12 - Tumour suppressor genes

Question 1

Proto-oncogene mutations

Answer 1

activate proteins (e.g. Ras) and are dominant
Ras permanently activated stimulates signalling cascade
Mutations - dominant - only one copy in diploid cells needed

Question 2

Tumour suppressor gene mutations

Answer 2

inactivate proteins and are recessive
Diploid cells - two copies of each chromosome –
One functional copy is enough to maintain tumour suppressor function
For cancer to develop both copies need to be mutated - cancer causing
mutations in tumour suppressors are, therefore, recessive

Question 3

Tumour suppressor genes

Answer 3

• Tumour-suppressor genes normally inhibit cell division
• Mutations in tumour suppressor genes reduce the inhibition of cell division – thereby stimulating cell proliferation
• One good (functional) copy of the gene is enough to inhibit cell division – so both copies must be mutated for cancer to result (recessive)

Question 4

Retinoblastoma protein (Rb)

Answer 4

Retinoblastoma - tumour of the retina
Two forms of the disease:
1. Familial (10%) - occurs in young children retinal tumours in both eyes
2. ‘Sporadic’ (90%) - occurs later in life - affects just one eye in most (2/3s) cases - leukocoria
The condition is RECESSIVE:
A. For cancer to arise both copies are inactivated by mutation (one is enough for normal function)
B. Only 1 in 106 cells becomes cancerous (recessive at cellular level) - average of 4 tumours per eye

Question 5

What is retinoblastoma caused by?

Answer 5

a mixture of inheritance, somatic mutation and mitotic recombinance
Frequency of retinoblastoma inheritance is too high for a rare, carcinogen-mediated mutation

Question 6

mitotic recombination in retinoblastoma

Answer 6

normal cell –1 good gene enough to inhibit cell division
Normal DNA duplication; results in duplication of normal and mutant choromosomes
Rare complication; simple exchange of genetic material between normal and mutant chromatids
Subsequent mitotic segregation of chromatids and cytokinesis - one cell homozygous (normal) and one cell homozygous (mutant)

Question 7

retinoblastoma protein inhibits the G1-S transition

Answer 7

pRb inhibits progression through the G1 checkpoint
pRb is bypassed allowing cells to replicate and divide
Inactivation of pRb by mutation leads to a loss of G1 checkpoint and uncontrolled cell division.
Cell division no longer requires stimulation by mitogens

Question 8

p53

Answer 8

• Called p53 because the size of the protein is 53 kDa
• Gene encoding p53 is mutated in a high percentage of cancers
• Main role - ‘to protect the cell against damaged DNA’
• Arrests the cell cycle giving DNA repair machinery more time to fix DNA before mitosis and passing on of the damaged DNA to daughter cells

Question 9

p53: Guardian of the genome

Answer 9

p53 is constitutively expressed but constantly degraded in absence of genetic damage
p53 is a transcriptional regulator of multiple proteins that halt the cell cycle while the genetic damage is repaired.
If DNA damage is too severe, cells can enter a permanently non-replicative state called senescence or undergo programmed cell death (apoptosis)

Question 10

p53 responds to DNA damage

Answer 10

1. DNA damaged in nucleus by, e.g. UV light
2. Signal is passed to p53 via a cascade of protein kinases each phosphorylating and activating each other
3. Activated p53 is a TRANSCRIPTION FACTOR - Triggers expression of proteins which inhibit cell cycle

Question 11

p53 (a transcription factor) alters gene expression

Answer 11

1. CDK inhibitor protein p21
2. DNA repair proteins are transcriptionally regulated by p53
3. genes involved in apoptosis

Question 12

1. CDK inhibitor protein p21

Answer 12

• p21 expression is up regulated by p53
• p21 is an inhibitor of Cyclin E-Cdk2
- Prevents G1 exit causing cell cycle to arrest
- Preventing duplication of damaged DNA and passing on of DNA to daughter cells

Question 13

2. DNA repair proteins are transcriptionally regulated by p53

Answer 13

• Expression up regulated by p53
• Allows cell to repair DNA during cell cycle arrest
• If DNA damage is unrepaired cells enter senescence or apoptosis. These pathways are regulated by p53

Question 14

3. Genes involved in apoptosis

Answer 14

• If the DNA is damaged so badly that it cannot be repaired p53 activates genes which cause the cell to commit suicide = APOPTOSIS

Question 15

apoptosis

Answer 15

‘A form of cell death in which a programmed sequence of events leads to the elimination of cells without releasing harmful substances into the surrounding area’
• Apoptosis is a specialised form of cell death that is regulated by activation of proteins called caspases.
• Apoptosis can be induced by factors within the cell - intrinsic apoptosis
• Apoptosis can be induced by specialised cells by activation of the FAS death receptor –extrinsic apoptosis

Question 16

apoptosis stages

Answer 16

1. Normal cell ‘realises’ that its DNA is beyond repair and so p53 activates apoptosis
2. Cell begins to shrink and invaginations form at the cell surface
3. Organelles become enclosed in vesicles. DNA in nucleus is dissolved by enzymes, cellular material is engulfed and degraded by phagocytes

Question 17

apoptosis vs necrosis

Answer 17

apoptosis differs from necrotic cell death (e.g. in wounds)
• The cell contents are not released, and digested cells are contained in liposomes that are engulfed by phagocytotic cells

Question 18

Regulation of apoptosis by p53

Answer 18

• p53 regulates intrinsic and extrinsic apoptosis.
• Intrinsic apoptosis i.e. signalling within the cell in response to p53 activation
– e.g. p53 enhances Bid synthesis - damages mitochondrial membranes
• Extrinsic apoptosis is mediated in response to stimuli outside of the cell
– e.g. p53 upregulates expression of death receptors on cell surface

Question 19

Intrinsic apoptosis regulated by p53

Answer 19

• When DNA is damaged p53 induces Bid expression
• Activated Bid induces Bax to form a pore in the mitochondrial membrane
• Release of cytochrome c activates caspases present in cytoplasm of the cell leading to cell death (apoptosis

Question 20

Regulation of Extrinsic apoptosis by p53

Answer 20

• Activation of p53 induces expression of Death receptors.
• Death receptors are activated by cells expressing Fas ligand e.g. Lymphocytes
• Fas ligand binding activates the Fas death receptor and recruits the Fas associated death domain (FADD)
• Formation of the DISC, activates caspases that regulate cellular disintegration - apoptosis

Question 21

Summary of tumour suppressor p53

Answer 21

p53 is a transcription factor that controls responses to DNA damage. It does this by:
• Upregulation of p21 expression– halts cell cycle by binding to Cdks allowing time for cell to repair damage
• Upregulation of genes that DNA repair mutations in DNA
• p53 activates genes that promote apoptosis if DNA can not be repaired
• Mutations in the p53 gene can cause cancer by allowing cells with damaged DNA to survive
• Accumulation of mutations drives cancer formation

Question 22

Example: colon cancer

Answer 22

• Loss of heterozygosity of a tumour suppressor gene (somatic mutation) = Small polyp forms (benign)
• Mutation in a proto-oncogene - increased cell proliferation At this stage tumour in check - p53 still okay - can aid DNA repair - badly mutated cells - apoptosis = Intermediate polyp (benign)
• p53 tumour suppressor mutates
  
  • rapid accumulation of mutations - increased survival of mutated cells (reduced apoptosis) = Tumour now malignant - may metastasise