Cancer Genetics

Colorectal cancer is the second leading cause of cancer death among men and women in the United States. The American Cancer Society estimates that there will be 147,500 new cases of colorectal cancer diagnosed in the U.S. in 2003. In the United States, a person is diagnosed with colorectal cancer approximately every 4 minutes.

Sporadic Colon Cancer
Hereditary Colon Cancer

Features of Hereditary Colon Cancer

Familial Adenomatous Polyposis (FAP)
Hereditary Non-Polyposis Colorectal Cancer (HNPCC)
Genetic Testing

SPORADIC COLON CANCER

The estimated general population risk for colorectal cancer is 2-6%, affecting approximately 1 in every 17 people, including both men and women. Most colon cancers occur sporadically, or cluster in susceptible families. Research suggests that cancer may be related to many different factors, such as diet, environmental exposures, hormonal factors, ethnicity, and genetics. Different interactions of some or all of these factors (and others) may be related to an individual's risk to one day develop cancer. The majority of colon cancer cases in the United States (greater than 90%) fall into this category, called sporadic cancer. Sporadic cancer refers to cancer that does not have a hereditary component and appears to have developed by random or chance occurrence.

HEREDITARY COLON CANCER

There are a small percentage of colon cancer cases (less than 10%) that have a genetic component. There are several features in a family that may be suggestive of a hereditary predisposition to cancer. If several members of a family over several generations are affected with colon cancer, especially at young (less than 50) ages, it is more suggestive of inherited susceptibility to colon cancer. Individuals in these families may have inherited a gene change that put them at a higher risk to develop colon cancer, regardless of other factors.

An inherited genetic change only makes an individual predisposed to developing certain cancers, and some people who inherit a changed gene will not develop cancer. Since one in three people will develop cancer at some point in his or her life, cancer can develop in several people in one family without there being an inherited mutation. It can be difficult to determine which familial cancers are due to inherited genes, which are due to shared environment and which are due to chance alone.

However, in some families there is a clear genetic cause to the cancer in the family. There have been several genes identified that are associated with hereditary colorectal cancer syndromes. Researchers have currently identified two main forms of hereditary colon cancer: familial adenomatous polyposis (FAP), and hereditary nonpolyposis colorectal cancer (HNPCC). FAP represents less than 1% of all colorectal cancer cases diagnosed by doctors each year, and HNPCC represents about 5%.

Features of Hereditary Colon Cancer

There are several factors that suggest that an individual or family is at risk for having a hereditary colon cancer syndrome (either FAP or HNPCC). These factors are:

FAMILIAL ADENOMATOUS POLYPOSIS (FAP)

FAP is an uncommon hereditary colon cancer syndrome that is associated with a very high lifetime risk for colon cancer. Familial means that the condition is inherited, and polyposis refers to the fact that individuals with this condition have many colon polyps. A colon polyp is an area of tissue overgrowth in the colon or rectum. Colon polyps can be benign (non-cancerous) or malignant (cancer- causing). Individuals with FAP develop hundreds of polyps in the colon, all of which have the potential to become malignant. It is important for individuals with FAP to be screened very carefully since they have a high risk of developing colon cancer. Individuals with FAP also have an increased risk for other types of cancer.

HEREDITARY NON-POLYPOSIS COLON CANCER (HNPCC)

HNPCC is a different hereditary cancer syndrome than FAP. Hereditary refers to the fact that this condition is inherited. Non-polyposis means that individuals who have HNPCC tend to have only a few polyps, maybe one or two. However, these polyps are the type that have the potential to become malignant, so individuals with HNPCC also need to be screened very closely for colon cancer. Individuals with HNPCC also have an increased risk for other types of cancer.

Although genetic testing for FAP and HNPCC is now widely available, genetic testing is not for everyone. In some cases, genetic testing may not provide useful information to the family, or genetic testing may not be necessary based on the family history. In other cases, after a thorough discussion of the risks and benefits of genetic testing, some individuals decide that they personally do not want that information. It is strongly recommended that any person who is considering pursuing genetic testing, or who is concerned about their family/personal cancer history receive genetic counseling from a qualified healthcare professional. Genetic counseling is a process where individuals can obtain information about their genetic risks, and make informed decisions about their own healthcare. To determine whether a person would benefit from a genetic counseling session, he or she should discuss their personal and/or family history of cancer with their primary care provider, oncologist, or other health professional.