Introduction to Tumour Suppressor genes

Question 1

What do tumour suppressor genes (TSGs) do?

Answer 1

• Encode proteins that maintain checkpoints in the cell cycle and to control genome stability
• Inhibit replication and proliferation of damaged cells

Question 2

How do TSGs carry out their function

Answer 2

• DNA damage repair
• Apoptosis

Question 3

Which TSGs carry out DNA damage repair?

Answer 3

• MLH1
• BRCA1/2

Question 4

Which TSG carries out apoptosis?

Answer 4

TP53

Question 5

When and who discovered TSGs?

Answer 5

Henry Harris in 1969 through somatic cell hybridisation

Question 6

When was the first TSG identified?

Answer 6

• In 1986, the RB1 gene was identified
• It causes retinoblastoma and it is a dominant gene

Question 7

What is the Knudson’s two-hit hypothesis in TSG inactivation?

Answer 7

• Discovered by Alfred Knudson in 1971
• Most loss-of-function mutations that occur in TSGs are recessive
• This is because typically, one normal allele is sufficient for cellular control
• This means that both copies on the homologous TSG need to be mutated in order for cancer to occur

Question 8

How does TSG inactivation occur for heritable genes?

Answer 8

• In a normal cell, there needs to be 2 mutations to lead to cancer
• In a germline cell, there is already 1 mutation which is inherited, therefore only 1 more mutation needs to occur to lead to cancer

Question 9

What are mutations in both alleles called?

Answer 9

Loss of heterozygosity

Question 10

What are 4 functions of TSGs?

Answer 10

1. Act as oncogene antagonists
2. Block cell proliferation (through cell cycle inhibitors and activation/repression of transcription factors
3. Induce apoptosis
4. DNA repair

Question 11

What do DNA repair genes do?

Answer 11

• Encode DNA repair enzymes which continuously monitor chromosomes to detect any damage caused by carcinogens or errors in DNA replication

Question 12

What is an example of damage that DNA repair enzymes fix?

Answer 12

• Double strand breaks caused by ionising radiation
• Repaired by homologous recombination

Question 13

What is the effect if the DNA repair gene loses function?

Answer 13

• Loss of gene results in acquisition of more mutations
• Defect of gene results in genomic instability and the acceleration in activation of oncogenes and loss of TSGs
• Tumours arising in patients as a result of inherited defects in DNA repair genes tend to have a very high mutational load

Question 14

What are the BRCA1/2 genes?

Answer 14

• DNA repair genes that encode proteins that repair DNA double strand breaks through homologous recombination

Question 15

Which cancers do BRCA1/2 increase the risk of?

Answer 15

1. Female/Male breast cancer
2. Ovarian cancer
3. Melanoma
4. Prostate cancer
5. Pancreatic cancer

Question 16

How are single strand breaks repaired by BRCA1/2?

Answer 16

They undergo base excision repair (BER) using the enzyme poly ADP-ribose polymerase (PARP)

Question 17

What is synthetic lethality?

Answer 17

• Occurs when the inhibition of two genes is lethal while the inhibition of each single gene is no
• Can be harnessed to selectively treat cancer by identifying inactive genes in a given cancer and targeting their synthetic lethal partners.

Question 18

How does synthetic lethality work to treat BRCA1/2 mutations in cancer?

Answer 18

• PARP inhibitors are used to inhibit PARP1 which results in an accumulation of single strand breaks
• This accumulation also leads to an accumulation in double strand breaks
• This draws the BRCA mutated cells to the site
• However, because they are mutated they’re unable to repair so the cell dies
• This stops cell growth and therefore tumour growth

Question 19

What are 4 examples of PARP inhibitors?

Answer 19

1. Olaparib
2. Rucaparib
3. Niraparib
4. Talazoparib

Question 20

What does the TP53 gene do?

Answer 20

Produces p53 protein

Question 21

What does p53 protein do?

Answer 21

• Acts as tumour suppressor to regulate cell division so there is no unregulated growth
• Located in cytoplasm and nucleus and binds to DNA
• Detects cellular stress and DNA damage and either signals for repair or apoptosis depending on the state of the cell

Question 22

Why is p53 referred to as ‘Guardian of the Genome’

Answer 22

It regulates cell division so no uncontrolled cell growth occurs therefore no tumours form

Question 23

Why are the levels of p53 low in normal cells?

Answer 23

The levels are low so they can be tightly regulated, if they were high, it is possible that they could work too much which would lead to excess apoptosis which could result in tissue damage

Question 24

How is p53 regulated?

Answer 24

• It is regulated by MDM2 gene which produces MDM2 protein and binds to p53
• This forms a complex which targets p53 to destruction in lysosomes
• This allows p53 to work via negative feedback

Question 25

Why is the TP53 gene mutation important?

Answer 25

• Over 50% of cancers contain mutations in TP53
• Most commonly affected TSG in human cancer
• In some cancers, TP53 mutations correlate with the pathological stage of cancer

Question 26

Why is the TP53 mutation different to other TSG mutations?

Answer 26

• In most TSG mutations, both copies of the gene need to be mutated to result in loss of function (recessive)
• However, in TP53, the mutation of only one copy can still have effects on the gene by reducing its function (dominant negative)

Question 27

What does the RB1 gene do?

Answer 27

Encodes retinoblastoma (Rb) protein

Question 28

What does Rb protein do?

Answer 28

• Prevents cell growth by inhibiting cell cycle until the cell is ready to divide
• It is phosphorylated to allow inactivation of E2F1 to prevent excess cell growth

Question 29

What happens if RB1 is mutated?

Answer 29

Rb protein loses its regulatory functions

Question 30

What is retinoblastoma?

Answer 30

• Childhood eye cancer
• Affects 1 in 20,000 children
• 80 cases/year
• 90% of cases develop before 5yrs old
• Treated with surgery/radiotherapy
• 98% of cases are cured

Question 31

What are the two types of retinoblastoma?

Answer 31

Sporadic and Familial

Question 32

What is sporadic retinoblastoma?

Answer 32

• Approx 60% of Rb cases
• No family history
• Single tumour
• Unilateral

Question 33

What is familial retinoblastoma?

Answer 33

• Approx 30% of Rb cases
• Family history
• Multiple tumours
• Bilateral
• 500x increased risk of subsequent osterosarcoma/other tumours

Question 34

What are 3 ways that the RB1 gene can be mutated?

Answer 34

1. Mutation or gene deletion
2. Loss of heterozygosity (part of the chromosome containing the RB1 sequence)
3. Promoter hypermethylation resulting in inactivation of transcription of RB1