Analysis of adaptive mutations selected during the consecutive passages of hepatitis E virus produced from an infectious cDNA clone

To characterize the genomic mutations of hepatitis E virus (HEV) during consecutive passages associated with adaptation to growth in cell culture, a cloned genotype 3 HEV [pJE03-1760F/wt, starting virus (SV)] was passaged 10 times in A549 cells, and the entire genomic sequence of the passage 10 (P10) progeny was determined. Compared to SV, P10 virus possessed two non-synonymous (T2808C and A5054G) and four synonymous mutations (C1213T, T2557C, C3118T and C4435T) in the ORF1. Full-length infectious cDNA clones with a single, double (T2808C and A5054G), or all six mutations, identical to P10, were constructed, and their replication capacity was compared. Four (C1213T, T2557C, T2808C and A5054G) of the six viruses with a single mutation grew more efficiently than SV. The P10 virus propagated more rapidly and grew more efficiently than SV and T2808C + A5054G and reached a higher viral load (95.1- and 8.5-fold, respectively) at 20 days post-inoculation. An immunofluorescence analysis revealed that a high percentage (>80%) of cells inoculated with the P10 virus expressed ORF2 proteins, while relatively low percentages (nearly 30% or 5%) inoculated with T2808C + A5054G or SV, respectively, expressed ORF2 proteins. We found that not only non-synonymous but also synonymous HEV mutations are independently associated with increased virus production.