Most normal cells undergo a programmed form of death (apoptosis). Activated oncogenes can cause those cells designated for apoptosis to survive and proliferate instead. Most oncogenes require an additional step, such as mutations in another gene, or environmental factors, such as viral infection, to cause cancer. Since the 1970s, dozens of oncogenes have been identified in human cancer. Many cancer drugs target the proteins encoded by oncogenes.
The first confirmed oncogene was discovered in 1970 and was termed src (pronounced sarc as in sarcoma). Src was in fact first discovered as an oncogene in a chicken retrovirus. Experiments performed by Dr. G. Steve Martin of the University of California, Berkeley demonstrated that the Src was indeed the oncogene of the virus. The first nucleotide sequence of v-src was sequenced in 1980 by A.P. Czernilofsky et al.
In 1976 Drs. Dominique Stehelin, J. Michael Bishop and Harold E. Varmus of the University of California, San Francisco demonstrated that oncogenes were activated proto-oncogenes, found in many organisms including humans. For this discovery, proving Todaro and Heubner's "oncogene theory", Bishop and Varmus were awarded the Nobel Prize in Physiology or Medicine in 1989.
A proto-oncogene is a normal gene that can become an oncogene due to mutations or increased expression. The resultant protein may be termed as oncoprotein. Proto-oncogenes code for proteins that help to regulate cell growth and differentiation. Proto-oncogenes are often involved in signal transduction and execution of mitogenic signals, usually through their protein products. Upon activation, a proto-oncogene (or its product) becomes a tumor-inducing agent, an oncogene. Examples of proto-oncogenes include RAS, WNT, MYC, ERK, and TRK. The MYC gene is implicated in Burkitt's Lymphoma, which starts when a chromosomal translocation moves an enhancer sequence within the vicinity of the MYC gene. The MYC gene codes for widely used transcription factors. When the enhancer sequence is wrongly placed, these transcription factors are produced at much higher rates. Another example of an oncogene is the Bcr-Abl gene found on the Philadelphia Chromosome, a piece of genetic material seen in Chronic Myelogenous Leukemia caused by the translocation of pieces from chromosomes 9 and 22. Bcr-Abl codes for a receptor tyrosine kinase, which is constitutively active, leading to uncontrolled cell proliferation. (More information about the Philadelphia Chromosome below)
The proto-oncogene can become an oncogene by a relatively small modification of its original function. There are three basic methods of activation:
- A mutation within a proto-oncogene, or within a regulatory region (for example the promoter region), can cause a change in the protein structure, causing
- An increase in the amount of a certain protein (protein concentration), caused by
- A chromosomal translocation (another type of chromosome abnormality)
- There are 2 different types of chromosomal translocations that can occur:
- translocation events which relocate a proto-oncogene to a new chromosomal site that leads to higher expression
- translocation events that lead to a fusion between a proto-oncogene and a 2nd gene (this creates a fusion protein with increased cancerous/oncogenic activity)
- the expression of a constitutively active hybrid protein. This type of mutation in a dividing stem cell in the bone marrow leads to adult leukemia
- Philadelphia Chromosome is an example of this type of translocation event. This chromosome was discovered in 1960 by Peter Nowell and David Hungerford, and it is a fusion of parts of DNA from chromosome 22 and chromosome 9. The broken end of chromosome 22 contains the "BCR" gene, which fuses with a fragment of chromosome 9 that contains the "ABL1" gene. When these two chromosome fragments fuse the genes also fuse creating a new gene: "BCR-ABL". This fused gene encodes for a protein that displays high protein tyrosine kinase activity (this activity is due to the "ABL1" half of the protein). The unregulated expression of this protein activates other proteins that are involved in cell cycle and cell division which can cause a cell to grow and divide uncontrollably (the cell becomes cancerous). As a result, the Philadelphia Chromosome is associated with Chronic Myelogenous Leukemia (as mentioned before) as well as other forms of Leukemia.
The expression of oncogenes can be regulated by microRNAs (miRNAs), small RNAs 21-25 nucleotides in length that control gene expression by downregulating them. Mutations in such microRNAs (known as oncomirs) can lead to activation of oncogenes. Antisense messenger RNAs could theoretically be used to block the effects of oncogenes.
There are several systems for classifying oncogenes, but there is not yet a widely accepted standard. They are sometimes grouped both spatially (moving from outside the cell inwards) and chronologically (parallelling the "normal" process of signal transduction). There are several categories that are commonly used:
|Growth factors, or mitogens||c-Sis||glioblastomas, fibrosarcomas, osteosarcomas, breast carcinomas, and melanomas||induces cell proliferation.|
|Receptor tyrosine kinases||epidermal growth factor receptor (EGFR), platelet-derived growth factor receptor (PDGFR), and vascular endothelial growth factor receptor (VEGFR), HER2/neu||Breast cancer, gastrointestinal stromal tumours, non-small-cell lung cancer and pancreatic cancer||transduce signals for cell growth and differentiation.|
|Cytoplasmic tyrosine kinases||Src-family, Syk-ZAP-70 family, and BTK family of tyrosine kinases, the Abl gene in CML - Philadelphia chromosome||colorectal and breast cancers, melanomas, ovarian cancers, gastric cancers, head and neck cancers, pancreatice cancer, lung cancer, brain cancers, and blood cancers||mediate the responses to, and the activation receptors of cell proliferation, migration, differentiation, and survival |
|Cytoplasmic Serine/threonine kinases and their regulatory subunits||Raf kinase, and cyclin-dependent kinases (through overexpression).||malignant melanoma, papillary thyroid cancer, colorectal cancer, and ovarian cancer||Involved in organism development, cell cycle regulation, cell proliferation, differentiation, cells survival, and apoptosis |
|Regulatory GTPases||Ras protein||adenocarcinomas of the pancreas and colon, thyroid tumors, and myeloid leukemia||involved in signalling a major pathway leading to cell proliferation.|
|Transcription factors||myc gene||malignant T-cell lymphomas and acute myleoid leukemias, breast cancer, pancreatic cancer, retinoblastoma, and small cell lung cancer ||-They regulate transcription of genes that induce cell proliferation.|
More detailed information for the above Table:
- Growth factors are usually secreted by either specialized or not specialized cells to induce cell proliferation in themselves, nearby cells, or distant cells. An oncogene may cause a cell to secrete growth factors even though it does not normally do so. It will thereby induce its own uncontrolled proliferation (autocrine loop), and proliferation of neighboring cells. It may also cause production of growth hormones in other parts of the body.
- Receptor Tyrosine Kinases add phosphate groups to other proteins to turn them on or off. Receptor kinases add phosphate groups to receptor proteins at the surface of the cell (which receive protein signals from outside the cell and transmit them to the inside of the cell). Tyrosine kinases add phosphate groups to the amino acid tyrosine in the target protein. They can cause cancer by turning the receptor permanently on (constitutively), even without signals from outside the cell.
- Ras is a small GTPase that hydrolyses GTP into GDP and phosphate. Ras is activated by growth factor signaling (i.e., EGF, TGFbeta) and acting like a binary switch (on/off) in growth signaling pathways. Downstream effectors of Ras include Raf, MEK, MEKK, MAPK, ERK, most of which in turn regulate genes that mediate cell proliferation.
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