Topoisomerase 3b is dispensable for replication of a positive-sense RNA virus--murine coronavirus

A recent study demonstrated that a DNA-RNA dual-activity topoisomerase complex, TOP3B-TDRD3, is required for normal replication of positive-sense RNA viruses, including several human flaviviruses and coronaviruses; and the authors proposed that TOP3B is a target of antiviral drugs. Here we examined this hypothesis by investigating whether inactivation of Top3b can inhibit the replication of a mouse coronavirus, MHV, using cell lines and mice that are inactivated of Top3b or Tdrd3. We found that Top3b-KO or Tdrd3-KO cell lines generated by different CRISPR-CAS9 guide RNAs have variable effects on MHV replication. In addition, we did not find significant changes of MHV replication in brains or lungs in Top3B–KO mice. Moreover, immunostaining showed that Top3b proteins are not co-localized with MHV replication complexes but rather, localized in stress granules in the MHV-infected cells. Our results suggest that Top3b does not have a universal role in promoting replication of positive-sense RNA virus, and cautions should be taken when targeting it to develop anti-viral drugs.