Revisiting Hepatitis B Virus: Challenges of Curative Therapies

Persistence of Hepatitis B Virus Infection: A Multi-Faceted Player for Hepatocarcinogenesis

Hepatitis B virus (HBV) infection has a multi-dimensional effect on the host, which not only alters the dynamics of immune response but also persists in the hepatocytes to predispose oncogenic factors. The virus exists in multiple forms of which the nuclear localized covalently closed circular DNA (cccDNA) is the most stable and the primary reason for viral persistence even after clearance of surface antigen and viral DNA. The second reason is the existence of pregenomic RNA (pgRNA) containing virion particles. On the other hand, the integration of the viral genome in the host chromosome also leads to persistent production of viral proteins along with the chromosomal instabilities. The interferon treatment or administration of nucleot(s)ide analogs leads to reduction in the viral DNA load, but the pgRNA and surface antigen clearance are a slow process and complete loss of serological HBsAg is rare. The prolonged exposure of immune cells to the viral antigens, particularly HBs antigen, in the blood circulation results in T-cell exhaustion, which disrupts immune clearance of the virus and virus-infected cells. In addition, it predisposes immune-tolerant microenvironment, which facilitates the tumor progression. Thus cccDNA, pgRNA, and HBsAg along with the viral DNA could be the therapeutic targets in the early disease stages that may improve the quality of life of chronic hepatitis B patients by impeding the progression of the disease toward hepatocellular carcinoma.

Rapid and Robust Continuous Purification of High-Titer Hepatitis B Virus for In Vitro and In Vivo Applications

Available treatments for hepatitis B can control the virus but are rarely curative. This led to a global initiative to design new curative therapies for the 257 million patients affected. Discovery and development of these new therapies is contingent upon functional in vitro and in vivo hepatitis B virus (HBV) infection models. However, low titer and impurity of conventional HBV stocks reduce significance of in vitro infections and moreover limit challenge doses in current in vivo models. Therefore, there is a critical need for a robust, simple and reproducible protocol to generate high-purity and high-titer infectious HBV stocks. Here, we outline a three-step protocol for continuous production of high-quality HBV stocks from supernatants of HBV-replicating cell lines. This purification process takes less than 6 h, yields to high-titer stocks (up to 1 × 10¹¹ enveloped, DNA-containing HBV particles/mL each week), and is with minimal equipment easily adaptable to most laboratory settings.

The RNA Binding Proteins YTHDC1 and FMRP Regulate the Nuclear Export of N6-Methyladenosine-Modified Hepatitis B Virus Transcripts and Affect the Viral Life Cycle

YTHDC1 and fragile X mental retardation protein (FMRP) bind N⁶-methyladenosine (m⁶A)-modified RNAs and facilitate their transport to the cytoplasm. Here, we investigated the role of these proteins in hepatitis B virus (HBV) gene expression and life cycle. We have previously reported that HBV transcripts are m⁶A methylated, and this modification regulates the viral life cycle. HBV is particularly interesting, as its DNA genome upon transcription gives rise to a pregenomic RNA (pgRNA), which serves as a template for reverse transcription to produce the relaxed circular DNA that transforms into a covalently closed circular DNA (cccDNA). While m⁶A modification negatively affects RNA stability and translation of viral transcripts, our current results revealed the possibility that it positively affects pgRNA encapsidation in the cytoplasm. Thus, it plays a differential dual role in the virus life cycle. YTHDC1 as well as FMRP recognize m⁶A-methylated HBV transcripts and facilitate their transport to the cytoplasm. In cells depleted with YTHDC1 or FMRP, viral transcripts accumulate in the nucleus to affect the viral life cycle. Most importantly, the core-associated DNA and subsequent cccDNA syntheses are dramatically affected in FMRP- or YTHDC1-silenced cells. This study highlights the functional relevance of YTHDC1 and FMRP in the HBV life cycle with the potential to arrest liver disease pathogenesis. IMPORTANCE YTHDC1 and FMRP have been recently implicated in the nuclear export of m⁶A modified mRNAs. Here, we show that FMRP and YTHDC1 proteins bind with m⁶A-modified HBV transcripts and facilitate their nuclear export. In the absence of FMRP and YTHDC1, HBV transcripts accumulate in the nucleus to reduce reverse transcription in HBV core particles and subsequently the cccDNA synthesis. Our study shows how m⁶A binding proteins can regulate the HBV life cycle by facilitating the nuclear export of m⁶A-modified HBV RNA.

Regulation of Hepatitis B Virus Replication by Cyclin Docking Motifs in Core Protein

Hepatitis B virus (HBV) capsid or core protein (HBc) consists of an N-terminal domain (NTD) and a C-terminal domain (CTD) connected by a short linker peptide. Dynamic phosphorylation and dephosphorylation of HBc regulate its multiple functions in capsid assembly and viral replication. The cellular cyclin-dependent kinase 2 (CDK2) plays a major role in HBc phosphorylation and, furthermore, is incorporated into the viral capsid, accounting for most of the "endogenous kinase" activity associated with the capsid. The packaged CDK2 is thought to play a role in phosphorylating HBc to trigger nucleocapsid disassembly (uncoating), an essential step during viral infection. However, little is currently known on how CDK2 is recruited and packaged into the capsid. We have now identified three RXL motifs in the HBc NTD known as cyclin docking motifs (CDMs), which mediate the interactions of various CDK substrates/regulators with CDK/cyclin complexes. Mutations of the CDMs in the HBc NTD reduced CTD phosphorylation and diminished CDK2 packaging into the capsid. Also, the CDM mutations showed little effects on capsid assembly and pregenomic RNA (pgRNA) packaging but impaired the integrity of mature nucleocapsids. Furthermore, the CDM mutations blocked covalently closed circular DNA (CCC DNA) formation during infection while having no effect on or enhancing CCC DNA formation via intracellular amplification. These results indicate that the HBc NTD CDMs play a role in CDK2 recruitment and packaging, which, in turn, is important for productive infection.IMPORTANCE Hepatitis B virus (HBV) is an important global human pathogen and persistently infects hundreds of millions of people, who are at high risk of cirrhosis and liver cancer. HBV capsid packages a host cell protein kinase, the cyclin-dependent kinase 2 (CDK2), which is thought to be required to trigger disassembly of the viral nucleocapsid during infection by phosphorylating the capsid protein, a prerequisite for successful infection. We have identified docking sites on the capsid protein for recruiting CDK2, in complex with its cyclin partner, to facilitate capsid protein phosphorylation and CDK2 packaging. Mutations of these docking sites reduced capsid protein phosphorylation, impaired CDK2 packaging into HBV capsids, and blocked HBV infection. These results provide novel insights regarding CDK2 packaging into HBV capsids and the role of CDK2 in HBV infection and should facilitate the development of antiviral drugs that target the HBV capsid protein.

Hepatitis B virus X protein recruits methyltransferases to affect cotranscriptional N6-methyladenosine modification of viral/host RNAs

Chronic hepatitis B virus (HBV) infections are one of the leading causes of cirrhosis and hepatocellular carcinoma. N6-methyladenosine (m⁶A) modification of cellular and viral RNAs is the most prevalent internal modification that occurs cotranscriptionally. Previously, we reported the dual functional role of m⁶A modification of HBV transcripts in the viral life cycle. Here, we show that viral HBV X (HBx) protein is responsible for the m⁶A modifications of viral transcripts. HBV genomes defective in HBx failed to induce m⁶A modifications of HBV RNAs during infection/transfection, while ectopic expression of HBx restores m⁶A modifications of the viral RNAs but not the mutant HBx carrying the nuclear export signal. Using chromatin immunoprecipitation assays, we provide evidence that HBx and m⁶A methyltransferase complexes are localized on the HBV minichromosome to achieve cotranscriptional m⁶A modification of viral RNAs. HBx interacts with METTL3 and 14 to carry out methylation activity and also modestly stimulates their nuclear import. This role of HBx in mediating m⁶A modification also extends to host phosphatase and tensin homolog (PTEN) mRNA. This study provides insight into how a viral protein recruits RNA methylation machinery to m⁶A-modify RNAs.

Tenofovir Disoproxil Fumarate Is Superior to Entecavir in Reducing Hepatitis B Surface Antigen for Chronic Hepatitis B in China: 2-Year Comprehensive Comparative Result of a Matched Comparative Study

Aim: Tenofovir disoproxil fumarate (TDF) and entecavir (ETV) are equally recommended as the first-line antiviral treatments for chronic hepatitis B (CHB) at present. We aimed to compare the long-term efficacy and safety between ETV and TDF therapy in CHB patients who had not received nucleoside analog treatment.

Method: In this single-center retrospective study, 414 patients who received ETV (290 patients) or TDF (124 patients) therapy at our center from January 2017 to May 2019 were included. To reduce the imbalance of baseline variables, propensity score matching (PSM) was employed to yield 124 pairs of patients at a ratio of 1:1 based on the treatment regimen.

Result: After PSM, the cumulative rate of patients who achieved complete virological response (CVR) was not different by drug therapy at each inspection time (1, 3, 6, 12, 18, and 24 months). Subgroup analysis on HBeAg status and level of HBV DNA demonstrated that evolution of proportion of achieving CVR was not significantly different between groups. Despite the insignificant incidence of HBsAg seroclearance in either group, patients in TDF group achieved higher on-treatment HBsAg decline at each inspection time (1, 3, 6, 9, 12, 18, and 24 months), 0.39, 0.51, 0.61, 0.64, 0.68, 0.76, and 0.91 log IU/mL, respectively; while the corresponding reduction were 0.27, 0.37, 0.40, 0.45, 0.48, 0.55, and 0.66 log IU/mL in ETV group (p < 0.05). In subgroup analysis, we found that the significant difference still existed in patients with high baseline HBsAg level (>3 log IU/mL). Additionally, the proportion of patients who achieved on-treatment HBsAg decline >1 log IU/mL in TDF and ETV group was 33.3 and 17.1% (p < 0.01) at the 12th month, 44.4 and 29.5% (p = 0.03) at the 24th month, respectively. Mean increase in serum creatinine from baseline was 0.10 and 0.08 mg/dL in TDF and ETV group (p = 0.11), with no patient experienced acute kidney injury.

Conclusions: TDF has higher potency in reducing HBsAg than ETV in this study. Considering the effect still existed in patients with high HBsAg level (>3 log IU/mL), TDF might be a superior therapeutic regimen combining with its relatively safety.

Hepatitis B virus cccDNA is formed through distinct repair processes of each strand

Hepatitis B virus (HBV) is a highly contagious pathogen that afflicts over a third of the world’s population, resulting in close to a million deaths annually. The formation and persistence of the HBV covalently closed circular DNA (cccDNA) is the root cause of HBV chronicity. However, the detailed molecular mechanism of cccDNA formation from relaxed circular DNA (rcDNA) remains opaque. Here we show that the minus and plus-strand lesions of HBV rcDNA require different sets of human repair factors in biochemical repair systems. We demonstrate that the plus-strand repair resembles DNA lagging strand synthesis, and requires proliferating cell nuclear antigen (PCNA), the replication factor C (RFC) complex, DNA polymerase delta (POLδ), flap endonuclease 1 (FEN-1), and DNA ligase 1 (LIG1). Only FEN-1 and LIG1 are required for the repair of the minus strand. Our findings provide a detailed mechanistic view of how HBV rcDNA is repaired to form cccDNA in biochemical repair systems.

HBV covalently closed circular DNA (cccDNA) enables and persists in chronic infection, but the molecular mechanism of its formation is unclear. Here, Wei and Ploss elucidate the detailed kinetics and biochemical steps by which the relaxed circular DNA is converted into cccDNA.

Dicoumarol, an NQO1 inhibitor, blocks cccDNA transcription by promoting degradation of HBx

BACKGROUND & AIMS

Current antiviral therapies help keep HBV under control, but they are not curative, as they are unable to eliminate the intracellular viral replication intermediate termed covalently closed circular DNA (cccDNA). Therefore, there remains an urgent need to develop strategies to cure CHB. Functional silencing of cccDNA is a crucial curative strategy that may be achieved by targeting the viral protein HBx.

METHODS

We screened 2,000 small-molecule compounds for their ability to inhibit HiBiT-tagged HBx (HiBiT-HBx) expression by using a HiBiT lytic detection system. The antiviral activity of a candidate compound and underlying mechanism of its effect on cccDNA transcription were evaluated in HBV-infected cells and a humanised liver mouse model.

RESULTS

Dicoumarol, an inhibitor of NAD(P)H:quinone oxidoreductase 1 (NQO1), significantly reduced HBx expression. Moreover, dicoumarol showed potent antiviral activity against HBV RNAs, HBV DNA, HBsAg and HBc protein in HBV-infected cells and a humanised liver mouse model. Mechanistic studies demonstrated that endogenous NQO1 binds to and protects HBx protein from 20S proteasome-mediated degradation. NQO1 knockdown or dicoumarol treatment significantly reduced the recruitment of HBx to cccDNA and inhibited the transcriptional activity of cccDNA, which was associated with the establishment of a repressive chromatin state. The absence of HBx markedly blocked the antiviral effect induced by NQO1 knockdown or dicoumarol treatment in HBV-infected cells.

CONCLUSIONS

Herein, we report on a novel small molecule that targets HBx to combat chronic HBV infection; we also reveal that NQO1 has a role in HBV replication through the regulation of HBx protein stability.

LAY SUMMARY

Current antiviral therapies for hepatitis B are not curative because of their inability to eliminate covalently closed circular DNA (cccDNA), which persists in the nuclei of infected cells. HBV X (HBx) protein has an important role in regulating cccDNA transcription. Thus, targeting HBx to silence cccDNA transcription could be an important curative strategy. We identified that the small molecule dicoumarol could block cccDNA transcription by promoting HBx degradation; this is a promising therapeutic strategy for the treatment of chronic hepatitis B.

The role of N6-methyladenosine modification in the life cycle and disease pathogenesis of hepatitis B and C viruses

N6-methyladenosine (m6A) is the most prevalent modification of mammalian cellular RNAs. m6A methylation is linked to epigenetic regulation of several aspects of gene expression, including RNA stability, splicing, nuclear export, RNA folding, and translational activity. m6A modification is reversibly catalyzed by methyltransferases (m6A writers) and demethylases (m6A erasers), and the dynamics of m6A-modified RNA are regulated by m6A-binding proteins (m6A readers). Recently, several studies have shown that m6A methylation sites have been identified in hepatitis B virus (HBV) transcripts and the hepatitis C virus (HCV) RNA genome. Here, we review the role of m6A modification in HBV/HCV replication and its contribution to liver disease pathogenesis. A better understanding of the functions of m6A methylation in the life cycles of HBV and HCV is required to establish the role of these modifications in liver diseases associated with these viral infections.

Hepatitis B Surface Antigen Suppresses the Activation of Nuclear Factor Kappa B Pathway via Interaction With the TAK1-TAB2 Complex

Chronic hepatitis B is a major health problem worldwide, with more than 250 million chronic carriers. Hepatitis B virus interferes with the host innate immune system so as to evade elimination via almost all of its constituent proteins; nevertheless, the function of HBsAg with respect to immune escape remains unclear. This study aimed to determine the role HBsAg plays in assisting HBV to escape from immune responses. We found that HBsAg suppressed the activation of the nuclear factor kappa B (NF-кB) pathway, leading to downregulation of innate immune responses. HBsAg interacted with TAK1 and TAB2 specifically, inhibiting the phosphorylation and polyubiquitination of TAK1 and the K63-linked polyubiquitination of TAB2. Autophagy is a major catabolic process participating in many cellular processes, including the life cycle of HBV. We found that HBsAg promoted the autophagic degradation of TAK1 and TAB2 via the formation of complexes with TAK1 and TAB2, resulting in suppression of the NF-κB pathway. The expression of TAK1, TAB2, and the translocation of NF-κB inversely correlated with HBsAg levels in clinical liver tissues. Taken together, our findings suggest a novel mechanism by which HBsAg interacts with TAK1-TAB2 complex and suppresses the activation of NF-κB signaling pathway via reduction of the post-translational modifications and autophagic degradation.

Management of Hepatitis B Virus in Allogeneic Hematopoietic Stem Cell Transplantation

The high morbidity of HBV reactivation following allogeneic hematopoietic stem cell transplantation (allo-HSCT) is partially due to the intense immunologic potency of complex therapeutic regimens, the use of antithymocyte globulin and calcineurin inhibitors to prevent graft versus-host disease (GVHD), prolonged immune reconstitution, and hematological malignancies infected with hepatitis B virus (HBV). Immunosuppression results in the reactivation of HBV replication from covalently closed circular DNA (cccDNA) residing in hepatocytes. However, the role of viral mutations during HBV reactivation needs to be validated. All individuals scheduled to receive allo-HSCT or wish to donate stem cells should be screened for hepatitis B surface antigen (HBsAg), antibodies to hepatitis B core (anti-HBc), and HBV-DNA. HBsAg-positive recipients of allo-HSCT have a high risk of HBV reactivation; thus, they should receive prophylactic antiviral therapy. The high barrier to resistance nucleos(t)-ide analogs (NAs) seems to be superior to the low barrier agents. Resolved-HBV recipients have a lower risk of HBV reactivation than HBsAg-positive recipients. Although prophylactic antiviral therapy remains controversial, regular monitoring of alanine transaminase (ALT) and HBV-DNA combined with preemptive antiviral treatment may be an optimized strategy. However, optimal antiviral therapy duration and time intervals for monitoring remain to be established. Accepting stem cells from HBsAg-positive donors is associated with a risk of developing HBV-related hepatitis. The overall intervention strategy, including donors and recipients, may decrease the risk of HBV-related hepatitis following HSCT from HBsAg positive stem cells. In this review, we summarize the issues of HBV in allo-HSCT, including HBV reactivation mechanism, HBsAg-positive recipients, HBV-resolved infection recipients, and donor-related factors, and discuss their significance.

HBV-Induced Increased N6 Methyladenosine Modification of PTEN RNA Affects Innate Immunity and Contributes to HCC

BACKGROUND AND AIMS

Epitranscriptomic modification of RNA has emerged as the most prevalent form of regulation of gene expression that affects development, differentiation, metabolism, viral infections, and most notably cancer. We have previously shown that hepatitis B virus (HBV) transcripts are modified by N6 methyladenosine (m⁶ A) addition. HBV also affects m⁶ A modification of several host RNAs, including phosphatase and tensin homolog (PTEN), a well-known tumor suppressor. PTEN plays a critical role in antiviral innate immunity and the development of hepatocellular carcinoma (HCC). Reports have shown that PTEN controlled interferon regulatory factor 3 (IRF-3) nuclear localization by negative phosphorylation of IRF-3 at Ser97, and PTEN reduced carcinogenesis by inhibiting the phosphatidylinositol-3-kinase (PI3K)/AKT pathway.

APPROACH AND RESULTS

Here, we show that HBV significantly increases the m⁶ A modification of PTEN RNA, which contributes to its instability with a corresponding decrease in PTEN protein levels. This is reversed in cells in which the expression of m⁶ A methyltransferases is silenced. PTEN expression directly increases activated IRF-3 nuclear import and subsequent interferon synthesis. In the absence of PTEN, IRF-3 dephosphorylation at the Ser97 site is decreased and interferon synthesis is crippled. In chronic HBV patient biopsy samples, m⁶ A-modified PTEN mRNA levels were uniformly up-regulated with a concomitant decrease of PTEN mRNA levels. HBV gene expression also activated the PI3K/AKT pathway by regulating PTEN mRNA stability in HCC cell lines.

CONCLUSIONS

The m⁶ A epitranscriptomic regulation of PTEN by HBV affects innate immunity by inhibiting IRF-3 nuclear import and the development of HCC by activating the PI3K/AKT pathway. Our studies collectively provide new insights into the mechanisms of HBV-directed immune evasion and HBV-associated hepatocarcinogenesis through m⁶ A modification of the host PTEN mRNAs.

Recent Update on the Role of Circular RNAs in Hepatocellular Carcinoma

After being overlooked for decades, circular RNAs (circRNAs) have recently generated considerable interest. circRNAs play a role in a variety of normal and pathological biological processes, including hepatocarcinogenesis. Many circRNAs contribute to hepatocarcinogenesis through sponging of microRNAs (miRs) and disruption of cellular signaling pathways that play a part in control of cell proliferation, metastasis and apoptosis. In most cases, overexpressed circRNAs sequester miRs to cause de-repressed translation of mRNAs that encode oncogenic proteins. Conversely, low expression of circRNAs has also been described in hepatocellular carcinoma (HCC) and is associated with inhibited production of tumor suppressor proteins. Other functions of circRNAs that contribute to hepatocarcinogenesis include translation of truncated proteins and acting as adapters to regulate influence of transcription factors on target gene expression. circRNAs also affect hepatocyte transformation indirectly. For example, the molecules regulate immune surveillance of cancerous cells and influence the liver fibrosis that commonly precedes HCC. Marked over- or under-expression of circRNA expression in HCC, with correlating plasma concentrations, has diagnostic utility and assays of these RNAs are being developed as biomarkers of HCC. Although knowledge in the field has recently surged, the myriad of described effects suggests that not all may be vital to hepatocarcinogenesis. Nevertheless, investigation of the role of circRNAs is providing valuable insights that are likely to contribute to improved management of a serious and highly aggressive cancer.

Multiple roles of PP2A binding motif in hepatitis B virus core linker and PP2A in regulating core phosphorylation state and viral replication

Hepatitis B virus (HBV) capsid or core protein (HBc) contains an N-terminal domain (NTD) and a C-terminal domain (CTD) connected by a short linker peptide. HBc plays a critical role in virtually every step of viral replication, which is further modulated by dynamic phosphorylation and dephosphorylation of its CTD. While several cellular kinases have been identified that mediate HBc CTD phosphorylation, there is little information on the cellular phosphatases that mediate CTD dephosphorylation. Herein, a consensus binding motif for the protein phosphatase 2A (PP2A) regulatory subunit B56 was recognized within the HBc linker peptide. Mutations within this motif designed to block or enhance B56 binding showed pleiotropic effects on CTD phosphorylation state as well as on viral RNA packaging, reverse transcription, and virion secretion. Furthermore, linker mutations affected the HBV nuclear episome (the covalently closed circular or CCC DNA) differentially during intracellular amplification vs. infection. The effects of linker mutations on CTD phosphorylation state varied with different phosphorylation sites and were only partially consistent with the linker motif serving to recruit PP2A-B56, specifically, to dephosphorylate CTD, suggesting that multiple phosphatases and/or kinases may be recruited to modulate CTD (de)phosphorylation. Furthermore, pharmacological inhibition of PP2A could decrease HBc CTD dephosphorylation and increase the nuclear HBV episome. These results thus strongly implicate the HBc linker in recruiting PP2A and other host factors to regulate multiple stages of HBV replication.

Hepatitis B virus (HBV) causes acute and chronic viral hepatitis, liver fibrosis, cirrhosis and cancer. The dynamic phosphorylation and dephosphorylation of the viral capsid protein (HBc), which are controlled by host cell protein kinases and phosphatases, play a critical role in regulating multiple stages of HBV replication. While a number of cellular kinases have been identified that mediate HBc phosphorylation, there is little information on cellular phosphatases that mediate its dephosphorylation. Herein we have identified a consensus binding motif in HBc for one of the major cellular phosphatases, the protein phosphatase 2A (PP2A). Genetic analysis of this motif revealed that it played multiple roles in regulating CTD phosphorylation state, as well as viral RNA packaging, reverse transcription, virion secretion, and formation of the nuclear HBV episome responsible for viral persistence. Furthermore, pharmacological inhibition of PP2A decreased HBc dephosphorylation and increased the nuclear episome, further supporting a role of PP2A in HBc dephosphorylation and HBV persistence. These results thus suggest that HBc recruits PP2A, among other host factors, to regulate HBc phosphorylation and dephosphorylation dynamics and HBV replication and persistence.

Characterization of Hepatitis B Precore/Core-Related Antigens

Current therapies rarely cure chronic hepatitis B virus (HBV) infection due to the persistence of the viral episome, the covalently closed circular DNA (cccDNA), in hepatocytes. The hepatitis B virus core-related antigen (HBcrAg), a mixture of the viral precore/core gene products, has emerged as one potential marker to monitor the levels and activities of intrahepatic cccDNA. In this study, a comprehensive characterization of precore/core gene products revealed that HBcrAg components included the classical hepatitis B virus core antigen (HBc) and e antigen (HBeAg) and, additionally, the precore-related antigen, PreC, retaining the N-terminal signal peptide. Both HBeAg and PreC antigens displayed heterogeneous proteolytic processing at their C termini resulting in multiple species, which varied with viral genotypes. HBeAg was the predominant form of HBcrAg in HBeAg-positive patients. Positive correlations were found between HBcrAg and PreC, between HBcrAg and HBeAg, and between PreC and HBeAg but not between HBcrAg and HBc. Serum HBeAg and PreC shared similar buoyant density and size distributions, and both displayed density and size heterogeneity. HBc, but not HBeAg or PreC antigen, was found as the main component of capsids in DNA-containing or empty virions. Neither HBeAg nor PreC protein was able to form capsids in cells or in vitro under physiological conditions. In conclusion, our study provides important new quantitative information on levels of each component of precore/core gene products as well as their biochemical and biophysical characteristics, implying that each component may have distinct functions and applications in reflecting intrahepatic viral activities.IMPORTANCE Chronic hepatitis B virus (HBV) infection afflicts approximately 257 million people, who are at high risk of progressing to chronic liver diseases, including fibrosis, cirrhosis, and hepatocellular carcinoma. Current therapies rarely achieve cure of HBV infection due to the persistence of the HBV episome, the covalently closed circular DNA (cccDNA), in the nuclei of infected hepatocytes. Peripheral markers of cccDNA levels and transcriptional activities are urgently required to guide antiviral therapy and drug development. Serum hepatitis B core-related antigen (HBcrAg) is one such emerging peripheral marker. We have characterized the components of HBcrAg in HBV-infected patients as well as in cell cultures. Our results provide important new quantitative information on levels of each HBcrAg component, as well as their biochemical and biophysical characteristics. Our findings suggest that each HBcrAg component may have distinct functions and applications in reflecting intrahepatic viral activities.

Hepatitis B virus P protein initiates glycolytic bypass in HBV-related hepatocellular carcinoma via a FOXO3/miRNA-30b-5p/MINPP1 axis

Background

Hepatitis B virus (HBV) infection is a crucial risk factor for hepatocellular carcinoma (HCC). However, its underlying mechanism remains understudied.

Methods

Microarray analysis was conducted to compare the genes and miRNAs in liver tissue from HBV-positive and HBV-negative HCC patients. Biological functions of these biomarkers in HBV-related HCC were validated via in vitro and in vivo experiments. Furthermore, we investigated the effect of HBV on the proliferation and migration of tumor cells in HBV-positive HCC tissue. Bioinformatics analysis was then performed to validate the clinical value of the biomarkers in a large HCC cohort.

Results

We found that a gene, MINPP1 from the glycolytic bypass metabolic pathway, has an important biological function in the development of HBV-positive HCC. MINPP1 is down-regulated in HBV-positive HCC and could inhibit the proliferation and migration of the tumor cells. Meanwhile, miRNA-30b-5p was found to be a stimulator for the proliferation of tumor cell through glycolytic bypass in HBV-positive HCC. More importantly, miRNA-30b-5p could significantly downregulate MINPP1 expression. Metabolic experiments showed that the miRNA-30b-5p/MINPP1 axis is able to accelerate the conversion of glucose to lactate and 2,3-bisphosphoglycerate (2,3-BPG). In the HBV-negative HCC cells, miRNA-30b-5p/MINPP1 could not regulate the glycolytic bypass to promote the tumorigenesis. However, once HBV was introduced into these cells, miRNA-30b-5p/MINPP1 significantly enhanced the proliferation, migration of tumor cells, and promoted the glycolytic bypass. We further revealed that HBV infection promoted the expression of miRNA-30b-5p through the interaction of HBV protein P (HBp) with FOXO3. Bioinformatics analysis on a large cohort dataset showed that high expression of MINPP1 was associated with favorable survival of HBV-positive HCC patients, which could lead to a slower progress of this disease.

Conclusion

Our study found that the HBp/FOXO3/miRNA-30b-5p/MINPP1 axis contributes to the development of HBV-positive HCC cells through the glycolytic bypass. We also presented miRNA-30b-5p/MINPP1 as a novel biomarker for HBV-positive HCC early diagnosis and a potential pharmaceutical target for antitumor therapy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-020-01803-8.

In vitro transcribed mRNA for expression of designer nucleases: Advantages as a novel therapeutic for the management of chronic HBV infection

Chronic infection with the hepatitis B virus (HBV) remains a significant worldwide medical problem. While diseases caused by HIV infection, tuberculosis and malaria are on the decline, new cases of chronic hepatitis B are on the rise. Because often fatal complications of cirrhosis and hepatocellular carcinoma are associated with chronic hepatitis B, the need for a cure is as urgent as ever. Currently licensed therapeutics fail to eradicate the virus and this is attributable to persistence of the viral replication intermediate comprising covalently closed circular DNA (cccDNA). Elimination or inactivation of the viral cccDNA is thus a goal of research aimed at hepatitis B cure. The ability to engineer nucleases that are capable of specific cleavage of a DNA sequence now provides the means to disable cccDNA permanently. The scientific literature is replete with many examples of using designer zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) and RNA-guided endonucleases (RGENs) to inactivate HBV. However, important concerns about safety, dose control and efficient delivery need to be addressed before the technology is employed in a clinical setting. Use of in vitro transcribed mRNA to express therapeutic gene editors goes some way to overcoming these concerns. The labile nature of RNA limits off-target effects and enables dose control. Compatibility with hepatotropic non-viral vectors is convenient for the large scale preparation that will be required for advancing gene editing as a mode of curing chronic hepatitis B.

Characterization of the Termini of Cytoplasmic Hepatitis B Virus Deproteinated Relaxed Circular DNA

The biosynthesis of hepatitis B virus (HBV) covalently closed circular DNA (cccDNA) requires the removal of the covalently linked viral polymerase from the 5' end of the minus strand [(-)strand] of viral relaxed circular DNA (rcDNA), which generates a deproteinated rcDNA (DP-rcDNA) intermediate. In the present study, we systematically characterized the four termini of cytoplasmic HBV DP-rcDNA by 5'/3' rapid amplification of cDNA ends (RACE), 5' radiolabeling, and exonuclease digestion, which revealed the following observations: (i) DP-rcDNA and rcDNA possess an identical 3' end of (-)strand DNA; (ii) compared to rcDNA, DP-rcDNA has an extended but variable 3' end of plus strand [(+)strand] DNA, most of which is in close proximity to direct repeat 2 (DR2); (iii) DP-rcDNA exhibits an RNA primer-free 5' terminus of (+)strand DNA with either a phosphate or hydroxyl group; and (iv) the 5' end of the DP-rcDNA (-)strand is unblocked at nucleotide G1828, bearing a phosphate moiety, indicating the complete removal of polymerase from rcDNA via unlinking the tyrosyl-DNA phosphodiester bond during rcDNA deproteination. However, knockout of cellular 5' tyrosyl-DNA phosphodiesterase 2 (TDP2) did not markedly affect rcDNA deproteination or cccDNA formation. Thus, our work sheds new light on the molecular mechanisms of rcDNA deproteination and cccDNA biogenesis.IMPORTANCE The covalently closed circular DNA (cccDNA) is the persistent form of the hepatitis B virus (HBV) genome in viral infection and an undisputed antiviral target for an HBV cure. HBV cccDNA is converted from viral genomic relaxed circular DNA (rcDNA) through a complex process that involves removing the covalently bound viral polymerase from rcDNA, which produces a deproteinated-rcDNA (DP-rcDNA) intermediate for cccDNA formation. In this study, we characterized the four termini of cytoplasmic DP-rcDNA and compared them to its rcDNA precursor. While rcDNA and DP-rcDNA have an identical 3' terminus of (-)strand DNA, the 3' terminus of (+)strand DNA on DP-rcDNA is further elongated. Furthermore, the peculiarities on rcDNA 5' termini, specifically the RNA primer on the (+)strand and the polymerase on the (-)strand, are absent from DP-rcDNA. Thus, our study provides new insights into a better understanding of HBV rcDNA deproteination and cccDNA biosynthesis.

Epitranscriptomic(N6-methyladenosine) Modification of Viral RNA and Virus-Host Interactions

N6-methyladenosine (m⁶A) is the most prevalent and internal modification of eukaryotic mRNA. Multiple m⁶A methylation sites have been identified in the viral RNA genome and transcripts of DNA viruses in recent years. m⁶A modification is involved in all the phases of RNA metabolism, including RNA stability, splicing, nuclear exporting, RNA folding, translational modulation, and RNA degradation. Three protein groups, methyltransferases (m⁶A-writers), demethylases (m⁶A-erasers), and m⁶A-binding proteins (m⁶A-readers) regulate this dynamic reversible process. Here, we have reviewed the role of m⁶A modification dictating viral replication, morphogenesis, life cycle, and its contribution to disease progression. A better understanding of the m⁶A methylation process during viral pathogenesis is required to reveal novel approaches to combat the virus-associated diseases.

Discovery and optimization of benzenesulfonamides-based hepatitis B virus capsid modulators via contemporary medicinal chemistry strategies

Hepatitis B is a vaccine-preventable, but potentially life-threatening liver infection caused by the Hepatitis B virus (HBV). It represents an important health burden, with 257 million active cases globally. Current HBV treatments using nucleos(t)ide analogs and pegylated interferons cannot alleviate the situation completely since they are unable to cure the infection or reduce the amount of viral covalently closed circular DNA (cccDNA). The HBV core protein is a small protein of 183 amino acids that participates in multiple essential functions in the HBV replicative cycle. Capsid assembly modulators that target the core protein are being developed. Sulfonamides are synthetic functional groups, found in several drugs. Herein, we provide a concise report focusing on the sulfamoylbenzamides as HBV capsid modulators, and medicinal chemistry strategies used in their design and development.

New Insights on Molecular Mechanism of Hepatitis B Virus Covalently Closed Circular DNA Formation

The chronic factor of the Hepatitis B Virus (HBV), specifically the covalently closed circular DNA (cccDNA), is a highly stable and active viral episomal genome established in the livers of chronic hepatitis B patients as a constant source of disease. Being able to target and eliminate cccDNA is the end goal for a genuine cure for HBV. Yet how HBV cccDNA is formed from the viral genomic relaxed circular DNA (rcDNA) and by what host factors had been long-standing research questions. It is generally acknowledged that HBV hijacks cellular functions to turn the open circular DNA conformation of rcDNA into cccDNA through DNA repair mechanisms. With great efforts from the HBV research community, there have been several recent leaps in our understanding of cccDNA formation. It is our goal in this review to analyze the recent reports showing evidence of cellular factor's involvement in the molecular pathway of cccDNA biosynthesis.

Virus spread in the liver: mechanisms, commonalities, and unanswered questions

The liver is home to five known human hepatitis viruses (hepatitis A virus-hepatitis E virus). Despite being phylogenetically unrelated, these viruses replicate and spread in the liver without causing apparent cytopathic effects, and all have evolved strategies to counteract antibody-mediated inhibition of virus spread. In this review, we discuss the current understanding regarding the spread mechanisms for these viruses with an attempt to extract common principles and identify key questions for future studies.

Single hepatocytes show persistence and transcriptional inactivity of hepatitis B

There is no cure for the more than 270 million people chronically infected with HBV. Nucleos(t)ide analogs (NUCs), the mainstay of anti-HBV treatment, block HBV reverse transcription. NUCs do not eliminate the intranuclear covalently closed circular DNA (cccDNA), from which viral RNAs, including pregenomic RNA (pgRNA), are transcribed. A key gap in designing a cure is understanding how NUCs affect HBV replication and transcription because serum markers yield an incomplete view of intrahepatic HBV. We applied single-cell laser capture microdissection and droplet digital PCR to paired liver biopsies collected from 5 HBV/HIV-coinfected persons who took NUCs over 2–4 years. From biopsy 1 to 2, proportions of HBV-infected hepatocytes declined with adherence to NUC treatment (P < 0.05); we extrapolated that eradication of HBV will take over 10 decades with NUCs in these participants. In individual hepatocytes, pgRNA levels diminished 28- to 73-fold during NUC treatment, corresponding with decreased tissue HBV core antigen staining (P < 0.01). In 4 out of 5 participants, hepatocytes with cccDNA but undetectable pgRNA (transcriptionally inactive) were present, and these were enriched in 3 participants during NUC treatment. Further work to unravel mechanisms of cccDNA transcriptional inactivation may lead to therapies that can achieve this in all hepatocytes, resulting in a functional cure.

Single-cell laser capture microdissection integrated with droplet digital PCR was used to study hepatocytes from individuals with chronic hepatitis B.

Synchronised infection identifies early rate-limiting steps in the hepatitis B virus life cycle

Hepatitis B virus (HBV) is an enveloped DNA virus that contains a partially double-stranded relaxed circular (rc) DNA. Upon infection, rcDNA is delivered to the nucleus where it is repaired to covalently closed circular (ccc) DNA that serves as the transcription template for all viral RNAs. Our understanding of HBV particle entry dynamics and host pathways regulating intracellular virus trafficking and cccDNA formation is limited. The discovery of sodium taurocholate co-transporting peptide (NTCP) as the primary receptor allows studies on these early steps in viral life cycle. We employed a synchronised infection protocol to quantify HBV entry kinetics. HBV attachment to cells at 4°C is independent of NTCP, however, subsequent particle uptake is NTCP-dependent and reaches saturation at 12 h post-infection. HBV uptake is clathrin- and dynamin dependent with actin and tubulin playing a role in the first 6 h of infection. Cellular fractionation studies demonstrate HBV DNA in the nucleus within 6 h of infection and cccDNA was first detected at 24 h post-infection. Our studies show the majority (83%) of cell bound particles enter HepG2-NTCP cells, however, only a minority (<1%) of intracellular rcDNA was converted to cccDNA, highlighting this as a rate-limiting in establishing infection in vitro. This knowledge highlights the deficiencies in our in vitro cell culture systems and will inform the design and evaluation of physiologically relevant models that support efficient HBV replication.

Hepatitis B virus biology and life cycle

Hepatitis B virus (HBV) specifically infects hepatocytes and causes severe liver diseases. The HBV life cycle is unique in that the genomic DNA (relaxed-circular partially double-stranded DNA: rcDNA) is converted to a molecular template DNA (covalently closed circular DNA: cccDNA) to amplify a viral RNA intermediate, which is then reverse-transcribed back to viral DNA. The highly stable characteristics of cccDNA result in chronic infection and a poor rate of cure. This complex life cycle of HBV offers a variety of targets to develop antiviral agents. We provide here an update on the current knowledge of HBV biology and its life cycle, which may help to identify new antiviral targets.

Hepatitis B virus cccDNA: Formation, regulation and therapeutic potential

Hepatitis B virus (HBV) infection remains a major public health concern worldwide with about 257 million individuals chronically infected. Current therapies can effectively control HBV replication and slow down disease progress, but cannot cure HBV infection. Upon infection, HBV establishes a pool of covalently closed circular DNA (cccDNA) in the nucleus of infected hepatocytes. The cccDNA exists as a minichromosome and resists to antivirals, thus a therapeutic eradication of cccDNA from the infected cells remains unattainable. In this review, we summarize the state of knowledge on the mechanisms underlying cccDNA formation and regulation, and discuss the possible strategies that may contribute to the eradication of HBV through targeting cccDNA.

Next-Generation Sequencing-Based Quantitative Detection of Hepatitis B Virus Pre-S Mutants in Plasma Predicts Hepatocellular Carcinoma Recurrence

Hepatocellular carcinoma (HCC) is among the most common and lethal human cancers worldwide. Despite curative resection, high recurrence of HCC remains a big threat, leading to poor patient outcomes. Hepatitis B virus (HBV) pre-S mutants, which harbor deletions over pre-S1 and pre-S2 gene segments of large surface proteins, have been implicated in HCC recurrence. Therefore, a reliable approach for detection of pre-S mutants is urgently needed for predicting HCC recurrence to improve patient survival. In this study, we used a next-generation sequencing (NGS)-based platform for quantitative detection of pre-S mutants in the plasma of HBV-related HCC patients and evaluated their prognostic values in HCC recurrence. We demonstrated that the presence of deletions spanning the pre-S2 gene segment and the high percentage of pre-S2 plus pre-S1 + pre-S2 deletions, either alone or in combination, was significantly and independently associated with poor recurrence-free survival and had greater prognostic performance than other clinicopathological and viral factors in predicting HCC recurrence. Our data suggest that the NGS-based quantitative detection of pre-S mutants in plasma represents a promising approach for identifying patients at high risk for HBV-related HCC recurrence after surgical resection in a noninvasive manner.

The cooperative complex of Argonaute-2 and microRNA-146a regulates hepatitis B virus replication through flap endonuclease 1

AIM

Hepatitis B virus (HBV) is a major cause of a variety of liver diseases. Existing antiviral drugs cannot eradicate HBV from our body, and the main reason is unclear on the molecular mechanism of HBV replication. Flap endonuclease 1 (FEN1) can repair relaxed circular DNA (HBV rcDNA) to covalently closed circular DNA (HBV cccDNA) that promotes HBV DNA replication, while its specific regulatory detail remains unclear. In addition, miR-146a is close related to regulation in HBV replication. This study aims to explore whether miR-146a regulates HBV cccDNA formation through FEN1.

MAIN METHODS

We investigated the expression of miR-146a, FEN1 and HBV copies in HBV stable replication cell line HepG2.2.15 and its parent cell line HepG2 transfected miR-146a and FEN1 plasmid by qRT-PCR and western blot, to identify the cooperation of Argonaute-2 (Ago2) and miR-146a by Ago2 siRNA and Ago2 RNA Binding Protein Immunoprecipitation (RIP).

KEY FINDINGS

Compared with the control group, we found that the expression of miR-146a was significantly up-regulated in HepG2.2.15, and the expression of FEN1 and HBV copies were also significantly up-regulated. On contrary, the expression of target gene of miR-146a, interleukin-1 receptor-associated kinase 1 (IRAK1) and tumor necrosis factor receptor-associated factor-6 (TRAF6), was significantly decreased in HepG2.2.15. With the use of Ago2 siRNA and then Ago2 RIP, we found that Ago2 performed as a carrier for miR-146a to promote HBV replication.

SIGNIFICANCE

The results suggest a novel miR-146a → FEN1 → HBV DNA regulatory axis in HBV replication life. Ago2 cooperates with miR-146a to regulate the transcription and expression level of FEN1 protein through the downstream target gene IRAK1/TRAF6, and to promote HBV replication.

Characterization of the antiviral effects of REP 2139 on the HBV lifecycle in vitro

During hepatitis B virus (HBV) infection, HBV subviral particles (SVP) are produced in large excess in comparison to infectious virions and account for the major source of HBV surface antigen (HBsAg) in the blood. This abundant circulating HBsAg has been postulated to promote HBV chronicity by inducing immune exhaustion against HBsAg. Nucleic acid polymers (NAPs) such as REP 2139 display promising antiviral activity against both HBV and hepatitis Delta virus (HDV) in clinical trials. REP 2139 is accompanied by clearance of HBsAg from blood with concomitant reappearance of anti-HBsAg antibodies. To decipher the mechanism-of-action of NAPs, a recently developed cell-based assay in human HepG2.2.15 cells was used (Blanchet et al., 2019). This assay recapitulates the HBsAg secretion inhibition observed in treated patients. In the present study, we analysed the antiviral effect of REP 2139 on the HBV lifecycle. Importantly, we confirm here the potent inhibitory activity of the compound on HBsAg secretion, and report minor or no effect on other viral markers such as intracellular DNA and RNA, and HBeAg or Dane particle secretion. Notably, intracellular HBsAg accumulation is prevented by proteasomal and lysosomal degradation.

N-Glycosylation and N-Glycan Processing in HBV Biology and Pathogenesis

Hepatitis B Virus (HBV) glycobiology has been an area of intensive research in the last decades and continues to be an attractive topic due to the multiple roles that N-glycosylation in particular plays in the virus life-cycle and its interaction with the host that are still being discovered. The three HBV envelope glycoproteins, small (S), medium (M) and large (L) share a very peculiar N-glycosylation pattern, which distinctly regulates their folding, degradation, assembly, intracellular trafficking and antigenic properties. In addition, recent findings indicate important roles of N-linked oligosaccharides in viral pathogenesis and evasion of the immune system surveillance. This review focuses on N-glycosylation’s contribution to HBV infection and disease, with implications for development of improved vaccines and antiviral therapies.

Role of Hepatitis B virus capsid phosphorylation in nucleocapsid disassembly and covalently closed circular DNA formation

Hepatitis B virus (HBV) delivers a partially double-stranded, relaxed circular (RC) DNA genome in complete virions to the host cell nucleus for conversion to the covalently closed circular (CCC) DNA, which establishes and sustains viral infection. An overlength pregenomic RNA (pgRNA) is then transcribed from CCC DNA and packaged into immature nucleocapsids (NCs) by the viral core (HBc) protein. pgRNA is reverse transcribed to produce RC DNA in mature NCs, which are then enveloped and secreted as complete virions, or delivered to the nucleus to replenish the nuclear CCC DNA pool. RC DNA, whether originating from extracellular virions or intracellular mature NCs, must be released upon NC disassembly (uncoating) for CCC DNA formation. HBc is known to undergo dynamic phosphorylation and dephosphorylation at its C-terminal domain (CTD) to facilitate pgRNA packaging and reverse transcription. Here, two putative phosphorylation sites in the HBc N-terminal domain (NTD), S44 and S49, were targeted for genetic and biochemical analysis to assess their potential roles in viral replication. The NTD mutant that mimics the non-phosphorylated state (N2A) was competent in all steps of viral replication tested from capsid assembly, pgRNA packaging, reverse transcription, to virion secretion, except for a decrease in CCC DNA formation. On the other hand, the phosphor-mimetic mutant N2E showed a defect in the early step of pgRNA packaging but enhanced the late step of mature NC uncoating and consequently, increased CCC DNA formation. N2E also enhanced phosphorylation in CTD and possibly elsewhere in HBc. Furthermore, inhibition of the cyclin-dependent kinase 2 (CDK2), which is packaged into viral capsids, could block CCC DNA formation. These results prompted us to propose a model whereby rephosphorylation of HBc at both NTD and CTD by the packaged CDK2, following CTD dephosphorylation during NC maturation, facilitates uncoating and CCC DNA formation by destabilizing mature NCs.

A novel TLR7 agonist as adjuvant to stimulate high quality HBsAg-specific immune responses in an HBV mouse model

Background

The global burden of hepatitis B virus (HBV) infection in terms of morbidity and mortality is immense. Novel treatments that can induce a protective immune response are urgently needed to effectively control the HBV epidemic and eventually eradicate chronic HBV infection.

Methods

We designed and evaluated an HBV therapeutic vaccine consisting of a novel Toll-like receptor 7 (TLR7) agonist T7-EA, an Alum adjuvant and a recombinant HBsAg protein. We used RNA-seq, ELISA and hTLR7/8 reporting assays to characterize T7-EA in vitro and real-time PCR to evaluate the tissue-retention characteristics in vivo. To evaluate the adjuvant potential, we administrated T7-EA intraperitoneally in a formulation with an Alum adjuvant and HBsAg in normal and HBV mice, then, we evaluated the HBsAg-specific immune responses by ELISA and Elispot assays.

Results

T7-EA acted as an hTLR7-specific agonist and induced a similar gene expression pattern as an unmodified TLR7 ligand when Raw 264.7 cells were exposed to T7-EA; however, T7-EA was more potent than the unmodified TLR7 ligand. In vivo studies showed that T7-EA had tissue-retaining activity with stimulating local cytokine and chemokine expression for up to 7 days. T7-EA could induce Th1-type immune responses, as evidenced by an increased HBsAg-specific IgG2a titer and a T-cell response in normal mice compared to mice received traditional Alum-adjuvant HBV vaccine. Importantly, T7-EA could break immune tolerance and induce persistent HBsAg-specific antibody and T-cell responses in an HBV mouse model.

Conclusions

T7-EA might be a candidate adjuvant in a prophylactic and therapeutic HBV vaccine.

Involvement of Host ATR-CHK1 Pathway in Hepatitis B Virus Covalently Closed Circular DNA Formation

Hepatitis B virus (HBV) chronically infects hundreds of millions of people and remains a major cause of viral hepatitis, cirrhosis, and liver cancer. HBV persistence is sustained by a viral nuclear episome that directs all viral gene expression needed to support viral replication. The episome is converted from an incomplete DNA precursor in viral particles in an ill-understood process. We report here that the incomplete DNA precursor is recognized by the host cell in a way similar to the sensing of damaged cellular DNA for subsequent repair to form the nuclear episome. Intense efforts are ongoing to develop novel antiviral strategies to eliminate CCC DNA so as to cure chronic HBV infection. Our results here provide novel insights into, and suggest novel ways of perturbing, the process of episome formation. Furthermore, our results inform mechanisms of cellular DNA damage recognition and repair, processes essential for normal cell growth.

The covalently closed circular (CCC) DNA of hepatitis B virus (HBV) functions as the only viral transcriptional template capable of producing all viral RNA species and is essential to initiate and sustain viral replication. CCC DNA is converted from a relaxed circular (RC) DNA, in which neither of the two DNA strands is covalently closed. As RC DNA mimics damaged cellular DNA, the host cell DNA damage repair (DDR) system is thought to be responsible for HBV CCC DNA formation. The potential role of two major cellular DDR pathways, the ataxia telangiectasia mutated (ATM) pathway and the ATM and Rad3-related (ATR) pathway, in HBV CCC DNA formation was thus investigated. Inhibition, or expression knockdown, of ATR and its downstream signaling factor CHK1, but not of ATM, decreased CCC DNA formation during de novo HBV infection, as well as intracellular CCC DNA amplification, when RC DNA from extracellular virions and intracellular nucleocapsids, respectively, is converted to CCC DNA. Furthermore, a novel RC DNA processing product with 5′ truncated minus strands was detected when the ATR-CHK1 pathway was inhibited, further indicating that this pathway controls RC DNA processing during its conversion to CCC DNA. These results provide new insights into how host cells recognize and process HBV RC DNA in order to produce CCC DNA and have implications for potential means to block CCC DNA production.

Interferon-stimulated gene 20 (ISG20) selectively degrades N6-methyladenosine modified Hepatitis B Virus transcripts

Interferon (IFN) stimulates a whole repertoire of cellular genes, collectively referred to as ISGs (Interferon-stimulated genes). ISG20, a 3´-5´ exonuclease enzyme, has been previously shown to bind and degrade hepatitis B Virus (HBV) transcripts. Here, we show that the N6-methyladenosine (m⁶A)-modified HBV transcripts are selectively recognized and processed for degradation by ISG20. Moreover, this effect of ISG20 is critically regulated by m⁶A reader protein, YTHDF2 (YTH-domain family 2). Previously, we identified a unique m⁶A site within HBV transcripts and confirmed that methylation at nucleotide A1907 regulates HBV lifecycle. In this report, we now show that the methylation at A1907 is a critical regulator of IFN-α mediated decay of HBV RNA. We observed that the HBV RNAs become less sensitive to ISG20 mediated degradation when methyltransferase enzymes or m⁶A reader protein YTHDF2 are silenced in HBV expressing cells. By using an enzymatically inactive form ISG20^D94G, we further demonstrated that ISG20 forms a complex with m⁶A modified HBV RNA and YTHDF2 protein. Due to terminal redundancy, HBV genomic nucleotide A1907 position is acquired twice by pregenomic RNA (pgRNA) during transcription and therefore the sites of methylation are encoded within 5´ and 3´ epsilon stem loops. We generated HBV mutants that lack m⁶A site at either one (5´ or 3´) or both the termini (5´& 3´). Using these mutants, we demonstrated that m⁶A modified HBV RNAs are subjected to ISG20-mediated decay and propose sequence of events, in which ISG20 binds with YTHDF2 and recognizes m⁶A-modified HBV transcripts to carry out the ribonuclease activity. This is the first study, which identifies a hitherto unknown role of m⁶A modification of RNA in IFN-α induced viral RNA degradation and proposes a new role of YTHDF2 protein as a cofactor required for IFN-α mediated viral RNA degradation.

Hepatitis B Virus (HBV) is a DNA virus but replicates through a transitional pregenomic RNA (pgRNA). Interferon stimulated antiviral RNase, ISG20 selectively binds to the lower epsilon stem loop of HBV RNA and causes their degradation. Surprisingly this ISG20 binding site is chemically modified by N6-methyladenosine addition to A1907 residue, which resides in the lower region of the epsilon stem loop. This single m⁶A site occurs twice due to terminal redundancy of sequences in the pgRNA. We demonstrated herein that IFN-α-induced ISG20 can selectively degrade m⁶A modified HBV RNA. Using a combined strategy of silencing cellular methyltransferases, m⁶A binding protein YTHDF2 and the m⁶A sites mutants, we show that HBV transcripts are resistant to either IFN-α treatment or ectopically introduced ISG20 mediated degradation. YTHDF2 is an m⁶A binding protein which makes the HBV RNAs less stable. YTHDF2 protein forms a complex with IFN-α stimulated ISG20 and executes the nuclease digestion of the recruited m⁶A modified transcripts. Absence of cellular m⁶A machinery (methyltransferases or m⁶A reader proteins) makes the HBV RNA unresponsive to ISG20 mediated decay. This study provides molecular explanation of IFN-α mediated degradation of m⁶A modified HBV RNAs.

Advances in Targeting the Innate and Adaptive Immune Systems to Cure Chronic Hepatitis B Virus Infection

“Functional cure” is being pursued as the ultimate endpoint of antiviral treatment in chronic hepatitis B (CHB), which is characterized by loss of HBsAg whether or not anti-HBs antibodies are present. “Functional cure” can be achieved in <10% of CHB patients with currently available therapeutic agents. The dysfunction of specific immune responses to hepatitis B virus (HBV) is considered the major cause of persistent HBV infection. Thus, modulating the host immune system to strengthen specific cellular immune reactions might help eliminate HBV. Strategies are needed to restore/enhance innate immunity and induce HBV-specific adaptive immune responses in a coordinated way. Immune and resident cells express pattern recognition receptors like TLRs and RIG I/MDA5, which play important roles in the induction of innate immunity through sensing of pathogen-associated molecular patterns (PAMPs) and bridging to adaptive immunity for pathogen-specific immune control. TLR/RIG I agonists activate innate immune responses and suppress HBV replication in vitro and in vivo, and are being investigated in clinical trials. On the other hand, HBV-specific immune responses could be induced by therapeutic vaccines, including protein (HBsAg/preS and HBcAg), DNA, and viral vector-based vaccines. More than 50 clinical trials have been performed to assess therapeutic vaccines in CHB treatment, some of which display potential effects. Most recently, using genetic editing technology to generate CAR-T or TCR-T, HBV-specific T cells have been produced to efficiently clear HBV. This review summarizes the progress in basic and clinical research investigating immunomodulatory strategies for curing chronic HBV infection, and critically discusses the rather disappointing results of current clinical trials and future strategies.

HBV Reactivation in Patients Undergoing Hematopoietic Stem Cell Transplantation: A Narrative Review

HBV reactivation (HBVr) can occur due to the ability of HBV to remain latent in the liver as covalently closed circular DNA and by the capacity of HBV to alter the immune system of the infected individuals. HBVr can occur in patients undergoing hematopoietic stem cell transplantation (HSCT) with a clinical spectrum that ranges from asymptomatic infection to fulminant hepatic failure. The risk of HBVr is determined by a complex interplay between host immunity, virus factors, and immunosuppression related to HSCT. All individuals who undergo HSCT should be screened for HBV. HSCT patients positive for HBsAg and also those HBcAb-positive/HBsAg-negative are at high risk of HBV reactivation (HBVr) due to profound and prolonged immunosuppression. Antiviral prophylaxis prevents HBVr, decreases HBVr-related morbidity and mortality in patients with chronic or previous HBV. The optimal duration of antiviral prophylaxis remains to be elucidated. The vaccination of HBV-naïve recipients and their donors against HBV prior to HSCT has an important role in the prevention of acquired HBV infection. This narrative review provides a comprehensive update on the current concepts, risk factors, molecular mechanisms, prevention, and management of HBVr in HSCT.