Role of VZV Latency Transcript (VLT) and ORF63 in latency and reactivation

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Varicella-zoster virus (VZV), a human alpha herpesvirus (αHV), establishes lifelong latent infection in ganglionic neurons of >90% humans worldwide, reactivating in one-third to cause herpes zoster (HZ), debilitating pain and stroke. How VZV maintains latency remains unclear. Despite the availability of two FDA-approved HZ vaccines, it is likely that HZ will remain a common and clinically important disease for many years to come. Currently, the VZV latent state is resistant to antiviral targeting, and the triggers of reactivation remain poorly defined. We argue that better knowledge of VZV latency and reactivation will establish the basis for new anti- VZV strategies to reduce the HZ burden. There is no appropriate animal model to study VZV latency and reactivation. To study these processes, our groups pioneered the study of naturally VZV-infected human trigeminal ganglia (TG) and a human embryonic stem cell (hESC)-derived cultured neuron model. We discovered the consistent detection of a novel spliced VZV RNA in human TG, the VZV latency-associated transcript (VLT), which has a genomic position similar to that in other neurotropic αHVs and is antisense to the VZV transactivator from open reading frame 61 (ORF61, ortholog of HSV ICP0). Notably, VLT encodes a protein of unknown function. Human TG also contains ORF63 RNAs, whose protein has roles in regulating viral gene expression and blocking apoptosis. Our overlying hypothesis is that VLT is important for VZV neuronal infection and the latent state, whereas ORF63 is involved in the initiation of reactivation. Our three specific aims are designed to address the VZV latent state genes in both cadaveric human TG and cultured neuron models of VZV latency and reactivation. In Specific Aim 1, we will perform in-depth in situ analyses of latently VZV-infected human TG and VZV-infected cultured hESC neurons to determine if specific human neuron subtypes host VZV latency and reactivation. In Specific Aim 2, we will dissect the mechanisms by which the VLT transcript and/or its encoding protein contribute to lytic, latent and reactivated VZV infections, by making several recombinant VZV VLT mutants and evaluating them in our hESC-derived neuron platform. Finally, Specific Aim 3 will determine the molecular chromatin modifications occurring at the VLT and ORF63 loci during VZV latency and reactivation in both human TG and hESC neurons, using chromatin immunoprecipitations to analyze CTCF binding, phospho/methyl switch of histone protein 3 and paused RNA polymerase II on VZV genomes. By sharing our complementary materials, methods and longstanding expertise in VZV biology, we are now uniquely positioned to characterize VZV latency ex vivo and systematically analyze the function of VLT and its encoded protein in vitro. Successful completion of our proposal will provide insight into molecular mechanisms that regulate VZV latency and reactivation. This will provide leads towards the development of novel intervention strategies that effectively target latent VZV and consequently the burden of HZ disease.