Lecture 20

Question 1

What are tumour suppressor genes?

Answer 1

Genes which decrease the rate/inhibit cell division

Question 2

How do TSG inhibit cell division?

Answer 2

• Inhibit cell division
• Promote cell death
• Negative regulation of proliferative signalling

Question 3

What type of mutations cause cancer and inhibit TSG

Answer 3

Point mutations
Chromosomal mutations
Genetically recessive

Question 4

What are retinoblastomas?

Answer 4

Most common eye tumour in children aged 0 to 4

Question 5

How are children screened for retinoblastomas?

Answer 5

• White light is shone in eye
  Red light reflect - Normal
  White light reflect - Retinoblastoma

Question 6

What percentage of children die from retinoblastomas

Answer 6

87%

Question 7

2 types of retinoblastomas

Answer 7

Sporadic (60%) - No family history, unilateral, single tumour in 1 eye
Hereditary - Bilateral, multiple tumours in both eyes

Question 8

Knudson’s 2 hit mutation hypothesis

Answer 8

• Retinoblastoma caused by 2 mutational events

One inherited by germinal cells

One occurs in somatic cells

Sporadic cases both occur in somatic cells

Question 9

Genetics of retinoblastoma

Answer 9

RB tumour suppressor gene
180kb gene
4.7kb mRNA
Familial retinoblastoma - Loss of wild type RB allele - Loss of heterozygosity

Question 10

What can lead to loss of heterozygosity?

Answer 10

Mitotic recombination

Question 11

pRB in G1 to S regulation

Answer 11

1. Progession through restiction point is pivotal in tumourigenesis
2. rb/rb cells - pRB often truncated and fails to bind to E2F
3. E2F free to activate genes for G1/S transition and S-phase
4. Loss of pRB causes sustained proliferative signalling

Question 12

p53 Tumour supressor gene

Answer 12

• Most commonly mutated gene in humans cancer - almost 50% of all sporadic mutations
• p53 protein - transcription factor named guardian of genome
• Li-Fraumeni syndrome associated with mutated p53 allele inheritance - 90% risk of cancer
• Tumours develop when p53 gene lost

Question 13

How does p53 accumulate in response to DNA damage?

Answer 13

1. MDM2-p53 disrupted by phosphorylation
2. Tetramerization of p53 blocks nuclear export, increasing nuclear concentration
3. Phosphorylated p53 interacts with histone acetylase p300
4. Acetylates histone and p53

Question 14

Major roles for p53

Answer 14

RB1 and accessory tumour suppressor proteins as brakes on cell divsion

Question 15

What genes does p53 activate?

Answer 15

• p21 - Inhibits CDKs which arrest cells at G1/S
• GADD45 - Binds to PCNA - Prevents PCNA becoming a processivity factor
• 14-3-3o - Binds and sequesters cdc25 phosphatase - Prevents removal of inhibitory phosphorylation on Y15 of Cdk1 - arrests cells at G2/M boundary
• Bax - Pro-apoptotic
• FAS cell surface death receptors - extrinsic apopotosis
• APAF1 - Activates caspases - Intrinsic apoptosis

Question 16

Downstream events trigged by p53

Answer 16

Loss of p53 causes:
- No activation of DNA damage checkpoint
- No cell cycle arrest
- Apoptosis inhibited

Question 17

What does increased genetic instability cause?

Answer 17

Increased risk of activated oncogenes or inhibition of TSGs

Question 18

Loss of p53 activity by inactivating mutation

Answer 18

Initiating mutation - may be caused by environment

Can cause skins and lung cancers e.g. basal cell carcinoma

Most p53 mutations recessive

Some have dominant negative effect - Can affect tetramer function

Question 19

Loss of p53 activity by degradation

Answer 19

~3000 women diagnosed annually with cervical cancer in UK
- Human papillomaviruses main cause of cervical cancer with type 16 and 18 - 70%
- HPV produces E6 and E7 oncoproteins
- E6 binds to p53 for targeted destruction
- E7 oncoprotein sequester Rb - degrades E2F activity

Question 20

Gatekeeper TSGs

Answer 20

• p53 mutation - Loss of G1/S, S and G2/M checkpoints
• p21 mutation - Loss of G1/S and S checkpoints
• RB mutation - Promotes proliferation, E2F uninhibited
• Bax mutation - No apoptosis

Question 21

Caretaker TSGs

Answer 21

Genes coding for protein mediating DNA repair

Loss of function leads to genome instability

Increases chances of loss of further TSGs and mutation of proto-oncogenes to oncogenes

Mutations cause familial cancer

Question 22

Lynch syndrome

Answer 22

• Autosomal dominant
• 80% chance of colon cancer
• Mutant allele of one of the mismatch genes MSH2, MLH1, PMS1, and PMS2
• Patients developing hereditary non-polyposis colon cancer (HNPCC) are heterozygous, tumours show LOH

Question 23

Xeroderma pigmentosum

Answer 23

• Autosomal recessive
• Skin cancer susceptibility
• Defective NER
• Failure to repair UV inflicted DNA damage

Question 24

Mutations in caretaker genes BRCA1 and BRCA2 linked to increased risk for breast and ovarian cancer

Answer 24

• 10% breast cancer associated with inherited mutation in BRCA1 or 2
• BCRA1 and BCRA2 large unrelated nuclear proteins
  -In normal cells BRCA1 and BRCA2 promote repair of DNA DSBs by high-fidelity homologous recombination - mediated by RAD51, human homologue of RecA
• Tumours show LOH