MORC3 restricts human cytomegalovirus infection by suppressing the major immediate‐early promoter activity

Abstract

During the long coevolution of human cytomegalovirus (HCMV) and humans, the host has formed a defense system of multiple layers to eradicate the invader, and the virus has developed various strategies to evade host surveillance programs. The intrinsic immunity primarily orchestrated by promyelocytic leukemia (PML) nuclear bodies (PML-NBs) represents the first line of defense against HCMV infection. Here, we demonstrate that microrchidia family CW-type zinc finger 3 (MORC3), a PML-NBs component, is a restriction factor targeting HCMV infection. We show that depletion of MORC3 through knockdown by RNA interference or knockout by CRISPR-Cas9 augmented immediate-early protein 1 (IE1) gene expression and subsequent viral replication, and overexpressing MORC3 inhibited HCMV replication by suppressing IE1 gene expression. To relief the restriction, HCMV induces transient reduction of MORC3 protein level via the ubiquitin-proteasome pathway during the immediate-early to early stage. However, MORC3 transcription is upregulated, and the protein level recovers in the late stages. Further analyses with temporal-controlled MORC3 expression and the major immediate-early promoter (MIEP)-based reporters show that MORC3 suppresses MIEP activity and consequent IE1 expression with the assistance of PML. Taken together, our data reveal that HCMV enforces temporary loss of MORC3 to evade its repression against the initiation of immediate-early gene expression.

MORC3 represses the HCMV major immediate early promoter in myeloid cells in the absence of PML nuclear bodies

“…The focus of this study was to understand the role of MORC3 upregulation and activity in the establishment and maintenance of latency. SUMOylated MORC3, associated with PML, is known to mediate transcriptional repression in a Daxx‐dependent manner 19–21 . However, our data suggest that MORC3‐mediated repression of the MIEP during latency is unlikely to involve SUMOylated MORC3 or its interaction with PML/Daxx for a number of reasons.…”

“…The data presented here demonstrate that MORC3 plays an important role in repressing viral MIEP activity to establish and maintain HCMV latency. This is in contrast to lytic infection where MORC3 is degraded to enhance MIEP activity 21,24 . Thus we propose a balanced model of MORC3 upregulation during latency and downregulation during reactivation/lytic infection.…”

“…This is in contrast to lytic infection where MORC3 is degraded to enhance MIEP activity. 21,24 Thus we propose a balanced model of MORC3 upregulation during latency and promoter to silence gene expression, which was lost upon degradation. Essentially, this was proposed as a fail-safe against manipulation by pathogen infection.…”

“…In contrast, deSUMOylated MORC3 forms NBs independently of PML and forms a repressive molecular DNA clamp 20 . In herpes simplex virus 1 (HSV‐1) or HCMV lytic infection, MORC3 is known to play a repressive role in a Daxx‐dependent manner and is targeted for degradation to enhance viral lytic gene expression 21,22 . Furthermore, it has also been demonstrated that MORC3 silences endogenous retroviruses again in a Daxx dependent manner.…”

“…We believe that this is likely accompanied by deSUMOylation of MORC3 by LUNA which may also enhance the ability of MORC3 to form a molecular clamp and associate with the MIEP on the viral genome. These repressive MORC3 NBs function to aid repression of the MIEP during latency in the absence of PML NBs.How the MORC3 NBs are removed before reactivation is not yet known but it is known that during lytic infection, MORC3 is degraded, although again the viral protein required for this is not known 21. Interestingly, MORC3 is also degraded during HSV-1 infection via the viral protein ICP022 thus degradation of MORC3 during lytic infection may be a common function to herpesviruses.…”