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An attenuated form of familial adenomatous polyposis (AFAP) has been identified. Individuals with AFAP develop fewer than 100 adenomatous polyps (average of 30 polyps) and the polyps are more likely to occur on the right side of the colon than in classic FAP. The risk of developing colon cancer is increased, but the average age of diagnosis is older (50 to 55 years of age) than in the classic form. Some of the other health problems associated with classic FAP also occur in the attenuated form, however, cases of congenital hypertrophy of the retinal pigment (CHRPE) and desmoid tumors are rarely seen.
Mutations in three specific areas of the APC gene have been associated with the AFAP phenotype. The polyp burden (number of polyps developed) and the risk of other FAP manifestations varies depending on in what area the mutation is located. One of these three areas of the gene is associated with a very variable number of polyps (ranging from two to 500). In families with such mutations, some relatives may have the AFAP phenotype whereas others have classic FAP.
Unlike FAP, not all persons with an AFAP-associated APC mutation will develop polyps, and the lifetime chance to develop polyps is unknown. The incidence of AFAP is also unknown, but thought to be about the same or less than classic FAP.