Genetic Determinants of Altered Virulence of Type O Foot-and-Mouth Disease Virus

Under different circumstances, the alteration of several viral genes may give an evolutionary advantage to the virus to maintain its prevalence in nature. In this study, a 70-nucleotide deletion in the small fragment (S fragment) of the viral 5′-untranslated region (5′-UTR) together with one amino acid insertion in the leader protein (L(pro)) that naturally occurred in several serotype O foot-and-mouth disease virus (FMDV) strains in China was identified. The properties of two field serotype O FMDV strains, with or without the 70-nucleotide deletion in the S fragment and the amino acid insertion in L(pro), were compared in vitro and in vivo. Clinical manifestations of FMD were clearly observed in cattle and pigs infected by the virus without the mutations. However, the virus with the mentioned mutations caused FMD outcomes only in pigs, not in cattle. To determine the role of the 70-nucleotide deletion in the S fragment and the single amino acid insertion in L(pro) in the pathogenicity and host range of FMDV, four recombinant viruses, with complete genomes and a 70-nucleotide deletion in the S fragment, a single amino acid insertion in L(pro), or both mutations, were constructed and rescued. It showed that deletion of 70 nucleotides in the S fragment or insertion of one amino acid (leucine) at position 10 of L(pro) partly decreased the viral pathogenicity of Mya-98 lineage virus in cattle and pigs. However, the virus with dual mutations caused clinical disease only in pigs, not in cattle. This suggested that the S fragment and L(pro) are significantly associated with the virulence and host specificity of FMDV. The naturally occurring dual mutation in the S fragment and L(pro) is a novel determinant of viral pathogenicity and host range for serotype O FMDV. IMPORTANCE FMD is probably the most important livestock disease in the world due to the severe economic consequences caused. The alteration of several viral genes may give the virus selective advantage to maintain its prevalence in nature. Here, we identified that a 70-nucleotide deletion in the S fragment combined with a single leucine insertion in the leader protein (L(pro)) is a novel determinant of restricted growth on bovine cells, which significantly contributes to the altered virulence of serotype O FMDV in cattle. A synergistic and additive effect of the 70-nucleotide deletion in the S fragment and the single leucine insertion in L(pro) on the virulence and host specificity of the virus was determined. These results will benefit efforts to understand the vial pathogenicity mechanism and molecular characteristics of FMDV.