Familial adenomatous polyposis (FAP) accounts for less than 1 percent of all colorectal cancers and can lead to multiple polyps in the colon and rectum. These growths usually start in adolescence and increase in number over time, sometimes to hundreds or even thousands of polyps.
A mutation in your APC gene causes FAP. The APC gene normally prevents the uncontrolled cell growth that leads to cancer. Most patients have a family history of the condition, but up to 25 percent have none. If you have a parent who had FAP, you have a 50 percent chance of developing the condition. Patients may also be at risk for stomach and small intestine polyps, as well as other types of tumors.
If left untreated, FAP may lead to colon or rectal cancer before age 40. Attenuated familial adenomatous polyposis (AFAP) is a milder form of FAP. A mutation in the APC gene causes this condition as well, but AFAP patients have fewer polyps than FAP patients — usually less than 100. Polyps and colorectal cancer start in AFAP patients at an average age of 44 and 56, respectively. An AFAP patient’s exact lifetime colorectal cancer risk isn’t known, but it seems to be higher than in people without the disease.