Lecture 15 - Checkpoint control Flashcards
How many checkpoints do we have in the cell cycle?
4 Checkpoints:
G1/S
G2/M
S checkpoint - DNA REPLICATION PAUSED
Mitotic spindle checkpoint - stops progression into anaphase if mitotic spindles are not assembled
Cancer cells can inactivate checkpoint control
What is PRB?
PRB - tumour supressor gene. Guardian of the R point
It stops cell cycle at G1 phase at the restriction point
need it to be INACTIVE in order to progress
What inactivates PRB?
Hyperphosphorylation of PRB inactives it and cell cycle progresses
Where is PRB phosphorylated?
Phosphorylised by Cyclin D in early G1
Then hyperphosphorylised by Cyclin E in mid-late G1
PRB when not phsopsphorylised is attached to a TF called E2F. PRB is now active and blocking everything
E2F dissociates when PRB is phosphorylised.
What does E2F transcribe?
E2F transcribes genes that allow the cell cycle into S phase
Cyclin E is also promoted by E2F
How is E2F stopped?
Inhibited by cyclin A/ Cdk2
Cyclin A causes it to be targeted by Ubiquitation for degradation.
What does Cyclin E degrade?
p27KIP1 - a CKI
Positive feedback route, causing cell cycle to be rapid and irreversible
What 3 ways can PRB be inactivated leading to cancer?
- Mutation - protein not expressed or protein is non-functional
- Phosphorylation of PRB is unregulated
- Interaction with viral proteins e.g human papilloma virus which causes cervical cancer
Why are cyclins hard drug targets?
Cyclins are hard pharmaceutical drug targets because they have no kinase activity and are located in the nucleus
cDK good target as they are kinases
What does E2F transcribe?
E2F transcribes genes that allow the cell to cycle into S phase
Cyclin E is also promoted by E2F
What is ther Palbociclib drug?
Palbociclib - a Cdk4/6 inhibitor normally used for breast cancer. Used with the drug taxanes
What are the 4 reasons people get resitant to the drug Palbociclib?
- Activation of upstream effectors
- Inactivation mutation in PRB
- Cdk 6 overexpression
- Sometimes downstream pathways can bypass first stage if they are active e.g Cylclin E
- Cells resistant to the drug are overexpressed MIC and Cyclin E.
What are the DNA damages in cell cycle?
- ATM - acts at G1/S checkpoint. Deals with double-stranded DNA breaks
- ATR - acts at S phase. Single-stranded DNA breaks. Mediates G2/M checkpoint
- WEE1 - At S/G2 and M checkpoint. Regulates Cdks
DDR - if damage they arrest cell or repair DNA
Inactivation of DDR = cancer
DDR inhibiting drugs -
What does CHK1 protein do?
CHK1 - Activated by ATR pathway, ensures DNA repair
How can CHK1s decrease tumour volume?
In MIC amplified cancer cells (blastoma) CHK1 expression is also increased.
Cells that overerexpress MIC are sensitive to CHK1 inhibitors
Decreases tumour volume
