19.06.03 cell cycle and cancer

Question 1

What are the stages of cell cycle

Answer 1

• Quiescent= G0
• Interphase= G1, S, G2,
• Cell division= M

Question 2

What happens in G0

Answer 2

Resting phase. Left cycle and stopped dividing.

Question 3

What happens in G1

Answer 3

• Growth phase
• proteins and RNA are synthesised.
• Chromosomes= single double helix

Question 4

What is the G1 checkpoint

Answer 4

Restriction point. After this, cells are committed to cell division

Question 5

What happens in S

Answer 5

• DNA synthesis replicates genetic material.

- Chromosomes= two sister chromatids

Question 6

What happens in G2

Answer 6

Cells continue to grow

Question 7

Purpose of the G2 checkpoint

Answer 7

Ensures enough cytoplasmic material necessary for mitosis and cytokinesis

Question 8

What happens in M

Answer 8

• Stops growing

- nuclear division (mitosis) and cell division (cytokinesis)

Question 9

Purpose of Metaphase checkpoint

Answer 9

Ensures the cell is ready to complete cell division.

Question 10

What proteins regulate the cell cycle

Answer 10

• Cyclins = regulatory subunit with no catalytic activity
• Cyclin-dependent kinases (CDKs)= catalytic subunit
• Form a heterodimer

Question 11

Are CDKs or cyclins constitutively expressed

Answer 11

• CDKs are constitutively expressed.

- Cyclins are only expressed in response to stimuli

Question 12

What factors are important in G1 checkpoint

Answer 12

DNA integrity, molecular signals, nutrients, cell size

Question 13

What happens in G1 checkpoint

Answer 13

• CDK4/6-cyclin D formed. This phosphorylates RB (retinoblastoma protein).
• RB protein releases E2F transcription factor
• Cyclin E expressed, which binds to CDK2. Leads to G1 to S phase transition

Question 14

What happens in G2 checkpoint

Answer 14

• CDK1 is activated by phosphorylation/de-phosphorylation of certain amino acids by Cyclin Activating kinase (CAK) and wee1 protein
• CDK1-cyclin B forms and leads to G2 to M phase transition

Question 15

What happens in M checkpoint

Answer 15

• Chromosomes assemble on metaphase plate
• Check to ensure sister chromatids attached correctly to spindle microtubules
• Activates anaphase-promoting complex (APC)
• CDK1-cyclin B disassembles (cyclin B is degraded)
• Separase is no longer inhibited so spindles are then cut
• Sister chromatids then separate. Cell enters anaphase

Question 16

Problems if checkpoints fail to activate

Answer 16

• Cells divide even when there is DNA damage or chromosome are misaligned.
• Leads to genome instability

Question 17

Cancer is a disease of

Answer 17

Uncontrolled cell division

Question 18

What defects in cancer cells lead to inappropriate activation of CDKs

Answer 18

• Overexpression of cyclin D1. Cyclin D1 regulates entry and progression through G1. Too much and could pass checkpoint without the appropriate growth factors.
• CDK inactivated by CDK inhibitors (CKI). Mutations in CKI genes means more CDK activation and more cell proliferation.

Question 19

What is an oncogene

Answer 19

Gene encoding cell proliferation and apoptosis controlling proteins

Question 20

What is a tumour suppressor gene

Answer 20

Gene encoding anti-proliferative and pro-apoptotic pathways. e.g. TP53 (encodes p53)

Question 21

3 functions of p53

Answer 21

1) Triggers production of CDK inhibitors, leading to cell cycle stopping at G1 checkpoint.
- CKIs bind to and inactivate CDK-cyclin complexes.
2) Activate DNA repair enzymes
3) if DNA is irreparable then p53 triggers cell death

Question 22

What regulates p53

Answer 22

• a ubiquitin ligase= MDM2 (murine double minute 2).

- Negative feedback loop as p53 induces expression of MDM2, which in turn degrades p53.

Question 23

What leads to p53 activation

Answer 23

• DNA damage
• Cell cycle abnormalities
• Hypoxia

Question 24

What proportion of cancers have mutated TP53

Answer 24

~50%

Question 25

Proportion of tumours with MDM2 gene amplification

Answer 25

~17%

Question 26

What interaction is a major target for cancer therapy

Answer 26

MDM2/p53 interaction

Question 27

How does ionising radiation and chemotherapeutic drugs work

Answer 27

-Genotoxic stresses increase p53 levels, leading to G1 or G2/M phase arrest and subsequent apoptosis if DNA can’t be repaired

Question 28

What disease is caused by p53 mutations

Answer 28

• Li-Fraumeni syndrome
• Patients often have multiple primary tumours. Typically osteosarcomas, leukaemia breast, brain and adrenal cortex tumours

Question 29

What disease is caused by RB1 mutations

Answer 29

Retinoblastoma

Question 30

How does pRB regulate proliferation

Answer 30

• pRB is active when dephosphorylated

- When active pRB binds and inactivates E2F1 transcription factor (which is required for cell cycle progression)

Question 31

How does pRB lead to S phase entry

Answer 31

• pRB is inactivated by phosphorylation.

- E2F1 is no longer inhibited and can let cell proceed to S phase.

Question 32

What regulates pRB activation

Answer 32

• CyclinD1-Cdk4 complex is a protein kinase. This phosphorylates and inactivates RB.
• CKI (CDK inhibitors) like p16 prevent Cyclin-CDK complex formation, so RB remains active and dephosphorylated

Question 33

What is CDKN2A

Answer 33

A cyclin-dependent kinase inhibitor

Question 34

What does CDKN2A encode

Answer 34

• p16INK4A

- p19ARF

Question 35

What does CDKN2A do

Answer 35

1) it inhibits the CDKs that inactivate pRB.
- Loss of CDKN2A function leads to loss of RB1 function and therefore inappropriate cell cycling.
2) Destabilises MDM2 there maintains levels of p53.
- Loss of CDKN2A function leads to excessive MDM2 levels and increased p53 degradation and loss of cell cycle control

Question 36

What disease is caused by CDKN2A mutations

Answer 36

CDKN2A gene is a melanoma susceptibility gene and its mutations are present in 20 to 40% of familial and 2 to 3% of sporadic melanomas

Question 37

Therapeutic targets

Answer 37

• Restoring p53 and Rb function

- Block MDM2 expression= higher levels of p53