Determination of Diabetogenicity Attributable to a Single Amino Acid, Ala⁷⁷⁶, on the Polyprotein of Encephalomyocarditis Virus

The best experimental evidence indicating that viruses have an etiological role in the pathogenesis of diabetes comes from studies of mice infected with EMC virus. For this study we generated mutant viruses from stocks of diabetogenic EMC-D and nondiabetogenic EMC-B viruses by serial passages of the viruses in L-cell cultures at high MOI. The genomic sequence information and the biological activities of three different plaque-purified diabetogenic variants of EMC virus (EMC-D, EMC-D1/6A, EMC-D2/4) and six different plaque-purified nondiabetogenic variants (EMC-B, EMC-BS, EMC-B1/G, EMC-DV1, EMC-D4/1B, EMC-D3/1) revealed that only one amino acid, Ala (776th amino acid on the polyprotein), is critical for the diabetogenicity of EMC virus. The G base at the nucleotide position 3155 (Ala[GCC]⁷⁷⁶ in the polyprotein) is unique to all diabetogenic variants, whereas the A base at the same position (Thr[ACC]⁷⁷⁶ in the polyprotein) is identical to all nondiabetogenic variants. A single-point mutation (G to A; Ala to Thr) results in the conversion of the diabetogenic variant into a nondiabetogenic variant of EMC virus. On the basis of these observations, we conclude that a single amino acid, Ala⁷⁷⁶, on the polyprotein of EMC virus appears responsible for the inducement of diabetes in susceptible mice. Conversion of Ala⁷⁷⁶ into Thr⁷⁷⁶ on the polyprotein by a point mutation, G to A at the nucleotide position 3155, results in the loss of diabetogenicity.