Patients homozygous for mutation of the ATM gene exhibit constitutional genetic instability and have a high risk of cancer. A-T heterozygotes also have an increased tendency to develop adenocarcinomas. Colorectal cancer (CRC) is the second most common cancer in western populations, and tumors of the right colon are typically highly genetically unstable. The DNA mismatch repair genes mutated in most familial and some sporadic CRCs account for one route by which cells acquire additional oncogenic mutations during the progression of malignancy. Mismatch repair defects, however, do not seem to account for the majority of CRCs. Because of its role in maintaining genomic stability, and the high risk of cancer to homozygotes, ATM is a candidate gene for inactivation in the evolution of chromosomal instability in tumor cells. We have examined 114 CRC patients for loss of heterozygosity (LOH) using six microsatellite markers tightly linked to the ATM locus. Our data suggest that LOH of this region is not associated with cancer of the proximal colon. In the distal colon, LOH was found in 23-31% of cases, which is moderately elevated above the non-specific LOH reported in tumors of this tissue. No correlations were found with regard to clinicopathological variables aside from tumor location.

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