Genetic testing for colon cancer: Should you consider it?

If a member of your family has been diagnosed with colon cancer, you might be concerned about your chances of also developing the disease. You might even be considering genetic testing for colon cancer. Using genetic testing to determine whether you're at increased risk of colon cancer may help you make decisions about your health care.

But keep in mind that inherited forms of colon cancer are rare, accounting for only about 5 percent of these cancers. The majority of colon cancers are considered sporadic, meaning they aren't linked to genes passed on from your family. Doctors aren't sure what causes most cases of colon cancer, though your exposure to carcinogens, your lifestyle and your diet may play a part.

Should you be tested?

If you think colon cancer runs in your family, talk to your doctor. He or she can help you determine whether your family might be carrying a genetic mutation.

Your doctor will want to know your family's health history. He or she will ask you questions about your family, including which members of your family have been diagnosed with cancer and at what age each was diagnosed.

In general, doctors consider genetic testing for people who have:

• A family history of colon cancer
• More than one close family member who has colon cancer, as well as related cancers, such as endometrial cancer
• A close family member who was diagnosed with colon cancer before age 50
• A close family member with a genetic syndrome, such as family adenomatous polyposis, known to increase the risk of colon cancer

If you meet any of these criteria, talk with your doctor about your family history and whether genetic testing is right for you.

Family history of colon cancer increases your risk

Some colon cancers are found in family clusters, but without a genetic link. Family clusters occur when more than one person in a family develops colon cancer — such as siblings or parents and children. In some families, that means something other than genes might have caused the colon cancer.

One theory to explain the clusters is that family members often have similar habits, which can increase their risk of disease. For instance, if you grew up in a family that ate mainly meat and skipped green leafy vegetables, you're likely to continue that habit in your adult life. Unfortunately, a diet heavy in meat and lacking in vegetables high in fiber might increase your risk and your family's risk of colon cancer.

Family clusters of colon cancer might also indicate that families are passing along genes that have yet to be identified. Genetics is an emerging science. In the future, more genetic links to cancer might be found.

Doctors do know that having a close relative with colon cancer doubles your risk of also getting the disease. But it isn't inevitable. If someone in your family has colon cancer, you can take preventive steps that might help decrease your risk, such as exercising and eating a healthy diet with a variety of fruits and vegetables.

Genetic syndromes that increase your risk of colon cancer

Genes associated with a higher risk of colon cancer are rare. In fact, only about 5 percent of colon cancers are linked to inherited genes. Genetic syndromes that can increase your risk of colon cancer include:

• Family adenomatous polyposis (FAP). FAP is caused by a mutated gene that you can inherit from either your mother or father. People with FAP have a 90 percent chance of developing colon cancer by age 45.
• Lynch syndrome, also called hereditary nonpolyposis colorectal cancer (HNPCC). Lynch syndrome is caused by an inherited mutation in one of four genes that regulate the replication of DNA in your cells. If you have Lynch syndrome, you have an 80 percent chance of developing colon cancer by age 75. Lynch syndrome also increases your risk of developing several other types of cancer.
• Peutz-Jeghers syndrome (PJS). PJS causes blotchy patches of discoloration on your lips and cheeks. It also causes numerous polyps in the small intestine and colon. If you have PJS, you have a 39 percent chance of developing colon cancer by age 64.
• Juvenile polyposis. Juvenile polyposis causes polyps throughout the gastrointestinal tract, starting in early childhood. People with juvenile polyposis have a 17 percent to 68 percent chance of developing colon cancer by age 60.

Your health care after genetic testing

If you know you have an increased risk of colon cancer, you can be proactive in reducing your risk of the disease. Your options might include:

• More frequent screenings. Your doctor might recommend more frequent screenings for colon cancer, such as colonoscopy or flexible sigmoidoscopy, and might start your screenings sooner. For instance, people at risk of FAP might begin colon cancer screenings in their teens. Your doctor might also recommend screening for related cancers, as well. For instance, women with Lynch syndrome are also at risk of endometrial and ovarian cancers.
• Surgical prevention. Removal of your colon (colectomy) can also reduce your risk of cancer. People with FAP and Lynch syndrome might consider this option.
• Medications. Certain drugs might reduce your risk of colon cancer if you carry the genetic mutation for FAP. A group of drugs known as COX-2 inhibitors, such as celecoxib (Celebrex) and sulindac (Clinoril), have shown promise in reducing colon cancer risk in people with FAP, though more study is needed.

You and your doctor will decide what preventive measure is best for you.

If your test reveals you don't carry a genetic mutation, that doesn't mean you're not at risk. If you have a family history of colon cancer, you're still at a higher risk than the general population. Your doctor might recommend you start colon cancer screenings sooner.

The American Cancer Society currently recommends that everyone, regardless of risk, be screened for colon cancer beginning at age 50.