What Is Colon Cancer Genetic Testing?

Colon cancer genetic testing is a blood test that can tell you whether you carry a rare changed, or mutated, gene that can cause colon cancer. Although most people who get colon cancer do not have one of these mutated genes, having them greatly increases your chance of getting colon cancer.

Colon cancer develops in the large intestine when cells change and grow out of control. Colon cancer is also called colorectal cancer because it can occur in both the colon and in the lowest section of the colon, which is called the rectum.

Colon cancer almost always begins as small growths on the inner wall of the colon called polyps. A doctor can find and remove polyps during a colonoscopy, a test in which a doctor uses a flexible video camera or scope to look at the inside of the colon. If a close member of your family, such as your brother, sister, or parent, has had colon cancer, doctors usually recommend screening with colonoscopy every 1 to 2 years starting at age 40, or 10 years earlier than the youngest age at which a member of your family was diagnosed with colorectal cancer.¹

The most common genetic changes occur in two conditions: familial adenomatous polyposis (FAP) and hereditary nonpolyposis colorectal cancer (HNPCC). In these conditions, screening often starts even sooner than age 40.

Familial adenomatous polyposis (FAP)

FAP develops because of a changed gene that causes hundreds or thousands of polyps to grow in the colon. The number of polyps increases with age. If one of your parents has FAP, you have a 50% chance of having the changed gene and the disease.

People with FAP have an almost 100% chance of getting colorectal cancer.^{2, 1} They can develop polyps in their 20s and 30s, or even earlier. People with FAP may decide to have their colons removed to prevent colon cancer.

Hereditary nonpolyposis colorectal cancer (HNPCC) or Lynch syndrome

HNPCC can cause polyps in the colon, but not as many as FAP. Changes in any of four or more different genes can cause HNPCC. These changes also increase the risk for other cancers, including cancer of the endometrium, ovaries, and other organs.

Having HNPCC does not mean you definitely will develop colon cancer, but it does increase your chances. If you have one parent who has HNPCC, you have a 50% chance of having one or more of these gene changes. If both parents have it, your chances of getting it are about 80%. (Not every person with the genes will get the condition, and not everyone with the condition will have the abnormal gene.)

If you have a strong family history of colon cancer, you may want to have a blood test to look for changed genes. Testing is most reliable when the family member that has colon cancer, FAP, or HNPCC also tests positive for the abnormal gene. Genetic testing may be recommended if:

• You have at least three relatives who have had colon cancer, and at least one of them is a parent, brother, or sister.
• Those relatives are spread over two generations in a row (for example, a grandparent and a parent).
• One of those relatives got cancer before age 50.

What Do the Results Mean?

Colon cancer genetic testing is a blood test that looks for the changed (mutated) genes that cause colon cancer. Although most people who get colon cancer do not have one of these mutated genes, having them greatly increases your chance of getting colon cancer.

A positive result means that you may have one of the changed genes that causes FAP or HNPCC. It also means that your chances of getting colon cancer are very high. A negative result means that none of these genes were found in your blood sample.

How Accurate Is the Test?

Although these blood tests are highly reliable, no test is 100% accurate. The test cannot tell you when or whether you will develop colon cancer. Testing negative for an inherited colon cancer gene does not mean you will never get colon cancer. It means your risk of colon cancer is about the same as that of the average person.

There are many different genetic mutations that can lead to FAP or HNPCC. Genetic testing does not find all of them. Because of this, a person may have a normal genetic test but still have an increased chance of having an inherited colon cancer.

Should I Be Tested?

The decision to be tested for genetic colon cancer is personal. You may have emotional, financial, and family reasons for taking or not taking the test.

You might choose to be tested because:

• You have received genetic counseling, understand the risks and benefits of testing, and feel that the benefits outweigh the risks. A genetic counselor can help you make well-informed decisions.
• You have a personal history of more than 20 colon polyps, especially at a young age.
• You have a parent, brother, or sister who has colon cancer, FAP, or HNPCC and they have tested positive for the abnormal gene.
• You would be worried not knowing whether you have an increased chance of getting cancer.
• You would change your decision about having children because of test results.

Why Would I Not Be Tested?

You might choose not to be tested because:

• No one in your family has had colon cancer, FAP, or HNPCC.
• Other people in your family have had colon cancer but you have regular screening.
• You would worry too much if you knew you had an increased chance of getting colon cancer.
• You would not change your decision about having children because of test results.
• Your health insurance would not pay for the test.

What Is Genetic Counseling?

Information from genetic testing can have a profound impact on your life. Genetic counselors are trained to explain the test and its results, but you make the decision about whether to have the test. A genetic counselor can help you make well-informed decisions. Ask to have genetic counseling before making a decision about testing. Genetic counseling can help you and your family:

• Understand medical facts, including what causes diseases, how a diagnosis is made, and what you may be able to do to help you manage a disease.
• Understand how your family history contributes to the development of a disease.
• Understand what you can do to help prevent a recurrence.
• Learn about caring for a family member with a genetic disease, including getting referrals to specialists or joining support groups.

Genetic counselors are trained to help you and your family make informed decisions. They are sensitive to the physical and emotional aspects of these decisions. Your privacy and confidentiality are carefully protected.

References

Citations

1. Winawer S, et al. (2003). Colorectal cancer screening and surveillance: Clinical guidelines and rationale—Update based on new evidence. Gastroenterology, 124(2): 544–560.

2. National Institutes of Health (2004). Familial adenomatous polyposis. Available online: http://ghr.nlm.nih.gov/condition=familialadenomatouspolyposis.

Other Works Consulted

• National Cancer Institute (2006). Genetics of Colorectal Cancer. Available online: http://www.cancer.gov/cancertopics/pdq/genetics/colorectal/healthprofessional.

• Winawer S, et al. (2003). Colorectal cancer screening and surveillance: Clinical guidelines and rationale—Update based on new evidence. Gastroenterology, 124(2): 544–560.

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