Familial adenomatous polyposis (FAP)

 

  1. A rare condition, usually due to a mutation in one of the two copies of a tumour suppressor gene called the adenomatous polyposis coli (APC) gene (see Table 1).
  2. Without treatment, those with proven FAP have a lifetime risk of colorectal cancer of almost 100%.
  3. Individuals with a mutated APC gene usually develop hundreds of adenomas throughout the colon and rectum that may appear in the teenage years or in early adult life. If left untreated, one or more of these adenomas will progress to cancer, often at an early age, so prophylactic surgery must be considered.
  4. Pathological lesions may occur outside the large colon, such as upper GI cancer (especially of the duodenum), desmoid tumours and osteomas.
  5. Inheritance of a mutated APC gene follows an autosomal dominant pattern. Sometimes there is no family history because a new mutation has occurred around the time of conception. This happens in 20 to 30% of cases.
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MUTYL-associated polyposis (MAP)

 

  1. A rare condition similar to FAP.
  2. Follows a pattern of autosomal recessive inheritance.
  3. Due to mutations in both copies of the base excision repair MUTYH gene (see Table 1).
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Management

  1. Flexible sigmoidoscopy yearly or second yearly starting from age 12 to 15 years until polyposis develops, then prophylactic colectomy.
  2. If family genetic testing is inconclusive and no polyposis develops, sigmoidoscopy reduced to every
    3 years after the age of 35 years, then change to population screening if examinations normal to age 55 years.
  3. Prophylactic surgery, eg restorative proctocolectomy, is appropriate for those with proven FAP.
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