Inherited Colorectal Cancer
Familial Adenomatous Polyposis (FAP) Syndrome
Familial adenomatous polyposis (FAP) is a rare condition characterized by the presence of more than hundreds or even thousands of benign polyps, or growths in the large intestine. It is thought to be present in about 1% of all people diagnosed with colorectal cancer each year. The polyps occur early in life, with 95% of people with FAP developing polyps by age 35, and are often detected in patients in their teens, with 50% developing polyps by age 15. If the colon is not surgically removed, there is almost a 100% chance that some of the polyps will develop into cancer, usually by age 40.
While most cases of FAP are inherited, nearly a third of the cases are the result of a spontaneous (newly-occurring) gene mutation, or abnormality. For those who develop a new gene mutation, it is possible to pass the FAP gene onto their children.
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 increase the risk for cancer, while fiber may not only speed up the passage of stool through the colon, but may be turned into chemicals that help prevent cells from growing out of control.