Lecture 5 - Oncogenes Flashcards
What did Peyton Ross discover in 1911?
that a filterable agent from a sarcoma of a chicken could rapidly cause cancer when injected into another chicke
- this marked to start of tumour virology
What are the 2 types of tumour retroviruses?
acutely transforming e.g. Rous sarcoma virus
slowly transforming e.g. avain leukosis virus
Briefly describe the lifecycle of a retrovirus
- virus envelope proteins attach to cell surface receptors
- virus capsid is released into cytoplasm
- Reverse Transcription: RNA –> DNA
- Provirus integrated into host genome
- transcribed by host cell machinery
- makes RNA and proteins for new virus particles
- provirus transmitted to daughter cells at mitosis
Describe the genome of slowly transforming retroviruses
Long Terminal Repeat at either end - contains transcription regulatory sequences - promotor and strong enhancer)
gag = group specific antigen (viral core protein)
pol = RNA-dependent DNA polymerase
env = envelope protein (binds target cell)
Describe the genome of acutely transforming retroviruses
LTR - gag - pol - env - v-onc - LTR
Cause tumours rapidly due to viral oncogene
The v-onc genes of acutely transforming retrovirus are homologous to genes in cellular DNA of the host species.
How was this proved?
- make radioactive ssDNA copy of v-src from Rous sarcoma virus
- mix with denatured DNA from normal, uninfected chickens
- the v-src probe annealed with a sequence present in the normal chicken DNA
- i.e. the viral oncogene has a very similar nucleotide sequence to a chicken gene i.e. one must be derived from the other
- the viral oncogene is a ‘pirated’ copy of a cellular gene
Similar experiments with other acutely transforming retroviruses rapidly identified scores of candidate cellular oncogenes. What does this suggest?
This suggested that the cellular homologues might also have potential to cause cancer
(the viruses themselves do not cause cancer in humans!)
The cellular homologue of the v-sis oncogene (from the Simian sarcoma virus) is a growth factor. What does this mean?
- cancer produces the GF and responds to it (autocrine stimulation)
- many osteosarcomas and gliomas both secrete PDGF and have PDGF receptors i.e. appear to produce a GF that stimulates their own proliferation
Retroviral oncogenes are derived from cellular genes involved in the signalling pathways driving cell proliferation. Describe this signalling pathway.
- GF’s e.g. EGF, PDGF
- Bind to their receptors
- Triggering a complex network of signalling cascades in the cytoplasm
- Leading to the nucleus, causing changes in gene expression
- Leading to cell cycle progression, DNA synthesis, mitosis etc.
ALV infection of chickens leads to bursal lymphomas (Cell tumours).
How?
What does the lag phase reflect?
- DNA analysis shows that in all tumours, an ALV provirus has integrated close to the c-myc gene
- Tumours must be initiated by provirus insertion near this gene
- in chronic viraemia, many cell become infected with retrovirus
- integration occurs at random in each infected cell
- inevitably, in some cells integrated occurs near the c-myc gene, where it can deregulate gene expression
Lag phase reflects progressive viraemia and then the time interval before a random provirus insertion event occurs sufficiently close to the c-myc gene
A variety of mutational changes to cellular oncogenes can enable their contribution towards the initiation and progression of human cancer. Give an example.
Expression of c-myc is deregulated by a chromosomal translocation in Burkitt’s Lymphoma
- translocation between c-myc gene on chromosome 8 and immunoglobulin gene loci on chromosomes 14, 2, or 22
- this leads to a constitutive increase in synthesis of a normal c-myc protein
What does c-myc do in normal cells and in cancer?
In normal cells, Myc accumulation transiently, shortly after growth factor stimulation
- accumated Myc forms heterodimers with Max, and activates transcription of growth related genes
- in non-stimulated normal cells, Max homodimers predominate, and repress transcription of these genes
- deregulated expression of Myc leads to constitutive expression of many growth related genes
What is a very common way of over expression of myc?
Gene amplification
- increases number of gene copies
- higher level of expression
- level of amplification of the myc gene in neuroblastoma is a prognostic indicator
How do tyrosine kinases work?
Growth factor binding causes conformational change, dimerisation and activation of tyrosine kinase activity of the cytoplasmic domain
- autophosphorylation further activates tyrosine kinase activity
- various cytoplasmic proteins bind to the tyrosine-phosphorylated receptor
- some recruited proteins become activated by tyrosine phosphorylation
- it facilitates interactions between recruited proteins or with other proteins or enzyme substrates already localised at the plasma membrane
What are the two ways in which typrisne kinases can be involved in cancer?
- Amino acid change or partial deletions
- constitutive TK activity
- GF independent signalling - Gene amplification and over-expression
- greater growth factor signal
