Familial Adenomatous Polyposis (FAP)

Key Features

Essentials of Diagnosis

• Inherited condition characterized by early development of hundreds to thousands of colonic adenomatous polyps and adenocarcinoma

• Variety of extracolonic manifestations including

  • Duodenal adenomas

  • Desmoid tumors

  • Osteomas

  • Extracolonic cancers (stomach, duodenum, thyroid)

• Attenuated variant with fewer than 100 colonic adenomas

• Genetic testing confirms mutation of adenomatous polyposis coli (APC) gene (90%) or MYH gene (8%)

• Prophylactic colectomy recommended to prevent otherwise inevitable colon cancer

General Considerations

• Classic form of familial adenomatous polyposis (FAP) characterized by development of hundreds to thousands of colonic adenomatous polyps and variety of extracolonic manifestations

• Of patients with classic FAP

  • Approximately 90% have a mutation in the APC gene that is inherited in an autosomal dominant fashion

  • 8% have mutations in the MUTYH gene that are inherited in an autosomal recessive fashion

• FAP arises de novo in 25% of patients in the absence of genetic mutations in the parents

• An attenuated variant of FAP also has been recognized in which an average of only 25 polyps (range of 1–100) develop

Demographics

• Affects 1:10,000 people

• Accounts for 0.5% of colorectal cancers

Clinical Findings

Differential Diagnosis

• Sporadic colorectal cancer

• Inflammatory bowel disease with multiple inflammatory polyps

• Other nonadenomatous polyposis syndromes: Peutz-Jeghers syndrome, juvenile polyposis

• Hereditary nonpolyposis colorectal cancer (Lynch syndrome) also is an inherited autosomal dominant condition associated with early-onset colorectal cancer but few adenomatous polyps

Diagnosis

Diagnostic Procedures

• Genetic counseling and testing should be offered to

  • Patients found to have multiple adenomatous polyps at endoscopy

  • First-degree family members of patients with FAP

• First genetic test is sequencing the APC gene to identify disease-associated mutations

• APC gene mutations are identified in 80% of patients with more than 1000, and 56% with 100–1000 polyps (ie, the classic phenotype of FAP)

• Guidelines also recommend that genetic testing be considered in individuals with as few as 10 adenomas to exclude a diagnosis of attenuated disease

• In patients with no detachable APC mutations, MUTYH mutational assessment is next step

• APC or MUTYH mutations are found in 10% and 7%, respectively, of patients with 20–100 adenomas and 5% and 4%, respectively, of those with 10–19 adenomas

Treatment

Medications

• Sulindac and celecoxib have been shown to decrease the number and size of polyps in the rectal stump but not the duodenum

Surgery

• Complete proctocolectomy with ileoanal pouch anastamosis or colectomy with ileorectal anastamosis is recommended after the development of polyposis, usually before age 20

• Ileorectal anastamosis affords superior bowel function but has 5% risk of rectal cancer

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