HE and HPP) inheritance.

Hereditary Spherocytosis (also known as: HS)

MIM 182900

Clinical features HS constitutes a group of disorders in which the red blood cell cytoskeletonis defective, resulting in impaired cell membrane stability. Presentationcan be with jaundice in neonates or with gall stones, episodic anemia, or splenomegaly in childhood or adult life. Most patients are only mildly to moderately affected, with minimal disability. Splenectomy, normally



delayed until after the age of 5 years to reduce the risk of pneumococcalinfection, is usually curative.

Age of onset Spherocytes are present in peripheral blood at birth.

Epidemiology The incidence in individuals of Northern European origin is approximately1 in 5,000. HS is much less common in other ethnic groups.

Inheritance Usually autosomal dominant. Up to 25% of cases result from autosomalrecessive inheritance.


Table 4. Hereditary spherocytosis (HS): chromosomal locations, genes, and mutational spectra.

Molecular The basic defect in HS lies in the red blood cell cytoskeleton. Thispathogenesis consists of a submembranous network composed mainly of actin

and a complex of α- and β-spectrin. This cytoskeleton is anchored to the cytoplasmic surface of the red cell lipid bilayer plasma membraneby ankyrin, which binds to β-spectrin in the cytoskeleton and band 3 in the cell membrane. Band 3 is a membrane-spanning protein that isinvolved in the transport of inorganic anions. Defects in the cytoskeletonlead to inadequate support for the lipid bilayer leading to a loss ofmembrane surface area which converts the red cells from biconcavediscs to spherocytes (see Figure 2). Spherocytes become trapped in the spleen (because of their reduced flexibility) where they areremoved by macrophages. This in turn leads to hemolysis and anemia.


1q21 SPTA1 (α-spectrin) Mainly missense mutations

8p11.2 ANK1 (ankyrin 1) Frame-shift and nonsense null mutations in autosomal dominant HS; missense and promoter mutations in autosomalrecessive HS

14q22–q23.2 SPTB (β-spectrin) Frame-shift and large deletions

17q21–q22 AE1 (anion exchanger Missense and nonsense point mutations,member 1, also known also duplicationsas solute carrier family4 and band 3)


Hereditary Spherocytosis192

Figure 2. Blood film from a patient with spherocytosis (courtesy of Dr ClaireChapman, Department of Haematology, Leicester Royal Infirmary, UK).

The precise relationship between genotype and phenotype is not fullyunderstood. Inheritance is autosomal dominant in about 75% of cases.These autosomal dominant cases are caused mainly by mutations inANK1 and SPTB, with AE1 mutations accounting for around 10% ofcases. Autosomal recessive inheritance is associated with mutations in ANK1, SPTA1, and AE1. The severity of HS appears to be modified by factors other than the primary gene defect.

Note that mutations in AE1 are also responsible for some forms ofhereditary renal tubular acidosis and Southern Asian ovalocytosis (MIM 166900).

Genetic diagnosis Specific mutation analysis is only available on a very limited research and counseling basis. The diagnosis is based on the presence of spherocytes on a

blood film (see Figure 2), an elevated mean corpuscular hemoglobinconcentration (>36%), and increased red blood cell osmotic fragility.Counseling is usually on the basis of autosomal dominant inheritancewith variable expression. Autosomal recessive inheritance should besuspected in severely affected patients.



Sickle Cell Anemia(also known as: sickle cell disease)

MIM 603903

Clinical features Affected children present with any or all of the classical triad of anemia,vaso-occlusive crises, and infection. The anemia results from sickling (see Figure 3) and hemolysis of red blood cells when oxygen tension isreduced. Clumping of sickled red blood cells causes painful vaso-occlusivecrises, which can result in ischemic damage to the limb bones, lungs,brain, kidneys, heart, and spleen. Functional asplenia contributes toincreased susceptibility to bacterial infection, particularly Streptococcuspneumoniae and Hemophilus influenza. In the worst cases, both qualityof life and overall life expectancy are severely compromised.

Figure 3. Blood film showing sickle-shaped red cells (courtesy of Dr ClaireChapman, Department of Haematology, Leicester Royal Infirmary, UK).

Age of onset Presentation is usually between the ages of 6 and 12 months.

Epidemiology The frequency of the hemoglobin (Hb)S carrier state, known as thesickle cell trait, is high in all populations that originate from areas inwhich malaria is, or has been, endemic. Typical frequencies are 20% in Africa, 8% in African Americans, and 5% in the Middle East andMediterranean regions.

HbC and HbD can also cause sickle cell anemia in the compoundheterozygous state. In Africa HbC carrier rates are similar to those for HbS. HbD has a carrier frequency of around 3% in parts of India.

Inheritance Sickle cell disease shows autosomal recessive inheritance. Sickle celltrait (ie, the heterozygous state) can be considered to show dominantinheritance as there is evidence that the carrier state can occasionally be symptomatic.


α-Thalassemia194


Gene HBB (hemoglobin β-globin gene)

Mutational spectrum Missense mutations, ie:HbS GAG (Glu6) → GTG (Val6)HbC GAG (Glu6) → AAG (Lys6)HbD GAA (Glu121) → CAA (Gln121)

Molecular The substitution of value for glutamic acid at the β6 residue (the sixth pathogenesis amino acid residue in the β chain) results in the formation of an α2β2

tetramer which is unstable in the deoxygenated form. When the oxygensaturation falls below 85%, α2β2 tetrameric polymers form rod-likestructures that cause red cells to become sickle-shaped. This, in turn,leads to increased blood viscosity (with clumping of sickled red bloodcells causing a risk of vaso-occlusion) and hemolysis of the deformedsickle cells (resulting in rapid red blood cell turnover and anemia).

Genetic diagnosis Specific mutation analysis for HbS is readily available by eitherand counseling polymerase chain reaction (using allele-specific oligonucleotides) or

Southern blotting (utilizing the fact that the mutation alters an MstIIcleavage site). Counseling is as for autosomal recessive inheritance.Population screening programs have been introduced with varyingsuccess in areas with a high incidence of carriers.

The prediction of phenotype in compound heterozygotes for different HBB mutations can be very difficult. Generally, compoundheterozygosity for HbS with either HbD or βº thalassemia results inmoderate to severe sickle cell disease. HbS with HbC tends to result in milder sickle cell disease.

αα-Thalassemia(includes hemoglobin H [HbH] disease)

MIM 141800

Clinical features These depend on how many of the four normal α-globin genes areintact. If all four are nonfunctional then there is profound intrauterineanemia, which manifests as hydrops fetalis with ascites and generalized



edema. This results in spontaneous pregnancy loss or neonatal death.Most of the hemoglobin (Hb) in these infants is Hb Barts (γ4 tetramers).Loss of three functional α-globin genes results in moderately severeanemia (Hb 8–10 g/100 mL) known as HbH (β4 tetramers) disease.Loss of two functional α-globin genes (α-thalassemia-1 or α-thalassemiatrait) leads to mild anemia (Hb 10–12 g/100 mL), which may beasymptomatic. Loss of a single functional α-globin gene is totallyasymptomatic (“silent carrier”).

Age of onset Hydrops fetalis presents in the second or third trimester. HbH diseaseusually presents in childhood with anemia and hemolysis.

Epidemiology Carrier frequencies are as high as 25% in parts of Africa, 20% inSoutheast Asia, and 5%–10% in Mediterranean regions.

Inheritance Hydrops fetalis and HbH disease show autosomal recessive inheritance.Mild disease associated with loss of two functional alleles can showautosomal dominant or recessive inheritance.

Chromosomal 16pter–p13.3location

Gene HBA (hemoglobin α-globin gene cluster)

Mutational Mainly deletions involving one or both pairs of α-globin genes. spectrum Missense, nonsense, splice-site, and frame-shift mutations have

also been found. These result in abnormal RNA processing ortranslation, or posttranslational instability.

Molecular Normally there are two closely contiguous α-globin genes on the short pathogenesis arm of each chromosome 16. Most cases of α-thalassemia are caused

by deletions resulting in loss of two (α-thalassemia-1), three (HbHdisease), or all four (hydrops fetalis) of these genes. The homology in and around all four α-globin genes predisposes to misalignment in meiosis I, with unequal crossing-over generating chromosomes with one and three genes instead of the normal two (see Figure 4).Selection for the clinically silent heterozygous state (ie, three α-globingenes) through relative immunity to malaria is thought to account for the high frequency of carriers in areas where malaria is endemic.


α-Thalassemia196

Figure 4. Diagrammatic representation of the α-globin gene cluster onchromosome 16. (a) shows normal alignment in meiosis I. (b) showsmisalignment with a crossover, which results in the recombinant deletion and triplication chromosomes shown in (c).

Subsequent unequal crossover events or larger deletions result in loss of both α-globin genes from a single chromosome. Thus, the heterozygousstate of two normal genes, also referred to as α-thalassemia-1 or α-thalassemia trait, can result from either an α–/α– genotype (deletionsin trans) or an αα /– – genotype (deletions in cis). The αα /– – genotype is mainly seen in Southeast Asia, which explains why hydrops fetalis,caused by a – –/– – genotype, is much more common in this populationthan in Africa, where heterozygotes usually have an α–/α– genotype.The clinical consequences of anoxia, hemolysis, and anemia areattributable to an imbalance in the ratio of α-globin to β-globin chainproduction, as illustrated by the presence of HbH (β4 tetramers)inclusions in red blood cells.



Genetic diagnosis Specific α-globin gene deletion or point mutation analysis is undertakenand counseling at a few specialized reference laboratories. Screening programs

detect carriers by measuring red blood cell indices and carrying out Hb electrophoresis.

ββ-Thalassemia(also known as: Cooley’s anemia. Includes thalassemia intermedia and thalassemia major)

MIM 141900

Clinical features Children with severe β-thalassemia (also known as thalassemia major or βº-thalassemia – mutations can result in either partial [β+] or complete [βº] absence of β-globin) usually present with hypochromicmicrocytic anemia and require regular life-long transfusions to maintaina hemoglobin (Hb) level above 10 g/dL. Extramedullary expansionresults in hepatosplenomegaly with mild facial coarsening. Regulartransfusion results in iron overload, leading to potentially irreversibledamage to the heart, liver, pancreas, and endocrine glands. Thesecomplications can be largely prevented by strict adherence to a rigorousregimen of iron chelation therapy. Children with β+-thalassemia tend to have milder anemia, which is not always transfusion dependent(thalassemia intermedia). Heterozygotes (β-thalassemia trait) are asymptomatic.

Age of onset Usually after 3 months of age when the β-globin gene becomes fully expressed.

Epidemiology The carrier frequency is 10%–20% in the Mediterranean region andapproximately 5% in the Indian subcontinent. Lower carrier frequenciesare observed in China, Southeast Asia, and the Middle East.


Chromosomal locus 11p15.5

Gene HBB (hemoglobin β-globin gene)

Mutational spectrum The mutational spectrum is very heterogeneous, with over 170 differentmutations identified. These include missense and nonsense pointmutations, splice-site mutations, insertions, and deletions. Common


von Willebrand Disease 198

mutations exist specific for different ethnic groups. Many mutationsreduce the quantity of mRNA (eg, promoter, RNA splicing and mRNAcapping or tailing mutations).

Molecular The reduction in β-globin chain synthesis results in an α to β chainpathogenesis imbalance, with excess α chains forming insoluble aggregates in erythroid

precursors in the bone marrow. This results in intramedullary hemolysisand ineffective erythropoiesis. The large numbers of erythroid precursorscause expansion of the bone marrow cavities and bone deformation.

δβ-thalassemia and hereditary persistence of fetal Hb are caused by large deletions involving both of the closely adjacent β- and δ-globin geneloci. Individuals who are homozygous for these deletions are only mildlyanemic because of compensatory increases in HbF production. Clinically,they are either asymptomatic or have mild thalassemia intermedia.

Genetic diagnosis Specific HBB mutation analysis is available on a limited basis becauseand counseling of the marked mutational heterogeneity. Linkage analysis using highly

polymorphic intragenic markers can be utilized for prenatal diagnosis in informative families. Carrier detection is achieved by measurement of red blood cell indices and Hb electrophoresis. Therapy with a compatible bone marrow transplant is potentially curative.

von Willebrand Disease (also known as: vWD; pseudohemophilia)

MIM 193400

Clinical features vWD is the most common inherited bleeding disorder and is classifiedinto three types on the basis of a quantitative defect (type I), a qualitativedefect (type II), or complete absence (type III) of von Willebrand factor(vWF). Type I (accounting for 70%–80% of all cases) is characterized by spontaneous bruising and mucosal bleeding from the nose andgastrointestinal tract, hemorrhage after surgery, and menorrhagia inwomen. Types IIA and IIB have a similar presentation. In types IIN and III, the clinical manifestations more closely resemble those seen in hemophilia A and B.

Age of onset Early childhood in severe forms (mainly type III)



Epidemiology Some studies have suggested incidence/prevalence figures as high as 1%, but the true figure is probably nearer 1 in 1,000.

Inheritance Autosomal dominant (types I, IIA and IIB); autosomal recessive (types IIN and III)


Gene VWF (von Willebrand factor)

Mutational Types I and II. Marked mutational heterogeneity with missense and spectrum nonsense point mutations, together with frame-shifts and deletions.

Type III. Mainly deletions and nonsense or frame-shift mutations.

Molecular VWF is composed of 52 exons spanning 178 kb. It encodes a pathogenesis 2,813-amino-acid protein, which contains four repeated domains

(designated A–D) that are present in multiple copies. Specific domainsare responsible for specific interactions. A1–A3 interact with collagen,heparin, and platelets; C1–C2 interact with platelets; D1–D3 interactswith heparin and factor VIII. VWF acts as a stabilizer for factor VIII and as an adhesive link between platelets and the blood vessel wall at sites of vascular injury.

Types I and II vWD are distinguishable on the basis of a quantitative or a qualitative defect in the VWF. In types IIA, IIB, and IIN, mutationsinvolve the A2, A1, and D1–3 domains, respectively. Thus, collagen and platelet binding are affected in types IIA and IIB, whereas in type IIN, the phenotype resembles that of mild hemophilia A.

In type III vWD, which can be considered as the homozygous state for type I, VWF is absent in the blood, resulting in the clinical features of both vWD and hemophilia A.

Genetic diagnosis Mutation analysis is available at a small number of specialized and counseling laboratories, although often a specific mutation cannot be identified.

Prenatal diagnosis has been achieved in a few families with severe type III disease using linkage analysis. The diagnosis of vWD is usuallymade using standard coagulation assays (factor VIII activity, vWFantigen level, ristocetin cofactor assay of vWF functional activity).Counseling in types I, IIA, and IIB is on the basis of autosomal dominant inheritance with incomplete penetrance.


200


12. Immunologic Disorders

Bruton Agammaglobulinemia 202

Chronic Granulomatous Disease 203

Severe Combined Immunodeficiency 205

Wiskott–Aldrich Syndrome 207


Bruton Agammaglobulinemia202

Bruton Agammaglobulinemia(also known as: X-linked agammaglobulinemia)

MIM 300300

Clinical features These reflect an isolated B-cell defect that manifests as multiplerecurrent bacterial infections such as pneumonia and meningitis.Without treatment affected boys develop chronic lung disease. They are also more susceptible to viral infections such as hepatitis, and to the development of rheumatoid-like arthritis. Treatment with regularinjections of gammaglobulin significantly reduces the incidence ofpyogenic infection and dramatically improves the long-term prognosis.

Age of onset Infection usually first occurs after the age of 3 months as the level of maternally acquired immunoglobulin G (IgG) declines.

Epidemiology The incidence in males is approximately 1 in 100,000.


Chromosomal Xq21.1–q22location

Gene BTK (Bruton tyrosine kinase)

Mutational Over 300 different mutations have been reported. These include spectrum missense and nonsense point mutations, as well as frame-shift

insertions and deletions. A mutational database is maintained athttp://www.uta.fi/imt/bioinfo.

Molecular BTK consists of 19 exons. These encode a 659-amino-acid cytoplasmicpathogenesis tyrosine kinase, known as Btk, which is expressed throughout B-cell

and myeloid differentiation. Btk is one of a family of tyrosine kinases that are activated by growth factors and which contain several domains.Most missense mutations involve the kinase domain. Other mutationsare distributed throughout the gene. No clear correlation has beenestablished between the specific mutation and the severity of thephenotype. The precise role of Btk in B-cell differentiation andproliferation is unknown.


Immunologic Disorders 203

Note that a specific splice-site mutation in BTK causes a rare form of Bruton agammaglobulinemia associated with growth hormonedeficiency (see p.165, MIM 307200).

Genetic diagnosis Mutation detection is offered by several laboratories, with 90%–95%and counseling of all mutations being detectable by single-strand conformation

polymorphism screening of genomic DNA. A small proportion ofmutations can only be detected by complementary DNA analysis orSouthern blotting. When a specific mutation has been identified themost desirable approach is to use mutation analysis for female carrierdetection. Alternatively, an attempt can be made to demonstratenonrandom X-chromosome inactivation in B cells using a double digest with a methylation-sensitive restrictive enzyme. Life-longtreatment with gammaglobulin should be instituted in males as soon as the diagnosis is made.

Chronic Granulomatous Disease (also known as: CGD)

MIM 233690 (cytochrome-b α-subunit deficiency)233700 (neutrophil cytosolic factor 1 deficiency)233710 (neutrophil cytosolic factor 2 deficiency)306400 (cytochrome-b β-subunit deficiency)

Clinical features CGD represents a group of disorders characterized by an inability ofneutrophils to kill bacteria, resulting in recurrent bacterial infection.Phagocytosis proceeds normally, but a defect in the cell’s ability togenerate activated oxygen radicals means that bacteria that do notgenerate hydrogen peroxide cannot be killed. Affected children presentwith recurrent infection, including abscesses, dermatitis, and pneumonia.Untreated CGD results in severe failure to thrive and ultimately death.

Age of onset Onset is in early infancy in typical severe cases. Milder cases canpresent in childhood or adult life.

Epidemiology The estimated worldwide incidence is approximately 1 in 250,000.

Inheritance Approximately 70% of cases show X-linked recessive inheritance. The remaining 30% are autosomal recessive.


Chronic Granulomatous Disease 204


Table 1. Chronic granulomatous disease: chromosomal locations, genes,

and mutational spectra.

Molecular CGD results from a defect in the neutrophils’ ability to convert oxygen topathogenesis superoxide. This process is mediated by NADPH oxidase, which involves

a small transmembrane electron-transport system. This results in theoxidation of NADPH and the generation of superoxide. The superoxidebecomes intracellular when invagination occurs during phagocytosis.Activation of oxidase requires an assembly of components, includingcytochrome-b (made up of α and β chains, encoded by CYBA and CYBB,respectively) and two neutrophil cytosolic proteins (p47-phox encodedby NCF1 and p67-phox encoded by NCF2). Mutations in any of thesegenes can result in the CGD phenotype.

Most cases (70%) of CGD are caused by mutations in CYBB. This holds a position of distinction in human genetics as the first disease gene to be identified by positional cloning (in 1986). The mutational spectrum is wide and includes “large” contiguous gene deletions, embracing theclosely adjacent loci for Duchenne muscular dystrophy (see p.4–6, MIM 310200) and the McLeod phenotype (MIM 314850), whichinvolves absence of a specific red blood cell protein and reduced levels of Kell blood group antigens. Approximately two thirds of autosomalrecessive cases of CGD are caused by homozygosity or compoundheterozygosity for a dinucleotide deletion in NCF1. This deletion results from misalignment with recombination between NCF1and its highly homologous pseudogenes, which carry the deletion.


1q25 NCF2 (neutrophil cytosolic Missense, nonsense and splice-sitefactor 2) mutations. Also insertions

and deletions

7q11.23 NCF1 (neutrophil cytosolic Mainly a 2-bp deletionfactor 1)

16q24 CYBA (cytochrome-b Mainly missense mutationsα-subunit)

Xp21.1 CYBB (cytochrome-b Very heterogeneous, with all formsβ-subunit) of mutations distributed throughout

the gene and its regulatory regions



Genetic diagnosis Specific mutation analysis and immunoblot assay for the relevant and counseling proteins are undertaken at a small number of specialist reference

laboratories. The diagnosis is usually made using the nitrobluetetrazolium test, which identifies neutrophils that cannot generatesuperoxide. Carrier detection for the X-linked form can be undertakenby examination of peripheral blood neutrophils for a subpopulation ofabnormal cells or, more reliably, by direct mutation analysis. In a largeseries of 131 kindreds, the mother was found to be a carrier in 89% of cases.

Severe Combined Immunodeficiency (also known as: SCID; Swiss-type agammaglobulinemia)

MIM 102700 (adenosine deaminase [ADA] deficiency)164050 (purine nucleoside phosphorylase [PNP] deficiency)300400 (X-linked form)

Clinical features Severe defects in both cellular and humoral immunity result in early susceptibility to bacterial, viral, and fungal infections. Infantsusually present with recurrent and persistent diarrhea, pneumonitis, and dermatitis. Common infectious agents include candida andPneumocystis carinii. Immunization with live agents (such as BCG [bacille Calmette–Guérin] and polio) can be life-threatening.Without treatment, the disease course is rapidly progressive with death by the age of 1–2 years. Milder forms present later and have a much better prognosis.

Age of onset Usually around the age of 2–3 months with recurrent infection

Epidemiology At the age of 1 year the estimated prevalence of the autosomal recessiveand X-linked recessive forms is approximately 3 in 1,000,000. Muchhigher figures have been noted in genetic isolates, such as the Mennonitesin Manitoba and in some native North American Indians. The X-linkedform accounts for around 50% of all cases.

Inheritance, See Table 2.chromosomallocation, gene, and mutational spectrum


Severe Combined Immunodeficiency 206

Table 2. Severe combined immunodeficiency: inheritance, chromosomal locations, genes,and mutational spectra.

Molecular In the USA it has been estimated that approximately 50% of SCID pathogenesis cases are of the X-linked form. The remainder show autosomal recessive

inheritance, with ADA and PNP deficiency accounting for around 15% of all cases. The remaining 35% of cases can be caused by anyone of several individually rare autosomal recessive immune defects,including major histocompatibility complex class I and II deficiency(bare lymphocyte syndrome [MIM 209920]) and interleukin (IL)-2receptor abnormalities.

ADA consists of 12 exons and 11 introns, with a large number of Alurepetitive elements that predispose to the formation of tiny deletionsthrough homologous recombination. The enzyme is present in alltissues, with highest activity in the thymus and other lymphoid tissues.Enzyme activity is usually reduced to less than 1% of normal in the celllines of affected patients. PNP consists of six exons and five introns andis ubiquitously expressed, with its highest levels in erythrocytes and thekidneys. Affected patients show extremely low levels of enzyme activity.With both ADA and PNP deficiency there is a correlation between thelevel of enzyme activity and clinical outcome, with levels greater than1% being associated with later onset of disease. Such levels are usuallycaused by intronic single base-pair substitutions, which cause exonskipping or activation of cryptic splice sites.

The precise mechanism by which ADA and PNP deficiency cause SCIDremains unclear. Normally, these enzymes catalyze sequential steps inthe metabolism of purine ribonucleosides and deoxyribonucleosides.

Form Inheritance Chromosomal Gene Mutational location spectrum

Adenosine Autosomal 20q13.11 ADA (adenosine Mainly missensedeaminase recessive deaminase) point mutations.deficiency Also small deletions

and splice-sitemutations

Purine Autosomal 14q13.1 PNP (purine Missense and nucleoside recessive nucleoside splice-site phosphorylase phosphorylase) mutations and deficiency small deletions

X-linked form X-linked Xq13 IL2RG Mainly missense recessive (interleukin-2 and nonsense

receptor, γ chain) mutations



The prevailing view is that substrate accumulation or alternativemetabolites have a toxic effect on lymphocyte differentiation andfunction, possibly by inhibition of ribonucleotide reductase, which isrequired for DNA replication in dividing cells. In reality, it is likely thatseveral different toxic effects contribute to the severe lymphocytopenia.

The relatively common X-linked form of SCID is caused by mutations in IL2RG, which encodes the γ chain of the IL-2 receptor. This subunit is shared with four other IL receptors, (IL-4, -7, -9, and -15), whichprobably accounts for the very severe phenotypic consequences ofIL2RG mutations. These include impaired growth and differentiation of T and B cells, and of cells of monocyte lineage. IL2RG consists ofeight exons with a wide spectrum of pathogenic missense and nonsensemutations that result in defective γ chains necessary for formation of the high and intermediate affinity IL cytokine family receptor.

Genetic diagnosis Diagnosis is usually made on the basis of reduced numbers of T cellsand counseling and very low levels of immunoglobulins. Specific mutation analysis for

ADA, PNP, and IL2RG, together with other known rarer genetic defects,is available at a small number of specialized reference laboratories. The mother is shown to be a carrier in around 50% of isolated cases of the X-linked form. If specific mutation analysis is not available for female carrier detection, this can be achieved by analysis of X-chromosome inactivation in T lymphocytes: carriers show anonrandom pattern. Hair root DNA has been used for analysis inchildren who have undergone successful bone marrow transplantationfrom a histocompatible sibling or other relative. Gene therapy usingautologous transduced hematopoietic stem cells has been appliedsuccessfully for ADA deficiency and the X-linked form.

Wiskott–Aldrich Syndrome (also known as: WAS)

MIM 301000

Clinical features These include the classic triad of thrombocytopenia, infection, andeczema, as well as a tendency to develop lymphoid malignancy. Thephenotype varies from severe infection with early death in childhood to a mild thrombocytopenia. Thrombocytopenia is the earliest feature,with platelets that are reduced in size and have a diminished half-life.


Wiskott–Aldrich Syndrome208

Eczema and infection with pyogenic bacteria develop in infancy inassociation with variable defects in humoral and cellular immunity.Other features can include nephropathy and hemolytic anemia. Bonemarrow transplantation restores normal platelet features and radicallyimproves the long-term outcome.

Age of onset Thrombocytopenia can present in the neonatal period.

Epidemiology WAS is rare, with an estimated incidence in the USA of 1 in 250,000male births.



Gene WAS, also known as WASP (WAS protein)

Mutational spectrum Missense mutations in exons 1–4. Nonsense, frame-shift, and splice-site mutations in exons 6–11.

Molecular WAS contains 12 exons and encodes a protein that is involved inpathogenesis both intracellular signaling and regulation of the actin cytoskeleton.

Its role in cell signaling, and hence growth and differentiation of earlyhematopoietic precursors, is mediated by interaction of the WAS proteinwith tyrosine kinases. Regulation of the cytoskeleton and cell movementinvolves interaction of the WAS protein with actin. Both intrafamilial and interfamilial variations have been demonstrated at the phenotypiclevel. Missense mutations resulting in reduced levels of a normally sizedWAS protein have been noted in mild X-linked thrombocytopenia (MIM313900), whereas nonsense and frame-shift mutations tend to result in the much more severe WAS phenotype. However, no consistentgenotype–phenotype correlation has emerged. Curiously, it has beenshown that an activating mutation in WAS can cause a rare X-linkedform of severe congenital neutropenia (MIM 300299).

Genetic diagnosis Mutation detection is available on a limited basis at designated and counseling specialized laboratories. This can also be utilized for prenatal

diagnosis and carrier detection. Carrier detection can be achieved by demonstrating nonrandom X-chromosome inactivation in B cells and T cells from peripheral blood. Counseling is on the basis of X-linked recessive inheritance.


13. Metabolic Disorders

Medium Chain Acyl-CoA Dehydrogenase Deficiency 210

Menkes Disease 211

Ornithine Transcarbamylase Deficiency 212

Phenylketonuria 214

Wilson Disease 215


Medium Chain Acyl-CoA Dehydrogenase Deficiency210

Medium Chain Acyl-CoA Dehydrogenase Deficiency(also known as: MCAD deficiency)

MIM 201450

Clinical features Presentation is usually with acute onset of hypoglycemia and vomitingleading to convulsions and coma, often following a period of fasting.Blood levels of ammonia are elevated and liver function tests areabnormal. The clinical presentation falls within the differential diagnosisof Reye’s syndrome and can be misdiagnosed as sudden infant deathsyndrome because of the high associated mortality.

Age of onset Usually between 3 months and 2 years. Some affected individuals first present in later childhood or adolescence and some remainasymptomatic throughout life.

Epidemiology The incidence in the UK and USA is approximately 1 in 15,000 to 1 in 20,000 live births. The carrier frequency is approximately 1 in 65.



Gene ACADM (acyl-CoA dehydrogenase, medium-chain)

Mutational spectrum Most mutations (90%) involve a 985A→G transition resulting in a Lys304Glu substitution (this is also referred to as Lys329Glu based on the precursor protein structure). The remaining 10% are mainlymissense, with small numbers of deletion and duplication frame-shifts.

Molecular ACADM contains 12 exons and encodes a 421-amino-acid proteinpathogenesis that catalyzes the initial reaction in the beta-oxidation of C4 to C12

straight-chain acyl-CoAs. This is one step in the complex sequence ofevents which releases energy through mitochondrial fatty acid oxidation.The common Lys304Glu mutation interferes with the assembly of themature homotetrameric enzyme and with its stability in mitochondria.Linkage disequilibrium studies are consistent with a common foundereffect. No straightforward genotype–phenotype correlation has emerged.


Metabolic Disorders 211

Genetic diagnosis Analysis of serum acylcarnitines by tandem mass spectrometry providesand counseling a rapid and reliable preclinical diagnosis and has been proposed as

a means of neonatal screening using dried blood spots. DNA analysis is widely available for the common mutation. Rare mutation detection by sequencing is undertaken on a very limited basis. Prenatal diagnosiscan be achieved by either DNA analysis or enzyme assay.

Menkes Disease(also known as: kinky hair disease)

MIM 309400

Clinical features The clinical features are a direct consequence of copper deficiency.Affected infants present with lethargy, failure to thrive, convulsions, and spasticity. Growth and development are severely delayed and death occurs in early childhood. Other characteristic features includehypothermia, Wormian bones, and fragile, steely, depigmented hair (see Figure 1), which on microscopy shows pili torti (see Figure 2).Some affected children have a less severe phenotype with longer survival.

Figure 1. Appearance of the hair Figure 2. Pili torti in a boy within a boy with Menkes disease. Menkes disease.

Age of onset Features are apparent at birth or develop in early infancy.

Epidemiology The incidence in live births has been estimated to be between 1 in 100,000 and 1 in 250,000.


Ornithine Transcarbamylase Deficiency212


Chromosomal Xq12–q13location

Gene ATP7A (Cu2+-transporting ATPase, α polypeptide) also known as MNK

Mutational spectrum Mainly insertions, deletions, nonsense, and splice-site mutations,resulting in protein truncation.

Molecular ATP7A contains 23 exons and encodes a 1,500-amino-acid pathogenesis copper-transporting ATPase with copper-binding, phosphorylation,

transduction, and ATP-binding domains. The protein is localized in themembrane of the trans-Golgi network. It cycles between the trans-Golgiand plasma membranes, facilitating the trafficking and cellular efflux of copper. Mutations result in accumulation of copper in the intestinalmucosa and kidney, with deficiency in other tissues, such as the centralnervous system. The mutational spectrum is very heterogeneous, with40% of patients in one series having truncating insertions or deletions.Splice-site mutations that allow a degree of normal mRNA processingresult in milder phenotypes, such as the occipital horn syndrome (MIM 304150, see p.112).

Genetic diagnosis The diagnosis is made by assay of serum copper and ceruloplasmin, and counseling both of which are low. Mutation and linkage analysis are available

in specific laboratories and are preferred to copper uptake studies for carrier detection and prenatal diagnosis because of the technicaldifficulties associated with copper uptake studies.

Ornithine Transcarbamylase Deficiency(also known as: OTC deficiency; ornithine carbamoyltransferase [OCT] deficiency)

MIM 311250

Clinical features In the classic severe form, affected male infants present soon after birth with lethargy, convulsions, coma, and severe hyperammonemia,leading to a rapidly fatal outcome. More mildly affected boys and someheterozygous females present later in childhood or in adult life withbehavioral disturbance and alterations of consciousness, particularlyafter a heavy protein load. Older patients often develop an aversion to



protein. These individuals usually have a good prognosis, but are at risk of developing hyperammonemia during intercurrent illness or episodes of fasting.

Age of onset Onset typically occurs after protein ingestion soon after birth.

Epidemiology The overall incidence has been estimated to be 1 in 80,000 live births.

Inheritance X-linked, with many female carriers showing mild manifestations


Gene OTC (ornithine transcarbamylase)

Mutational spectrum This is very heterogeneous, with over 200 unique mutations reported.Large deletions account for 10%–15% of all mutations. The rest consistmainly of missense point mutations with smaller numbers of nonsense,splice-site, and small deletion mutations.

Molecular OTC contains 10 exons and encodes the OTC enzyme, which catalyzespathogenesis the conversion of ornithine and carbamoyl phosphate to citrulline.

OTC is a homotrimeric mitochondrial enzyme that is expressed almostexclusively in liver. It is synthesized as a subunit in cytoplasm as aprecursor with an NH2 extension. The NH2 extension is required formitochondrial uptake prior to cleavage in the mitochondria. Mutationsare distributed throughout the gene and most are unique to individualfamilies. An exception is a recurring Arg109Gln substitution, whichaccounts for 10% of all missense mutations. Those mutations that result in severe neonatal disease are clustered in important functional or structural domains in the interior of the protein at sites of enzymeactivity or at the interchain surface. Mutations associated with late-onset disease are located on the surface of the protein.

Genetic diagnosis Biochemical methods are unreliable for carrier detection, so mutationand counseling analysis is the preferred method. This is undertaken at several specialist

laboratories. Prenatal diagnosis can only be achieved by liver biopsy,which is hazardous, or by molecular analysis based on specific mutationdetection or linkage. The male to female mutation rate has been shownto be very high (ie, most mutations originate in males), indicating that


Phenylketonuria214

the mothers of most isolated affected males are carriers. At present, only approximately 80% of all mutations can be identified suggestingthat some mutations probably occur in introns or control sequences.

Phenylketonuria(also known as: PKU; phenylalanine hydroxylase deficiency)

MIM 261600

Clinical features Children develop severe mental retardation with eczema andhypopigmentation of hair and skin. Neurologic sequelae includespasticity, hyperactivity, convulsions, and occasional autistic behavior.Careful dietary restriction of phenylalanine up to the age of 12 yearsresults in normal intellectual development.

Most children with elevated blood phenylalanine levels (detectedthrough neonatal screening programs) have either transient or mildhyperphenylalaninemia (enzyme level >5%). This usually conveys a good prognosis and does not require treatment.

Age of onset Congenital

Epidemiology The average incidence in western and northern European populations is 1 in 10,000 live births (the carrier frequency is 1 in 50). Lowerincidences of 1 in 50,000 and 1 in 143,000 have been noted in black Americans and the Japanese, respectively.



Gene PAH (phenylalanine hydroxylase)

Mutational This is very heterogeneous with over 350 separate mutations reported spectrum (www.mcgill.ca/pahdb). Most of these are missense, with small

numbers of splice-site and nonsense mutations.

Molecular PAH is 90 kb long and is transcribed into a 2.4-kb mRNA. It encodes pathogenesis the enzyme responsible for the conversion of phenylalanine to tyrosine.

The active enzyme exists as a trimer or tetramer – most mutations



impair enzyme activity by causing protein instability or abnormalaggregation. Mutations in the N-terminal domain show clustering in residues 46–48 and affect the ability of the enzyme to bind to phenylalanine.

Although many mutations have been reported, they tend to beassociated with a small number of haplotypes. There is some evidenceto support heterozygote advantage, possibly resulting from protectionagainst spontaneous miscarriage. Two of the most common northernEuropean mutations are an Arg408Trp substitution and a GT→ATsubstitution in the 5 splice site of intron 12, which results in skipping of exon 12 during RNA splicing.

Genetic diagnosis The diagnosis of PKU is based on conventional biochemical assay. and counseling However, the enzyme is only expressed in the liver so prenatal diagnosis

can be achieved only by molecular analysis (either specific mutationdetection or linkage analysis). Mutation detection is undertaken at a small number of laboratories.

In 1%–3% of children with severe hyperphenylalaninemia, the basicdefect lies not in PAH but in one of the enzymes involved in the synthesisor recycling of tetrahydrobiopterin (BH4), the cofactor for PAH in theconversion of phenylalanine to tyrosine. These enzymes includedihydropteridine reductase (MIM 261630), pterin 4-α-carbinolaminedehydratase (MIM 264070), and 6-pyruvoyltetrahydropterin synthase(MIM 261640). Treatment for these enzyme deficiencies, all of whichshow autosomal recessive inheritance, involves both a low phenylalaninediet and supplementation with neurotransmitters, which are deficient as a result of lack of BH4 (which acts as a cofactor for two otherenzymes, tyrosine hydroxylase and tryptophan hydroxylase).

Wilson Disease(also known as: hepato-lenticular degeneration)

MIM 277900

Clinical features Increased copper deposition in the brain, cornea, and liver, results inprogressive neurologic and liver disease. Neurologic involvement caninclude behavioral disturbance, dystonia, tremor, and spasticity, withoccasional frank psychiatric illness. Copper deposition in the cornea


Wilson Disease216

results in a characteristic brown–green collarette of discoloration knownas a Kayser–Fleischer ring. Hepatic involvement culminates in cirrhosisand liver failure. Other findings can include hemolytic anemia and renalFanconi syndrome.

Age of onset Late childhood or early adult life

Epidemiology The incidence worldwide is estimated to be between 1 in 30,000 and 1 in 55,000. Wilson disease is particularly common in Sardinia.



Gene ATP7B (Cu2+-transporting ATPase β polypeptide), also known as WND

Mutational Mainly missense mutations with a common His1069Gln mutation spectrum in European populations. Also nonsense mutations, deletions,

and insertions.

Molecular ATP7B encodes a 1,411-amino-acid copper-transporting ATPase withpathogenesis close homology to the Menkes syndrome copper-transporting protein

(see p.211–2). In the liver, ATPase is localized to the trans-Golgi network,where it is responsible for the transport of copper into the hepatocytesecretory pathways for incorporation into ceruloplasmin and for excretionof copper into bile. As with the Menkes syndrome protein, the WNDATPase contains multiple domains including copper-binding andATP-binding domains. The common European His1069Gln mutationdisrupts ATP-binding. Truncating mutations tend to be associated withboth a more severe phenotype and the onset of liver disease in childhood.

Genetic diagnosis Serum copper and ceruloplasmin levels are usually low, while urinary and counseling copper excretion is raised, as is the level of copper in liver tissue.

Mutation analysis can be undertaken at a small number of centers and can be used for prenatal and preclinical diagnosis. Counseling is as for autosomal recessive inheritance with a carrier frequency of approximately 1 in 100.


14. Renal Disorders

Alport Syndrome 218

Beckwith–Wiedemann Syndrome 220

Cystinosis 224

Orofaciodigital Syndrome Type I 225

Polycystic Kidney Disease 226


Alport Syndrome218

Alport Syndrome(also known as: AS; nephropathy and deafness)

MIM 104200 (autosomal dominant form [ADAS])203780 (autosomal recessive form [ARAS])301050 (X-linked form [XLAS])308940 (Alport syndrome and diffuse leiomyomatosis [ASDL])

Clinical features AS is characterized by renal, cochlear, and ocular involvement.Presentation is usually with persistent microhematuria, or occasionalgross macroscopic hematuria in childhood followed by proteinuria andeventual end-stage renal disease in middle age. Sensorineural hearingloss develops in early adult life and is progressive. Anterior lenticonus (ie, protrusion of the central part of the lens into the anterior chamber)becomes apparent in early adulthood and is sometimes associated withlens opacities and recurrent corneal erosion. In the very rare ASDL form,leiomyomata develop in childhood in the upper airways, esophagus, and female genital tract.

Age of onset Presentation is usually in late childhood or early adult life withhematuria and/or sensorineural hearing loss.

Epidemiology AS has an estimated incidence of approximately 1 in 50,000live births.

Inheritance Around 80% of cases show X-linked inheritance with heterozygousfemales showing only asymptomatic microhematuria, although a fewdevelop progressive renal disease. Of the remaining cases, 15% showautosomal recessive and 5% autosomal dominant inheritance.


Molecular In most, if not all, forms of AS the basic defect is in abnormal expressionpathogenesis of type IV collagen genes in the basement membrane (where type IV

collagen is the main structural component). Six type IV collagen geneshave been cloned and are located pairwise on chromosomes 13q34(COL4A1 and COL4A2), 2q36–q37 (COL4A3 and COL4A4), andXq22.3 (COL4A5 and COL4A6). They encode type IV α collagen


Renal Disorders 219

Table 1. Alport syndrome: forms, chromosomal locations, genes, and mutational spectra.

chains; these share a similar structure with a carboxy-terminalnoncollagenous domain (NC1), a long collagenous domain (with a repetitive glycine-X-Y triplet sequence), and a short noncollagenousamino-terminal sequence. These chains form heterotrimers, which in turn form a nonfibrillar network in the basement membrane. This acts as a scaffold for the deposition of other matrix components.

In normal renal development, there is a switch from COL4A1 andCOL4A2 expression in early childhood to COL4A3, COL4A4, andCOL4A5 expression as glomerular maturation proceeds. This switchusually does not occur in AS, resulting in the accumulation of type IV α1and α2 chains together with types V and VII collagen in the glomerularbasement membrane, possibly as a result of compensatory activation of the relevant genes. This accumulation of inappropriate collagen mayaccount for the observed electron microscopic changes of thickeningand splitting of the glomerular basement membrane, with progressiveglomerulosclerosis. It is not clear why a mutation in one of the genesencoding the α3–α4–α5 network should prevent expression of the othergenes. The explanation may lie in a proposed dominant-negative effectresulting from integration of a single abnormal chain in a trimericmolecule, as observed in osteogenesis imperfecta (see p.119–24).

Form Chromosomal Gene Mutational Spectrumlocation

ADAS 2q36–q37 Unknown Unknown(autosomaldominant)

ARAS 2q36–q37 COL4A3, Nonsense mutations and deletions (autosomal COL4A4 with a loss of function effectrecessive)

XLAS Xq22.3 COL4A5 Large deletions (20%), missense (X-linked) (35%–40%), splice-site (15%),

and nonsense mutations or small frame-shift deletions/insertions (25%–30%), with either loss of function or dominant-negative effects

ASDL Xq22.3 COL4A5, Large deletions, including the 5′(Alport COL4A6 exons of both COL4A5 and COL4A6syndromeand diffuseleiomyomatosis


Beckwith–Wiedemann Syndrome220

There is only a weak correlation between genotype and phenotype, with some evidence that glycine substitutions and splice-site mutationstend to be associated with a later onset of end-stage renal failure.Antiglomerular basement membrane nephritis develops in 10%–20% of patients receiving a renal transplant due to the development ofantibodies to the type IV α3 chain. This is most likely to occur when a large deletion or nonsense mutation in COL4A3 or COL4A5 leads to absence of the NC1 domain of the type IV α3 chain in the patient’sbasement membrane, with immunologic intolerance for this domain in transplanted kidneys.

The rare ASLD is caused by large deletions that remove the 5′ ends of the COL4A5 and COL4A6 genes, which are located close together in a head-to-head orientation. The deletion breakpoint in COL4A6is always in the second intron. It is not known how these deletionspredispose to the widespread development of leiomyomata.

Genetic diagnosis Mutation analysis is available on a limited basis for XLAS. If a specificand counseling mutation cannot be identified, then linkage analysis can be utilized for

carrier detection and prenatal diagnosis. Approximately 90% of XLAScarriers show microhematuria. Mutation analysis is available on aresearch basis only for ARAS. Up to 50% of ARAS carriers showmicrohematuria, so it can sometimes be difficult to distinguish ADASfrom ARAS. ADAS is rare and shows clinical overlap with the Epstein(MIM 153650) and Fechtner (MIM 153640) syndromes, in whichhereditary nephritis and deafness are associated with thrombocytopeniaand large platelets. These very rare autosomal dominant disorders arecaused by mutations in MYH9 (which encodes the nonmuscle myosinheavy chain 9).

Beckwith–Wiedemann Syndrome(also known as: BWS; exomphalos-macroglossia-gigantism [EMG] syndrome)

MIM 130650

Clinical features As reported by Beckwith in 1963 and Wiedemann in 1964, BWS is characterized by variable overgrowth with macroglossia, ear pits or creases, visceromegaly, hemihypertrophy, and abdominal walldefects, including exomphalos and umbilical hernia. Potentially seriouscomplications include neonatal hypoglycemia (over 50% of cases)


Renal Disorders 221

and childhood-onset tumors (7%–20% of cases). Of these, the mostcommon is Wilms tumor, with an estimated incidence of 7%–10% by age 4 years. Other reported tumors include hepatoblastoma,neuroblastoma, adrenal carcinoma, and rhabdomyosarcoma.

Age of onset The diagnosis is usually apparent at birth and can be suspected on the basis of ultrasound findings of visceromegaly and exomphalos in the second trimester.

Epidemiology The incidence in live-born infants is approximately 1 in 13,000. All ethnic groups are affected.

Inheritance Autosomal dominant, with exclusively maternal transmission in 15% of cases. The remaining 85% of cases are sporadic and are caused by several different epigenetic mechanisms, as discussed in thefollowing sections.


Genes CDKN1C (cyclin-dependent kinase inhibitor 1C)H19 (also know as ASM1 – adult skeletal muscle)IGF2 (insulin-like growth factor 2)KVLQT1 (potassium channel, voltage-gatedKQT-like subfamily, member 1), KVLQT1-AS (KVLQT1 antisense)

Mutational spectrum The genetic mechanisms involved in BWS are complex and have notand molecular been fully elucidated. They involve disturbance in the expression of pathogenesis a cluster of genes located in the BWS critical region at 11p15. Most

of these genes are imprinted through differential methylation, dependingon the parent of origin. The BWS critical region is thought to contain at least two imprinting centers plus at least 12 other genes, includingthe five listed above (see Figure 1).

Numerous chromosomal and molecular abnormalities have beenidentified in patients with BWS. Mechanistically, these can beconsidered under the following headings.

Paternal uniparental disomy (UPD)This is present in approximately 10%–20% of sporadic cases, usually in the form of mosaicism for a normal cell line and a cell line showingsegmental 11p15 paternal UPD, probably arising as a result of somatic


Beckwith–Wiedemann Syndrome222

Figure 1. Schematic representation of the Beckwith–Wiedemann syndromecritical region (BWSCR) on the short arm of chromosome 11.

(mitotic) recombination involving homologous nonsister chromatids. The demonstration of paternal UPD implies loss of the normal maternal imprint.

Chromosome abnormalitiesA visible chromosome abnormality can be identified in around 1% of all cases. This usually involves a paternally derived 11p15duplication, a maternally derived 11p15 deletion, or rearrangementwith an 11p15 breakpoint. It is hypothesized that any such breakpointwill disturb the setting of the normal maternal imprint.

Germline mutations in CDKN1CMissense and nonsense substitutions, and deletions and insertions have been identified in CDKN1C in approximately 40% of familial and5% of sporadic cases. CDKN1C is expressed almost exclusively from the maternally derived chromosome 11. It encodes a regulatory kinasethat inhibits cell proliferation in the G1 phase of the cell cycle.

Errors of imprinting and methylationBased on expression studies of several imprinted genes in the BWS gene cluster, defects in at least two imprinting centers are suspected.KVLQT1-AS (also known as LIT1) is normally expressed from the

Chromosome II

Centromere BWSCR

11p15

pter

Maternal chromosome

Paternal chromosome

Not expressedExpressed

CD

KN

IC

KVL

QTI

KVL

QTI

-AS

IGF2

H19


Renal Disorders 223

paternal allele only, but KVLQT1-AS shows loss of imprinting in40%–50% of sporadic cases, with expression from the maternal allelealso. This gene encodes an antisense transcript for KVLQT1, which isnormally only expressed from the maternal allele. However, in manyBWS cases it is also found to be expressed from the paternal allele. The role of these two genes in causing the features of BWS is not known.(Note that mutations in KVLQT1 can cause Jervell and Lange-Nielsensyndrome, MIM 220400.) Evidence for the involvement of a secondimprinting center stems from expression studies of H19 and IGF2, two closely linked genes telomeric of KVLQT1. These show reciprocalimprinting with paternal expression of IGF2 and maternal expression of H19. IGF2 encodes a fetal growth factor, whereas H19 encodes an mRNA that suppresses tumor growth. In approximately 10%–15% of BWS patients, this IGF2/H19 domain shows an abnormal imprinting pattern that is independent of the imprinting status at the KVLQT1 locus.

The precise causal mechanisms in BWS are not well understood. As a gross oversimplification, but a useful aide mémoire, it can behelpful to consider the overgrowth in BWS as a consequence of eitheroverexpression of paternally expressed growth promoting genes such as IGF2 (as a result of chromosomal duplication, paternal UPD oraberrant imprinting), or underexpression of maternally derived growthsuppressing genes such as CDKN1C (as a result of chromosome deletion or disruption or an imprinting error).

Genetic diagnosis Detailed chromosome analysis should be undertaken in all cases, and counseling focusing specifically on chromosome 11p15. Specific mutation and

imprinting studies are available at only a few specialized researchlaboratories. Regarding genotype–phenotype correlation, there is a suggestion that hemihypertrophy shows a positive association withpaternal UPD and that the risk of tumor formation is greatest when there is abnormal methylation of H19. The recurrence risk is very low for cases due to paternal UPD and 50% for maternally inheritedmutations in CDKN1C. A recurrence risk of 5% has been suggested for isolated cases of unknown cause.


Cystinosis224

Cystinosis

MIM 219800 (infantile nephropathic cystinosis)219900 (juvenile or adolescent nephropathic cystinosis)219750 (adult nonnephropathic cystinosis)

Clinical features Affected children present with polyuria, polydipsia, dehydration, failureto thrive, and rickets, all as a result of renal Fanconi syndrome. Withouttreatment, end-stage renal failure develops between 8 and 12 years of age.Other characteristic features include photophobia due to the depositionof cystine crystals in the cornea, hypothyroidism, myopathy, and blindness.

Age of onset Features of Fanconi syndrome are evident from 6–12 months of age.

Epidemiology Cystinosis affects all ethnic groups with an incidence of 1 in 100,000 to 1 in 200,000 live births.



Gene CTNS

Mutational Over 50 mutations have been described, most of which result in loss spectrum of function. About 50% of patients of northern European ancestry are

homozygous for a 57-kb intragenic deletion that results in a loss of the first 10 exons of the gene. Smaller deletions, insertions, missense,nonsense, and splice-site mutations have also been described. Patients with nephropathic cystinosis have two severe mutations that result in almost complete lack of protein expression. Patients with nonnephropathic cystinosis have one severe mutation and one mild mutation, so that some protein function is retained.

Molecular CTNS has 12 exons and encodes a 367-amino-acid protein calledpathogenesis cystinosin. Cystinosin is an integral lysosomal membrane protein with

seven transmembrane domains that is expressed in fetal and adultkidneys, pancreas, and skeletal muscle. Cystine produced from proteindegradation is transported by cystosin from the lysosome to thecytoplasm, where it can be reutilized in protein synthesis. Lack of


Renal Disorders 225

cystinosin causes cystine to accumulate in lysosomes. Accumulation of cystine in various tissues is responsible for the clinical features of cystinosis. Cystinosis is therefore a lysosomal storage disorder.

Genetic diagnosis The diagnosis of cystinosis can be confirmed by white-cell cystine assay.and counseling Another useful diagnostic test is slit-lamp examination to look for the

characteristic corneal crystals, although crystal deposition might not be evident in infancy. CTNS mutation analysis is available from a fewspecialized laboratories and is useful for carrier detection. Counseling is on an autosomal recessive basis. Prenatal diagnosis is available bycystine assay using cultured chorionic villi or cultured amniocytes.

Orofaciodigital Syndrome Type I(also known as: OFD1; oral-facial-digital syndrome type I)

MIM 311200

Clinical features The orofaciodigital syndromes consist of a heterogeneous group of ninediscrete entities with overlapping clinical features. The oral findings in OFD1 include a midline cleft lip, tongue clefts or nodules, multiplefrenula, and a high-arched or cleft palate. Facial features include aprominent forehead with hypertelorism and a broad nasal bridge withalar hypoplasia. The hands show variable brachydactyly, clinodactyly,and syndactyly, while preaxial polysyndactyly is the most characteristicfinding in the feet. Mild mental retardation is reported in up to 40% ofcases, often in association with a central nervous system malformationsuch as hydrocephalus or partial agenesis of the corpus callosum.Adult-onset polycystic kidney disease, with the cysts being of glomerular origin, is a common complication.

Age of onset The dysmorphic features are present at birth (they can usually also be detected using ultrasound during the second trimester).

Epidemiology OFD1 is by far the most common of the OFD syndromes, all of which are rare.

Inheritance X-linked dominant, with intrauterine lethality in males



Polycystic Kidney Disease226

Gene CXORF5 (chromosome X open reading frame 5), also known as OFDI

Mutational Missense, nonsense, splice-site, and frame-shift mutationsspectrum

Molecular CXORF5 was one of the first transcripts to be assigned to the pathogenesis X chromosome. It contains 23 exons with two transcripts subject to

alternative splicing, which are widely expressed. The locus is not subjectto normal X-chromosome inactivation and the identity of the proteinproduct is unknown. This is predicted to contain many coiled-coilα-helical domains, similar to those in the protein products of genes thatcause adult polycystic kidney disease types 1 and 2 (see next entry).This observation may explain the relatively high incidence of polycystickidney disease in OFD1.

Genetic diagnosis The diagnosis is based on the pattern of clinical findings rather and counseling than molecular analysis, which is only available on a research basis.

Counseling is as for X-linked dominant inheritance with male lethality.

Polycystic Kidney Disease(also known as: PKD. Includes adult polycystic kidney disease [APKD], infantile or autosomalrecessive polycystic kidney disease [ARPKD], polycystic kidneys, and hepatic disease)

MIM See Table 2.

Clinical findings Both forms of APKD usually present in the third to fifth decades with hematuria, abdominal pain or swelling, and uremia, although the onset of hypertension is usually earlier (second or third decade).Ultrasonography shows enlargement of the kidneys, with multiple largediscrete cysts distributed throughout both kidneys. Approximately 75%of affected individuals develop end-stage renal failure by the age of70 years. Known associations include polycystic liver disease andintracerebral aneurysms. Cysts may also be present in the pancreas and spleen. The disease runs a milder course in APKD2 than in APKD1, with end-stage renal disease occurring at average ages of 69 and 53 years, respectively.

ARPKD usually presents at birth with massive bilateral nephromegalyand respiratory problems due to associated oligohydramnios.


Renal Disorders 227

Table 2. Polycystic kidney disease: forms, MIM numbers, inheritance chromosomal locations, genes, and mutational spectra.

Approximately 30% of affected children die in infancy. Fifty percent of those who survive develop end-stage renal failure by the age of 10 years. Congenital hepatic fibrosis is a constant finding and leads to liver dysfunction in later childhood.

Age of onset APKD can present in the neonatal period or in childhood, but usuallypresents in middle age. ARPKD is often first suspected on ultrasoundscanning during pregnancy or at delivery because of dystocia caused by the enlarged kidneys.

Epidemiology APKD affects approximately 1 in 1,000 individuals worldwide. ARPKDhas an estimated incidence of approximately 1 in 20,000 live births witha somewhat higher frequency in the Afrikaner population of South Africa.

Inheritance, See Table 2.chromosomallocation, gene, and mutational spectrum

Molecular Mutations in PKD1 account for 85% of APKD families. PKD1 containspathogenesis 46 exons and encodes a large protein known as polycystin 1. This

associates with the cell membrane at points of cell–cell contact andcontains 11 transmembrane domains, with a coiled-coil domain in the cytoplasmic tail. PKD2 contains 15 exons and encodes a smaller (968-amino-acid) protein known as polycystin 2. This contains six transmembrane domains, with cytoplasmic N- and C-termini.

Form MIM Inheritance Chromosomal Gene Mutational location spectrum

APKD1 173900 Autosomal 16p13.3 PKD1 Mainly nonsense or frame-shift (adult polycystic dominant (polycystic kidney mutations, with some missensekidney disease disease 1) point mutations and in-frame type 1) deletions. All have a loss of

function effect

APKD2 173910 Autosomal 4q12–22 PKD2 Nonsense and splice-site (adult polycystic dominant (polycystic kidney mutations, frame-shift deletions, kidney disease disease 2) and insertions. All have a loss type 2) of function effect

ARPKD 263200 Autosomal 6p21.1–p12 PKHD1 Missense and nonsense point (autosomal recessive (fibrocystin) mutations. Also small deletionrecessive and insertion frame-shiftspolycystic kidney disease)


Polycystic Kidney Disease228

Polycystin 1 and 2 are believed to interact by binding of the coiled-coildomain of polycystin 1 with the C-terminal tail of polycystin 2 toproduce calcium-permeable cation channels in the renal tubular cell membrane.

Renal cyst formation has been shown to involve a “two-hit” mechanisminvolving both a germ-line and a somatic mutation in PKD1 and/or PKD2.When mutations in both APKD genes are involved, this is referred to as a “transheterozygous” state. Rare examples of individuals with bothAPKD and tuberous sclerosis are the result of contiguous gene deletionsinvolving the closely adjacent PKD1 and TSC2 loci on chromosome 16.The renal disease in these individuals is usually much more severe thanin typical APKD1.

ARPKD is caused by mutations in PKHD1, which contains 66 exons andencodes a large protein, known as fibrocystin; this is expressed in adultkidney, liver, and pancreas. It has been proposed that fibrocystin is areceptor protein that acts in renal tubule and biliary duct differentiation.

Genetic diagnosis Specific mutation analysis for PKD1 is difficult because of the presenceand counseling of multiple silent copies of part of the gene elsewhere on chromosome 16.

Linkage analysis for both APKD1 and APKD2 is readily available forpreclinical and prenatal diagnosis. However, demand for prenataldiagnosis is very limited because very few families wish to pursuetermination of pregnancy for adult PKD as they don’t perceive it to be a serious enough problem. Linkage analysis can also be used for the prenatal diagnosis of ARPKD. Clinically, APKD shows fullpenetrance by age 30 years, with all heterozygotes showing cysts on renal ultrasonography.


15. Abbreviations


Genetics for Pediatricians 230

ADA adenosine deaminase ADAS autosomal dominant form of Alport syndrome AHO Albright’s hereditary osteodystrophy AIS androgen insensitivity syndrome AMN adrenomyeloneuropathy APKD adult polycystic kidney disease ARAS autosomal recessive form of Alport syndrome AS Alport or Angelman syndrome ASDL Alport syndrome and diffuse leiomyomatosisAT ataxia–telangiectasiaATM ataxia–telangiectasia mutatedATR-X X-linked α-thalassemia and mental retardation BBS Bardet–Biedl syndrome BCG bacille Calmette–GuérinBEK bacterially expressed kinaseBWS Beckwith–Wiedemann syndrome CAH congenital adrenal hyperplasia CAL café-au-lait CBAVD congenital bilateral absence of the vas deferens CF cystic fibrosis CFTR cystic fibrosis transmembrane conductance regulator CGD chronic granulomatous disease CHN congenital hypomyelinating neuropathyCK creatine kinaseCNS central nervous system CRM cross-reacting material DI diabetes insipidus DMD Duchenne muscular dystrophyEEG electroencephalograph EGF epidermal growth factor EMG exomphalos-macroglossia-gigantism ERG electroretinogram FA Fanconi anemia FISH fluorescence in situ hybridization FMRP FMR protein FMTC familial medullary thyroid carcinoma FSHMD facioscapulohumeral muscular dystrophy G6PD glucose-6-phosphate dehydrogenase


Abbreviations 231

GAG glycosaminoglycanGAP GTPase-activating protein GDNF glial cell-line derived neurotrophic factor GH growth hormone GHR growth hormone receptorGROD granular osmiophilic depositHb hemoglobin HD Huntington disease HE hereditary elliptocytosis HGPRT hypoxanthine-guanine phosphoribosyl transferaseHME hereditary multiple exostoses HMSN hereditary motor and sensory neuropathyHNPP hereditary neuropathy with liability to pressure palsiesHOS Holt–Oram syndrome HPE holoprosencephaly HPP hereditary pyropoikilocytosisHS hereditary spherocytosis or Hunter syndrome HSCR Hirschsprung diseaseIg immunoglobulin IGF insulin-like growth factor IL interleukin KGFR keratinocyte growth factor receptor LCA Leber congenital amaurosis LGMD limb-girdle muscular dystrophyMASA mental retardation, aphasia, shuffling gait,

and adducted thumbs MBD methyl-CpG binding domain MCAD medium chain acyl-CoA dehydrogenase MD myotonic dystrophyMDS Miller–Dieker syndrome MEBD muscle–eye–brain disease MEN2 multiple endocrine neoplasia type 2 MIM Mendelian inheritance in manMPS mucopolysaccharidosisMTC medullary thyroid carcinoma NBF nucleotide-binding folds NCL neuronal ceroid lipofuscinosis NCV nerve conduction velocity



NF1 neurofibromatosis type 1 OCT ornithine carbamoyltransferase OFD1 orofaciodigital syndrome type I OI osteogenesis imperfectaOIC osteogenesis imperfecta congenita OIT osteogenesis imperfecta tarda ORCC outwardly rectifying chloride channels OTC ornithine transcarbamylase PCD primary ciliary dyskinesia PDS Pendred syndromePHP pseudohypoparathyroidism PKD polycystic kidney disease PKU phenylketonuria PNP purine nucleoside phosphorylase PPHP pseudopseudohypoparathyroidism PPT palmitoyl-protein thioesterase PROMM proximal myotonic myopathypVHL protein product of the VHL gene PWS Prader–Willi syndrome SCBH subcortical band heterotopia SCID severe combined immunodeficiency SCS Saethre–Chotzen syndrome SMA spinal muscular atrophySVAS supravalvular aortic stenosis TRD transcriptional repression domain TS tuberous sclerosis TSH thyroid-stimulating hormone UPD uniparental disomy VCFS velocardiofacial syndrome VEP visual evoked potential VHL Von Hippel–Lindau disease VLCFA very long-chain fatty acidvWD von Willebrand disease vWF von Willebrand factorWAGR Wilms’ tumor, aniridia, genitourinary anomalies,

mental retardationWAS Wiskott–Aldrich Syndrome WS Waardenburg syndrome


Abbreviations 233

WS Williams syndrome X-ALD X-linked adrenoleukodystrophy XLAG X-linked lissencephaly with ambiguous genitalia XLAS X-linked form of Alport syndrome XLIS X-linked form of isolated classical lissencephaly


234


Glossary


Genetics for Pediatricians236

A

Adenine (A) One of the bases making up DNA and RNA (pairs with thyminein DNA and uracil in RNA).

Agarose gel See electrophoresis.electrophoresis

Allele One of two or more alternative forms of a gene at a given location(locus). A single allele for each locus is inherited separately from eachparent. In normal human beings there are two alleles for each locus(diploidy). If the two alleles are identical, the individual is said to behomozygous for that allele; if different, the individual is heterozygous.

For example, the normal DNA sequence at codon 6 in the beta-globingene is GAG (coding for glutamic acid), whereas in sickle cell disease the sequence is GTG (coding for valine). An individual is said to be heterozygous for the glutamic acid → valine mutation if he/shepossesses one normal (GAG) and one mutated (GTG) allele. Suchindividuals are carriers of the sickle cell gene and do not manifestclassical sickle cell disease (which is autosomal recessive).

Allelic heterogeneity Similar/identical phenotypes caused by different mutations within a gene. For example, many different mutations in the same gene are now known to be associated with Marfan’s syndrome (FBN1gene at 15q21.1).

Amniocentesis Withdrawal of amniotic fluid, usually carried out during the secondtrimester, for the purpose of prenatal diagnosis.

Amplification The production of increased numbers of a DNA sequence.

1. In vitroIn the early days of recombinant DNA techniques, the only way toamplify a sequence of interest (so that large amounts were available fordetailed study) was to clone the fragment in a vector (plasmid or phage)and transform bacteria with the recombinant vector. The transformationtechnique generally results in the “acceptance” of a single vector moleculeby each bacterial cell. The vector is able to exist autonomously withinthe bacterial cell, sometimes at very high copy numbers (eg, 500 vectorcopies per cell). Growth of the bacteria containing the vector, coupled


Glossary 237

with a method to recover the vector sequence from the bacterial culture,allows for almost unlimited production of a sequence of interest. Cloningand bacterial propagation are still used for applications requiring eitherlarge quantities of material or else exceptionally pure material.

However, the advent of the polymerase chain reaction (PCR) has meantthat amplification of desired DNA sequences can now be performedmore rapidly than was the case with cloning (a few hours cf. days), and it is now routine to amplify DNA sequences 10 million-fold.

2. In vivoAmplification may also refer to an increase in the number of DNAsequences within the genome. For example, the genomes of manytumors are now known to contain regions that have been amplifiedmany fold compared to their nontumor counterparts (ie, a sequence orregion of DNA that normally occurs once at a particular chromosomallocation may be present in hundreds of copies in some tumors). It isbelieved that many such regions harbor oncogenes, which, whenpresent in high copy number, predispose to development of themalignant phenotype.

Aneuploid Possessing an incorrect number (abnormal complement) of chromosomes.The normal human complement is 46 chromosomes, any cell thatdeviates from this number is said to be aneuploid.

Aneuploidy The chromosomal condition of a cell or organism with an incorrectnumber of chromosomes. Individuals with Down’s syndrome aredescribed as having aneuploidy, because they possess an extra copy of chromosome 21 (trisomy 21), making a total of 47 chromosomes.

Anticipation A general phenomenon that refers to the observation of an increase inseverity, and/or decrease in age of onset, of a condition in successivegenerations of a family (see Figure 1). Anticipation is now known, inmany cases, to result directly from the presence of a dynamic mutationin a family. In the absence of a dynamic mutation, anticipation may be explained by “ascertainment bias”. Thus, before the first dynamicmutations were described (in Fragile X and myotonic dystrophy), it wasbelieved that ascertainment bias was the complete explanation foranticipation. There are two main reasons for ascertainment bias:

1. Identical mutations in different individuals often result in variableexpressions of the associated phenotype. Thus, individuals within a


family, all of whom harbor an identical mutation, may have variation in the severity of their condition.

2. Individuals with a severe phenotype are more likely to present to the medical profession. Moreover, such individuals are more likely to fail to reproduce (ie, they are genetic lethals), often for social, rather than direct physical reasons.

For both reasons, it is much more likely that a mildly affected parent will be ascertained with a severely affected child, than thereverse. Therefore, the severity of a condition appears to increasethrough generations.

Anticodon The 3-base sequence on a transfer RNA (tRNA) molecule that is complementary to the 3-base codon of a messenger RNA(mRNA) molecule.

Ascertainment bias See anticipation.

Autosomal disorder A disorder associated with a mutation in an autosomal gene.

Autosomal dominant An autosomal disorder in which the phenotype is expressed in (AD) inheritance the heterozygous state. These disorders are not sex-specific. Fifty

percent of offspring (when only one parent is affected) will usuallymanifest the disorder (see Figure 2). Marfan syndrome is a goodexample of an AD disorder; affected individuals possess one wild-type (normal) and one mutated allele at the FBN1 gene.


Figure 1. Autosomal dominant inheritance with anticipation. In many disordersthat exhibit anticipation, the age of onset decreases in subsequent generations. It may happen that the transmitting parent (grandparent in this case) is unaffectedat the time of presentation of the proband (see arrow). A good example isHuntington’s disease, caused by the expansion of a CAG repeat in the codingregion of the huntingtin gene. Note that this pedigree would also be consistentwith either gonadal mosaicism or reduced penetrance (in the carrier grandparent).


Glossary 239

Autosomal recessive An autosomal disorder in which the phenotype is manifest in the(AR) inheritance homozygous state. This pattern of inheritance is not sex-specific and

is difficult to trace through generations because both parents mustcontribute the abnormal gene, but may not necessarily display thedisorder. The children of two heterozygous AR parents have a 25%chance of manifesting the disorder (see Figure 3). Cystic fibrosis (CF) is a good example of an AR disorder; affected individuals possess twomutations, one at each allele.

Figure 3. Autosomal recessive (AR) inheritance.

Autosome Any chromosome, other than the sex chromosomes (X or Y), that occursin pairs in diploid cells.

B

Barr body An inactive X chromosome, visible in the somatic cells of individualswith more than one X chromosome (ie, all normal females and all maleswith Klinefelter’s syndrome). For individuals with nX chromosomes, n–1Barr bodies are seen. The presence of a Barr body in cells obtained byamniocentesis or chorionic villus sampling used to be used as anindication of the sex of a baby before birth.

Base pair (bp) Two nucleotides held together by hydrogen bonds. In DNA, guaninealways pairs with cytosine, and thymine with adenine. A base pair is also the basic unit for measuring DNA length.

Figure 2. Autosomal dominant (AD) inheritance.



C

Carrier An individual who is heterozygous for a mutant allele (ie, carries one wild-type [normal copy] and one mutated copy of the geneunder consideration).

CentiMorgan (cM) Unit of genetic distance. If the chance of recombination between two loci is 1%, the loci are said to be 1 cM apart. On average, 1 cM implies a physical distance of 1 Mb (1,000,000 base pairs) but significant deviations from this rule of thumb occur becauserecombination frequencies vary throughout the genome. Thus ifrecombination in a certain region is less likely than average, 1 cM may be equivalent to 5 Mb (5,000,000 base pairs) in that region.

Centromere Central constriction of the chromosome where daughter chromatidsare joined together, separating the short (p) from the long (q) arms (see Figure 4).

Chorionic villus Prenatal diagnostic procedure for obtaining fetal tissue at an earliersampling (CVS) stage of gestation than amniocentesis. Generally performed after

10 weeks, ultrasound is used to guide aspiration of tissue from the villus area of the chorion.

Chromatid One of the two parallel identical strands of a chromosome, connectedat the centromere during mitosis and meiosis (see Figure 4). Beforereplication, each chromosome consists of only one chromatid. Afterreplication, two identical sister chromatids are present. At the end of mitosis or meiosis, the two sisters separate and move to oppositepoles before the cell splits.

Chromatin A readily stained substance in the nucleus of a cell consisting of DNA and proteins. During cell division it coils and folds to form the metaphase chromosomes.

Chromosome One of the threadlike “packages” of genes and other DNA in the nucleusof a cell (see Figure 4). Humans have 23 pairs of chromosomes, 46 intotal: 44 autosomes and two sex chromosomes. Each parent contributesone chromosome to each pair.


Chromosomal A disorder that results from gross changes in chromosome dose.

disorder May result from addition or loss of entire chromosomes or just portionsof chromosomes.

Clone A group of genetically identical cells with a common ancestor.

Codon A 3-base coding unit of DNA that specifies the function of a correspondingunit (anticodon) of transfer RNA (tRNA).

Complementary DNA synthesized from messenger RNA (mRNA) using reverseDNA (cDNA) transcriptase. Differs from genomic DNA because it lacks introns.

Complementation The wild-type allele of a gene compensates for a mutant allele of thesame gene so that the heterozygote’s phenotype is wild-type.

Complementation A genetic test (usually performed in vitro) that determines whether analysis or not two mutations that produce the same phenotype are allelic.

It enables the geneticist to determine how many distinct genes areinvolved when confronted with a number of mutations that have similar phenotypes.

Glossary 241

Nucleus

Chromosome

Chromatid

q arm

p arm

Telomere

Centromere

DNA Double helix

AT

A T

AT

CG

AFigure 4. Chromosome structure. T



Occasionally it can be observed clinically. Two parents who both sufferfrom recessive deafness (ie, both are homozygous for a mutationresulting in deafness) may have offspring that have normal hearing. If A and B refer to the wild-type (normal) forms of the genes, and a and b the mutated forms, one parent could be aa,BB and the otherAA,bb. If alleles A and B are distinct, each child will have the genotypeaA,bB and will have normal hearing. If A and B are allelic, the child will be homozygous at this locus and will also suffer from deafness.

Compound An individual with two different mutant alleles at the same locus.heterozygote

Concordant A pair of twins who manifest the same phenotype as each other.Consanguinity Sharing a common ancestor, and thus genetically related. Recessive

disorders are seen with increased frequency in consanguineous families.

Consultand An individual seeking genetic advice.

Contiguous gene A syndrome resulting from the simultaneous functional imbalance syndrome of a group of genes (see Figure 5). The nomenclature for this group

of disorders is somewhat confused, largely as a result of the history of their elucidation. The terms submicroscopic rearrangement/deletion/duplication and microrearrangement/deletion/duplication are often usedinterchangeably. Micro or submicroscopic refer to the fact that suchlesions are not detectable with standard cytogenetic approaches (where the limit of resolution is usually 10 Mb, and 5 Mb in only the most fortuitous of circumstances). A newer, and perhaps morecomprehensive, term that is currently applied to this group of disordersis segmental aneusomy syndromes (SASs). This term embraces thepossibility not only of loss or gain of a chromosomal region that harborsmany genes (leading to imbalance of all those genes), but also offunctional imbalance in a group of genes, as a result of an abnormality ofthe machinery involved in their silencing/transcription (ie, methylation-based mechanisms that depend on a master control gene).

In practice, most contiguous gene syndromes result from theheterozygous deletion of a segment of DNA that is large in molecularterms but not detectable cytogenetically. The size of such deletions is usually 1.5–3.0 Mb. It is common for one to two dozen genes to be involved in such deletions, and the resultant phenotypes are often


14

complex, involving multiple organ systems and, almost invariably,learning difficulties. A good example of a contiguous gene syndrome is Williams’ syndrome, a sporadic disorder that is due to a heterozygousdeletion at chromosome 7q11.23. Affected individuals havecharacteristic phenotypes, including recognizable facial appearance and typical behavioral traits (including moderate learning difficulties).Velocardiofacial syndrome is currently the most common microdeletionknown, and is caused by deletions of 3 Mb at chromosome 22q11.

Crossing over Reciprocal exchange of genetic material between homologouschromosomes at meiosis (see Figure 6).

Cytogenetics The study of the structure of chromosomes.

Cytosine (C) One of the bases making up DNA and RNA (pairs with guanine).

Cytotrophoblast Cells obtained from fetal chorionic villi by chorionic villus sampling(CVS). Used for DNA and chromosome analysis.

Glossary 243

Williams’ syndrome region:1.5–2.5 Mb in size.

Figure 5. Schematic demonstrating the common deletion found in Williams’ syndrome,at 7q11.23. The common deletion is not detectable using standard cytogeneticanalysis (even high resolution), despite the fact that the deletion is at least 1.5 Mb in size. In practice, only genomic rearrangements that affect at least 5–10 Mb aredetectable, either by standard cytogenetic analysis or, in fact, any technique whoseendpoint involves analysis at the chromosomal level. Such deletions are termedmicrodeletions or submicroscopic deletions. Approximately 20 genes are known to be involved in the 7q11.23 microdeletion, and work is underway to determinewhich genes contribute to which aspects of the Williams’ syndrome phenotype.

7

p

q

22

21

15.3 15.215.1

13

12

11.2

11.1

11.1

11.21

11.22

11.23

21.1

21.3

21.2

22.1

31.1

31.3

31.2

32

33

34

35

36


A A

BC

BC

A A

BC

BC

Figure 6. Schematic demonstrating the principle of recombination (crossing over). On average, 50 recombinations occur per meiotic division (1–2 per chromosome). Loci that are far apart on the chromosome are more likely to be separated duringrecombination than those that are physically close to each other (they are said to be linked, see linkage), ie, A and B are less likely to cosegregate than B and C. Note that the two homologues of a sequence have been differentially labeled according to their chromosome of origin.


D

Deletion A particular kind of mutation that involves the loss of a segment of DNA from a chromosome with subsequent re-joining of the two extantends. It can refer to the removal of one or more bases within a geneor to a much larger aberration involving millions of bases. The termdeletion is not totally specific, and differentiation must be made betweenheterozygous and homozygous deletions. Large heterozygous deletionsare a common cause of complex phenotypes (see contiguous genesyndrome); large germ-line homozygous deletions are extremely rare, but have been described. Homozygous deletions are frequentlydescribed in somatic cells, in association with the manifestation of the malignant phenotype. The two deletions in a homozygous deletion need not be identical, but must result in the complete absence of DNA sequences that occupy the “overlap” region.


Glossary 245

Denature Broadly used to describe two general phenomena:

1. The “melting” or separation of double-stranded DNA (dsDNA) into its constituent single strands, which may be achieved using heat or chemical approaches.

2. The denaturation of proteins. The specificity of proteins is a result oftheir 3-dimensional conformation, which is a function of their (linear)amino acid sequence. Heat and/or chemical approaches may result in denaturation of a protein – the protein loses its 3-dimensionalconformation (usually irreversibly) and, with it, its specific activity.

Diploid Having two sets of chromosomes. The number of chromosomesin most human somatic cells is 46. This is double the number found in gametes (23, the haploid number).

Discordant A pair of twins who differ in their manifestation of a phenotype.

Dizygotic The fertilization of two separate eggs by two separate sperm resulting in a pair of genetically nonidentical twins.

DNA The molecule of heredity. DNA normally exists as a double-stranded(deoxyribonucleic (ds) molecule; one strand is the complement (in sequence) of the other.acid) The two strands are joined together by hydrogen bonding, a noncovalent

mechanism that is easily reversible using heat or chemical means. DNAconsists of four distinct bases: guanine (G), cytosine (C), thymine (T),and adenine (A). The convention is that DNA sequences are written in a 5 to 3 direction, where 5 and 3 refer to the numbering of carbonson the deoxyribose ring. A guanine on one strand will always pair with a cytosine on the other strand, while thymine pairs with adenine. Thus,given the sequence of bases on one strand, the sequence on the other is immediately determined:

5 –AGTGTGACTGATCTTGGTG–33 –TCACACTGACTAGAACCAC–5

The complexity (informational content) of a DNA molecule residesalmost completely in the particular sequence of its bases. For a sequenceof length “n” base pairs, there are 4n possible sequences. Even forrelatively small n, this number is astronomical (4n = 1.6 x 1060

for n = 100).



The complementarity of the two strands of a dsDNA molecule is a veryimportant feature and one that is exploited in almost all moleculargenetic techniques. If dsDNA is denatured, either by heat or bychemical means, the two strands become separated from each other. If the conditions are subsequently altered (eg, by reducing heat), the two strands eventually “find” each other in solution and re-anneal to form dsDNA once again. The specificity of this reaction is quite high,under the right circumstances – strands that are not highly complementaryare much less likely to re-anneal compared to perfect or near perfectmatches. The process by which the two strands “find” each otherdepends on random molecular collisions, and a “zippering” mechanism,which is initiated from a short stretch of complementarity. This propertyof DNA is vital for the polymerase chain reaction (PCR), Southernblotting, and any method that relies on the use of a DNA/RNA probeto detect its counterpart in a complex mix of molecules.

DNA chip A “chip” or microarray of multiple DNA sequences immobilized on a solid surface (see Figure 7). The term chip refers more often tosemiconductor-based DNA arrays, in which short DNA sequences(oligos) are synthesized in situ, using a photolithographic process akin to that used in the manufacture of semiconductor devices for the electronics industry. The term microarray is much more general and includes any collection of DNA sequences immobilized onto a solid surface, whether by a photolithographic process, or by simple“spotting” of DNA sequences onto glass slides.

The power of DNA microarrays is based on the parallel analysis that theyallow for. In conventional hybridization analysis (ie, Southern blotting),a single DNA sequence is usually used to interrogate a small number of different individuals. In DNA microarray analysis, this approach is reversed – an individual’s DNA is hybridized to an array that maycontain 30,000 distinct spots. This allows for direct information to be obtained about all DNA sequences on the array in one experiment.DNA microarrays have been used successfully to directly uncover point mutations in single genes, as well as detect alterations in gene expression associated with certain disease states/cellulardifferentiation. It is likely that certain types of array will be useful in the determination of subtle copy number alterations, as occurs in microdeletion/microduplication syndromes.


Glossary 247

DNA methylation Addition of a methyl group (–CH3) to DNA nucleotides (often cytosine).Methylation is often associated with reduced levels of expression of a given gene and is important in imprinting.

DNA replication Use of existing DNA as a template for the synthesis of new DNA strands.In humans and other eukaryotes, replication takes place in the cellnucleus. DNA replication is semiconservative – each new double-stranded molecule is composed of a newly synthesized strand and a pre-existing strand.

Figure 7. DNA chip. DNA arrays (or “chips”) are composed of thousands of “spots” of DNA, attached to a solid surface (normally glass). Each spot contains a different DNA sequence. The arrays allow for massively parallel experiments to be performed on samples. In practice, two samples are applied to the array. One sample is a control(from a “normal” sample) and one is the test sample. Each sample is labeled withfluorescent tags, control with green and test with red. The two labeled samples arecohybridized to the array and the results read by a laser scanner. Spots on the arraywhose DNA content is equally represented in the test and control samples yield equalintensities in the red and green channels, resulting in a yellow signal. Spots appearing as red represent DNA sequences that are present at higher concentration in the testsample compared to the control sample and vice versa.



Dominant Manifesting a phenotype in the heterozygous state. Individuals with(traits/diseases) Huntington’s disease, a dominant condition, are affected even though

they possess one normal copy of the gene.

Dynamic/ The vast majority of mutations known to be associated with humannonstable mutation genetic disease are intergenerationally stable (no alteration in the

mutation is observed when transmitted from parent to child). However, a recently described and growing class of disorders result from thepresence of mutations that are unstable intergenerationally. Thesedisorders result from the presence of tandem repeats of short DNAsequences (eg, the sequence CAG may be repeated many times intandem), see Table 1. For reasons that are not completely clear, the copy number of such repeats may vary from parent to child (usuallyresulting in a copy number increase) and within the somatic cells of a given individual. Abnormal phenotypes result when the number ofrepeats reaches a given threshold. Furthermore, when this threshold has been reached, the risk of even greater expansion of copy number in subsequent generations increases.

E

Electrophoresis The separation of molecules according to size and ionic charge by an electrical current.

Agarose gel electrophoresisSeparation, based on size, of DNA/RNA molecules through agarose.Conventional agarose gel electrophoresis generally refers toelectrophoresis carried out under standard conditions, allowing the resolution of molecules that vary in size from a few hundred to a few thousand base pairs.

Polyacrylamide gel electrophoresisAllows resolution of proteins or DNA molecules differing in size by only 1 base pair.

Pulsed field gel electrophoresis(Also performed using agarose) refers to a specialist technique thatallows resolution of much larger DNA molecules, in some cases up to a few Mb in size.


Glossary 249

Empirical Based on observation, rather than detailed knowledge of, eg, modesrecurrence of inheritance or environmental factors.risk – recurrence risk

Endonuclease An enzyme that cleaves DNA at an internal site (see also restriction enzyme).

Euchromatin Chromatin that stains lightly with trypsin G banding and contains active/potentially active genes.

Euploidy Having a normal chromosome complement.

Disorder Protein/location Repeat Repeat Normal Pre-mutation Full mutation Type MIMlocation range

Progressive myoclonus cystatin B C4GC4G Promoter 2–3 12–17 30–75 AR 254800epilepsy of Unverricht- 21q22.3 CGLundborg type (EPM1)

Fragile X type A (FRAXA) FMR1 Xq27.3 CGG 5’UTR 6–52 ~60–200 ~200–>2,000 XLR 309550

Fragile X type E (FRAXE) FMR2 Xq28 CGG 5 C’UTR 6–25 – >200 XLR 309548

Friedreich’s ataxia (FRDA) frataxin 9q13 GAA intron 1 7–22 – 200–>900 AR 229300

Huntington’s disease (HD) huntingtin 4p16.3 CAG ORF 6–34 – 36–180 AD 143100

Dentatorubal-pallidoluysian atrophin 12p12 CAG ORF 7–25 – 49–88 AD 125370atrophy (DRPLA)

Spinal and bulbar androgen receptor CAG ORF 11–24 – 40–62 XLR 313200muscular atrophy Xq11-12(SBMA – Kennedy syndrome)

Spinocerebellar ataxia ataxin-1 6p23 CAG ORF 6–39 – 39–83 AD 164400type 1 (SCA1)

Spinocerebellar ataxia ataxin-2 12q24 CAG ORF 15–29 – 34–59 AD 183090type 2 (SCA2)

Spinocerebellar ataxia ataxin-3 CAG ORF 13–36 – 55–84 AD 109150type 3 (SCA3) 14q24.3-q31

Spinocerebellar ataxia PQ calcium CAG ORF 4–16 – 21–30 AD 183086type 6 (SCA6) channel 19p13

Spinocerebellar ataxia ataxin-7 CAG ORF 4–35 28–35 34–>300 AD 164500type 7 (SCA7) 3p21.1-p12

Spinocerebellar ataxia SCA8 13q21 CTG 3’UTR 6–37 – ~107–2501 AD 603680type 8 (SCA8)

Spinocerebellar ataxia SCA10 ATTCT intron 9 10–22 – 500–4,500 AD 603516type 10 (SCA10) 22q13-qter

Spinocerebellar ataxia PP2R2B 5q31-33 CAG 5’UTR 7–28 – 66–78 AD 604326type 12 (SCA12)

Myotonic dystrophy (DM) DMPK 19q13.3 CTG 3’UTR 5–37 ~50–180 ~200–>2,000 AD 160900

Table 1. “Classical” repeat expansion disorders. 1Longer alleles exist but are not associated with disease. AD: autosomaldominant; AR: autosomal recessive; ORF: open reading frame (coding region); 3 UTR: 3 untranslated region(downstream of gene); 5 UTR: 5 untranslated region (upstream of gene); XLR: X-linked recessive.



Exon Coding part of a gene. Historically, it was believed that all of a DNAsequence is mirrored exactly on the messenger RNA (mRNA) molecule(except for the presence of uracil in mRNA compared to thymine inDNA). It was a surprise to discover that this is generally not the case.The genomic sequence of a gene has two components: exons andintrons. The exons are found in both the genomic sequence and themRNA, whereas the introns are found only in the genomic sequence.The mRNA for dystrophin, an X-linked gene associated with Duchennemuscular dystrophy (DMD), is 14,000 base pairs long but the genomicsequence is spread over a distance of 1.5 million base pairs, because of the presence of very long intronic sequences. After the genomicsequence is initially transcribed to RNA, a complex system ensuresspecific removal of introns. This system is known as splicing.

Expressivity Degree of expression of a disease. In some disorders, individualscarrying the same mutation may manifest wide variability in severity of the disorder. Autosomal dominant disorders are often associated with variable expressivity, a good example being Marfan’s syndrome.Variable expressivity is to be differentiated from incomplete penetrance,an all or none phenomenon that refers to the complete absence of a phenotype in some obligate carriers.

F

Familial Any trait that has a higher frequency in relatives of an affected individualthan the general population.

FISH Fluorescence in situ hybridization (see In situ hybridization).

Founder effect The high frequency of a mutant allele in a population as a result of its presence in a founder (ancestor). Founder effects are particularlynoticeable in relative genetic isolates, such as the Finnish or Amish.

Frame-shift Deletion/insertion of a DNA sequence that is not an exact multiple mutation of 3 base pairs. The result is an alteration of the reading frame of the

gene such that all sequence that lies beyond the mutation is missence (ie, codes for the wrong amino acids) (see Figure 8). A premature stopcodon is usually encountered shortly after the frame shift.


Glossary 251

G

Gamete (germ cell) The mature male or female reproductive cells, which contain a haploidset of chromosomes.

Gene An ordered, specific sequence of nucleotides that controls thetransmission and expression of one or more traits by specifying thesequence and structure of a particular protein or RNA molecule. Mendeldefined a gene as the basic physical and functional unit of all heredity.

Gene expression The process of converting a gene’s coded information into the existing,operating structures in the cell.

Gene mapping Determines the relative positions of genes on a DNA molecule and plotsthe genetic distance in linkage units (centiMorgans) or physical distance(base pairs) between them.

Genetic code Relationship between the sequence of bases in a nucleic acid and the order of amino acids in the polypeptide synthesized from it

C A T G T T T T C C C C C A C C C A

Mutant ATG TTT TCC CCC ACC CAA(protein) Met Phe Ser Pro Thr Gln

Normal ATG TTT TCC CCA CCC AAC(protein) Met Phe Ser Pro Pro Asn

Figure 8. Frame-shift mutation. This example shows a sequence of PITX2 in a patientwith Rieger’s syndrome, an autosomal dominant condition. The sequence graph showsonly the abnormal sequence. The arrow indicates the insertion of a single cytosine (C)residue. When translated, the triplet code is now out of frame by 1 base pair. This totallyalters the translated protein’s amino acid sequence. This leads to a premature stop codonlater in the protein and results in Rieger’s syndrome.

PITX2 sequence



(see Table 2). A sequence of three nucleic acid bases (a triplet) acts as a codeword (codon) for one amino acid or instruction (start/stop).

Genetic counseling Information/advice given to families with, or at risk of, genetic disease.Genetic counseling is a complex discipline that requires accuratediagnostic approaches, up-to-date knowledge of the genetics of thecondition, an insight into the beliefs/anxieties/wishes of the individualseeking advice, intelligent risk estimation, and, above all, skill incommunicating relevant information to individuals from a wide variety of educational backgrounds. Genetic counseling is most often carriedout by trained medical geneticists or, in some countries, specialistgenetic counselors or nurses.

Genetic heterogeneityAssociation of a specific phenotype with mutations at different loci. The broader the phenotypic criteria, the greater the heterogeneity(eg, mental retardation). However, even very specific phenotypes may be genetically heterogeneous. Isolated central hypothyroidism is a good example: this autosomal recessive condition is now known to be associated (in different individuals) with mutations in the TSH β

2nd 2nd 2nd 2nd

T C A G

1st T TTT Phe [F] TCT Ser [S] TAT Tyr [Y] TGT Cys [C] T 3rd

TTC Phe [F] TCC Ser [S] TAC Tyr [Y] TGC Cys [C] C

TTA Leu [L] TCA Ser [S] TAA Ter [end] TGA Ter [end] A

TTG Leu [L] TCG Ser [S] TAG Ter [end] TGG Trp [W] G

1st C CTT Leu [L] CCT Pro [P] CAT His [H] CGT Arg [R] T 3rd

CTC Leu [L] CCC Pro [P] CAC His [H] CGC Arg [R] C

CTA Leu [L] CCA Pro [P] CAA Gln [Q] CGA Arg [R] A

CTG Leu [L] CCG Pro [P] CAG Gln [Q] CGG Arg [R] G

1st A ATT Ile [I] ACT Thr [T] AAT Asn [N] AGT Ser [S] T 3rd

ATC Ile [I] ACC Thr [T] AAC Asn [N] AGC Ser [S] C

ATA Ile [I] ACA Thr [T] AAA Lys [K] AGA Arg [R] A

ATG Met [M] ACG Thr [T] AAG Lys [K] AGG Arg [R] G

1st G GTT Val [V] GCT Ala [A] GAT Asp [D] GGT Gly [G] T 3rd

GTC Val [V] GCC Ala [A] GAC Asp [D] GGC Gly [G] C

GTA Val [V] GCA Ala [A] GAA Glu [E] GGA Gly [G] A

GTG Val [V] GCG Ala [A] GAG Glu [E] GGG Gly [G] G

Table 2. The genetic code. To locate a particular codon (eg, TAG, marked in bold) locatethe first base (T) in the left hand column, then the second base (A) by looking at the toprow, and finally the third (G) in the right hand column (TAG is a stop codon). Note theredundancy of the genetic code – for example, three different codons specify a stopsignal, and threonine (Thr) is specified by any of ACT, ACC, ACA, and ACG.


chain at 1p13, the TRH receptor at 8q23, or TRH itself at 3q13.3–q21.There is no obvious distinction between the clinical phenotypes associatedwith these two genes. Genetic heterogeneity should not be confusedwith allelic heterogeneity, which refers to the presence of differentmutations at the same locus.

Genetic locus A specific location on a chromosome.

Genetic map A map of genetic landmarks deduced from linkage (recombination)analysis. Aims to determine the linear order of a set of genetic markersalong a chromosome. Genetic maps differ significantly from physicalmaps, in that recombination frequencies are not identical acrossdifferent genomic regions, resulting occasionally in large discrepancies.

Genetic marker A gene that has an easily identifiable phenotype so that one candistinguish between those cells or individuals that do or do not have the gene. Such a gene can also be used as a probe to mark cell nuclei or chromosomes, so that they can be isolated easily or identified fromother nuclei or chromosomes later.

Genetic screening Population analysis designed to ascertain individuals at risk of eithersuffering or transmitting a genetic disease.

Genetically lethal Preventing reproduction of the individual, either by causing death prior toreproductive age, or as a result of social factors making it highly unlikely(although not impossible) that the individual concerned will reproduce.

Genome The complete DNA sequence of an individual, including the sexchromosomes and mitochondrial DNA (mtDNA). The genome ofhumans is estimated to have a complexity of 3.3 x 109 base pairs(per haploid genome).

Genomic Pertaining to the genome. Genomic DNA differs from complementaryDNA (cDNA) in that it contains noncoding as well as coding DNA.

Genotype Genetic constitution of an individual, distinct from expressed features (phenotype).

Germ line Germ cells (those cells that produce haploid gametes) and the cells from which they arise. The germ line is formed very early in embryonicdevelopment. Germ line mutations are those present constitutionally

Glossary 253



in an individual (ie, in all cells of the body) as opposed to somaticmutations, which affect only a proportion of cells.

Giemsa banding Light/dark bar code obtained by staining chromosomes with Giemsastain. Results in a unique bar code for each chromosome.

Guanine (G) One of the bases making up DNA and RNA (pairs with cytosine).

H

Haploid The chromosome number of a normal gamete, containing one each of every individual chromosome (23 in humans).

Haploinsufficiency The presence of one active copy of a gene/region is insufficient tocompensate for the absence of the other copy. Most genes are not“haploinsufficient” – 50% reduction of gene activity does not lead to an abnormal phenotype. However, for some genes, most often thoseinvolved in early development, reduction to 50% often correlates withan abnormal phenotype. Haploinsufficiency is an important componentof most contiguous gene disorders (eg, in Williams’ syndrome,heterozygous deletion of a number of genes results in the mutantphenotype, despite the presence of normal copies of all affected genes).

Hemizygous Having only one copy of a gene or DNA sequence in diploid cells. Males are hemizygous for most genes on the sex chromosomes, as they possess only one X chromosome and one Y chromosome(the exceptions being those genes with counterparts on both sexchromosomes). Deletions on autosomes produce hemizygosity in both males and females.

Heterochromatin Contains few active genes, but is rich in highly repeated simplesequence DNA, sometimes known as satellite DNA. Heterochromatinrefers to inactive regions of the genome, as opposed to euchromatin,which refers to active, gene expressing regions. Heterochromatin stainsdarkly with Giemsa.

Heterozygous Presence of two different alleles at a given locus.

Histones Simple proteins bound to DNA in chromosomes. They help to maintain chromatin structure and play an important role in regulating gene expression.


Glossary 255

Holandric Pattern of inheritance displayed by mutations in genes located onlyon the Y chromosome. Such mutations are transmitted only from father to son.

Homologue or Two or more genes whose sequences manifest significant similarityhomologous gene because of a close evolutionary relationship. May be between species

(orthologues) or within a species (paralogues).

Homologous Chromosomes that pair during meiosis. These chromosomes containchromosomes the same linear gene sequences as one another and derive from

one parent.

Homology Similarity in DNA or protein sequences between individuals of the same species or among different species.

Homozygous Presence of identical alleles at a given locus.

Human gene therapy The study of approaches to treatment of human genetic disease, usingthe methods of modern molecular genetics. Many trials are under waystudying a variety of disorders, including cystic fibrosis. Some disordersare likely to be more treatable than others – it is probably going to beeasier to replace defective or absent gene sequences rather than dealwith genes whose aberrant expression results in an actively toxic effect.

Human genome Worldwide collaboration aimed at obtaining a complete sequence of theproject human genome. Most sequencing has been carried out in the USA,

although the Sanger Centre in Cambridge, UK has sequenced one third of the genome, and centers in Japan and Europe have also contributedsignificantly. The first draft of the human genome was released in thesummer of 2000 to much acclaim. Celera, a privately funded venture,headed by Dr Craig Ventner, also published its first draft at the same time.

Hybridization Pairing of complementary strands of nucleic acid. Also known as re-annealing. May refer to re-annealing of DNA in solution, on amembrane (Southern blotting) or on a DNA microarray. May also be used to refer to fusion of two somatic cells, resulting in a hybrid that contains genetic information from both donors.



I

Imprinting A general term used to describe the phenomenon whereby a DNAsequence (coding or otherwise) carries a signal or imprint that indicatesits parent of origin. For most DNA sequences, no distinction can be made between those arising paternally and those arising maternally(apart from subtle sequence variations); for imprinted sequences this is not the case. The mechanistic basis of imprinting is almost alwaysmethylation – for certain genes, the copy that has been inherited fromthe father is methylated, while the maternal copy is not. The situationmay be reversed for other imprinted genes. Note that imprinting of a generefers to the general phenomenon, not which parental copy is methylated(and, therefore, usually inactive). Thus, formally speaking, it is incorrectto say that a gene undergoes paternal imprinting. It is correct to say thatthe gene undergoes imprinting and that the inactive (methylated) copy is always the paternal one. However, in common genetics parlance,paternal imprinting is usually understood to mean the same thing.

In situ hybridization Annealing of DNA sequences to immobilized chromosomes/cells/(ISH) tissues. Historically done using radioactively labeled probes, this is

currently most often performed with fluorescently tagged molecules(fluorescent in situ hybridization – FISH, see Figure 9). ISH/FISHallows for the rapid detection of a DNA sequence within the genome.

Incomplete Complete absence of expression of the abnormal phenotype in a proportionpenetrance of individuals known to be obligate carriers. To be distinguished from

variable expressivity, in which the phenotype always manifests inobligate carriers, but with widely varying degrees of severity.

Index case – proband The individual through which a family medically comes to light. For example, the index case may be a baby with Down’s syndrome. Can be termed propositus (if male) or proposita (if female).

Insertion Interruption of a chromosomal sequence as a result of insertion ofmaterial from elsewhere in the genome (either a different chromosome, or elsewhere from the same chromosome). Such insertions may result in abnormal phenotypes either because of direct interruption of a gene(uncommon), or because of the resulting imbalance (ie, increaseddosage) when the chromosomes that contain the normal counterparts of the inserted sequence are also present.


Glossary 257

Intron A noncoding DNA sequence that “interrupts” the protein-codingsequences of a gene; intron sequences are transcribed into messengerRNA (mRNA), but are cut out before the mRNA is translated into a protein (this process is known as splicing). Introns may containsequences involved in regulating expression of a gene. Unlike the exon, the intron is the nucleotide sequence in a gene that is notrepresented in the amino acid sequence of the final gene product.

Inversion A structural abnormality of a chromosome in which a segment isreversed, as compared to the normal orientation of the segment. An inversion may result in the reversal of a segment that lies entirely on one chromosome arm (paracentric) or one that spans (ie, contains)the centromere (pericentric). While individuals who possess an inversionare likely to be genetically balanced (and therefore usually phenotypicallynormal), they are at increased risk of producing unbalanced offspringbecause of problems at meiosis with pairing of the inversion chromosomewith its normal homologue. Both deletions and duplications may result, with concomitant congenital abnormalities related to genomicimbalance, or miscarriage if the imbalance is lethal.

Figure 9. Fluorescence in situ hybridization. FISH analysis of a patient with acomplex syndrome, using a clone containing DNA from the region 8q24.3. In additionto that clone, a control from 8pter was used. The 8pter clone has yielded a signal on both homologues of chromosome 8, while the “test” clone from 8q24.3 hasyielded a signal on only one homologue, demonstrating a (heterozygous) deletion in that region.



K

Karyotype A photomicrograph of an individual’s chromosomes arranged in a standard format showing the number, size, and shape of eachchromosome type, and any abnormalities of chromosome number or morphology (see Figure 10).

Kilobase (kb) 1000 base pairs of DNA.

Knudson hypothesis See tumor suppressor gene

L

Linkage Coinheritance of DNA sequences/phenotypes as a result of physicalproximity on a chromosome. Before the advent of molecular genetics,linkage was often studied with regard to proteins, enzymes, or cellularcharacteristics. An early study demonstrated linkage between the Duffy blood group and a form of autosomal dominant congenitalcataract (both are now known to reside at 1q21.1). Phenotypes may also be linked in this manner (ie, families manifesting two distinct Mendelian disorders).

During the recombination phase of meiosis, genetic material isexchanged (equally) between two homologous chromosomes. Genes/DNA sequences that are located physically close to each other areunlikely to be separated during recombination. Sequences that lie far apart on the same chromosome are more likely to be separated. For sequences that reside on different chromosomes, segregation will always be random, so that there will be a 50% chance of twomarkers being coinherited.

Linkage analysis An algorithm designed to map (ie, physically locate) an unknown gene(associated with the phenotype of interest) to a chromosomal region.Linkage analysis has been the mainstay of disease-associated geneidentification for some years. The general availability of large numbers of DNA markers that are variable in the population (polymorphisms),and which therefore permit allele discrimination, has made linkageanalysis a relatively rapid and dependable approach (see Figure 11).However, the method relies on the ascertainment of large familiesmanifesting Mendelian disorders. Relatively little phenotypic


Glossary 259

heterogeneity is tolerated, as a single misassigned individual (believedto be unaffected despite being a gene carrier) in a pedigree maycompletely invalidate the results. Genetic heterogeneity is anotherproblem, not within families (usually) but between families. Thus,conditions that result in identical phenotypes despite being associatedwith mutations within different genes (eg, tuberous sclerosis) are oftenhard to study. Linkage analysis typically follows a standard algorithm:

1. Large families with a given disorder are ascertained. Detailed clinicalevaluation results in assignment of affected vs. unaffected individuals.

2. Large numbers of polymorphic DNA markers that span the genomeare analyzed in all individuals (affected and unaffected).

3. The results are analyzed statistically, in the hope that one of themarkers used will have demonstrably been coinherited with thephenotype in question more often than would be predicted by chance.

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Figure 10. Schematic of a normal human (male) karyotype. (ISCN 550 ideogram produced by the MRC HumanGenetics Unit, Edinburgh, reproduced with permission.)



The LOD score (logarithm of the odds) gives an indication of thelikelihood of the result being significant (and not having occurred simply as a result of chance coinheritance of the given marker with the condition).

Linkage Association of particular DNA sequences with each other, more oftendisequilibrium than is likely by chance alone (see Figure 12). Of particular relevance

to inbred populations (eg, Finland), where specific disease mutationsare found to reside in close proximity to specific variants of DNAmarkers, as a result of the founder effect.

2kb

Probe

2kb 5kb 2kb

5kb 2kb

5kb 2kb 2kb

5kb 2kb 2kb

5kb 2kb 2kb 2kb

In the example above, note that the (affected) mother has a 5-kb band in addition to a 2-kb band. All the unaffected individuals have the small band only, all those who areaffected have the large band. The unaffected individuals must have the mother’s 2-kbfragment rather than her 5-kb fragment, and the affected individuals must have inheritedthe 5-kb band from the mother (as the father does not have one) – note that thoseindividuals who only show the 2-kb band still have two alleles (one from each parent),they are just the same size and so cannot be differentiated. Thus, it appears that the 5-kbband is segregating with the disorder. The results in a family such as this are suggestive butfurther similar results in other families would be required for a sufficiently high LOD score.

X XX

The probe recognizes a DNA sequence adjacent to a restriction site (see arrow) that is polymorphic (present on some chromosomes but not others). When such a site ispresent, the DNA is cleaved at that point and the probe detects a 2-kb fragment. Whenabsent, the DNA is not cleaved and the probe detects a fragment of size (2 + 3) kb =5 kb. X refers to the points at which the restriction enzyme will cleave the DNA. Therecognition sequence for most restriction enzymes is very stringent – change in just one nucleotide will result in failure of cleavage. Most RFLPs result from the presence of a single nucleotide polymorphism that has altered the restriction site.

Figure 11. Schematic demonstrating the use of restriction fragment lengthpolymorphisms (RFLPs) in linkage analysis.

3kb


Glossary 261

Linkage map A map of genetic markers as determined by genetic analysis (ie,recombination analysis). May differ markedly from a map determined by actual physical relationships of genetic markers, because of thevariability of recombination.

Locus The position of a gene/DNA sequence on the genetic map. Allelic genes/sequences are situated at identical loci in homologous chromosomes.

Locus heterogeneity Mutations at different loci cause similar phenotypes.

+–

––

++

–+

–+

––

Mutant allele

Mutant allele

Mutant allele

Many generations

Marker A Marker B

Figure 12. Schematic demonstrating the concept of linkage disequilibrium.

A gene is physically very close to marker B and further from marker A. Markers A and B, both on the same chromosome, can exist in one of two forms : +/–. Thus thereare four possible haplotypes, as shown. If the founder mutation in the gene occurredas shown, then it is likely that even after many generations the mutant allele willsegregate with the – form of marker B, as recombination is unlikely to have occurredbetween the two. However, since marker A is further away, the gene will now oftensegregate with the – form of marker A, which was not present on the originalchromosome. The likelihood of recombination between the gene and marker A will depend on the physical distance between them, and on rates of recombination. It is possible that the gene would show a lesser but still significant degree of linkagedisequilibrium with marker A.



LOD (Logarithm A statistical test of linkage. Used to determine whether a result is of the Odds) score likely to have occurred by chance or to truly reflect linkage. The LOD

score is the logarithm (base 10) of the likelihood that the linkage ismeaningful. A LOD score of 3 implies that there is only a 1:1,000chance that the results have occurred by chance (ie, the result would be likely to occur once by chance in 1,000 simultaneous studiesaddressing the same question). This is taken as proof of linkage (see Figure 11).

Lyonization The inactivation of n–1 X chromosomes on a random basis in anindividual with n X chromosomes. Named after Mary Lyon, thismechanism ensures dosage compensation of genes encoded by theX chromosome. X chromosome inactivation does not occur in normalmales who possess only one X chromosome, but does occur in one ofthe two X chromosomes of normal females. In males who possess morethan one X chromosome (ie, XXY, XXXY, etc.), the rule is the same andonly one X chromosome remains active. X-inactivation occurs in earlyembryonic development and is random in each cell. The inactivationpattern in each cell is faithfully maintained in all daughter cells.Therefore, females are genetic mosaics, in that they possess twopopulations of cells with respect to the X chromosome: one populationhas one X active, while in the other population the other X is active. This is relevant to the expression of X-linked disease in females.

M

Meiosis The process of cell division by which male and female gametes (germcells) are produced. Meiosis has two main roles. The first is recombination(during meiosis I). The second is reduction division. Human beings have46 chromosomes, and each is conceived as a result of the union of twogerm cells; therefore, it is reasonable to suppose that each germ cell willcontain only 23 chromosomes (ie, the haploid number). If not, then thefirst generation would have 92 chromosomes, the second 184, etc. Thus,at meiosis I, the number of chromosomes is reduced from 46 to 23.

Mendelian Refers to a particular pattern of inheritance, obeying simple rules: each inheritance somatic cell contains two genes for every characteristic and each pair

of genes divides independently of all other pairs at meiosis.


Glossary 263

Mendelian A catalogue of human Mendelian disorders, initiated in book form Inheritance in by Dr Victor McKusick of Johns Hopkins Hospital in Baltimore, USA.Man (MIM/OMIM) The original catalogue (produced in the mid-1960s) listed approximately

1500 conditions. By December 1998, this number had risen to 10,000and by November 2003 the figure had reached 14,897. With theadvent of the Internet, MIM is now available as an online resource, free of charge (OMIM – Online Mendelian Inheritance in Man). The URL for this site is: http://www.ncbi.nlm.nih.gov/omim/. The onlineversion is updated frequently, far faster than is possible for the printversion; therefore, new gene discoveries are quickly assimilated into thedatabase. OMIM lists disorders according to their mode of inheritance:

1 - - - - (100000– ) Autosomal dominant (entries created beforeMay 15, 1994)

2 - - - - (200000– ) Autosomal recessive (entries created beforeMay 15, 1994)

3 - - - - (300000– ) X-linked loci or phenotypes

4 - - - - (400000– ) Y-linked loci or phenotypes

5 - - - - (500000– ) Mitochondrial loci or phenotypes

6 - - - - (600000– ) Autosomal loci/phenotypes (entries created after May 15, 1994).

Full explanations of the best way to search the catalogue are available at the home page for OMIM.

Messenger RNA The template for protein synthesis, carries genetic information from(mRNA) the nucleus to the ribosomes where the code is translated into protein.

Genetic information flows: DNA → RNA → protein.

Methylation See DNA methylation.

Microdeletion Structural chromosome abnormality involving the loss of a segment thatis not detectable using conventional (even high resolution) cytogeneticanalysis. Microdeletions usually involve 1–3 Mb of sequence (the resolutionof cytogenetic analysis rarely is better than 10 Mb). Most microdeletionsare heterozygous, although some individuals/families have been describedwith homozygous microdeletions. See also contiguous gene syndrome.



Microduplication Structural chromosome abnormality involving the gain of a segment that may involve long sequences (commonly 1–3 Mb), which are,nevertheless, undetectable using conventional cytogenetic analysis.Patients with microduplications have three copies of all sequenceswithin the duplicated segment, as compared to two copies in normalindividuals. See also contiguous gene syndrome.

Microsatellites DNA sequences composed of short tandem repeats (STRs), such asdi- and trinucleotide repeats, distributed widely throughout the genomewith varying numbers of copies of the repeating units. Microsatellites are very valuable as genetic markers for mapping human genes.

Missense mutation Single base substitution resulting in a codon that specifies a differentamino acid than the wild-type.

Mitochondrial Ambiguous term referring to disorders resulting from abnormalitiesdisease/disorder of mitochondrial function. Two separate possibilities should

be considered.

1. Mutations in the mitochondrial genome (see Figure 13). Suchdisorders will manifest an inheritance pattern that mirrors the manner in which mitochondria are inherited. Therefore, a mother will transmit a mitochondrial mutation to all her offspring (all of whom will beaffected, albeit to a variable degree). A father will not transmit thedisorder to any of his offspring.

2. Mutations in nuclear encoded genes that adversely affectmitochondrial function. The mitochondrial genome does not code for all the genes required for its maintenance; many are encoded in the nuclear genome. However, the inheritance patterns will differmarkedly from the category described in the first option, and will be indistinguishable from standard Mendelian disorders.

Each mitochondrion possesses between 2–10 copies of its genome, and there are approximately 100 mitochondria in each cell. Therefore,each cell possesses 200–1,000 copies of the mitochondrial genome.Heteroplasmy refers to the variability in sequence of this large number of genomes – even individuals with mitochondrial genome mutations are likely to have wild-type alleles. Variability in the proportion ofmolecules that are wild-type may have some bearing on the clinicalvariability often seen in such disorders.


Glossary 265

Mitochondrial DNA The DNA in the circular chromosome of mitochondria. MitochondrialDNA is present in multiple copies per cell and mutates more rapidly than genomic (nuclear) DNA.

Mitosis Cell division occurring in somatic cells, resulting in two daughter cellsthat are genetically identical to the parent cell.

Monogenic trait Causally associated with a single gene.

Monosomy Absence of one of a pair of chromosomes.

Monozygotic Arising from a single zygote or fertilized egg. Monozygotic twins are genetically identical.

Mosaicism or mosaic Refers to the presence of two or more distinct cell lines, all derived from the same zygote. Such cell lines differ from each other as a resultof DNA content/sequence. Mosaicism arises when the genetic alterationoccurs postfertilization (postzygotic). The important features that needto be considered in mosaicism are:

The proportion of cells that are “abnormal”. In general, the greater the proportion of cells that are abnormal, the greater the severity of the associated phenotype.

The specific tissues that contain high levels of the abnormal cell line(s).This variable will clearly also be relevant to the manifestation of anyphenotype. An individual may have a mutation bearing cell line in atissue where the mutation is largely irrelevant to the normal functioningof that tissue, with a concomitant reduction in phenotypic sequelae.

Mosaicism may be functional, as in normal females who are mosaic for activity of the two X chromosomes (see Lyonization).

Figure 13. Mitochondrial inheritance. This pedigree relates to mutations in themitochondrial genome.



Mosaicism may occasionally be observed directly. X-linked skindisorders, such as incontinentia pigmenti, often manifest mosaicchanges in the skin of a female, such that abnormal skin is observedalternately with normal skin, often in streaks (Blaschko’s lines), which delineate developmental histories of cells.

Multifactorial A type of hereditary pattern resulting from a complex interplay inheritance of genetic and environmental factors.

Mutation Any heritable change in DNA sequence.

N

Nondisjunction Failure of two homologous chromosomes to pull apart during meiosis I,or two chromatids of a chromosome to separate in meiosis II or mitosis.The result is that both are transmitted to one daughter cell, while theother daughter cell receives neither.

Nondynamic Stably inherited mutations, in contradistinction to dynamic mutations,(stable) mutations which display variability from generation to generation. Includes all

types of stable mutation (single base substitution, small deletions/insertions, microduplications, and microdeletions).

Nonpenetrance Failure of expression of a phenotype in the presence of the relevant genotype.

Nonsense mutation A single base substitution resulting in the creation of a stop codon (see Figure 14).

Northern blot Hybridization of a radiolabeled RNA/DNA probe to an immobilized RNAsequence. So called in order to differentiate it from Southern blotting,which was described first. Neither has any relationship to points on thecompass. Southern blotting was named after its inventor Ed Southern

Nucleotide A basic unit of DNA or RNA consisting of a nitrogenous base – adenine,guanine, thymine, or cytosine in DNA, and adenine, guanine, uracil, or cytosine in RNA. A nucleotide is composed of a phosphate moleculeand a sugar molecule – deoxyribose in DNA and ribose in RNA. Manythousands or millions of nucleotides link to form a DNA or RNA molecule.


Glossary 267

O

Obligate carrier See obligate heterozygote.

Obligate An individual who, on the basis of pedigree analysis, must carry heterozygote the mutant allele.(obligate carrier)

Oncogene A gene that, when over expressed, causes neoplasia. This contrasts with tumor suppressor genes, which result in tumorigenesis when their activity is reduced.

P

p Short arm of a chromosome (from the French petit) (see Figure 4).

G G A C T G T C C T C T G A G

Mutant ACT GTC CTC TGA(protein) Thr Val Leu STOP

Normal ACT GTC CTC TGC(protein) Thr Val Leu Cys

Figure 14. Nonsense mutation. This example shows a sequence graph of collagen II(α1) in a patient with Stickler syndrome, an autosomal dominant condition. Thesequence is of genomic DNA and shows both normal and abnormal sequences (thepatient is heterozygous for the mutation).The base marked with an arrow has been changed from C to A. When translated thecodon is changed from TGC (cysteine) to TGA (stop). The premature stop codon in thecollagen gene results in Stickler syndrome.

Collagen IIα1 sequence



Palindromic A DNA sequence that contains the same 5 to 3 sequence on both sequence strands. Most restriction enzymes recognize palindromic sequences.

An example is 5 –AGATCT–3 , which would read 3 –TCTAGA–5 on the complementary strand. This is the recognition site of BglII.

Pedigree A schematic for a family indicating relationships to the proband andhow a particular disease or trait has been inherited (see Figure 15).

Penetrance An all-or-none phenomenon related to the proportion of individuals withthe relevant genotype for a disease who actually manifest the phenotype.Note the difference between penetrance and variable expressivity.

Phenotype Observed disease/abnormality/trait. An all-embracing term that does not necessarily imply pathology. A particular phenotype may be theresult of genotype, the environment or both.

Physical map A map of the locations of identifiable landmarks on DNA, such asspecific DNA sequences or genes, where distance is measured in base pairs. For any genome, the highest resolution map is the complete nucleotide sequence of the chromosomes. A physical map should be distinguished from a genetic map, which depends on recombination frequencies.

Plasmid Found largely in bacterial and protozoan cells, plasmids are autonomouslyreplicating, extrachromosomal, circular DNA molecules that are distinctfrom the normal bacterial genome and are often used as vectors inrecombinant DNA technologies. They are not essential for cell survivalunder nonselective conditions, but can be incorporated into the genomeand are transferred between cells if they encode a protein that wouldenhance survival under selective conditions (eg, an enzyme that breaksdown a specific antibiotic).

Pleiotropy Diverse effects of a single gene on many organ systems (eg, the mutationin Marfan’s syndrome results in lens dislocation, aortic root dilatation,and other pathologies).

Ploidy The number of sets of chromosomes in a cell. Human cells may be haploid (23 chromosomes, as in mature sperm or ova), diploid(46 chromosomes, seen in normal somatic cells), or triploid (69 chromosomes, seen in abnormal somatic cells, which results in severe congenital abnormalities).


Glossary 269

Point mutation Single base substitution.

Polygenic disease Disease (or trait) that results from the simultaneous interaction ofmultiple gene mutations, each of which contributes to the eventualphenotype. Generally, each mutation in isolation is likely to have a relatively minor effect on the phenotype. Such disorders are notinherited in a Mendelian fashion. Examples include hypertension,obesity, and diabetes.

Polymerase chain A molecular technique for amplifying DNA sequences in vitro (seereaction (PCR) Figure 16). The DNA to be copied is denatured to its single strand form

and two synthetic oligonucleotide primers are annealed to complementaryregions of the target DNA in the presence of excess deoxynucleotides anda heat-stable DNA polymerase. The power of PCR lies in the exponentialnature of amplification, which results from repeated cycling of the“copying” process. Thus, a single molecule will be copied in the first cycle,resulting in two molecules. In the second cycle, each of these will alsobe copied, resulting in four copies. In theory, after n cycles, there will be2n molecules for each starting molecule. In practice, this theoretical limitis rarely reached, mainly for technical reasons. PCR has become a standardtechnique in molecular biology research as well as routine diagnostics.

4

Male, female - unaffected

Sex not known

Male, female – affected

4 unaffected females

Deceased, affected female

Consanguineous marriage

Abortion/stillbirth

Twins

Monozygotic twins

Heterozygote (AR)

Heterozygote (X-linked)

Propositus/proband

Figure 15. Symbols commonly used in pedigree drawing.



Polymorphism May be applied to phenotype or genotype. The presence in a populationof two or more distinct variants, such that the frequency of the rarest isat least 1% (more than can be explained by recurrent mutation alone). A genetic locus is polymorphic if its sequence exists in at least twoforms in the population.

Premutation Any DNA mutation that has little, if any, phenotypic consequence butpredisposes future generations to the development of full mutations with phenotypic sequelae. Particularly relevant in the analysis ofdiseases associated with dynamic mutations.

Proband (propositus) The first individual to present with a disorder through which a pedigree– index case can be ascertained.

Probe General term for a molecule used to make a measurement. In moleculargenetics, a probe is a piece of DNA or RNA that is labeled and used to detect its complementary sequence (eg, Southern blotting).

Promoter region The noncoding sequence upstream (5 ) of a gene where RNApolymerase binds. Gene expression is controlled by the promoter region both in terms of level and tissue specificity.

Protease An enzyme that digests other proteins by cleaving them into smallfragments. Proteases may have broad specificity or only cleave a particular site on a protein or set of proteins.

Protease inhibitor A chemical that can inhibit the activity of a protease. Most proteaseshave a corresponding specific protease inhibitor.

Proto-oncogene A misleading term that refers to genes that are usually involved insignaling and cell development, and are often expressed in activelydividing cells. Certain mutations in such genes may result in malignanttransformation, with the mutated genes being described as oncogenes.The term proto-oncogene is misleading because it implies that suchgenes were selected for by evolution in order that, upon mutation,cancers would result because of oncogenic activation. A similar problem arises with the term tumor suppressor gene.

Pseudogene Near copies of true genes. Pseudogenes share sequence homologywith true genes, but are inactive as a result of multiple mutationsover a long period of time.


Glossary 271

Purine A nitrogen-containing, double-ring, basic compound occurring in nucleicacids. The purines in DNA and RNA are adenine and guanine.

Pyrimidine A nitrogen-containing, single-ring, basic compound that occurs innucleic acids. The pyrimidines in DNA are cytosine and thymine, and cytosine and uracil in RNA.

Q

q Long arm of a chromosome (see Figure 4).

R

Re-annealing See hybridization

Recessive Manifest only in homozygotes. For the X chromosome, recessivity(traits, diseases) applies to males who carry only one (mutant) allele. Females who carry

X-linked mutations are generally heterozygotes and, barring unfortunateX-inactivation, do not manifest X-linked recessive phenotypes.

Figure 16. Schematic illustrating the technique of polymerase chain reaction (PCR).

P1P2

1st Cycle

2nd Cycle

3'

5'

5'

3'

3' 5'

5' 3'

3' 5'

5' 3'

3'

5'

5'

3'

3'P1

P2

Genomic double-stranded DNA

95°C DENATURATIONTemperature is loweredto ~50°C to permitannealing of primers totheir complementaryDNA sequence

Temperature is elevatedto the optimal heat(~72°C) for thethermophilic polymerase,resulting in primerextension

Denaturation andannealing of primers



Reciprocal The exchange of material between two non-homologous chromosomes.translocation

Recombination The creation of new combinations of linked genes as a result of crossingover at meiosis (see Figure 6).

Recurrence risk The chance that a genetic disease, already present in a member of a family, will recur in that family and affect another individual.

Restriction enzyme Endonuclease that cleaves double-stranded (ds)DNA at specificsequences. For example, the enzyme BglII recognizes the sequenceAGATCT, and cleaves after the first A on both strands. Most restrictionendonucleases recognize sequences that are palindromic – thecomplementary sequence to AGATCT, read in the same orientation,is also AGATCT. The term “restriction” refers to the function of theseenzymes in nature. The organism that synthesizes a given restrictionenzyme (eg, BglII) does so in order to “kill” foreign DNA – ”restricting”the potential of foreign DNA that has become integrated to adverselyaffect the cell. The organism protects its own DNA from the restrictionenzyme by simultaneously synthesizing a specific methylase thatrecognizes the same sequence and modifies one of the bases, such that the restriction enzyme is no longer able to cleave. Thus, for everyrestriction enzyme, it is likely that a corresponding methylase exists,although in practice only a relatively small number of these have been isolated.

Restriction fragment A restriction fragment is the length of DNA generated when DNA islength polymorphism cleaved by a restriction enzyme. Restriction fragment length varies(RFLP) when a mutation occurs within a restriction enzyme sequence. Most

commonly the polymorphism is a single base substitution, but it mayalso be a variation in length of a DNA sequence due to variable numbertandem repeats (VNTRs). The analysis of the fragment lengths afterDNA is cut by restriction enzymes is a valuable tool for establishingfamilial relationships and is often used in forensic analysis of blood, hair, or semen (see Figure 11).

Restriction map A DNA sequence map, indicating the position of restriction sites.

Reverse genetics Identification of the causative gene for a disorder, based purely onmolecular genetic techniques, when no knowledge of the function of the gene exists (the case for most genetic disorders).


Glossary 273

Reverse transcriptase Catalyses the synthesis of DNA from a single-stranded RNA template.Contradicted the central dogma of genetics (DNA → RNA → protein) and earned its discoverers the Nobel Prize in 1975.

RNA (ribonucleic RNA molecules differ from DNA molecules in that they contain acid) a ribose sugar instead of deoxyribose. There are a variety of types

of RNA (including messenger RNA, transfer RNA, and ribosomal RNA) and they work together to transfer information from DNA to the protein-forming units of the cell.

Robertsonian A translocation between two acrocentric chromosomes, resulting translocation from centric fusion. The short arms and satellites (chromosome

segments separated from the main body of the chromosome by a constriction and containing highly repetitive DNA) are lost.

S

Second hit hypothesis See tumor suppressor gene

Segmental aneusomy A general term designed to encompass microdeletion/microduplicationsyndrome (SAS) syndrome, contiguous gene syndrome, and any situation that results in

loss of function of a group of genes at a particular chromosome location,irrespective of genomic copy number (ie, loss of function may be relatedto mutations in master control regions, which affect the expression ofmany genes). See also contiguous gene syndrome.

Sex chromosomes Refers to the X and Y chromosomes. All normal individuals possess 46 chromosomes, of which 44 are autosomes and two are sexchromosomes. An individual’s sex is determined by his/her complementof sex chromosomes. Essentially, the presence of a Y chromosomeresults in the male phenotype. Males have an X and a Y chromosome,while females possess two X chromosomes. The Y chromosome is smalland contains relatively few genes, concerned almost exclusively with sexdetermination and/or sperm formation. By contrast, the X chromosome is a large chromosome that possesses many hundreds of genes.

Sex-limited trait A trait/disorder that is almost exclusively limited to one sex and oftenresults from mutations in autosomal genes. A good example of asex-limited trait is breast cancer. While males are affected by breastcancer, it is much less common (~1%) than in women. Females



are more prone to breast cancer than males, not only because theypossess significantly more breast tissue, but also because their hormonalmilieu is significantly different. In many cases, early onset bilateralbreast cancer is associated with mutations either in BRCA1 or BRCA2,both autosomal genes. An example of a sex-limited trait in males is male pattern baldness, which is extremely rare in premenopausal women.The inheritance of male pattern baldness is consistent with autosomaldominant, not sex-linked dominant, inheritance.

Sex-linked dominant See X-linked dominant

Sex-linked recessive See X-linked recessive

Sibship The relationship between the siblings in a family.

Silent mutation One that has no (apparent) phenotypic effect.

Single gene disorder A disorder resulting from a mutation on one gene.

Somatic cell Any cell of a multicellular organism not involved in the production of gametes.

Southern blot Hybridization with a radiolabeled RNA/DNA probe to an immobilizedDNA sequence (see Figure 17). Named after Ed Southern (currentlyProfessor of Biochemistry at Oxford University, UK), the technique has spawned the nomenclature for other types of blot (Northern blotsfor RNA and Western blots for proteins).

Splicing Removal of introns from precursor RNA to produce messenger RNA(mRNA). The process involves recognition of intron–exon junctions and specific removal of intronic sequences, coupled with reconnection of the two strands of DNA that formerly flanked the intron.

Start codon The AUG codon of messenger RNA recognized by the ribosome to beginprotein production.

Stop codon The codons UAA, UGA, or UAG on messenger RNA (mRNA) (seeTable 2). Since no transfer RNA (tRNA) molecules exist that possessanticodons to these sequences, they cannot be translated. When theyoccur in frame on an mRNA molecule, protein synthesis stops and theribosome releases the mRNA and the protein.


Glossary 275

Synergistic This refers to the phenomenon whereby the manifestation of a phenotypeheterozygosity normally associated with complete loss of function of a single gene

(ie, that gene has two mutations) may be associated with heterozygousmutations in two distinct genes that inhabit the same or related pathways.

T

Telomere End of a chromosome. The telomere is a specialized structure involvedin replicating and stabilizing linear DNA molecules.

Teratogen Any external agent/factor that increases the probability of congenitalmalformations. A teratogen may be a drug, whether prescribed or illicit,or an environmental effect, such as high temperature. The classicalexample is thalidomide, a drug originally prescribed for morning sickness,which resulted in very high rates of congenital malformation in exposedfetuses (especially limb defects).

Termination codon See stop codon.

A B

Figure 17. Southern blotting.



Thymine (T) One of the bases making up DNA and RNA (pairs with adenine).

Transcription Synthesis of single-stranded RNA from a double-stranded DNA template(see Figure 18).

Transfer RNA An RNA molecule that possesses an anticodon sequence (complementary(tRNA) to the codon in mRNA) and the amino acid which that codon specifies.

When the ribosome “reads” the mRNA codon, the tRNA with thecorresponding anticodon and amino acid is recruited for protein synthesis.The tRNA “gives up” its amino acid to the production of the protein.

Translation Protein synthesis directed by a specific messenger RNA (mRNA), (see Figure 19). The information in mature mRNA is converted at theribosome into the linear arrangement of amino acids that constitutesa protein. The mRNA consists of a series of trinucleotide sequences,known as codons. The start codon is AUG, which specifies thatmethionine should be inserted. For each codon, except for the stopcodons that specify the end of translation, a transfer RNA (tRNA)molecule exists that possesses an anticodon sequence (complementaryto the codon in mRNA) and the amino acid which that codon specifies.The process of translation results in the sequential addition of aminoacids to the growing polypeptide chain. When translation is complete,the protein is released from the ribosome/mRNA complex and may

Figure 18. Schematic demonstrating the process of transcription. The sense strandhas the sequence CTC (coding for leucine). RNA is generated by pairing with theantisense strand, which has the sequence GAG (the complement of CTC). The RNAproduced is the complement of GAG, CUC (essentially the same as CTC, uracilreplaces thymine in RNA).

DNA

5'

RNA

CTC

CUC

GAG

3'

RNA polymerase

Sense strand

Antisense strand


Glossary 277

then undergo posttranslational modification, in addition to folding into its final, active, conformational shape.

Translocation Exchange of chromosomal material between two or morenonhomologous chromosomes. Translocations may be balanced orunbalanced. Unbalanced translocations are those that are observed in association with either a loss of genetic material, a gain, or both. As with other causes of genomic imbalance, there are usually phenotypicconsequences, in particular mental retardation. Balanced translocationsare usually associated with a normal phenotype, but increase the risk of genomic imbalance in offspring, with expected consequences(either severe phenotypes or lethality). Translocations are described by incorporating information about the chromosomes involved (usuallybut not always two) and the positions on the chromosomes at which thebreaks have occurred. Thus t(11;X)(p13;q27.3) refers to an apparentlybalanced translocation involving chromosome 11 and X, in which thebreak on 11 is at 11p13 and the break on the X is at Xq27.3

Figure 19. Schematic of the process of translation. Messenger RNA (mRNA) istranslated at the ribosome into a growing polypeptide chain. For each codon, there is a transfer RNA (tRNA) molecule with the anticodon and the appropriate amino acid. Here, the amino acid leucine is shown being added to the polypeptide. The nextcodon is GUC, specifying valine. Translation happens in a 5 to 3 direction along themRNA molecule. When the stop codon is reached, the polypeptide chain is releasedfrom the ribosome.

tRNA with anticodon GAG,charged with Leucine

Amino (NH2)terminus of protein

Growingpolypeptidechain LEU

G A GC U C G U C

mRNA5' 3'

Ribosome

Ribosome moves to next codon



Triallelic inheritance The association of a phenotype with three mutations. The classicalexample is Bardet–Biedl syndrome, in which some individuals onlymanifest the phenotype when three independent mutations are present(two on one gene and another on one of several genes implicated in this disorder). Triallelic inheritance has been trumpeted as providing an insight into the no-man’s land that lies between Mendelian andpolygenic disorders.

Triplet repeats Tandem repeats in DNA that comprise many copies of a basictrinucleotide sequence. Of particular relevance to disorders associatedwith dynamic mutations, such as Huntington’s chorea (HC). HC isassociated with a pathological expansion of a CAG repeat within thecoding region of the huntingtin gene. This repeat codes for a tract of polyglutamines in the resultant protein, and it is believed that theincrease in length of the polyglutamine tract in affected individuals is toxic to cells, resulting in specific neuronal damage.

Trisomy Possessing three copies of a particular chromosome instead of two.

Tumor suppressor Genes that act to inhibit/control unrestrained growth as part of normalgenes development. The terminology is misleading, implying that these genes

function to inhibit tumor formation. The classical tumor suppressor geneis the Rb gene, which is inactivated in retinoblastoma. Unlike oncogenes,where a mutation at one allele is sufficient for malignant transformationin a cell (since mutations in oncogenes result in increased activity, whichis unmitigated by the normal allele), both copies of a tumor suppressorgene must be inactivated in a cell for malignant transformation to proceed.Therefore, at the cellular level, tumor suppressor genes behave recessively.However, at the organismal level they behave as dominants, and anindividual who possesses a mutation in only one Rb allele still has an extremely high probability of developing bilateral retinoblastomas.

The explanation for this phenomenon was first put forward by Knudsonand has come to be known as the Knudson hypothesis (also known asthe second hit hypothesis). An individual who has a germ-line mutationin one Rb allele (and the same argument may be applied to any tumorsuppressor gene) will have the mutation in every cell in his/her body. It is believed that the rate of spontaneous somatic mutation (definedfunctionally, in terms of loss of function of that gene by whatevermechanism) is of the order of one in a million per gene per cell division.


Glossary 279

Given that there are many more than one million retinal cells in eacheye, and many cell divisions involved in retinal development, the chancethat the second (wild-type) Rb allele will suffer a somatic mutation is extremely high. In a cell that has acquired a “second hit”, there will now be no functional copies of the Rb gene, as the other allele isalready mutated (germ-line mutation). Such a cell will have completelylost its ability to control cell growth and will eventually manifest as aretinoblastoma. The same mechanism occurs in many other tumors, the tissue affected being related to the tissue specificity of expression of the relevant tumor suppressor gene.

U

Unequal crossing Occurs between similar sequences on chromosomes that are not over properly aligned. It is common where specific repeats are found

and is the basis of many microdeletion/microduplication syndromes (see Figure 20).

Uniparental disomy In the vast majority of individuals, each chromosome of a pair is derived(UPD) from a different parent. However, UPD occurs when an offspring receives

both copies of a particular chromosome from only one of its parents.UPD of some chromosomes results in recognizable phenotypes whereasfor other chromosomes there do not appear to be any phenotypicsequelae. One example of UPD is Prader–Willi syndrome (PWS), which can occur if an individual inherits both copies of chromosome 15from their mother.

Uniparental Uniparental disomy in which the two homologues inherited from theheterodisomy same parent are not identical. If the parent has chromosomes A,B

the child will also have A,B.

Uniparental Uniparental disomy in which the two homologues inherited from isodisomy the same parent are identical (ie, duplicates). So, if the parent has

chromosomes A,B then the child will have either A,A or B,B.

Uracil (U) A nitrogenous base found in RNA but not in DNA, uracil is capable of forming a base pair with adenine.



V

Variable expressivity Variable expression of a phenotype: not all-or-none (as is the case withpenetrance). Individuals with identical mutations may manifest variableseverity of symptoms, or symptoms that appear in one organ and not in another.

Variable number of Certain DNA sequences possess tandem arrays of repeated sequences.tandem repeats Generally, the longer the array (ie, the greater the number of copies(VNTR) of a given repeat), the more unstable the sequence, with a consequent

wide variability between alleles (both within an individual and betweenindividuals). Because of their variability, VNTRs are extremely useful for genetic studies as they allow for different alleles to be distinguished.

W

Western blot Like a Southern or Northern blot but for proteins, using a labeledantibody as a probe.

Figure 20. Schematic demonstrating (i) normal homologous recombination and (ii) homologous unequal recombination, resulting in a deletion and a duplication chromosome.

A1 B1 C1

C2B2A2

A1Equal (normal)recombinationat meiosis

Unequal (abnormal)recombination at meiosis

Meiotic exchange (crossing over)

B1

Repeats 1 and 2 represent identicalrepeated sequences in differentpositions on the chromosome.These are likely to have no function.

C2

C1B2A2

A1 B1 C1

C2B2A2

A2 B2 B1 C1

A1 C2

Product 1Duplication of region Band all genes within it

Product 2Deletion of region Band all genes within it


Glossary 281

X

X-autosome Translocation between the X chromosome and an autosome.translocation

X chromosome See sex chromosomes.

X-chromosome See lyonization.inactivation

X-linked Relating to the X chromosome/associated with genes on the X chromosome.

X-linked recessive X-linked disorder in which the phenotype is manifest in (XLR) homozygous/hemizygous individuals (see Figure 21). In practice,

it is hemizygous males that are affected by X-linked recessive disorders,such as Duchenne’s muscular dystrophy (DMD). Females are rarelyaffected by XLR disorders, although a number of mechanisms have beendescribed that predispose females to being affected, despite beingheterozygous.

X-linked dominant X-linked disorder that manifests in the heterozygote. XLD disorders

(XLD) result in manifestation of the phenotype in females and males (seeFigure 22). However, because males are hemizygous, they are moreseverely affected as a rule. In some cases, the XLD disorder results in male lethality.

Y

Y chromosome See sex chromosomes.

Z

Zippering A process by which complementary DNA (cDNA) strands that haveannealed over a short length undergo rapid full annealing along theirwhole length. DNA annealing is believed to occur in two main stages. A chance encounter of two strands that are complementary results in a short region of double-stranded DNA (dsDNA), which if perfectlymatched, stabilizes the two single strands so that further re-annealing



Figure 21a. X-linked recessive inheritance – A. Most X-linked disorders manifestrecessively, in that heterozygous females (carriers) are unaffected and males, who arehemizygous (possess only one X chromosome) are affected. In this example, a carriermother has transmitted the disorder to three of her sons. One of her daughters is alsoa carrier. On average, 50% of the male offspring of a carrier mother will be affected(having inherited the mutated X chromosome), and 50% will be unaffected. Similarly,50% of daughters will be carriers and 50% will not be carriers. None of the femaleoffspring will be affected but the carriers will carry the same risks to their offspring as their mother. The classical example of this type of inheritance is Duchennemuscular dystrophy.

Figure 22. X-linked dominant inheritance. In X-linked dominant inheritance, theheterozygous female and hemizygous male are affected, however, the males areusually more severely affected than the females. In many cases, X-linked dominantdisorders are lethal in males, resulting either in miscarriage or neonatal/infantiledeath. On average, 50% of all males of an affected mother will inherit the gene and be severely affected; 50% of males will be completely normal. Fifty percent of femaleoffspring will have the same phenotype as their affected mother and the other 50%will be normal and carry no extra risk for their offspring. An example of this type ofinheritance is incontinentia pigmenti, a disorder that is almost always lethal in males(males are usually lost during pregnancy).

Figure 21b. X-linked recessive inheritance – B. In this example the father is affected.Because all his sons must have inherited their Y chromosome from him and their X chromosome from their normal mother, none will be affected. Since all his daughtersmust have inherited his X chromosome, all will be carriers but none affected. For thistype of inheritance, it is clearly necessary that males reach reproductive age and arefertile – this is not the case with Duchenne’s muscular dystrophy, which is usuallyfatal by the teenage years in boys. Emery-Dreifuss muscular dystrophy is a goodexample of this form of inheritance, as males are likely to live long enough to reproduce.


Glossary 283

of their specific sequences proceeds extremely rapidly. The initial stage is known as nucleation, while the second stage is called zippering.

Zygote Diploid cell resulting from the union of male and female haploid gametes.




Index



Page numbers in italics indicate tables.Page numbers in bold indicate figures.vs indicates a comparison or differential diagnosis.

ABCD1 63–4ACADM 210acetylhydrolase 1B 46–7achondrogenesis II 122achondroplasia 108–10acrocephalosyndactyly

type I see Apert syndrometype III see Saethre–Chotzen syndrome (SCS)type V see Pfeiffer syndrome

acrocephaly 144acylcarnitines 211ADA 206, 206–7ADAMTS2 112Addison disease 62adenosine deaminase (ADA) deficiency 205–7adrenal carcinoma 221adrenal steroid 21-hydroxylase 161–2adrenoleukodystrophy, X-linked 62–4adrenomyeloneuropathy (AMN) 62adult nonnephropathic cystinosis 223–5adult polycystic kidney disease

type 1 (APKD1) 226–7type 2 (APKD2) 226–8

AE1hereditary elliptocytosis 189hereditary renal tubular acidosis 192hereditary spherocytosis 191Southeast Asian ovalocytosis 192

agammaglobulinemia, X-linked see Bruton agammaglobulinemiaaganglionic megacolon see Hirschsprung diseaseagenesis of the corpus callosum, X-linked 67agyria spectrum see lissencephalyAIPL1 76, 77, 78AIS (androgen insensitivity syndrome) 158–9Alagille syndrome 174–5Albright’s hereditary osteodystrophy (AHO) (pseudohypoparathyroidism) 169–71aldosterone synthesis 161

congenital adrenal hyperplasia 160aldosterone synthetase deficiency 162α1-antitrypsin deficiency 175–7α-fetoprotein (AFP) 2Alport syndrome (AS) 218–20

genes 122androgen insensitivity syndrome (AIS) 158–9androgen receptor

androgen insensitivity syndrome 158–9Sotos syndrome 154

anemia 193Angelman syndrome (AS) 30–3

Rett syndrome vs 31, 62anion exchange member 1

hereditary elliptocytosis 189hereditary spherocytosis 191

aniridia 70–2ANK1 191ankyrin 1 191α1-antitrypsin deficiency 175–7Antly–Bixler syndrome 147Apert syndrome 144–6

fibroblast growth factor receptors 145AQP2 163aquaporin-2 163, 164


Index 287

aqueduct stenosis, X-linked (X-linked hydrocephalus) 66–8AR

androgen insensitivity syndrome 158–9CAG repeat 159

areflexia 10, 11, 27arginine vasopressin (AVP) 163, 163–4arginine vasopressin receptor 2 163, 164aristaless-related homeobox protein 48artemin 95arterio-hepatic disorder (Alagille syndrome) 174–5arthrochalasis multiplex congenita 112arthro-ophthalmopathy, hereditary (Stickler syndrome) 122, 125–7ARX 47, 48, 50AS see Alport syndrome (AS); Angelman syndrome (AS)ASM1 (H19) 221ASM1(H19) 223asplenia with cardiovascular anomalies (laterality defects) 137–8ataxia–telangiectasia (AT) 2–4ATB7B 216ATM 2–4ATP7A

Ehlers–Danlos syndrome 112Menkes disease 212

atrial septal defects (ASDs)Holt–Oram syndrome 136laterality defects 137Noonan syndrome 138

ATR-X syndrome 64–6autosomal dominant hyperplasia 81autosomal dominant keratitis 71autosomal recessive polycystic kidney disease (ARKPD) 226–8AVP 163, 163–4AVPR2 163, 164

bacterially expressed kinase (BEK) 145, 147Bardet–Biedl syndrome (BBS) 72–4bare lymphocyte syndrome 206Barth syndrome 130Batten disease see neuronal ceroid lipofucinosis (NCL)BBS (Bardet–Biedl syndrome) 72–4BBS1 72, 73BBS2 72, 73BBS4 72, 73BBS6 73B-cell lymphomas 2Beals’ syndrome 119Beare–Stevenson cutis gyrata syndrome 147Becker muscular dystrophy 5–6Beckwith–Wiedemann syndrome (BWS) 220–3Bethlem myopathy 122bilateral pseudoglioma 79birth weights, Sotos syndrome 153bone marrow failure, Fanconi anemia 182Bourneville–Pringle syndrome (tuberous sclerosis) 101–3brachycephaly

Apert syndrome 144Saethre–Chotzen syndrome 151

brain tumors, retinoblastoma 100BRCA2 183bronchiectasis, primary ciliary dyskinesia 139Bruton agammaglobulinemia 202–3

associated with growth hormone deficiency 203Bruton tyrosine kinase 202–3BTK

Bruton agammaglobulinemia 202–3growth hormone deficiency 165



butterfly vertebrae, Alagille syndrome 174BWS (Beckwith-Wiedemann syndrome) 220–3

cadherin 23nonsyndromal hearing loss 84Usher syndrome 89

café-au-lait macules 96CAG repeat, AR 159CAH (congenital adrenal hyperplasia) 160–2, 162calpain 3 19, 20cancer 93–105CAPN3 19, 20carcinoembryonic antigen 2cardiomyopathy

hypertrophic, Noonan syndrome 138X-linked 6

cardio-respiratory disorders 129–42cartilage oligomeric matrix protein 124–5“cascade screening”, cystic fibrosis 133cataracts, congenital 71catecholamines 96CAV3 19, 20caveolin 3 19, 20CBP/CREBBP 151CDH23

nonsyndromal hearing loss 84Usher syndrome 89, 89–90

CDKN1C 221, 222cerebral gigantism (Sotos syndrome) 153–4cerebral malformations 29–68ceruloplasmin

Menkes disease 212Wilson disease 216

CF (cystic fibrosis) 131–3CFTR 131–3CGD (chronic granulomatous disease) 203–5CGG repeats, fragile X syndrome 34–6Charcot–Marie–Tooth disease see hereditary motor and sensory neuropathy (HMSN)chondrosarcomas, hereditary multiple exostoses 115chorea, Huntington disease 41Christmas disease (hemophilia B) 187–8chromosome 1

chronic granulomatous disease 204cobblestone lissencephaly 49collagen gene disorders 122congenital adrenal hyperplasia 162congenital hypomyelinating neuropathy 15Dejerine–Sottas syndrome 14hereditary elliptocytosis 189hereditary motor and sensory neuropathy 13, 14, 15hereditary spherocytosis 191Hirschsprung disease association 179infantile neuronal ceroid lipofucinosis 54Leber congenital amaurosis 76limb-girdle muscular dystrophy 20medium chain acyl-CoA dehydrogenase deficiency 210nonsyndromal hearing loss 84ocular-scoliotic Ehlers–Danlos syndrome 112Stickler syndrome II 126Usher syndrome 89van der Woude syndrome 155Waardenburg syndrome 91

chromosome 2Alport syndrome 219arterial/vascular Ehlers–Danlos syndrome 112Bardet–Biedl syndrome 72


Index 289

classical Ehlers–Danlos syndrome 112collagen gene disorders 122familial hypermobility Ehlers–Danlos syndrome 112Hirschsprung disease association 179holoproscencephaly 36limb-girdle muscular dystrophy 20nonsyndromal hearing loss 84Waardenburg syndrome 91

chromosome 3Bardet–Biedl syndrome 72collagen gene disorders 122Fanconi anemia 183hereditary motor and sensory neuropathy 13Hirschsprung disease association 179limb-girdle muscular dystrophy 20nonsyndromal hearing loss 84panhypopituitarism 168proximal myotonic myopathy 25Usher syndrome 89von Hippel–Lindau disease 104Waardenburg syndrome 91

chromosome 4achondroplasia 109adult polycystic kidney disease type 2 227Crouzon syndrome 147fascioscapulohumeral muscular dystrophy 7Huntington disease 42limb-girdle muscular dystrophy 21nonsyndromal hearing loss 84polycystic kidney disease 227Rieger syndrome 80

chromosome 5dermatosparaxis Ehlers–Danlos syndrome 112growth hormone receptor defects 167hereditary motor and sensory neuropathy 15Hirschsprung disease association 179limb-girdle muscular dystrophy 20, 21nonsyndromal hearing loss 84panhypopituitarism 168primary ciliary dyskinesia 140Sotos syndrome 153spinal muscular atrophy 27Treacher Collins syndrome 154Usher syndrome 89

chromosome 6autosomal recessive polycystic kidney disease 227collagen gene disorders 122congenital adrenal hyperplasia 160Fanconi anemia 183laterality defects 137Leber congenital amaurosis 76limb-girdle muscular dystrophy 20nonsyndromal hearing loss 84polycystic kidney disease 227Stickler syndrome III 126

chromosome 7arthrochalasis multiplex congenita 112chronic granulomatous disease 204cobblestone lissencephaly 49collagen gene disorders 122cystic fibrosis 131Greig syndrome 148hereditary motor and sensory neuropathy 13holoproscencephaly 36limb-girdle muscular dystrophy 20nonsyndromal hearing loss 84



osteogenesis imperfecta 120Pendred syndrome 87Saethre–Chotzen syndrome 151Williams syndrome 141

chromosome 8congenital adrenal hyperplasia 162hereditary motor and sensory neuropathy 13, 14, 15hereditary multiple exostoses 116hereditary spherocytosis 191nonsyndromal hearing loss 84Pfeiffer syndrome 150Waardenburg syndrome 91

chromosome 9classical Ehlers–Danlos syndrome 112cobblestone lissencephaly 49collagen gene disorders 122Fanconi anemia 183Friedreich Ataxia 8Fukuyama muscular dystrophy 49holoproscencephaly 36limb-girdle muscular dystrophy 21nonsyndromal hearing loss 84panhypopituitarism 168primary ciliary dyskinesia 140tuberous sclerosis 102Walker–Warburg syndrome 49

chromosome 10Apert syndrome 144collagen gene disorders 122congenital adrenal hyperplasia 162congenital hypomyelinating neuropathy 15Crouzon syndrome 147Dejerine–Sottas syndrome 14hereditary motor and sensory neuropathy 13, 14, 15Hirschsprung disease association 179multiple endocrine neoplasia type 2 94nonsyndromal hearing loss 84Pfeiffer syndrome 150Usher syndrome 89

chromosome 11aniridia 70ataxia–telangiectasia 2Bardet–Biedl syndrome 72Beckwith–Wiedemann syndrome 221, 222Fanconi anemia 183hereditary motor and sensory neuropathy 15hereditary multiple exostoses 116late infantile neuronal ceroid lipofucinosis 54nonsyndromal hearing loss 84sickle cell anemia 194β-thalassemia 197Usher syndrome 89

chromosome 12collagen gene disorders 122Holt–Oram syndrome 136nephrogenic diabetes insipidus 163Noonan syndrome 138phenylketonuria 214Stickler syndrome I 126von Willebrand disease 199

chromosome 13connexin 26 gene defect 84, 85Fanconi anemia 183Hirschsprung disease association 179holoproscencephaly 36late infantile neuronal ceroid lipofucinosis 54


Index 291

limb-girdle muscular dystrophy 20nonsyndromal hearing loss 84retinoblastoma 99Rieger syndrome 80Waardenburg syndrome 91Wilson disease 216

chromosome 14α1-antitrypsin deficiency 176hereditary elliptocytosis 189hereditary spherocytosis 191Leber congenital amaurosis 76nonsyndromal hearing loss 84severe combined immunodeficiency 206Usher syndrome 89

chromosome 15Angelman syndrome 30, 31Bardet–Biedl syndrome 72limb-girdle muscular dystrophy 20Marfan syndrome 118nonsyndromal hearing loss 84Prader–Willi syndrome 31, 59

chromosome 16adult polycystic kidney disease type 1 227Bardet–Biedl syndrome 72chronic granulomatous disease 204Fanconi anemia 183hereditary motor and sensory neuropathy 13nonsyndromal hearing loss 84polycystic kidney disease 227α-thalassemia 195

chromosome 17arthrochalasis multiplex congenita 112classical Ehlers–Danlos syndrome 112collagen gene disorders 122cystinosis 224Dejerine–Sottas syndrome 14growth hormone deficiency 165hereditary elliptocytosis 189hereditary motor and sensory neuropathy 13, 14, 15hereditary neuropathy with liability to pressure palsies 15hereditary spherocytosis 191Leber congenital amaurosis 76limb-girdle muscular dystrophy 20, 21lissencephaly 48Miller–Dieker syndrome 48neurofibromatosis type 1 97nonsyndromal hearing loss 84osteogenesis imperfecta 120Usher syndrome 89

chromosome 18, holoproscencephaly 36chromosome 19

Dejerine–Sottas syndrome 14hereditary motor and sensory neuropathy 14hereditary multiple exostoses 116Hirschsprung disease association 179Leber congenital amaurosis 76limb-girdle muscular dystrophy 21myotonic dystrophy 25pseudoachondrodysplasia 124

chromosome 20adenosine deaminase deficiency 206Alagille syndrome 174Bardet–Biedl syndrome 72collagen gene disorders 122Hirschsprung disease association 179neurohypophyseal diabetes insipidus 163



pseudohypoparathyroidism 171severe combined immunodeficiency 206Waardenburg syndrome 91

chromosome 21collagen gene disorders 122holoproscencephaly 36nonsyndromal hearing loss 84Usher syndrome 89

chromosome 22DiGeorge/Shprintzen syndrome 134Hirschsprung disease association 179nonsyndromal hearing loss 84Waardenburg syndrome 91

chromosome analysisaniridia 71lissencephaly 51

chronic granulomatous disease (CGD) 203–5classic hemophilia (hemophilia A) 185–7claudin 14 84CLDN14 84clear cell renal cell carcinomas, von Hippel–Lindau disease 103cleft lip/palate, van der Woude syndrome 155clinical examination

Ehlers–Danlos syndrome 114tuberous sclerosis 103

CLN1 54, 55CLN2 54, 56CLN3 54, 56CLN5 54, 56–7Coat’s disease 79cobblestone lissencephaly 46COCH 84cochlin 84COL1A1

Ehlers–Danlos syndrome 112, 113–4, 122osteogenesis imperfecta 120–1, 122osteoporosis 122

COL1A2Ehlers–Danlos syndrome 112, 113–4, 122osteogenesis imperfecta 120–1, 122osteoporosis 122

COL2A1 122, 126–7Stickler syndrome I 122, 126, 126

COL3A1 112, 113, 122COL4A3 122, 218–20, 219COL4A4 122, 218–9, 219COL4A5 122, 218–20, 219COL4A6

Alport syndrome 219leiomyomatosis 122

COL5A1 111, 112, 113–4, 122COL5A2 111, 112, 113–4, 122COL6A1 122COL6A2 122COL6A3 122COL7A1 122COL9A1 122COL9A2 122COL9A3 122COL10A1 122COL11A1 122, 126, 127COL11A2

nonsyndromal hearing loss 84Stickler syndrome III 122, 126, 127Weissenbacher–Zweymuller syndrome 122

COL17A1 122


Index 293

COL18A1 122collagen

type IIα2 84type IV 218–20

combined pituitary hormone deficiency (panhypopituitarism) 167–9COMP 124–5congenital adrenal hyperplasia (CAH) 160–2, 162congenital cataracts 71congenital central hyperventilation syndrome 95congenital hypomyelinating neuropathy (CHN) 11, 15congenital intestinal aganglionosis see Hirschsprung diseasecongenital lipoid adrenal hyperplasia 162connective tissue disorders 107–27connexin

26/30 8426 gene defect 85–631 84

conotruncal anomaly facial syndrome (DiGeorge/Shprintzen syndrome) 133–5convulsions, Angelman syndrome 30Cooley’s anemia (β-thalassemia) 197–8copper deficiency, Menkes disease 211copper deposition, Wilson disease 215–6copper-transporting ATPase

Menkes disease 212Wilson disease 216

cortisol synthesis 161congenital adrenal hyperplasia 160

craniofacial disorders 143–56craniofacial dysostosis see Crouzon syndromecraniosynostosis

Crouzon syndrome 146Saethre–Chotzen syndrome 151

CRB1 76, 77–8creatine kinase (CK) 6CREBBP/CBP 151cross-reacting material (CRM) positive, hemophilia A 186Crouzon syndrome 146–8

FGFR2 150fibroblast growth factor receptors 145

Crouzon syndrome with acanthosis nigricans 147–8fibroblast growth factor receptors 145

CRX1 76, 77, 78CTNS 224cutaneous neurofibromas 96cutis laxa, Ehlers–Danlos syndrome vs 110–1CX26/30

connexin 26 gene defect 85–6nonsyndromal hearing loss 84

CX31 84CX32 (GLB1) 12, 13, 16, 17CXORF5(OFDI) 225–6CYBA 204, 204CYBB 204, 204–5cyclin-dependent kinase inhibitor 1C 221CYP11B1 162CYP11B2 162CYP17 162CYP21 160–2cystic fibrosis (CF) 131–3cystic fibrosis transmembrane conductance regulator (CFTCR) 131–3cystinoin 224cystinosis 223–5cytochrome-b

α-subunit 204β-subunit 204



DCX 47, 48DDP 84deafness with goiter (Pendred syndrome) 86–7Dejerine–Sottas syndrome 10dementia, Huntington disease 41depigmentation, Waardenburg syndrome 90dermatosparaxis Ehlers–Danlos syndrome 11217, 20-desmolase deficiency 159dexamethasone, congenital adrenal hyperplasia 162DFNA5 84diabetes insipidus (DI) 163–4

nephrogenic 163, 164neurohypophyseal 163, 163–4

diaphyseal aclasis (hereditary multiple exostoses) 115–7DiGeorge/Shprintzen syndrome 133–5dihydropteridine reductase 215dilated cardiomyopathy

Barth syndrome 130LMNA 19

DMD 4–6DMD (Duchenne muscular dystrophy) 4–6DMPK 25–6DNAH5 140DNAI1 140DNA methylation, Beckwith–Wiedemann syndrome 222–3doublecortin 46–47, 48Down’s syndrome, Hirschsprung disease association 177Duchenne muscular dystrophy (DMD) 4–6Dunnigan type partial lipodystrophy 19dynein 46–47, 140, 140–1dysarthria, Friedreich ataxia 8DYSF 19, 20dysferlin 19, 20dystonia canthorum 90dystrophia myotonica protein kinase 25–6dystrophin 5–6

ECE1 179eczema

phenylketonuria 214Wiskott–Aldrich syndrome 207–8

EDN3Hirschsprung disease association 179Waardenburg syndrome 91, 92

EDNRBHirschsprung disease association 179Waardenburg syndrome 91, 92

EGR2 12, 13, 14, 15Ehlers–Danlos syndrome 110–4

cutis laxa vs 110–1genes/chromosomal locations 112, 122

elastin 141–2electroencephalography (EEG)

adult neuronal ceroid lipofucinosis 55infantile neuronal ceroid lipofucinosis 54

electromyography 27electron microscopy, Ehlers–Danlos syndrome 114electroretinography (ERG)

infantile neuronal ceroid lipofucinosis 54juvenile retinoschisis 74

elliptocytes 189–90elliptocytosis, hereditary 189–90ELN 141–2Emery–Dreifuss muscular dystrophy

limb-girdle muscular dystrophy vs 22–3LMNA 19


Index 295

endocrine disorders 157–71endothelin 3 179endothelin-converting enzyme-1 179endothelin receptor, type B 179EPB41 189epidermolysis bullosa 122

junctional 122Epstein syndrome, Alport syndrome overlap 220erythrocyte cytoskeleton

hereditary elliptocytosis 190hereditary spherocytosis 191–2

exomphalos-macroglossia-gigantism (EMG) syndrome (Beckwith-Wiedemann syndrome) 220–3EXT1

hereditary multiple exostoses 116, 116–7Langer–Giedion syndrome 117

EXT2 116, 116–7EXT3 116exudative vitreoretinopathy, X-linked 79EYA4 84

F8 186–7F9 188facial features

achondroplasia 108Alagille syndrome 174DiGeorge/Shprintzen syndrome 133–4holoproscencephaly 36Miller–Dieker syndrome 45pseudohypoparathyroidism 169Rubenstein–Taybi syndrome 151Stickler syndrome 125Treacher Collins syndrome 154Williams syndrome 141X-linked α-thalassemia and mental retardation syndrome 64

facioscapulohumeral muscular dystrophy (FSHMD) 7–8limb-girdle muscular dystrophy vs 22–3

factor VIII deficiency (hemophilia A) 185–7factor IX deficiency (hemophilia B) 187–8Fallot’s tetralogy, DiGeorge/Shprintzen syndrome 134familial medullary thyroid carcinoma see multiple endocrine neoplasia type 2 (MEN2)family history, Ehlers–Danlos syndrome 114FANCA 182, 183FANCC 182, 183FANCD2 183FANCE 183FANCF 183FANCG 183Fanconi anemia 182–3Fanconi pancytopenia (Fanconi anemia) 182–3favism (glucose-6-phosphate dehydrogenase deficiency) 183–5FBN1 118–9FBN2 119FCMD

cobblestone lissencephaly 49Fukuyama muscular dystrophy 49, 50–1

Fechtner syndrome, Alport syndrome overlap 220α-fetoprotein (AFP) 2FGFR1 150, 150FGFR2

Antly–Bixler syndrome 147Apert syndrome 144–6, 145, 147Beare–Stevenson cutis gyrata syndrome 147Crouzon syndrome 147, 147–8, 150Jackson–Weiss syndrome 147Pfeiffer syndrome 147, 150, 150



FGFR3achondroplasia 109, 109–10, 110Crouzon syndrome 147, 147–8hypochondroplasia 109, 110severe achondroplasia with developmental delay and acanthosis nigricans 110thanatophoric dysplasia 109, 110

fibrillin 1 118–9fibroblast growth factor receptors

achondroplasia 109, 109–10Apert syndrome 145Crouzon syndrome 145

with acanthosis nigricans 145Muenke syndrome 145Pfeiffer syndrome 145

fibrocystin 227, 228FKRP 21, 22“flip inversions”, hemophilia A 186, 186, 187fluorescence in situ hybridization (FISH)

Angelman syndrome 32, 33aniridia 71DiGeorge/Shprintzen syndrome 134–5, 135lissencephaly 51neurofibromatosis type 1 97Prader–Willi syndrome 60Rubenstein–Taybi syndrome overlap 151Williams syndrome 142

FMR1 34–514-3-3ε 47, 48fragile X syndrome 34–6Franceschetti’s sign, Leber congenital amaurosis 75frataxin 9FRDA 9freckling, neurofibromatosis type 1 96Friedreich Ataxia 8–9FSHMD see facioscapulohumeral muscular dystrophyfukutin

cobblestone lissencephaly 49Fukuyama muscular dystrophy 49, 50–1

fukutin-related protein 21, 22Fukuyama muscular dystrophy 49

G4.5 (TAZ) 130G6PD 184–5gait abnormalities, X-linked adrenoleukodystrophy 62GARS 13, 16gastrointestinal disorders 173–9GDAP1 14, 15, 16GDNF 179gel electrophoresis, α1-antitrypsin deficiency 176GH1 165, 165–6GHR 167GLI3 148–9glial cell-line derived neurotrophic factor (GDNF) 95

Hirschsprung disease association 179glucose-6-phosphate dehydrogenase deficiency 183–5GNAS1 170, 171Gorlin syndrome 38granular osmophilic deposits (GRODs) 54Greig cephalopolysyndactyly syndrome see Greig syndromeGreig syndrome 148–9

Rubenstein–Taybi syndrome overlap 151growth hormone

deficiency 164–6receptor defects 166–7

guanine nucleotide-binding (Gs) protein 170, 171GUCY2d 76, 77, 78


Index 297

H19 (ASM1) 221, 223Haddad syndrome 95happy puppet syndrome see Angelman syndrome (AS)harmonin 88, 89HBA 195–6HBB

sickle cell anemia 194β-thalassemia 197–8

HbH inclusions, X-linked α-thalassemia and mental retardation syndrome 65–6HD (Huntington disease) 41–3HD1A8 84hearing disorders 83–92

Alport syndrome 218heart–hand syndrome (Holt–Oram syndrome) 135–6Heinz bodies 185hemangioblastomas, von Hippel–Lindau disease 103hematologic disorders 181–99hematuria

Alport syndrome 218polycystic kidney disease 226

hemoglobin α-globin gene 196α-thalassemia 195–7

hemoglobin β-globin genesickle cell anemia 194β-thalassemia 197–8

hemoglobin H (HbH) disease 194–7hemophilia A 185–7hemophilia B 187–8hepatic disorders 173–9hepatoblastoma, Beckwith–Wiedemann syndrome 221hepato-lenticular degeneration (Wilson disease) 215–6HERC2 60hereditary arthro-ophthalmopathy (Stickler syndrome) 122, 125–7hereditary elliptocytosis 189–90hereditary motor and sensory neuropathy (HMSN) 10–7

clinical features 9–10molecular pathogenesis 11–2, 16–7

hereditary multiple exostoses (HME) 115–7hereditary neuropathy with liability to pressure palsies (HNPP) 11hereditary pyropoikilocytosis 189hereditary renal tubular acidosis 192hereditary spherocytosis 190–2heterotaxy (laterality defects) 137–8hexacosanoate 64HGPRT deficiency (Lesch–Nyhan syndrome) 43–4Hirschsprung disease 177–9

disease association 177RET 95Waardenburg syndrome 92

HMSN see hereditary motor and sensory neuropathyholoproscencephaly (HPE) 36–9Holt–Oram syndrome (HOS) 135–6HRPT 44HSCR see Hirschsprung diseaseHSD3B2 162Hunter syndrome (HS) 40–1huntingtin 42Huntington disease (HD) 41–3Hutchinson–Gilford syndrome 19hydrocephalus, X-linked 66–8hydrops fetalis 195–611β-hydroxylase deficiency 16217α-hydroxylase deficiency 159

congenital adrenal hyperplasia 16221-hydroxylase deficiency (congenital adrenal hyperplasia) 160–2, 162



3β-hydroxysteroid dehydrogenase deficiency 159congenital adrenal hyperplasia 162

17β-hydroxysteroid dehydrogenase deficiency 159hyperammonemia, ornithine transcarbamylase deficiency 212–3hyperphosphatemia, pseudohypoparathyroidism 169hypertrophic cardiomyopathy, Noonan syndrome 138hypocalcemia, pseudohypoparathyroidism 169hypochondrogenesis 122hypochondroplasia 109, 110hypoglycemia, medium chain acyl-CoA dehydrogenase deficiency 210hypopigmentation

Angelman syndrome 32phenylketonuria 214Prader–Willi syndrome 60

hypothyroidism, pseudohypoparathyroidism 169hypotonia

Pelizaeus–Merzbacher syndrome 57Prader–Willi syndrome 59spinal muscular atrophy 27

hypoxanthine-guanine phosphoribosyl transferase deficiency (Lesch–Nyhan syndrome) 43–4

IDS 40–1iduronate 2-sulfatase 40–1IGF2 221, 223IL2RG 206, 207immotile cilia syndrome see primary ciliary dyskinesiaimmunoglobulin levels, ataxia–telangiectasia 2immunohistochemistry

Duchenne muscular dystrophy 6limb-girdle muscular dystrophy 22

immunologic disorders 201–8imprinting

Beckwith–Wiedemann syndrome 222–3Prader–Willi syndrome 60pseudohypoparathyroidism 170

infantile hypercalcemia (Williams syndrome [WS]) 141–2infantile nephropathic cystinosis 223–5infantile Refsum disease 76infantile spasms, X-linked 50infections

sickle cell anemia 193Wiskott–Aldrich syndrome 207–8

insulin-like growth factor (IGF)1 166insulin-like growth factor (IGF)2 221interferon regulatory factor 6 155–6interleukin-2 receptor, γ chain 206, 207intestinal aganglionosis, congenital see Hirschsprung diseaseIRF6 155–6iridogoniodysgenesis type II (Rieger syndrome) 80–1isoelectric focusing, α1-antitrypsin deficiency 176isolated fovea hypoplasia 71isomerism (laterality defects) 137–8IT15 42–3Ivemark syndrome (laterality defects) 137–8

Jackson–Weiss syndrome 147JAG1 174–5jagged 1 174–5Jansky–Bielschowski disease see neuronal ceroid lipofucinosis (NCL)jaundice

Alagille syndrome 174glucose-6-phosphate dehydrogenase deficiency 184hereditary spherocytosis 190

Jervell syndrome 223joint laxity, Ehlers–Danlos syndrome 110Joubert syndrome 76


Index 299

Juberg–Marsidi syndrome 65junctional epidermolysis bullosa 122juvenile nephropathic cystinosis 223–5juvenile retinoschisis 74–5

Kartagener syndrome see primary ciliary dyskinesiaKayser–Fleischer ring, Wilson disease 216keratinocyte growth factor receptor (KGFR)

Apert syndrome 145–6Crouzon syndrome 147

KIF1B 13, 16kinky hair disease see Menkes diseaseKlein–Waardenburg syndrome see Waardenburg syndromeKneist dysplasia 122Knobloch syndrome 122Kufs disease see neuronal ceroid lipofucinosis (NCL)Kugelberg–Welander syndrome 27–8KVLQT1

Beckwith–Wiedemann syndrome 221, 222–3Jervell syndrome 223Lange–Nielsen syndrome 223

L1CAM 67–8L1 cell-adhesion molecule 67–8lamin A

hereditary motor and sensory neuropathy 13, 16limb-girdle muscular dystrophy 18, 20

lamin B 16lamin C

hereditary motor and sensory neuropathy 13limb-girdle muscular dystrophy 18

Lange–Nielsen syndrome 223Langer–Giedion syndrome 117Laron dwarfism (growth hormone receptor defects) 166–7laterality defects 137–8laughter, Angelman syndrome 30LCA see Leber congenital amaurosisLeber congenital amaurosis (LCA) 75–9

clinical features 75–6genes/chromosomal location 76molecular pathogenesis 78

leiomyomatosis 122Lesch–Nyhan syndrome 43–4leucocoria, retinoblastoma 98Leyden hemophilia B 188Leydig cell hyperplasia 159LGMD see limb-girdle muscular dystrophyLHX3 168, 168limb-girdle muscular dystrophy (LGMD) 18–23

Emery–Dreifuss muscular dystrophy vs 22–3fascioscapulohumeral muscular dystrophy vs 22–3molecular pathogenesis 18–9, 22

lim homeobox 3 168linkage analysis

ataxia–telangiectasia 4congenital adrenal hyperplasia 162neuronal ceroid lipofucinosis 55

lipoid adrenal hyperplasia, congenital 162lipoid congenital adrenal hyperplasia 159lip-pit syndrome (van der Woude syndrome) 155–6LIS1 (PAFAH1B1)

lissencephaly 46–7, 48Miller–Dieker syndrome 48

Lisch nodules, neurofibromatosis type 1 98lissencephaly 45–51

clinical features 45–6



genes/molecular pathogenesis 46–7, 48–9, 50–1X-linked 45

LITAF 12, 13LMNA

hereditary motor and sensory neuropathy 13, 16limb-girdle muscular dystrophy 18–9, 20

Louis-Bar syndrome (ataxia–telangiectasia [AT]) 2–4Lowe syndrome 52–317, 20-lyase deficiency 162lysyl hydroxylase 113–4

Madelung deformity 115magnetic resonance imaging (MRI)

Pelizaeus–Merzbacher syndrome 57, 58–9subcortical band heterotopia 45X-linked adrenoleukodystrophy 63X-linked hydrocephalus 66

major histocompatibility complex (MHC) deficiency 206male pseudohermaphroditism 159

androgen insensitivity syndrome 158malignant melanomas, retinoblastoma 100mandibuloacral dysplasia 19mandibulofacial dystosis (Treacher Collins syndrome) 154–5O-mannose β-1,2-N-acetylglucosaminyltransferase-1 50O-mannosyl transferase 1 49, 50Marfanoid habitus, multiple endocrine neoplasia type 2 94Marfan syndrome 117–9Marshall syndrome 125–7MASA syndrome 67McCune–Albright syndrome 170Mckusick–Kaufman syndrome 72, 73MD (myotonic dystrophy) 23–6MECP2 61–2medium chain acyl-CoA dehydrogenase deficiency 210–1melanomas, malignant, retinoblastoma 100MEN2 see multiple endocrine neoplasia type 2Menkes disease 211–2

Ehlers–Danlos syndrome 114mental retardation 29–68

phenylketonuria 214X-linked 50

metabolic disorders 209–16metaphyseal chondrodysplasia 122methylation analysis

Angelman syndrome 32, 33Prader–Willi syndrome 60

methyl-CpG-binding protein 2 61–23α-methylglutaconic aciduria II (Barth syndrome) 130microhematuria, Alport syndrome 220Miller–Dieker syndrome (MDS) 45, 47, 48MITF 91, 92MKKS 72, 73Mowat–Wilson syndrome 179MPZ 12, 13, 14, 15, 17MTMR2 15, 16–7mucopolysaccharidosis type II (MPS II) (Hunter syndrome [HS]) 40–1mucoviscidosis (cystic fibrosis) 131–3Muenke syndrome 150

fibroblast growth factor receptors 145multiple cartilogenous exostoses (hereditary multiple exostoses [HME]) 115–7multiple endocrine neoplasia type 2 (MEN2) 94–6

RET 178multiple epiphyseal dysplasia (MED) 122, 125multiple osteochondromatosis (hereditary multiple exostoses [HME]) 115–7muscle weakness

Duchenne muscular dystrophy 4


Index 301

myotonic dystrophy 23–4muscular dystrophy of muscle–eye–brain disease (MEBD) 50MYH9 84, 220MYO3A 84MYO6 84MYO7A

nonsyndromal hearing loss 84Usher syndrome 88–9, 89

MYO15 84myosin heavy chain 9 84myosin

IIIA 84V 84VI 84VIIA 84, 88, 89

myotilin 18myotonic dystrophy (MD) 23–6myotubularin-related protein 2 16–7

NADPH oxidase 204NCF1 204, 204NCF2 204, 204NCL see neuronal ceroid lipofucinosis (NCL)NDN 59–60NDP 79–80NDRG1 15, 17necdin 59–60NEFL 12, 13, 14Tnephrogenic diabetes insipidus 163, 164nephropathy and deafness see Alport syndrome (AS)nerve conduction velocities (NCVs) 10–1, 13–5neuroblastoma, Beckwith–Wiedemann syndrome 221neurocutaneous disorders 93–105neurofibromatosis type 1 96–8

Noonan syndrome 139neurofibromin 97–8neurohypophyseal diabetes insipidus 163, 163–4neuroimaging

holoproscencephaly 39X-linked adrenoleukodystrophy 63

neurologic disorders 1–28neuronal ceroid lipofucinosis (NCL) 53–7

adult 54, 55infantile 53–4, 54juvenile 54, 54late infantile 54, 54

neurturin 95neutropenia, Barth syndrome 130neutrophil cystolic factor 1 204neutrophil cystolic factor 2 204NF1 97–8Nijmegen breakage syndrome 3nonsyndromal hearing loss 84Noonan syndrome 138–9

neurofibromatosis type 1 139“normal transmitting males”, fragile X syndrome 35Norrie disease 79–80NSD1 153–4nuclear receptor SET-domain protein I 153–4nystagmus, Pelizaeus–Merzbacher syndrome 57

obesity, Bardet–Biedl syndrome 72occipital horn syndrome 112OCRL1 52–3octocadherin 89–99oculo-cerebro-renal syndrome (Lowe syndrome) 52–3



oculodigital sign, Leber congenital amaurosis 75OFD1 (orofaciodigital syndrome type I [OFD1]) 225–6OI see osteogenesis imperfectaOndine’s curse 95oral-facial-digital syndrome type I (orofaciodigital syndrome type I [OFD1]) 225–6ornithine transcarbamylase deficiency 212–4orofaciodigital syndrome type I (OFD1) 225–6osteoarthritis 122osteochondrosarcomas, hereditary multiple exostoses 116–7osteogenesis imperfecta (OI) 119–24

clinical features 120, 121mutational mechanisms 123radiologic features 121, 123

osteogenesis imperfecta congenita (OIC) see osteogenesis imperfecta (OI)osteogenesis imperfecta tarda (OIT) see osteogenesis imperfecta (OI)osteoporosis 122osteosarcoma

hereditary multiple exostoses 115retinoblastoma 100

OTC 213–4OTOA 84otoancorin 84OTOF 84otoferlin 84outwardly rectifying chloride channels 132

P1 176–7pachygyria spectrum see lissencephalyPAFH1B1 see LIS1 (PAFAH1B1)PAH 214–5Pallister–Hall syndrome 149palmitoyl-protein thioesterase (PPT) 55panhypopituitarism 167–9Partington syndrome 50patent ductus arteriosus, Noonan syndrome 138paternal uniparental disomy (UPD), Beckwith–Wiedemann syndrome 221PAX3 91, 91–2PAX6 70–2PCDH15 89, 90PDS 84PDS (Pendred syndrome) 86–7pedigree analysis, hereditary motor and sensory neuropathy 17Pelizaeus–Merzbacher syndrome 57–9Pendred syndrome (PDS) 86–7pendrin 84periaxin 14, 16peroneal muscular atrophy see hereditary motor and sensory neuropathy (HMSN)persephin 95personality changes

Huntington disease 41Williams syndrome 141

Peter’s anomalyPAX6 71PITX2 81

Pfeiffer syndrome 149–50FGFR2 147fibroblast growth factor receptors 145

phagocytosis, chronic granulomatous disease 203phenylalanine decarboxylase deficiency (phenylketonuria [PKU]) 214–5phenylketonuria (PKU) 214–5pheochromocytomas

multiple endocrine neoplasia type 2 96von Hippel–Lindau disease 103

phosphatidylinositol-4,5-bisphosphatase 52–3PHP (pseudohypoparathyroidism) 169–71


Index 303

Pierre Robin anomalyStickler syndrome 125Weissenbacher–Zweymuller syndrome 127

PIT1 168, 168pituitary dwarfism (growth hormone deficiency) 164–6pituitary-specific transcription factor 1 168PITX2 80, 81PKD1 227, 227–8PKD2 227, 227–8PKHD1 227, 228PKU (phenylketonuria) 214–5plagiocephaly, Saethre–Chotzen syndrome 151PLOD1 112, 113PLP1 58, 58PMP22 11–2, 13, 14, 15, 17PNP 206, 206–7polycystic kidney disease (PKD) 226–8polycystin 1 227polycystin 2 227polydipsia, cystinosis 224polyhydramnios, myotonic dystrophy 24polysplenia syndrome (laterality defects) 137–8polyuria

cystinosis 224diabetes insipidus 163

POMGnT1cobblestone lissencephaly 49, 50muscular dystrophy of muscle–eye–brain disease 50

POMT1 49, 50popliteal pterygium syndrome 156postaxial polydactyly type A1 149posterior embryotoxon 174POU3F4 84POU4F3 84Prader–Labhardt–Willi syndrome see Prader–Willi syndrome (PWS)Prader–Willi syndrome (PWS) 30, 59–60

chromosome 15 31, 59preaxial polydactyly type IV 149presenile cataracts, myotonic dystrophy 23primary ciliary dyskinesia 139–41

situs inversus 137, 140primordial dwarfism (growth hormone deficiency) 164–6progeria 19progressive ataxias 1–28PROMM (proximal myotonic myopathy) 23–6PROP1 168, 168prophet of POT1 168protease inhibitor 1 176–7protein-tyrosine phosphatase, nonreceptor-type 11 138–9proteolipid protein 1 58protocadherin 15 89proximal myotonic myopathy (PROMM) 23–6PRX 14, 16pseudoachondrodysplasia 124–5pseudoachondrodysplastic spondyloepiphyseal dysplasia 124–5pseudohemophilia (von Willebrand disease) 198–9pseudohypoparathyroidism (PHP) 169–71pseudopseudohypoparathyroidism (PPHP) 169–71psychomotor regression, Rett syndrome 61PTCH

Gorlin syndrome 38holoproscencephaly 36, 38, 39

PTPN11 138–9pulmonary stenosis

Alagille syndrome 174Noonan syndrome 138



purine nucleoside phosphorylase (PNP) deficiency 205–7PWS see Prader–Willi syndromepyropoikilocytosis, hereditary 1896-pyruvoyltetrahydropterin synthase 215

RAB7 16radiologic examination, tuberous sclerosis 103RAS-associated protein-7 16RAS-MAP kinase pathway 95RB1 99–100receptor tyrosine kinase 1795α-reductase deficiency 159reelin

cobblestone lissencephaly 49lissencephaly 51

Refsum disease, infantile 76RELN

cobblestone lissencephaly 49lissencephaly 51

renal disorders 217–26Bardet–Biedl syndrome 72

renal Fanconi syndromecystinosis 224Lowe syndrome 52

renal tubular acidosis, hereditary 192restriction fragment length polymorphism analysis (RFLP), Angelman syndrome 32RET

Haddad syndrome 95Hirschsprung disease 95

association 178–9, 179multiple endocrine neoplasia type 2 94–6, 178

retinal angiomas, von Hippel–Lindau disease 103retinal dystrophy, Bardet–Biedl syndrome 72retinal examination, juvenile neuronal ceroid lipofucinosis 54retinitis pigmentosa

Leber congenital amaurosis vs 76Usher syndrome 87–8

retinoblastoma 98–100retinoschisin 75retinoschisis, X-linked (juvenile retinoschisis) 74–5Rett syndrome 61–2

Angelman syndrome vs 31, 62infantile neuronal ceroid lipofucinosis vs 53

rhabdomyosarcoma, Beckwith–Wiedemann syndrome 221Rieger syndrome 80–1RPE65 76, 77, 78RPGRIP1 76, 77, 78RS1 75Rubenstein–Taybi syndrome 151

Saethre–Chotzen syndrome (SCS) 152–3Rubenstein–Taybi syndrome overlap 151

Santavuori–Haltia–Hagberg disease see neuronal ceroid lipofucinosis (NCL)α-sarcoglycan 19, 20, 22β-sarcoglycan 21, 22δ-sarcoglycan 21, 22γ-sarcoglycan 20, 22sarcoglycanopathies 18sarcomas, retinoblastoma 100SBF2 15, 17screening, von Hippel–Lindau disease 105SCS see Saethre–Chotzen syndromeSenior–Loken syndrome 76SET-binding factor 2 17severe achondroplasia with developmental delay and acanthosis nigricans (SADDAN) 110severe combined immunodeficiency (SCID) 205–7


Index 305

severe congenital neutropenia, WAS 208sex-determining factor-related box 10 179SGCA 19, 20SGCB 21, 22SGCD 21SGCG 20“shaker” 88SHH 36, 37, 38Shprintzen syndrome (DiGeorge/Shprintzen syndrome) 133–5sickle cell anemia 193–4sickle cell trait 193SIP1 179Sipple syndrome see multiple endocrine neoplasia type 2 (MEN2)situs ambiguus (laterality defects) 137–8situs inversus

Kartagener syndrome 140primary ciliary dyskinesia 139–40

SIX3 36, 37, 38skeletal disorders 107–27skin hyperextensibility, Ehlers–Danlos syndrome 110SLC26A4 87SMA (spinal muscular atrophy) 27–8small nucleoribonucleoprotein N 59–60SMN1 27–8SNRPN 59–60, 60Sotos syndrome 153–4Southeast Asian ovalocytosis 190, 192SOX-10 91, 179spastic paraparesis, X-linked 67spastic paraplegia, X-linked 58spectrins


spherocytosis, hereditary 190–2Spielmeyer–Vogt–Sjögren disease see neuronal ceroid lipofucinosis (NCL)spinal muscular atrophy (SMA) 27–8spondyloepimetaphyseal dysplasia (SEMD) 122spondylopepiphyseal dysplasia congenita (SEDC) 122SPTA1


SPTBhereditary elliptocytosis 189hereditary spherocytosis 191

StAR 162stature, growth hormone deficiency 164Steinert disease (myotonic dystrophy [MD]) 23–6stereocilin 84Stickler syndrome 122, 125–7strabismus, retinoblastoma 98STRC 84subcortical band heterotopia (SCBH) 45supravalvular aortic stenosis (SVAS), Williams syndrome 141supravalvular pulmonary stenosis (SVPS), Williams syndrome 141survival of motor neurons interacting protein 179Swiss-type agammaglobulinemia (severe combined immunodeficiency [SCID]) 205–7

tafazzin 130TAZ (G4.5) 130T-BOX

DiGeorge/Shprintzen syndrome 134–5Holt–Oram syndrome 136

TBX1 134–5TBX5 135, 136TCAP 21, 22T-cell leukemias, ataxia–telangiectasia 2



TCOF1 154–5TECTA 84tectorin 84telethonin 21, 22testicular feminization syndrome (androgen insensitivity syndrome [AIS]) 158–9testosterone biosynthesis defects 159tetrahydrobiotin (BH4) 215TFCP2L3 84TGIF 36, 37, 38α-thalassemia 194–7

and mental retardation syndrome, X-linked 64–6β-thalassemia 197–8thanatophoric dysplasia 109, 109–10, 110thirst, diabetes insipidus 163thrombocytopenia

Wiskott–Aldrich syndrome 207–8X-linked 208

thymic aplasia, DiGeorge/Shprintzen syndrome 133thyroidectomy, multiple endocrine neoplasia type 2 96thyroid stimulating hormone (TSH) deficiency 168titin cap 22titin immunoglobulin domain protein 18, 20TMCI 84TMIE 84TMPRSS3 84transforming growth factor β-induced factor 38Treacher Collins syndrome 154–5Treacher Collins–Franceschetti syndrome 154–5treacle (TCOF1) 154–5triallelic inheritance, Bardet–Biedl syndrome 74TRIM32 21, 22trisomy 8, X-linked hydrocephalus 66“trisomy rescue”, Angelman syndrome 31TSC1 102–3TSC2 102–3TS complex (tuberous sclerosis) 101–3TTID 18, 20tuberin 102–3tuberous sclerosis 101–3tumor suppressor genes

EXT1 116–7EXT2 116–7RB1 99–100VHL 104–5

TWIST 151–2

UBE3A 30–1ultrasonography, polycystic kidney disease 226ureate levels, Lesch–Nyhan syndrome 43uremia, polycystic kidney disease 226USH2A 89, 90USH3A 89, 90usher IC 84usherin 89, 90Usher syndrome 87–90USHIC

nonsyndromal hearing loss 84Usher syndrome 88–9, 89

valvular stenosis, Alagille syndrome 174van der Woude syndrome 155–6vaso-occlusive crisis, sickle cell anemia 193velocardiofacial syndrome (VCFS) (DiGeorge/Shprintzen syndrome) 133–5ventricular septal defects (VSDs)

DiGeorge/Shprintzen syndrome 134Holt–Oram syndrome 136


Index 307

laterality defects 137Noonan syndrome 138

very-long chain fatty acids (VLCFAs) 63–4VHL 104–5visual disorders 69–81

Lowe syndrome 52Pelizaeus–Merzbacher syndrome 57

visual evoked potentials (VEPs) 54von Hippel–Lindau disease (VHL) 103–5von Reckinghausen’s disease see neurofibromatosis type 1von Willebrand disease 198–9VWF 199

Waardenburg–Shah syndrome see Waardenburg syndromeWaardenburg syndrome 90–2

Hirschsprung disease 92, 177Wagenmann–Froboese syndrome see multiple endocrine neoplasia type 2 (MEN2)WAGR syndrome (aniridia) 70–2Walker–Warburg syndrome 46, 51“waltzer” mice 89–90WAS 208Weissenbacher–Zweymuller syndrome 125–7

COL11A2 122Pierre Robin anomaly 127Stickler syndrome 127

Werdnig–Hoffmann disease 27–8WFS1 84Williams–Beuren syndrome (Williams syndrome [WS]) 141–2Williams syndrome (WS) 141–2Wilm’s tumor, aniridia, genitourinary anomalies mental retardation syndrome 70–2Wilms tumor, Beckwith–Wiedemann syndrome 220–1Wilson disease 215–6Wiskott–Aldrich syndrome (WAS) 207–8Wolfram syndrome 84WS (Williams syndrome) 141–2WT1 71

X-chromosomeAlport syndrome 219androgen insensitivity syndrome 158Barth syndrome 130Bruton agammaglobulinemia 202chronic granulomatous disease 204collagen gene disorders 122Duchenne muscular dystrophy 4fragile X syndrome 34glucose-6-phosphate dehydrogenase deficiency 184growth hormone deficiency 165hemophilia A 186hemophilia B 187hereditary motor and sensory neuropathy 13Hunter syndrome 40juvenile retinoschisis 74laterality defects 137Lesch–Nyhan syndrome 43Lowe syndrome 52Menkes disease 212nephrogenic diabetes insipidus 163nonsyndromal hearing loss 84Norrie disease 79occipital horn syndrome 112ornithine transcarbamylase deficiency 213orofaciodigital syndrome type I 225Pelizaeus–Merzbacher syndrome 58Rett syndrome 61severe combined immunodeficiency 206



Wiskott–Aldrich syndrome 208X-linked lissencephaly 48X-linked lissencephaly with ambiguous genitalia 48

X-linked disordersadrenoleukodystrophy 62–4agammaglobulinemia see Bruton agammaglobulinemiaagenesis of the corpus callosum 67aqueduct stenosis 66–8α-thalassemia and mental retardation syndrome 64–6cardiomyopathy 6complicated spastic paraparesis 67exudative vitreoretinopathy 79hydrocephalus 66–8infantile spasms 50lissencephaly (XLIS) 45lissencephaly with ambiguous genitalia (XLAG) 46mental retardation 50myoclonic epilepsy with mental retardation and 50nuclear protein 65retinoschisis (juvenile retinoschisis) 74–5spastic paraplegia 58thrombocytopenia 208

XNP 65–6

Zellweger syndrome, Leber congenital amaurosis 76ZIC2 36, 36, 38ZIC3 137–8zinc finger protein 3 137–8zinc finger protein 9 25, 26zinc finger protein 127 59–60ZNF9 25, 26ZNF127 59–60


Genetics for Pediatricians

Documents

neuronal ceroid lipofucinosis

nerve conduction velocities

becker muscular dystrophy

duchennes muscular dystrophy

duchenne muscular dystrophy

spinal muscular atrophy

congenital adrenal hyperplasia

girdle muscular dystrophy