A Noncanonical Function of Polycomb Repressive Complexes Promotes Human Cytomegalovirus Lytic DNA Replication and Serves as a Novel Cellular Target for Antiviral Intervention.

Chromatin-based modifications of herpesviral genomes play a crucial role in dictating the outcome of infection. Consistent with this, host cell multiprotein complexes, such as polycomb repressive complexes (PRCs), were proposed to act as epigenetic regulators of herpesviral latency. In particular, PRC2 has recently been shown to contribute to the silencing of human cytomegalovirus (HCMV) genomes. Here, we identify a novel proviral role of PRC1 and PRC2, the two main polycomb repressive complexes, during productive HCMV infection. Western blot analyses revealed strong HCMV-mediated upregulation of RING finger protein 1B (RING1B) and B lymphoma Moloney murine leukemia virus insertion region 1 homolog (BMI1) as well as of enhancer of zeste homolog 2 (EZH2), suppressor of zeste 12 (SUZ12), and embryonic ectoderm development (EED), which constitute the core components of PRC1 and PRC2, respectively. Furthermore, we observed a relocalization of PRC components to viral replication compartments, whereas histone modifications conferred by the respective PRCs were specifically excluded from these sites. Depletion of individual PRC1/PRC2 proteins by RNA interference resulted in a significant reduction of newly synthesized viral genomes and, in consequence, a decreased release of viral particles. Furthermore, accelerated native isolation of protein on nascent DNA (aniPOND) revealed a physical association of EZH2 and BMI1 with nascent HCMV DNA, suggesting a direct contribution of PRC proteins to viral DNA replication. Strikingly, substances solely inhibiting the enzymatic activity of PRC1/2 did not exert antiviral effects, while drugs affecting the abundance of PRC core components strongly compromised HCMV genome synthesis and particle release. Taken together, our data reveal an enzymatically independent, noncanonical function of both PRC1 and PRC2 during HCMV DNA replication, which may serve as a novel cellular target for antiviral therapy.IMPORTANCE Polycomb group (PcG) proteins are primarily known as transcriptional repressors that modify chromatin and contribute to the establishment and maintenance of cell fates. Furthermore, emerging evidence indicates that overexpression of PcG proteins in various types of cancers contributes to the dysregulation of cellular proliferation. Consequently, several inhibitors targeting PcG proteins are presently undergoing preclinical and clinical evaluation. Here, we show that infection with human cytomegalovirus also induces a strong upregulation of several PcG proteins. Our data suggest that viral DNA replication depends on a noncanonical function of polycomb repressor complexes which is independent of the so-far-described enzymatic activities of individual PcG factors. Importantly, we observe that a subclass of inhibitory drugs that affect the abundance of PcG proteins strongly interferes with viral replication. This principle may serve as a novel promising target for antiviral treatment.