Herpesvirus latency is generally thought to be governed by epigenetic modifications, but the dynamics of viral chromatin at early timepoints of latent infection are poorly understood. Here, we report a comprehensive spatial and temporal analysis of DNA methylation and histone modifications during latent infection with Kaposi Sarcoma-associated herpesvirus (KSHV), the etiologic agent of Kaposi Sarcoma and primary effusion lymphoma (PEL). By use of high resolution tiling microarrays in conjunction with immunoprecipitation of methylated DNA (MeDIP) or modified histones (chromatin IP, ChIP), our study revealed highly distinct landscapes of epigenetic modifications associated with latent KSHV infection in several tumor-derived cell lines as well as de novo infected endothelial cells. We find that KSHV genomes are subject to profound methylation at CpG dinucleotides, leading to the establishment of characteristic global DNA methylation patterns. However, such patterns evolve slowly and thus are unlikely to control early latency. In contrast, we observed that latency-specific histone modification patterns were rapidly established upon a de novo infection. Our analysis furthermore demonstrates that such patterns are not characterized by the absence of activating histone modifications, as H3K9/K14-ac and H3K4-me3 marks were prominently detected at several loci, including the promoter of the lytic cycle transactivator Rta. While these regions were furthermore largely devoid of the constitutive heterochromatin marker H3K9-me3, we observed rapid and widespread deposition of H3K27-me3 across latent KSHV genomes, a bivalent modification which is able to repress transcription in spite of the simultaneous presence of activating marks. Our findings suggest that the modification patterns identified here induce a poised state of repression during viral latency, which can be rapidly reversed once the lytic cycle is induced.
A characteristic feature of herpesviruses is their ability to establish a latent infection during which most of the viral genes are silenced. As a consequence, no viral progeny is produced and the host cell remains viable. While the viral genome may persist in the nucleus of such cells indefinitely, it retains the ability to re-enter the lytic cycle and produce new virions if conditions in the cell become unfavorable. The molecular requirements for the establishment of latency are poorly understood, but are thought to depend on epigenetic modifications of the viral episome. Here, we report a genome-wide screen to investigate DNA methylation and histone modification patterns associated with latent infection by Kaposi Sarcoma-associated herpesvirus (KSHV), a tumor virus linked to the development of several cancers. We find that latency is likely to be determined by modifications commonly associated with genes that are transcriptionally “poised”. The promoters of such genes harbor activating as well as repressive histone marks such that they are silenced, but they can be rapidly activated upon removal of the repressive marks. Our findings thus may explain how KSHV achieves efficient quiescence during latency, yet retains the potential to quickly revert to a fully active state upon induction of the lytic cycle.