Role of the histone H3 lysine 9 methyltransferase Suv39 h1 in maintaining Epsteinn-Barr virus latency in B95-8 cells

The ability of Epstein-Barr Virus (EBV) to establish latent infection is associated with infectious mononucleosis and a number of malignancies. In EBV, the product of the BZLF1 gene (ZEBRA) acts as a master regulator of the transition from latency to the lytic replication cycle in latently infected cells. EBV latency is primarily maintained by hypoacetylation of histone proteins in the BZLF1 promoter by histone deacetylases. Although histone methylation is involved in the organization of chromatin domains and has a central epigenetic role in gene expression, its role in maintaining EBV latency is not well understood. Here we present evidence that the histone H3 lysine9 (H3K9) methyltransferase suppressor of variegation 3-9 homolog1 (Suv39h1) transcriptionally represses BZLF1 in B95-8 cells by promoting repressive trimethylation at H3K9 (H3K9me3). Suv39h1 significantly inhibited basal expression and ZEBRA-induced BZLF1 gene expression in B95-8 B cells. However, mutant Suv39h1 lacks the SET domain responsible for catalytic activity of histone methyl transferase and thus had no such effect. BZLF1 transcription was augmented when Suv39h1 expression was knocked down by siRNA in B95-8 cells, but not in Akata or Raji cells. In addition, treatment with a specific Suv39h1 inhibitor, chaetocin, significantly enhanced BZLF1 transcription. Furthermore, chromatin immunoprecipitation assays revealed the presence of Suv39h1 and H3K9me3 on nucleosome histones near the BZLF1 promoter. Taken together, these results suggest that Suv39h1-H3K9me3 epigenetic repression is involved in BZLF1 transcriptional silencing, providing a molecular basis for understanding the mechanism by which EBV latency is maintained.