Microsatellite instability in gastric cancer is closely associated with hMLH1 hypermethylation at the proximal region of the promoter.

Most sporadic gastric cancer with the microsatellite instability (MSI) phenotype is linked with hypermethylation (HM) of hMLH1. However, a part of gastric cancer with hMLH1 HM does not show MSI, suggesting a region-specific effect of hMLH1 promoter methylation on developing MSI. To test this possibility, we measured the methylation level in 3 distinct areas of hMLH1 promoter and compared them with MSI in 129 sporadic gastric cancer patients. Three areas of hMLH1 promoter, from distal toward proximal, were designated as hMLH1-A, hMLH1-B, and hMLH1-C, respectively. The methylation level was measured by fluorescence-based real-time methylation specific PCR. MSI status was tested using a panel of 5 microsatellite markers (BAT25, BAT26, D2S123, D5S346, and D17S250). Gastric cancers with no HM in hMLH1-A (n=105, 81.4%) also showed no HM in 2 other regions of hMLH1 promoter. On the other hand, the cancers with HM in hMLH1-A (n=24, 18.6%) showed various levels of methylation in 2 other regions. In most cases, the methylation value was the highest in hMLH1-A and the lowest in hMLH1-C. We found the MSI phenotype in 12 cancers (13%) of 92 tested cases and these cancers were all associated with HM in the region of hMLH1-C. A third of hypermethylated cancers in the hMLH1-A region did not show the MSI phenotype. The survival of the patients with HM in hMLH1-C was significantly better than that of patients without HM (P<0.05). These results suggest that HM in the proximal region of hMLH1 promoter, hMLH1-C in this study, plays a critical role in the progression of gastric cancer with MSI. The complete association between HM in hMLH1-C and MSI phenotype with gastric cancer provides an alternative diagnostic tool for detecting a favorable prognostic subgroup with MSI by using simple methylation analysis.