Epigenetic alterations induced by transient Epstein-Barr virus infection.

摘要

Epstein-Barr virus (EBV) is a gammaherpesvirus associated with lymphoid and epithelial tumors. Although EBV is a known tumor virus, incomplete association of EBV with any given tumor type has confounded its role in tumorigenesis. For example, only 15% of gastric carcinomas are EBV-positive compared to virtually all undifferentiated nasopharyngeal carcinomas. Upon entry into a cell, the viral genome is chromatinized and EBV rapidly shuts off its viral gene expression program via methylation of its viral promoters to evade the host immune response. A similar restricted pattern of viral gene expression is present in EBV-positive tumors. We hypothesize that EBV infection of tumor cells results in virus-induced epigenetic alterations involving DNA methylation and chromatin reprogramming providing a selective advantage to tumor cells.;To understand the role of EBV in a setting of incomplete association, we established a transient model of infection. Carcinoma cell lines were infected with recombinant EBV, passaged with selective pressure, and allowed to lose viral genomes by withdrawal of selection. Global gene expression comparing EBV-negative, transiently-infected clones to uninfected controls identified expression changes in ∼ 4% of genes. Downregulated genes included PYCARD and E-cadherin, a tumor suppressor gene hypermethylated in EBV-positive gastric and nasopharyngeal carcinomas. PYCARD was found to have increased DNA methylation in the transiently infected, EBV-negative clones relative to controls. In contrast to PYCARD, transient EBV infection did not alter DNA methylation of the E-cadherin promoter; rather formation of repressive chromatin was responsible for E-cadherin suppression. Decreased histone acetylation at the 5' end of the E-cadherin gene were observed in the EBV-negative, transiently-infected clone relative to the uninfected/mock controls. Through the use of inhibitors and specific shRNA, HDAC1 has been identified as critical for mediating the repression of E-cadherin. Interestingly, infected clones displayed a significant increase in motility and invasion that was maintained even after loss of the virus. These results suggest that EBV can stably alter gene expression in a heritable fashion in formerly infected cells, while its own contribution to the oncogenic process is masked. The results have the potential to expand EBV's role in tumorigenesis to include "hit-and-run" mechanisms of carcinogenesis.