What is colon cancer?
Cells normally grow and divide only when they are needed to keep our bodies functioning properly. But sometimes, the mechanisms that regulate cell growth stop working and cells divide out of control to form tumors. This is called cancer. When cancer develops in the cells lining the colon (the first part of the large intestine), it is called colon cancer.
People who have a history of colon cancer in their family are at greater risk of getting the disease themselves. The risk increases when a relative has had the disease before age 50. These families are considered high-risk, because they may have inherited one of two rare genetic conditions: FAP (familial adenomatous polyposis) or HNPCC (hereditary non-polyposis colon cancer).
FAP is caused by mutations of the APC (adenomatous polyposis coli) gene on chromosome 5. APC is a tumor suppressor gene, which means that it prevents uncontrolled cell growth. People who inherit a mutated form of this gene develop growths called polyps in their colon. By age 15, they may have hundreds of these polyps. Polyps are not cancerous at first, but if they aren't treated, they will develop into colon cancer.
HNPCC (also called Lynch syndrome) is caused by mutations in one of several genes that fix damaged DNA. People who inherit one of these mutations have a much greater risk of accumulating mutations that will lead to uncontrolled cell growth and cancer.
How do people get colon cancer?
FAP and HNPCC are both inherited in an autosomal dominant pattern. If a parent has FAP or HNPCC, his or her children run a 50 percent risk of inheriting the mutated gene. Usually when a person inherits a defective gene it does not necessarily mean he or she will develop a malignant cancer. However, the APC gene strikingly predisposes one to colon cancer. People who inherit one bad copy of the APC gene are practically guaranteed to develop colon cancer by age 40. Similarly, people who inherit one bad copy of a gene associated with HNPCC have an 80 percent chance of getting colon cancer before. HNPCC also increases a person's risk of developing other cancers, including ovarian, stomach, brain, and liver.
What are the symptoms of colon cancer?
Colon cancer affects the stomach and bowels. Common symptoms include: diarrhea or constipation, blood in the stool, vomiting, bloating, cramps, and unexplained weight loss.
How do doctors diagnose colon cancer?
When a patient shows symptoms of colon cancer, his or her doctor can screen for the disease using one of several tests:
- Fecal Occult Blood Test (FOBT) - Colon cancer can sometimes cause tiny dots of blood, too small for the eye to see, in the feces The FOBT test uses a special chemical to check the patient's stool sample for these traces of blood.
- Flexible-Sigmoidoscopy - Using a thin flexible tube called a simoidoscope, the doctor looks inside the patient's colon for growths called polyps.
- Double Contrast Barium Enema (DCBA) - A silvery-white metallic substance called barium is inserted up the patient's colon through the rectum. The barium outlines the patient's colon on an x-ray screen.
- Colonoscopy - Using a thin instrument called a colonoscope, the doctor looks inside the patient's colon. During the procedure, the doctor removes pieces of tissue (called a biopsy) to test them for cancer. If the doctor finds any polyps, he or she can also remove them. A newer method, called virtual colonoscopy, looks at the colon without going into the body, with an MRI or CT scan.
- DNA-Based stool test - This test examines DNA taken from a patient's stool sample to look for genetic defects associated with colon cancer.
How is colon cancer treated?
Colon cancer is very treatable. In fact, about 90 percent of patients survive the disease after treatment. First, doctors stage the disease to see how far it has progressed. If the cancer has not spread to other tissues of the body, it can be treated with:
special chemicals (chemotherapy) or radiation (powerful x-rays) that kill all rapidly dividing cells in the body,
including cancer cells
surgery to remove the polyps and/or cancerous part of the colon
Interesting facts about colon cancer
People who have FAP can develop hundreds and even thousands of polyps in their colon, whereas people with HNPCC develop relatively few.
The progression from a benign to a malignant cancer typically requires multiple mutations that allow cells to acquire new and abnormal characteristics, such as an increased growth rate, inability to adhere or stick to neighboring cells, propensity to migrate to other places in the body, etc. For example, at least seven mutations are required to produce a malignant colon tumor.
Inherited cancers often provide clues about the genes mutated in noninherited (sporadic) cancers. For example, mutations in the APC gene are found not only in FAP tumors but in 85% of all sporadic colon tumors as well.