Author Affiliations: Center of Infectious Diseases, West China Hospital of Sichuan University; Division of Infectious Diseases, State Key Laboratory of Biotherapy and Cancer Center, West China Hospital of Sichuan University, and Collaborative Innovation Center for Biotherapy, Chengdu 610041, China (Wang ML and Tang H)

Corresponding Author: Hong Tang, MD, PhD, Center of Infectious Diseases, West China Hospital of Sichuan University; Division of Infectious Diseases, State Key Laboratory of Biotherapy and Cancer Center, West China Hospital of Sichuan University, and Collaborative Innovation Center for Biotherapy, Chengdu 610041, China (Tel: +86-28-85422650; Fax: +86-28-85423052; Email: htang6198@hotmail.com)

 

© 2016, Hepatobiliary Pancreat Dis Int. All rights reserved.

doi: 10.1016/S1499-3872(16)60064-4

Published online January 13, 2016.

 

 

Contributors: TH proposed the study. WML performed the research, wrote the first draft, collected and analyzed the data. Both authors contributed to the design and interpretation of the study and to further drafts. TH is the guarantor.

Funding: This study was supported by a grant from the National Natural Science Foundation of China (81071363).

Ethical approval: Not needed.

Competing interest: No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.

 

 

BACKGROUND: The long-term use of nucleos(t)ide analogues causes drug resistance and mutations in the HBV reverse transcriptase (RT) region of the polymerase gene. The RT region overlaps the HBV surface gene (S gene) and therefore, the mutations in the RT region simultaneously modify S gene sequence. Certain mutations in the RT region bring about truncated S proteins because the corresponding changed S gene encodes a stop codon which results in the loss of a large portion of the C-terminal hydrophobic region of HBV surface protein. The rtA181T/sW172*, rtM204I/sW196* and rtV191I/sW182* are the most frequently reported drug-resistant mutations with C-terminal truncation, these mutations have oncogenic potential.

 

DATA SOURCES: PubMed and Web of Science were searched using terms: “hepatitis B virus”, "HBV drug resistance mutation”, "HBV surface protein”, "HBV truncation”, "hepatocellular carcinoma”, "rtA181T/sW172*”, "rtM204I/sW196*”, "rtV191I/sW182*”, and relevant articles published in English in the past decades were reviewed.

 

RESULTS: The rtA181T/sW172* and rtV191I/sW182* mutants occurred more frequently than the rtM204I/sW196* mutant both in chronic hepatitis B patients and the HBV-related hepatocellular carcinoma tissues. Although these mutations occur naturally, nucleos(t)ide analogues therapy is the main driving force. These mutations may exist alone or coexist with other HBV mutations. All these three mutants impair the virion secretion and result in HBV surface protein retention and serum HBV DNA level reduction. These mutations possess potential carcinogenic properties. The three mutations are resistant to more than one nucleos(t)ide analogue and therefore, it is difficult to treat the patients with the truncated mutations.

 

CONCLUSIONS: Nucleos(t)ide analogues induce drug resistance and HBV S gene truncated mutations. These mutations have potential carcinogenesis.

 

(Hepatobiliary Pancreat Dis Int 2016;15:579-586)

 

KEY WORDS: hepatitis B virus; drug resistance mutation; surface protein; C-terminal truncation; oncogenic potential; hepatocellular carcinoma

In approximately 2 billion people infected worldwide, HBV infection has long been a tough public health problem resulting in 600 000-1 200 000 annual deaths associated with cirrhosis or hepatocellular carcinoma (HCC).^[1] Antiviral treatment for chronic hepatitis B (CHB) with nucleos(t)ide analogues (NAs), including lamivudine, telbivudine, entecavir, adefovir and tenofovir, has shown remarkable progress in inhibiting HBV replication without significant side effects. However, an indefinite period of treatment with NAs results in drug resistance which weakens the antiviral effects of NAs. Moreover, NAs cause HBV mutations, particularly HBV surface gene (S gene) truncated mutations which have potential carcinogenesis.^[2]

 

As a minute human DNA virus, HBV is an enveloped virus containing partially double-stranded circular DNA about 3.2 kb in length.^[3] Despite of its small size, the HBV genome has highly precise structure with at least four partially overlapping reading frames: C, P, X and S gene. The S gene is composed of 1185 base pairs, encodes the HBV surface proteins, including the S protein (226 amino acids), M protein (281 amino acids), and L protein (389-400 amino acids); these proteins are viral transmembrane envelope proteins and required for infectivity of HBV and antibody production. These three proteins share the same C-terminal (stop codon) no matter where the translations begin (start codons). Their major functions are the envelopment of nucleocapsids and subsequent assembly into empty subviral particles called HBsAg, which is a key target for the human immune system.^[4] The mutations in the S gene lead to the immune evasion of HBV.^{[4, 5]} A large number of evidence suggested that HBV could induce HCC indirectly by inhibiting host immune responses against the infected hepatocytes.^[6] It is noteworthy that the S gene is overlapped totally by the reverse transcriptase (RT) domain of the polymerase gene (P gene, Fig.). Owing to this structural characteristic, the nucleotide mutation in the S gene is accompanied by RT mutation and vice versa.^[7] One type of the nucleotide mutations in the RT domain could encode premature stop codons in the corresponding S gene, resulting in truncation of the last dozens of amino acids at the C-terminal of S proteins. rtA181T/sW172*, rtM204I/sW196* and rtV191I/sW182* are the truncated mutations that are found in the highest frequency and decrease the antiviral effect and have oncogenic potential.^[8] This review aimed to analyze the publications on these mutations and explore the possible managements of the patients harboring these mutations.

 

 

HBV rtA181T mutant is the point mutation (TGGCTC→TGACTC) at position 181 of the P gene in the RT domain, causing the alanine (A)→threonine (T) mutation in the polymerase. Due to the complete overlap between S gene and the RT domain, the mutation (sW172*) replaces the amino acid tryptophan (W) at position 172 of the S gene with a termination codon (UGA), resulting in the truncation of the last 55 amino acids of the S proteins. Likewise, rtM204I/sW196* (ATGGAT→ATAGAT) and rtV191I/sW182* (GTGGTT→GTGATT) are the same type of nucleotide substitutions in the RT domain and S gene and also induce truncated surface proteins with loss of the last 31 and 45 amino acids, respectively, in the C-terminals of the S proteins (Table 1 and Fig.).

 

 

The rtA181T/sW172* mutation was discovered by Yeh et al^[9] in 2000. It was found in patients treated with adefovir, lamivudine, telbivudine or clevudine.^{[10, 11]} This mutation was detectable in all HBV genotypes and in the form of quasispecies with the wild-type. Linear regression analysis revealed that age and cirrhosis were the independent predictors of the emergence of this mutation.^[12] However, Warner and Locarnini^[10] performed a review in the SeqHepB database and found that rtA181T/sW172* occurred in 2 of 830 patients without antiviral therapy. They^[13] later pointed out that this mutation is associated with primary drug resistance. Ding et al^[14] found this mutation in 1 of 139 NA-naïve patients. A national survey in France^[15] assumed that this mutation was naturally selected even though it initially presented with wild-type. Lai and Yeh^[16] found rtA181T/sW172* mutation in a patient with advanced HBV-related HCC, who surprisingly has never received antiviral therapy, and their data indicated that rtA181T/sW172* mutation could emerge spontaneously without previous antiviral treatment.

 

The rtV191I/sW182* mutation was detected by Seignères and colleagues^[17] in 2 of 28 patients with naïve famciclovir treatment for 48 weeks. This mutation, thereafter, was detected not only in patients treated with adefovir,^[18] tenofovir,^[19] especially with lamivudine,^[20] but also in those never treated with NA^[21] and those with HBV/HIV-coinfection.^[19] Similar to the rtA181T/sW172* mutation, this mutation was accompanied with the wild-type^[6] and detected in all HBV genotypes.

 

The rtM204I/sW196* mutation was not well investigated because of its low frequency. This mutation is just detectable in patients treated with lamivudine^[22] and telbivudine.^[23]

 

Despite of the natural occurrence, these three mutations are mainly induced by NAs.

 

 

The epidemiology of the drug-resistant truncated HBsAg mutants is not clear. The incidence of rtA181T/sW172* mutation slightly differs in various studies. Warner and Locarnini in 2008 reviewed 1440 patients with chronic hepatitis B (CHB) in the SeqHepB database^[10] and found that 23 of 611 (3.76%) patients treated with NAs had rtA181T/sW172* mutation; in comparison, only 2 of 830 (0.24%) patients without NAs treatment had this mutation. Villet et al^[24] reported that this mutation was observed in 4 of 10 consecutive CHB patients with NAs treatment. Servant-Delmas et al^[15] found only 2 (0.21%) patients with rtA181T/sW172* mutation in 940 CHB patients. In an investigation from Turkey,^[25] rtA181T/sW172* was found in 2% (1/46) of CHB patients. The frequency of rtA181T/sW172* mutation was significantly higher in patients with drug resistance, particularly with adefovir- and lamivudine-resistance.^[26] This variant was detected in 1% and 11% of lamivudine- and adefovir-resistant cases, respectively.^[10] Sayan’s study demonstrated the similar mutation rate^[25] which is 2% and 11% of lamivudine- and adefovir-resistant cases, respectively. Yeh et al^[12] found the mutant in 10 (8.1%) of the 123 lamivudine-resistant patients. The incidences are higher in other studies, which detected the mutant in 5/13 of adefovir-resistant patients^[27] and 5/10 of patients^[28] with drug-resistant truncated HBsAg mutations. It is noteworthy that Lai et al^[29] found rtA181T/sW172*in 2/8 of HBV-related HCC patients undergoing lamivudine therapy.

 

The mutation rate of rtV191I/sW182* in patients treated with NAs is close to that of rtA181T/sW172*. Meldal et al^[21] found that 1 of 84 (1.19%) Malaysian NA-naïve blood donors had rtV191I/sW182* mutation, while Chinese researchers^[14] detected 4 in 139 (2.88%) of NA-naïve patients. Yang et al^[18] discovered this mutation in 1 of 39 patients under treatment with adefovir at 24 weeks. Wakil et al^[20] and Kazim et al^[30] found this mutation in 1 of 26 (3.85%) and 57 (1.75%) Indian patients under lamivudine treatment, respectively. Pollicino et al^[31] detected 1 carrying the rtV191I/sW182* mutation in 53 (1.89%) lamivudine-resistant patients. Amini-Bavil-Olyaee et al^[32] and Sheldon et al^[19] found this mutation in an HBV/HIV coinfected patient treated with lamivudine-tenofovir. More importantly, this mutation was identified in 6 of 25 (24%) HBcAg(+) HBV-related HCC tumor tissues, and the 6 tissues were of genotype C.^[33] A significantly higher occurrence rate was reported by Lee et al,^[6] the rtV191I/sW182* mutation was detected in 73 (26.5%) of 275 genotype C HBV patients and, importantly, the incidence was 31.0% (35/113) in genotype C HBV-related HCC cancerous tissues, while Huang et al^[33] found that 12% (6/50) of the HCC patients were detected via test of their blood. The high rate reported by Lee et al^[6] could be attributed to the detection method (real-time PCR) or specific HBV genotype C.

 

The data of rtM204I/sW196* mutation are relatively less than those of the other two. Salpini et al^[26] detected 3 mutations from 204 (1.5%) in CHB patients. The analysis of the SeqHepB database found that more than 10% of lamivudine-treated CHB patients had this mutation. Verheyen et al^[28] demonstrated only one of 23 HBV strains with rtM204I harbors rtM204I/sW196* mutation. This is attributed to code degeneracy resulting in substituted mutation (sW196S/L), which is more often than rtM204I/sW196*, and is likely due to the selection pressure from the antiviral treatments. However, Damerow et al^[22] analyzed 827 CHB patients from 29 articles and did not find rtM204I/sW196*. It deserves attention that rtM204I/sW196* emerged in 1 of 8 HBV-related HCC patients treated with lamivudine as reported by Lai et al.^[29]

 

Similarly, these three truncated mutations are usually detectable in wild-type HBV in a low percentage because the mutants need wild-type to pack and secrete.

 

Since the three truncation mutations were developed from the wild-type, the mutants usually coexisted with the wild-type in the viral population.^[33] The rtA181T/sW172* mutation, according to clonal analysis of HBV RT sequences in patients with this mutation, exist alone or coexist mostly with N236T, rtI233V, M204V/I, L80V, N236+N238T or L80V+M204I. The rtV191I/sW182* mutation could be accompanied by rtA181T, I53S/I, rtI233V. In line with the rtA181T/sW172* mutation, the rtM204I/sW196* mutation could be alone or associated frequently with L80I/V, L180M, or L80I/V+L180M and occasionally with rtV173L/M.

 

 

Investigators found that the rtA181T/sW172* mutation shows dominant replication deficiency both in vitro and in vivo. Yeh et al^[9] transfected HepG2 cells with rtA181T/sW172* mutation, they found that 24-hour later, the level of HBV DNA from the transfected HepG2 cells was 6 times lower than that from wild-type. Yatsuji et al^[34] did not find that the mutation affected replication. Warner and Locarnini^[10] found that the mutation had significantly less intracellular replicative intermediates than wild-type virus, and that no extracellular HBV DNA was detected via Southern blotting. In contrast to the findings in vitro, the rtA181T/sW172* mutation interestingly increased and prolonged HBV DNA expression in liver tissues of mouse model compared to that of wild-type, suggesting that the mutation may increase HBV replication capability in the liver, while the serum HBV DNA level was reduced, as described above.^[35] The replication and serum HBV DNA discrepancy are still unclear, probably due to the secretion impairment of the cells infected with mutation. But Kim et al^[27] showed that, not as the same result in vitro, the HBV DNA levels had no distinct difference in 13 patients with this mutation compared with other 9 patients as a control group, which was similarly observed by Warner and Locarnini.^[10] This may be due to secretory defect or a dominant negative effect on wild-type HBV vision secretion.^[10] With regard to its atypical viral load rebound, the selection of rtA181T/sW172* will mask the diagnosis of drug resistance if serum HBV DNA levels are the only indicator. Using complementation assay, Ahn et al^[36] found that the truncated PreS1 is responsible for decreased replication of this mutant. In addition, the replicative capacity of this mutant is totally restored in the administration of lamivudine, suggesting that this mutation is dependent on lamivudine.^[9]

 

Amini-Bavil-Olyaee et al^[32] reported that the rtV191I/sW182* mutation impaired the replicative capacity of HBV in Huh7 cells; this deficiency could be restored by concomitant precore or basal core promoter mutations as found in HBeAg(-) CHB, suggesting that rtV191I/sW182* may get more survival advantages in HBeAg(+) patients. Similar to rtA181T/sW172*, the serum HBV DNA level of patients with this mutation is significantly lower than that of patients with non-rtV191I/sW182* mutation. The potential mechanism is, according to that of the rtA181T/sW172*, that the truncated S proteins may interrupt the formation of normal virions, resulting in a loss of infectivity and, in turn, DNA levels, supported by the observation in this study of the HBV virions formation failure with this mutation. Another possible mechanism underlying is that the truncated S protein could stress the endoplasmic reticulum and the later produce reactive oxygen species via an expanded protein response pathway, which contributes to the decrease of HBV DNA levels.^[6]

 

The rtM204I mostly coexists with sW196S/L mutation rather than sW196* as mentioned above and this is why the research on the mutation of rtM204I/sW196* is rare. There are just two studies demonstrating that this mutant appears to be replication impaired.^{[37, 38]} Compared to wild-type virus, this mutation weakens HBV DNA replicative intermediate signals and reduces total intracellular HBV DNA. The related mechanism might be similar to the other two truncated mutations.

 

Compared to the wild-type HBV virus, the introduction of rtA181T/sW172* mutation results in dominant HBsAg secretion failure.^{[10, 16, 35]} Warner and Locarnini^[10] verified that the rtA181T/sW172* mutation decreases the HBsAg molecular weight of nearly 6 kDa; the truncated surface proteins were only detectable intracellularly (Huh7 cells) and this mutant was not detectable in the supernatant. The intracellular surface proteins present at similar levels to wild-type. However, the mutant proteins have lower levels of glycosylation. Immunohistochemistry demonstrated that the surface proteins from HBV rtA181T/sW172* transfected Huh7 cells showed better granules without packaging, and also exhibited mainly greater staining intensity than those from HBV wild-type Huh7 cells, which proved intracellular retention. However, it is not clear which step is impaired in the secreting pathway. Despite the dominant secretion defect of rtA181T/sW172* was verified in vitro, the serum HBsAg titer in patient infected with this mutant is similar to that with non-rtA181T/sW172*.^[27] Dai and coworkers^[35] injected rtA181T/sW172* mutant plasmid into BALB/C mice, they found that the serum HBsAg level of the rtA181T/sW172* was very low. Lai and Yeh^[16] examined a patient with HBV-related HCC and found that this patient had rtA181T/sW172* mutation seropositive for HBeAg but seronegative for HBsAg which indicated that rtA181T/sW172* mutation causes secretion failure rather than production impairment.

 

Lee and colleagues^[6] introduced rtV191I/sW182* mutation to Huh7 cells and found that the truncated S protein was not secreted to supernatant. Because of the impairment of secretion, the truncated S protein escapes from the humoral immune response.

 

rtM204I/sW196* mutation, like other two truncated mutants described above, blocks the expression of small envelope proteins and therefore exhibits lower infectivity and replication ability than wild-type virus.^[37]

 

Lai and Yeh in 2008 first reported the relationship between rtA181T/sW172* and HCC.^[16] They found rtA181T/sW172* mutant both in the serum and hepatoma samples in a 39-year-old NA-naïve patient with advanced HCC. When the rtA181T/sW172* mutant was transferred to HepG2 cells, this mutant was capable of activating the human c-Myc and Simian virus 40 promoters and resulting in carcinogenesis. Injecting rtA181T/sW172* mutant transfected NIH3T3 cells which stably express this mutant to null mice, the laboratory results demonstrated the tumourigenicity of this mutant.^[29] A Kaplan-Meier analysis, based partially on the occurrence of HCC in 3 of 10 rtA181T/sW172*-positive patients with lamivudine-resistance, revealed that emergence of sW172* increased the risk of hepatoma occurrence in the subsequent antiviral therapy.^[12] As HBsAg in rtA181T/sW172* mutant is retained and accumulated in the endoplasmic reticulum, the grp78 gene might be activated in one signal pathway of endoplasmic reticulum, resulting in the occurrence of HCC.

 

In 2012, Lee et al^[6] found that the percentage of rtV191I/sW182* was higher in patients with HCC and cirrhosis than that in the HBV carriers (HCC 30.4%, cirrhosis 33.3% and HBV carrier 16.7%). Because cirrhosis is regarded as a precancerous state, rtV191I/sW182* may have a more important contribution to cirrhosis development rather than to HCC, which should be elucidated in future studies. Further experiments with NIH3T3, Huh7 and HEK-293 cells showed that the clone numbers are significantly increased in rtV191I/sW182* transfected cells compared with mock and wild-type counterparts. These experiments suggested that the truncated S protein could hamper the cell cycle checkpoint at the G1/S phase through inhibiting the expression of p53-p21 axis. This mutation is confirmed by a transacting activity of the mutant protein, which in turn may contribute to HCC via accumulation of the mutations. Additionally, PCNA and CDK4, the proliferation-related genes which are responsible for the G1/S transition, were also up-regulated by the truncated S protein, which may stem from the down-regulation of p21.

 

A study read at the 22nd Conference of the Asian Pacific Association for the Study of the Liver^[39] reported the oncogenic potential of rtM204I/sW196* mutation. This mutant was transfected mutation into NIH3T3 cells. The behaviors of these cells were different: much softer agar clones, more proliferation ability, less apoptosis under the incubation of camptothecin, and increased tumorigenesis in nude mice when compared with mock control. Microarray analysis showed that rtM204I/sW196* upregulated Hif1A, HLF and ER81 expression, and down-regulated miR-335 and TGF-b expression in mutant-transfected NIH3T3 cells.

 

Although the studies above demonstrated the oncogenic potential of the three truncated S protein mutants, the mechanism of pathogenicity is still unclear. Truncated preS2/S sequence in integrated HBV DNA of hepatocyte is a possible mechanism.^[40] And this trans-activation strictly required the location of the truncation points within a defined region of surface gene, named the transactivity-ON-region, which located between codons 122 and 139 in the S region.^[41] Based on this, the virus might theoretically have the transactivation activity when a truncation point of surface gene in the transactivity-ON-region. The truncation point of rtA181T/sW172* is located at the border of the transactivity-ON-region. Lauer and coworkers^[41] indeed found the transactivation activity of this mutant. But the mutation points of other two truncation mutants (rtM204I/sW196* and rtV191I/sW182*) are out of the transactivity-ON-region, inconsistent with the possible way like rtA181T/sW172*. Regarding the non-uniform pathway among these mutations, the specific mechanism for the oncogenic potential requires further studies.

 

Generally, drug sensitivity is evaluated by the median effective concentration (EC50) or the median inhibitory concentration (IC50). Drug sensitivity test is performed in succession by transfection of plasmid or virus strains harboring the target mutation into cells, assessment of HBV replication after adding NAs into cells, then comparison with the EC50/IC50 of the wild-type and judging drug resistance.

 

Currently, in vivo studies suggest that rtA181T/sW172* is the primary resistance mutation to adefovir^[42] and it also confers cross resistance to both lamivudine and telbivudine.^[24] The results by Villet et al^[24] revealed that rtA181T/sW172* mutant not only decreased susceptibility to the L-nucleosides lamivudine (5.7-10.8 folds) and telbivudine, but also to the alkyl phosphonates adefovir and tenofovir; the addition of rtN236T to rtA181T/sW172* increased the resistance level to lamivudine, adefovir and tenofovir. However, single rtN236T remains sensitive to lamivudine, and rtA181T/sW172* only remains sensitive to entecavir (Table 2).^{[19, 25, 35]} Notably, the combined therapy of lamivudine and adefovir enhanced rtA181T/sW172* to evolve to rtA181T/sW172S. Compared to sW172*, sW172S is mildly resistant to adefovir and lamivudine and highly sensitive to entecavir and tenofovir. rtV191I/sW182* mutation is resistant to lamivudine, and remains sensitive to adefovir and tenofovir. The patient showed a lg6 decrease in serum HBV DNA following adefovir therapy (Table 2).^{[32, 43]}

 

Compared with rtA181T/sW172*, the rtM204I/sW196* mutation confers lamivudine and telbivudine resistance and do not confer cross-resistance to adefovir or tenofovir (Table 2). It is the predominantly detected mutation in the YMDD motif, that is essential for RT activity, inducing viral breakthrough during lamivudine treatment.^[13]

 

 

There is so far no clinical trial for antiviral therapy in CHB patients with drug resistance truncated mutations, the following are some preliminary data in clinical treatments.

 

At first, the rtA181T/sW172* mutation is resistant to lamivudine, but lamivudine increases the incidence of rtA181T/sW172* mutations.^{[9, 24, 34, 44]} This mutation may induce cross-resistance to lamivudine and adefovir.^[24] However, all the clones harboring the rtA181T mutation are sensitive to entecavir. Patients with rtA181T mutation induced by lamivudine, a combination of tenofovir+ lamivudine might be the optimal choice because location at rt204 has been concomitantly selected by lamivudine exposure. In patients with sW172* mutants selected under adefovir therapy, the addition of entecavir may be recommended according to their cross-resistance profile (rtA181T/sW172*). The European Association for the Study of the Liver approved that entecavir or tenofovir+emtricitabine are the optimized options,^[45] and the definitive recommendations indeed needs further clinical observation and cost-benefit studies.

 

A study^[6] demonstrated that rtV191I/sW182* remains sensitive to adefovir in vitro. This study was conducted by enzymatic assay of recombinant HBV polymerase and, importantly, not the replicative HBV mutant in vivo for resistant analyses. The optimal choice for this mutation is therefore still tenofovir.

 

Related data suggested that the rtM204I/sW196* mutation reduces the sensitivity to entecavir but remains sensitive to adefovir or tenofovir.^[46] Thus, it is recommended to add tenofovir (or adefovir if tenofovir is not available). However, more attention is needed when multiple mutations (rtL80I+rtL180M) occur in addition to this mutation, which requires combined therapy of high-level NAs (entecavir or tenofovir).

 

It is unknown whether long-term combination therapy with the currently approved therapies is safe or not, but adding-on a recommended second drug without cross-resistance is the only efficient strategy.

 

 

Among the oncogenic potential HBV mutants, one special group of the NAs-related-resistant mutants that catch much concerns of molecular virologists, the rtA181T/sW172* and rtV191I/sW182* mutants occurred more frequently than rtM204I/sW196* mutant both in CHB patients and those with HBV-related HCC. According to the mutant prevalence, more studies on rtA181T/sW172* and rtV191I/sW182* mutants should be carried out. However, relevant studies revealed these three mutants could impair the virion secretion, resulting in HBV surface protein retention, serum HBV DNA level reduction and carcinogenesis. The three mutations are resistant to more than one NA; therefore it is important to make appropriate therapies for the truncated mutations, especially for multiple mutations.

 

The NAs are widely used to inhibit HBV replication and complications, drug resistance may occur when they are on long-term use, especially the mutation in the RT domain of HBV, which is always a veiled threat in CHB treatment. Mutations not only cause the lower effectiveness of the NAs or inability to suppress HBV replication, but also the truncated mutations lead to carcinogenesis. Obviously, hepatologists are facing the challenge of nonclassical CHB infection caused by mutants with altered antigenicity and viral fitness. To the present, studies on the drug-resistant truncated mutation are limited, which contribute to the inadequate understanding of the insufficient real pathogenesis of these mutations. With the recent progress in testing and treating for HBV resistance truncated mutations, however, it is still necessary to comprehensively understand the effects of these mutations on replication, secretion, drug resistance of HBV as well as the mechanism of carcinogenesis. Only the detailed characterization of HBV truncated mutations is demonstrated, can we make the right choice for rescue therapy or prevent drug resistance and improve the outcome of patients with CHB.

 

 

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