Integrated hepatitis B virus DNA sequences specifying the major viral core polypeptide are methylated in PLC/PRF/5 cells.

The methylation of various hepatitis B virus (HBV) DNA sequences was examined using the restriction endonucleases Hpa II and Msp I. HBV DNA from virions (Dane particles) and virus-infected liver tissue was digested with Hpa II or Msp I and fractionated by electrophoresis in agarose gels, and the restriction enzyme cleavage pattern was examined by Southern blot analysis. No methylation of the 5' C-C-G-G 3' recognition sequence was detected in either virion DNA or HBV DNA from infected liver tissue. The tissue culture cell line PLC/PRF/5, derived from a human hepatoma, possesses HBV DNA exclusively integrated at several sites. Digestion of PLC/PRF/5 DNA with Hpa II and Msp I revealed that the integrated HBV DNA sequences were methylated. Further analysis using probes specific for various regions of the HBV genome showed that some of the hepatitis B viral DNA sequences, including those specifying the major surface antigen polypeptide, were methylated infrequently or not at all. In contrast, the viral DNA sequences coding for the major core polypeptide were extensively methylated. Because the surface antigen is expressed in these cells while the core antigen is not, our results suggest that DNA methylation could account for the selective expression of HBV genes in this hepatoma cell line.