We have shown previously that the EBNA 1 and latent membrane protein encoding regions of the Epstein—Barr virus (EBV) genome are highly methylated at CCGG sequences in the Burkitt's lymphoma (BL)-derived cell line Rael, but are unmethylated in a lymphoblastoid cell line (LCL) harbouring the same virus. To examine whether this is a regular phenomenon, we compared the methylation patterns of selected regions (HI C, W, H, M, E, K and N fragments) of EBV DNA in representative EBV-carrying cell types of normal and neoplastic origin. Analysis of II and I cleavage patterns showed that all probed regions were highly methylated in all six BL biopsy samples, but hypomethylated in the four LCLs immortalized by the virus. EBV DNA was also highly methylated in the nude mouse-passaged C15 nasopharyngeal carcinoma strain and partially methylated in the C18 strain. Eight BL lines propagated , ranging from a typical BL group I to a more LCL-like group III phenotype, showed heterogeneous levels of methylation. Rael, the only stable group I cell line, carried highly methylated viral genomes. The other cell lines, which have drifted to an LCL-like blastic phenotype to various degrees, showed more moderate or low viral DNA methylation. Two sublines of the BL cell line Jijoye, which could be classified as groups II and III, respectively, showed a corresponding difference in EBV DNA methylation. To assess the possible influence of hypomethylated linear EBV DNA molecules produced in lytically infected cells, the virus producer P3HR-1 and Jijoye M13 lines were compared for DNA methylation before and after treatment with phosphonoformic acid (PFA), an inhibitor of the viral DNA polymerase. PFA treatment resulted in a shift towards a more methylated pattern in both regions (HI W and E) assayed, but had no effect on virus non-producer lines (Rael, CB-M1-Ral-STO and Jijoye p79).


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