Epigenetics of viral persistence induced by the association of latent herpes simplex virus 1 with promyelocytic leukemia nuclear bodies (PML-NBs) – EPIPRO

The comprehension of the molecular mechanisms that contribute to the establishment and then maintenance of a persistent virus in its lethargic state is essential to understand the reasons for the resurgence of the associated disease once the virus reactivates. The EPIPRO project is designed to understand one of the key aspects of the interaction between a persistent virus, the herpes simplex virus type 1 (HSV-1), and the host cell, focusing on the role of promyelocytic leukemia (PML) nuclear bodies (NBs) as nuclear epigenetic relays of the intrinsic antiviral response.
HSV-1 is a common human pathogen, which stays a major public health problem. HSV-1 induces recurrent infections, mainly in the face and occasionally in the eyes, which results in severe ocular pathologies. Primary infection occurs in oral mucosa, and is followed by a secondary infection called latency occurring in ganglia of the peripheral nervous systems (PNS). Following various triggering factors, HSV-1 reactivates and spreads towards the face and the central nervous system (CNS). When the virus reactivates in the eyes each episode of ocular herpes can affect the visual prognosis. Indeed, herpes simplex keratitis (HSK) is still today the first infectious cause of blindness in western countries. The comprehension of the molecular mechanisms that regulates the balance between latency and reactivation of HSV-1 is crucial for the understanding of the biology of the virus. This is also a major challenge for the design of new therapeutic approaches based on preventing the virus to reactivate rather than stopping the lytic cycle. Indeed, antiviral therapies are administrated at a time often so late that the virus has already caused irreversible damage. This is particularly true when reactivation affects noble tissues (eye or brain), or in the cases of reactivations provoked by severe immunodeficiencies like in patients co-infected by other viruses such as HIV.
In two recent studies, we showed that the positioning and the distribution of the latent HSV-1 genomes in the nucleus of infected neurons are not random. We showed that the establishment of HSV-1 latency in mice and human trigeminal ganglia neurons correlated with the formation of PML nuclear bodies containing HSV-1 genomes (named viral DNA-containing PML-NBs, vDCP-NBs). Using Pml KO mice we demonstrated that Pml was directly implicated in the formation of the vDCP-NBs and in the transcriptional control of the non-coding viral RNA, called LAT (for Latency Associated Transcript), which is synthesized during the latency process. As a consequence, all PML-NBs-associated viruses were transcriptionally silent, suggesting an epigenetic mechanism of HSV-1 genomes repression associated with the vDCP-NBs. Indeed, the in vivo analysis of the first moments of neuron infection demonstrated that the formation of vDCP-NBs starts very early during the process of latency establishment.
Our published and unpublished data show that vDCP-NBs contain the histone variant H3.3 chaperones, i.e., the Daxx-ATRX and HIRA complexes. This highly suggests that vDCP-NBs are involved in a HSV-1-specific and latency-associated chromatinization. Moreover, previous studies demonstrated that HSV-1 remains as a latent extrachromosomal chromatinized DNA in the nucleus of infected neurons of the PNS and CNS. The project will (i) characterize the involvement of vDCP-NBs in the H3.3-dependent HSV-1 chromatinization in cultured cells and mice; (ii) study the mechanism of vDCP-NB-dependent HSV-1 latency establishment and reactivation in human induced Pluripotent Stem Cells (hiPSCs)-derived neuron models; (iii) study the PNS and CNS spread of HSV-1 infection in mice and human, and the role of PML-NBs and type I interferon in preventing CNS infection.