Oncogenes Flashcards
How was Rous Sarcoma Virus (RSV) discovered? (5)
- Sarcoma removed from the breast of a chicken, ground up and filtered
- Filtrate injected into another chicken
- Chicken develops sarcoma
- Concluded only a virus would be small enough to pass through the filter
- Thought that cancer was an infectious disease
What happens when you infect cells with RSV in vitro? (2)
- Cells form foci
- Cells show similar metabolism to those isolated from tumours
(RSV transforms cells in vitro)
What is cell transformation?
Conversion of a normal cell into a cancer cell
What are foci?
Clusters of tumour cells
What are the 2 possible explanations for the transformed RSV cells passed on their phenotype to other cells?
- RSV particle transforms the progenitor cells and then transforms their offspring so the virus must be present to maintain the transformed phenotype
- RSV particle transforms the progenitor cells which somehow transmit the phenotype to their offspring in the absence of the virus
How was it discovered how RSV cells pass on their phenotype? (5)
- Made a temperature sensitive RSV mutant where the viral proteins are functional at 37 degrees but not 41
- Infected cells at 37 degrees, saw the transformation
- Raised the temperature to 41 degrees, transformation was lost
- Reverting back to 37 degrees restored the transformation
- Concluded that the presence of the virus is required to maintain the transformed phenotype
What are the properties of transformed (cancer) cells? (9)
- Altered morphology
- Loss of contact inhibition
- Anchorage independent growth
- Ability to proliferate indefinitely (immortalisation)
- Can grow without growth factors
- High saturation density
- Inability to halt proliferation in the absence of growth factors
- Increased uptake of glucose
- Tumorigenicity
What are the viral constituents of RSV? (4)
- Gag
- Pol
- Env
- Src
Which RSV genes are required for viral replication? (3)
- Gag
- Pol
- Env
Which RSV gene is required for transformation?
Src
What is the difference between c-src and v-src? (3)
- Src is present in the genome of normal organisms in the c-src form
- C-src is a proto-oncogene
- V-src is an oncogene
What is an oncogene?
A gene capable of transforming a normal cell into a cancer cell
What is a proto-oncogene?
A precursor of an active oncogene
What causes cancer? (2)
- Cancer is induced by mutagens which mutate growth-controlling genes
- If mutagens mutate proto-oncogenes they induce the formation of tumours
What is segment recombination? (2)
- Cut up segments of the proto-oncogene and the oncogene
- Recombine using enzymes to identify which bit of the proto-oncogene changes to induce cancer
What mutation occurs in Ras to make it oncogenic? (2)
- G12V
- G12C
What are examples of proto-oncogenes? (2)
- Ras
- Myc
How does wildtype Ras work? (4)
- Inactive Ras bound to GDP
- GEF causes Ras to dissociate from GDP and bind to GTP
- Ras is active when bound to GTP and activates downstream targets
- GAP causes GTP hydrolysis so Ras is bound to GDP and inactivated again
What kind of molecule is Ras?
Small GTPase
What impact does a G12V/G12C mutation have on Ras signalling
Ras is unable to hydrolyse GTP to GDP so remains active and can’t be switched off, even in the absence of growth factors
What are the 3 ras genes in the human genome?
- H-ras
- K-ras
- N-ras
Which mutated form of ras is the most frequent driver of tumour development? (2)
- K-ras
- KRAS G12C mutation is the most prevalent in lung cancers
What is AMG510? (3)
- New drug AMG510 is a specific inhibitor for G12C mutated K-ras
- AMG510 binds to KRAS G12C in the GDP-bound form and prevents binding to GTP so it can’t be activated
- No effect on wildtype Ras
What are the results from AMG510 testing? (3)
- AMG510 inhibits tumour growth in mice
- Efficiency improves in combination with chemotherapy/MEK inhibitor
- AMG510 is not effective on mice lacking an immune system
What is MEK?
Downstream target of Ras
What happens when you combine AMG510 with anti-PD1 (immunotherapy)?
AMG510 potentiates immunotherapy
How does AMG510 potentiate immunotherapy? (3)
- AMG510 boosts the expression of pro-inflammatory cytokines
- Increases infiltration of tumours by T-cells and dendritic cells
- The T-cell response is long term, reintroducing KRAS G12C cancer cells after being cured had no effect
What is the problem that arises with KRAS inhibition therapy?
Rapid adaptive resistance
What are the mechanisms of adaptive resistance to KRAS inhibition? (5)
- Mutations to the drug binding site on the KRAS so the inhibitor can’t bind
- Mutations in the other KRAS allele which compensates for the inhibition of the original mutant KRAS
- Amplification of KRAS which decreases the efficacy of the inhibitor
- Additional G12 mutations
- Mutations in effectors downstream of KRAS
What are the 3 members of the myc family?
- c-myc
- N-myc
- L-myc
What kind of molecule are the myc proteins?
Growth-promoting transcription factors
What are the 3 mechanisms by which the myc oncogene can arise?
- Gene amplification
- Chromosomal translocation
- Pro-virus integration
Where is N-myc amplification seen?
In 30% of childhood neuroblastoma cases
How does gene amplification of myc cause cancer?
More copies of the gene results in too much myc protein being made causing uncontrolled growth
How does chromosomal translocation make myc oncogenic? (2)
- In Burkitt lymphoma, myc gene is translocated onto a different chromosome which puts it under the control of an immunoglobulin promoter in lymphoid cells
- Causes excessive production of myc protein and uncontrolled proliferation of lymphoid cells
Where is chromosomal translocation of myc seen?
Burkitt lymphoma
How does pro-virus integration of myc cause cancer? (2)
- Avian Leukosis Virus (ALV) can insert into the genome, transcriptional promoter
- If AVL inserts upstream of myc, causes increased expression and high levels of myc protein = uncontrolled growth
How do growth factor receptor mutations cause cancer?
Truncated versions of epidermal growth factor (EGF) receptor can be constitutively active in the absence of growth factors
How is the Ras proto-oncogene activated?
Point mutation (G12 substitution) causing a constitutively active protein
How is the Myc proto-oncogene activated?
Gene amplification/chromosomal translocation/insertional mutagenesis causing overexpression of normal myc protein
How is the EGFR proto-oncogene activated?
Structural changes causing constitutive activation
