Tolerance of human MSH2(+/-) lymphoblastoid cells to the methylating agenttemozolomide

Members of hereditary nonpolyposis colon cancer (HNPCC) families harboring heterozygous germline mutations in the DNA mismatch repair genes hMSH2 or h MLH1 present with tumors generally two to three decades earlier than indivi duals with nonfamilial sporadic colon cancer. We searched for phenotypic fe atures that might predispose heterozygous cells from HNPCC kindreds to mali gnant transformation. hMSH2(+/-) lymphoblastoid cell lines were found to be on average about 4-fold more tolerant than wild-type cells to killing by t he methylating agent temozolomide, a phenotype that is invariably linked wi th impairment of the mismatch repair system. This finding was associated wi th an average 2-fold decrease of the steady-state level of hMSH2 protein in hMSH2(+/-) cell lines. In contrast, hMLH1(+/-) heterozygous cells were ind istinguishable from normal controls in these assays. Thus, despite the fact that HNPCC families harboring mutations in hMSH2 or hMLH1 cannot be distin guished clinically, the early stages of the carcinogenic process in hMSH2 a nd hMLH1 mutation carriers may be different. Should hMSH2(+/-) colonocytes and lymphoblasts harbor a similar phenotype, the increased tolerance of the former to DNA-damaging agents present in the human colon may play a key ro le in the initiation of the carcinogenic process.