Interstrain Recombinants of Human Cytomegalovirus Reveal Complex Genetic Correlates and Epistasis Influencing Glycoprotein Display, Virion Infectivity and Spread Characteristics
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Most of the nucleotide diversity in the human cytomegalovirus (HCMV) genome is due to approximately 17 genes with 2-14 alleles each. These allelic genes are interspersed among longer stretches of highly conserved sequences with signatures of extensive recombination that would shuffle the allelic genes into a vast number of allelic haplotypes. Bacterial artificial chromosome clones derived from 3 independent clinical isolates (TB40/e (TB), TR and Merlin (ME)) display dramatic differences in the abundance of entry-mediating glycoproteins gH/gL/gO and gH/gL/UL128-131, virion infectivity and efficiency of cell-free and cell-to-cell modes of spread. Of these, TB and ME are the most phenotypically different and share only 2 of the 17 allelic genes. A set of recombinant HCMV was generated by coinfecting cells with TB and ME and restriction fragment length polymorphism (RFLP) analyses demonstrated complex crossover patterns. Most recombinants were either “TB-like” with much more gH/gL/gO than gH/gL/UL128-131, or “ME-like” with much more gH/gL/UL128-131. This correlated with a TB or ME UL128 sequence, consistent with a G/T polymorphism affecting UL128 pre-mRNA splicing. One recombinant had a gH/gL/gO:gH/gL/UL128-131 ratio of 0.8, suggesting genetic determinants beyond UL128. Virion infectivity correlated with TB versus ME-like glycoprotein display, but intragroup variability indicated additional factors and variability in spread efficiency and the contribution of cell-free and cell-to-cell spread modes indicated an influence of characteristics beyond virion infectivity. Results suggest that the relationships among these three phenotypes are not strictly causal and that all three phenotypes are genetically complex and influenced by epistasis among polymorphic loci across the genome.
IMPORTANCE
The emerging picture of the genetic diversity of HCMV in vivo prompts a reevaluation of how in vitro characterized viral phenotypes, such as the abundance of the various glycoproteins displayed on the virion envelope and tendency towards cell-free or direct cell-to-cell spread, reflect viral characteristics in vivo . A small sampling of the in vivo genetic diversity of HCMV has been examined in the laboratory and the genetic correlates of in vitro phenotypes are unclear. Engineered mutations can yield different phenotypic effects when introduced on to different genetic backgrounds, suggesting complex epistasis among polymorphic loci across the genome. By generating a set of recombinants derived from two phenotypically and genotypically distinct parental clones, this work offers an approach to characterize complex genetic correlates of viral phenotypes, which in turn may enhance efforts to link genomics data with in vivo viral behavior and clinical outcomes.
