Table 4. Absolute Risks of Colorectal Cancer for Mutation Carriers in Hereditary Colorectal Cancer Syndromes continued...
Familial Adenomatous Polyposis (FAP)
FAP is one of the most clearly defined and well understood of the inherited colon cancer syndromes.[1,9,10] It is an autosomal dominant condition, and the reported incidence varies from 1 in 7,000 to 1 in 22,000 live births, with the syndrome being more common in Western countries. Autosomal dominant inheritance means that affected persons are genetically heterozygous, such that each offspring of a patient with FAP has a 50% chance of inheriting the disease gene. Males and females are equally likely to be affected.
Classically, FAP is characterized by multiple (>100) adenomatous polyps in the colon and rectum developing after the first decade of life. Variant features in addition to the colonic polyps may include polyps in the upper gastrointestinal (GI) tract, extraintestinal manifestations such as congenital hypertrophy of retinal pigment epithelium, osteomas and epidermoid cysts, supernumerary teeth, desmoid formation, and other malignant changes such as thyroid tumors, small bowel cancer, hepatoblastoma, and brain tumors, particularly medulloblastoma (see Table 5).
Table 5. Extracolonic Tumor Risks in Familial Adenomatous Polyposis
|Malignancy||Relative Risk||Absolute Lifetime Risk (%)|
|Adapted from Giardiello et al.,Jagelman et al.,Sturt et al.,Lynch et al.,Bülow et al.,Burt et al.,and Galiatsatos et al.|
|a The Leeds Castle Polyposis Group.|
FAP is also known as familial polyposis coli, adenomatous polyposis coli (APC), or Gardner syndrome (colorectal polyposis, osteomas, and soft tissue tumors). Gardner syndrome has sometimes been used to designate FAP patients who manifest these extracolonic features. However, Gardner syndrome has been shown molecularly to be a variant of FAP, and thus the term Gardner syndrome is essentially obsolete in clinical practice.
Most cases of FAP result from mutations of the APC gene on chromosome 5q21. Individuals who inherit a mutant APC gene have a very high likelihood of developing colonic adenomas; the risk has been estimated to be more than 90%.[1,9,10] The age at onset of adenomas in the colon is variable: By age 10 years, only 15% of FAP gene carriers manifest adenomas; by age 20 years, the probability rises to 75%; and by age 30 years, 90% will have presented with FAP.[1,9,10,20,21] Without any intervention, most persons with FAP will develop colon or rectal cancer by the fourth decade of life.[1,9,10] Thus, surveillance and intervention for APC gene mutation carriers and at-risk persons have conventionally consisted of annual sigmoidoscopy beginning around puberty. The objective of this regimen is early detection of colonic polyps in those who have FAP, leading to preventive colectomy.[22,23]