Gene Mutation and level of protein

Question 1

What are the effects of mutation on protein function?

Answer 1

• loss of function
• gain of function
• acquisition of novel property
• expression at wrong time/wrong place

Question 2

What happens in ataxia telangiectasia?

Answer 2

-ATM gene is lost

Question 3

What is allelic heterogenity?

Answer 3

different mutations at the same locus lead to the same or very similar phenotypes. These allelic variations can arise as a result of natural selection processes, as a result of exogenous mutagens, genetic drift, or genetic migration.

Question 4

Why does loss of function occur?

Answer 4

gene deletion which leads to a reduction in gene dosage

Question 5

What can the severity of a disease due to a loss of function mutation be correlated to?

Answer 5

the amount of function lost

Question 6

What does a gain of function mutation result in?

Answer 6

• increase in abundance of protein

- increase in the ability to perform one or more functions

Question 7

How does a FCFR3 mutation cause dwarfism?

Answer 7

• Binding of FGF to extracellular domain activates intracellular kinase domain-initaites signal cascade
• In endochondral bone, FGFR3 activation inhibitors proliferation of chondrocytes-slowing down the formation of bone (helps to co-ordinate growth)
• FGFR3 mutation increases its activity (gain of function)
• Cause FGF independent activation of tyrosine kinase domain
• Causes inappropriate inhibition of chondrocyte proliferation (leads to shortening of bones = achondroplasia)

Question 8

Is there allelic heterogeneity in achondroplasia?

Answer 8

no there is only one position where an amino acid change in FGFR3 receptor can cause it

Question 9

Give an example of a novel property mutation

Answer 9

• Sickle cell anaemia (Glu6Val mutation in Beta-globin decreases solubility of deoxygenated Hb)
• causes it to form stiff fibrous palmers that distort RBC

Question 10

What type of diseases is acquisition of novel function common/uncommon?

Answer 10

• rare in inherited disease

- common in cancer

Question 11

What is a eugenic disorder?

Answer 11

when two genes affect the disorder (usually genes produce proteins that work together in the same process)

Question 12

What type of disorder ataxia telangiectasia?

Answer 12

-autosomal recessive disorder

Question 13

What happens in ataxia telangiectasia?

Answer 13

• progressive cerebellar degeneration (wheel chair by teen)
• speech difficulties
• abnormal eye movements
• immune deficiency
• large increased risk of malignant disease
• increased chromosome instability
• increased radio-sensitivity

Question 14

What is ocular telangiectasia?

Answer 14

enlarged blood vessels in the eye

Question 15

How can you test for it?

Answer 15

• use part of the blood sample to check various features of the disorder (e..g chromosome radio-sensitivity)
• use the remainder of the blood sample to make a lymphoblastic cell line (LCL) (can make cells from lymphocytes)

Question 16

What dies radio sensitivity indicate?

Answer 16

-an inability to repair DNA double strand breaks

Question 17

What is the function of the ATM protein?

Answer 17

to protect our cells against DNA/chromosome damage

Question 18

What can you do once the cell line is available?

Answer 18

-look at the level of the relevant protein

Question 19

What does a western blot tell you?

Answer 19

• whether there is any protein present and how much

- this can give us a clue about the type of mutations present

Question 20

What does western blot not tell you?

Answer 20

-if there is a protein, it doesn’t tell you whether it is still functional

Question 21

What type of disorder is ataxia telangiectasia, what types of alleles do patients have?

Answer 21

• recessive

- affected individuals are either homozygous for the ATM mutation or compound heterozygous (two different mutations)

Question 22

What happens in the ATM protein is totally absent?

Answer 22

• patient likely to have A-T
• suggest that the two mutations in the patient are null and probably both truncating mutations leading to instability (and loss) of the protein

Question 23

What happened if there are normal levels of ATM protein?

Answer 23

• patient may not have A-T (protein is normal ATM protein and present at normal levels)
• the patient may have A-T but expresses a normal level of the protein, but it is a mutant protein. This may happen due to 2 missense mutations (if the protein is mutant its function should be affected)
• The patient may have A-T but caused by a mutation in a different gene so ATM would be normal

Question 24

What is the protein level related to?

Answer 24

the type of mutation present

Question 25

How can you obtain the DNA sequence?

Answer 25

1. sequence the individual coding axons (need to know the exon/intron structure)
2. extract RNA from the cell and make a copy DNA (cDNA). This can then be sequenced (only get 1 sequence which could be different to the exon sequence)

Question 26

What does a private mutation mean?

Answer 26

each patient has a mutation which is unique to them

Question 27

Problem:
Father is not a carrier for A-T, mother is a carrier. Both children inherit the same chromosomes, why is one child affected and one not?

Answer 27

-mutation in paternal germ cell so affected child has mutations in both alleles