Date Published: September 13, 2018
Publisher: Public Library of Science
Author(s): Sharon E. Hopcraft, Samantha G. Pattenden, Lindsey I. James, Stephen Frye, Dirk P. Dittmer, Blossom Damania, Michael Lagunoff.
Kaposi’s sarcoma-associated herpesvirus (KSHV) is the etiologic agent of three human malignancies, the endothelial cell cancer Kaposi’s sarcoma, and two B cell cancers, Primary Effusion Lymphoma and multicentric Castleman’s disease. KSHV has latent and lytic phases of the viral life cycle, and while both contribute to viral pathogenesis, lytic proteins contribute to KSHV-mediated oncogenesis. Reactivation from latency is driven by the KSHV lytic gene transactivator RTA, and RTA transcription is controlled by epigenetic modifications. To identify host chromatin-modifying proteins that are involved in the latent to lytic transition, we screened a panel of inhibitors that target epigenetic regulatory proteins for their ability to stimulate KSHV reactivation. We found several novel regulators of viral reactivation: an inhibitor of Bmi1, PTC-209, two additional histone deacetylase inhibitors, Romidepsin and Panobinostat, and the bromodomain inhibitor (+)-JQ1. All of these compounds stimulate lytic gene expression, viral genome replication, and release of infectious virions. Treatment with Romidepsin, Panobinostat, and PTC-209 induces histone modifications at the RTA promoter, and results in nucleosome depletion at this locus. Finally, silencing Bmi1 induces KSHV reactivation, indicating that Bmi1, a member of the Polycomb repressive complex 1, is critical for maintaining KSHV latency.
Kaposi’s sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus 8, is the causative agent of three human malignancies, the endothelial cell cancer Kaposi’s sarcoma (KS), as well as two B cell cancers, primary effusion lymphoma (PEL) and multicentric Castleman’s disease (reviewed in ). KSHV establishes latent infections, where only a few viral genes and microRNAs are expressed, but can be reactivated from latency to the lytic phase of the viral life cycle, where all viral genes are expressed, the viral genome is replicated, and progeny virions are released. Although the majority of KSHV positive cells in PEL and KS are latently infected, the virus undergoes spontaneous reactivation in a fraction of these cells . It is thought that spontaneous reactivation contributes to KSHV maintenance, and that certain lytic proteins shape the clinical pathology of KS [3, 4].
In this study, we aimed to identify host chromatin-modifying proteins that are critical for the maintenance of KSHV latency. To this end, we screened a collection of inhibitors that target chromatin associated proteins, including histone writers, readers, and erasers, for their ability to stimulate KSHV reactivation. Romidepsin and Panobinostat, two HDAC inhibitors not previously shown to induce KSHV reactivation, as well as (+)-JQ1, a bromodomain inhibitor, and PTC-209, a Bmi1 inhibitor, induced KSHV lytic gene transcription. While Romidepsin and Panobinostat stimulated expression of all viral genes (Fig 3A), only a subset of viral genes were induced by (+)-JQ1 and PTC-209 (Fig 3B), suggesting that modulation of different epigenetic signals may activate viral gene transcription through different pathways. This possibility will be explored in future investigation. Romidepsin, Panobinostat, and PTC-209 induced viral DNA replication and the production of infectious virions. Histone modifications at the RTA promoter were altered by treatment with these compounds, where Romidepsin and Panobinostat increased H3 acetylation, and PTC-209 decreased H2AK119ub. Furthermore, each of these compounds induced nucleosome depletion at the RTA promoter. These data indicate that histone deacetylation and monoubiquitination at the RTA promoter aid KSHV latency, and conversely, that these marks need to be reversed to allow RTA transcription.