Causes

In most cases of colon or rectal cancers the cause or causes are unknown. Defects in genes that normally protect against cancer play the major role in causing polyp cells to continously spread and become cancerous. Some of these cases are caused by inherited genetic defects, and such patients usually have family histories of colorectal cancer. Most of genetic mutations involved in colon cancers, however, appear to arise spontaneously (no strong family history) rather than being inherited. In such cases, environmental or other factors trigger genetic changes in the intestine that lead to cancer.

Inherited Genetic Factors

About 6% of cases of colon cancer are due to inherited facts.

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APC Gene and Familial Adenomatous Polyposis (FAP). When the adenomatous polyposis coli (APC) gene is normal, it helps suppress tumor growth. In its defective form, it permits high levels of the protein beta-catenin to accumulate, which accelerates cell growth leading to polyps. Various genetic mutations that affect the APC gene directly or indirectly have been identified:

• Familial adenomatous polyposis (FAP) is a rare and serious disorder in which the patient inherits an APC mutation from either parent. It occurs in about 1 in 8,000 people. During early adulthood, hundreds to thousands of polyps grow in the colon. FAP causes less than 1% of all cases of colorectal cancer, but if untreated, virtually everyone who inherits this condition develops cancer before the age of 40. Many of the deaths attributed to FAP can be prevented with early and aggressive surgical treatment.
• Non-inherited mutations of the APC gene have been detected in nearly all patients with spontaneous colon cancers.