L8- Tumour supressor genes

Question 1

what is a tumour suppressor gene

Answer 1

a gene that has inhibitory functions on cell growth, poliferation and survival. It maintains genomic integrity so protects against cancer.

Question 2

what mutations affect TSGs?

Answer 2

loss of function mutations leading to inactivation

usually recessive mutations - both alleles of the gene must be mutated to have an affect

Question 3

what is Knudson’s 2-hit hypothesis and what does it tell us about retinoblastoma

Answer 3

‘Retinoblastoma is a cancer caused by two mutational events’. i.e Both copies of the RB gene (both alleles) must be eliminated or inactivated to develop retinoblastoma.
The 2-hit hypothesis explains the relationship between hereditary and non-hereditary forms of retinoblastoma.

Question 4

what mutations specifically affect Rb

Answer 4

• nonsense (stop) = truncated protein,
• frameshift = change in reading frame
• splice site mutations = affects how exons are joined together = frameshifts= truncated protein.

> mutations distributed throughout gene (this is the opposite to oncogenes in which mutations occur at hotspots)

Question 5

what are the ‘second hit’ mutations/alterations in Rb that cause loss of the second allele

Answer 5

second hit, also called loss of heterozygosity (LOH)
- point mutations, although in second allele the chance is low
– Epigenetic gene silencing e.g. promoter methylation
– Other mechanisms leading to loss of second allele e.g. non-disjunction, recombination, chromosomal loss, deletion.

Question 6

describe how non-disjunction leads to LOH

Answer 6

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. Results in daughter cells with abnormal chromosome numbers (aneuploidy)

1. initial heterozygous Rb gene (chromatids), undergoes S phase replication to give one mutant Rb allele (chromosome)
2. non-disjunction at mitosis = uneven separation
3. natural subsequent loss of extra chromosome (purely by chance which chromosome is lost)
4. may = homozygous mutant alleles or retention of heterozygosity.

Question 7

what role does the Rb gene have and what is the consequence of the mutation

Answer 7

RB1 controls G1-S checkpoint by binding to E2F TF.
Normal: In RB1 hyperphosphorylated form it is bound to E2F, keeping it inactive.
Hyperphosphorylated RB1 = releases E2F –> goes on to express DNA synthesis genes, driving cell cycle.
Hyperphosphorylation of RB1 caused by mitogenic signalling (growth factors) which increase cyclin dependent kinase activity e.g. cyclin D/CDK4/6 or cyclin E/CDK2.
- Loss of Rb regulation leads to inappropriate S phase entry and uncontrolled poliferation.