Familial Adenomatous Polyposis

Diagnosis

FAP is often diagnosed when patients are identified with many (sometimes hundreds) of adenomatous polyps in their colon (see colon polyps). Unfortunately, this may not happen until cancer has already developed.

For people known to be at risk for the disorder, screening should begin in the early teenage years. Read more about screening methods for colon cancer.

FAP is also sometimes diagnosed with genetic testing, using a blood sample. Genetic testing is complicated, but can be helpful to:

• Establish a diagnosis in someone who has some, but not all, of the features of FAP (for example, someone who is young and has polyps but not in the large numbers typical of FAP)
• Identify which members in a family with a history of FAP carry the mutation so they get early evaluation and treatment
• Make a prenatal diagnosis of FAP

Genetic counseling is recommended prior to undergoing genetic testing. Genetic counselors meet with patients to discuss:

• The likelihood of finding a gene mutation
• Accuracy of genetic testing
• Medical usefulness of identifying a genetic mutation
• Medical interpretation of not finding a mutation
• Costs, waiting time for a result, and implications for insurance coverage
• Possible psychological reactions to genetic testing
• Implications for cancer prevention for patients and relatives

Genetic Testing

Genetic testing is usually done in two phases: in the first, mutation scanning, the gene is examined to see if it looks or behaves differently from normal APC (adenomatous polyposis coli) genes. If it does, direct sequencing is performed to find the region of the gene where the mutation has occurred.

In about 10 to 20 percent of people with the clinical features of FAP (large numbers of adenomatous polyps) mutation scanning does not find mutations; in this case, direct sequencing is not performed. This doesn't mean FAP is not present, but rather that current testing methods aren't able to detect the mutation.

Two other methods used at Mayo to find genetic mutations are linkage analysis, an indirect way of identifying differences by comparing gene sequences; and duplication/deletion analysis, used to identify duplicate or missing pieces of genes.