High recombination rate of hepatitis C virus revealed by a green fluorescent protein reconstitution cell system

Genetic recombination is an important evolutionary mechanism for RNA viruses and can facilitate escape from immune and drug pressure. Recombinant hepatitis C virus (HCV) variants have rarely been detected in patients, suggesting that HCV has intrinsic low recombination rate. Recombination of HCV has been demonstrated in vitro between non-functional genomes, but its frequency and relevance for viral evolution and life cycle has not been clarified. We developed a cell-based assay to detect and quantify recombination between fully viable HCV genomes, using the reconstitution of green fluorescent protein (GFP) as a surrogate marker for recombination. Here, two GFP-expressing HCV genomes carrying different inactivating GFP mutations can produce a virus carrying a functional GFP by recombining within the GFP region. Generated constructs allowed quantification of recombination rates between markers spaced 603 and 553 nucleotides apart by flow cytometry and next-generation sequencing (NGS). Viral constructs showed comparable spread kinetics and reached similar infectivity titers in Huh7.5 cells, allowing their use in co-transfections and co-infections. Single-cycle co-transfection experiments, performed in CD81-deficient S29 cells, showed GFP expression in double-infected cells, demonstrating genome mixing and occurrence of recombination. Quantification of recombinant genomes by NGS revealed an average rate of 6.1 per cent, corresponding to 49 per cent of maximum detectable recombination (MDR). Experiments examining recombination during the full replication cycle of HCV, performed in Huh7.5 cells, demonstrated average recombination rates of 5.0 per cent (40.0 per cent MDR) and 3.6 per cent (28.8 per cent MDR) for markers spaced by 603 and 553 nucleotides, respectively, supporting a linear relationship between marker distance and recombination rates. First passage infections using recombinant virus supernatant resulted in comparable recombination rates of 5.9 per cent (47.2 per cent MDR) and 3.5 per cent (28.0 per cent MDR), respectively, for markers spaced by 603 and 553 nucleotides. We developed a functional cell-based assay that, to the best of our knowledge, allows for the first time detailed quantification of recombination rates using fully viable HCV constructs. Our data indicate that HCV recombines at high frequency between highly similar genomes and that the frequency of recombination increases with the distance between marker sites. These results have implication for our understanding of HCV evolution and emphasize the importance of recombination in the reassortment of mutations in the HCV genome.