What Is Inherited Colon Cancer?

Colorectal cancer is common. Approximately 130,000 cases of colorectal cancers are diagnosed in American men and women each year. Colorectal cancer arises because of damage to some of the genes that control colon cell growth and death. In most cases the genes are mutated because of dietary factors and aging. In about 10 percent of people with colorectal cancer, a critical mutation is passed from a parent to a child.

Inherited colorectal cancer is suspected when colorectal cancers occur in multiple generations of a family. The colorectal cancers and polyps usually occur at a young age, and tumors and cancers may also occur in other organs. Through an investigation of the family tree, inherited colorectal cancer can be documented. The inherited colorectal cancer syndromes include Hereditary Non-polyposis Colorectal Cancer (HNPCC) and Familial Adenomatous Polyposis (FAP).

Over the last decade, a series of scientific discoveries has advanced our understanding of the causes of colorectal cancer. The most exciting of these is related to genetics, or the body’s DNA code. With the discovery of the genes that cause HNPCC and the gene that causes FAP, we now understand the mechanism of cancer development in these families. Identification of the gene for HNPCC and FAP permits blood testing of family members to see if they have, or have not, inherited the disease. These tests help to make an early diagnosis before the disease occurs while enabling physicians to recommend appropriate chemoprevention, surveillance and treatment strategies. The tests also allow for scientific research on the prevention and cure of inherited colorectal cancer.

What Role Do Genetics Play?

As genetics is a complicated subject, some background information on genes and inheritance is important in a discussion of colorectal cancer.

Genes are made of DNA and come in pairs. They are stored inside cells in packages called chromosomes. Genes carry the codes that control how our bodies develop. Each cell in our body contains thousands of different genes. Each gene provides the recipe for protein construction. Proteins have distinct jobs: some determine our hair color, the ability of our livers to regulate cholesterol, or the height to which we will grow.

What is the Effect of Mutated Genes?

Damaged (or mutated) genes may result in abnormalities in protein construction. Sometimes a mutation has no effect on protein production, and there is no disease associated with it. Sometimes the mutation produces an abnormal protein, but the protein is able to work to some extent, and the disease is mild. Sometimes, the mutation produces no protein, or a protein that cannot work at all. Then the disease may be severe.

In addition, there may be factors in our diet, our body and in other genes that have an effect on the way a mutation produces disease. This is obvious in families where people have the same mutation but different types of disease.

How Are Children Affected?

A parent passes only one of the two copies of each gene on to the fetus through an egg or sperm cell. The genes passed on are reproduced in every cell in the baby’s body. If a parent has one damaged and one good gene, there is a 50-percent chance of passing the damaged gene onto the child. If passed on, the child’s gene is damaged and may predispose the child to certain diseases, such as colorectal cancer, for example.

Sometimes the parent's genes are normal, but the genes are damaged at the moment a sperm fertilizes an egg. If this happens, even though the parent’s genes are normal and working well, the child will have damaged genes. This new mutation can be passed on from generation to generation.

In the cases of HNPCC and FAP, a person inherits a normal gene from one parent and a damaged one from the other parent, or a person acquires a damaged gene at conception. All the cells in that person's body will then contain one normal and one abnormal gene. Over time, the normal gene may become damaged too. When there are two damaged genes, the cells are not under normal control and cancer develops.

Currently, there is no genetic treatment for HNPCC or FAP. However, understanding the inheritance of the disease, ways to make the diagnosis, and the appropriate treatment and testing options is the first step to cancer prevention.