Cell culture-adaptive mutations in the NS5B gene of hepatitis C virus with delayed replication and reduced cytotoxicity

Remarkable advances have been made through recent demonstrations of hepatitis C virus (HCV) replication in cultured cells transfected with in vitro synthesized viral genome RNA. From the HCV JFH1 (genotype 2a) subcultured successively in Huh-7 cells we have identified several missense mutations near the junction of NS5A and NS5B genes. Reverse genetic analysis indicated that two mutations in the N-terminal region of NS5B replicase caused delayed viral RNA replication and protein expression in the early stage of infection. However, the mutant viruses showed significantly alleviated effects on cell growth inhibition, proteolysis of viral proteins, apoptotic DNA cleavage, and induction of antiviral responses, giving rise to a 100-fold higher titer compared to the parental JFH1 virus in a more extended time period. These results suggested that delayed replication and reduced cytotoxicity can be characteristic features of cell culture-adaptive mutants with enhanced infectivity.