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.
cancer almost always begins as small growths on the inner wall of the colon
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 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.
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 need 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 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.