The Genetics of Colon and Rectal Cancers

About 5 to 10% of all colorectal cancers are caused by an inherited mutation, while the remaining 90 to 95% are caused by a mutation of a gene that is acquired during a person’s lifetime (non-genetic).  

Inherited gene mutations

Cancer is caused by certain changes to genes that control the way our cells function, especially how they grow and divide. Some mutations called inherited (hereditary) mutations, are ones that can be passed from generation to generation. These mutations are present throughout a person’s life in nearly every cell of the body. Common genetic syndromes, which are disorders caused by one or more abnormalities in the genome, have been linked to colorectal cancer.

These genetic syndromes may include:

  • Familial adenomatous polyposis (FAP) and Gardner syndrome: caused by inherited changes in the APC gene, which is a tumor suppressor gene that helps keep cell growth in check. Changes in the APC gene can result in hundreds of polyps to form in the colon.
  • Lynch syndrome (hereditary non-polyposis colon cancer, or HPCC): caused by changes in genes that help damaged DNA repair itself. Mutations of genes like MLH1, MSH2, MLH3, MSH6, PMS1, and PMS2 keep DNA errors from being fixed. This can affect growth-regulating genes, thus leading to the possible development of cancer.
  • Peutz-Jeghers syndrome: caused by inherited changes in the tumor-suppressor gene STK11 (LKB1).
  • MUTYH-associated polyposis: caused by mutations in the MUTYH gene, which plays a role in how the cell checks the accuracy of DNA when cells divide.

Acquired gene mutations

Some mutations simply happen during a person’s lifetime. Unlike inherited mutations that affect the entire body, acquired mutations only affect cells that come from the original mutated cell. Certain risk factors such as smoking, diet, and UV radiation from the sun may play a role in the formation of these mutations, but overall, the reason behind what causes most of them is unclear.

Genetic testing for colorectal cancer

Genetic tests for hereditary cancer syndromes can tell whether a person from a family that shows signs of such a syndrome has one of the mutations listed above. These tests can also show whether family members without obvious disease have inherited the same mutation as a family member who carries a cancer-associated mutation.

Factors that suggest a genetic contribution to colorectal cancer include:

  • A strong family history of colorectal cancer and/or polyps
  • Multiple cancers in a patient with colorectal cancer
  • Being of an early age at diagnosis

Genetic tests for mutations that cause hereditary cancer syndromes are usually requested by a person’s doctor or another healthcare provider. Genetic counseling can help people consider the risks, benefits, and limitations of genetic testing in their particular situations.

At Compass Oncology our oncologists are here to help you determine if you are a good candidate for genetic testing. If any of the above listed genetic factors apply to you, or if you have not been diagnosed, but would like to know if you could be affected by colorectal cancer that seems to be in your family, please contact us to schedule an appointment with one of our experts in cancer-risk reduction.  Find a location in Portland, OR or Vancouver, WA that is closest to you.


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