3.3. cell cycle: tumour suppressors and oncoproteins

Question 1

name two oncoproteins in the cell cycle

Answer 1

1. G1 cyclin

2. CDK

Question 2

what is the function of G1 cyclin in the cell cycle?

Answer 2

If gene is overexpressed or inappropriately expressed then G1 cyclin present when shouldn’t be. Increased cycling through the cell cycle without needing growth factor signalling

Question 3

what is the function of CDK?

Answer 3

If gene is mutated to produce a constitutively active or overexpressed CDK protein→ increased cell cycling.

Question 4

name two tumour suppressors which play an important role in regulating the cell cycle

Answer 4

1. RB (retinoblastoma)

2. p53

Question 5

describe the RETINOBLASTOMA Tumour Suppressor Gene

Answer 5

• The retinoblastoma gene was discovered in a rare childhood cancer affecting the nerve cells of the eye
• Inherited–a deletion or loss of function mutation in one copy of the RB1gene in germ line and then loss of heterozygosity in a somatic cell
• Noninherited/ sporadic –coincidence of two mutations in the same somatic cell
• The RB1gene is lost in other tumour types
• The RB1gene encodes a nuclear phosphoprotein RB which suppresses growth during the G1 phase of the cell cycle

Question 6

discuss the Function of Retinoblastoma protein

Answer 6

• The RB1 gene encodes a nuclear phosphoprotein RB which suppresses the cell cycle during the G1 phase.
• The RB protein, when hypophosphorylated binds to the transcription factor E2F.
• When CDK2 binds G1 cyclins and is activated, it phosphorylates RB resulting in a hyperphosphorylated form that no longer binds E2F
• The released E2F can then act as a transcription factor and results in increased expression of genes including the gene for S cyclins thus ensuring the progression of the cell cycle.
• If RB is lost or mutated so that it cannot bind E2F, the cell cycle would continue without needing the activation of CDK2.
• The RB1 gene is lost in retinoblastoma and other tumour types
• E2F gene has also been found amplified or mutated in cancers resulting in overexpressed E2F protein or one that can’t bind RB–increased cell cycling.

Question 7

Viral proteins and RB regulation

Answer 7

•Many DNA tumour viruses produce viral proteins which bind to RB, releasing E2F and stimulating S phase progression
–Human Herpes Virus 8 (HHV-8, Kaposi’s Sarcoma Herpes Virus)-LANA-1 protein
–HPV (human papilloma virus): E7 protein

Question 8

where is the TP53 gene found

Answer 8

on chromosome 17p

Question 9

The TP53gene

Answer 9

•There are a high proportion of missense mutations resulting in a protein that differs from wild-type by just one amino acid.
•The Li-Fraumenicancer predisposition syndrome is associated with TP53mutation. It is characterized by early-onset cancers of diverse types.
•It encodes for p53
–P53 is a transcription factor

Question 10

describe the function of MDM2 (E3 ligase) in p53 protein regulation

Answer 10

*binds to amino terminal and attaches Ubto p53 carboxyl terminal → degradation. MDM2 also blocks the transactivation domain of p53.
•In normal cells, levels of p53 are low

Question 11

describe the regulation of p53 protein

Answer 11

•Increased oncoprotein activity results in increased amounts of E2F transcription factor which increases expression of an inhibitor of MDM2.
•This inhibitor prevents MDM2 from ubiquitinatingp53 and thus prevents degradation of p53 → stabilised p53 →p53 can now function as a transcription factor
* p53 increases transcription of MDM2 so it regulates its own function -core regulation, thus p53 and MDM2 form a negative-feedback loop
*Phosphorylation and acetylation of C terminal (oligomerisation domain) promotes oligomerisation→ Four monomers bind together to form tetramers
*Tetrameric p53 binds to short genomic sequences, called p53-responsive elements (p53RE) in DNA via DNA binding domain and initiates gene transcription of proteins regulating cell cycle, DNA repair and apoptosis
*Because p53 forms tetramers, one mutant allele resulting in a non-functional protein will result in loss of function as the abnormal protein binds to the normal protein and inhibits its function –thus p53 is an exception to rule that both tumour suppressor genes need to be lost or mutated

Question 12

list oncogenic DNA viruses which produce viral proteins that bind to p53 and inactivate its function

Answer 12

1. Human Herpes Virus 8 (HHV-8, Kaposi’s Sarcoma Herpes Virus)-LANA-2
2. Hepatitis B virus -HBx
3. HPV: E6 protein

Question 13

list the functions of p53 protein

Answer 13

• It functions as the “guardian of the genome”
• Able to bind DNA and control the transcription of genes
• The p53protein plays a role in cell cycle arrest by preventing transition from G1 to S andG2 to M–Increases transcription of p21 and an inhibitor of cdc25
• If DNA damaged -delays cell-cycle progression to allow for DNA repair. Increases transcription of DNA repair proteins e.g. MSH, MLH,
• If DNA can’t be repaired -targets cell for apoptosis.–Increases transcription of Fas(Death receptor) and BAX
• OR If DNA cannot be repaired, p53 induces replicative senescence (permanent cell cycle arrest)
• p53 inhibits angiogenesis
• p53 blocks conversion from epithelial cell to a mesenchymal cell type (EMT) needed for metastasis

Question 14

why does mutation of p53 result in oncogenesis?

Answer 14

P53 gene is mutated in many cancers and loss of function results in oncogenesis because:
•cell cycle is not halted, DNA is not repaired, apoptosis or senescence is not initiated and EMT and angiogenesis is not inhibited

Question 15

discuss p53 targeted therapy

Answer 15

•Targeting p53 is difficult
–Mutant p53 in tumours function must be restored not inhibited.
–There are a broad range of p53 mutations in human tumours thus the target is heterogeneous.
–Overexpressed p53 has been shown to be linked to aging