Genetics and Colorectal Cancer

Only about five to 10 percent of colorectal cancers are passed from parents to their children though it is possible. Several gene mutations, or abnormalities, that cause colorectal cancer, and allow it to be transmitted to family members, have been found.

The two most common inherited colorectal cancer syndromes are hereditary nonpolyposis colorectal cancer (HNPCC) and familial adenomatous polyposis (FAP). They can affect both men and women, and the children of people who carry these genes have a 50 percent chance of inheriting the disease-causing gene.

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This is the most common form of hereditary colorectal cancer, accounting for about three percent of all colorectal cancer diagnoses each year. If you’ve been diagnosed with HNPCC, chances are you have at least three family members and two generations with colorectal cancer, with cancer that develops before age 50. Although not everyone who inherits the HNPCC gene will develop colorectal cancer, the risk is very high — about 80 percent. People with HNPCC also have a higher risk of developing other cancers, such as kidney, ovarian, uterine, renal pelvis, small intestine, and stomach cancer.

We can evaluate the pattern of colorectal cancer in your family in order to determine if the family has HNPCC. Families considered to be “HNPCC families” must display certain criteria indicating a pattern of colon cancer throughout generations.

These are referred to as the Amsterdam Criteria, and include:

At least three members (two in small families) with colon or rectal cancer or two members with colorectal cancer and one with endometrial cancer, with other early-onset Lynch syndrome defined cancer.
At least two successive generations with colon or rectal cancer.
Two family members with the disease are first-degree relatives (i.e. parents, brothers, sisters or children) of another family member with colon and rectal cancer.
At least one member affected at or before age 50.
The familial adenomatous polyposis syndrome is excluded from the family member’s diagnosis.

Familial adenomatous polyposis (FAP) is a rare condition characterized by the presence of more than hundreds or even thousands of benign polyps, or growths in the large intestine. It is thought to be present in about one percent of all people diagnosed with colorectal cancer each year. The polyps occur early in life, with 95 percent of people with FAP developing polyps by age 35, and are often detected in patients in their teens, with 50 percent developing polyps by age 15. If the colon is not surgically removed, there is a 100 percent chance that some of the polyps will develop into cancer, usually by age 40.

While most cases of FAP are inherited, nearly a third of the cases are the result of a spontaneous or newly-occurring gene mutation, or abnormality. For those who develop a new gene mutation, it is possible to pass the FAP gene onto their children.

Genes are tiny segments of DNA that control how cells function, such as telling them when to divide and grow. One copy of each gene comes from your mother; the other comes from your father.

In 1991, researchers made a significant breakthrough in the diagnosis of FAP. They identified the gene – called APC – that is responsible for the condition. This gene mutation can be detected in 82 percent of patients with FAP. The exact lifetime risk of developing colon cancer in people who have inherited this gene abnormality is about 100 percent. Families in which this gene mutation occurs may or may not have one or more family members with colorectal cancer or polyps.