HBx oncoprotein induces chromosome instability in hepatocarcinogenesis via dysregulation of putative tumor suppressor, TAX1BP2

Abstract

Introduction and Project Objective: Chronic hepatitis B virus infection has been implicated in development of hepatocellular carcinoma (HCC). Evidence indicates that the HBV X protein (HBx) is involved in the aberration of centrosomes, which are major microtubule-organizing centres for regulating spindle assembly and bipolarity. It is suggested that cancer is a multi-step accumulation process of genomic instability resulted from centrosome dysfunction. Herein we present a mechanistic study on the roles of HBx in inducing genomic instability through centrosome aberration. Methods: Two stable expression systems of HBx were achieved in cell-lines. Centrosome dynamics were investigated by various types of microscopy as well as biochemical methods. Genomic instability was evaluated in terms of chromosome number and mitotic aberration. The detailed mechanism of chromosome instability was elucidated in cell-lines as well as on clinical HCC samples using molecular assays. Result: HBx expression resulted centrosome ultrastructure disorganization as well as centrosome numerical defects. Further investigation suggests that HBx binds to a centrosome protein called TAX1BP2, which was previously shown to be an intrinsic block of centrosome over-duplication and a tumor suppressor being down-regulated in HCC. Restoring TAX1BP2 was found to block the effect of HBx in centrosome aberrations. Moreover, we found that HBx expression resulted in aberrant mitosis and chromosome mis-segregation. Elucidation of the mechanisms revealed that HBx upregulated mitotic gatekeeper pumilio 2 through down-regulation of its upstream inhibitor NORAD. Similar dysregulation of pumilio 2 and NORAD were found in clinical HCC samples. Conclusion: Taken together, herein we have provided evidence on the mechanism of genomic instability induced by hepatitis B virus X protein via TAX1BP2 and pumilio 2. These findings shed light on potential blockage of genomic instability induced by hepatitis B virus infections.