Comparison of poliovirus recombinants: accumulation of point mutations provides further advantages

Abstract

The roles of recombination and accumulation of point mutations in the origin of new poliovirus (PV) characteristics have been hypothesized, but it is not known which are essential to evolution. We studied phenotypic differences between recombinant PV strains isolated from successive stool specimens of an oral PV vaccine recipient. The studied strains included three PV2/PV1 recombinants with increasing numbers of mutations in the VP1 gene, two of the three with an amino acid change IAT in the DE-loop of VP1, their putative PV1 parent and strains Sabin 1 and 2. Growth of these viruses was examined in three cell lines: colorectal adenocarcinoma, neuroblastoma and HeLa. The main observation was a higher growth rate between 4 and 6 h post-infection of the two recombinants with the IAT substitution. All recombinants grew at a higher rate than parental strains in the exponential phase of the replication cycle. In a temperature sensitivity test, the IATsubstituted recombinants replicated equally well at an elevated temperature. Complete genome sequencing of the three recombinants revealed 12 (3), 19 (3) and 27 (3) nucleotide (amino acid) differences from Sabin. Mutations were located in regions defining attenuation, temperature sensitivity, antigenicity and the cis-acting replicating element. The recombination site was in the 59 end of 3D. In a competition assay, the most mutated recombinant beat parental Sabin in all three cell lines, strongly suggesting that this virus has an advantage. Two independent intertypic recombinants, PV3/PV1 and PV3/PV2, also showed similar growth advantages, but they also contained several point mutations. Thus, our data defend the hypothesis that accumulation of certain advantageous mutations plays a key role in gaining increased fitness.

Growth kinetic analysis of bi-recombinant poliovirus vaccine strains

“…Moreover, sequences of Sabin type 1 origin are regularly found in SX/S1 recombinant strains [4,50,[55][56][57][58]. This observation correlates with the increased fitness of SX/S1 recombinants compared to those of S1/SX recombinants.…”

Sporadic Isolation of Sabin-Like Polioviruses and High-Level Detection of Non-Polio Enteroviruses during Sewage Surveillance in Seven Italian Cities, after Several Years of Inactivated Poliovirus Vaccination

“…The presence of intertypic recombination in the 3D region of the poliovirus isolated in Milan and Palermo is interesting, even though similar recombinations were previously detected in poliovirus isolated in several other countries (41,60,61). Increased transmissibility over that of Sabin parental strains has been suggested to be the consequence of a recombinant event (62).…”

Mechanisms of genetic variation in polioviruses

“…Theoretical explanations for the benefit of recombination fall into two categories: (1) recombination provides rapid adoption and spread of advantageous traits 42, 43, and (2) it permits elimination of detrimental mutations and genes 44. Recent phenotypic comparison of OPV‐derived PV recombinants shows that recombination per se is not necessarily a key to a genome with better phenotypic characteristics as not all studied recombinant strains outgrew their vaccine parents 45. However, recombination may serve as a means of gaining a set of advantageous point mutations selected and fixed in previous generations 45.…”