The HUSH-SETDB1-MORC2 epigenetic repressor complex restricts herpesvirus infection in association with PML nuclear bodies

The establishment of latent herpes simplex virus 1 (HSV-1) infection is controlled by promyelocytic leukemia nuclear bodies (PML NBs). Viral genomes are recruited to PML NBs structures and chromatinized by repressive H3.3K9me3 modified H3.3 histone variant to form viral DNA-containing PML-NBs (vDCP NBs). Exactly how this occurs is unclear. Here we identify an essential role for the HUSH complex and its SETDB1 and MORC2 effectors in the establishment and maintenance of latent herpes simplex virus 1 (HSV-1) infection. ChIP-seq analyses conducted on latently/quiescently infected primary human fibroblasts highlight the association of the H3K9me3 mark with the entire viral genome. We demonstrate the necessity of HUSH, SETDB1, and MORC2 in the establishment of repressive heterochromatin in vDCP NBs. Depletion of these components prior to viral infection results in reduced H3K9me3 levels across the entirety of latent/quiescent HSV-1 genomes, with minimal impact on the cellular genome as a whole. Once latency is established, depletion of HUSH, SETDB1, or MORC2 by shRNAs or the HIV-2ROD Vpx protein, induces the reactivation of HSV-1 in infected primary human fibroblasts as well as in human induced pluripotent stem cell-derived sensory neurons (hiPSDN). We discovered that the viral protein ICP0 induces MORC2 degradation via the proteasome machinery. This process is concurrent with ICP0 capability to initiate full HSV-1 reactivation, as well as the significant reactivation of HSV-1 upon MORC2 depletion in hiPSDN. Our data demonstrate the potent antiviral restriction activity of the HUSH/SETDB1/MORC2 complex to a non-integrated human herpesvirus, its close association with PML NBs, and introduces a new target for anti-herpesvirus therapy.