Tumour Supresssor Genes Flashcards
Characteristics of TSG
Protein products of TSG generally function to negatively regulate cell growth and/or cell cycle checkpoints that ensure genomic integrity in response to genotoxic stress
Loss of TSG function means loss of growth suppression and or check point control enhancing proliferative potential
Are TSG dominant or recessive in cancer
Recessive so both alleles must be inactives/removed to stop tumour suppression
This is explained by what hypothesis
Knudson’s two hit hypothesis
Example of retinoblastoma
There is one mutation in the germ line -found in every cell
Only needs 1 random mutation to occur to cause cancer
Tends to cause bilateral retinoblastoma
Early onset of disease
Also increased risk of other cancers such as osteosarcomas
In non hereditary forms - 2 mutation are needed -somatic mutations in a single cell for it to lose its TSG properties and become cancerous
Tends to cause unilateral retinoblastoma
De novo mutations
These can occur in the germ line these lead to familial patterns of inheritance in future generations
What is retinoblastoma
Cancer of the retina
Tumour displaces normal retina
Optic nerve used to metastasise
Symptoms - white light reflection, squint
90% cure rate in developed countries
Bilateral diagnosed early unilateral later diagnosed
Familial inheritance
Large deletions
Single base mutations
Small length mutations
Most mutations are associated with almost complete penetrance
Autosomal dominant
Autosomal dominant at gene level so both alleles need to be inactivated but it is an autodominant disease very likely to get it
Multiple tumors
Somatic mutations
Gene deletions Base substitutions Small length mutations Single tumours, unilateral, later onset Fewer people
How the second mutation occurs
Hypermethylation of the 5’ end affecting RB1
There is inactivation of the promoter that drives expression of Rb
Leads to loss of Rb
Or
Loss of heterozygosity more common
Due to mitotic recombination, mitotic nondysjunction and large deletions
Function of Rb protein in the cell cycle
Rb is important for restriction point control between G1 and S where the cell becomes committed to completing the cell cycle
What drives the cell cycle
Cyclins and CDKs (cyclin dependent kinases)
In the G1 phase CDK4/6 and E-CDK2 are active
pRb is a target for CDK activity
What is pRB function
Transcriptional depressor
What does pRB complex with and what does that do
PRB complexes with E2F family of transcription factors and suppresses their activity
What is HDAC and what does it do?
Histone deacetylases
These down regulate transcription by removing acetyle groups from histones
P300 what is it what does it do
It is an acetylase and oppose the action of HDAC
Add acetyle groups to histones
What happens in early G1 phase
PRB becomes phosphorylated by CDK4/6
This causes dissociation of HDAC
Allow for p300 to act
This then primes the promoter elements for genes that are important in the S phase of the cell cycle
Transcription of these S phase genes are activated
What happens in late G1 phase
CDK2 phosphorylates pRB
This makes pRB move away from E2F
E2F factors drive transcription
What regulates pRb preventing it from allowing the cells cycle move forward into s phase
P16
P16 inhibits CDK 4 dependent phosphorylation of pRb
So pRb remains dephosphorylated and associated with E2F
Inhibiting transcription
Leads to growth and cellular senescence
What is p16ink4a
It is a TSG that can be inactivated in human cancer
If inactivated it cannot inhibit pRb
Leading to its phosphorylation and dissociates from E2F
Activating transcription
Usually happens via a mutation and then epigenetic silencing (both alleles are targeted)
So there is no block to transcription
P53 and what does it do
Protects against genetic instability
Between 30-50% of all human cancers carry missense mutations that lead to mutant p53
p53 may be inactivated but not necessaryily it that is mutated
This can be done by inactivating signals upstream and downstream to p53
P53 germline mutations cause
Li-Fraumeni and Li-Fraumeni like syndrome
What are germ line p53 mutations associated with
Clinically and genetically heterogeneous
Autosomal dominant
Causes early onset and multiple tumours within an individual
Soft tissue sarcomas and breast cancer
Also leads to multiple affected family members
Very rare
At what stage in cancer progression is p53 inactivated
Late stages
E.g. Epithelial cancer
Late stage adenoma before becoming a carcinoma
Not required for the initiation of the cancer but for its progression.
What is p53 activated by
DNA damage Oncogenes expression Hypoxia Loss of adhesion Ribosomal Reactive oxygen species
What do cellular stressors do to p53 to activate it
Phosphorylate it by mediators such as ATM and Chk2 from DNA damage
P53 is
A sequence specific functional transcription factor
It activates the transcription of proteins that are needed for its roles
P53 roles
Pro-apoptotic activity
Growth arresting activity
Induces cellular senescence
Inhibits angiogenesis
When mutated what does p53 lose the ability to do
Loses the ability to activate transcription
Loses growth suppressive activities
When p53 has a gain of function mutation what can it do?
Has oncogenic potential
And can promote genetic instability
Also increase drug resistance
How does p53 act alone or?
Acts as a tetramer to activate gene transcription
Binds to the response element of the promoter
Drives transcription of genes that are important in growth arrest and apoptosis
How do p53 activity regulated by cellular stresses
Regulated by post translational modification
What determines p53 activity
The cellular stress appropriately modifies p53 - acetylation, ubiquitylation, phosphorylation
This makes p53 go to a certain promoter region
Here it can activate the appropriate gene to achieve the desired effect
This is done via mediators - ATM, Chk2
These are protein kinases that cause phosphorylation of p53, also TSG which may. E inactivated in cancers
What inhibits p53
Mdm2
This can also be inhibited by p19ARF
Which means p53 will become activated
Role of p53 in G1-S checkpoint control after a DNA double strand break
Activation of protein complexes that recognise DNA DSB
ATM kinase recruitment to the DSB
Internal phosphorylation activates ATM
ATM can then phosphorylate p53 directly enhancing its transcription activity
Phosphorylate CHK2 activating it which then phosphorylates p53 and prevents mdm2 inhibition of p53
P53 prevents CDK4/6 CDK2 so prevents the cell cycle continuing
How does p53 induce apoptosis
Transcriptional and non- transcriptional mechanisms
Express proapoptotic molecules such as puma and Bax
Regulation of BCL2 proteins by post transcriptional mechanisms
Activate apoptosomes and caspases which lead to cell death
P53 mdm2 regulatory feedback loop
Active p53 induces mdm2 expression
Mdm2 promotes the ligation of many ubiquitin molecule to p53
This targets the p53 for degradation in the proteosome
Reduces level of p53
P53 controls its own activity
Where do most of the p53 mutations occur
Occur in the sequence specific DNA binding area
This prevents p53 from binding to DNA
Leads to loss of p53 transcriptional activity in cancer
What does BRAC1 gene do?
Predisposes women to early onset breast carcinoma and ovarian tumours
Greater incidence in breast than ovarian
1/500 woman have germ line mutation in the BRCA1 gene
Accounts for 60% of inherited breast cancer
5% of all breast cancer and 12% of early onset
May be associated with some prostate cancers
What is BRCA1 important founder mutation
185delAg
Found in 1% of the Ashkenazic Jewish population
Role of BRCA1
Interacts with lots of anti-apoptosis proteins
c-Myc, ATM, ATR, CHk2
Also regulates ubiquitylation, thought that via this pathway BRCA1 can control DNA damage response, DNA repair, transcription, chromatin remodelling etc.
BRCA1 role in the DNA damage checkpoint signalling
In DNA damage
ATM and ATR are activated they phosphorylate BRCA1
ATM dependent phosphorylation of BRCA1 prevents the progression of the cell into S phase
ATR dependent phosphorylation of BRCA1 stops the cell progressing from G2 to mitosis
Loss of BRCA1 loss of damage checkpoint
BRCA1 role in repair
Encourage homologous recombination and DNA repair
Phosphorylation of BRCA1 can promote gene conversion
Most high fidelity form of DNA repair
Inhibits more error prone NHEJ mechanism of DNA repair
BRCA2 ( Franconi anaemia complementation group D1)
Predisposes to early onset breast cancer and ovarian tumours
Breast more than ovarian
Inc risk of prostate cancer, gall bladder, and bile duct cancer, stomach cancer and malignant melanoma
BRCA2 mutation found in the Ashkenazic Jewish women
6174delT
Role of BRCA2 in DNA repair
Associates with RAD51
After damage you need to resect one strand away allowing binding of RPA (single strand binding protein, stabilises)
Associates with RAD51 and promotes association of RAD51 onto the single strand stretch of DNA so RAD51 nucleoprotein filaments form on the DNA allowing binding to the homologous chromosome and form recombinations for DNA recombination and repair
BRCA1 and BRCA2
Make equal contribution to early onset breast cancer
Somatic mutations in either gene are not a major feature of sporadic breast cancer
Differences in histology between BRCA1 and 2
1 - higher mitotic rate and more lymphatic infiltration
2 - higher tubule formation
Loss of either result in increased sensitivity to ionising radiation
