br HEV Mutations and Their Functional
HEV Mutations and Their Functional Role RNA viruses exhibit high genetic variability by rapid evolution with estimated mutation rate ranging from 10 to 10 substitutions per nucleotide per strand copying (Sanjuan et al., 2010). The HEV mutation rates were estimated indirectly from clinical isolates as 1.5 base substitutions per site per year and were similar to those reported for hepatitis C viruses (Takahashi et al., 2004). Mutations can occur frequently over the entire HEV genome during propagation and consecutive passages for leptomycin b manufacturer to cell culture (Lorenzo et al., 2008). High variability and frequent selection of mutations in the HEV genome is due to the transcription process. The viral RdRp, which lacks the proof-reading ability of DNA polymerases, likely increases the variations in the HEV genome. On the other hand, the selection pressure imposed by antiviral drugs and host immune responses may additionally contribute to increased HEV variability (Lhomme et al., 2014a). Although a quasispecies is linked to mutations, not all mutations in the viral genome shall generate viable virus quasispecies (Lauring and Andino, 2010).
HEV Mutations and Clinical Relevance An in-frame deletion of 246bp in ORF3 of Indian HEV strain isolated from clinical samples has been reported, however, the functional and clinical relevance of this deletion were unknown (Ray et al., 1992). An analysis of 22 HEV-4 full-length sequences from patients with fulminant and acute hepatitis found that the substitutions at the positions C1816 and U3148 were significantly associated with fulminant hepatitis (Inoue et al., 2006). However, only the mutation U3148 was confirmed to be associated with fulminant hepatitis by an additional analysis of 16 HEV-4 isolates. Further analysis of 86 HEV isolates showed that the U3148 variant revealed a stronger association with fulminant hepatitis in comparison to other variants (C3148 or G3148), and was associated with lower prothrombin activity (Inoue et al., 2006). A comparable study extended the results using 28 HEV-4 full-length sequences (from fulminant and acute hepatitis) and identified the C5907 variant most significantly associated with fulminant hepatitis (Inoue et al., 2009). An additive effect of both U3148 and C5907 mutations on fulminant hepatitis development was confirmed by further analysis of full-length sequences of 28 HEV-4 and 11 HEV-3 isolates as well as 35 partial sequences (Inoue et al., 2009). However, the mechanism of how U3148 and C5907 mutations influence hepatitis E progression is unresolved as these mutations are silent substitutions not changing the aa (Table 2). The ORF1 mutation V1213A corresponding to the aa substitution V239A in the Hel domain was found in all patients with more severe hepatitis but not in the patient with mild clinical course indicating that the V239A mutation can be associated with increased virulence (Takahashi et al., 2009). Notably, the V239A substitution in the Hel domain may enhance helicase activity and subsequently increase HEV replication (Ahola et al., 2000). Six aa changes in HEV-1 ORF1 including T563C (aa F179S), G977A (A317T), C2232T (T735I), T3355C (L1110F), G3386A (V1120I), and T4344A (F1439Y), were identified to be significantly associated with fulminant hepatic failure (Mishra et al., 2013). A total of 22 nucleotide substitutions were identified in the ORF1 region of 55 HEV sequences obtained from patients with acute viral hepatitis and those with acute liver failure. Most of these mutations including two non-synonymous substitutions C4476G (C1483W) and A4616C (N1530T) in the RdRp were found only in the HEV sequences from acute liver failure patients (Borkakoti et al., 2016). The mutations C1483W and N1530T were significantly associated with high viral load, abnormal prothrombin time, high bilirubin, and high mortality (Borkakoti et al., 2016). The association of HEV mutations in ORF2 with disease severity was investigated. Six substitutions including C5927T, C5933T, T6014C, C6032T, G6098A, C6104T, and a novel amino acid mutation P259S in ORF2 were identified to be significantly associated with fulminant liver failure (Borkakoti et al., 2014). These results indicate that non-synonymous substitutions can be associated with virulence and may affect viral replication (mutations in the RdRp) and especially enhance host immune response via modifying the antigen epitopes (non-silent mutations in ORF2 region) (Table 2). The abundance of mutations in the HEV genome from patient isolates is probably due to selective immune pressure. These mutations enable the virus to better adapt and modulate the host immune responses that lead to severity of complications.