Lesson 7.1 : Metabolic Diseases Inborn Errors Of Metabolism (IEM)

Lesson 7.1 : Metabolic Diseases

Inborn Errors Of Metabolism (IEM)

A primer on metabolic disease in the neonate...

What is a metabolic disease?

• “Inborn errors of metabolism”• inborn error : an inherited (i.e.

genetic) disorder• metabolism : chemical or physical

changes undergone by substances in a biological system

• “any disease originating in our chemical individuality”

What is a metabolic disease?

• Garrod’s hypothesis

product deficiency

substrate excess toxic

metabolite

A

D

B C

What is a metabolic disease?

• Small molecule disease– Carbohydrate– Protein– Lipid– Nucleic Acids

• Organelle disease– Lysosomes– Mitochondria– Peroxisomes– Cytoplasm

How do metabolic diseases present in the

neonate ??• Acute life threatening illness

– encephalopathy - lethargy, irritability, coma– vomiting– respiratory distress

• Seizures, Hypertonia• Hepatomegaly (enlarged liver)• Hepatic dysfunction / jaundice• Odour, Dysmorphism, FTT (failure to

thrive), Hiccoughs

How do you recognize a metabolic disorder ??

• Index of suspicion– eg “with any full-term infant who has no

antecedent maternal fever or PROM (premature rupture of the membranes) and who is sick enough to warrant a blood culture or LP, one should proceed with a few simple lab tests.

• Simple laboratory tests– Glucose, Electrolytes, Gas, Ketones, BUN

(blood urea nitrogen), Creatinine– Lactate, Ammonia, Bilirubin, LFT– Amino acids, Organic acids, Reducing subst.

Index of suspicionFamily History

• Most IEM’s are recessive - a negative family history is not reassuring!

• CONSANGUINITY, ethnicity, inbreeding• neonatal deaths, fetal losses• maternal family history

– males - X-linked disorders – all - mitochondrial DNA is maternally inherited

• A positive family history may be helpful!

Index of suspicionHistory

• CAN YOU EXPLAIN THE SYMPTOMS?

• Timing of onset of symptoms– after feeds were started?

• Response to therapies

Index of suspicionPhysical examination

• General – dysmorphisms (abnormality in shape or size), ODOUR

• H&N - cataracts, retinitis pigmentosa• CNS - tone, seizures, tense fontanelle• Resp - Kussmaul’s, tachypnea• CVS - myocardial dysfunction• Abdo - HEPATOMEGALY• Skin - jaundice

Index of suspicionLaboratory

• ANION GAP METABOLIC ACIDOSIS• Normal anion gap metabolic acidosis• Respiratory alkalosis• Low BUN relative to creatinine• Hypoglycemia

– especially with hepatomegaly– non-ketotic

A parting thought ...

• Metabolic diseases are individually rare, but as a group are not uncommon.

• There presentations in the neonate are often non-specific at the outset.

• Many are treatable.• The most difficult step in diagnosis is

considering the possibility!

INBORN ERRORS OF METABOLISM

Inborn Errors of Metabolism

An inherited enzyme deficiency leading to the disruption of normal bodily metabolism

• Accumulation of a toxic substrate (compound acted upon by an enzyme in a chemical reaction)

• Impaired formation of a product normally produced by the deficient enzyme

Three Types

• Type 1: Silent Disorders• Type 2: Acute Metabolic Crises• Type 3: Neurological Deterioration

Type 1: Silent Disorders

• Do not manifest life-threatening crises

• Untreated could lead to brain damage and developmental disabilities

• Example: PKU (Phenylketonuria)

PKU• Error of amino acids metabolism• No acute clinical symptoms• Untreated leads to mental retardation • Associated complications: behavior

disorders, cataracts, skin disorders, and movement disorders

• First newborn screening test was developed in 1959

• Treatment: phenylalaine restricted diet (specialized formulas available)

Type 2: Acute Metabolic Crisis

• Life threatening in infancy• Children are protected in utero by

maternal circulation which provide missing product or remove toxic substance

• Example OTC (Urea Cycle Disorders)

OTC

• Appear to be unaffected at birth• In a few days develop vomiting,

respiratory distress, lethargy, and may slip into coma.

• Symptoms mimic other illnesses• Untreated results in death• Treated can result in severe

developmental disabilities

Type 3: Progressive Neurological Deterioration• Examples: Tay Sachs disease

Gaucher diseaseMetachromatic

leukodystrophy• DNA analysis show: mutations

Mutations

• Nonfunctioning enzyme results: Early Childhood - progressive loss of motor and cognitive skillsPre-School – non responsive stateAdolescence - death

Other Mutations• Partial Dysfunctioning Enzymes

-Life Threatening Metabolic Crisis

-ADH-LD-MR

• Mutations are detected by Newborn Screening and Diagnostic Testing

Treatment

• Dietary Restriction• Supplement deficient product• Stimulate alternate pathway• Supply vitamin co-factor• Organ transplantation• Enzyme replacement therapy• Gene Therapy

Children in School

• Life long treatment• At risk for ADHD

LD MR

• Awareness of diet restrictions• Accommodations

Inborn errors of metabolism

Definition:

Inborn errors of metabolism occur from a group of rare genetic disorders in whi

ch the body cannot metabolize food co mponents normally. These disorders ar

e usually caused by defects in the enzy mes involved in the biochemical pathw

ays that break down food components. Alternative Names:

- Galactosemia nutritional consideratio - ns; Fructose intolerance nutritional co

nsiderations; Maple sugar urine disease - (MSUD) nutritional considerations; Phe

- nylketonuria (PKU) nutritional conside - rations; Branched chain ketoaciduria n

utritional considerations

Background:

Inborn errors of metabolism (IEMs) in dividually are rare but collectively are

common. Presentation can occur at an y time, even in adulthood.

Diagnosis does not require extensive knowledge of biochemical pathways o

r individual metabolic diseases.

An understanding of the broad clinical manifestations of IEMs provides the b

asis for knowing when to consider the diagnosis.

Most important in making the diagnos is is a high index of suspicion.

Successful emergency treatment dep ends on prompt institution of therapy

aimed at metabolic stabilization.

A genetically determined biochemical disorder in which a specific enzyme defect produces a metabolic block that may have pathologic consequences at birth (e.g., phenylketonuria) or in later life (e.g., diabetes mellitus); called also enzymopathy and genetotrophic disease.

Metabolic disorders testabl e on Newborn Screen

CongenitalHypothyroidism

Phenylketonuria (PK U)

Galactosemia Galactokinase deficie

ncy

Maple syrup urine dis ease

Homocystinuria Biotinidase deficienc

y

Classification Inborn Errors of Small mo lecule Metabolism

Example: Galactosemia

Lysosomal storage disease s

Example: Gaucher's Di sease Disorders of Energy Meta

bolism Example Glycogen Stor age Disease

Other more rare classes of metabolism error Paroxysmal disorders

Transport disorders Defects in purine and p yrimidine metabolism

Receptor Defects

Categories of IEMs are as follows:

Disorders of protein metabolism (eg, ami no acidopathies, organic

acidopathies, and urea cycle defects)

Disorders of carbohydrate metabolism (e g, carbohydrate intolerance

disorders, glycogen storage disorders, di sorders of gluconeogenesis

and glycogenolysis)

Lysosomal storage disorders

Fatty acid oxidation defects

Mitochondrial disorders

Peroxisomal disorders

Pathophysiology:

Single gene defects result in abnormali ties in the synthesis or catabolism of pr

oteins, carbohydrates, or fats.

Most are due to a defect in an enzyme o r transport protein, which results in a b

lock in a metabolic pathway.

Effects are due to toxic accumulations of substrates before the block, interme

diates from alternative metabolic path ways, and/or defects in energy product

ion and utilization caused by a deficien cy of products beyond the block.

Nearly every metabolic disease has several forms that vary in age of onset, clinical severity and, often, mode of inh

eritance.

Frequency: In the US: The incidence,

collectively, is estimated to be 1 in 5000 live births. The frequencies for each individual IEM vary, but most are very rare. Of term infants who develop symptoms of sepsis without known risk factors, as many as 20% may have an IEM.

Internationally: The overall incidence is similar to that of US. The frequency for individual diseases varies based on racial and ethnic composition of the population.

Mortality/Morbidity:

IEMs can affect any organ system a nd usually do affect multiple organ

systems.

Manifestations vary from those of a - cute life threatening disease to sub

acute progressive degenerative disorder.

Progression may be unrelenting - with rapid life threatening deterior ation over hours, episodic with inte

rmittent decompensations and asy mptomatic intervals, or insidious wi

th slow degeneration over decades.

Purine metab

olism

Adenine phosphoribosyltra nsferase deficiency

The normal function of adenine pho

sphoribosyltransferase (APRT) is th

e removal of adenine derived as me

tabolic waste from the polyamine p

athway and the alternative route of

adenine metabolism to the extreme

- ly insoluble 2,8 dihydroxyadenine,

which is operative when APRT is ina

ctive. The alternative pathway is ca

talysed by xanthine oxidase.

- Hypoxanthine guanine phosp horibosyltransferase (HPRT, E

242 8C . . . )

HGPRTcatalyses the transfer of the phosphoribosyl moiety - - of PP ribose P to the 9 positio

n of the purine ring of the bas es hypoxanthine and guanine to form inosine monophospat

e (IMP) and guanosine monop hosphate (GMP) respectively.

HGPRT is a cytoplasmic enzym e present in virtually all tissue s, with highest activity in brai

n and testes.

The salvage p athway of the

purine bases, hypoxanthine

and guanine, to IMP and G

MP, respectiv ely, catalysed

by HG PRT (1) in the presen -ce of PP ribos

- e P. The defec t in HPRT is sh

own.

The importance of HPRT in the norm

al interplay between synthesis and s

alvage is demonstrated by the bioch

emical and clinical consequences as

sociated with HPRT deficiency.

Gross uric acid overproduction resul

ts from the inability to recycle either

hypoxanthine or guanine, which inte

rrupts the inosinate cycle producing

a lack of feedback control of synthes

is, accompanied by rapid catabolism

-of these bases to uric acid. PP ribose

- P not utilized in the salvage reactio

n of the inosinate cycle is considered

to provide an additional stimulus to

de novo synthesis and uric acid over

production.

• The defect is readily detectable in er ythrocyte hemolysates and in cultur

e fibroblasts.• HGPRT is determined by a gene on t

- he long arm of the x chromosome at26Xq .

• -The disease is transmitted as an X li nked recessive trait.

• - Lesch Nyhan syndrome• Allopurinal has been effective reduci

ng concentrations of uric acid.

Phosphoribosyl pyrophosphate synth

etase (PRPS, EC 2.7.6.1) catalyses th

e transfer of the pyrophosphate grou

- - p of ATP to ribose 5 phosphate to for

- - m PP ribose P.

The enzyme exists as a complex aggr

egate of up to 32 subunits, only the 1

6 and 3 2 subunits having significa

nt activity. It requires Mg2+ , is activa

ted by inorganic phosphate, and is su

bject to complex regulation by differe

- nt nucleotide end products of the pat

- - hways for which PP ribose P is a subs

trate, particularly ADP and GDP.

Phosphoribosyl pyrophosphate synthetase superactivity

- - PP ribose P acts as an allosteric regulator of the first specific reacti

on of de novo purine biosynthesis, i n which the interaction of glutamin - - e and PP ribose P is catalysed by a

midophosphoribosyl transferase, p roducing a slow activation of the a

midotransferase by changing it fro m a large, inactive dimer to an activ

e monomer.

Purine nucleotides cause a rapid re versal of this process, producing th

e inactive form.

Variant forms of PRPS have been de scribed, insensitive to normal regul

atory functions, or with a raised sp ecific activity. This results in contin - - uous PP ribose P synthesis which st

imulates de novo purine production , resulting in accelerated uric acid f

ormation and overexcretion.

- - The role of PP ribose P in the de novo s ynthesis of IMP and adenosine (AXP) a

nd guanosine (GXP) nucleotides, and t he feedback control normally exerted by these nucleotides on de novo purin

e synthesis.

Purine nucleoside phosphorylase (PNP, EC 2.4.2.1)

PNP catalyses the degradation of th e nucleosides inosine, guanosine or

their deoxyanalogues to the corres ponding base. The mechanism appears to be the accumulation of p

urine nucleotides which are toxic to T and B cells.

Although this is essentially a revers ible reaction, base formation is fav

oured because intracellular phosph ate levels normally exceed those of

- - -either ribose , or deoxyribose 1 pho sphate.

The enzyme is a vital link in the 'ino sinate cycle' of the purine salvage p

athway and has a wide tissue distribution.

Purine nucleotide phosphoryla se deficiency

The necessity of purine nucleoside phosphoryla se (PNP) for the normal catabolism and salvage of both nucleosides and deoxynucleosides, resu

lting in the accumulation of dGTP, exclusively, i n the absence of the enzyme, since kinases do n ot exist for the other nucleosides in man. The la ck of functional HG PRT activity, through absenc

e of substrate, in PNP deficiency is also apparen t.

The importance of adenosine deaminase (ADA) for the catabolism of dA, but not A, and the resu

ltant accumulation of dATP when ADA is defecti ve. A is normally salvaged by adenosine kinase

(see Km values of A for ADA and the kinase, AK) and deficiency of ADA is not significant in this si

tuation

Adenine deaminase deficiency

The role of AMPDA in the deamination of AMP to IMP, and the recorversion of the latter to A MP via AMPS, thus completing the purine nucl

eotide cycle which is of particular importance in muscle.

Myoadenylate deaminase (A MPDA) deficiency

Purine and pyrimidine degradation

PRPP synthesis

1 =ribokinase 2=ribophosphate pyrophosphokinase 3=phosphoribosyl transferase

Salvage pathway of purine

PRPP Purine ribonucleotide

purine PPi

Adenine + PRPP Adenylate + PPi

(AMP)

Mg 2+

APRTase

Catalyzed by adenine phosphoribosyl transferase (APRTase)

IMP and GMP interconversion

Hypoxanthine + PRPP Inosinate + PPi

( IMP)

Mg 2+

HGPRTase

Guanine + PRPP Guanylate + PPi

(GMP)

Mg 2+

HGPRTase

HGPRTase = Hypoxanthine-guanine phosphoribosyl transferase

purine reused

1 =adenine phosphoribosyl transferase

2=HGPRTase

Formation of uric acid from hypox anthine and xanthine catalysed by

xanthine dehydrogenase (XDH).

Intracellular uric acid crystal under polarised li - ght (left) and under non polarised light (right)

With time, elevated levels of uric acid in the blood may lead to deposits around joints. Ev

- entually, the uric acid may form needle like crystals in joints, leading to acute gout attac

ks. Uric acid may also collect under the skin as tophi or in the urinary tract as kidney sto

nes.

Additional Gout Foot Sites: Inflamation In Joints Of Big Toe, Small Toe And Ankle

- Gout Early Stage: No Joint Damage

- Gout Late Stage: Arthritic Joint

Disorders of pyrimi dine metabolism

The UMP synthase (UMPS) complex, a bifunction al protein comprising the enzymes orotic acid ph

-osphoribosyltransferase (OPRT) and orotidine 5'- monophosphate decarboxylase (ODC), which c

atalyse the last two steps of the de novo pyrimid ine synthesis, resulting in the formation of UMP.

Overexcretion formation can occur by the altern ative pathway indicated during therapy with OD

C inhibitors.

Hereditary oro tic aciduria

Dihydropyrimidine dehydrogenase (DHPD) is responsib - le for the catabolism of the end products of pyrimidine

metabolism (uracil and thymine) to dihydrouracil and d ihydrothymine. A deficiency of DHPD leads to accumul

ation of uracil and thymine. Dihydropyrimidine amidoh ydrolase (DHPA) catalyses the next step in the further

catabolism of dihydrouracil and dihydrothymine to ami no acids. A deficiency of DHPA results in the accumulat ion of small amounts of uracil and thymine together wit

h larger amounts of the dihydroderivatives.

The role of uridine monophosphat e hydrolases (UMPH) 1 and 2 in th e catabolism of UMP, CMP, and dC

MP (UMPH 1), and dUMP and dTM P (UMPH 2).

- CDP choline phosphotransferase ca talyses the last step in the synthesi

s of phosphatidyl choline. A deficie ncy of this enzyme is proposed as t

he metabolic basis for the selective - accumulation of CDO choline in the

erythrocytes of rare patients with a n unusual form of haemolytic anae

mia.

- CDP choline phosphotr ansferase deficiency

WHAT IS TYROSINEMIA?

Hereditary tyrosinemia is a genetic in born error of metabolism associated with severe liver disease in infancy. T

he disease is inherited in an autosom al recessive fashion which means that in order to have the disease, a child m

ust inherit two defective genes, one fr om each parent. In families where bot

h parents are carriers of the gene for t he disease, there is a one in four risk t hat a child will have tyrosinemia.

About one person in 100 000 is affecte d with tyrosinemia globally.

HOW IS TYROSINEMIA CAUSED ?

Tyrosine is an amino acid which is foun d in mostanimal and plantproteins. Th e metabolismof tyrosine in humans tak es place primarily in the liver.

Tyrosinemia is caused by an absence of the enzyme fumarylacetoacetate hyd

rolase (FAH) which is essential in the metabolismof tyrosine. The absence of

FAH leads to an accumulation of toxic metabolic products in various body tiss

ues, which in turn results in progressive damage to the liver and kidneys.

WHAT ARE THE SYMPTOMS OF TYROSINEM IA?

The clinical features of the disease ten to fall int o two categories, acute and chronic.

- In the so called acute form of the disease, abnor malities appear in the first month of life. Babies

may show poor weight gain, an enlarged liver a nd spleen, a distended abdomen, swelling of th

e legs, and an increased tendency to bleeding, particularly nose bleeds. Jaundice may or may n

ot be prominent. Despite vigorous therapy, deat h from hepatic failure frequently occurs betwee n three and nine months of age unless a liver tr

ansplantation is performed.

Some children have a more chronic form of tyro sinemia with a gradual onset and less severe cli

nical features. In these children, enlargement of the liver and spleen are prominent, the abdome

n is distended with fluid, weight gain may be po or, and vomiting and diarrhoea occur frequently

. Affected patients usually develop cirrhosis and its complications. These children also require liv er transplantation.

Methionine synthesis

Homocystinuria

Homocystinuria

1Figure : the structures of tyrosine, phenylalanine and homogentisic acid

Phenylketonuria

Maple syrup urine disease

Albinism

This excess can be caused by an increase in pro - duction by the body, by under elimination of uri

c acid by the kidneys or by increased intake of f oods containing purines which are metabolized

to uric acid in the body. Certain meats, seafood, dried peas and beans are particularly high in pu rines. Alcoholic beverages may also significantl

y increase uric acid levels and precipitate gout attacks.

Pyruvate kinase (PK) deficiency:

This is the next most common red cell enzymopathy after G6PD deficiency, b

ut is rare. It is inherited in a autosomal recessive pattern and is the commone

- st cause of the so called "congenital n- on spherocytic haemolytic anaemias"

(CNSHA).

PK catalyses the conversion of phosph oenolpyruvate to pyruvate with the ge

neration of ATP. Inadequate ATP gener ation leads to premature red cell deat

h.

There is considerable variation in the s everity of haemolysis. Most patients ar

e anaemic or jaundiced in childhood. G allstones, splenomegaly and skeletal d eformities due to marrow expansion m

ay occur. Aplastic crises due to parvov irus have been described.

Hereditary hemolytic anemia

Blood film: PK deficiency:

Characteristic "prickle cells" may be seen.

Drug induced hemolytic anemia

Glycogen storage disease

Case Description

A female baby was delivered normally after an uncomplicated pregnancy. At the time of the infant’s second immunization, she became fussy and was seen by a pediatrician, where examination revealed an enlarged liver. The baby was referred to a gastroenterologist and later diagnosed to have Glycogen Storage Disease Type IIIB

GlycogenosesDisord

erAffected Tissue Enzyme Inheritance Gene Chromosome

Type 0 Liver Glycogen synthase AR GYS2[125] 12p12.2[121]

Type IA Liver, kidney, intestine

Glucose-6-phosphatase AR G6PC[96] 17q21[13][94]

Type IB Liver Glucose-6-phosphate transporter (T1)

AR G6PTI[57][104] 11q23[2][81][104]

[155]

Type IC Liver Phosphate transporter AR 11q23.3-24.2[49][135]

Type IIIA

Liver, muschle, heart

Glycogen debranching enzyme

AR AGL 1p21[173]

Type IIIB

Liver Glycogen debranching enzyme

AR AGL 1p21[173]

Type IV Liver Glycogen phosphorylase AR PYGL[26] 14q21-22[118]

Type IX Liver, erythrocytes, leukocytes

Liver isoform of -subunit of liver and muscle phosphorylase kinase

X-Linked PHKA2 Xp22.1-p22.2[40][68][162]

[165]

Liver, muscle, erythrocytes, leukocytes

Β-subunit of liver and muscle PK

AR PHKB 16q12-q13[54]

Liver Testis/liver isoform of γ-subunit of PK

AR PHKG2 16p11.2-p12.1[28][101]

Glycogen

Type 0

Type I

Type II

Glycogen Storage Diseases

Type IV

Type VII

Glycogen Storage DiseaseType IIIb

• Deficiency of debranching enzyme in the liver needed to completely break down glycogen to glucose

• Hepatomegaly and hepatic symptoms– Usually subside with age

• Hypoglycemia, hyperlipidemia, and elevated liver transaminases occur in children

GSD Type III

Type III

Debranching Enzyme

• Amylo-1,6-glucosidase– Isoenzymes in liver, muscle and heart– Transferase function– Hydrolytic function

Genetic Hypothesis

• The two forms of GSD Type III are caused by different mutations in the same structural Glycogen Debranching Enzyme gene

Amylo-1,6-Glucosidase Gene

• The gene consists of 35 exons spanning at least 85 kbp of DNA

• The transcribed mRNA consists of a 4596 bp coding region and a 2371 bp non-coding region

• Type IIIa and IIIb are identical except for sequences in non-translated area

• The tissue isoforms differ at the 5’ end

Mutated Gene

• Approximately 16 different mutations identified

• Most mutations are nonsense• One type caused by a missense

mutation

Where Mutation Occurs

• The GDE gene is located on chromosome 1p21, and contains 35 exons translated into a monomeric protein

• Exon 3 mutations are specific to the type IIIb, thus allowing for differentiation

Inheritance

• Inborn errors of metabolism• Autosomal recessive disorder• Incidence estimated to be between

1:50,000 and 1:100,000 births per year in all ethnic groups

• Herling and colleagues studied incidence and frequency in British Columbia– 2.3 children per 100,000 births per year

Inheritance

• Single variant in North African Jews in Israel shows both liver and muscle involvement (GSD IIIa)– Incidence of 1:5400 births per year– Carrier frequency is 1:35

Inheritance

G g

G

g

GG Gg

Gg gg

GG = normal

Gg = carrier

Gg = GSD

Both parents are carriers in the case.

Inheritance

normal

carrier

GSD“Baby”

Clinical Features

• Hepatomegaly and fibrosis in childhood

• Fasting hypoglycemia (40-50 mg/dl)

• Hyperlipidemia

• Growth retardation

• Elevated serum transaminase levels (aspartate aminotransferase and alanine aminotransferase > 500 units/ml)

Common presentation

• Splenomegaly

• Liver cirrhosis

Clinical FeaturesLess Common

Galactosemia is an inherited disorder that affects the way the body breaks down certain sugars. Specifically, it affects the way the suga

r called galactose is broken down. Galactose c an be found in food by itself. A larger sugar call ed lactose, sometimes called milk sugar, is bro

ken down by the body into galactose and gluco se. The body uses glucose for energy. Because

- -of the lack of the enzyme (galactose 1 phosph ate uridyl transferase) which helps the body br eak down the galactose, it then builds up and

becomes toxic. In reaction to this build up of g alactose the body makes some abnormal che

micals. The build up of galactose and the other chemicals can cause serious health problems li

ke a swollen and inflamed liver, kidney failure, stunted physical and mental growth, and catar

acts in the eyes. If the condition is not treated there is a 70% chance that the child could die.

Lysomal storage diseases The pathways are shown for the formation a

nd degradation of a variety of sphingolipids, with the hereditary metabolic diseases indic ated.

Note that almost all defects in sphingolipid metabolism result in mental retardation and

the majority lead to death. Most of the disea ses result from an inability to break down sp

- hingolipids (e.g., Tay Sachs, Fabry's disease).