L14 - Inborn Errors of Metabolism Flashcards
Which substances often act as cofactors?
Vitamins (B12m B6)
trace elements (Mg Zinc)
Why can deficiency of vitamins and trace elements cause problems?
Vitamins and trace elements are often cofactors - lack of cofactors can lead to lack of activation of important enzymes
Accumulation of a toxin can be a mechanism of disease. What accumulates in patients with urea cycle defects?
Ammonia (NH3) - very toxic
What are the clinical effects of acute hyperammonaemia toxicity?
- lethargy
- poor feeding
- vomiting
- tachypnoea (metabolic alkalosis)
- convulsions
- coma
- death
What is porphyria?
Disease in which there is abnormal metabolism of haemoglobin
Get accumulation of porphyrins
What are the 2 types of porphyria?
- acute porphyria
- photosensitive porphyria
What are the steps in the pathway between ALA and HAEM?
ALA
PBG
Uroporphyrin
Coproporphyrin
Protoporphyrin
Haem
What are the signs of acute porphyria?
- severe abdominal pain
- pain in chest, legs, back
- constipation or diarrhoea
- vomiting
- insomnia
- palpitations
- hypertension
- anxiety or restlessness
- seizures
- mental changes
- breathing problem
- muscle pain/tingling/weakness/paralysis
- red or brown urine
What are the signs of photosensitive porphyria?
- sensitivity to light
- sudden painful erythema and oedema
- blisters taking weeks to heal
- itching
- fragile skin
- increased hair growth
- red or brown urine
Fatty acid oxidation results in what product?
Acetyl CoA
Goes on to make ketones or enter the TCA cycle
Fatty acid oxidation occurs in which organelle?
Mitochondria
What is androgen insensitivity syndrome?
AIS is when a person who is genetically male (who has one X and one Y chromosome) is resistant to male hormones (androgens) - and so the person has some physical traits of a female, but the genetics a male.
What does the basic urine metabolism screen consist of?
- spot tests
- organic acids
- amino acids
- sugar chromatography
- oligosaccharides/sialic acids
- mucopolysaccharides
How can amino acids be tested?
By amino acid TLC (thin layer chromatography)
- can identity argininosuccinic acid which is a product of urea cycle defects
What is homocystinuria?
A disorder of methionine metabolism, leading to an abnormal accumulation of homocysteine and its metabolites in urine
What are the clinical problems of homocystinuria?
- mental retardation
- marfinoid habitus
- ectopia lentis
- osteoporosis
- thromboembolism
What is hyperhomocystinaemia?
Medical condition characterised by an abnormally high level of homocysteine in the blood, conventionally described as above 15 µmol/L
What percentage of the general population have hyperhomocystinaemia?
5%
There is increased incidence of hyperhomocystinaemia in patients with which diseases?
- stroke
- peripheral vascular disease
- coronary artery disease
What are the 3 classic organic acidaemias?
- propionic acidaemia
- isovaleirc acidaemia
- methyl malonic acidameia
What are the benefits of diagnosis of IEMs?
- treatment, improve prognosis
- identify cause of clinical problem
- genetic counselling
- IEM act as models for other disorders
How are neural tube defects diagnosed?
- maternal serum and amniotic fluid AFP
- ultrasound scan at 16 weeks
How is down syndrome diagnosed?
1st trimester - PAPA, HCG and nuchal translucency
2nd trimester - maternal serum AFP, HCG, inhibin and estriol
Test on the ascent: free foetal DNA
Enzyme deficiencies cause problems clinically due to what 3 outcomes?
- accumulation of metabolites prior the enzyme
- reduction in new products of the enzyme
- spontaneous production of new compounds due to inc accumulation (can be toxic)
how is co-factor deficiency treated?
cofactor supplementation
what is a common urea cycle defect?
OTC
Ornithine transcarbamylase deficiency
X-linked
how is an acute Ornithine transcarbamylase deficiency patient treated?
- reduced protein intake
- not fasting for too long
Low protein diet; dialysis; liver transplant
what is the porphyrins pathway?
biosynthetic pathway to make haem
what happens if there is a complete block of the porphyrins pathway?
Complete block of this pathway is incompatible with life (no Hb for oxygen delivery)
can’t sustain life
what happens if there is a partial block at the top of the porphyrins pathway?
accumulation of ALA and PBG
presenting acute porphyria.
what happens if there is a partial block at the bottom of the porphyrins pathway?
accumulation of porphyrin
molecules, presenting photosensitive porphyria.
how can an enzyme deficiency cause an energy deficiency?
if an enzyme is missing from:
- oxidative phosphorylation
- glycolysis
- fatty acid oxidation
- TCA cycle
what does a partial defect in androgen insensitivity syndrome
result in?
ambiguous genitalia
what is the presentation of androgen insensitivity syndrome?
primary amenorrhoea, infertility [only resent in puberty]
how is androgen insensitivity syndrome treated?
surgical resection of residual gonads
how is androgen insensitivity syndrome treated?
surgical resection of residual gonads
how are inborn errors of metabolism diagnosed\/
- pre-symptomatic screening
- investigation of symptomatic individuals
how do you investigate a symptomatic individual for inborn errors of metabolism?
● test body fluids for abnormal metabolites
● measure enzyme activities [more demanding and time consuming]
● histochemical / immunochemical staining
● DNA analysis
