The Initial Hepatitis B Virus-Hepatocyte Genomic Integrations and Their Role in Hepatocellular Oncogenesis

Hepatitis B virus (HBV) remains a dominant cause of hepatocellular carcinoma (HCC). Recently it was shown that HBV and woodchuck hepatitis virus (WHV) integrate into hepatocyte genome minutes after invasion. Retrotransposons and transposable sequences were frequent sites of the initial insertions suggesting a mechanism for spontaneous HBV DNA disperse throughout hepatocyte genome. Several somatic genes were also identified as early insertional targets in infected hepatocytes and woodchuck livers. Head-to-tail joints (HTJs) dominated amongst fusions indicating their creation by non-homologous-end-joining (NHEJ). Their formation coincided with robust oxidative damage of hepatocyte DNA. This was associated with activation of the poly(ADP-ribose) polymerase 1 (PARP1)-mediated dsDNA repair as reflected by augmented transcription of PARP1 and XRCC1, the PARP1 binding partner, OGG1, a responder to oxidative DNA damage, and by increased activity of NAD+, a marker of PARP1 activation, and HO1, an indicator of cell oxidative stress. The engagement of the PARP1-mediated NHEJ repair pathway explains HTJ format of the initial merges. The findings showed that HBV and WHV are immediate inducers of oxidative DNA damage, hijack dsDNA repair to integrate into hepatocyte genome and by this may initiate pro-oncogenic process. Tracking initial integrations may uncover early markers of HCC and help to explain HBV-associated oncogenesis.