Novel Synthesis and Phenotypic Analysis of Mutant Clouds for Hepatitis E Virus Genotype 1

Enhanced fitness of hepatitis C virus increases resistance to direct-acting antivirals

Drug resistance mutations of hepatitis C virus (HCV) negatively impact viral replicative fitness. RNA viruses are known to change their replication behaviour when subjected to suboptimal selection pressure. Here, we assess whether mutation supply in HCV is sufficiently large to allow the selection of its variants during dual or triple direct-acting antiviral (DAA) treatment associated with augmented virus fitness or impairment. We engineered randomly mutagenized full-genome libraries to create a highly diverse population of replication-competent HCV variants in cell culture. These variants exhibited escape when treated with NS5A/NS5B inhibitors (daclatasvir/sofosbuvir), and relapse on treatment with a combination of NS3/NS5A/NS5B inhibitors (simeprevir or paritaprevir/daclatasvir/sofosbuvir). Analysis of the relationship between virus fitness and drug resistance of JFH1-derived NS5A-5B variants showed a significant positive correlation (P=0.003). At the earliest time points, intracellular RNA levels remain unchanged in both the subgenomic replicon and infection assays, whereas extracellular RNA levels increased upto ten-fold compared to wild-type JFH1. Beneficial substitutions hyperstimulated phosphatidylinositol 4-phosphate during DAA treatment, and showed decreased dependence on cyclophilins during cyclosporine A treatment, indicating an interplay of virus-host molecular mechanisms in beneficial substitution selection that may necessitate infectious virus production. This comprehensive study demonstrates a possible role for HCV fitness of overcoming drug-mediated selection pressure.

HCV Replicon Systems: Workhorses of Drug Discovery and Resistance

The development of direct-acting antivirals (DAAs) has revolutionized the state-of-the art treatment of HCV infections, with sustained virologic response rates above 90%. However, viral variants harboring substitutions referred to as resistance-associated substitutions (RASs) may be present in baseline levels and confer resistance to DAAs, thereby posing a major challenge for HCV treatment. HCV replicons have been the primary tools for discovering and evaluating the inhibitory activity of DAAs against viral replication. Interest in replicon systems has further grown as they have become indispensable for discovering genotype-specific and cross-genotype RASs. Here, we review functional replicon systems for HCV, how these replicon systems have contributed to the development of DAAs, and the characteristics and distribution of RASs for DAAs.

Genome-Wide Mutagenesis of Hepatitis C Virus Reveals Ability of Genome To Overcome Detrimental Mutations

To gain insight into the impact of mutations on the viability of the hepatitis C virus (HCV) genome, we created a set of full-genome mutant libraries, differing from the parent sequence as well as each other, by using a random mutagenesis approach; the proportion of mutations increased across these libraries with declining template amount or dATP concentration. The replication efficiencies of full-genome mutant libraries ranged between 71 and 329 focus-forming units (FFU) per 10⁵ Huh7.5 cells. Mutant libraries with low proportions of mutations demonstrated low replication capabilities, whereas those with high proportions of mutations had their replication capabilities restored. Hepatoma cells transfected with selected mutant libraries, with low (4 mutations per 10,000 bp copied), moderate (33 mutations), and high (66 mutations) proportions of mutations, and their progeny were subjected to serial passage. Predominant virus variants (mutants) from these mutant libraries (Mutant^l, Mutant^m, and Mutant^h, respectively) were evaluated for changes in growth kinetics and particle-to-FFU unit ratio, virus protein expression, and modulation of host cell protein synthesis. Mutant^m and Mutant^l variants produced >3.0-log-higher extracellular progeny per ml than the parent, and Mutant^h produced progeny at a rate 1.0-log lower. More than 80% of the mutations were in a nonstructural part of the mutant genomes, the majority were nonsynonymous, and a moderate to large proportion were in the conserved regions. Our results suggest that the HCV genome has the ability to overcome lethal/deleterious mutations because of the high reproduction rate but highly selects for random, beneficial mutations.IMPORTANCE Hepatitis C virus (HCV) in vivo displays high genetic heterogeneity, which is partly due to the high reproduction and random substitutions during error-prone genome replication. It is difficult to introduce random substitutions in vitro because of limitations in inducing mutagenesis from the 5' end to the 3' end of the genome. Our study has overcome this limitation. We synthesized full-length genomes with few to several random mutations in the background of an HCV clone that can recapitulate all steps of the life cycle. Our study provides evidence of the capability of the HCV genome to overcome deleterious mutations and remain viable. Mutants that emerged from the libraries had diverse phenotype profiles compared to the parent, and putative adaptive mutations mapped to segments of the conserved nonstructural genome. We demonstrate the potential utility of our system for the study of sequence variation that ensures the survival and adaptation of HCV.