EPIGENETIC REGULATION OF QUIESCENT HERPES SIMPLEX VIRUS TYPE 1 GENE EXPRESSION

HSV-1 is a ubiquitous human pathogen with a biphasic lifecycle. During latency, gene expression is globally repressed, most likely due to epigenetic mechanisms. The HSV protein ICP0 is a promiscuous transactivator of gene expression, and is required for efficient viral reactivation from latency. Evidence indicates that ICP0 interacts with a number of cellular pathways that mediate chromatin structure. Using a cell culture model of latency, the effects of cell-type, ICP0 expression, and ICP0 functional domains on the chromatin structure of viral genomes was examined. During viral entry into quiescence, ICP0 expression increased gene expression and histone hyperacetylation, while limiting the association of histones and heterochromatin with the viral genome. In the absence of ICP0, expression from viral promoters was rapidly repressed, and heterochromatin formed on viral promoters in a cell-type specific manner. Once quiescence was fully established, HSV genomes were found in a highly heterochromatic state. Heterochromatin, measured by the presence of heterochromatin protein 1γ and trimethylation of histone H3 lysine 9, was rapidly removed upon provision of ICP0 in trans. The changes in epigenetic structure were global, and preceded reactivation of gene expression. Overexpression of ICP0 resulted in the removal of histones from the quiescent genome, an effect not seen when ICP0 was expressed at physiological levels. This indicated that ICP0 may be mediating its effects through multiple functions, potentially through the enzymatic function of its RING finger (RF) E3 ubiquitin ligase domain when expressed at low levels, and through physical protein-protein interaction when overexpressed. The effects on chromatin structure and gene activation of quiescent HSV were determined by superinfection with mutants of the RF domain and the C-terminus, which is important for the disruption of the CoREST/HDAC repressor complex. The RF domain was necessary and sufficient for reactivation of gene expression, but full transcriptional activation and removal of heterochromatin and histones required both the RF and C-terminus of ICP0. These results indicate that ICP0 relieves repression of gene expression through interactions with multiple cellular pathways. Expression of ICP0 removes epigenetic repression at multiple levels of chromatin, including heterochromatin, histone deposition, and the acetylation of histones