Disorders Of Aromatic Amino Acid Metabolism (22082013)

INBORN ERRORS OF AROMATIC AMINO ACID METABOLISM

(Phenylalanine & Tyrosine)

Dr.Shrikant Raut22/08/2013

• A 1 yr old girl is brought to her pediatrician’s office with concerns about her development. She had an uncomplicated birth at term. The mother reports that the baby is not achieving the normal milestones for a baby of her age. She also reports an unusual odor to her urine and some areas of hypopigmentation on her skin and hair. On examination, the girl is noted to have some muscle hypotonia and microcephaly. The urine collected is said to have mousy odor.

• What is the most likely diagnosis ?

• PHENYLKETONURIA

• Autosomal recessive• Metabolic genetic disorder• Incidence of 1 in 10,000 live births ( 1 in 18,300 in India )• Mutation in the gene coding for phenylalanine hydroxylase

(PAH)• It is this enzyme that is necessary to convert phenylalanine

into another amino acid called tyrosine• If this chemical pathway can not occur, phenylalanine builds

up in the bloodstream and brain tissue, causing mental retardation and central nervous system problems

• Discovered by Norwegian physician Ivar Folling , thus also called as Folling’s disease

Normal serum phenylalanine levels are 1 to 2 mg%

Increase in phenylalanine levelsDecrease in tyrosine levels

Signs & Symptoms• Untreated children are normal at birth , but fail to attain early

developmental milestones• Seizures • Microcephaly• Progressive impairment of cerebral function Executive function ( cognitive abilities ) Information processing speed ADHD ( late manifestation ) Learning disabilities( late manifestation ) basically untreated PKU show symptoms of mental retardation by

the age of 1 yr• Musty or Mousy odor of skin ,hair ,sweat and urine (due to

phenylacetate accumulation)• Tendency towards hypopigmentation and eczema

Diagnosis

Normal serum tyrosine levels : Neonate = 7.6 – 18.0 µg/ml Adult = 3.9- 15.8 µg/ml

TLC

Newborns are screened for PKU

• All US babies are screened for PKU by heel-prick test.

• Performed 2-7 days after birth and repeat at 2 weeks of age.

• Blood tested for excess phenylalanine.• Blood placed on agar plate with bacteria

that need phenylalanine to grow.• Healthy babies’ blood doesn’t have extra

phenylalanine, so bacteria can’t grow• Babies with PKU have extra

phenylalanine, so bacteria grow

Guthrie test for PKU

Bacterial plate with newborn blood samples

Negative controls: no bacterial growth

Positive blood test results: bacterial halo = PKU

Negative blood test results: no bacterial growth = healthy babies

Positive controls : increasing phenylalanine concentrations give bacterial halos

http://www.childrenshospital.org/cfapps/research/data_admin/Site2940/mainpageS2940P4sublevel15.html

• Infants may still be breastfed to provide the benefits of breast milk , but quantity should be monitered

• Top fed with special infant formula called Lofenalac ( low in phenylalanine )

• Kuvan ( sapropterin dihydrochloride , BH4 , tetrahydrobiopterin)

• Glycomacropeptide ( protein derived from goat milk , which is free of Phe , but it is to be substituted with Tyr and Trp )

• Large neutral amino acid therapy ( LNAA )

Sickenly sweet Aspartame• Aspartame = artificial sweetener made from

amino acids phenylalanine and aspartic acid• Found in “diet”, “light”, and “sugar-free” foods• Highly toxic to people with PKU

Other modalities of treatment

• Enzyme substitution therapy ( PEG-PAL ) : Phenylalanine ammonia lyase is joined to polyethylene glycol.

PAL breaks down Phe to transcinnamic acid and ammonia which further break down into non-toxic compounds that can be easily handled by the body.

• Therapeutic liver cell ( hepatocyte ) repopulation• Gene therapy

Types of hyperphenylalaninemia

• Classical PKU 400 disease causing mutations have been seen on the PAH

gene located on chromosome number 12• Persistent hyperphenylalaninemia Decreased PAH enzyme activity , can be managed by

temporary dietary therapy• Transient mild hyperphenylalaninemia Maturational delay in PAH enzyme• Tetrahydrobiopterin-deficient hyperphenylalaninemia

Tetrahydrobiopterin-deficient hyperphenylalaninemia

• Rare form of hyperphenylalaninemia when PAH is normal• Defect in the gene coding for dihydrobiopterine reductase , thus

BH4 cannot be replinished and made available for PAH enzyme for its activity.

• Additional supplements required• Differentiate with the help of prolactin levels ( prolactin levels

normal in treated classical PKU )

Alkaptonuria

Alkaptonuria

• Also known as Black bone disease or Black urine disease• Autosomal recessive inheritance• Metabolic genetic disorder• Incidence is 2 to 5 per million live births• Deficiency in homogentisate 1,2-dioxygenase, an enzyme

which converts homogentisic acid (HGA ) to maleylacetoacetic acid in the tyrosine degradation pathway

• Accumalation of HGA at 2000 times the normal rate• The gene coding for homogentisate 1,2-dioxygenase is located

on chromosome number 3• First described by Sir Archibald Garrod in 1901 in London as

one of his tetrad

• Alkaptonuria has three major features : 1) HGA accumulates in the blood and appears in the urine, which on standing

is oxidized to a black pigment (alkapton), hence the name Black urine disease

2) Ochronosis ( bluish-black pigmentation of connective tissue ) Accumulation of HGA and its oxidation product benzoquinone acetate in

the connective tissue• Brown pigmentation of the sclera is observed midway between the

cornea and the outer and inner canthi at the insertin of the recti muscles. Pigment deposition may also be seen in the conjunctiva and cornea. The pigmentation does not affect vision .

• Ear cartilage pigmentation is first seen in the concha and antihelix, and later in the tragus. The cartilage is slate blue or gray and feels irregular or thickened. Calcification of the ear cartilage may be observed on radiographs.

• Pigment also appears in cerumen and in perspiration, causing discoloration of clothing.

• A deep purple discoloration may be seen on the skin of the hands, corresponding to the underlying tendons, or in the web between the thumb and index finger.

3) Arthiritis

Natural History

• During childhood , AKU is asymptomatic apart from the urine turning dark on standing

• AKU does not cause cognitive impairment or developmental delay

• From the third decade of life signs and symptoms due to ochronosis begin to manifest ( low back pain may be the presenting symptom at this age indicating the involvement of the spine )

• By the fifth decade arthritis manifests in the large joints • Other organ involvement Aortic and Mitral Valve calcification Renal stones and Prostate stones

Diagnosis

• Biochemical testing. The diagnosis of alkaptonuria is based on the detection of a significant amount of HGA in the urine by gas chromatography-mass spectrometry analysis. The amount of HGA excreted per day in individuals with alkaptonuria is usually between one and eight grams .

• A normal 24-hour urine sample contains 20-30 mg of HGA.• HGD molecular genetic testing to confirm the diagnosis• Prenatal diagnosis in high-risk families

Treatment

• No cure for AKU• Only treatment offered to patients is palliative, pain

management till joint collapse and joint replacement surgery• Important to avoid sports and exercise that put too much stress

on joints• Dietary restriction does not help much , but red meat can be

avoided just to control the Phe and Try levels• Nitisinone a triketone herbicide an inhibitor of 4-hydroxyphenyl

dioxygenase , the enzyme that produces HGA Drug approved for the use in tyrosinemia type 1 , clinical trials

going on for its use in AKU Reduces HGA formation at the expense of elevated Tyr levels ,

which may cause photophobia and rarely corneal crystals

Tyrosinemia

Tyrosinemia type I

• Also known as hepato-renal tyrosinemia / Tyrosinosis / Hereditary infantile Tyrosinemia

• Most severe form of tyrosinemia• It is inhereted in an autosomal recessive pattern with an

incidence of 1 in 1,00,000 ( incidence is quite common in Quebec ,Canada of 1 in 16,000 )

• Deficiency of enzyme fumaryl acetoacetate hydrolase• This leads to build up of fumaryl acetoacetate and succinyl

acetone• This enzyme is expressed more in the liver and proximal renal

tubular epithelial cells

Pathophysiology

• Fumaryl acetoacetate along with its metabolites accumulate in the hepatocytes and renal tubular cells, causing oxidative damage and DNA damage along with dysfunctional gene expression which alters the metabolic processes like protein synthesis and damages these organs

• The symptoms of Tyrosinemia type 1 are due to accumulation of tyrosine and its metabolites ( Succinylacetone, Succinylacetoacetate and fumarylacetone ) in the liver, kidney and central nervous system

• Succinylacetone inhibits enzyme δ-ALA dehydratase in the liver and circulating RBC’s

Signs and Symptoms

• The so-called acute form is present at birth or during the few first months of life.

• Infants with the acute form of tyrosinemia type 1 exhibit rapid onset of symptoms,

• usually beginning with failure to thrive. Additional early symptoms include:• • Fever• • Diarrhea/bloody stools• • Vomiting• • Enlarged liver• • Tendency to bruise easily• • Jaundice• • Lethargy• • Irritability• • Some infants may have a distinctive cabbage-like odor to the skin and urine Untreated acute tyrosinosis progress towards life-threatening liver failure

and succumb to the same by 8 months of age

• Chronic form of Tyrosinemia type 1 Characterized by a more gradual onset

and less severe expression of symptoms Present with hepato-splenomegaly and

failure to thrive Developmental delay and repeated

acute neurological episodes like acute polyneuropathy and

altered mental status Cirrhosis of liver ( increased risk of HCC ) Renal Fanconi syndrome• Untreated chronic cases survive upto 10

years of age

Diagnosis

• Diagnosed as a result of newborn screening• Suspicion of diagnosis in infants who display failure to thrive

and hepatomegaly in first 3 months of life• Detection of Succinylacetone in urine and decreased activity

of FAH enzyme in liver tissue or cultured fibroblasts confirms the diagnosis

• Elevated plasma Tyrosine, Phenylalanine and Methionine levels

• Increased urinary excretion of δ-ALA• Markedly elevated AFP• Prolonged PT and pTT• Prenatal diagnosis : Succinylacetone levels in amniotic fluid

and FAH enzyme activity in amniotic fluid cells.

Treatment

• Prompt indentification and treatment may prevent severe liver, kidney and neurological problems and child can experience a normal growth

• 1) Dietary treatment : Low protein diet that contains limited amounts of Phe and Tyr

• 2) Medical Treatment : Nitisinone, inhibitor of 4-hydroxyphenylpyruvate dioxygenase.

• 3) Liver transplant : infants with end stage liver failure

Tyrosinemia Type II

• Also known as Oculocutaneous Tyrosinemia / Richner Hanhart Syndrome

• Autosomal recessive inheritance• Deficiency of enzyme tyrosine aminotransferase due to

mutation in the gene located on chromosome number 16• Incidence is 1 in 2,50,000• It affects the eyes , skin and mental development• Manifestations are due to accumulation of tyrosine and its

metabolites by an unknown mechanism

Clinical Manifestation

• Excessive lacrimation, redness, pseudodendritic keratitis, corneal deposits, corneal ulcers followed by scarring

Usually manifest in the first year of age• Non-pruritic , hyperkeratotic papules and plaques principally

located on palms and soles ( palmoplantar hyperkeratosis ). The lesions are painful and progressive and associated with

hyperhidrosis Usually manifest after 1 year of age• Central nervous system involvement is variable and usually

manifests as intellectual deficit

Diagnosis and Treatment

• Detection of high levels of plasma and urinary tyrosine• Management : Dietary limitation of Phe and Tyr. Oral retinoids for skin lesions. Oculocutaneous manifestations resolve with dietary control. Its unknown wether CNS symptoms resolve with dietary control.

Tyrosinemia type III• Very rare disorder• Autosomal recessive inheritance• Deficiency of enzyme 4-hydroxyphenylpyruvate dioxygenase,

leading to accumulation of 4-hydroxyphenylpyruvate and its metabolites like 4-hydroxyphenylacetate and 4-hydroxyphenyllactate along with increased plasma levels of tyrosine

• Deficiency is due to mutation in the gene coding for the enzyme , locate don chromosome number 12

• It is characterized by mild mental retardation, seizures and intermittent ataxia observed during the infancy period

• Not much is studied about the disease

Albinism

Albinism

• It is a group of genetic disorder that is potrayed by the absence or diminished pigmentation in the skin, eyes and hair of the organism.

• Albinism at first was considered an innate error of metabolism in the genetic makeup of organisms in 1908 by scientist Sir Archibald Garrod

• Two general types : 1) Ocular Albinism 2) Oculocutaneous Albinism

Signs and Symptoms

• It affects the pigmentation of skin, hair and eyes• Visual problems : 1) Photophobia & decreased visual acuity 2) Nystagmus , Astigmatism 3) Strabismus 4) Optic Chiasma abnormalities The above manifestations are because poor development of

retinal pigment epithelium and foveal hypoplasia Iris, being pigmented normally controls the amount of light

falling on the retina ( through pupil ). In albinism the iris is completely trans-illuminated and and

displays the colour of retina itself

• Gene Therapy▫ Improve visual acuity by implementing healthy OA1 protein into the

RPE cells (Anderson, et. al, 2004). ▫ This enriched visual acuity and improving overall eye sight of those

affected with albinism (Anderson, et. al, 2004).

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