Lecture 3 - TSG Flashcards
What are tumour suppressor genes
- Absence leads to cancerous phenotype
- Suppress growth or promote apoptosis - breaks of cell proliferation
- Recessive at cellular genetic level - dominant inheritance in cancer predisposition syndromes
- RB and p53 prime examples
Pro-growth pathways overview
- Oncogenes short circuit normal growth regulatory pathways
- Upregulation of TF that control proliferation expression genes
- Leads to increased cyclin D-Cdk4 activity
Pro-growth pathway to overcome G1/S checkpoint
- Mitogen bind mitogen receptor
- Ras activated
- Ras actibvation activates MAP kinase
- MAP kinase activates gene regulatory protein
- gene regulatory protein causes immediate early gene expression
- Myc causes delayed-response gene expression
- Active G1-Cdk activated -> G1/S checkpoint
Hallmarks of cancer
Inducing angiogenesis
Resisiting cell death
Sustaining proliferative signalling
Enabling replicative immortality
Activating invasion and metastasis
Evading growth suppressors
Rb binding E2F pathway in normal cells
- Active Rb binds inactive E2F
- E2F activates using cyclin D-cdk4 and inactive Rb released
- Active E2F protein -> S-phase gene transcription but positive feedback can cause returning back to previous step
- S-phase gene transcription -> G1/S cyclin (cyclin E) and S-cyclin (Cyclin A)
- Cyclin E and A -> Active S-Cdk -> DNA synthesis
- Cyclin E and A can however be reverted back to activation of E2F step by positive feedback
Rb binding E2F pathway in cancer cells
Rb protein is missing
G1-cdk missing
Constantly active E2F protein leads to excessive DNA synthesis and overexpression of protein
What types of mutations do tumour suppressors show in inactivating in RB1
Nonsense - lots
Missense - few
Frameshift - lots
Splice site
Promoter
More damaging mutations more common
Knudson’s two-hit hypothesis for Retinoblastoma
- Rare cancer in children
- Knudson studied cases on retinoblastoma before properly understood
- 2 forms:
- Unilateral - one eye - No family history tumour can be removed and patients live long healthy lives
- Bilateral - Both eyes - often family history of disease. Even if tumours removed, more risk of cancer later in life
Knudson’s results
- Bilateral group usually diagnosed earlier in life than unilateral group
Familial retinoblastoma (a cancer-predisposition syndrome):
Already 1 mutant Rb allele
Two mutant Rb copies after first somatic mutation
Sporadic retinoblastoma:
No mutant Rb alleles->
First somatic mutation->
1 mutant Rb allele->
Second somatic mutation->
2 mutant Rb alleles
TP53 most common mutations in cancers
Ovary
Colorectum
Head and neck
Oesophagus
How does p53 work?
Lack of nucleotides, UV, ionizing radiation, oncogene signalling, hypoxia, transcription blockage->
-> p53
-> Cell cycle arrest -> Senescence or return to proliferation OR DNA repair OR block of angiogenesis OR Apoptosis
Different p53 genes
p21CIP1/WAF1 - Cell cycle arrest
XPA - DNA repair
TSP-1 - Angiogenesis inhibition
Killer/DR5 - Apoptosis
p53 pathway
Without DNA damage:
p53 bound by Mdm2 where p53 undergos ubiquitylation and degradation in proteasomes
With DNA damage:
ATM/ATR kinase activated
Chk1/Chk2 kinase activated
p53 phosphorylated, and Mdm2 and ubiquitin degraded by proteosome
Forms stable, active p53 that binds to regulatory region of p21 gene
p21 mRNA formed via transcription (CDKN1A)
p21 protein forms via translation - inhibit G1/S-Cdk and S-Cdk
CDKN1A
Cell cycle arrest gene
Transcribes RB and DREAM
Oncogenic signalling pathway of p53
p53 bound by Mdm2 where p53 undergos ubiquitylation and degradation in proteasomes
Oncogenic signalling cause excessive Myc production
Arf binds inactive Mdm2
Stable active p53 -> cell cycle arrest or apoptosis
In absence of p53, excessive Myc is expressed which can lead to carcinomas and other forms of cancer
Why don’t elephants get cancer
Large and long lived, yet risk of cancer is <5%.
Multiple copies of p53 (20 genes, 40 alleles)
Very unlikely all alleles would mutate
Enabling replicative immortality: telomerases
- Ends of chromosomes protected by telomeres from damage
- Telomeres get successively shorter with successive generations, leading to a CRISIS
- Normally only expressing in germ/stem cells
- Cancer cells become immortal by expressing TERT
- Telomerase positive in neuroblastoma decreases prognosis, and high telomerase in Ewing’s sarcoma decreases prognosis
How do telomerases resynthesize telomeres
Parental strand and incomplete, newly synthesized lagging strand
Telomerase binds and telomere synthesis begins
Telomerase extends 3’ end (RNA-templated DNA synthesis)
Completion of lagging strand by DNA polymerase
The Warburg effect in deregulation of cellular energetics
Glucose taken up by GLUT1 -> glycolysis
Glycolysis -> Pyruvate by PK-M1
Pyruvate -> acetyl CoA by PDH and release of 4ATP
Acetyl CoA -> Krebs cycle
Requires O2 and leaves little for biosynthetic pathways
PET/CT scanning
- Patient given small amount of PET radiotracer: 18F fluorodeoxyglucose, which is recognised as glucose by cells
- Cancers uptake large amounts of glucose/ FDG
- PET scans and CT scans detect radiation and reveal tumour location
