CORRECTION OF HYPERMUTABILITY, N-METHYL-N'-NITRO-N-NITROSOGUANIDINE RESISTANCE, AND DEFECTIVE-DNA MISMATCH REPAIR BY INTRODUCING CHROMOSOME-2 INTO HUMAN TUMOR-CELLS WITH MUTATIONS IN MSH2 AND MSH6

The human DNA mismatch repair genes hMSH2 and hMSH6 encode the protein s that, together, bind to mismatches to initiate repair of replication errors, Human tumor cells containing mutations in these genes have st rongly elevated mutation rates in selectable genes and at microsatelli te loci, although mutations in these genes cause somewhat different mu tator phenotypes. These cells are also resistant to killing by certain drugs and are defective in mismatch repair, Because the elevated muta tion rates in these cells may lead to mutations in additional genes th at are causally related to the other defects, here we attempt to estab lish a cause-effect relationship between the hMSH2 and hMSH6 gene muta tions and the observed phenotypes. The endometrial tumor cell line HEC 59 contains mutations in both alleles of hMSH2. The colon tumor cell l ine HCT15 contains mutations in hMSH6 and also has a sequence change i n a conserved region of the coding sequence for DNA polymerase delta, a replicative DNA polymerase, We introduced human chromosome 2 contain ing the wild-type hMSH2 and hMSH6 genes into HEC59 and HCT15 cells, In troduction of chromosome 2 to HEC59 cells restored microsatellite stab ility, sensitivity to N-methyl-N'-nitro-N-nitrosoguanidine treatment, and mismatch repair activity, Transfer of chromosome 2 to HCT15 cells also reduced the mutation rate at the HPRT locus and restored sensitiv ity to N-methyl-N'-nitro-N-nitrosoguanidine treatment and mismatch rep air activity, The results demonstrate that the observed defects are ca usally related to mutations in genes on chromosome 2, probably hMSH2 o r hMSH6, but are not related to sequence changes in other genes, inclu ding the gene encoding DNA polymerase delta.