Molecular biology and replication of hepatitis E virus

Abstract

Hepatitis E virus (HEV), a single-stranded, positive-sense RNA virus, is responsible for acute hepatitis E epidemics in many developing countries, and the virus is also endemic in some industrialized countries. Hepatitis E is a recognized zoonotic disease, and several animal species, including pigs, are potential reservoirs for HEV. The genome of HEV contains three open reading frames (ORFs). ORF1 encodes the nonstructural proteins, ORF2 encodes the capsid protein, and ORF3 encodes a small multifunctional protein. The ORF2 and ORF3 proteins are translated from a single, bicistronic mRNA. The coding sequences for these two ORFs overlap each other, but neither overlaps with ORF1. Whereas the mechanisms underlying HEV replication are poorly understood, the construction of infectious viral clones, the identification of cell lines that support HEV replication, and the development of small animal models have allowed for more detailed study of the virus. As result of these advances, recently, our understanding of viral entry, genomic replication and viral egress has improved. Furthermore, the determination of the T=1 and T=3 structure of HEV virus-like particles has furthered our understanding of the replication of HEV. This article reviews the latest developments in the molecular biology of HEV with an emphasis on the genomic organization, the expression and function of genes, and the structure and replication of HEV.

Keywords: animal model; hepatitis E; hepatitis E virus; molecular biology; replication; zoonosis.

Figure 1

A phylogenetic tree based on the complete genomic sequences of representative HEV strains from each genotype. The tree was constructed using the neighbor-joining method with 1000 bootstrap analyses using MEGA5. The scale bar represents 0.05 nucleotide substitutions per position. The bootstrap values are labeled at the major nodes. The GenBank accession numbers of the HEV genomes are included in parenthesis after each strain.

Figure 2

A schematic diagram of the genomic and subgenomic organization of the HEV genome. The three ORFs are labeled and shown as boxes with the putative ORF1 domains indicated inside the box. Modified from Cao et al. (2010). CRE, cis-reactive element; Hel, helicase; HEV, hepatitis E virus; HVR, hypervariable region; JR, junction region; MT, methytransferase; NCR, noncoding region; ORF, open reading frame; PCP, a papain-like cysteine protease; RdRp, RNA-dependent RNA polymerase; SL, stem–loop structure; X, macro domain; Y, Y domain.

Figure 3

The secondary structure of the CREs of a genotype 1 HEV (strain Sar-55) as predicted using mfold. (A) The secondary structures at the 3' end of HEV genomic region (nt 7082–7192 Aⁿ) with the SL1 and SL2 labeled.^, (B) Negative-polarity complement of the JR in the HEV genome (nt 5096–5157). The regulatory SL is labeled, and the arrows indicate the start sites of the subgenomic mRNA, ORF2 and ORF3.

Figure 4

Proposed life cycle of HEV.^,,, Step a: HEV attaches to the cell surface via HSPGs, HSC70 or other putative attachment receptor(s) and then enters the cell via a unknown specific cellular receptor.^, Step b: The HEV virion penetrates the membrane and enters the cells. HSP90 and Grp78 may be involved in this transport. The virion then uncoats and releases the positive-sense genomic RNA into the cytoplasm of the cell. Step c: The positive-sense genomic viral RNA serves as the template to translate the ORF1 nonstructural polyprotein in the cytoplasm. Step d: The viral RdRp synthesizes an intermediate, replicative negative-sense RNA from the positive-sense genomic RNA that (step e) serves as the template for the production of positive-sense, progeny viral genomes. Step f: The ORF2 and ORF3 proteins are translated from the subgenomic, positive-stranded RNA, and (step g) the ORF2 capsid protein packages the genomic viral RNA and assembles new virions. Step h: The nascent virions are transported to the cell membrane. The ORF3 protein facilitates the trafficking of the virion, and (step i) the nascent virions are released from the infected cells. Modified from various studies.^,,