Transcript Inborn errors of Metabolism (IEM)

Inborn Errors of Metabolism
Dr Mick Henderson
Biochemical Genetics
Leeds Teaching Hospitals
What this seminar will cover
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Overview of principles in IEM
Neonatal screening
Approach to diagnosis
Investigating suspected cases
– Examples from disorders of intermediary
metabolism
It won’t and cant be a comprehensive review of all IEM
Inborn Errors of Metabolism
• Individually rare, collectively a
significant health problem
• Approx 1:1-2,000 live births
• 25-50 in Yorkshire p.a.
• Most present in childhood
Enzyme Deficiencies
z
a
b
c
c’ase
active
d
cofactor
c’ase
inactive
IEM are not just enzyme defects,
can also be transport proteins, receptors or structural components
Mechanisms of Disease
• accumulation of a toxin
• energy deficiency
• deficient production of essential
metabolite / structural
component
Investigation of IEM’s
• Population Screening
-Neonatal screening programme
• Individual Case Investigation
- based in clinical presentation
Neonatal screening in the UK
• Phenylketonuria
• Hypothyroidism
• Cystic fibrosis
• MCAD deficiency
• Sickle disease
Leeds now involved in pilot for expanded
screening, GA1, IVA, LCHADD, Hcys,
MSUD
www.expandedscreening.org
Neonatal screening card
Guthrie sample: Good sample
Front
Back
Guthrie sample: repeat needed
Front
Back
Leeds Neonatal Screening
Pathway
Approach to Diagnosis
Investigations likely to be directed by presentation
Acute vs Chronic
Acute: may be medical emergency, hypoglycaemia,
hyperammonaemia, metabolic acidosis
Chronic: more difficult, need to decide how many
investigations to persue
Specific clinical features that immediately suggest a
disorder or group e.g.
• Dislocated optic lenses
• Acute abdominal pain, red urine, hyponatraemia
Metabolic Investigations
• Most lab’s have a urine “metabolic screen”
• Common initial profile:
Urine; Organic acids
Amino acids
Sugar Chromatography
Oligosaccharides
Mucopolysaccharides
Blood; Amino acids
Acylcarnitines
Further tests
Mutation analysis
CF mutations
Enzyme assays
White cells
Biopsies
Fibroblast studies
N.B. Genetic testing is changing fast, increasing use of
NGS for arrays and whole exome sequencing
Examples from different IEM groups
Amino acid metabolism:
Urea cycle
disorders
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
Phenylketonuria
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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
Classical PKU
Phenylketones  
Phenylalanine  
Pre-block
metabolite
increases
Tetrahydrobiopterin
(reduced)
X
Phenylalanine
Hydroxylase
Dihydrobiopterin
reductase
Dihydrobiopterin
(Oxidised)
Tyrosine 
Post-block
metabolite
decreases
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
High Phenylalanine
Plasma Amino Acid Profile, PKU
High phe
Low tyrosine
Metabolism of homocysteine
tetrahydrofolate
cobalamin
methionine
Methyl donor
reactions
betaine
MTHF reductase
5-me tetrahydrofolate
homocysteine
pyridoxine
cystathionine synthase
cystathionine
homocystine
cysteine
Molecular Forms
COOH
Homocysteine
CHCH2 CH2SH
NH 2
COOH
COOH
CHCH2 CH2SSCH2CH2 CH
NH 2
NH 2
Homocystine
Natural History of Clasical
Homocystinuria
Mudd et al Am J Hum Genet 1985; 31: 1-31
• 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%
A tragic Case
Female, 21 years old
Pregnant
Normal delivery
Died 3 days post partum
Saggital vein thrombosis
Tests Performed
Urine
Positive cyanide-nitroprusside test
Chromatography: homocystine
Plasma total homocysteine:
152 mol/L (ref <18)
No comment on haematology report as to
significance
Follow Up
Aunt had similar event some years before
Siblings:
Total homocysteine (mol/L)
F, 20y:
F, 18y:
M,16y:
M, 7y:
3
268
263
4
Urea cycle & Hyperammonaemia
Hyperammonaemia variable depends on:
- nature of IEM
- nutrition, protein intake
- renal clearance of metabolites
- Lyonisation (for x-linked OTC)
Plasma Ammonia
• Lithium heparin
• Phone lab to request urgently
• Transport immediately to lab
Delays cause falsely high ammonia
• Avoid contamination: smoking
The Urea Cycle
NH3 + Bicarbonate
Converts highly
toxic ammonia to
less toxic urea
Carbamyl phosphate synthetase
Carbamyl Phosphate
Ornithine Transcarbamylase (OTC)
Citrulline
Ornithine
Urea
Argininosuccinate Synthase
Arginase
Arginine
Argininosuccinic acid
Argininosuccinate Lyase
Disorders of urea
cycle
Marked
hyperammonaemia
NH3 
Side pathway
utilised
Carbamyl phosphate synthetase
Carbamyl Phosphate
Orotic acid
Ornithine Transcarbamylase (OTC)
OTC deficiency
Urea 
Citrulline
Ornithine
Impaired urea
synthesis
Arginase
Arginine
Argininosuccinic acid
Argininosuccinate Lyase
Arginino succinic aciduria
OTC deficiency
NH3 
Side pathway
utilised
Marked hyperammonaemia
Carbamyl phosphate synthetase
Carbamyl Phosphate
Orotic acid
Ornithine Transcarbamylase (OTC)
Urea 
X
Ornithine
Citrulline
Impaired urea
synthesis
Argininosuccinate Synthase
Arginase
Arginine
Argininosuccinic acid
Argininosuccinate Lyase
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)
OTC presentation - infancy
– 12- 72 hours of age
– Lethargic and poor feeding
– Abnormal respirations
– vomiting
– Seizure
– Decreasing conscious level
– If untreated die
Differential diagnosis is sepsis / meningitis
– Initial investigations
• 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 –
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
Hyperammonaemia Treatment
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Protein restriction
Antibiotics
Benzoate
Phenylbutyrate /Phenylacetate
Arginine
Carbaglu
Dialysis
Alternative pathway treatment
• Benzoate
• phenylbutyrate
Benzoate Therapy
Benzoate
+
Glycine
Hippurate
Phenylbutyrate/acetate Therapy
Phenylbutyrate
Phenylbutyryl CoA
Phenylacetate
Glutamine
Phenylacetylglutamine
Urine
N-Carbamoyl-L-glutamic acid
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Carbaglu
Marketed by Orphan
N-acetylglutamate analogue
Stimulates Carbamyl phosphate synthetase
Particularly useful in NAGS def
Arginine supplementation
Arginine deficiency common in many urea
cycle defects
Neonates and young infants have high
requirement for arginine
Argininosuccinic aciduria
NH3 normal,  or 
Why can
ammonia be
normal ?
Carbamyl phosphate synthetase
Carbamyl Phosphate
Ornithine Transcarbamylase (OTC)
Urea 
Citrulline 
Ornithine
Argininosuccinate Synthase
Arginase
Argininosuccinic acid 
 Arginine
X
Argininosuccinate Lyase
Arginino succinic aciduria
NH3
Carbamyl phosphate synthetase
Carbamyl Phosphate
Ornithine Transcarbamylase (OTC)
Ornithine
Citrulline
Urea
Argininosuccinate Synthase
Arginase
Arginine
Urea cycle effectively becomes a
“linear pathway” provided arginine
intake is adequate
Argininosuccinic acid (ASA)
Renal clearance of ASA
is much higher than for
citrulline. NH3 is
excreted as ASA
Small MW organic acids are intermediates in most
metabolic pathways
amino acids
drugs, diet
cholesterol
neurotransmitters
Organic acids
purines
pyrimidines
carbohydrates
microorganisms
fatty acids
Clinical indications
Acute, intoxication
• Unexplained metabolic acidosis
• Hyperammonaemia
• Hypoglycaemia
• Lactic acidaemia
• ketonuria
Chronic
• Developmental delay
• Fits or seizures
• Liver disease
Branched chain amino acid catabolism
Leucine
2-Oxoisocaproic
Valine
Isoleucine
2-Oxoisvaleric
2-Oxo-3-methylvaleric
Isovaleryl-CoA
3-Methylcrotonyl-CoA
2-Methylbutyryl
2-Methylmalonic acid
semialdehyde
Triglyl-CoA
2-Methyl-3OHbutyryl-CoA
3-Methylglutaconyl-CoA
3-OH-3-MethylglutarylCoA
Propionyl CoA
2-Methylmalonyl-CoA
Succinyl CoA
Acetyl-CoA
Case history
• Male baby
• Hx of miscarriages
Parents First cousins
3 other children
• Admitted Local District Hospital @ 5days
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Feeding difficulty
Lethargy
?sepsis
?IEM
Case cont
Na
K
Urea
Creatinine
Bicarb
Glu
Day 7:
156
3.5
24.5
123
12
22
Ammonia 1850mol/L (ref<40)
Transferred to Leeds
Urine organic acids:
Gross inc. in isovalerylglycine and 3OHisovaleric acid
Diagnosis
Isovaleric acidaemia
Day 9
Died in hospital
Hyperammonaemia in organic acidaemias
propionyl CoA
methylmalonyl CoA
glutamate
ATP + CO2 + NH3
-ve
N-acetyl glutamate
+ acetyl CoA
+ve
CPS
synthetase
NAG synthetase
Carbamoyl phosphate
UREA CYCLE
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
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)
Methyl Malonic Acidaemia:
other treatments
• Protein restriction
• Carnitine supplements
• Antibiotics
• Management of CRF
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
MCAD; the biochemical defect
ketones
mitochondria
Acyl carnitines
acetyl CoA
fatty
acids
carnitine
fatty acid oxidation
dicarboxylic acids
TCA
cycle
URINE
Ketone production pathways
Triglycerides
Glucagon, Cortisol
Growth hormone, Adrenaline
Insulin
Free fatty acids + Glycerol
Mitochondrial
-oxidation
Acetyl CoA
TCA cycle
Ketones
(3-hydroxybutyrate, acetoacetate)
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.
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
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
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
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
Galactosaemia management
• Primary source of galactose is lactose therefore:
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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
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
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Incidence: ~ 1 in 200,000 live births
Defective lysosomal membrane transport protein for
cystine, cystinosin
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Cystinosis: Diagnosis
Clinical - Opthalmology
 Corneal cystine crystals on slit-lamp
examination of eye
Biochemical
 White cell cystine (definitive)
 CVS / amniotic fluid prenatal available
Lysosomal metabolism
Amino acids
Specific
TP’s
Amino acids
Hydrolysis
Proteins
Treatment of cystinosis with cystagon
Cystine
X
cystinosin
Cysteamine
Cystine
Cysteine
Cysteamine
Cysteamine-cysteine
Lysine
TP
Cysteamine-cysteine