Review Article

Hepatitis E—A Concise Review of Virology, Epidemiology, Clinical Presentation and Therapy

M. C. Donnelly; L. Scobie; C. L. Crossan; H. Dalton; P. C. Hayes; K. J. Simpson

Disclosures

Aliment Pharmacol Ther. 2017;46(2):126-141. 

In This Article

The Molecular Biology of Hepatitis E Virus

HEV is a small (27–34 mm) single-stranded RNA virus,[9] approximately 7.2 kb in length with 7 currently recognised genotypes.[10] It is known to be non-enveloped in bile and faeces, and is present in blood, coated in a lipid membrane. HEV consists of a 5' short noncoding region (NCR), ORF1 (which encodes the nonstructural proteins), ORF3 (which encodes a small multifunctional protein), ORF2 (which encodes the capsid protein) and a 3'NCR followed by a polyadenosine tail approximately 150–200 bases long (Figure 2). A novel fourth open reading frame (ORF4) has been identified, situated within the ORF1 region of the HEV genome, encoding a 20 kDa (139–158aa long) protein during endoplasmic reticulum stress which interacts with host and viral proteins to control the activity of the viral RNA-dependent RNA polymerase.[11] However, ORF4 is unique to, although universal amongst, genotype 1 HEV strains.

Figure 2.

Organisation of the Hepatitis E virus(HEV) genome. A schematic diagram of the genomic and subgenomic organisation of the HEV genome. The open reading frames are shown as boxes and labelled. The noncoding features are labelled and the putative domains of open reading frame(ORF)1 are also shown. In genotype 1 strains, a putative ORF4 has been identified and falls within the ORF1 coding region. Modified from Cao & Meng 2012. CRE, cis-reactive element; Hel, Helicase; HVR, hypervariable region; JR junction region; MT, methyltransferase; NCR, noncoding region; PCP, papain-like cysteine protease; RdRp, RNA dependent RNA polymerse; SL, stem-loop structure

ORF1 and ORF2/3 are separated by a junction region (JR), whereas ORF2 and ORF3 overlap and are transcribed as a bicistronic subgenomic mRNA.[12] The genome contains two cis-reactive elements (CREs), one in the JR and one overlapping the 3' end of ORF2 and the 3'NCR.[13,14] The sequence and stem-loop structure of the CRE in the JR is essential for replication and may also serve as the promoter for the subgenomic region.[12] The 3'NCR CRE localises the RNA-dependent RNA polymerase.[15] The 5' end of both the genomic and subgenomic RNA is capped.[16] A 76 nucleotide region in the 5'NCR is responsible for binding the ORF2 protein and is considered to play a role in viral assembly.[17] The coding region of ORF1 begins immediately after the 5'NCR and extends over 5082 nucleotides. ORF1 encodes a 1693 amino acid polyprotein with a molecular mass of approximately 186 kDa and several putative functional domains (Figure 2) including; a methyltransferase (MT) domain for 5' capping, a Y domain of unknown function, a papain-like cysteine protease (PCP) domain, a proline rich region that contains a hypervariable region, a X domain of unknown function, a helicase (Hel) domain, and a RNA-dependent RNA polymerase (RdRp).[18] However, the protease activity of the PCP domain is a subject of some debate, with some reports suggesting that this domain may instead de-ubiquinate proteins, preventing the prosomal degradation of protein required for viral replication.[19] The capsid protein of HEV is expressed by ORF2. The ORF2 protein contains 4 domains; the N terminals, arginine rich, shell, middle and protruding domain. Neutralising antibodies have been shown to bind to the P and M domains, suggesting that these play a role in cell binding and entry.[20] Studies involving the expression of a truncated form of ORF2 by a baculovirus expression system in insect cells have produced HEV like particles.[21] Such virus like particles have been shown to bind to heparan sulfate proteoglycans (HSPGs) and HSPG expression on the cell surface is required for in vitro infection.[22] ORF3 encodes a small 114 amino acid protein approximately 13 kDa in size. The ORF3 protein is believed to interact with several cell proteins to facilitate HEV replication, for example; interacts with hemopexin (affecting iron homoeostasis), binds to SH3 domain containing proteins (which function in the signal transduction pathway and promote cell survival) and interacts with Tsg101 and α1-microglobulin to facilitate the sorting of the endosomal sorting complexes required for transport (ESCRT).[23–25] However, ORF3 protein's key role is believed to be in viral assembly and egress, with phosphorylated ORF3 interacting with the capsid protein.[26] Interestingly monoclonal antibodies to ORF3 were able to bind nascent virions but not faecal virions.[27] Further studies indicated that virions circulating in human serum banded at a lower sucrose gradient in comparison to faecally derived virions and were not neutralised by the presence of antibodies in cell culture systems in the way faecally derived virions were suggesting that virions circulating in sera are protected by a membrane, containing, at least in part, ORF3.[28]

Negative sense intermediate replicates are detected during replication and attachment receptors have been identified but overall, the viral life cycle of HEV has not been extensively studied.[29,30] This is, in part, due to the slow progress in the development of reliable culture methods for HEV. Similarly, in vivo studies have been hampered due to the absence of a small animal model, until recently; with three studies achieving viral inoculation of human liver chimeric mice with HEV.[31–33] Interestingly, these reports indicated greater success using genotype 1 strains as inoculants (in contrast to in vitro studies) and all achieved greater infection and viraemia rates in inoculated mice using faecally derived HEV virions, in comparison to serum or culture derived. Two studies examined the use of ribavirin in inoculated chimeric mice and demonstrated the treatment to be successful in reducing viraemia in therapeutics.[31,33] However, ribavirin-induced anaemia was common in the treated mice (a side effect documented in humans).[31]

HEV Phylogeny

All HEV strains belong to the family Hepeviridae, which has not been assigned to any order. The family Hepeviridae contains 2 genera; Piscihepevirus and Orthohepevirus. Piscihepevirus contains one species; piscihepevirus A, which contains all known cutthroat trout strains of hepatitis E. The Orthohepevirus genus contains four species; orthohepevirus A,orthohepevirus B,orthohepevirus C and orthohepevirus D. All avian strains are contained in the species orthohepevirus B. The strains isolated from humans and pigs are all assigned to orthohepevirus A, as are the strains which infect camels, deer, rabbits, mongooses and some rat strains. Orthohepevirus C contains all strains isolated from ferrets and some rat strains. Orthohepevirus D contains only strains isolated from bats[34]

HEV has seven known genotypes; genotypes 1–4 and 7 displaying human tropism.[35] While genotypes 1 and 2 infect only humans, genotype 3 and 4 strains have been isolated from various animals and genotype 7 strains have been isolated from camels.[36] HEV appears to be unique amongst human hepatitis viruses, as recombination events appear to alter the replicative capacity, tissue specificity and pathogenicity of HEV.[37]

Genotypes 1 and 2

Genotypes 1 and 2 are endemic in developing countries, where they cause water-borne outbreaks. These are obligate human pathogens, transmitted via the faeco-oral route and clinical presentation with genotype 1 or 2 infection is indistinguishable from any other cause of acute viral hepatitis.

Genotypes 3 and 4

The most common mode of HEV transmission in developed countries is believed to be food-borne zoonosis.[38] Evidence of HEV as a zoonosis first came from the detection of HEV in pigs with a high homology to HEV strains found in humans.[39] Since then, many potential animal reservoirs for HEV have been identified.[40] Only genotypes 3 and 4 of the species orthohepevirus A, and genotype 7, are recognised as zoonotic and circulate mainly in developed countries.[40] Host species for genotypes 3 and 4 include pigs, deer, rabbits, mongoose, cattle, sheep and horses[39,41–45] Food-borne zoonosis of HEV has been documented in several case reports, and undercooked or raw pork has now been identified as a significant risk factor for human HEV infection. Transmission via contaminated shellfish[46] and soft fruits[47] is also recognised as a potential source of food-borne transmission. Transmission via blood transfusion and transplantation has also been well documented.

Genotype 7

To date, only one incidence of genotype 7 infection in humans has been documented; this was a case of a liver transplant recipient in the United Arab Emirates who regularly consumed camel milk and meat.[48] However, the incidence of camelid HEV in humans in countries with large camel populations deserves further attention.

Importantly, the virus can present as two quite distinct clinical conditions: large epidemics in endemic areas (genotype 1 in Africa and Asia, genotype 2 in Mexico and Africa: sporadic cases are recognised but less common than epidemics) and isolated cases amongst asymptomatic individuals in developed countries (genotype 3 and 4). HEV is a virus with 'two faces', behaving in a remarkable contrast between developing and developed countries and according to genotype[49] ( Table 1 ).

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