What are Oncogenes?
An oncogene is any gene which is instrumental in the development of cancer. There are several different types of oncogenes, classified according to the way in which they activate to contribute to cancer development.
Oncogenes contribute to cancer development because they mutate in a way that changes their function, or increases the rate at which they are expressed as proteins. These are normal genes which, when mutated for any reason, can turn a cell cancerous. These genes are called proto-oncogenes. The gene is referred to as a proto-oncogene in its normal unmutated state, and as an oncogene when it mutates.
For a more detailed explanation of how oncogenes can cause cancer, read this BrightHub article.
Examples of Oncogenes
HER-2/neu (erbB-2): This gene codes for a protein called human epidermal growth factor receptor type 2. The protein is located on the surface of certain cell types, and helps control the response of cells to proteins called growth factors. When growth factors come into contact with the receptor, the cell is stimulated to begin dividing more rapidly.
In up to thirty percent of breast cancers, this gene is expressed at a higher level than in normal cells. This means the breast cancer cells have more of the growth factor receptor on their surfaces. As a result, they respond more strongly to growth factors, and begin to divide constantly, and spread to other parts of the body readily.
hTERT: Chromosomes become slightly shorter each time a cell divides. This shortening of chromosomes limits the number of times a cell can divide before it becomes non-viable. At this point, the cell dies. Many types of cancer can develop as a result of the abnormal activation of a gene called hTERT, which codes for a protein enzyme called telomerase. In fetal cells, this enzyme prevents telomeres from shortening, but the hTERT gene is inactive in adult cells. If the gene becomes active in adulthood, cancer is the result, as telomerase prevents telomeres shortening, and promotes overgrowth of the cell.
myc: Mutant forms of this gene are known to cause several types of cancers, including B-cell leukemia, Burkitt’s lymphoma, and some forms of lung cancer. The myc protein is a transcription factor, which means it is involved in the process by which DNA is transcribed into messenger RNA.
ras: Ras gene products are involved in signaling pathways that help control transcription of genes that are involved in cell growth and differentiation. When the ras gene is altered, its protein product is also altered, and it can no longer turn off the regulatory switch that controls cell growth. Overexpression or mutation of the ras gene can therefore lead to uncontrolled cell growth, and the development of cancer. Mutations in the ras gene have been found in many types of cancer, including pancreatic, lung, thyroid, bladder, and ovarian cancers.
src: In certain types of breast, colon, and lung cancers, the src gene is overexpressed. The src gene product is a type of protein called a tyrosine kinase, and is involved in cell regulation. Mutations in this gene can cause cells to begin dividing without restraint. Src was the first oncogene to be discovered, when it was identified as a cancer-causing gene in chickens.
American Cancer Society: Oncogenes, Tumor Suppressor Genes, and Cancer
Carlo M. Croce, M.D: Oncogenes and Cancer In New England Journal of Medicine, January 31, 2008.
Emory University: Important Oncogenes
Heidi Chial, Ph.D: Proto-oncogenes to Oncogenes to Cancer