Inborn errors of metabolism

Aseem JainResident of pathology

JNMC

• Many childhood conditions are caused by single gene mutations that encode specific proteins.

• These mutations can result in alteration of primary protein structure or the amount of protein synthesized.

• The function of protein, whether it is an enzyme, receptor, transport vehicle, membrane component or structural element may be compromised or abolished.

• These hereditary biochemical disorders are termed as inborn errors of metabolism.

• This term was coined by ‘Garrod’.

ClassificationDisorders Examples

Disorders of Amino acid metabolism Phenylketonuria, Alkaptonuria, Homocyteinuria,

Disorders of Carbohydrate metabolism

Galactosemia, Glycogen storage diseases

Disorders of Porphyrin metabolism Acute intermittent porphyrias

Disorders of Purine / Pyrimidine metabolism

Lesch-Nyhan syndrome

Disorders of Steroid metabolism Congenital adrenal hyperplasia

Disorders of Mitochondrial function Kearns-Sayre syndrome

Disorders of Peroxisomal function Zellweger syndrome

Disorders of Lysosomes Lysosomal storage diseases

Phenylalanine & Tyrosine

• Hyperphenylalaninemia depends on the degree of enzyme deficiency and may vary from very high plasma concentrations (>20mg/dl) to mildly elevated levels (2-10mg/dl).

• Classic Phenylketonuria(PKU) - >20mg/dl plasma concentration of phenylalanine ; Autosomal recessive ; M=F ; 1 in 11000 births.

• Hyperphenylalaninemia – 2-10mg/dl plasma concentrations of phenylalanine.

Hyperphenylalaninemia

• Clinical features – 1. Affected infant is normal at birth.2. Severe mental retardation by 6 months.3. Only 2-5% have normal IQ.4. The infants are lighter in complexion than their

siblings.5. Seborrhoeic or eczematoid rash.6. Urine/Sweat – musty or mousy odor.7. Neurologic signs.8. Microcephaly.9. Widely spaced teeth.10. Growth retardation.

• Diagnosis – 1. Plasma phenylalanine is elevated.2. Urine - Typical musty/mousey odor. - Phenistix reagent strips are available.

- At 30 seconds following immersion into urine, the color of the test area is noted and compared with the chart provided.- Gray to Gray green color indicates positive test.- This test detects 5-10mg/dl phenylalanine.

• Maternal PKU

Alkaptonuria

• Rare autosomal recessive disorder.• Incidence – 1 in 250,000 births.• First human inborn error of metabolism to be

discovered.• Discovered by Garrod.• Caused by deficiency of Homogentisate oxidase.• The gene encoding this enzyme is mapped to

chromosome 3q13.3.

• Clinical features – 1. Onchronosis – most evident in ear, nose and cheek.2. Arthritis – involves large joints (spine, hip & knee).3. Blackening of urine on standing.4. High incidence of heart disease.

Homogentisic Benzoquinone Alkapton acid acetate

Oxidation

Polyphenoloxidase

Polymerization

• Diagnosis – • Urine – Ferric chloride & Nitric acid tests.• 2ml Urine + 2 drops 10%FeCl3 = Dark blue color.

(Ferric chloride test).• 4ml 3% silver nitrite + 0.5ml Urine + mix + Few drops

of Ammonium hydroxide = Black color. (Silver nitrite test).

• Other disorders of tyrosine metabolism – 1. Tyrosinemia type I / Hepatorenal tyrosinemia2. Tyrosinemia type II / Occulocutaneous tyrosinemia3. Tyrosinemia type III

Homocystinuria

• Clinical features – 1. FTT & developmental delay.2. Subluxation of ocular lens (ectopia lentis).3. Astigmatism, glaucoma, cataract, optic atrophy may

occur.4. Progressive intellectual disability.5. Normal IQ.6. Psychiatric and behavioral disturbances(50%).7. Convulsions(20%).8. Skeletal abnormalities.9. Fair complexion, blue eyes and malar flush.10. Thromboembolic episodes.

• Diagnosis – • Elevation of both methionine and homocystine in

body fluids are diagnostic.• Assay of enzyme in liver biopsy specimen• DNA analysis

Glycogen storage diseases

Type (GSD)

Defect Clinical features Lab findings Genetics

Ia (Von Gierke disease)

Glucose 6 phosphatase. Gene on chromosome 17q21

Doll like face Thin extremities, short stature, protuberant abdomen, skin xanthomas, retinal changes, hepatomegaly

1. Hypoglycemia, 2. lactic acidosis, 3. hyperuricemia, 4. Dyslipidemia,5. Normal liver transaminaseDiagnosis – Liver biopsy

Autosomal recessive

IIIa (Cori’s/ Forbes disease/Limit dextrinosis)

Debranching enzyme.Gene on chromosome 1p21.

Hepatomegaly, Growth retardation, Muscle weakness, Cardiomyopathy

1. Dyslipidemia,2. hypoglycemia, 3. ↑ hepatic transaminases,4. fasting ketosis, 5. Normal lactate and uric

acid6. EMG shows myopathyDiagnosis- abnormal glycogen and abnormal enzyme activity on liver and muscle biopsy

Autosomal recessive

Type (GSD) Defect Clinical features

Lab findings Genetics

IV (also calledAnderson’sdisease, amylopectinosis)

GlycogenbranchingEnzyme.Gene on chromosome 3p21.

Hepatosplenomegaly, failure to thrive, liver cirrhosis, portal hypertension, ascites,esophageal varices, fatal by 5 years of age

1. Tissue deposition of amylopectin like material

2. Deficiency of branching enzyme in liver

Autosomal Recessive

VI (also calledHers disease)

Glycogenphosphorylase(in liver)

Hepatomegaly, growth retardation in early childhood

1. Mild hypoglycemia, 2. Dyslipidemia,3. ketosis4. Normal uric acid and lactic acidDiagnosis: Abnormal enzymeactivity in biopsy of affected tissues

Autosomal recessive

Type (GSD)

Defect Clinical features Lab findings Genetics

IXa Phosphorylasekinase (inliver)

Hepatomegaly, protuberantabdomen, growth retardation,delay in motordevelopment

Mild dyslipidemia, mildly elevatedliver transaminases, fasting ketosis,mild hypoglycemiaDiagnosis: Abnormal enzymeactivity in biopsy of affected tissues

X linked

Type (GSD) Defect Clinical features Lab findings

V (McArdle disease)

Musclephosphorylase

Exercise intolerance, muscleCramps, Dark red urine after intense exercise,Usually presents in 2nd or 3rd Decade

Myoglobinuria, ↑ creatinine kinase at rest and increases after exercise, ↑ ammonia and ↑ uric acid with exerciseDiagnosis - enzymaticevaluation of muscle

VII (Tarui disease)

Phosphofructokinase

Same as in type V + Severe exercise intolerance ormyopathy in childhood,compensated hemolytic anemia, more hyperuricemia than type V

↑ Creatine kinase ↑ bilirubin, reticulocytosis,HyperuricemiaDiagnosis - biochemical orhistochemical demonstrationof enzyme defect

II/Pompe disease

Lysosomal acidalpha glucosidase(GAA) (acidmaltase)

Muscle weakness, feeding problem, macroglossia, hepatomegaly, Cardiomyopathies, swallowing difficulties

↑ Creatine kinase, ↑ aspartateTransaminase.• Diagnosis - decreased or absentacid alpha-glucosidase (GAA)activity in muscle

Galactosemia

• Inability to metabolize galactose occurs in the galactosemias, which may be caused by inherited defects of galactokinase, uridyl transferase, or 4-epimerase though deficiency of uridyl transferase is best known.

• It has an autosomal recessive mode of inheritance.• The most common mutation of the GALT gene is the

Q188R mutation on chromosome 9.• 1 in 60,000 live births.• Typically presents by second half of 1st week of life.

• Clinical features – 1. Hypoglycemia,2. Vomiting,3. Diarhhoea,4. Irritability,5. Feeding difficulties,6. Failure to thrive,7. Jaundice,8. Hepatomegaly,9. Easy bruisibility,10. Increased risk of developing cerebral edema,11. Viterous hemorrhage12. E.coli sepsis

• Diagnosis – 1. Hyperbilirubinemia,2. Elevated liver transaminases, 3. Metabolic acidosis, 4. Galactosuria, 5. Glycosuria,6. Hypoglycemia, 7. Abnormal clotting measurements,8. Liver biopsy,9. Thin layer chromatography,10. Microbiological and fluorometric assays are used in

the screening of the newborn to detect galactosemia.

Mucopolysaccharoidoses

• Hereditary, progressive diseases caused by mutations of genes encoding for lysosomal enzymes needed to degrade glycosaminoglycans (GAGs).

• The enzymes involved in the degradation of these molecules cleave terminal sugars from polysaccharide chains.

• When there is a block in the removal of a terminal sugar, the remainder of the polysaccharide chain is not further degraded and these chains accumulate within the lysosomes in various tissues and organs.

MPSType

Name Enzyme defect

Gene/chromosome

Accumulated GAGs

Clinical features

I-H Hurler disease

Alpha-L-iduronidase

IDUA/ 4p16.3

HS,DS Mental retardation, Corneal clouding,URTI, Large tongue, prominent forehead, joint stiffness, short stature

II Hunter disease

Iduronate sulfate sulfatase

IDS/ Xq27.3-28

HS,DS Features same as Hurler syndrome but are less severe

IIIA,B,C,D

Sanfillipo A,B,C,D

•HSS•NAADG•AGNAT•NAG6S

SGSH/17qNAGLU/17qHGSNAT/8pGNS/12q

HS Behavioral problems, sleeping disorder, aggression, progressive dementia, coarse hair, clear cornea

IV A,B

Morquio A,B

N acetyl glucosamine 6 sulfataseBeta galactosidase

GALNS/16q

GLB/3p

KS Short trunk dwarfism, fine corneal opacities, bone dysplasia, height below 125cm

• MPS type VI (Maroteaux lamy disease)• MPS type VII (Sly disease)• MPS type IX (Hyaluronidase deficiency)

• The accumulated mucopolysaccarides are generally found in mononuclear phagocytic cells, endothelial cells, smooth muscle cells and fibroblasts.

• Common sites – Spleen, liver, bone marrow, lymph nodes, blood vessels and heart.

• Hepatosplenomegaly, skeletal deformities, valvular lesions & subendothelial arterial deposits & lesions in brain are common threads that run through all MPS.

Gaucher disease

• Multisystemic lipidoses.• Hematologic abnormalities, organomegaly, skeletal

involvement.• Most common lysosomal storage diseases.• 3 types – type 1, 2, 3.• Results from deficient activity of lysosomal

hydrolase, glucocerebrosidase which is encoded a gene located on chromosome 1q21-q31.

• The enzymatic defect results in accumulation of glucocerebroside in the cells off RE system and their progressive deposition leads to infiltration of bone marrow, hepatosplenomegaly and skeletal problems.

• Clinical features – • Early childhood to late adulthood.• Bruising/ Epistaxis• Fatigue• Hepatosplenomegaly• Growth retardation• Bone pain• Pathological fractures• Reduced but detectable glucocerebrosidase activity.

• Gaucher type 2 disease (infantile or rapid neuronopathic form) is rare and characterized by rapid neurodegenerative course with extensive visceral involvement and death within 1st year of life.

• Clinical features – • Increased tone of muscles• Failure to thrive• Strabismus• Organomegaly (liver and spleen)• Stridor• Virtually no glucocerebrosidase activity.

• Morphology – • Accumulation of glucocerbrosides

within phagocytic cells.• The distended phagocytic cells –

Gaucher cells.• Gaucher cells – often enlarged, up

to 100 µm in diameter, having one or more eccentrically placed dark nuclei and fibrillary cytoplasm likened to crumbled piece of paper.

• Gaucher cells are found in spleen, liver, bone marrow, lymph nodes, tonsils, thymus and Peyer patches.

• PAS stain is intensely positive.

• Diagnosis – • Measurement of glucocerebrosidase activity in

peripheral blood leukocytes or in extracts of cultured skin fibroblasts.

• Chitotriosidase, an enzyme synthesized by macrophages, is markedly elevated in patients with Gaucher disease.

• Biopsy.

Niemann Pick disease

• 3 types – Type A,B,C.• Type A & B are two related disorders that are

characterized by lysosomal accumulation of sphingomyelin due to inherited deficiency of sphingomyelinase enzyme.

• Type A – severe infantile form with extensive neurologic involvement, marked visceral accumulations of sphingomyelin and early death within first 3 years of life.

• Clinical features – 1. Protuberant abdomen2. Failure to thrive3. Vomiting 4. Fever5. Generalized lymphadenopathy6. Progressive deterioration of psychomotor function.

• Diagnosis – • Biochemical assays for sphingomyelinase activity in

liver and bone marrow.

• Morphology – • Affected cells become enlarged,

sometimes to 90µm in diameter, secondary to distension of lysosomes with sphingomyelin and cholesterol.

• Plenty vacuoles of relatively uniform size are created imparting foaminess to the cytoplasm.

• The lipid laden phagocytic foam cells are widely distributed in spleen, liver, lymph nodes, BM, tonsils, GIT and lungs.

• Brain – gyri are shrunken and sulci are widened

• Vacuolations and ballooning of neurons.

• Retinal cherry red spot.

Cystic fibrosis

• Also known as mucoviscidosis.• It is a widespread disorder in epithelial transport

affecting fluid secretion in exocrine glands and epithelial lining of the respiratory, gastrointestinal and reproductive tracts.

• In many infants, this disorder leads to abnormally viscid mucous secretions, which obstruct organ passages, leading to most of the clinical features of this disease.

• Clinical features –

• Diagnosis –

• Sequencing of CFTR gene – gold standard• Persistently elevated sweat electrolyte

concentrations• Measurement of nasal transepithelial potential

difference in vivo•

Wilson’s disease

• Autosomal recessive disorder• Accumulation of toxic levels of copper in many

tissues & organs principally in liver, brain and eye.• Gene ATB7B (chromosome 13) encodes a 7.5kB

transcript for a transmembrane copper transporting ATPase, located on the hepatocyte canalicular membrane.

• Defective biliary excretion leads to copper accumulation in liver causing toxic liver injury.

• Clinical features – • Neuropsychiatric manifestations• Acute or chronic liver disease• Kayser-Fleischer rings

• Diagnosis – • Decreased serum ceruloplasmin• Increased hepatic copper content• Increased urinary excretion of copper

References

• Nelson textbook of pediatrics 1st south east asia edition

• Robbins and cotrans basic pathology 7th edition

• Henry’s clinical diagnosis and management 22nd edition

• Harper illustrated biochemistry 30th edition

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