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By Louis Neipris, M.D., HealthAtoZ writer
Most colon cancer cases strike randomly without any known family link. The extent to which heredity influences the chance of developing the disease varies among families. Some risks for colon cancer can be reduced with lifestyle changes, such as eating more fiber. However, no one can change the hereditary risk.
Fortunately, colon cancer is usually curable if caught in time. If someone in your family has colon cancer, your doctor may suggest genetic testing to find it early.
Genes and Cancer
Genes are found on chromosomes. One of their roles is to carry information that determines hereditary characteristics such as what color eyes you have. Strands of DNA linked together make up a gene. Sometimes the correct order of how the DNA molecules are supposed to line up gets changed. When that happens, it's called a mutation. Mutations can be caused by environmental factors such as radiation or toxins that damage your DNA. Or gene mutations can be inherited.
Oncogenes, a subset of genes that normally regulate cell growth, may be part of the development of cancer if they undergo mutations over years. One such gene (APC gene) regulates cell division and, if damaged, may lead to one of the inherited forms of colon cancer: familial adenomatous polyposis (FAP). Minor alterations probably act together in causing other types of colon cancer.
FAP and hereditary nonpolyposis colorectal cancer (HNPCC) are the two types of colon cancer caused by heredity. Only 5 percent of colon cancers are inherited. Tests can help determine your risk. However, just knowing that colon cancer runs in your family helps your doctor monitor you.
FAP
A child with FAP develops thousands of benign (noncancerous) polyps in the large intestine by age 10. By age 35, one or more of these polyps can become cancerous. FAP does not have a cure. However, a colectomy (surgical removal of the large intestine), if done early enough, significantly reduces the risk of recurrence. But careful surveillance must continue even after colectomy. A test to screen for FAP is available to family members of a patient with FAP, which helps in monitoring.
HNPCC
HNPCC is the most common of inherited colorectal cancers and accounts for about 5 to 8 percent of all cases. Similar to FAP, noncancerous polyps develop at a young age. However, there are fewer polyps and the chances for malignant transformation are slightly lower.
You can get a HNPCC genetic test, which detects mutations in the five associated genes. This is not recommended as a general screening tool. Rather, it is offered only to people with three or more relatives with colon cancer.
Limitations of testing
The genetic screening tests for FAP and HNPCC are not perfect. A doctor will develop a follow-up plan for screening and monitoring if the genetic testing results are positive. However, if someone has negative genetic test results but a strong family history of colon cancer, the doctor will watch that person closely nonetheless.
Conventional screening saves lives
Testing for blood in the stool is a non-invasive method for detecting colon cancer. Early in its course, a tumor growing inside the colon may cause bleeding. A chemical is applied to a card containing a sample of your stool. This is called a Fecal Occult Blood Test, or FOBT. A color change means the presence of blood. However, this test does not find every cancer. The American Cancer Society recommends that along with a FOBT every year, you have a flexible sigmoidoscopy every five years. Or, you can have a double contrast barium enema every 5 years or a colonoscopy every 10 years.
Colonoscopy or sigmoidoscopy can view the colon directly and can pick up polyps or changes. If a polyp, or precancerous lesion, is seen on colonoscopy or sigmoidoscopy, it is removed.
Keep in mind
- Colon cancer can be cured in most cases if the precancerous polyp (or early cancer lesion) is found early and removed.
- Know your family history for colon cancer. You may need to be screened earlier than the general population (age 40 instead of 50).
- If you don't have a family history of colon cancer, you still need to be screened, usually beginning at age 50.
- Reduce your risk for colon cancer. Stick to a high fiber diet, rich in colorful fruit, vegetables and whole grains. Limit your intake of red meat.
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External Sources
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National Cancer Institute
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Memorial Sloan Kettering Cancer Center: Colorectal Cancer
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American Cancer Society
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This article was reviewed and updated June 2007.
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