Cancer biology 2

Question 1

What is Ras?

Answer 1

-first proto-oncogene + oncogene discovered
-a protein that has enzymatic activity, but isn’t a classical enzyme

Question 2

What 2 states does Ras exist in and how are they characterised?

Answer 2

-on state = characterised by binding GTP
-off state = characterised by binding GDP

Question 3

How does Ras change from the off to on state?

Answer 3

Another protein (exchange protein) removes GDP, allowing GTP to bind

Question 4

How does Ras change from the on to off state?

Answer 4

Hydrolyses GTP slowly

Question 5

What are Glu 61 and Gly 12 in Ras close to, why is it critical and what happens if they become mutated?

Answer 5

-close to terminal phosphate
-critical for movement between its 2 forms
-mutations convert Ras from proto-oncogene to oncogene

Question 6

How does Ras turn on?

Answer 6

-growth factor binds to receptor tyrosine kinase in plasma membrane
-brings 2 subunits together, phosphorylating themselves inside the membrane
-allows assembly of Grb2 + Sos (Sos = exchanger protein)
-Sos binds to Ras binding domain on several proteins that switch on all the things proliferating cells do

Question 7

What happens if there’s a mutation in Ras?

Answer 7

-loses its GTPase activity
-remains in active state no matter if growth factor isn’t present

Question 8

Describe the experiment to determine if there are tumour suppressor genes

Answer 8

-fuse a cancer + normal cell
-hypothesis = normal cells express tumour suppressor genes that’re lost during oncogenesis
-result - get a normal mouse, so fusing cell results in no cancer
-suggests there’s something dominant in normal cells that suppresses cancer cells

Question 9

What is the argument for and against the existence of tumour suppressor genes?

Answer 9

-for = loss of growth suppressor gene more likely than gain of function oncogene mutations
-against = loss of both alleles of putative growth suppressor genes unlikely

Question 10

What does Knudsen’s one/two-hit hypothesis provide evidence for?

Answer 10

-tumour suppressor gene hypothesis
-that cancer requires loss of both wild-type alleles
-for the basis of inherited predisposition to cancer

Question 11

What is the difference in the effect of a mutation in an oncogene or tumour suppressor gene?

Answer 11

-oncogenes = activating, gain of function, dominant
-tumour suppressor gene = inactivating, loss of function, recessive

Question 12

What is the difference in the number of alleles mutated to exert an effect in an oncogene and tumour suppressor gene?

Answer 12

-oncogenes = 1
-tumour suppressor gene = 2

Question 13

What is the difference in the effect on function of protein produced in an oncogene or tumour suppressor gene?

Answer 13

-oncogene = enhances
-tumour suppressor gene = reduced

Question 14

Are cancer cells genetically stable or unstable?

Answer 14

Genetically unstable

Question 15

What are the causes of genetic instability in cancer cells?

Answer 15

Defects in:
-DNA repair pathways
-correction mechanisms for DNA replication errors
-correction mechanisms for DNA segregation errors

Question 16

What is P53 and where is it found?

Answer 16

-an example of a cell cycle checkpoint gene
-normal cells responds to cellular stresses through 1 signalling pathway = stable, active p53
-found at end of G1, in S phase, M phase

Question 17

What happens if p53 is mutated?

Answer 17

-mutated in almost all types of cancer
-mutations in p53 + associated pathways disrupt intrinsic apoptosis
-checkpoint loss contributes to genetic instability

Question 18

What is Rb and where does it operate?

Answer 18

-it’s a tumour suppressor gene
-cell cycle checkpoints are often disrupted in cancer
-it operates at the restriction point (R point)

Question 19

What does a mutated Rb cause?

Answer 19

-can lead to development of cancer
-its mutated in retinoblastoma