POINT MUTATIONS INVOLVED IN THE ATTENUATION NEUROVIRULENCE ALTERNATION IN TYPE-1 AND 2 ORAL POLIO VACCINE STRAINS DETECTED BY SITE-SPECIFICPOLYMERASE CHAIN-REACTION/

One of the problems raised by the use of the attenuated oral polioviru s vaccine (OPV) Sabin strains is the generic instability of the attenu ated phenotype upon multiplication in vivo. Nucleotide sites critical for attenuation were identified for each of the three poliovirus serot ypes. One important position lies in the 5' non-coding region of the g enome of each of the three OPV strains, at nucleotide 480 in type 1, 4 81 in type 2 and 472 in type 3. Point mutations at these positions wer e usually selected upon multiplication in vivo as substitutions of the vaccine-type residue. The reversion was found to correlate with an in creased degree of neurovirulence. To screen easily for this mutation i n a great number of strains, we developed a site-specific polymerase c hain reaction method based on the property of the Tag polymerase to el ongate only primers with a perfect homology at the 3' extremity. We sc reened for this mutation in Jive type I and nine type 2 polio vaccine- derived strains isolated from vaccine-associated paralytic poliomyelit is (VAPP) cases and in 16 such strains isolated from healthy vaccinees . All 14 strains isolated from VAPP presented the reversion. Of the ei ght pairs of type I isolates from healthy vaccinees, four presented th e reversion 3 days after vaccine administration and all but one at 7 d ays postvaccination. These results support the involvement of the 5' n on-coding specific nucleotide sites in the reversion to neurovirulence of attenuated polio vaccine strains upon multiplication in the human gut. The mutation is rapidly selected in type I vaccine poliovirus, wh ich is reputed to be the most stable of the three polio vaccine strain s.