THE ONCOGENIC POTENTIAL OF HEPATITIS B VIRUS (TRANSFORMATION)

A growing body of evidence has accumulated suggesting a causal association between chronic Hepatitis B Infection and primary liver cancer. Geographic areas known to be endemic for HBV overlap with areas known to have a high incidence of hepatocellular carcinoma. Another suggestive line of evidence is the presence of integrated forms of the virus in 85% of liver tumors from HBV carriers.; In order to define a possible role for the virus in hepatocarcinogenesis cloned viral DNA was used to test its oncogenic potential. A head to tail dimer of the viral genome which had been cloned into bacterial plasmid, pBR322, was used to transfect NIH 3T3 cells and the resulting cultures were observed for foci of transformation. The parental plasmid, pBR322, served as a negative control.; The results of the assay were expressed in foci per microgram of transfected DNA. The mean transformation frequency for pBR322 was 0.047 foci per microgram while that of cloned viral DNA was 0.300 foci per microgram. Statistical analysis using the paired T test revealed that the probability of this result occurring by chance was 0.0012, (P = .0012).; Viral transformants were tested for anchorage independence by growth in soft agar. None of the transformants from control plates formed colonies but four of the seven HBV transformants were able to grow in semi-solid medium, (P = .0147). These soft agar clones were tested for tumorigenicity in NIH Swiss nude mice. None of the mice injected with parental 3T3 cells developed tumors but ten of the eleven mice injected with virally transformed cell lines developed subcutaneous, nonmetastasizing tumors in a matter of a few weeks.; Virally transformed lines were analyzed by slot blot hybridization and shown to contain only sub-genomic quantities of viral DNA. The transforming region of the viral genome was localized by testing sub-genomic clones for activity and by restriction inactivation. These methods identified two possible fragments responsible for transformation. The low frequency of transformation, the small size of the possible transforming regions and the very long incubation period for HBV related hepatocellular carcinomas suggest a mechanism other than that of a viral oncogene.;...