Genetic characterization and codon usage bias of full-length Hepatitis E virus sequences shed new lights on genotypic distribution, host restriction and genome evolution.

Hepatitis E virus (HEV) is present in different species and ecological niches. It has been divided into 4 major mammalian genotypes. In this study, 3 new full-length genomes of swine HEV were sequenced and the results did not reveal any particular host determinant in comparison with human isolates belonging to the same genotype. Nucleotide composition and codon usage bias were determined to characterize HEV host restriction and genome evolution. Peculiar nucleotide bias was observed for A and C nucleotides in all HEV genotypes. Apart from the ORF1 hypervariable region and the ORF2/3 overlapping region, no nucleotide bias was observed between the 3 codon positions. CpG dinucleotides were also shown to be under-represented in HEV as in most RNA viruses. The effective number of codon used in HEV genome was high, indicating a lack of codon bias. Correspondence analysis of the relative synonymous codon usage was performed and demonstrated that evolution of HEV is not driven by geographical or host factors, but is representative of HEV phylogeny. These results confirm that HEV genome evolution is mainly based on mutational pressure. Natural selection, for instance involving fine-tuning translation kinetics and escape from the host immune system, may also play a role in shaping the HEV genome, particularly in the ORF1 hypervariable region and the ORF2/3 overlapping region. These regions might be involved in host restriction. Finally this study revealed the need to re-evaluate the possible subtyping classification.