Kinetics of Facultative Heterochromatin and Polycomb Group Protein Association with the Herpes Simplex Viral Genome during Establishment of Latent Infection

Abstract

ABSTRACT The herpes simplex virus (HSV) genome is associated with heterochromatic histone modifications, including trimethylation of the lysine 27 residue of histone H3 (H3K27me3), during latent infection of neurons. Here we have examined the kinetics of general chromatin and H3K27me3 association with the viral genome during establishment of latent infection. Using both wild-type virus and a mutant virus that is unable to undergo replication in neurons, we found that histone H3 associates with viral gene promoters by 7 days postinfection (dpi). Levels of H3K27me3 were low at 7 dpi but increased dramatically by 14 dpi. Hence, general chromatin association and/or other factors may play a key role(s) in the initial silencing of lytic genes, and H3K27me3 may play a role in further suppression of the genome and/or the maintenance of latency. A component of Polycomb repressive complex 2 (PRC2), which mediates the addition of K27me3 to histone H3 (Suz12), was also recruited by 14 dpi. We have shown previously that the levels of H3K27me3 during latent infection are increased in the presence of the latency-associated transcript (LAT). However, the initial targeting of PRC2 was not found to be dependent on the LAT. We found that a component of the PRC1 complex (Bmi1), which binds to H3K27me3, was not enriched at promoters found previously to be enriched for H3K27me3. Our results are consistent with (i) chromatinization of viral DNA or other mechanisms causing the initial silencing of HSV lytic genes and (ii) facultative heterochromatin maintaining that silencing during latent infection of neurons. IMPORTANCE The human pathogen herpes simplex virus (HSV) hides for the lifetime of the host in peripheral neurons. The mechanism by which HSV is able to shut off its gene expression and persist in neurons is not known. Here we show that the HSV DNA first associates with histone H3, with later recruitment of Polycomb repressor complex 2 (PRC2) and trimethylation of the lysine 27 residue of histone H3 (H3K27me3), a modification associated with heterochromatin. This work indicates that the initial silencing of HSV gene expression is not correlated with enrichment of H3K27me3 and that PRC2 may be recruited to already-silenced genes to further silence gene expression and/or maintain gene silencing. We demonstrate that recruitment of PRC2 is not dependent upon expression of the noncoding HSV latency-associated transcripts, indicating the presence of unknown triggers for PRC2 recruitment during the establishment of latent infection.