Detection of Epstein–Barr virus infection in thymic epithelial tumors by nested PCR and Epstein–Barr-encoded RNA ISH

Abstract Background：Epstein–Barr virus (EBV) is well known to be associated with a lot of tumors, including lymphoma, nasopharyngeal carcinoma, EBV associated gastric carcinoma and some other carcinomas with similar lymphoepithelioma-like features. However, the association between EBV and thymic epithelial tumors (TETs) is inconclusive as reports in this regard are not entirely consistent and the methods employed are of different sensitivity and specificity. The geographical difference of the patients is also one of the reasons for the different point of views. Methods：In our study, we examined 72 thymomas, including 3 cases of type A thymomas, 27 cases of type AB, 6 cases of type B1, 26 cases of type B2 and 10 cases of type B3 thymomas, and 15 thymic carcinomas to detect the viral genome at both DNA and RNA levels. The genome DNA of fresh tissues were first screened by nested polymerase chain reaction (PCR), which could be regarded as the most sensitive method to detect small amounts of DNA. Then all the tissue blocks were further submitted for viral localization by Epstein–Barr-encoded RNA (EBER) ISH. Results：Nested PCR results showed that none of type A, eight (29.6%) type AB, one (16.7%) type B1, fifteen (57.7%) type B2 and four (40.0%) type B3 were positive for EBV genome. None of them detected EBER expression except for one case of B2 thymoma. Fourteen (93.3%) thymic carcinomas were positive for EBV by nested PCR, of which three displayed weak nuclear signals within the tumor cells by EBER ISH. Conclusions：These results showed that nested PCR was a sensitive method for screening the EBV genome in thymic epithelial tumors. As the malignancy of thymoma increases, the rate of EBV infection became higher. Thymic carcinomas were well associated with the Epstein–Barr virus. We further analyzed the association between EBV infection and myasthenia gravis. We found that EBV infection rate was higher in the thymomas with myasthenia gravis. This gave us a hint that EBV may also played some roles in thymoma-related myasthenia gravis.