Tumour biology Lecture 9 Flashcards
When are the cell cycle checkpoints?
R point in G1 G1/S
Replication point in S phase G2/M
Anephase checkpoint before mitosis
Spindle checkpoint in M phase
Within Cancer, tumour surppressor genes are lost, so cancer cells can endleslly proiferate etc
Steroid hormones can pass through plasma membrane and interact with a intracellular rectpror. BUt protein hormones cant pass throguh so has to interact with receptors at the cell surface.
What is the Rb restriction point?
As Rb progresses through G1, it becomes hyperphosphorylated (inactive) so then the cell cycle can continue.
Steps:
- pRb Binds E2F in hypophosphorylated form during G0/G1. Rb represses its activity by forming a repressor complex with histone deacetylase and acetylase
- Cdk4/6 and Cyclin D phosphorylate causing dissociation of the histone acetylation complex
- This allows for acetylation of chromatin in the region of the promoter, which primes the gene for transcription
- In late Rb is then phosphorylated by Cyclin E and Cdk2, which allows Rb to dissociate from E2F, allowing it to drive transcription of genes important for S phase entry
How is Rb regulated?
By p16 Cdk inhibitor
- In cellular stress p16 is upregulated and inhibits Cdk 4/6 activity, stopping Rb from being phosphorylated, so the cell cant move into S phase
- This can stimulate cellular senescence
What is the DNA damage response to checkpoint arrest?
DSB- MRN➡ ATM➡ BRCA1/53BP1➡ Chk2/p53- effectors
SSB- RPA➡ ATR/ATRIP➡ TopBP1/Claspin➡ Chk1- effector
Facilitated and amplified by Rad17 and H2AX
Describe telomeres
Nucleoprotein structures that protect chromosome ends
10-15kb in humans
Maintained by telomerase (hTERT) and an RNA template (TERC)
Telomere length is maintained by telomerasein cancer to immortalise the cell.
Normally, after replication, the telomere length is reduced.
Telomere DNA is associated with the proteins TRF1/2
Forms a D loop of ssDNA protected by Pot1 and a T loop of dsDNA, this is so out does not look like a DSB
Describe senescence
Stress and Abberent Signalling Induced Senescence (STASIS)
Inhibitors of cdks- p21 and p16
STASIS = Prematurely induced senescence (without telomeres shortening)
- Induced by stress or aberrant signalling - not by telomere shortening
- Happens with DNA damage or oncogene activity
- Increases Cdk inhibitors
Senescence = Post relpicative (dont respond to growth signals) Flattened morphology, metabollicaly active, altered gene expression, change in tissue homeostasis.
Normally the progressive loss of telomeres induces a DNA damage response and this triggeres senescence - potent tumour suppresor.
Telomere ends are protected by
TRF1 complex has C-terminal mid DNA binding domain
-POT1 binds ssDNA within telomere, protecting it from DNA damage signalling
TRF2 complex is made up of MRN and Ku86 which shelter the telomeric end. When exposed this induces a DNA damage response immedietly.
Describe regulation of telomeric chromatin
Regulation is by pRB and SUV39H
In cancer if you looose any of these, you loose the strucutre of chormatin - the telomer becomes deortected and relomerase can exptend the life span
pRB is involved in regulating the chromatin structure
SUV39H is a mathylase which add methyl groups onto chromatin. Without SUV29H, the protein has a different conformation. Telomerase can now acess and extend the telomere. Normaly telomerase has restircted acess with SUV39H.
RB regulates H4K20Me3
SUV3aH2 regulates H3K9Me3 Loss of either results in the extension of telomeres and opening of chromatin
What is ALT?
Alternative lengthening of telomeres
Strand invasion of the template molecule and formation of HR intermediate.ALT is controlled by HR, finds a homologous bit of DNA and extends telomere through HR.
Copying
Dissolution of HR intermediate
Possible filling of complementary strands
What induces M1 senescence and how can cancer cells bypass it?
DNA damage induces M1 senescence➡ ATM➡ p53➡ p21➡ growth arrest
Normal Cell:
- Stable conformation and protection of telomere
- Each division shortens telomeres leading to them becoming unprotected and exposing SSB/DSBs
- This activates p53 which induces p21 expression
- This induces cellular senescence
Bypass by cancer cells:
- Loss of p53 or Rb so p21 can no longer be stimulated and p16 is no longer active
- Once telomeres erode there is no check
- Causes further chromosome abnormalities through chromosome end to end fusions leading to tumourigenesis
What induces M2 senescence?
In cells that bypass M1 senescence, telomers continue to erode, they cant protect chromosomes and apoptosis is triggered, leads to chromsome abnormalities, in cells inactive of p53 and Rb, there is no check on proliferation, leading to accumilation of mutations
Many short telomeres induce the DNA damage response ATM/ATRIP dependent- components of TRF2-shelterin complex
Telomeres continue to shorten causing a Dicenteic chromosomes to be made that trigger apoptosis
Describe cell immortalization
Mutations in critical genes and maintainence of telomeres via telomerase or ALT pathways
Causes loss of tumour surpressor genes and activation of telomerase. This leads to the cell becoming immortal and leads to a cancer phenotype
(ALT maintains telomere length in telomerase)
Telomere loss is accelerated in premature aging - mutation deletion of TRF2 complex leads to accelerated telomere loss.
In cancer telomerase is switched on which causes decline of telomeres
Describe telomerase as a possible cancer therapy
Oligonucleotides complementary to the TERC template (prevents telomerases binding to telomeres)
Targeting telomerase- expressing cells with suicide gene or oncolytic viruses
Telomerase targeted immunotherapy - vaccine to hTERT peptides brings around T-cell immune response
