19.06.03 cell cycle and cancer Flashcards
What are the stages of cell cycle
- Quiescent= G0
- Interphase= G1, S, G2,
- Cell division= M
What happens in G0
Resting phase. Left cycle and stopped dividing.
What happens in G1
- Growth phase
- proteins and RNA are synthesised.
- Chromosomes= single double helix
What is the G1 checkpoint
Restriction point. After this, cells are committed to cell division
What happens in S
- DNA synthesis replicates genetic material.
- Chromosomes= two sister chromatids
What happens in G2
Cells continue to grow
Purpose of the G2 checkpoint
Ensures enough cytoplasmic material necessary for mitosis and cytokinesis
What happens in M
- Stops growing
- nuclear division (mitosis) and cell division (cytokinesis)
Purpose of Metaphase checkpoint
Ensures the cell is ready to complete cell division.
What proteins regulate the cell cycle
- Cyclins = regulatory subunit with no catalytic activity
- Cyclin-dependent kinases (CDKs)= catalytic subunit
- Form a heterodimer
Are CDKs or cyclins constitutively expressed
- CDKs are constitutively expressed.
- Cyclins are only expressed in response to stimuli
What factors are important in G1 checkpoint
DNA integrity, molecular signals, nutrients, cell size
What happens in G1 checkpoint
- 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
What happens in G2 checkpoint
- 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
What happens in M checkpoint
- 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
Problems if checkpoints fail to activate
- Cells divide even when there is DNA damage or chromosome are misaligned.
- Leads to genome instability
Cancer is a disease of
Uncontrolled cell division
What defects in cancer cells lead to inappropriate activation of CDKs
- 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.
What is an oncogene
Gene encoding cell proliferation and apoptosis controlling proteins
What is a tumour suppressor gene
Gene encoding anti-proliferative and pro-apoptotic pathways. e.g. TP53 (encodes p53)
3 functions of p53
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
What regulates p53
- a ubiquitin ligase= MDM2 (murine double minute 2).
- Negative feedback loop as p53 induces expression of MDM2, which in turn degrades p53.
What leads to p53 activation
- DNA damage
- Cell cycle abnormalities
- Hypoxia
What proportion of cancers have mutated TP53
~50%
Proportion of tumours with MDM2 gene amplification
~17%
What interaction is a major target for cancer therapy
MDM2/p53 interaction
How does ionising radiation and chemotherapeutic drugs work
-Genotoxic stresses increase p53 levels, leading to G1 or G2/M phase arrest and subsequent apoptosis if DNA can’t be repaired
What disease is caused by p53 mutations
- Li-Fraumeni syndrome
- Patients often have multiple primary tumours. Typically osteosarcomas, leukaemia breast, brain and adrenal cortex tumours
What disease is caused by RB1 mutations
Retinoblastoma
How does pRB regulate proliferation
- pRB is active when dephosphorylated
- When active pRB binds and inactivates E2F1 transcription factor (which is required for cell cycle progression)
How does pRB lead to S phase entry
- pRB is inactivated by phosphorylation.
- E2F1 is no longer inhibited and can let cell proceed to S phase.
What regulates pRB activation
- 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
What is CDKN2A
A cyclin-dependent kinase inhibitor
What does CDKN2A encode
- p16INK4A
- p19ARF
What does CDKN2A do
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
What disease is caused by CDKN2A mutations
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
Therapeutic targets
- Restoring p53 and Rb function
- Block MDM2 expression= higher levels of p53
