GENETIC-BASIS OF THE NEUROVIRULENCE OF TYPE-1 POLIOVIRUSES ISOLATED FROM VACCINE-ASSOCIATED PARALYTIC PATIENTS

We examined four type 1 polioviruses isolated from the stools of patie nts with vaccine-associated paralytic poliomyelitis in China. All of t hese isolates were shown to be Sabin derived viruses by restriction fr agment length polymorphism assay after polymerase chain reaction and b y sequencing of the viral genome encoding the viral coat protein, VP1. However, the same analysis of the 3D coding region suggested that two of the four isolates had the sequence of wild type poliovirus in the tested region. Furthermore there were also point mutations in the 5' n on-coding region. One was a single base change from U to C at nucleoti de position 525, and the other three were from G to A at position 480. All the four strains were more neurovirulent than Sabin type 1 virus in transgenic mice with human poliovirus receptor gene. The data showe d that the nucleotide positions of type 1 poliovirus which were identi fied to be in favor of the high neurovirulence were indeed changed dur ing natural transmission, and suggested that the point mutation alone or a recombination of the vaccine type with wild type genome results i n an acquisition of neurovirulence.