The discovery of certain types of genes that contribute to the development of cancer has been an extremely important milestone for cancer research. More than 90 percent of cancers are believed to have some type of genetic alteration (or mutation). Some of these mutations are inherited, while others are random, which means they occur by chance or as a result of environmental exposures (usually over many years). There are three main types of genes that can affect cell growth and that are found to be mutated in certain cancers:
1. Oncogenes: These genes regulate the normal growth of cells. Oncogenes can be described as a cancer "switch." Something "flips the switch" that makes the oncogenes unable to control the normal growth of cells and allows abnormal cancer cells to begin to grow. Why this happens is not known.
2. Tumor Suppressor Genes: Tumor suppressor genes are genes that are able to recognize when damaged cells are growing and reproducing abnormally (e.g. cancer cells). Normally functioning tumor suppressor genes will then interrupt the reproduction of damaged cells until the damage is corrected. However, if the tumor suppressor genes are mutated they will not function properly and tumor growth may occur.
3. Mismatch Repair Genes: Mismatch repair genes help recognize errors when DNA is copied to make a new cell. If the DNA does not "match" perfectly, these genes repair the mismatch and correct the error. If these genes are not working properly, however, errors in DNA can be transmitted to new cells, causing them to be damaged. Usually the number of cells in any of our body tissues is tightly controlled so that new cells are made for normal growth and development, as well as to replace dying cells. Ultimately, cancer is a loss of this balance due to genetic alterations that "tip the balance" in favor of uncontrolled cell growth.