Most scientists believe that cancer happens when several genes of a particular group of cells become mutated. Some people may have more inherited mutations than others, and even with the same amount of environmental exposure, some people are simply more likely to develop cancer.
The following types of genes contribute to cancer:
- Tumor suppressor genes are protective genes. Normally, they suppress (limit) cell growth by monitoring the rate at which cells divide into new cells, repairing mismatched DNA (a cause of mutations), and controlling cell death. When a tumor suppressor gene is mutated (due to heredity or environmental factors), cells continue to grow and can eventually form a tumor. BRCA1, BRCA2, and p53 are examples of tumor suppressor genes. In fact, nearly 50% of all cancers involve a missing or damaged p53 gene.
- Oncogenes turn a healthy cell into a cancerous one. HER2/neu and ras are two common oncogenes.
- DNA repair genes fix any mistakes made when DNA is replicated (copied). Mistakes that aren’t fixed become mutations, which may eventually lead to cancer, especially if the mutation occurs in a tumor suppressor gene or oncogene.
DNA Repair Genes
A third type of gene associated with cancer is the group involved in DNA repair and maintenance of chromosome structure. Environmental factors, such asionizing radiation, UV light, and chemicals, can damage DNA. Errors in DNA replication can also lead to mutations. Certain gene products repair damage to chromosomes, thereby minimizing mutations in the cell. When a DNA repair gene is mutated its product is no longer made, preventing DNA repair and allowing further mutations to accumulate in the cell. These mutations can increase the frequency of cancerous changes in a cell. A defect in a DNA repair gene called XP (Xeroderma pigmentosum) results in individuals who are very sensitive to UV light and have a thousand-fold increase in the incidence of all types of skin cancer. There are seven XP genes, whose products remove DNA damage caused by UV light and other carcinogens. Another example of a disease that is associated with loss of DNA repair is Bloom syndrome, an inherited disorder that leads to increased risk of cancer, lung disease, and diabetes. The mutated gene in Bloom syndrome, BLM, is required for maintaining the stable structure of chromosomes. Individuals with Bloom syndrome have a high frequency of chromosome breaks and interchanges, which can result in the activation of oncogenes.