Mechanisms of Gene Regulation by EBV EBNA-1 Protein

DESCRIPTION (provided by applicant): Epstein-Barr virus (EBV) is an extremely successful pathogen, able to persist lifelong as a latent infection within B lymphocytes with little overt disease. However, a breakdown in immune surveillance, e.g., as a consequence of AIDS, remains a significant risk factor for development of EBV-associated lymphoma, underscoring the highly evolved equilibrium that exists between this potentially oncogenic herpesvirus and the host immune system. This equilibrium is dependent on a selective down-regulation of EBV latency-associated gene expression during establishment of persistent infection that ultimately restricts expression to viral genes critical for maintenance of persistence, while precluding those with acute transforming properties and/or which encode dominant epitopes recognized by the EBV-specific T-cell surveillance. A pivotal process in this transition to restricted latency is a promoter switching event that enables exclusive expression of the essential EBV genome-maintenance protein, EBNA-1, from the promoter Qp, which can be negatively autoregulated through two EBNA-1 binding sites immediately downstream of its transcription start site. Our recent efforts to define the mechanism of EBNA-1 repression revealed that it acts not by inhibition of transcription, as originally believed, but by suppression of pre-mRNA processing. The principal significance of this autoregulation, furthermore, has recently become apparent. Although EBNA-1 was earlier thought to be "invisible" to the host immune surveillance as a consequence of its ability to inhibit in cis its degradation by the cell proteasome, thereby preventing presentation of EBNA-1 peptide epitopes in association with HLA class I molecules, subsequent studies indicated that cytotoxic T cells that recognize EBNA-1 not only exist, but that they are directed towards peptides generated during actual synthesis of EBNA-1, not by the degradation of mature EBNA-1. Thus, resistance to proteasomal degradation is secondary to the autoregulated expression of EBNA-1 as the primary mechanism employed by EBV to restrict EBNA-1-specific T-cell killing. Further, recently described anti-apoptotic properties of EBNA-1 suggest that it may have tumorigenic potential. We hypothesize, therefore, that the autoregulatory function of EBNA-1 is highly critical to EBV persistence and its associated pathogenic potential: it ensures sufficient EBNA-1 for genome maintenance, while limiting EBNA-1 synthesis below a threshold that, if exceeded, would subject latently infected B cells to elimination by EBNA-1-specific cytotoxic T cells, and potentially oncogenic transformation. We propose three specific aims to help us reach our long-term objective of defining the contribution of EBNA-1 autoregulation to EBV biology, immune evasion and pathogenesis: 1) Elucidate the influence of EBNA-1 on pre-mRNA processing;2) Define the molecular mechanism of EBNA-1 autoregulation;and 3) Elucidate the contributions of EBNA-1 autoregulation to the growth and restricted programs of EBV latency. PUBLIC HEALTH RELEVANCE: Epstein-Barr virus (EBV) is a herpesvirus that has significant potential to cause cancer in its human host, particularly within individuals that become immune suppressed as a consequence of AIDS, for example. Through this research we hope to gain a better understanding of how infection by EBV may be prevented or treated. Specifically, we seek to elucidate the mechanism by which a key EBV protein, EBNA-1, regulates its own expression, and to determine the respective importance of this autoregulatory mechanism in the different forms of EBV infection within B lymphocytes. We hypothesize that the autoregulatory function of EBNA-1 ensures that it is expressed at levels necessary to perform its essential role in propagation of the EBV DNA genome, but below levels that would be detected by the host immune system and that might promote malignancy.