Familial adenomatous polyposis is a rare, inherited condition that causes extra tissue (polyps) to form in your large intestine and in the upper part of your small intestine (duodenum). If untreated, the polyps in the large intestine almost always become cancerous by age 40.
Most people with familial adenomatous polyposis eventually need surgery to remove the upper part of the large intestine (colon) to prevent cancer. The polyps in the duodenum can also develop cancer, but they can usually be managed by removing them regularly.
Familial adenomatous polyposis is caused by a defect in the adenomatous polyposis coli (APC) gene. Most people inherit the genetic abnormality from a parent. But in about 30 percent of cases, the genetic mutation occurs spontaneously at conception.
The abnormal gene causes hundreds or even thousands of polyps to grow in your colon and rectum, usually starting by your midteens. The polyps are nearly 100 percent certain to develop into colon cancer or rectal cancer by the time you are 40.
Familial adenomatous polyposis can cause other complications:
- Duodenal polyps. These polyps grow in the upper part of your small intestine and may become cancerous. But with careful monitoring, duodenal polyps can often be detected and removed before cancer develops.
- Periampullary polyps. These polyps arise in the duodenum, close to where the bile and pancreas ducts enter it (ampulla). Periampullary polyps also may become cancerous but can often be detected and removed before cancer develops.
- Desmoids. These noncancerous masses can arise anywhere in the body, but often develop in the stomach area (abdomen). Desmoids can cause serious problems if they grow into nerves or blood vessels or exert pressure on other organs in your body.
- Other cancers. Rarely, familial adenomatous polyposis can cause cancer to develop in your thyroid gland, central nervous system, adrenal glands, liver or other organs.
- Noncancerous skin tumors.
- Noncancerous bone tumors.
- Pigment changes in the retina of your eye.
- Dental abnormalities.
Familial adenomatous polyposis has these variations:
- Attenuated familial adenomatous polyposis causes fewer polyps than classic familial adenomatous polyposis, usually less than 100. You have a high likelihood of developing colon cancer by age 60 if the polyps are not removed.
- Gardner's syndrome is a form of familial adenomatous polyposis that also causes bony and dental growths.
MYH-associated polyposis (MAP)
This rare genetic disorder is caused by a different gene (the MYH gene), but has similar colon and duodenum risks. The number of polyps can vary, with up to 40 percent of people with MAP having 10 to 100 polyps, and up to 30 percent having more than 100 polyps. The risk of developing colon cancer is estimated to be 80 percent by age 70, if the polyps cannot be controlled with colonoscopy or if the colon and rectum are not removed.
At Mayo Clinic, people with MAP have colonoscopies every year, beginning at age 25.
Nov. 19, 2012