Inactivation of a DNA repair gene may be an early step in the development of sporadic colorectal tumors, and detection of the molecular basis for this inactivation may ultimately be useful in risk assessment for colorectal cancer, according to a new study in the September 21 issue of the Journal of the National Cancer Institute.
Cancers can arise from a region of cells with a "field defect", cells that appear normal but that have an underlying molecular defect. The molecular basis for the field defect is easy to understand when tumors develop from cells that all have similar genetic abnormalities, such as those that occur in patients who are genetically predisposed to a cancer or those who undergo an exposure to a carcinogen such as tobacco smoke. However, how the field defect may arise in most sporadic cancers in unclear.
In colorectal cancer, the DNA repair gene O6-methylguanine-DNA methyltransferase (MGMT) is often methylated--that is, the gene's promoter region has methyl groups added to it, which inactivate the gene. Jean-Pierre J. Issa, M.D., of the University of Texas M. D. Anderson Cancer Center in Houston, and colleagues hypothesized that MGMT promoter methylation could be one of the characteristics of the field defect in colorectal cancer, so they studied MGMT promoter methylation in the tumor, adjacent mucosa, and nonadjacent mucosa of 95 colorectal cancer patients and in the colon mucosa of 33 subjects without cancer.
MGMT promoter methylation was found in 50% to 94% of colorectal tumors, depending on the method of detection, and normal-appearing colon mucosa up to 10 cm away from the tumor had detectable MGMT methylation. In addition, the authors detected MGMT methylation in the normal colon mucosa of several subjects who did not have cancer.