Epigenetic Regulation of Lytic and Latent Herpes Simplex Virus 1 Infection

Abstract

Epigenetic regulation plays a major role in whether the herpes simplex virus 1 (HSV-1) will initiate viral gene expression and lytic infection or instead suppress its gene expression and establish a latent infection. Prior to this study, it was known that cells respond to naked DNA by assembling chromatin to silence foreign genetic material. However, during lytic infection of epithelial cells, viral proteins VP16 and ICP0 have been implicated in limiting chromatin association and promoting euchromatic histone modifications on the HSV-1 genome. We hypothesized that the viral genome would also be subject to silencing by heterochromatin modification during lytic infection. To test this we examined the association of chromatin and heterochromatic modifications during lytic infection with WT viruses and ICP0-null mutant viruses. We found that heterochromatin modifications H3K9me3 and H3K27me3 associate initially with all viruses, but were removed rapidly during infection with WT HSV-1. ICP0-null viruses were not able to remove histones or heterochromatin, indicating a role for ICP0 in reversing epigenetic silencing. In latent infection, HSV-1 undergoes epigenetic silencing as a means to suppress gene expression and persist in neurons. Surprisingly, in this study, we find that ICP0-null viruses accumulate less heterochromatin on lytic gene promoters relative to WT viruses. This suggests that ICP0 may function to promote infection of neurons, or assist in the establishment or maintenance of latent infection. Additionally, during latency the viral genome maintains active expression from the latency-associated transcript (LAT) region, and this region retains markers of euchromatin that are excluded from the lytic viral genes. The insulator protein, CTCF, binds to a site downstream of this region between the LAT and ICP0 promoters. We find that during latent infection, deletion of this site promoted accumulation of H3K27me3 at the LAT promoter and reduced reactivation competence of the virus, but surprisingly enhanced LAT expression. This suggests that CTCF balances epigenetic repression to promote latency and maintain reactivation competence. In summary, this dissertation suggests that during lytic infection HSV reverses cell-mediated epigenetic repression and promotes viral gene expression, while during latency, the virus co-opts epigenetic mechanisms to maintain a silenced but poised genome.