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Familial adenomatous polyposis
Familial adenomatous polyposis (FAP) is characterized by the development of hundreds to thousands of adenomas in the rectum and colon during the second decade of life.
ORPHA:733Classification level: Disorder
FAP has a birth incidence of about 1/8,300, manifests equally in both sexes, and accounts for less than 1% of colorectal cancer (CRC) cases. In the EU, prevalence is estimated at 1/11,300-1/37,600.
Most patients are asymptomatic for years until the adenomas are large and numerous, and cause rectal bleeding or even anemia, or cancer develops. Generally, cancers start to develop a decade after appearance of the polyps. Nonspecific symptoms may include constipation or diarrhea, abdominal pain, palpable abdominal masses and weight loss. FAP may present some extraintestinal manifestations such as osteomas, dental abnormalities, congenital hypertrophy of the retinal pigment epithelium (CHRPE), desmoid tumors, and extracolonic cancers (thyroid, liver, bile ducts and central nervous system). A less aggressive variant of FAP, attenuated FAP (AFAP; see this term), is characterized by fewer colorectal adenomatous polyps (usually 10 to 100), later age of adenoma appearance and a lower cancer risk. Some lesions (skull and mandible osteomas, dental abnormalities, and fibromas on the scalp, shoulders, arms and back) are indicative of the Gardner syndrome, whereas the association of FAP and medulloblastoma is referred to as the Turcot syndrome (see these terms).
Classic FAP results from a germline mutation in the APC gene (5q21-q22). Most patients (about 70%) have a family history of colorectal polyps and cancer. In one patient subset, a MUTYH mutation (1p34.1) causes a recessively inherited polyposis condition, MUTYH-related familial adenomatous polyposis (see this term), which is characterized by a slightly increased risk of developing CRC and polyps/adenomas in both the upper and lower gastrointestinal tract.
Diagnosis is based on family history, clinical findings, and large bowel endoscopy or full colonoscopy. Whenever possible, the clinical diagnosis should be confirmed by genetic testing.
Differential diagnoses include other disorders causing multiple polyps (Peutz-Jeghers syndrome, familial juvenile polyposis or hyperplastic polyposis, hereditary mixed polyposis syndrome, and Lynch syndrome; see these terms).
Presymptomatic and prenatal (amniocentesis and chorionic villous sampling), and even preimplantation genetic testing, is possible. Referral to a geneticist or genetic counselor is mandatory.
Classic FAP is inherited in an autosomal dominant manner. When the APC mutation in the family has been identified, genetic testing of all first-degree relatives should be performed.
Management and treatment
Cancer prevention and maintaining a good quality of life are the main goals of management, and regular and systematic follow-up and supportive care should be offered to all patients. By the late teens or early twenties, colorectal cancer prophylactic surgery is advocated. The recommended alternatives are total proctocolectomy, and ileoanal pouch or ileorectal anastomosis for AFAP. Duodenal cancer and desmoids are the two main causes of mortality after total colectomy. They need to be identified early and treated. Upper endoscopy is necessary for surveillance to reduce the risk of ampullary and duodenal cancer. Patients with progressive tumors and unresectable disease may respond or stabilize with a combination of cytotoxic chemotherapy and surgery. Celecoxib has received marketing authorization by the US Food and Drug Administration (FDA) and the European Medicines Agency to be used as an adjunctive therapy in patients with FAP.
Patients with FAP carry a 100% risk of CRC. However, this risk is reduced significantly when patients enter a screening-treatment program.