Colorectal cancer is a malignant tumor which develops in the colon or the rectum. The American Cancer Society estimates 106,100 new cases of colon cancer, 52,010 in men and 54,090 in women, and 40,870 new cases of rectal cancer, 23,580 in men and 17,290 in women, will be diagnosed this year. (1)
The vision of the future is genome (genetic information) research. Scientists have discovered several genes which contribute to the susceptibility of developing two types of colon cancer. The first gene is FAP (familial adenomatous polyposis). Individuals with this syndrome develop multiple polyps in the colon. Polyps are usually benign but some can become malignant and require removal. Persons with mutations of the FAP gene have a nearly 100% chance of developing colon cancer by age 40. The second gene is HNPCC (hereditary nonpolypsosis colorectal cancer). Individuals with a HNPCC gene mutation have an estimated 80% lifetime chance of developing colon or rectal cancer. The risk of inheriting these mutated genes from an affected parent is 50% for both women and men. (2)
In a study, published in Genome Research, an international team of researchers led by Richard Houlston, MD, of The Institute of Cancer Research in the U.K., examined the biology underlying common variants on chromosome 18 and the association with colorectal cancer. A large group of patients diagnosed with colorectal cancer and control subjects were involved in the study. The researchers identified all variants in this chromosomal region that were common to the colorectal cancer patients. The focus was directed on the variant which was most strongly associated with colorectal cancer. This variant, a single nucleotide polymorphism (SNP) reduced the cellular levels of a nearby gene, SMAD7. Disruption of SMAD7 had previously been implicated in the progression of colorectal cancer. The researchers confirmed this and concluded that an association between the risk of colorectal cancer and this chromosomal region exists. (3)
Gene testing, though not perfect, can identify some individuals who carry the gene for FAP and some HPNCC cases. The test involves examining DNA in lymphocytes (white blood cells) and looking for mutations in the gene. The current test for HPNCC is of limited value because only a few mutations on two of the four genes can be identified. Testing is not recommended on the general population because of the limitations. However, individuals in families at high risk may consider the testing. For those who test positive for FAP, frequent surveillance can detect the cancer at an early and more treatable stage. (2)
(1) www.cancer.org
(2) www.genome.gov
(3) Genome-Wide Association Study Points to Colorectal Cancer Culprit
Gastroenterology July 2009
Maryann Gromisch is a registered nurse with clinical experience in the areas of medicine, surgery, critical care.