Metabolic Testing Wk1 Flashcards
What are inherited metabolic disorders?
Heterogeneous group of genetic conditions
Individually rare, collectively numerous
1 in 1000 live births
Ranges from neonatal lethal - benign
Diagnosed in infancy = life long treatment/monitoring
Once inherited metabolic disorders are identified genetically they can be tested for by…
Antenatal diagnosis
Genetic counselling required
Autosomal recessive mode of inheritance
One copy required from both unaffected parents
Autosomal dominant
One copy from parent
X-linked
Gene is located on X chromosome
Mitochondrial (maternal) inheritance
Males carry mothers mtDNA, but only females pass to children
Mitochondrial heteroplasmy
Mutations in enzymes affect metabolism by..
If pathway is blocked, substrates will accumulate = toxic/cause build up of toxic metabolites.
After blocked pathway compounds become deficient
Potential treatment for mutations/metabolism
Restriction of substrates/removal of metabolites
Improve/replace defective enzyme
Replace deficient compounds (after mutation line)
Strategies for IMD treatment
Treatment goal| Principle used |Example
Substrate reduction |Dietary restriction |Low Phe in PKU
ditto |Decrease flux upstream of defective enzyme
ditto |Removal of substrate |Dialysis
Correct product deficiency |Supply the product /substrate |Give glucose in glycogen storage disease
Decrease toxicity of metabolites |Remove toxic metabolites by conjugation|Sodium benzoate /phenylbutyrate in hyperammonaemia
Stimulate residual enzyme activity |Increase cofactor (vitamin), give drug |Vit B12 in methylmalonic aciduria
Enzyme replacement |Organ transplant, enzyme therapy, gene therapy|Liver/ bone marrow transplant
Laboratory testing for IMDs
Blood test
Glucose hypo/hyperglycaemia
Ammonia hyperammonaemia is a medical emergency
Acid-base status metabolic acidosis in cancer
Liver function clotting factor changes can indicate abnormal liver function
Techniques used lab testing IMDs
Basic biochemical stains, eg ninhydrin and TLC for amino acids
Fluorimetry
Chromatography (HPLC, GC, ion-exchange)
Mass spectroscopy
DNA sequencing
Who do u investigate? ££££
Family history = X-linkage inheritance
Clinical features
Developmental regression evidence
Age of onset
Symptoms =/ feeding
Classes of IMD
• Amino acid metabolism
• Urea cycle
• Organic acid metabolism
• Fatty acid metabolism
• Carbohydrate metabolism
• Glycogen storage
• Gluconeogenesis
• Lysosome function
• Peroxisome function
• Respiratory chain
• Mitochondrial DNA
abnormalities
• Cholesterol biosynthesis and
bile acid synthesis
• Haeme metabolism
(Porphyrias)
• Creatine metabolism
• Glycosylation
• Neurotransmitter
• Purine and pyrimidine
Screening priorities
The condition
The test
The treatment
The program
English newborn screening program
Heel prick test, 5-8 days old
Phenylketonuria (PKU)
Amino acid disorder
Recognise PKU early = important
Increased phenylalanine conc = developmental delay
In neonates conc rises quickly after milk
Treatment : low protein + Phenylalanine-free amino acid supplement
1-3% of Phenylketonuria (PKU) dont respond to low Phe diet
Because they have deficiency in biopterin metabolism
Cofactor for Phe hydrolyse
BH2 needs to be reduced back to BH4 by dihydrobiopterin reductase
Fatty acid oxidation disorders
MCAD deficiency
Hypoketotic hypoglycaemia
Test : octanoyl carnitine
Treatment: avoid fasting,
Antenatal testing
Metabolic testing biomarkers
Biological molecule whose conc changes in response to a specific disease
E.g. enzyme
Cell signalling molecule
Proteins
Molecule of intermediate metabolism
Good biomarker?
Not present in blood of healthy ppl
Specific to particular tissue
Structurally/functionally stable
Easy to assay
Changes in con related to disease
Enzymes as biomarkers
Measure activity using spectrophometer + colorimetric assay
Product/substrate coloured
Diff to measure enzyme levels as a protein
