Evidence for a Hepatitis B Virus Short RNA Fragment Directly Targeting the Cellular RRM2 Gene.

The hepatitis B virus (HBV) is one of the smallest but most highly infectious human pathogens. With a DNA genome of only 3.2 kb and only four genes, HBV successfully completes its life cycle by using intricate processes to hijack the host machinery. HBV infects non-dividing liver cells in which dNTPs are limited. As a DNA virus, HBV requires dNTPs for its replication. HBV induces the ATR-mediated cellular DNA damage response pathway to overcome this constraint. This pathway upregulates R2 (RRM2) expression in generating an active RNR holoenzyme catalyzing de novo dNTP synthesis. Previously we reported that ERE, a small RNA fragment within the HBx ORF, is sufficient to induce R2 upregulation. Interestingly, there is high sequence similarity between ERE and a region within the R2 5'UTR that we named R2-box. Here, we established a mutant cell line in the R2-box region of the R2 gene using CRISPR-Cas9 technology to investigate the R2 regulation by ERE. This cell line expresses a much lower R2 level than the parental cell line. Interestingly, the HBV infection and life cycle were severely impaired. These cells became permissive to HBV infection upon ectopically R2 expression. These results validate the requirement of the R2 gene expression for HBV replication. Remarkably, the R2-box mutated cells became ERE refractory, suggesting that the homology region between ERE and R2 gene is critical for ERE-mediated R2 upregulation. Thus, along with the induction of the ATR pathway of the DNA damage response, ERE might also directly target the R2 gene via the R2-box.