Purpose: Four DNA mismatch repair genes have been identified as being susceptible genes for hereditary nonpolyposis colorectal cancer. Deficiency of one of the mismatch repair genes causes the replication error phenotype in more than 80 percent of patients with hereditary nonpolyposis colorectal cancer and in 10 to 30 percent of patients with sporadic colorectal cancer. To determine which mismatch repair gene is lacking the function in patients with replication error-positive colorectal cancer, several approaches have been used at the nucleic acid and protein levels. We studied replication error in 40 samples of randomly selected colorectal cancers and expression of hMSH2 and hMLH1 proteins analyzed by immunoblot in the tumor and normal tissues of the replication error-positive and replication error-negative samples.
Materials and methods: Frozen tumor and normal tissues were obtained from 40 Japanese patients who had colorectal cancer. According to the Amsterdam criteria, those patients were classified as having 39 sporadic and 1 unknown colorectal cancers. Genomic DNA was extracted from tumor and normal tissues for determining replication error with eight microsatellite markers. Expression of hMSH2 and hMLH1 proteins in cell lysates of tumor and normal tissues of 16 patients was analyzed by immunoblot.
Results: The replication error phenotype was found in 6 (15 percent) of the 39 sporadic cases. hMLH1 protein was not detected in two of the six replication error-positive tumor tissues and not in the normal tissues, indicating that the tumor cells of the two patients had severe mutations in both alleles of the hMLH1 gene. Another four replication error-positive and ten replication error-negative tumors and normal tissues expressed hMLH1 protein. hMSH2 protein was detected in all samples.
Conclusion: hMLH1 protein was undetectable in the two tumor tissues of the six replication error-positive samples of sporadic colorectal cancer. The detection procedure used here may have potential use for determining a dysfunctional mismatch repair gene product.