Viral N6-methyladenosine upregulates replication and pathogenesis of human respiratory syncytial virus

N6-methyladenosine (m6A) is the most prevalent internal modification of mRNAs in most eukaryotes. Likewise, viral RNAs may acquire m6A methylation during replication within these cells. Here we show that RNAs of human respiratory syncytial virus (RSV), a medically important non-segmented negative-sense (NNS) RNA virus, are modified by m6A within discreet regions and that these modifications enhance viral replication and pathogenesis. Overexpression of m6A binding proteins significantly enhanced RSV replication and gene expression. Knockdown of m6A methyltransferases decreased viral replication and gene expression whereas knockdown of m6A demethylases had the opposite effect. The G gene contained the most abundant m6A modifications. Recombinant RSV expressing a G gene lacking different m6A sites, resulted in RSVs with various degrees of defects in replication in A549 cells, primary well differentiated human airway epithelial (HAE) cultures, upper and lower respiratory tract of cotton rats, and were also less pathogenic to the lungs of cotton rats. One of the m6A-deficient rgRSVs, rgRSV-G12, was completely attenuated yet retained high immunogenicity in cotton rats. Moreover, a small molecule that inhibits S-adenosyl-L-homocysteine (SAH) hydrolase, thereby reducing the cellular SAH pool and viral RNA m6A, also inhibited RSV replication in HAE cells. Collectively, our results demonstrate viral m6A methylation upregulates RSV replication and pathogenesis and identify viral m6A methylation as a target for rational design of live attenuated vaccine candidates and for novel antiviral therapeutic agents for RSV. RSV virions were purified from supernatants of RSV-infected Hela and A549 cell lines. Virus derived mRNA as well as host mRNA were purified from total RNA, which were extracted from RSV infected cells.