Interaction of mutant hepatitis B X protein with p53 tumor suppressor protein affects both transcription and cell survival.

This study examines the differential activities between wild-type Hepatitis B virus X protein (WtHBx) and a mutant HBx (MutHBx), which bears a hotspot mutation at nucleotides 1,762 and 1,764, resulting in a lysine to methionine change at codon 130 and a valine to isoleucine change at codon 131. This mutation leads to hepatocellular carcinoma, and we evaluated how WtHBx and MutHBx proteins differ in their interactions with the p53 tumor suppressor protein. This was experimentally addressed through co-immunoprecipitation assays examining the interaction between WtHBx and MutHBx proteins with p53, reporter assays determining the impact of the HBx proteins on p53-mediated gene transcription, and clonogenic survival assays evaluating the effect of HBx on cell growth in lines of varying p53-expression status. Both WtHBx and MutHBx proteins physically interact with p53 protein, but have different impacts on p53-mediated gene transcription. WtHBx did not effect p53-mediated gene transcription, whereas MutHBx inhibited it (P < 0.01). MutHBx inhibited colony formation in p53-proficient cells (P < 0.01), but not p53-deficient lines. Although both HBx proteins interact with p53, they affect p53-mediated gene transcription differently. WtHBx has no effect, whereas MutHBx inhibits it. In clonogenic survival assays, MutHBx inhibited cell growth in p53-proficient cells rather than enhanced it. This suggests that for MutHBx to behave oncogenically, the p53 pathway must be crippled or absent. This study has identified some important novel ways in which WtHBx and MutHBx differentially interact with p53 and this could begin to form the cellular explanation for the association between this particular mutant and liver cancer.