Do We Know What Causes Colorectal Cancer?

Researchers have found several factors that can increase a person’s risk of colorectal cancer, but it’s not yet clear exactly how all of these factors might cause this cancer.

Cancer is caused by changes in the DNA inside cells. DNA is the chemical in our cells that makes up our genes, which control how our cells function. We usually look like our parents because they are the source of our DNA. But DNA affects more than just how we look.

Some genes help control when our cells grow, divide into new cells, and die:

  • Certain genes that help cells grow, divide, and stay alive are called oncogenes.
  • Genes that help keep cell division under control or cause cells to die at the right time are called tumor suppressor genes.

Cancers can be caused by DNA mutations (defects) that turn on oncogenes or turn off tumor suppressor genes. Changes in several different genes are usually needed to cause colorectal cancer.

Inherited gene mutations

Some DNA mutations can be passed from generation to generation and are found in all cells in the body. When this happens, we say the mutations are inherited. A small portion of colorectal cancers are caused by inherited gene mutations. Many of these DNA changes and their effects on the growth of cells are now known. For example:

  • Familial adenomatous polyposis (FAP) and Gardner syndrome are caused by inherited changes in the APC gene. The APC gene is a tumor suppressor gene; it normally helps keep cell growth in check. In people with inherited changes in the APC gene, this “brake” on cell growth is turned off, causing hundreds of polyps to form in the colon. Over time, cancer will nearly always develop in one or more of these polyps because new gene mutations occur in the cells of the polyps.
  • Lynch syndrome (hereditary non-polyposis colon cancer, or HNPCC) is caused by changes in genes that normally help a cell repair faulty DNA. A mutation in one of the DNA repair enzyme genes like MLH1, MSH2, MLH3, MSH6, PMS1, or PMS, can allow DNA errors to go uncorrected. These errors will sometimes affect growth-regulating genes, which may lead to the development of cancer. TGFBR2 is another gene linked to HNPCC. It helps regulate cell growth.
  • Peutz-Jeghers syndrome is caused by inherited changes in the STK11 gene. This seems to be a tumor suppressor gene.

Genetic tests can detect gene mutations associated with these inherited syndromes. If you have a family history of colorectal polyps or cancer or other symptoms linked to these syndromes, you may want to ask your doctor about genetic counseling and genetic testing. The American Cancer Society recommends discussing genetic testing with a qualified cancer genetics professional before any genetic testing is done. For more on this, see Genetic Testing: What You Need to Know.

Acquired gene mutations

Some gene mutations happen during a person’s lifetime and are not passed on. They affect only cells that come from the original mutated cell. These DNA changes are due to acquired mutations.

In most cases of colorectal cancer, the DNA mutations that lead to cancer are acquired during a person’s life rather than having been inherited. Certain risk factors probably play a role in causing these acquired mutations, but so far it’s not known what causes most of them.

There doesn’t seem to be a single genetic pathway to colorectal cancer that is the same in all cases. In many cases, the first mutation occurs in the APC gene. This leads to an increased growth of colorectal cells because of the loss of this “brake” on cell growth. Further mutations may then occur in genes such as KRAS, TP53, and SMAD4. These changes can lead the cells to grow and spread uncontrollably. Other genes that aren’t known yet are probably involved as well.

For more about how genes changes can lead to cancer, see Genes and Cancer.

The American Cancer Society medical and editorial content team
Our team is made up of doctors and master’s-prepared nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

Last Medical Review: October 15, 2015 Last Revised: January 20, 2016

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