Clinicopathologic and genetic features of nonfamilial colorectal carcinomas with DNA replication errors

DNA replication errors (RERs) are closely associated with hereditary nonpolyposis colorectal carcinoma (HNPCC). Recently, alterations in DNA mismatch repair genes, including hMSH2, hMLH1, and hPMS2, have been implicated in the pathogenesis of HNPCC: Several studies have demonstrated RER in 13-17% of nonfamilial colorectal carcinomas. It is unclear, however, as to whether or not these RER positive nonfamilial colorectal carcinomas are incomplete forms of HNPCC or are caused by incidental alterations of DNA mismatch repair genes. Consequently, the authors studied the characteristics of RER positive nonfamilial colorectal carcinomas, placing particular emphasis on hMSH2 and hMLH1 gene mutations.

Fresh or frozen samples of 103 nonfamilial colorectal carcinomas were examined for RERs using the polymerase chain reaction (PCR) and specific microsatellite primers. The authors also identified mutations of the hMSH2 and hMLH1 genes in RER positive samples by a PCR single strand conformational polymorphism analysis followed by direct nucleotide sequencing.

The incidence of RER was 15.7% (17/103) in nonfamilial colorectal carcinomas, and only 1 case, which was found in the ascending colon, showed a somatic mutation at exon 12 in the hMSH2 gene. Neither germline nor somatic mutations of the hMSH2 or hMLH1 genes could be found in any of the remaining RER positive tumors. RER positive nonfamilial carcinomas tended to be located more frequently in the right colon. There was no increased prevalence in young patients, and the clinicopathologic characteristics of HNPCC were absent in the patients with RER positive nonfamilial colorectal carcinoma.

Based on these findings, the carcinogenesis of RER positive nonfamilial colorectal carcinoma is considered different from that of HNPCC: