Molecular characterization of enterovirus 71 sibling strains for thermal adaption in Vero cells with adaptive laboratory evolution.

Enterovirus 71 is the main pathogen that causes severe and fatal hand-foot-mouth-disease (HFMD) cases. As the enterovirus virus mutation has implications for pathogenesis, vaccine development, antiviral therapy, and epidemiological disease management of the virus. In this study, we investigated the variations of enterovirus 71 in thermal adaption, using the method of adaptive laboratory evolution. The sibling virus strains were isolated from a 2-year-old severe case of HFMD (#100) and her symptomless close contact (#101). Both strains were cultured in Vero cells by serial passage of 36 generations at the temperatures of 28.0 °C, 33.0 °C and 39.5 °C to construct adaptive lineages. According to the comparative analysis of phenotypes between adapted strains and parental strains, differences in growth rate were observed in the sibling lineages and a larger plaque was found mainly in the hot adapted strains for lineage #101. Two sets of adaptive strains from six time points (parental, 12th 17th, 31st, 35th passage and endpoint) were sequenced and analyzed by both Sanger sequencing and Next Generation Sequencing. Several variations in most coding genes and one reverse mutation in 5'UTR was observed, along with the identity of 99.8% for complete genome for both lineages. Notably, thermal specific non-synonymous mutations were found in the gene of VP1\VP3\3A\2C\3C. Moreover, the concurrent mutations A292G, A434G and A355C/T of sibling lineages in VP1 showed quantificational trace with distinguishing patterns for different temperatures, which were suspected to be the thermo-sensitive mutation hotspots. These results highlight the possible rules of thermal adaption in enterovirus 71, produce a novel picture of genome evolution of the virus, and shed light on viral variation and evolution.