Molecular genetic analysis of revertants from a poliovirus mutant that is specifically adapted to the mouse spinal cord

Mouse adaptation of a sub-genogroup B5 strain of human enterovirus 71 is associated with a novel lysine to glutamic acid substitution at position 244 in protein VP1

Most human enterovirus 71 (HEV71) strains infect only primates and are unable to cause clinically apparent infection in mice. Here we describe a mouse-adapted HEV71 strain that belongs to sub-genogroup B5 with increased virulence in newborn BALB/c mice. The mouse-virulent strain was initially selected by serial passage of a HEV71 clinical isolate (HEV71-B5) in Chinese hamster ovary (CHO) cells (CHO-B5), followed by serial passage in newborn mice. Virus from the fifth mouse passage was cultured twice on Vero cells and designated as MP-B5. MP-B5 induces severe disease of high mortality in newborn mice in a dose-dependent manner. Skeletal muscle is the primary site of virus replication and results in severe myositis. CHO-B5 harbours a single amino acid substitution (K(149) → I) in the VP2 capsid protein. Five additional nucleotide sequence changes were identified in MP-B5, two of which are located in the 5' UTR and the three within the open reading frame (ORF). Two of the ORF mutations resulted in deduced amino acid changes in the capsid protein VP1: S(241) → L and K(244) → E; the third ORF mutation was a synonymous C → T change at nucleotide position 6072 within the 3D polymerase gene. Infectious cDNA clone-derived mutant virus populations of HEV71 belonging to sub-genogroup B3 (CHO-26 M) that contain the VP1 mutations identified in MP-B5 were generated in order to determine the mutation(s) responsible for mouse virulence. Only viruses expressing the VP1 (K(244) → E) mutation were virulent in 5-day-old BALB/c mice, indicating that the VP1 (K(244) → E) change is the critical genetic determinant of mouse adaptation and virulence in this model.

The molecular basis of mouse adaptation by human enterovirus 71

A mouse-adapted strain of human enterovirus 71 (HEV71) was selected by serial passage of a HEV71 clinical isolate (HEV71-26M) in Chinese hamster ovary (CHO) cells (CHO-26M) and in newborn BALB/c mice (MP-26M). Despite improved growth in CHO cells, CHO-26M did not show increased virulence in newborn BALB/c mice compared with HEV71-26M. By contrast, infection of newborn mice with MP-26M resulted in severe disease of high mortality. Skeletal muscle was the primary site of replication in mice for both viruses. However, MP-26M infection induced severe necrotizing myositis, whereas CHO-26M infection caused only mild inflammation. MP-26M was also isolated from whole blood, heart, liver, spleen and brain of infected mice. CHO-26M harboured a single mutation within the open reading frame (ORF), resulting in an amino acid substitution of K149→I in the VP2 capsid protein; two further ORF mutations that resulted in amino acid substitutions were identified in MP-26M, located within the VP1 capsid protein (G145→E) and the 2C protein (K216→R). Infectious cDNA clone-derived mutant virus populations containing the mutations identified in CHO-26M and MP-26M were generated in order to study the molecular basis of CHO cell and mouse adaptation. The VP2 (K149→I) change was responsible only for improved growth in CHO cells and did not lead to increased virulence in mice. Of the two amino acid substitutions identified in MP-26M, the VP1 (G145→E) mutation alone was sufficient to increase virulence in mice to the level observed in MP-26M-infected mice.