Identification of a natural mutant of HBV X protein truncated 27 amino acids at the COOH terminal and its effect on liver cell proliferation

Aim: To identify mutants of the hepatitis B virus (HBV) X (HBx) gene and investigate the effect of the natural mutant on liver cell proliferation. Methods: We identified natural mutants of the HBx gene from 188 sera and 48 tissues of Chinese patients infected with HBV by PCR, respectively. Based on the identification of the mutants of HBx gene, we cloned the fragments of the mutants into the pcDNA3 vector. The biological activities of the mutants were investigated. Results: We identified a natural mutant of the HBx gene with deletion from 382 to 401 base pairs from 3 sera out of 188 patients, which resulted in the expression deletion of the HBx protein from the 128th amino acid at the COOH terminal. The similar mutant with deletion from 382 base pair at the COOH terminal was identified from 5 cases of genomes out of 48 hepatocellular carcinoma tissues. Regarding the biological activities of the mutant, we found that the mutant of the HBx protein failed to induce apoptosis by transient transfection, but promoted proliferation of human liver immortalized L-O2 cells by stable transfection, compared with the wild-type HBx protein. The data showed that the proliferation of the mutant stably-trans-fected L-O2-X-Sera cells and fragment stably-transfected L-O2-X Delta 127 cells was enhanced by the BrdU incorporation assay and flow cytometry analysis. Lu-ciferase reporter gene assay showed that the transcriptional activities of NF-kappa B, survivin, and human telomerase reverse transcriptase were upregulated, and Western blot analysis revealed that the expression levels of c-Myc and proliferating cell nuclear antigen (PCNA) were upregulated in the cells. Conclusion: Our findings suggest that the natural HBx mutant truncated 27 amino acids at the COOH terminal promotes cell proliferation.