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Colon Cancer

Developing Cancer, Major Genes Involved, Familial Adenomatous Polyposis (fap), Hereditary Nonpolyposis Colon CancerOther Risk Factors

Colon cancer is the second leading cause of cancer death in the United States, occurring in approximately 5 percent of the population and resulting in roughly 55,000 deaths annually. New cases of colorectal cancer are diagnosed in approximately 90 per 100,000 people annually. The majority of cases occur in individuals older than age fifty. Of those who suffer from colorectal malignancy, an estimated 40 percent will die from the disease.

Colon cancer-related health-care costs, consisting of outpatient visits, hospitalizations, hospice and home health care, medications, and physician services, exceed $5 billion per year. This figure does not include the indirect costs of wages lost and reduced productivity.

A colonic polyp, or visible growth, is created by the overgrowth of colonic epithelial cells. Such uncontrolled cell division is not always cancerous, but may become malignant. DNA mutations of the colonic epithelial cells—the cause of the overgrowth—may be hereditary.


The Adenomatous Polyposis Coli (APC) gene is found on chromosome 5 and is a tumor suppressor gene. Both alleles of the gene must be inactivated for tumor growth to occur. In the normal cell, the APC gene plays a role in regulating the cycle of cellular division and replication, as well as in cell-to-cell communication, thereby suppressing tumor development. Mutations of APC result in a loss of gene function, thus allowing unregulated cellular proliferation. APC mutations are found in the majority of common colon polyps and cancers and in patients with FAP, and they may be one of the earliest genetic alterations in the adenoma-carcinoma sequence.


The K-ras gene plays an active role in cellular signaling and promoting cell growth. The normal gene exists in both an active and inactive form. However, in the abnormal state, the active form predominates and results in a continually growth-stimulated state.


The normal p53 gene is responsible for regulating cells with damaged DNA by directing abnormal cells either to halt the cycle of cell division or to die as the result of a process called apoptosis. Like APC, the p53 gene is a tumor suppressor. With the p53 mutation, the gene no longer functions, and this permits the uninhibited proliferation of cells that may have damaged DNA. p53 mutations are seen in more than half of colorectal cancers.

In addition to the well-described genetic syndromes of FAP and HNPCC, other factors that place an individual at increased risk include a personal or family history of colon cancer and the presence of inflammatory bowel disease (e.g., ulcerative colitis and Crohn's disease). Population studies support an association between the development of colon cancer and a high-fat, low-fiber diet, although a cause-and-effect relationship has not been proved.

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Medicine EncyclopediaGenetics in Medicine - Part 1