lecture 3 - oncogenes Flashcards
What is the definition of a proto-oncogene and what are the three main ways they can be converted into oncogenes?
A gene that encodes a protein able to transform cells. The normal function of the proteins is to drive cell growth - they are pro-proliferative.
Gene amplification, chromosomal translocation and point mutations.
How was the src oncogene discovered?
Peyton found that sarcomas in chickens could be transmitted - they made the assumption that the sarcomas observed were mediated by viral particles.
They thought cancer was an infectious disease
if they injected the RNA tumour virus (retrovirus) into other chickens, they also got cancer
they discovered the v-src oncogene, which was however also present in normal cells
the viral genome was converted into cDNA to insert into the chicken genome, next to the c-src gene
when cut out, it cut a part of the c-src with it
this showed that the cancer was from the genes within the chicken, not the introduction of the virus
What are the molecular details explaining why v-src was oncogenic (hint: tyrosine)?
Tyrosine kinases have protein interaction domains that bind phosphorylated tyrosine residues
Src has a tyrosine residue in its c terminus, which is phosphorylated (unlike in most resting cells)
because of this, the molecule folds up - the SH2 domain binds to the phosphorylated tyrosine and folds up, masking the active site of src
when cut out of the chicken genome, the c terminal tyrosine was cut out - src was therefore active all the time and over proliferation occurred.
What is seen in cells over expressing src gene compared to normal cells? And name three src inhibitors.
Cells do nots top growing and they pile on top of each other, compared to normal cells that grow in a flat membrane and stop when they contact other cells
Dasatinib, saracatinib and bosutinib - block the ATP binding site, which kinases use (ATP) as their source of phosphate, meaning src is starved of phosphate and it cannot phosphorylate substrates.
What is HPV and how does it cause cancer?
It is an oncogenic DNA virus (not retrovirus) that permanently transforms host cells to make them produce viral proteins
can cause cervical cancer - needs three hits to turn normal cells into cancer cells
this virus does not actually cause mutations, but it produces molecules that can
the E5 subunit causes prolonged activation of PDGFR (undergoes mutations to make it constitutively active)
Vaccine Gardasil 9 protects against 9 types of HPV
What can mutations in EGFR/HER/ERBB receptors cause?
Ligand independence - constitutive activation of receptor - no longer need ligand binding to stay activated
over expression, due to amplification - produces more receptors on cell surface
HER2+ breast cancer can be caused by this
Describe the EGFR signalling pathway
Binding of EGF leads to activation of the receptor. The receptor dimerises and brings intracellular kinase domains into close proximity. Grb2 associates with SOS, which helps exchange GDP for GTP, thus activating ras.
Ras activates Raf, which activates MEK, then ERK, which drives expression of cyclins that drive the cell cycle.
Ras is frequently mutated, as well as Raf in melanoma
In the TM region, a mutation changing the valine to glutamine causes the chains to constitutively dimerise, which signals for growth independent of any ligand.
How can HER2 status be detected?
Biopsy staining - take a sample of the tissue, stain it with antibodies for the HER2 protein.
If you have more HER2 receptors, there will be more staining.
How can HER2+ breast cancer be treated?
HER2+ cells are associated with a more aggressive tumour phenotype, increased recurrence and reduced survival rate.
Treatment - aim is to block ligand binding - can use monoclonal antibodies
Herceptin, cetuximab and pertuzumab are offered to HER2+ patients - usually given in combo with chemo to reduce side effects of chemo.
How do Herceptin (and other related drugs) work?
Decreases activation of signalling pathways and may induce down regulation of receptor. Uncouples Src activation, increases PTEN activation. Promotes antibody dependent cellular cytotoxicity (ADCC).
What happens when ras is mutated and how can it be used as a therapeutic target?
It loses the ability to hydrolyse GTP back to GDP, so it remains active
It is difficult to target as it is not a kinase and has no ATP binding site
Fatty acid modification can be targeted, such as using farnesyltransferase inhibitors
can use antisense oligonucleotides and MEK or Raf inhibitors
What is one therapeutic agent that can be used against ras? extra reading
Farnesyltransferase inhibitors
Addition of a farnesyl group is a PTM of ras proteins
this processing results in stable localisation of ras
the inhibitors mimic the carboxy terminal motif of ras and compete for binding to farnesyltransferase
has not translated well to humans - ras can be modified by GGT, which results in transfer of ras to a different isoprenoid group that supports its activity
