HEV and Its Genetic Variability: Contribution from the ORF4 Region

Hepatitis E virus (HEV) is one of the causative agents for liver inflammation worldwide. The open reading frame 4 (ORF4) has been demonstrated to enhance the replication in HEV Genotype 1 (G1) isolates through regulation of an internal ribosome entry site (IRES)-like RNA element. HEV accounts for up to 30% mortality rate in pregnant women with the highest incidences reported for HEV G1. The HEV physiopathogenesis and adaptation to new hosts are attributed to its large genomic variability and constant evolution. Although previous investigations have reported extensive genetic diversity in HEV strains, data on the prevalence of genomic variability in ORF4 protein remains unexplored. The present study attempted a detailed characterization of the factors contributing to genetic variability in HEV ORF4 which included mutational, entropy and selection pressure analyses. The sequences of ORF4 genomes were retrieved and a total of three different datasets were built, representative of each host Human, Rat and Ferret, and comparative analyses were performed. Resulting alignments of ORF4 protein genes inspected for different datasets revealed the prevalence of both synonymous and non-synonymous mutations. Datasets I, II and III were observed with 1, 63 and 23 entropy sites, respectively, with Dataset I possessing the least variation and Dataset II with the largest variation. Several non-synonymous mutations in conjunction with higher entropy values were observed in the II (Rat) and III (Ferret) datasets, however, limited variation was observed in the I dataset (Human). Selection pressure analysis revealed ORF4 protein genes under both purifying (I) and positive (II and III) selection. A higher transition to transversion ratio was observed signifying bias towards transition in ORF4 genomes. This chapter aims to collect information and discuss new advances in the role of ORF4 heterogeneity in HEV variability. These findings are likely to augment information about the molecular evolution, adaptation and biology of this emerging viral protein.