lecture 2 - oncogenes and TS Flashcards
How do human tumors arise?
tumor viruses carrying viral oncogenes causing cells to transform upon infection
non-viral carcinogenesis
give egs.
environmental: radiation, cigarettes, mutagens
genetic: familial
how do we prove that oncogenes exist in our genome?
- obtain cells without prior viral infection and are not cancerous
- treat them with carcinogens
- collect DNA of transformed cells
- mix the DNA fragments with reactants to be transformed into other cells (transfection)
- inject isolated DNA fragments into mouse (will form tumor)
this proves the recipient cell’s ability to form cancer
define oncogenes/proto-oncogenes
oncogenes (which drive cancer) are mutated form of normal cellular genes (proto-oncogenes) which contribute to the regulation of cell growth and survival
2 classic examples of proto-oncogenes/oncogenes
RAS and MYC
role of RAS in the receptor-signal transduction pathway
RAS binds to RTK to trigger downstream signaling cascades
3 types of RAS protein
- KRAS
- HRAS
- NRAS
Each type exist in different tissues
what is a common mutation in the RAS proto-oncogene?
a single base pair difference at the 12th amino acid, G12V (glycine to valine) mutation and G12D (glycine to aspartatic acid) + some point mutations at #13 and #61
What is the mechanism in RAS in normal condition vs after mutation?
- RAS binds to EGFR
- complex recrutis SOS
- promotes GDP exchange for GTP
- active GTP-bound RAS
- mutation of RAS leads to greater activity such that it chronically bnds to and activates multiple downstream effectors
Common mutation in MYC
- multiple copies of the MYC gene
- reciprocal chromosomal translocation of Ch8 (contains proto-oncogene) and Ch14 –> MYC overexpresison
*Ch14 has the IgH chain regulatory elements (which has high, constitutive expression)
*MYC gene now controlled by IgH regulatory elements –> abnormally high amounts of MYC protein
what are TSGs?
TSGs (tumorsuppressor genes) are:
- anti-growth genes
- tumor forms when they are inactivated/lost
- key role in cancer pathogenesis
- may be more impotant than oncogenes in cancer development
how can we proof the existence of TSGs?
Cell fusion expt:
- 2 cells fuse (1 normal, 1 cancerous)
- the oncogene in the cancer cell should drive the hybrid cell to maintain its cancerous state
- but in actuality the hybrid cell is no longer tumorigenic
- suggesting existence of TSGs
describe the difference in dominance of oncogenes vs TSGs
Oncogenes act as dominant factor
TSGs act as recessive factor
*1 mutant allele is sufficient to drive cancer
describe the 2 forms of retinoblastoma (RB)
- unilateral RB: those born in families with no RB history
- single tumor in 1 eye
- if removed, pose no further risk of RB or other cancers
- sporadic - familial RB: either parent suffered from RB
- multiple tumors in both eyes
- even if removed, patient has high chance of getting other cancers
- bilateral (affecting both eyes
Dr Alfred Knudson Jr’s findings
He studied the kinetics of unilateral and bilateral RB
- Unilateral: 2 hit theoretical curve -> 2 random events are needed for unilateral RB
- Bilateral: 1 hit theoretical curve -> 1 single random event is needed for familial RB
where is RB gene loacted?
on Ch13
mechanisms leading to the LOH
- 2 independent mutation events in the same gene, on both alleles - extremely rare
- mitotic recombination during the G2 and M phase
- gene conversion due to DNA replication error leading to LOH
- nondisjunction during mitosis - daughter cell retains both chromatids of a chr leading to triploidy (1 normal chr + 2 chr containing mutant RB allele) descendents might shed extra chr (could shed the normal chr)
Why can LOH events be used to find TSGs?
High percentage of allelic deletions in particular regions suggest key/major TSGs presence in these chromosomal regions
Classic examples of TSGs and the function of their gene product
TP53 —> transcription factor (TF)
NF1 —> Ras-GAP
Describe the effect of NF1 protein on RAS signalling
- GAP (gene product of NF1) on Ch17 q-arm, is the main GTPase-activating protein
- RAS is inactivated by hydrolysing GTP —> GDP
- This stimulates the GTPase activity of Ras by >10000-folds —> counter the activity of RAS (tumor suppressor function)
How does RB (fail to) regulate cell proliferation in the cell cycle (upon the activation of mitogenic signalling)?
Mitogenic signalling activate oncogenic G1 cyclins.
This inhibits pRb –> pRb releases E2F.
E2F is activated (transactivation) and promotes S-phase entry —> proliferation
oncogenic G1 cyclins: D1, 2, 3, cyclin E, cyclin A
Cdks: Cdk1, 2, 3, 4, 6
What is the mechanism of P53?
- P53 is a transcriptional activator of P21 –> activates CDK inhibitors –> prevent the inhibition of pRb
- P53 causes cell cycle arrest and regulates cell survival vs apoptosis
How does the mitogenic signalling pathway prevent P53 from regulating cell cycle progression?
mitogenic signals lead to complex formation between PP2A with CCNG1 (Cyclin G1) –> dephosphorylation of Mdm2 (inactive) –> Mdm2 (active) will inhibit and degrade P53
