What Is a Genetic Testing for Colon Cancer?

Only 5% of all colon cancers occur in families with a history of colon cancer and identifiable genetic defects.

An individual is likely to belong to a family with FAP (Familial Adenomatous Polyposis) if he or she has more than 100 adenomatous colon polyps or is a first-degree relative (parent, sibling, or child) of a person who has more than 100 adenomatous colon polyps. The number of polyps is less in some families, a condition referred to as attenuated FAP. Therefore, individuals who have between 20 and 100 adenomatous colon polyps or are first-degree relatives of individuals with 20 to100 adenomatous colon polyps also may belong to a family with FAP.

There has been much excitement during the past decade because of the identification of abnormal or defective genes (mutations) associated with colon cancer in families where colon cancer is common. When a defective gene can be identified, it is possible to examine other members of the family to see if they also carry the defective gene. Those individuals who carry the defective gene are at a very high risk (75%-100%) for developing colon cancer. The reason for the excitement is that if an individual is found to have the defective gene, his or her colon can be carefully monitored and then removed before the cancer occurs.

Only 5% of all colon cancers occur in families with a history of colon cancer and identifiable genetic defects. Therefore, genetic testing as it exists today is useful for only a minority of the about 130,000 people each year who are destined to develop colon cancer. Nevertheless, genetic testing is important because the risk is so extremely high among individuals who are found to have the genetic defect. In addition, more defective genes are likely to be found during the next few years, and this will make genetic testing valuable for an increasing number of individuals who will develop colon cancer.

At present, there are two types of familial colon cancer in which defective genes can be identified. One type of cancer is associated with a strong family history of colon polyps. The other type of colon cancer is not associated with a family history of colon polyps. The polyp-associated cancerous disease is called familial adenomatous polyposis (FAP). (Adenomatous polyps are a type of polyp that have the potential to become cancerous.) The nonpolyp-associated cancerous disease is called hereditary nonpolyposis colorectal cancer (HNPCC).

Almost all mutations that cause FAP occur in one gene, referred to as the APC gene. If an individual has FAP, that family member should have their APC gene examined for mutations. If a mutation is found, the same mutation can be sought in other family members. If a family member has the same mutation, then he or she will probably develop colon cancer. If that person does not have the mutation, he or she is not at an increased risk for colon cancer. If there is no member in the family who clearly has FAP and can undergo genetic testing, then genetic testing has little value for other family members.

Mutations that cause HNPCC occur in several different genes. If an individual in a family suspected of being in an HNPCC family has colon cancer, tissue from the cancer can be examined to determine if a mutation is present. If there is a mutation, then other family members can be examined for the mutation. If they have the mutation, then they probably will develop colon cancer. If they do not have the mutation, they are not at increased risk for colon cancer. If there is no member in the family with colon cancer and an identifiable mutation, then genetic testing of family members has little value.

An individual is likely to belong to a family with HNPCC and require genetic testing if:

• three or more relatives have had colon cancer (or another cancer associated with HNPCC such as uterine, small bowel, urethral, or renal pelvic cancer) and at least one of the relatives is a first-degree relative,
• two or more generations of the family have colon cancer, or
• one or more relatives were diagnosed with colon cancer before age 50.

These criteria for identifying HNPCC are referred to as the Amsterdam II Criteria. The Amsterdam II Criteria have been modified in order to identify additional individuals who should undergo genetic testing for HNPCC. These include people with:

• two or more colon cancers,
• with colon cancer and a first-degree relative with colon cancer or another cancer associated with HNPCC before age 50, or an adenomatous colon polyp before the age of 40,
• colon or uterine cancer before age 50, and
• an adenomatous colon polyp before age 50. The expanded Amsterdam II Criteria are referred to as the modified Bethesda Criteria.