Background. The aberrant promoter methylation of the mismatch repair gene, hMLH1, is associated with microsatellite instability (MSI) in cancer cells and often is associated with a favorable prognosis. Methods. Pancreatic endocrine neoplasms (PENs) were obtained from 48 patients who underwent surgical resection. Methylation-specific polymerase chain reaction was used to detect methylation in the hMLH1 promoter. Tumor MSI at loci BAT26, BAT25, D2S123, D5S346, and D17S250 was determined with microsatellite polymerase chain reaction. Results. Hypermethylation of the hMLH1 promoter was present in 11 of 48 PENs (23%). Five of the 11 hMLH1-methylated PENs were found to be microsatellite unstable, and MSI was restricted to PENs with hMLH1 hypermethylation. Tumor recurrence at 2 years after surgical resection was significantly less common among the hMLH1-methylated PENs (11%), compared with the unmethylated PENs (35%; P = .035). Patients with hMLH1-methylated PENs experienced improved 5-year survival (100%) compared with patients with unmethylated tumors (56%; P = .010). Likewise, MSI-positive PENs were associated with improved survival compared with MSI-negative tumors (100% vs 59%; P = .017) at 5 years. Conclusion. As in hereditary nonpolyposis colorectal cancer in which MSI is associated with improved survival, methylation of hMLH1 leads to MSI in PENs and affords a favorable prognosis.
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