Mechanisms of Gene Regulation by EBV EBNA-1 Protein

Sample, Jeffery

Abstract

hin 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 regulated 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 antiapoptotic 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) Define the mechanism of EBNA-1 autoregulation; 2) Elucidate the contributions of EBNA-1 autoregulation to the growth and restricted programs of latency; and 3) Define the respective roles of Qp and Fp, an alternative adjacent EBNA-1 promoter, in EBV infection.