br HEV Mutations and Vaccination Prevention of HEV

HEV Mutations and Vaccination Prevention of HEV infection in endemic areas is based on the implementation of appropriate hygiene and sanitary measures to avoid fecal-oral transmission. In regions where HEV infection is sporadic, the consumption of raw food should be avoided. HEV infection can be prevented with effective HEV vaccines and ORF2 is widely used as a target for vaccine development (Zhang et al., 2015). Mutations in ORF2 may lead to a failure of an adaptive cellular immune response in vaccinated individuals. Therefore, mutations occurring in ORF2 sensitively affecting the ORF2 protein structure is one of the challenges for the protective efficiency of HEV vaccine programs. On the other hand, a number of mutations resulting in HEV attenuation (e.g., F51L, T59A, S390L, N562Q/D/P/Y, L477T, L613T and HVR deletion) may provide a basis for the development of live-attenuated vaccines against HEV (Cordoba et al., 2011; Pudupakam et al., 2009; Zhang et al., 2008). A strategy for viral attenuation and vaccine development has been proposed based on mutating the conserved active site lysine residue to arginine of the viral RdRp (Weeks et al., 2012). Therefore, HEV mutations in the RdRp domain and deletions in the HVR domain have repercussions for the development of live, attenuated HEV vaccines.
Clinical Relevance of HEV Mutations in Antiviral Therapy Although no HEV-specific treatment options show significant antiviral activity, the effectiveness of PEG-interferon-α in combination with ribavirin, and ribavirin alone for HEV infection have been recently documented (Kamar et al., 2014b; Peron et al., 2016; Wedemeyer et al., 2012). However, ribavirin treatment failure was reported in patients with chronic hepatitis E (Debing et al., 2014, 2016b; Gisa et al., 2015; Lhomme et al., 2015; Todt et al., 2016) (Table 2). The entire HEV sequences before, during and after treatment courses were compared and a nucleotide substitution (G>A) resulting in a G1634R disability in the C-terminal region of the HEV-3 RdRp was identified (Debing et al., 2014). Although showing no effect on ribavirin resistance, the variants 1634R and 1634K significantly contribute to an increased efficiency of viral replication and infectivity compared to the wild-type G1634 (Debing et al., 2014). Comparable results confirmed a similar role for HEV-1 (Debing et al., 2014). This result was further supported by a clinical observation that plasma HEV-RNA levels were significantly increased in patients infected with 1634R mutant compared to non-1634R mutant viruses (Lhomme et al., 2015). These findings suggest that G1634R/K may not be a direct antiviral resistance mutation but partially involved in ribavirin treatment failure by enhancing HEV replication. By analyzing 63 HEV sequences from solid-organ transplant patients with chronic hepatitis E, the prevalence of the G1634R mutation was shown to be higher in non-sustained virologic response (SVR) compared to SVR patients (Lhomme et al., 2015). This observation is supported by evidence that the proportion of the G1634R mutation was rapidly increasing in patients with ribavirin treatment failure (Parvez, 2013; Xu et al., 2016). Antiviral resistance mutations (G1634R/K) can occur in HEV genome under ribavirin treatment, since ribavirin has been recently demonstrated to cause mutations in the HEV genome as well as in other RNA viruses (Debing et al., 2016b). Ribavirin therapy can lead to an increased HEV variability (ORF1, ORF2 and ORF3) over time, especially in the RdRp domain (Todt et al., 2016). However, the presence of the 1634R mutation neither leads to absolute ribavirin resistance nor influences the response to re-treatment with ribavirin (Galante et al., 2015; Lhomme et al., 2015). This result is in line with a finding showing that the ribavirin treatment failure is not directly caused by the G1634R mutation (Debing et al., 2014). Besides the described G1634R mutation, two other substitutions (Y1320H, K1383N) in the RdRp domain, as well as two mutations (A723V, A647T) and a 282bp-insertion (a duplicated 258bp HVR-derived and a 24bp RdRp-derived fragments) in the HVR were identified. Notably, the frequency of these substitutions (Y1320H, K1383N, G1634R and A723V) increased during the course of ribavirin treatment (Debing et al., 2016b). The G1634R and Y1320H mutations enhanced viral replication but did not affect ribavirin susceptibility, whereas the K1383N mutation abrogated viral replication and was associated with increased ribavirin sensitivity by affecting the binding activity of the RdRp domain to guanosine-5′-triphosphate (GTP) (Debing et al., 2016b). Although Y1320H and G1634R/K can compensate for the harmful effect on viral replication caused by the K1383N mutation (Debing et al., 2016b), the interaction among these mutations and their functional role is yet to be understood. The A723V mutation had no effect on viral replication whereas the 282bp-insertion in the HVR significantly increases viral replication. However, the artificial insertion and deletion of the 24bp RdRp-derived fragment reduced viral replication compared to wild-type HEV, suggesting a role of the insertion and also other unknown factors (Debing et al., 2016b). In addition, four additional substitutions (D1384G, K1398R, V1479I and Y1587F) together with the known mutations K1383N and G1634R in ORF1 were identified. Of those, the mutation G1634R could be detected in low frequencies before ribavirin therapy (Todt et al., 2016). These additional mutations (D1384G, K1398R, V1479I and Y1587F) were associated with increased ribavirin sensitivity, and with higher HEV replication (Todt et al., 2016).