Host-cell-determined methylation of specific Epstein-Barr virus promoters regulates the choice between distinct viral latency programs.

Epstein-Barr virus (EBV) is capable of adopting three distinct forms of latency: the type III latency program, in which six EBV-encoded nuclear antigens (EBNAs) are expressed, and the type I and type II latency programs, in which only a single viral nuclear protein, EBNA1, is produced. Several groups have reported heavy CpG methylation of the EBV genome in Burkitt's lymphoma cell lines which maintain type I latency, and loss of viral genome methylation in tumor cell lines has been correlated with a switch to type III latency. Here, evidence that the type III latency program must be inactivated by methylation to allow EBV to enter the type I or type II restricted latency program is provided. The data demonstrates that the EBNA1 gene promoter, Qp, active in types I and II latency, is encompassed by a CpG island which is protected from methylation. CpG methylation inactivates the type III latency program and consequently allows the type I or II latency program to operate by alleviating EBNA1-mediated repression of Qp. Methylation of the type III latency EBNA gene promoter, Cp, appears to be essential to prevent type III latency, since EBNA1 is expressed in all latently infected cells and, as shown here, is the only viral antigen required for activation of Cp. EBV is thus a pathogen which subverts host-cell-determined methylation to regulate distinct genetic programs.     

Epstein-barr virus (EBV) nuclear protein 2-induced disruption of EBV latency in the Burkitt's lymphoma cell line Akata: analysis by tetracycline-regulated expression

The Burkitt's lymphoma (BL) cell line Akata retains the latency I program of Epstein-Barr virus (EBV) gene expression and cross-linking of its surface immunoglobulin G (IgG) by antibodies results in activation of viral replication. When EBV nuclear antigen 2 (EBNA2) was artificially expressed by a constitutive expression vector, the Cp EBNA promoter remained inactive and accordingly the latency III program was not induced. In contrast, expression of LMP2A and activity of the Fp lytic promoter were activated. Consistent with this Fp activity, the rate of spontaneous activation of the EBV replicative cycle was increased significantly, suggesting the possibility that EBNA2 can induce EBV replication. The efficiency of anti-IgG-induced activation of the viral replication was reduced in Akata cells expressing EBNA2. To obtain more direct evidence for EBNA2-induced activation of the EBV replicative cycle, this protein was next expressed by a tetracycline-regulated expression system. EBNA2 was undetectable with low doses (<0.5 microgram/ml) of tetracycline, while its expression was rapidly induced after removal of the antibiotic. This induced expression of EBNA2 was immediately followed by expression of EBV replicative cycle proteins in up to 50% of the cells, as shown by indirect immunofluorescence and immunoblot analysis. These results suggest an unexpected potential of EBNA2 to disrupt EBV latency and to activate viral replication.     

Epstein-Barr virus nuclear antigen 1 replication and segregation functions in nasopharyngeal carcinoma cell lines

Nasopharyngeal carcinomas (NPC) are usually Epstein-Barr virus (EBV) positive, but, with the exception of C666-1 cells, these cells lose the EBV genomes when grown in culture. Maintenance of EBV requires the viral EBV nuclear antigen 1 (EBNA1) protein, which ensures the replication and mitotic segregation of the genomes through interactions with OriP. Here we compare the abilities of C666-1 and NPC cells that have lost EBV genomes to replicate and segregate OriP plasmids. We found that either cell line can replicate and maintain OriP plasmids for extended periods under conditions where low levels of EBNA1 are expressed but that high EBNA1 levels selectively interfered with mitotic segregation.