EBV infection and mismatch repair deficiency mediated by loss of hMLH1 expression contribute independently to the development of multiple synchronous gastric carcinomas

Background To explore the possible association between EBV, microsatellite instability (MSI), and alterations of hMLH1 protein, 282 tumors from 141 patients with multiple synchronous gastric carcinomas (MSGC) were studied. Methods In situ hybridization for EBV-encoded small RNA and hMLH1 immunohistochemistry were performed in tissue microarrays. In 19 MSGC cases with altered hMLH1 expression, methylation analyses by MethyLight and MSI tests were performed. Results Loss of hMLH1 was found in 19 of 141 MSGC patients (13.5%) and 26 of 282 MSGC tumors (9.2%). hMLH1 loss was associated with differentiated histology (P = 0.03). Out of the 38 tumors from 19 hMLH1-negative MSGCs, 12 tumors from six cases (31.6%) showed concurrent methylation of hMLH1 and MSI-high in both multiple tumors. EBV was found in 31 of 141 MSGC patients (21.9%) and 49 of 282 MSGC tumors (17.4%) and was significantly associated with undifferentiated histology and a location within the upper third of the stomach (P < 0.002). EBV was not observed in any of the tumors that had a loss of hMLH1 expression. Conclusions Considering that EBV-associated GCs show global CpG island methylation, our findings suggest that EBV infection allows the gastric mucosa to escape from aberrant methylation of hMLH1 and induces a malignant pathway independent of MSI.