Bromodomain proteins regulate human cytomegalovirus latency and reactivation allowing epigenetic therapeutic intervention

Abstract

Reactivation of human cytomegalovirus (HCMV) from latency is a major health consideration for recipients of stem-cell and solid organ transplantations. With over 200,000 transplants taking place globally per annum, virus reactivation can occur in more than 50% of cases leading to loss of grafts as well as serious morbidity and even mortality. Here, we present the most extensive screening to date of epigenetic inhibitors on HCMV latently infected cells and find that histone deacetylase inhibitors (HDACis) and bromodomain inhibitors are broadly effective at inducing virus immediate early gene expression. However, while HDACis, such as myeloid-selective CHR-4487, lead to production of infectious virions, inhibitors of bromodomain (BRD) and extraterminal proteins (I-BETs), including GSK726, restrict full reactivation. Mechanistically, we show that BET proteins (BRDs) are pivotally connected to regulation of HCMV latency and reactivation. Through BRD4 interaction, the transcriptional activator complex P-TEFb (CDK9/CycT1) is sequestered by repressive complexes during HCMV latency. Consequently, I-BETs allow release of P-TEFb and subsequent recruitment to promoters via the superelongation complex (SEC), inducing transcription of HCMV lytic genes encoding immunogenic antigens from otherwise latently infected cells. Surprisingly, this occurs without inducing many viral immunoevasins and, importantly, while also restricting viral DNA replication and full HCMV reactivation. Therefore, this pattern of HCMV transcriptional dysregulation allows effective cytotoxic immune targeting and killing of latently infected cells, thus reducing the latent virus genome load. This approach could be safely used to pre-emptively purge the virus latent reservoir prior to transplantation, thereby reducing HCMV reactivation-related morbidity and mortality.

HCMV Antivirals and Strategies to Target the Latent Reservoir

“…With this in mind, Groves et al (2021) built on this work and screened a new generation of HDACis that had lower toxicity and resulted in a substantially higher number of monocytes expressing lytic HCMV antigens in a model of a long-term treatment regime of latently infected monocytes [ 102 ]. However, in these long-term treatment studies, both old and new HDACis also resulted in full reactivation of infectious virus.…”

“…In the study by Groves et al (2021), an I-BET, GSK726 was far superior to HDACis in its ability to induce IE gene expression in latently infected CD14+ monocytes [ 102 ]. Furthermore, unlike most new generation HDACis tested, GSK726 did not result in full reactivation of the virus, but the expression of a select group of lytic antigens, even after long-term incubation.…”

“…HCMV lytic gene expression normally occurs in a tightly controlled, orderly cascade of immediate early (IE), then early (E), followed by DNA replication and late (L) gene expression. RNAseq confirmed that GSK726, unliked the HDACis, was activating a certain set of viral promoters rather than initiating the lytic life cycle [ 102 ].…”

“…I-BET treatment actually resulted in a decrease in the transcription of viral genes with roles in interfering with antigen presentation and T cell recognition: UL21.5, US8, UL18, US10, UL6-11, US2, US3, US6, and US11. Genes which were not activated by I-BET treatment likely depended on Brd4 activity for expression; at certain promoters, Brd4 is actually required to recruit PTEFb [ 102 ]. Additionally, it was found that I-BETs actually inhibited viral DNA replication in permissive fibroblasts [ 102 ].…”

“…Genes which were not activated by I-BET treatment likely depended on Brd4 activity for expression; at certain promoters, Brd4 is actually required to recruit PTEFb [ 102 ]. Additionally, it was found that I-BETs actually inhibited viral DNA replication in permissive fibroblasts [ 102 ].…”