Programmed splicing derangement as new EBV host cell shut-off mechanism

About This Grant

Summary The Epstein Barr virus is a DNA tumor virus that is associated with human pathologies including Hodgkin’s lymphoma, non-Hodgkin’s lymphoma, stomach cancer, nasopharyngeal carcinoma and autoimmune diseases. EBV is particularly problematic in the HIV/AIDS population where EBV associated lymphomas are especially prevalent. While more than 90% of the world’s population carries EBV, the virus typically exists in a “latent” state with little impact on the host. In response to certain stimuli or local microenvironmental cues, however, EBV enters the viral lytic replication program, leading to viral spread both within and between hosts. Despite the known role of viral latency proteins in EBV associated cancers, there are well-established links between elevated lytic replication and the onset of EBV associated cancers. Further, general elevation of EBV lytic replication is observed in the context of HIV co-infection (+ or – ART), likely contributing to the increased susceptibility of HIV infected individuals to EBV associated lymphomas and autoimmune diseases. With minimal genetic content, viruses are highly dependent on host cell resources for their replication and they elicit extensive alterations of host cell metabolic processes to facilitate efficient virus replication. One of the most conserved and well studied virus-host interactions in herpesvirus replication is “host shut off” where virus encoded factors degrade host cell RNAs destined for translation, freeing up translation resources for dedicated production of high amounts of viral structural proteins. Recently, the Glaunsinger lab showed that despite inducing global Pol III activation of host B2 SINE elements, the murine γ-herpesvirus, MHV68, inhibits host Pol II transcription as a second arm of host shut off, further promoting preferential viral protein synthesis. Using EBV reactivation models that facilitate interrogation of transcriptome changes in pure reactivating cell populations, we have gained insights into remarkable and unexpected interactions between EBV and the host cell transcriptome. Unlike MHV68, we found that EBV sustains cell Pol II gene expression at canonical promoters during lytic replication and strikingly, causes transcription at >10,000 new Pol II initiation sites across the cell genome. While the reason for the broad induction of predominantly non-coding Pol II (EBV) or Pol III (MHV68) transcription across host genomes is unclear, it could relate to some role in remodeling nuclear structure or redistribution of nuclear resources. Our studies also revealed that EBV reactivation induces widespread, noncanonical exon skipping, the extent of which surpasses the degree of exon skipping observed upon severe depletion of most spliceosome components. Preliminary analysis of KSHV reactivation similarly revealed widespread induction of exon skipping indicating that splicing disruption is not unique to EBV. Previous studies have shown that exon skipping can cause either nuclear retention or cytoplasmic nucleolytic degradation by the cellular nonsense mediated RNA decay (NMD) pathway; and we show that nearly 50% of exon skipping events observed during reactivation are NMD candidates. We hypothesize that EBV (and KSHV) lytic replication induces extensive non-canonical exon skipping of cell transcripts resulting in either nuclear retention or degradation through the cytoplasmic NMD pathway as a second, new arm of host shut off. While classic host shut off has been studied for many years, how specificity for cell transcripts is achieved has been largely enigmatic. Notably, herpesviral lytic genes exhibit a remarkably consistent feature of being primarily mono-exonic (i.e. unspliced). We hypothesize that splicing derangement is a new arm of host shut off that facilitates selective targeting of spliced cell transcripts to free up resources for high-level production of viral proteins. In this proposal, we will test this hypothesis, we will begin to address the mechanisms through which EBV induces splicing derangement and we will begin to address the consequences of splicing derangement on host and viral gene expression.