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Learning About Colon CancerWhat do we know about heredity and colon cancer?Colon cancer, a malignant tumor of the large intestine, affects both men and women. In the year 2000, there were an estimated 130,200 cases diagnosed. The vast majority of colon cancer cases are not hereditary. However, approximately 5 percent of individuals with colon cancer have a hereditary form. In those families, the chances of developing colon cancer is significantly higher than in the average person. Scientists have discovered several genes contributing to a susceptibility to two types of colon cancer:
Together, FAP and HNPCC gene mutations account for approximately 5 percent of all colorectal cancers. These hereditary cancers typically occur at an earlier age than sporadic (non-inherited) cases of colon cancer. The risk of inheriting these mutated genes from an affected parent is 50 percent for both males and females. The genes that cause these two syndromes were relatively easy to discover because they exert strong effects. Other genes that cause susceptibility to colon cancer are harder to discover because the cancers are caused by an interplay among a number of genes, which individually exert a weak effect.
Is there a test for hereditary colon cancer?Gene testing can identify some individuals who carry genes for FAP and some HPNCC cases of colon cancer. However, the tests are not perfect at this point in time. So, some families may have alterations in the FAP or HNPCC gene that can not be detected. The test for FAP syndrome involves examining DNA in blood cells called lymphocytes (white blood cells), looking for mutations in the APC gene. No treatment to reduce cancer risk is currently available for people with FAP. But for those who test positive, frequent surveillance can detect the cancer at an early, more treatable stage. Because of the early age at which this syndrome appears, the test may be offered to people under 18 who have a parent known to carry the mutated gene. Researchers hope that an easier test, now experimental, will become available in three to five years. This new test examines a stool sample and looks for cancer cells sloughed off by the APC gene. Genetic tests for HNPCC are of limited value since the current test can identify only a few mutations on two genes that cause HNPCC (hMSH2 and hMLH1). There are no clinical tests for the other two HNPCC genes. Because of the limitations of available tests for hereditary colon cancer, testing is not recommended for the general population. However, individuals in families at high risk may consider testing. Genetic counselors can help individuals make decisions regarding testing.
Current NHGRI Clinical Research on Hereditary Colon CancerCurrently, NHGRI is conducting two clinical research studies on colon cancer:
Additional Resources for Information on Hereditary Colon Cancer
Last Reviewed: May 13, 2008 |
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