Transcript Inborn errors of Metabolism (IEM)
Inborn Errors of Metabolism
Dr Mick Henderson
Biochemical Genetics Leeds Teaching Hospitals
What this seminar will cover
• 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
cofactor
c c’ase active c’ase inactive IEM are not just enzyme defects, can also be transport proteins, receptors or structural components
d
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
Front
Guthrie sample: repeat needed
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:
phenylketonuria (PKU) maple Syrup Urine Disease (MSUD) homocystinuria
Urea cycle disorders
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
• 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
Phenylketones
Classical PKU
Dihydrobiopterin reductase
Phenylalanine
Pre-block metabolite increases
Tetrahydrobiopterin (reduced)
Phenylalanine
X 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
Low tyrosine High phe
Metabolism of homocysteine
tetrahydrofolate methionine cobalamin
MTHF reductase
5-me tetrahydrofolate homocystine betaine Methyl donor reactions
homocysteine
pyridoxine
cystathionine synthase
cystathionine cysteine
Molecular Forms
COOH CHCH2 CH2SH NH 2 Homocysteine COOH COOH CHCH 2 CH 2 SSCH 2 CH 2 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 : F, 20y: F, 18y: M,16y: M, 7y:
Total homocysteine (
mol/L)
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
Converts highly toxic ammonia to less toxic urea
Ornithine NH 3 + Bicarbonate
Carbamyl phosphate synthetase
Carbamyl Phosphate
Ornithine Transcarbamylase (OTC)
Citrulline Urea
Argininosuccinate Synthase Arginase
Arginine Argininosuccinic acid
Argininosuccinate Lyase
Disorders of urea cycle
Side pathway utilised
NH 3
Marked hyperammonaemia
Carbamyl phosphate synthetase
Orotic acid Carbamyl Phosphate
Ornithine Transcarbamylase (OTC)
Citrulline OTC deficiency Urea
Impaired urea synthesis
Ornithine
Arginase
Argininosuccinic acid Arginine
Argininosuccinate Lyase
Arginino succinic aciduria
OTC deficiency
Side pathway utilised
Orotic acid
Marked hyperammonaemia
NH 3
Carbamyl phosphate synthetase
Urea
Impaired urea synthesis
Ornithine Carbamyl Phosphate
Ornithine Transcarbamylase (OTC)
X
Citrulline
Argininosuccinate Synthase Arginase
Argininosuccinic acid Arginine
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 • Protein restriction • Antibiotics • Benzoate • Phenylbutyrate /Phenylacetate • Arginine • Carbaglu • Dialysis
Alternative pathway treatment
• Benzoate • phenylbutyrate
Benzoate Benzoate Therapy + Glycine Hippurate
Phenylbutyrate/acetate Therapy Phenylbutyrate
Phenylbutyryl CoA
Phenylacetate Glutamine Phenylacetylglutamine Urine
N-Carbamoyl-L-glutamic acid • 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
Why can ammonia be normal ?
NH 3 normal,
or
Carbamyl phosphate synthetase
Urea
Ornithine Carbamyl Phosphate
Ornithine Transcarbamylase (OTC)
Citrulline
Argininosuccinate Synthase Arginase
Arginine Argininosuccinic acid
X
Argininosuccinate Lyase
Arginino succinic aciduria
Ornithine NH 3
Carbamyl phosphate synthetase
Carbamyl Phosphate
Ornithine Transcarbamylase (OTC)
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. NH 3 is excreted as ASA
Small MW organic acids are intermediates in most metabolic pathways amino acids neurotransmitters carbohydrates drugs, diet Organic acids micro organisms cholesterol purines pyrimidines 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 2-Oxoisvaleric Isoleucine 2-Oxo-3-methylvaleric Isovaleryl-CoA 3-Methylcrotonyl-CoA 3-Methylglutaconyl-CoA 3-OH-3-Methylglutaryl CoA 2-Methylmalonic acid semialdehyde Propionyl CoA 2-Methylmalonyl-CoA Succinyl CoA 2-Methylbutyryl Triglyl-CoA 2-Methyl-3OHbutyryl-CoA Acetyl-CoA
Case history
• Male baby • Hx of miscarriages Parents First cousins 3 other children • Admitted Local District Hospital @ 5days • Feeding difficulty • Lethargy • ?sepsis • ?IEM
Day 7: Na K Urea Creatinine Bicarb Glu
Case
cont
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 glutamate + acetyl CoA propionyl CoA methylmalonyl CoA -ve N-acetyl glutamate ATP + CO 2 + NH 3 +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… • Post-B12, MMA in urine… 4359 3332 5181 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 Acyl carnitines fatty acids carnitine ketones
mitochondria
acetyl CoA fatty acid oxidation TCA cycle dicarboxylic acids 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: – 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 Incidence: ~ 1 in 200,000 live births Defective lysosomal membrane transport protein for cystine,
cystinosin
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 Cysteamine Cystine Cysteamine Cysteine Cysteamine -cysteine
Lysine TP
Cysteamine -cysteine