Repair-defective Human Tumor Cell Lines Expression of hMLH1 in Sporadic Colon Tumors and Mismatch

Somatic mutations in DNA mismatch repair genes have been observed in sporadic tumors as well as ceHlines and xenografts derived from such tumors implicating genetic defects of mismatch repair genes in the devel opment of such tumors. However, the proportion of sporadic tumors in which mismatch repair genes have been inactivated has not been deter mined accurately. We have analyzed 66 sporadic colorectal tumors for the expression of hMLH1 by immunohistochemistry and identified 4 tumors that do not express hMLH1. These four colorectal tumors, a colon tumor cell line (SW4S) and an endometrial tumor cell line (AN3CA), did not express hMLH1, despite the absence of mutations in its coding sequence. Cytosine methylation of the hMLHJ promoter region was found in these four colorectal tumors, whereas cytosine methylation of the hMLHJ pro moter region was absent in adjacent normal tissue or in nine tumors that expressed hMLH1. In addition, cytosine methylation of the hMLHJ pro moter region was observed in the 5W48 and AN3CA cell lines that do not express hMLH1 but not in four tumor cell lines known to express hMLH1 mRNA. Our data indicate that DNA methylation is likely to be a common mode of mismatch repair gene inactivation in sporadic tumors.