A NOVEL HBV RNA ELEMENT INDUCES THE R2 CELL CYCLE GENE, THE KEY COMPONENT IN dNTPs SYNTHESIS

HBV infects quiescent hepatocytes, which are deficient in dNTPs, the critical precursors of HBV replication. We found that HBV activates the expression of the R2 subunit of the Ribonucleotide Reductase (RNR) holoenzyme, the cell cycle gene that is critical for generation of dNTPs. We recently published that HBV induces the DNA damage response (DDR) Chk1-E2F1 axis to target R2 gene. We next wanted to address the question of which of the known HBV ORFs is responsible for DDR activation. To this end, we knocked down each of the ORFs. Remarkably, we found that none of the ORFs is important. Surprisingly, however, HBx gene in isolation was sufficient to upregulate R2 expression in quiescent cells. These enigmatic findings led us to investigate the possibility that the mRNA rather than the ORF encoded protein is involved in R2 upregulation. We have identified a 52 bases long region within the HBx mRNA, sufficient to induce R2 expression. By inhibiting the Chk1-E2F1 DDR axis we validated that the non-coding HBV mRNA function induces R2 expression via this pathway. We currently conduct experiments to investigate how the HBV mRNA induces DDR. By RNA pull-down mass-spec approach we found several candidate binding proteins that may mediate this effect. These findings exemplify the dual functionality of the HBV mRNA in controlling virus-host cell interactions, and add another level of complexity to what was known about the viral genome. Moreover, the fact that this RNA element plays an essential role in the viral life cycle makes it a potential target for antiviral therapy.