Hypermethylation of the hMLH1 gene promoter in human gastric cancers with microsatellite instability

Human gastric carcinoma shows a higher prevalence of microsatellite instabi lity (MSI) than does any other type of sporadic human cancer. The reasons f or this high Frequency of MSI are not get known. In contrast to endometrial and colorectal carcinoma, mutations of the DNA mismatch repair (MMR) genes hMLH1 or hMSH2 have not been described in gastric carcinoma. However, hype rmethylation of the hMLH1 MMR gene promoter is quite common in MSI-positive endometrial and colorectal cancers. This hypermethylation has been associa ted with hMLH1 transcriptional blockade, which is reversible with demethyla tion, suggesting that an epigenetic mechanism underlies hMLH1 gene inactiva tion and MMR deficiency. Therefore, we studied the prevalence of hMLH1 prom oter hypermethylation in a total of 65 gastric tumors: 18 with frequent MSI (MSI-H), 8 with infrequent MSI(MSI-L), and 39 that were MSI negative. We f ound a striking association between hMLH1 promoter hypermethylation and MSI : of 18 MSI-H tumors, 14 (77.8%) showed hg permethylation, whereas 6 of 8 M SI-L tumors (75%) were hypermethylated at hMLH1. In contrast, only; 1 of 39 (2.6%) MSI-negative tumors demonstrated hMLH1 hypermethylation (P < 0.0001 for MSI-H or MSI-L, versus MSI-negative). Moreover, hypermethylated cancer s demonstrated diminished expression of hMLH1 protein by both immunohistoch emistry and Western blotting, whereas nonhypermethylated tumors expressed a bundant hMLH1 protein. These data indicate that hypermethylation of hMLH1 i s strongly associated with MSI in gastric cancers and suggest an epigenetic mechanism by which defective MMR occurs in this group of cancers.