Specific Histone Tail Modification and Not DNA Methylation Is a Determinant of Herpes Simplex Virus Type 1 Latent Gene Expression

Abstract

During herpes simplex virus type 1 (HSV-1) latency, gene expression is tightly repressed except for the latency-associated transcript (LAT). The mechanistic basis for this repression is unknown, but its global nature suggests regulation by an epigenetic mechanism such as DNA methylation. Previous work demonstrated that latent HSV-1 genomes are not extensively methylated, but these studies lacked the resolution to examine methylation of individual CpGs that could repress transcription from individual promoters during latency. To address this point, we employed established models to predict genomic regions with the highest probability of being methylated and, using bisulfite sequencing, analyzed the methylation profiles of these regions. We found no significant methylation of latent DNA isolated from mouse dorsal root ganglia in any of the regions examined, including the ICP4 and LAT promoters. This analysis indicates that methylation is unlikely to play a major role in regulating HSV-1 latent gene expression. Subsequently we focused on differential histone modification as another epigenetic mechanism that could regulate latent transcription. Chromatin immunoprecipitation analysis of the latent HSV-1 DNA repeat regions demonstrated that a portion of the LAT region is associated with histone H3 acetylated at lysines 9 and 14, consistent with a euchromatic and nonrepressed structure. In contrast, the chromatin associated with the HSV-1 DNA polymerase gene located in the unique long segment was not enriched in H3 acetylated at lysines 9 and 14, suggesting a transcriptionally inactive structure. These data suggest that histone composition may be a major regulatory determinant of HSV latency.

Modulation of reactivation of latent herpes simplex virus 1 in ganglionic organ cultures by p300/CBP and STAT3

“…Second, to remain latent, viral DNA must be extensively remodeled into facultative heterochromatin (42)(43)(44). The results that we present are that inhibition of STAT3 or activation of p300/CBP is capable of modifying latent viral DNA to the point where it can replicate and be susceptible to antiviral drugs.…”

Herpes simplex viral-vector design for efficient transduction of nonneuronal cells without cytotoxicity

“…Recent findings have shown that, during latency, the cellular CTCF protein is enriched on the two HSV CTRLs and that acetylated histone H3, an epigenetic beacon of active transcription, accumulates in the LAT region between these elements (19,(43)(44)(45). These findings suggested that CTRLs may act as boundary elements to shield the LAT locus against the spreading of repressive epigenetic modifications (29), and it was shown in short-term plasmid transfection assays that a fragment encompassing CTRL2 combines the functions of a typical insulator (19).…”

Transcriptional coactivator HCF-1 couples the histone chaperone Asf1b to HSV-1 DNA replication components

“…Upon stimulation, HCF-1 is rapidly transported to the nucleus and recruited to the viral IE promoters during the initiation of viral reactivation (12)(13)(14). As repressive and activating chromatin modifications correlate with latency and reactivation, respectively (15)(16)(17)(18)(19)(20), it has been hypothesized that HCF-1 modification complexes may play a central role in this process, in a manner analogous to its role in the initiation of viral lytic infection.…”