Polycomb repressive complex 2 silences human cytomegalovirus transcription in quiescent infection models.

Chromatin-based regulation of herpesviral transcriptional programs is increasingly appreciated as a mechanism for modulating infection outcomes. Transcriptionally permissive euchromatin predominates during lytic infection, whereas heterochromatin domains refractory to transcription are enriched at lytic genes during latency. Reversibly silenced facultative heterochromatin domains are often enriched for histone H3 trimethylated on lysine 27 (H3K27me3), a modification catalyzed by Polycomb repressive complex 2 (PRC2). The requirement for PRC2 in suppressing the human cytomegalovirus (HCMV) lytic transcriptional program during latency has not been thoroughly evaluated. Therefore, we disrupted PRC2 activity in the highly tractable THP1 and NT2D1 quiescent-infection models by treating cells with small-molecule inhibitors of PRC2 activity. Compared to control cells, disruption of PRC2 in HCMV-infected THP1 or NT2D1 cells resulted in significant increases in viral transcript levels and the detection of viral protein. Using chromatin immunoprecipitation, we demonstrated that enrichment of H3K27me3, deposited by PRC2, correlates inversely with lytic transcriptional output, suggesting that PRC2 catalytic activity at viral chromatin directly represses lytic transcription. Together, our data suggest that PRC2-mediated repression of viral transcription is a key step in the establishment and maintenance of HCMV latency.