The latent infection of Epstein-Barr Virus (EBV) is associated with human lymphoid and epithelial cell malignancies. Reversible epigenetic silencing of EBV-encoded oncogenes is known to provide the virus with a survival strategy that includes controlling B-cell maturation and evading immune system detection. The cellular signals and mechanisms regulating the epigenetic silencing have not been fully elucidated. We have investigated the chromatin organization and epigenetic modifications in the control regions of the major latency transcripts of EBV in different latency types. Our studies suggest that epigenetic modifications play a key role in regulating the different gene expression programs of EBV latency. We focus on the control of the major latency promoters Cp and Qp, and how they are regulated by the tumor suppressor protein Rb (aim1), the viral- encoded origin binding protein EBNA1 (aim 2), and the chromatin boundary factor CTCF (aim 3). Rb and Rb-associated factors bind to Cp and regulate a cell cycle-dependent change in chromatin modifications. We propose to explore the role of Rb and Rb-associated factors in the epigenetic silencing that occurs at Cp in type I latency, and cell cycle regulation in type III.
For aim 2, we focus on the role of EBNA1 in the transcription enhancement of Cp and how this is regulated in different cell types. We propose to identify a B-cell specific mediated of EBNA1 enhancer function and whether enhancer-promoter activation correlates with changes in viral chromosome conformation.
In aim 3, we focus on the role of CTCF in regulating the interactions between enhancers and promoters, and in insulating active promoters from silent genes during latency. The long-term goal of this project is to understand the epigenetic mechanisms regulating viral gene expression during latency, and how these mechanisms contribute to viral pathogenesis.
EBV is an important human pathogen that has been linked to multiple malignances and lymphoproliferative disorders. EBV can escape immune detection and clearance by several mechanisms including the ability to reversibly down-regulate immunogenic viral proteins. This down-regulation is known to be regulated by epigenetic changes in the latent viral chromosome. In this application, we propose to investigate key regulatory steps in the formation of epigenetic silencing during EBV latent infection.
In aim 1 we focus on the process by which the major latency promoter is transcriptionally silenced by the Rb tumor suppressor protein, and how this leads to histone H3 K9 and DNA methylation.
In aim 2, we focus on the mechanism of EBNA1 enhancer activation of Cp and the requirement for a B-cell specific coactivator. This B-cell coactivator may account for the type II latency observed in epithelial carcinomas where LMP1 is expressed in the absence of EBNA2.
In aim 3, we explore whether higher order chromatin structure and chromatin organizing factors, like CTCF, are responsible for transcriptional silencing observed in type I latent infections. These studies will help to elucidate mechanisms of viral gene regulation in different latency types. This is clinically relevant because eradication of the immune-resistant latent forms of EBV (type I and type II) will be necessary to eliminate many viral-associated cancers. The recent advent of lytic-induction therapies will also require a better understanding of the mechanisms that control transcription silencing specific for the latent viral chromosome. The proposed studies should further our understanding of EBV latent cycle gene regulation and host-virus interactions.
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