Knockout of the antiviral genes mx1 or mx3 modulates the expression of paralogous genes in a salmonid cell line

Mx dynamin-like GTPases genes are Interferon Stimulated Genes (ISGs) encoding the Mx (myxovirus resistance) proteins that have an antiviral effect against a wide array of RNA viruses as well as some DNA viruses. In salmonids, the mx genes reside in four distinct chromosomal loci, that encode proteins clustering together into separate phylogenetic clades. To understand the contribution of Mx to the innate antiviral resistance, we knocked out the mx1 or mx3 gene by generating single mutant CHSE-derived cell lines by CRISPR/Cas9 genome editing. sgRNA were designed within the first coding exon of mx1 (LOC112247236), or mx3 (LOC112247235) genes, located 27 kb apart on the same chromosome LG02. Mx1, Mx2 and Mx3 proteins were quantified by western blotting. Only the Mx3 protein was found induced in the wild type EC cells after stimulation with recombinant Atlantic salmon interferon A2. mx1 gene knockout reduced the up-regulation of Mx3 protein expression, while mx3 gene knockout resulted in the induction of Mx1 and Mx2 proteins with and without additional stimulation. This was observed at the transcriptional level with the induction of mx1 gene was increased in mx3^-/- cells compared to wild type cells. These results illustrate the complex interplay between mx1 and mx3 genes in salmonids.