High-level expression of hepatitis B virus HBx gene and hepatocarcinogenesis in transgenic mice

We studied the development of liver tumors in male HBx gene transgenic mice. Of two lineages studied, in the lineage with the lowest HBx gene expression liver tumors developed only in an incidence comparable with that in normal CD-1 strain, whereas 84% of male mice with a high level of the HBx gene product succumbed to liver neoplasia, indicating that continued HBx gene expression higher than a certain threshold level may be necessary for the development of hepatic neoplasia. Sixty-five mice from a lineage with a high level of HBx expression were then followed throughout their 24-mo lifespan. The livers of transgenic mice showed foci of cellular alteration with cytoplasmic vacuolations around the central veins from the age of 2 mo, but these foci did not expand progressively by the age of 12 mo. Immunostaining demonstrated such hepatocytes had higher expression of HBx protein than surrounding cells. Neoplastic lesions including liver cell adenomas and hepatocellular carcinomas developed from the age of 13 mo. By bromodeoxyuridine labeling analysis, hepatocytes in altered foci were found to have increased DNA synthesis, whereas no labeling was observed in age- and sex-matched nontransgenic littermate controls. Furthermore, DNA content analysis revealed the existence of several small aneuploid peaks in the transgenic liver before the age of tumor development. These results suggest that the continued expression of HBx gene may initiate a complex process to hepatocellular carcinoma by inducing DNA synthesis and placing large numbers of hepatocytes subjective to secondary events for transformation.