Introduction to tumour suppressor genes

Question 1

Define tumour suppressor genes (TSG)

Answer 1

TSG encodes proteins that maintain the checkpoints and control genome stability. inhibit replication and proliferation of damaged cells by :

• repair of DNA damage (BRCA1/2)
• apoptosis (TP53)

Question 2

What was the first TSG discovered

Answer 2

• RB1 - 1986 (retinoblastoma)

Question 3

What is the two-hit hypothesis?

Answer 3

• development of retinoblastoma requires two mutations, which are now known to correspond to the loss of both of the functional copies of the tumour susceptibility gene

Question 4

Explain TSG inactivation

Answer 4

• most loss of function mutations that occur in TSG re recessive in nature - one normal allele is sufficient for the cellular control

Question 5

Describe loss of heterozygosity

Answer 5

• additional loss of the normal functional allele
• ‘second hit’
• allele imbalance by which are heterozygous somatic cell becomes homozygous bc one of the 2 alleles is lost
• this form of chromosome instability is sufficient to provide selective growth advantage and has been recognised as a major cause of tumourigenesis

Question 6

What are the functions of TSG?

Answer 6

• oncogene antagonists
• block proliferation (cell cycle inhibitors)
• DNA repair
• induce apoptosis

Question 7

DNA repair genes

Answer 7

• endode DNA repair enzymes
• continuously monitoring the screening chromosomes and DNA to detect damaged nucleotides
• damage is often consequence of environmental agents
• DNA repair enzymes detect and repair errors - can be ds breaks - caused by ionising radiation
• single strand breaks caused. by exposure to x-rays
• ds breaks are repaired by homologous recombination or non-homologous end joining
• single strand breaks are repaired by base excision repair (BER) systems

Question 8

What happens if there are defects in DNA repair genes?

Answer 8

• knockout of the DNA repair function of one or more DNA repair gene leads to sequential acquisition of more mutations
• defects in DNA repair genes cause genomic instability and accelerate the activation of oncogenes and the loss of tumour suppressors
• tumour arising in patients as a result of inherited defects in DNA repair genes tend to have a high mutational load.

Question 9

Describe BRCA1 + 2

Answer 9

• repair DNA double strand breaks by homologous recombination

Question 10

Describe PARP

Answer 10

• poly-ADP ribose polymerase

- proteins in charge of fixing single strand breaks (BER)

Question 11

Define synthetic lethality

Answer 11

• blocking the function of PARP proteins which leads to BRCA mutated cell death

Question 12

Why are PARP inhibitors used as targeted therapy?

Answer 12

• function to treat cancer carrying BRCA1/2 mutations
• block catalytic action of single strand break repair function of PARP1 - leads to accumulation of ss breaks in those cells = accumulation of ds breaks
• BRCA1/2 can’t repair ds breaks as mutated = death of BRCA1/2 cells

Question 13

List some PARP inhibitors

Answer 13

• olaparib
• rucaparib
• nireaparib
• talazoparib

Question 14

Describe TP53

Answer 14

• TP53 = gene which makes p53 = protein
• detects cellular stress, esp. DNA damage
• induced G2 cell cycle arrest
• if failure to repair damage induced apoptosis
• TF activator

Question 15

How does p53 act in the cells

Answer 15

• high levels of are due to inactive p53 protein
• normal p53 regulated by negative feedback
• high levels of p53 - accelerated ageing
• main regulator is MDM2 - triggers degradation of p53 through ubiquitnation - occurs when p53 is not phosphorylated
• phosphorylation disrupts p53-MDM2 complex
• p53 becomes active and translocate to nucleus and bind to DNA - activating transcription of different genes
• p53 self regulates own levels

Question 16

Why is p53 important?

Answer 16

• over 50% of cancers contain mutations in the TP53 gene
• most commonly affected TSG in cancer
• missense mutations in hotspots

Question 17

Describe the function of the mutant p53

Answer 17

• mutant p53 suppresses the ability of wild type p53 copy in inducing cell cycle arrest
• mutations exert dominant activity over recessive
• one mutation is sufficient to lose normal function

Question 18

What are current advances in treatment?

Answer 18

• small molecules (MIRA-1, PRIMA-1) - can restore wild-type p53 functions

Question 19

Describe RB1

Answer 19

• Gatekeeper - encodes RB protein
• prevents cell growth by inhibiting cell cycle until cell is ready to divide
• phosphorylation = inactivation

Question 20

What is the effect of Rb phosphorylation?

Answer 20

• phosphorylation permits cell preparation through s phase because Rb cannot repress E2F1
• E2F1 TF which activates genes involved in cell cycle progression

Question 21

Describe the stats of RB1

Answer 21

• 1 in 20,000 children
• 90% present before 5 years of age
• treatment - surgery and radiotherapy
• 98% cases cured

Question 22

What are the two types of retinoblastoma?

Answer 22

sporadic
- 60% of cases
- no family cases
- single tumour 
- unilateral
familial
- 30% of cases
- family history
- multiple tumours
- bilateral

Question 23

Describe second hits in retinoblastoma

Answer 23

1. mutation or gene deletion
2. loss of heterozygosity - by somatic recombination, gene conversion, chromosome disjunction
3. promoter hypermethylation