Oncogenes Flashcards
What are Oncogenes?
- Mutated genes which can cause cancer
Tumour viruses (Rous sarcoma virus)
- In animals many retroviruses cause cancer, however not many in humans
Normal Retrovirus:
LTR = Promoter
gag = viral core protein
pol = reverse transcriptase
env = virion glycoprotein spikes
src = encodes the Src protein which causes cell
transformation
Properties of RSV-transformed cells
Identifying proto-oncogenes
- Proto-oncogenes identified in normal human DNA by Southern blotting using a Src specific probe
- Found cellular sequences that hybridised viral Src gene in multiple species
V-sis
- Shown to be a mutated but closely related product to the B-chain of platelet-derived growth factor
- Autocrine growth stimulation
V-erbB
- Shown to be a highly truncated & point mutated version of the Epidermal growth factor (EGF) receptor
- EGF receptor is a member of the receptor tyrosine kinase family
Activation by insertional mutagenesis
- After integration of virus into the host DNA, expression of adjacent host genes is brought under the control of regulatory elements of the virus
- Myc gene is suceptible
Finding oncogenes by gene transfer
- Transforming activity detected in DNA of ~ 20% of human tumour samples tested
- Only detects dominant active oncogenes
- Commonly detects Ras, Neu, Met, Mas
Identifying mutated proto-oncogenes in the DNA of chemically transformed cells
Ras
- Involved in driving proliferation of cells
- Found by transfection of DNA into NIH 3T3 cells
- Point mutations in Ras genes found in ~ 30% of all human tumours
Ras Mutations
- Normally inactive when bound to GDP
- Growth factor stimulates Guanine Exchange Factor
- GDP released and GTP binds (Exchange)
- Changes shape of RAS into active form
- Most common sites (hotspots) are at AA 12, 13, 59, 61
- Mutations at 12 reduces affinity for nucleotides
- 13, 59, 61 affect GTP Hydrolysis
Activating RAS Mutations
Activating Ras mutants act by increasing the time spent in the active conformation
By increasing the rate of nucleotide exchange OR by affecting activation of GTPase activity required to return Ras to the ‘off’ state
Tumour types from Ras mutations
Chromosomal translocation
Translocations in human tumours that deregulate proto-oncogene expression and thereby create oncogenes
Ras Pathway regulates Myc expression
Excessive Myc protein causes cell cycle arrest / apotosis
- P19 ARF binds to inhibit mdm2
- mdm2 breaks down p53, when inhibited p53 builds up
PI 3-Kinase
- Catalytic subunit PI3KCA identified as an oncogene
- May be activated by point mutation in 20-30% of all tumours
- Other tumours lacking these mutations, this
pathway may be activated by deletion of the tumour suppressor gene PTEN which encodes a lipid phosphatase that
dephosphorylated PI3K products
PI3K Pathway
PI3K antagonises apoptotic signals
Chronic myelogenous
leukaemia
9:22 Translocation
- Lose a bit of 22 chromosome and fused on 9
- Activates Abl oncogene by creating a BCR-Abl fusion
BCR-Abl fusion
- Abl able to phosphorylate 2 tyrosine residues on BCR
- One site allows it to bind GRB2 to Ras pathway
- Increases cell cycle & decreases apoptosis
Regulation of B-catenin stability/localisation
- When Wnt present GSK switches off and B-catenin does not get phosphorylated
- Accumulated in cytosol and transported into nucleus to function as transcription factor
