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Inherited Colorectal Cancer

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What Is the FAP Gene?

Genes are tiny segments of DNA that control how cells function, such as telling them when to divide and grow. One copy of each gene comes from your mother; the other comes from your father.

In 1991, researchers made a significant breakthrough in the diagnosis of FAP. They identified the gene - called APC - that is responsible for the condition. This gene mutation can be detected in 82% of patients with FAP. The exact lifetime risk of developing colon cancer in people who have inherited this gene abnormality is about 100%. Families in which this gene mutation occurs may or may not have one or more family members with colorectal cancer or polyps.

What Causes Gene Mutations to Occur?

Gene mutations, or changes in the structure of a gene, happen for several reasons. Chemicals in stools, or feces, can cause mutations in the colon and rectum. These chemicals are found in the food we eat, or are produced from these foods during the process of digestion. Mutations are likely to occur in the cells lining the colon and rectum, because stool usually stays in the large intestine, or colon, for 24-36 hours, allowing time for cancer-causing chemicals, called carcinogens, to have an effect. In comparison, it only takes 2 to 4 hours for stool to get through the small intestine, making cancer extremely rare in this area.

This helps explain why certain foods may promote colon cancer and others may protect against it. For example, it is believed that red meat and animal fat may cause cancer, while fiber may not only speed up the passage of stool through the colon, but may be turned into chemicals that prevent cells from growing out of control.

What Is the Difference Between FAP and HNPCC?

There are 2 main differences between FAP and HNPCC, and they include:

  1.  Number of genes mutated. In FAP, there is only one gene, the APC gene, which is abnormal. In HNPCC, there are several gene, mutations which may be responsible for the development of the condition.
  2. Presence of polyps, or growths that can become cancer. FAP is characterized by the presence of more than 100 benign polyps. In HNPCC, affected people have fewer polyps, but these polyps can become cancerous more quickly than normal.

Other Forms of Inherited Polyposis Syndromes

There are other, very rare forms of inherited polyposis syndromes that are associated with an increased risk of colorectal cancer. These include:

  •  Juvenile polyposis (JP). Patients may have anywhere from 5 to 500 polyps, or growths, mostly in the colon and rectum, which usually begins before the age of 10. The stomach and small intestine are less commonly affected. These patients are also at increased risk of bowel cancer.
  • Peutz-Jehgers syndrome (PJS). Patients with PJS typically develop dozens to thousands of benign polyps, or growths, in the stomach and intestines, primarily in the small intestine. The growths can become malignant or can cause obstruction of the bowel.

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