LAUSR

The hepatitis B virus (HBV) is a major human pathogen. The HBV genome encodes four overlapping open-reading frames (ORFs) including the S-ORF, C-ORF, P-ORF, and X-ORF. Among them, polymerase which encoded by P-ORF is the target of all approved nucleos(t)ide analogues (NAs) for antiviral therapy. NAs are generally efficient and well tolerated. However, resistance to non-first line antiviral agents is a major issue affecting long-term therapy [1]. HBx encoded by X-ORF is a nonstructural protein that serves multiple functions during the various stages of chronic HBV infection (CHB) through interaction with a number of host proteins. In this issue of Hepatology International, Lin et al. identified HBx mutants that emerged in patients experiencing lamivudine (LAM) resistance or entecavir (ETV) suboptimal response by sequence analysis and characterized their roles in the HBV replication cycle. Co-transfection of these HBxmutant plasmids and HBV replication-competent clone into cell lines result in increased nuclear-to-cytoplasmic ratios of core antigen and HBV-DNA, as well as the level of nuclear covalently, closed circular DNA (cccDNA). These results demonstrate that HBx mutants can emerge during LAM or ETV therapy and compensate for the replication suppression of NAs by increased cccDNA. This is an innovative study in which the researchers extend the resistant-associated mutations from polymerase to the X gene, and explore the effect of these mutations on HBV replication in vitro [2]. Mutations provide possibilities for the virus to survive a complex environment. Mutations often arise spontaneously in viral quasispecies followed by some of them were subsequently selected as predominant strains for their adaption in changing conditions. We have known that HBV resistance to NAs is achieved through polymerase mutations. Antiviral agents always target important virological mechanisms; as a result, resistance mutation often led to decreased replication fitness. There are many documented examples of the fitness costs that are associated with NA resistance in HBV antiviral therapy. In most cases, the cost can be ameliorated by the acquisition of compensatory mutations. Hughes et al. classified the common resistance-compensatory mechanisms into four categories [3]. Two of these compensating mechanisms that often appear in antiviral resistance are compensation by an intragenic mutation and compensation by an intergenic mutation. According to the above definitions, regular HBV initial primary mutation and compensatory mutation introduced by Lok et al. belong to compensation by an intragenic mutation, in which primary mutations in the reverse transcription (RT) region of polymerase cause an amino acid substitution that results in reduced susceptibility to an antiviral agent while secondary RT mutations cause amino acid substitutions that restore functional defects in viral polymerase activity (i.e., replication fitness) associated with primary drug resistance. For example, typically rtM204V/I is a primary lamivudine resistance mutation which causes a greater than a 100-fold decrease in susceptibility to LAM in phenotypic assays. rtL180M is the most common compensatory mutation which restores the replication fitness of HBV polymerase that harbors the rtM204V/I mutation [4, 5]. The mutations reported by Lin et al. are involved above intergenic compensation between polymerase and HBx. The results not only enrich our knowledge on the resistancecompensation mechanism of HBV but also provide more thoughts on the role of HBx mutations. The development of NA resistance can be divided into three phases during HBV antiviral therapy. In the first phase, the wild-type dominates quasispecies at the initiation of antiviral therapy. Along with the application of NAs, a rapid decline was observed in virus load; in the second phase, accompanied by the rapid elimination of wild-type virus, * Jinlin Hou [email protected]