Inborn errors of metabolism

Question 1

How many live births are affected by inborn errors of metabolism?

Answer 1

1:1-2000 live births

Question 2

Inborn errors of metabolism are caused by genetic defects that code for what 3 types of protein (in general)?

Answer 2

1) transport proteins
2) structural proteins
3) enzymes

Question 3

Why, in inborn errors of metabolism, do you tend not to have complete blockage of a metabollic pathway?

Answer 3

A full blockage in a pathway is unlikely to be compatible with life
Inborn errors of metabolism tend to be caused by reduced function of an enzyme (/structural/transport protein) thus you have a range of severity in each disorder

Question 4

What kind of genetic defect tends to occur in inborn errors of metabolism?

Answer 4

A mutation which leads to over expression or under expression of a gene, doing something too much or too little is what causes the problems

Question 5

What is biochemical genetics?

Answer 5

The study/investigation of (primarily) genetic disorders that affect the metabolic pathways of cells

Question 6

Why do enzymes catalysing reactions give you a degree of control of the rate of reaction?

Answer 6

Can be controlled by over or under expression of genes

Question 7

In a metabolic pathway involving multiple enzymes, what are the 3 possible damaging consequences of a malfunctioning enzyme towards the end of the pathway?

Answer 7

1) Dont produce enough of the end-point product
2) Intermediates produced prior to that enzyme can accumulate and can themselves be toxic
3) Intermediates that build up prior to that enzyme could be converted to something else which is a toxic substance

Question 8

How can you work out which enzyme is malfunctioning in a metabolic pathway?

Answer 8

By measuring levels of intermediates and end point products to work out where the blockage occurs

Question 9

How can vitamin and nutrition deficiencies sometimes mimic inborn errors of metabolism symptoms?

Answer 9

Many enzymes require a co-factor to carry out their function, these are often vitamins, they can give you a biochemical and clinical phenotype similar to the genetic phenotype so you need to work out if there is an actual problem

Question 10

How can vitamins be useful in treating inborn errors of metabolism?

Answer 10

There is usually some residual enzyme activity there, you can give a vitamin supplement thus providing excess of the cofactor and maximise what is actually there, - this can decrease the symptoms

Question 11

Give the 3 broad mechanisms of disease of inborn errors of metabolism?

Answer 11

1) Accumulation of a toxin
2) Energy deficiency
3) Deficient production of essential metabolite / structural component

Question 12

What substance accumulates in patients with urea cycle defects?

Answer 12

Ammonia

Question 13

Give the 7 clinical effects of acute hyperammonaemia toxicity?

Answer 13

1) Lethargy
2) Poor feeding
3) Vomiting
4) Tachypnoea
5) Convulsions
6) Coma
7) Death

Question 14

What is porphyria?

Answer 14

Accumulation of porphyrins in the haem synthesis pathway

Question 15

What is the haem synthesis pathway?

Answer 15

ALA - BPG - Uroporphrin - Corpoporphrin - Protoporphrin - Haem

Question 16

Where do defects occur in the haem synthesis pathway in acute porphyria (and what are the 13 possible signs)?

Answer 16

In the first 2 steps

1) Severe abdo pain
2) Pain in the chest, legs, or back
3) Constipation or diarrhoea
4) Vomiting
5) Insomnia
6) Palpitations
7) Hypertension
8) Anxiety or restlessness
9) Seizures
10) Mental changes
11) Breathing problems
12) Muscle pain/tingling/paralysis
13) Red or brown urine

Question 17

Where do the defects in the haem synthesis pathway occur in photosensitive porphyria (and what are the 7 possible signs)?

Answer 17

In the last 3 steps

1) Sensitivity to sunlight or artificial light
2) Sudden painful erythema and oedema
3) Blisters that take weeks to heal
4) Itching
5) Fragile skin
6) Increased hair growth
7) Red or brown urine

Question 18

In what disorders of metabolism does energy deficiency cause crisis presentations?

Answer 18

Defects of fatty oxidation

Pt can be very well and then be hit by infection and need to use their fats and present in a crisis of energy deficiency

Question 19

Androgen insensitivity syndrome is caused by an enzyme defect leading to what?

Answer 19

Defective receptors

Question 20

What is androgen insensitivity syndrome, how does it present and what surgery is usually required?

Answer 20

Genetic males with a healthy female phenotype, normal breast development, absent pubic hair. Partial defect results in ambiguous genitalia.
Presentation - primary amenorrhoea, infertility
Surgery - usually requires surgical resection of residual gonads

Question 21

Why is their much clinical heterogeneity with inborn errors of metabolism? 3

Answer 21

1) genetic variability of lesions, most disorders are multi-alleic
2) Variability of other aspects/ components of metabolism
3) Environment
Often genotype phenotype correlation is poor

Question 22

In which 2 broad ways are IEM diagnosed?

Answer 22

1) Pre-symptomatic screening

2) Investigation of symptomatic individuals

Question 23

Is everyone in the population screened for IEM?

Answer 23

Some IEM are screened for in the whole population as part of routine screening
Some IEM are only screened for in selected groups

Question 24

What 4 things can investigation of symptomatic individuals include in diagnosing IEM?

Answer 24

1) testing body fluids for abnormal metabolites
2) measuring enzyme activities
3) histochemical/ immunochemical staining
4) DNA analysis

Question 25

IEM are usually bottom of the list for diagnosis give a key trigger about a babies progression which may indicate a diagnosis of IEM?

Answer 25

A baby who was well at birth and suddenly had problems (as they had become completely reliant on their own metabolism rather than their mothers)

Question 26

In basic urine metabolic screening in Leeds which 6 things are carried out?

Answer 26

1) Spot tests
2) Organic acids
3) Amino acids
4) Sugar chromatography
5) Oligosaccharides/Sialic Acids
6) Mucopolysaccharides

Question 27

How does amino acid TLC (test of the urine work)?

Answer 27

Basic test with chromatography and different amino acids run to different points

Question 28

Instead of urine amino acid chromatography what more precise technique can be carried out?

Answer 28

Automated amino acid analysis

Gives you quantitative data

Question 29

What is homocysteinuria?

Answer 29

Disorder of methionine metabolism leading to an accumulation of homocysteine and its metabolites in the blood and urine

Question 30

What are the 5 clinical problems of homocysteinuria?

Answer 30

1) Mental retardation
2) Marfinoid habitus
3) Ectopia lentis
4) Osteroporosis
5) Thromboembolism

Question 31

What percentage of the population is hyperhomocysteinaemia present in?

Answer 31

5% of the general population

Question 32

Which set of diseases has hyperhomocysteinaemia been linked to?

Answer 32

Vascular diseases (stroke, peripheral vascular disease and coronary artery disease)

Question 33

Why are SW organic acids important in metabolism?

Answer 33

They are intermediates in most metabolic pathways

Question 34

Classic organic acidaemias are what kind of defects?

Answer 34

Defects in branched chain amino acid catabolism

Question 35

Name 3 classic organic acidaemias?

Answer 35

1) Propionic acidaemia
2) Isovaleric acidaemia
3) Methyl malonic acidaemia

Question 36

What are the 4 main benefits to diagnosing IEMs?

Answer 36

1) Can then give treatment to improve diagnosis
2) Enables you to identify the cause of a clinical problem
3) Allows you to carry out genetic counselling
4) IEM act as models for other disorders

Question 37

How are neural tube defects screened for prenatally?

Answer 37

Maternal serum and amniotic fluid AFT

US scan at 16 weeks

Question 38

How is Down’s Syndrome screened for prenatally?

Answer 38

1) 1st trimester: PAPA, HCG and nuchal translucency
2) 2nd trimester: maternal serum AFP, HCG, inhibin and estriol
3) Test on the ascent: free fetal DNA