Genomic structure of grass carp Mx2 and the association of its polymorphisms with susceptibility/resistance to grass carp reovirus.

Mx (myxovirus-resistant) proteins are induced by interferon and inhibit viral replication in many vertebrates. In the present study, the organization of grass carp (Ctenopharyngodon idella) Mx2 (CiMx2) gene sequence was elucidated and its single nucleotide polymorphisms (SNPs) were investigated to explore their association with susceptibility/resistance to grass carp reovirus (GCRV). The CiMx2 genomic sequence is composed of 8108 bp, containing 12 exons and 11 introns. Exon size ranges from 29 to 648 bp, and intron size varies from 89 to 1925 bp. Five SNPs were discovered in the complete sequence of CiMx2 genomic sequence and four of them were found to be located in introns. The 7 C/T locus is a non-synonymous mutation. The genotype and allele distribution were examined by PCR-RFLP in susceptible and resistant fish. The results indicate that genotypes at the 1191 C/A and 1205 G/A loci are significantly associated with the resistance of grass carp to GCRV (P<0.05). To further verify the correlation, an additional infection experiment was carried out. The mortality in CC genotype individuals (0%) at 1191 C/A locus was significantly lower than that in AA (61.11%) and AC (71.17%) genotypes (P<0.05). At 1205 G/A site, no AA genotype individual was found; the mortality in AG genotype group was 53.06%, which was significantly lower than that in GG genotype group (90.48%) (P<0.05). The results demonstrate that genotype 1191 AC, 1191 AA and 1205 GG individuals are susceptible to GCRV, while 1191 CC and 1205 AG are resistant. The 7 C/T and 528 C/T loci are in high linkage disequilibrium, however, no significant association was found between the haplotype and resistance to GCRV (P>0.05). These results provide potential markers for further investigation of selective breeding of resistant grass carp to GCRV.