The Dihydroquinolizinone Compound RG7834 Inhibits the Polyadenylase Function of PAPD5 and PAPD7 and Accelerates the Degradation of Matured Hepatitis B Virus Surface Protein mRNA

Hepatitis B virus (HBV) mRNA metabolism is dependent upon host proteins PAPD5 and PAPD7 (PAPD5/7). PAPD5/7 are cellular, noncanonical, poly(A) polymerases (PAPs) whose main function is to oligoadenylate the 3' end of noncoding RNA (ncRNA) for exosome degradation. HBV seems to exploit these two ncRNA quality-control factors for viral mRNA stabilization, rather than degradation. RG7834 is a small-molecule compound that binds PAPD5/7 and inhibits HBV gene production in both tissue culture and animal study. We reported that RG7834 was able to destabilize multiple HBV mRNA species, ranging from the 3.5-kb pregenomic/precore mRNAs to the 2.4/2.1-kb hepatitis B virus surface protein (HBs) mRNAs, except for the smallest 0.7-kb X protein (HBx) mRNA. Compound-induced HBV mRNA destabilization was initiated by a shortening of the poly(A) tail, followed by an accelerated degradation process in both the nucleus and cytoplasm. In cells expressing HBV mRNA, both PAPD5/7 were found to be physically associated with the viral RNA, and the polyadenylating activities of PAPD5/7 were susceptible to RG7834 repression in a biochemical assay. Moreover, in PAPD5/7 double-knockout cells, viral transcripts with a regular length of the poly(A) sequence could be initially synthesized but became shortened in hours, suggesting that participation of PAPD5/7 in RNA 3' end processing, either during adenosine oligomerization or afterward, is crucial for RNA stabilization.