Overview of Inborn errors of metabolism

Inborn errors of metabolism - metabolic aspects and investigations for medical students and practitioners

1. Inborn errors of metabolism
Dr.S.Sethupathy,M.D.,Ph.D.,
Professor and Head,
Dept. of Biochemistry,
Rajah Muthiah Medical college,
Annamalai University.

2. 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”

3. What is a metabolic disease?
•Garrod’s hypothesis
product
deficiency
substrate excess
toxic metabolite
A
D
B
C

4. What is a metabolic disease?
•Small molecule
disease
–Carbohydrate
–Protein
–Lipid
–Nucleic Acids
•Organelle
disease
–Lysosomes
–Mitochondria
–Peroxisomes
–Cytoplasm

5. 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

6. How do you recognize a
metabolic disorder ??
•Index of suspicion
–eg “with any full-term infant who has no
antecedent maternal fever or PROM-
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.

7. Index of suspicion
Family 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!

8. Index of suspicion
History
•CAN YOU EXPLAIN THE SYMPTOMS?
•Timing of onset of symptoms
–after feeds were started?
•Response to therapies

9. Index of suspicion
Physical 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

10. Index of suspicion
Laboratory
•ANION GAP METABOLIC ACIDOSIS
•Normal anion gap metabolic acidosis
•Respiratory alkalosis
•Low BUN relative to creatinine
•Hypoglycemia
–especially with hepatomegaly
–non-ketotic

11. 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!

12. 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

14. Three Types
•Type 1: Silent Disorders
•Type 2: Acute Metabolic Crises
•Type 3: Neurological Deterioration

15. Type 1: Silent Disorders
•Do not manifest life-threatening crises
•Untreated could lead to brain damage and
developmental disabilities
•Example: PKU (Phenylketonuria)

16. PKU
•Error of amino acids metabolism
•No acute clinical symptoms
•Untreated leads to mental retardation
•Behavior disorders, cataracts, skin
disorders, and movement disorders
•First newborn screening test was
developed in 1959
•Treatment: phenylalaine restricted diet

17. 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)

18. 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

19. Type 3: Progressive
Neurological Deterioration
•Examples: Tay Sachs disease
Gaucher disease
Metachromatic leukodystrophy
•DNA analysis show: mutations

20. Mutations
•Nonfunctioning enzyme results:
Early Childhood - progressive loss of
motor and cognitive skills
Pre-School – non responsive state
Adolescence - death

21. Other Mutations
•Partial Dysfunctioning Enzymes
-Life Threatening Metabolic Crisis
-ADH
-LD
-MR
•Mutations are detected by Newborn
Screening and Diagnostic Testing

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

23. Children in School
•Life long treatment
•At risk for ADHD
LD
MR
•Awareness of diet restrictions
•Accommodations

24. Examples from different IEM groups
Amino acid metabolism: phenylketonuria (PKU)
maple Syrup Urine Disease (MSUD)
homocystinuria
arginino succinnic aciduria
OTC deficiency
Organic Acidaemias propionic acidaemia
methyl malonic aciduria
isovaleric acidaemia
Fat Oxidation Defects: MCAD deficiency
Carbohydrate Metabolism: glycogen storage disorders
galactosaemia
Lysosomal storage disorders: gaucher and Fabry diseases
mucopolysaccharidoses
Transport protein defects: cystic Fibrosis
cystinuria
cystinosis
Mitochondrial disorders: Pearson syndrome
cytochrome oxidase def
Urea cycle
disorders

25. Phenylketonuria
•Affects 1: 10,000 Caucasian births
•Severe mental retardation untreated
•Excellent prognosis if treated from birth
•Screening test: bloodspot phenylalanine
•Confirm diagnosis with plasma phe
measurements
–no need to meas enzyme or DNA

26. Classical PKU
Phenylalanine ­ ­
Phenylalanine
Hydroxylase
Tyrosine ¯¯
X
Tetrahydrobiopterin
(reduced)
Dihydrobiopterin
(Oxidised)
Dihydrobiopterin
reductase
Pre-block
metabolite
increases
Post-block
metabolite
decreases
Phenylketones ­ ­

27. Treatment
•Low phenylalanine diet
–requires careful monitoring
–risk of tyrosine insufficiency
–risk vitamin and trace element deficiencies
•? biopterin supplementation (sapropterin)
•Large Neutral Amino Acids (val, leu, ileu) supplements
•Diet for life
•Management of PKU pregnancies

28. High Phenylalanine
Low tyrosine
Plasma Amino Acid Profile, PKU
High phe

29. Metabolism of homocysteine
homocysteine
methionine
betaine
tetrahydrofolate
cobalamin
cystathionine
cystathionine synthasepyridoxine
cysteine
5-me tetrahydrofolate
MTHF reductase
homocystine
Methyl donor
reactions

30. Natural History of Clasical
Homocystinuria
Lens dislocation:
–82% dislocated by age 10 years
•Osteoporosis (x-ray):
–64% with osteoporosis by age 15 yrs
•Vascular events:
–27% had an event by age 15 years
•Death:
–23% will not survive to age 30 years
•Mental Retardation – approx 50%

31. Urine
Positive cyanide - Nitroprusside test
Chromatography: Homocystine
Plasma total homocysteine:
Increased (ref <18 mmol/L )
Tests Performed

32. Plasma Ammonia
• Lithium heparin tube
• Request urgently
• Transport immediately to lab
Delays cause falsely high ammonia
• Avoid contamination:smoking

33. The Urea Cycle

Carbamyl Phosphate
Citrulline
Argininosuccinic acid
Arginine
Ornithine
NH
3
+ Bicarbonate
Ornithine Transcarbamylase (OTC)
Argininosuccinate Synthase
Argininosuccinate Lyase
Carbamyl phosphate synthetase
Arginase
Urea
Converts highly
toxic ammonia to
less toxic urea

34. Disorders of urea
cycle

Carbamyl Phosphate
Citrulline
Argininosuccinic acid
Arginine
Ornithine
NH
3
­­
Ornithine Transcarbamylase (OTC)
Argininosuccinate Lyase
Carbamyl phosphate synthetase
Arginase
Urea ¯¯
Orotic acid
Side pathway
utilised
Marked
hyperammonaemia
Impaired urea
synthesis
Arginino succinic aciduria
OTC deficiency

35. OTC deficiency

Carbamyl Phosphate
Citrulline
Argininosuccinic acid
Arginine
Ornithine
NH
3
­­
Ornithine Transcarbamylase (OTC)
Argininosuccinate Synthase
Argininosuccinate Lyase
Carbamyl phosphate synthetase
Arginase
Urea ¯¯
X
Orotic acid
Side pathway
utilised
Marked hyperammonaemia
Impaired urea
synthesis

36. OTC deficiency
•OTC deficiency is x-linked
•Males and female homozygotes are
severely affected
•Female heterozygotes are variably
affected due to random x inactivation
(Lyonisation)

37. OTC presentation - infancy
–12- 72 hours of age
–Lethargic and poor feeding
–Abnormal respirations
–vomiting
–Seizure
–Decreasing conscious level
–If untreated die

38. Differential diagnosis is sepsis / meningitis
–Initial investigations
•Blood Gas – respiratory alkalosis
•FBC
•U+E, Ca2+, glucose – increase anion gap
(anion gap Na – (HCO3+Cl) normally 8 to 11)
•Lactate, ammonia
•LFT (often abnormal)
•Urine and blood cultures and ?LP (also test urine for
ketones)
–Initial management
•commence IV antibiotics...
•review management with results of investigations –

39. Interpreting Ammonia
•Abnormal
•>200 µmol/l premature neonates
•>100 µmol/l term neonates
•>40 µmol/l in older infants
Prognosis depends on duration and degree of
hyperammonaemia
•<500 = 94% surivival
•>1000 = 34% survival

40. Hyperammonaemia Treatment
•Protein restriction
•Antibiotics
•Benzoate
•Phenylbutyrate /Phenylacetate
•Arginine
•Carbaglu
•Dialysis

41. Benzoate Therapy
GlycineBenzoate
Hippurate
+

42. Phenylbutyrate/acetate Therapy
Phenylbutyrate
Phenylacetate
Glutamine
Phenylacetylglutamine
Phenylbutyryl CoA
Urine

43. N-Carbamoyl-L-glutamic acid
•Carbaglu
•Marketed by Orphan
•N-acetylglutamate analogue
•Stimulates Carbamyl phosphate synthetase
•Particularly useful in NAGS def

44. Arginine supplementation
Arginine deficiency common in many urea
cycle defects
Neonates and young infants have high
requirement for arginine

45. Argininosuccinic aciduria

Carbamyl Phosphate
Citrulline ­
Argininosuccinic acid ­­
¯ Arginine
Ornithine
NH
3
normal, ­ or ­­
Ornithine Transcarbamylase (OTC)
Argininosuccinate Synthase
Argininosuccinate Lyase
Carbamyl phosphate synthetase
Arginase
Urea ¯¯
X
Why can
ammonia be
normal ?

46. Arginino succinic aciduria

Carbamyl Phosphate
Citrulline
Argininosuccinic acid (ASA)­­
Arginine
Ornithine
NH
3
Ornithine Transcarbamylase (OTC)
Argininosuccinate Synthase
Carbamyl phosphate synthetase
Arginase
Urea
Renal clearance of ASA
is much higher than for
citrulline. NH
3
is
excreted as ASA
Urea cycle effectively becomes a
“linear pathway” provided arginine
intake is adequate

47. Small MW organic acids are intermediates in most
metabolic pathways
Organic acids
neurotransmitters
purines
pyrimidines
drugs, diet
micro-
organisms
amino acids carbohydrates
cholesterol
fatty acids

48. Clinical indications
Acute, intoxication
•Unexplained metabolic acidosis
•Hyperammonaemia
•Hypoglycaemia
•Lactic acidaemia
•ketonuria
Chronic
•Developmental delay
•Fits or seizures
•Liver disease

49. Branched chain amino acid catabolism
Leucine Valine Isoleucine



2-Oxoisocaproic 2-Oxoisvaleric 2-Oxo-3-methylvaleric
3-Methylcrotonyl-CoA
Isovaleryl-CoA
Propionyl CoA
Acetyl-CoA
2-Methyl-3OHbutyryl-CoA
Triglyl-CoA

2-Methylbutyryl
2-Methylmalonyl-CoA
Succinyl CoA
3-Methylglutaconyl-CoA
3-OH-3-Methylglutaryl-
CoA
2-Methylmalonic acid
semialdehyde

50. Hyperammonaemia in organic
acidaemias

glutamate
+ acetyl CoA
N-acetyl glutamate
NAG synthetase
Carbamoyl phosphate
ATP + CO
2
+ NH
3
+ve
CPS
synthetase
propionyl CoA
methylmalonyl CoA
-ve
UREA CYCLE

54. Methyl Malonic Acidaemia
•Episodes of metabolic acidosis triggered
by intercurrent illness
•Hyperammonaemia, ketoacidosis
•Vomiting
•Poor weight gain
•Progressive loss of renal function
•Hypotonia and later learning difficulties
•Seizures

55. Methyl Malonic Acidaemia: B12 Treatment
•Vitamin B12 is a co-factor
Test for B12 responsiveness
•Pre-B12, MMA in urine…4359
3332
5181
•Post-B12, MMA in urine…279
982
472
(units= umol/mmol creatinine)

56. Methyl Malonic Acidaemia:
other treatments
•Protein restriction
•Carnitine supplements
•Antibiotics
•Management of CRF

57. MCAD deficiency
•A fat oxidation defect
•Unable to mobilise full energy from fat
during fasting
•Prolonged fasting → hypoglycaemia
•Impaired ketone production
–Hypoketotic response to hypoglycaemia
•Incidence ~ 1 in 10,000

58. MCAD; the biochemical defect
fatty
acids
fatty acid oxidation
acetyl CoA
TCA
cycle
mitochondria
ketones
dicarboxylic acids URINE
Acyl carnitines
carnitine

59. MCAD deficiency
•Before screening ~25% of diagnoses were post mortem
•Crisis often follows D&V, chest infections, etc i.e.
prolonged fasting when present with lethargy and decrease
conscious level or seizures
•Hypoglycaemia severe (→ zero) and no ketones in urine
•Neonatal screening now mandated UK-wide
•Untreated ongoing problems liver and brain damage,
coma, and sudden death.

60. Insulin
Triglycerides
Glucagon, Cortisol
Growth hormone, Adrenaline
Free fatty acids + Glycerol
Mitochondrial
b-oxidation
Acetyl CoA
Ketones
TCA cycle
(3-hydroxybutyrate, acetoacetate)
Ketone production pathways

61. MCAD deficiency
•Treatment in crisis: ABC in A&E
•Check BM for all unconsious children if low then
bloods hypoglycaemia screen
•IV dextrose
•Recovery time relatively high
•Slow recovery is partly due to accumulation of
toxic metabolites
•Dieticians: Emergency advice/packs

62. Classical Galactosaemia: Initially
•1 week old, F, term delivery
•Milk feeds established, poor feeder and
failing to thrive
•Vomiting, diarrhoea, jaundice, hepatomegaly
•LFT’s:Bilirubin – 371
Conj Bili – 136 (ie around 30%)
ALT – 199
Alk Phos – 2293
•Cataracts
•Deranged clotting

63. Metabolic Investigations
Prolonged jaundice screen
- Obstructive jaundice – remember Gal-1-PUTas well
as biliary atresia
Metabolic investigations
•Urine Organic acids: NAD
•Urine amino acids: generalised amino aciduria
•Urine sugar chromatography: NAD
Galactosaemia screen: Absent activity

64. Galactosaemia Diagnosis: Pitfalls
• Galactosuria: False Neg & Pos results
• False Neg: If no galactose intake
• Galactosaemia blood test is red cell enzyme
• Invalid if child has been blood transfused

65. Galactosaemia management
•Primary source of galactose is lactose therefore:
–Stop breast feeding
–Involve a dietician
–Lactose free formula (or soy formula)
–Lactose free diet
•Good prognosis but even if well treated there are
long term complications
–short stature
–Female infertility

66. Nephropathic Cystinosis
Presents with:

Fanconi syndrome: generalised aminoaciduria
glycosuria
phosphaturia

Polyuria, Polydipsia

Failure to thrive

Renal failure

Rickets

Fair complexion

Photophobia – Cysteine crystals on slit lamp examination
 Incidence: ~ 1 in 200,000 live births

Defective lysosomal membrane transport protein for
cystine, cystinosin

67. Cystinosis: Diagnosis
Clinical - Opthalmology

Corneal cystine crystals on slit-lamp
examination of eye

Biochemical

White cell cystine (definitive)

CVS / amniotic fluid prenatal available

68. Cysteamine
Cysteine
Cystine
Cysteamine-cysteine
Lysine
TP
Cysteamine-cysteine
Cysteamine
cystinosin
Cystine
X
Treatment of cystinosis with cystagon

78. Adenine phosphoribosyltransferase
(APRT)- remove adenine from the polyam
ine pathway
In deficiency , extremely insoluble 2,8-
dihydroxyadenine formed by xanthine oxi
dase.

79. Hypoxanthine-guanine
phosphoribosyltransferase

80. Gross uric acid overproduction
results from the inability to recycle either h
ypoxanthine or guanine
Unutilized PP-ribose-P provides an
additional stimulus to de novo synthesis a
nd uric acid overproduction.

81. •In erythrocyte hemolysates and
culture fibroblasts.
•HGPRT -a gene on the long arm
of the x-chromosome at Xq26.
•The disease is transmitted as an
X-linked recessive trait.
•Lesch-Nyhan syndrome
•Allopurinal reduce uric acid.

82. PPRP synthetase overactive
•It requires Mg
2+
, is activated by
inorganic phosphate
•Feed back inhibition mainly by ADP
and GDP.
•Over activity- overproduction of
purines leading to excess uric acid

84. The importance of adenosine deaminase (ADA) for the catabolism of dA, but not A,
and the resultant accumulation of dATP when ADA is defective. A is normally salvag
ed by adenosine kinase (see K
m
values of A for ADA and the kinase, AK) and deficie
ncy of ADA is not significant in this situation
Adenine deaminase deficiency

85. Purine and pyrimidine degradation

86. PRPP synthesis

88. 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

91. Uric acid crystals

92. 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

93. UMP synthase (UMPS) complex, a bifunctional protein
comprising the enzymes orotic acid phosphoribosyltransferase
(OPRT) and orotidine-5'-monophosphate decarboxylase (ODC)
Catalyse the last two steps of the de novo pyrimidine synthesis
Deficiency causes orotic aciduria.

94. WHAT IS TYROSINEMIA?
Hereditary tyrosinemia is a genetic
inborn error of metabolism associated with
severe liver disease in infancy.
An autosomal recessive fashion
one person in 100 000 is affected with
tyrosinemia globally.

95. HOW IS TYROSINEMIA CAUSED?
Metabolism of tyrosine - primarily
in the liver.
Deficiency of the enzyme
fumarylacetoacetate hydrolase (FAH)
Leads to an accumulation of toxic
metabolic products
It results in progressive damage to the
liver and kidneys.

96. Homocystinuria

98. Phenylketonuria

101. PKU Brothers-MR with fair hair
and skin

102. MSUD

104. Albinism

106. Pyruvate kinase (PK) deficiency:
Next most common red cell enzymopathy after
G6PD deficiency, but is rare.
Autosomal recessive
Causes "congenital non-spherocytic
haemolytic anaemias" (CNSHA).
Inadequate ATP - premature red cell death.
Patients are anaemic or jaundiced in
childhood. Gallstones, splenomegaly
Skeletal deformities due to marrow expansion

107. Hereditary hemolytic anemia

108. Blood film: PK deficiency:
Characteristic "prickle cells" may be seen.

109. Case Description
A female baby - after an
uncomplicated pregnancy.
she became fussy and on
examination revealed an
enlarged liver.
Glycogen Storage Disease Type
IIIB

110. Glycogen Storage Disease
Type 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

111. 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

112. Galactosemia
an inherited disorder
lack of the enzyme (galactose-1-phosphate uridyl
transferase) which helps the body break down the g
alactose – builds up and becomes toxic.
Swollen and inflamed liver, kidney failure, stunted
physical and mental growth, and cataracts in the ey
es. If the condition is not treated there is a 70% cha
nce that the child could die.

116. Hunter syndrome with umblical
hernia

119. •The specific diagnosis is made on the
basis of the patient's age at onset, the
level of neurologic stunting, the amount of
corneal clouding, and other clinical
features.
•With the exception of Morquio syndrome,
all the mucopolysaccharidoses are
marked by excessive urinary excretion of
glycosaminoglycans; -
•Dermatan sulfate, keratan sulfate,
chondroitin sulfate, hyaluronan and
heparan sulfate

120. Testing for IEM

121. •Thank you