Oncogenes

2) Oncogenes

Oncogenes vs proto-oncogenes? –
 Proto-oncogene = NORMAL unmutated gene which
regulates cell proliferation = GOOD ANGEL
 Oncogene = arises from a gain-of-function mutation
 over-activates genes involved in cell proliferation
= EVIL DEMON



1) The discovery of src (retroviral transfer) –
 1941 – Rous transformed normal cells  tumour cells
 In culture by infecting them with a virus
 Varmus + Bishop showed src gene was originally an avian cellular
gene and was ‘stolen’ by the RSV virus & turned into an oncogene
by a viral promotor


2) The discovery of RAS (mammalian cell transfection) –
 Begins with viruses in the 60s: HRAS + KRAS
 By the 80s it became possible to transfect DNA into mammalian cells without viral
vectors
 Could fragments of DNA from mammalian tumour cells transform normal cells into
cancer cells?

DNA transfection assay:
 DNA from tumour cell lines
 Transfected into mouse cultured cells
 Transformed cells identified by growth and loss of
contact inhibition
 Transformed cells produced tumours in athymic (nude)
mice

Multiple RAS genes exist:
1981-1984 – human homologues of HRAS, KRAS and
NRAS were discovered
HRAS = from DNA in EJ/T24 bladder
carcinoma line
KRAS = from DNA in lung and colon cancers
NRAS = from DNA in neuroblastomas

These oncogenes differed from the wildtype RAS gene by 1 single
point mutations

, 3) The discovery of the Philadelphia chromosome and ABL
(chromosomal translocations) –
 1960 – Peter Nowell and his PhD student described an unusual
small chromosome
 It was present in leukocytes from patients with chronic
myelogenous leukaemia (CML)
 Called it Philadelphia chromosome after its city of discovery
 Present in 95% of cases of CML

Reciprocal translocation:
 Early 1980s – 3 labs identified Philadelphia chromosome was: a reciprocal
translocation between Chr 9 and 22
 Translocation effects ABL gene on Chr 9 & fuses it to BCR gene on Chr 22
 BCE-ABL fusion = ONCOGENE


4) MYC (oncogene amplification) –
 1977- Discovered by Gilbert in neuroblastoma cell lines
 Double minute chromosomes are ‘small chromatin bodies whose origin and function
are uncertain’
 In neuroblastoma (up to 80%) they exist in multiple copies and contain the gene N-
MYC

5) More up to date: IDH1 (Next Generation DNA Sequencing) –
 Discovered by Kenneth Kinnzler
 By lab sequencing 20,611 protein coding genes from 22 human glioblastoma (GBS)
tumour samples
 In 5 of the tumours they identified a mutated gene not previously known to be
mutated in GBS = IDH1
 All 5 tumours had the same point mutation
 IDH1 has been found to be mutated in many types of glioma, colon cancer, and AML

Other oncogenes identified by Next Generation DNA Sequencing:
 Study involving IDH1 took around 3 years – compared with >60 for SRC
 Almost all future oncogenes are likely to be discovered this way
 Systemic sequencing of cancer genomics is now underway in many labs;
o The Cancer Genome Project
o Genomics England, 100,000 genomes project
o Other oncogenes found this way include: IDH2, EZH2


Summary: Oncogenes discovered via –
1) Retroviruses
2) Transfection (tumour  normal)
3) Chromosomal translocations
4) Chromosomal amplifications
5) New genetic technologies