What Is Colon Cancer Genetic Testing?
Colon cancer genetic testing is a blood test that can tell you whether you carry rare changed, or mutated, genes 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, talk with your doctor about colonoscopy screening.
The most common genetic changes related to colon cancer are familial adenomatous polyposis (FAP) and hereditary non-polyposis 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 can cause hundreds 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.
Almost everyone who has FAP will get colorectal cancer if they are not treated. They can develop polyps in their 20s and 30s, or even earlier. People who have FAP usually need to have their colons removed to prevent colon cancer.
Hereditary non-polyposis colorectal cancer (HNPCC) or Lynch syndrome
HNPCC can cause polyps in the colon, but not as many as FAP. Changes in any of one or more different genes can cause HNPCC. These changes also increase the risk of getting other cancers, including cancer of the endometrium, ovaries, stomach, urinary tract, small intestine, skin, brain, and liver.
Having the gene change related to HNPCC does not mean you definitely will develop colon cancer. But it does increase your chances of getting colon cancer at a younger age. And the cancer is more likely to grow faster. It also increases your chances of other cancers, so screening for endometrial or other cancers may be important. If a person has an HNPCC gene change, he or she has a 50% chance of passing this gene on to a child.
If you have a very strong family history of colon cancer, you may want to have a blood test to look for changed genes. These things are common in families with HNPCC:
- You have at least three relatives who have had colon cancer, endometrial cancer, or another HNPCC-related cancer, and at least one of the relatives 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 had colorectal or endometrial cancer before age 50.
Testing is more useful when the family member who has colon cancer, FAP, or HNPCC also tests positive for the abnormal gene. If your relative who has the condition does not test positive, this means he or she does not have the changed gene. If your relative doesn't have the changed gene, most likely you don't either, so you would not need to be tested for this changed gene. But you would still have a high risk. This means you would need to be watched closely by your doctor and have careful screening.
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, without treatment, your chances of getting colon cancer are very high.
If you have a positive result, you will be able to take action that may help you and your family members live longer. This includes starting screening at a younger age and having screening tests more often.
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. Also, there is the chance that if you do test positive, you may have trouble getting life insurance, long-term care insurance, or disability insurance.
You might choose to be tested because:
- You have a family history of colon cancer. This means you have a parent, brother, sister, or child who has colon cancer, FAP, or HNPCC and they have tested positive for the abnormal gene.
- You have a personal history of more than 20 colon polyps, especially at a young age.
- You have received genetic counselling, understand the risks and benefits of testing, and feel that the benefits outweigh the risks. A genetic counsellor can help you make well-informed decisions.
What Is Genetic Counselling?
Information from genetic testing can have a profound impact on your life. Genetic counsellors are trained to explain the test and its results, but you make the decision about whether to have the test. A genetic counsellor can help you make well-informed decisions. Ask to have genetic counselling before making a decision about testing. Genetic counselling 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 colon cancer or a recurrence.
- Learn about caring for a family member who has a genetic disease, including getting referrals to specialists or joining support groups.
Genetic counsellors 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.
Current as of: August 22, 2019
Author: Healthwise Staff
Medical Review: E. Gregory Thompson, MD - Internal Medicine
Brian O'Brien, MD, FRCPC - Internal Medicine
Adam Husney, MD - Family Medicine
Kathleen Romito, MD - Family Medicine
Kenneth Bark, MD - General Surgery, Colon and Rectal Surgery