A Role for the Host DNA Damage Response in Hepatitis B Virus cccDNA Formation-and Beyond?

HBV cccDNA and Its Potential as a Therapeutic Target

Chronic hepatitis B virus infection continues to be a major health burden worldwide. It can cause various degrees of liver damage and is strongly associated with the development of liver cirrhosis and hepatocellular carcinoma. Covalently closed circular DNA in the nucleus of infected cells cannot be disabled by present therapies which may lead to HBV persistence and relapse. In this review, we summarized the current knowledge on hepatitis B virus covalently closed circular DNA and its potential role as a therapeutic target.

Antiviral therapy may decrease HBx, affecting cccDNA and MSL2 in hepatocarcinogenesis

Chronic hepatitis B virus (HBV) is the leading cause of hepatocellular carcinoma (HCC). Covalently closed circular DNA (cccDNA) is an intermediate in the life cycle of HBV. HBV-encoded X protein (HBx), a key viral oncoprotein, can be specifically ubiquitylated by male specific lethal 2 (MSL2), which causes upregulation of HBx activity and promotes transcription, cell proliferation and tumor growth. The present study compared the levels of cccDNA, MSL2 mRNA and HBx mRNA in tumor and peri-tumor tissues, and clarified the effect of antiviral therapy on these indicators. Levels of intrahepatic cccDNA, MSL2 mRNA and HBx mRNA were determined using quantitative PCR in patients with HBV-associated HCC who had undergone liver surgery. A total of 50 patients were included in the present study. Prior to surgery, 31 patients had undergone antiviral treatment. Intrahepatic cccDNA levels were significantly higher in the tumor tissues compared with the peri-tumor tissues (P=0.001), particularly in the hepatitis B e antigen-positive (P=0.008), tumor recurrence (P=0.002) and <3 cm tumor size (P=0.003) groups. Furthermore, in patients with preoperative cirrhosis, levels of cccDNA and MSL2 mRNA were significantly higher in tumor tissues compared with that in peri-tumor tissues (P<0.001 and P=0.023, respectively). The expression levels of HBx mRNA in antiviral-treated tumors and peri-tumor tissues were significantly lower compared with those in untreated tissues (P=0.026 and P=0.035). The levels of cccDNA and MSL2 mRNA in the HBx-positive group were significantly higher in tumor tissues compared with those in peri-tumor tissues (P=0.026 and P=0.013). In conclusion, cccDNA participated in the tumorigenesis of HBV-associated HCC, and antiviral therapy was found to modulate hepatocarcinogenesis by decreasing the levels of HBx to inhibit the tumorigenic effect of MSL2 and cccDNA.

DNA Polymerase alpha is essential for intracellular amplification of hepatitis B virus covalently closed circular DNA

Persistent hepatitis B virus (HBV) infection relies on the establishment and maintenance of covalently closed circular (ccc) DNA, a 3.2 kb episome that serves as a viral transcription template, in the nucleus of an infected hepatocyte. Although evidence suggests that cccDNA is the repair product of nucleocapsid associated relaxed circular (rc) DNA, the cellular DNA polymerases involving in repairing the discontinuity in both strands of rcDNA as well as the underlying mechanism remain to be fully understood. Taking a chemical genetics approach, we found that DNA polymerase alpha (Pol α) is essential for cccDNA intracellular amplification, a genome recycling pathway that maintains a stable cccDNA pool in infected hepatocytes. Specifically, inhibition of Pol α by small molecule inhibitors aphidicolin or CD437 as well as silencing of Pol α expression by siRNA led to suppression of cccDNA amplification in human hepatoma cells. CRISPR-Cas9 knock-in of a CD437-resistant mutation into Pol α genes completely abolished the effect of CD437 on cccDNA formation, indicating that CD437 directly targets Pol α to disrupt cccDNA biosynthesis. Mechanistically, Pol α is recruited to HBV rcDNA and required for the generation of minus strand covalently closed circular rcDNA, suggesting that Pol α is involved in the repair of the minus strand DNA nick in cccDNA synthesis. Our study thus reveals that the distinct host DNA polymerases are hijacked by HBV to support the biosynthesis of cccDNA from intracellular amplification pathway compared to that from de novo viral infection, which requires Pol κ and Pol λ.

CCC DNA is the most refractory HBV replication intermediate under long-term antiviral therapies and is responsible for the viral rebound after treatment cessation. Therefore, understanding the biosynthesis and maintenance of cccDNA minichromosome is crucial for the development of novel antiviral therapeutics to cure chronic HBV infection. Although it has been clearly demonstrated that cccDNA biosynthesis relies on host cellular DNA repair machinery, the molecular pathways that convert rcDNA into cccDNA remain to be identified. Here we report that DNA polymerase alpha (Pol α) as well as Pol δ and ɛ are required for converting rcDNA into cccDNA through intracellular cccDNA amplification. This finding adds novel molecular insights on cccDNA biosynthesis. Further understanding the mechanism of cccDNA synthesis should reveal molecular targets for developing therapeutic agents to eradicate cccDNA and cure chronic hepatitis B.

Quantification of Hepatitis B Virus Covalently Closed Circular DNA in Infected Cell Culture Models by Quantitative PCR

Persistence of the human hepatitis B virus (HBV) requires the maintenance of covalently closed circular (ccc)DNA, the episomal genome reservoir in nuclei of infected hepatocytes. cccDNA elimination is a major aim in future curative therapies currently under development. In cell culture based in vitro studies, both hybridization- and amplification-based assays are currently used for cccDNA quantification. Southern blot, the current gold standard, is time-consuming and not practical for a large number of samples. PCR-based methods show limited specificity when excessive HBV replicative intermediates are present. We have recently developed a real-time quantitative PCR protocol, in which total cellular DNA plus all forms of viral DNA are extracted by silica column. Subsequent incubation with T5 exonuclease efficiently removes cellular DNA and all non-cccDNA forms of viral DNA while cccDNA remains intact and can reliably be quantified by PCR. This method has been used for measuring kinetics of cccDNA accumulation in several in vitro infection models and the effect of antivirals on cccDNA. It allowed detection of cccDNA in non-human cells (primary macaque and swine hepatocytes, etc.) reconstituted with the HBV receptor, human sodium taurocholate cotransporting polypeptide (NTCP). Here we present a detailed protocol of this method, including a work flowchart, schematic diagram and illustrations on how to calculate "cccDNA copies per (infected) cell".

A dual role for SAMHD1 in regulating HBV cccDNA and RT-dependent particle genesis

Chronic hepatitis B is one of the world's unconquered diseases with more than 240 million infected subjects at risk of developing liver disease and hepatocellular carcinoma. Hepatitis B virus reverse transcribes pre-genomic RNA to relaxed circular DNA (rcDNA) that comprises the infectious particle. To establish infection of a naïve target cell, the newly imported rcDNA is repaired by host enzymes to generate covalently closed circular DNA (cccDNA), which forms the transcriptional template for viral replication. SAMHD1 is a component of the innate immune system that regulates deoxyribonucleoside triphosphate levels required for host and viral DNA synthesis. Here, we show a positive role for SAMHD1 in regulating cccDNA formation, where KO of SAMHD1 significantly reduces cccDNA levels that was reversed by expressing wild-type but not a mutated SAMHD1 lacking the nuclear localization signal. The limited pool of cccDNA in infected Samhd1 KO cells is transcriptionally active, and we observed a 10-fold increase in newly synthesized rcDNA-containing particles, demonstrating a dual role for SAMHD1 to both facilitate cccDNA genesis and to restrict reverse transcriptase-dependent particle genesis.

Hepatitis B virus core protein phosphorylation: Identification of the SRPK1 target sites and impact of their occupancy on RNA binding and capsid structure

Hepatitis B virus (HBV) replicates its 3 kb DNA genome through capsid-internal reverse transcription, initiated by assembly of 120 core protein (HBc) dimers around a complex of viral pregenomic (pg) RNA and polymerase. Following synthesis of relaxed circular (RC) DNA capsids can be enveloped and secreted as stable virions. Upon infection of a new cell, however, the capsid disintegrates to release the RC-DNA into the nucleus for conversion into covalently closed circular (ccc) DNA. HBc´s interactions with nucleic acids are mediated by an arginine-rich C terminal domain (CTD) with intrinsically strong non-specific RNA binding activity. Adaptation to the changing demands for nucleic acid binding during the viral life cycle is thought to involve dynamic phosphorylation / dephosphorylation events. However, neither the relevant enzymes nor their target sites in HBc are firmly established. Here we developed a bacterial coexpression system enabling access to definably phosphorylated HBc. Combining Phos-tag gel electrophoresis, mass spectrometry and mutagenesis we identified seven of the eight hydroxy amino acids in the CTD as target sites for serine-arginine rich protein kinase 1 (SRPK1); fewer sites were phosphorylated by PKA and PKC. Phosphorylation of all seven sites reduced nonspecific RNA encapsidation as drastically as deletion of the entire CTD and altered CTD surface accessibility, without major structure changes in the capsid shell. The bulk of capsids from human hepatoma cells was similarly highly, yet non-identically, phosphorylated as by SRPK1. While not proving SRPK1 as the infection-relevant HBc kinase the data suggest a mechanism whereby high-level HBc phosphorylation principally suppresses RNA binding whereas one or few strategic dephosphorylation events enable selective packaging of the pgRNA/polymerase complex. The tools developed in this study should greatly facilitate the further deciphering of the role of HBc phosphorylation in HBV infection and its evaluation as a potential new therapeutic target.

Effectiveness of PCR primers for the detection of occult hepatitis B virus infection in Mexican patients

BACKGROUND

Occult hepatitis B infection (OBI) is defined as the presence of hepatitis B virus (HVB) DNA in the liver of HBsAg negative individuals with or without detectable viral DNA in serum. OBI is a diagnostic challenge as it is characterized by a very low viral load, intermittently detectable through time. Individuals with OBI can develop chronic hepatic disease, including liver cirrhosis and hepatocellular carcinoma. The aim of this work was to produce tools to improve OBI detection of the HVB genotypes prevalent in Mexico.

METHODS

We designed and tested primers to detect OBI in serum samples by nested and real-time PCR. Conserved sites in the viral genome were determined by alignment of the most frequent HBV genotypes in Mexico (H, G/H, F and D) and primers spanning the entire viral genome were designed for first round and nested PCR. Primers were tested in serum samples of 45 patients not co-infected with hepatitis C virus or with HIV, out of a group of 116 HBsAg (-)/anti-HBc (+) individuals. Primers were also tested in a control group with chronic HBV. Nested PCR products obtained from HBsAg (-)/anti-HBc (+) were sequenced and used to design primers for real-time PCR (SYBR Green).

RESULTS

The most effective primer pairs to detect HBV products by nested PCR targeted ORF regions: PreS2/P, S/P, X/PreC, and C; while by real-time PCR they targeted ORF regions PreS2/P, S/P, X, and C. Out of the 45 HBsAg (-)/anti-HBc (+) patients tested, the viral genome was detected in 28 (62.2%) and 34 (75.5%), with nPCR and real-time PCR respectively.

CONCLUSION

Primers designed for real-time PCR detected up to 75.5% of suspected OBI Mexican patients, with or without liver disease, which represents an improvement from previous PCR strategies.

Clinical Implications of Hepatitis B Virus RNA and Covalently Closed Circular DNA in Monitoring Patients with Chronic Hepatitis B Today with a Gaze into the Future: The Field Is Unprepared for a Sterilizing Cure

. Chronic hepatitis B virus (HBV) infection has long remained a critical global health issue. Covalently closed circular DNA (cccDNA) is a persistent form of the HBV genome that maintains HBV chronicity. Decades of extensive research resulted in the two therapeutic options currently available: nucleot(s)ide analogs and interferon (IFN) therapy. A plethora of reliable markers to monitor HBV patients has been established, including the recently discovered encapsidated pregenomic RNA in serum, which can be used to determine treatment end-points and to predict the susceptibility of patients to IFN. Additionally, HBV RNA splice variants and cccDNA and its epigenetic modifications are associated with the clinical course and risks of hepatocellular carcinoma (HCC) and liver fibrosis. However, new antivirals, including CRISPR/Cas9, APOBEC-mediated degradation of cccDNA, and T-cell therapies aim at completely eliminating HBV, and it is clear that the diagnostic arsenal for defining the long-awaited sterilizing cure is missing. In this review, we discuss the currently available tools for detecting and measuring HBV RNAs and cccDNA, as well as the state-of-the-art in clinical implications of these markers, and debate needs and goals within the context of the sterilizing cure that is soon to come.

Host functions used by hepatitis B virus to complete its life cycle: Implications for developing host-targeting agents to treat chronic hepatitis B

Similar to other mammalian viruses, the life cycle of hepatitis B virus (HBV) is heavily dependent upon and regulated by cellular (host) functions. These cellular functions can be generally placed in to two categories: (a) intrinsic host restriction factors and innate defenses, which must be evaded or repressed by the virus; and (b) gene products that provide functions necessary for the virus to complete its life cycle. Some of these functions may apply to all viruses, but some may be specific to HBV. In certain cases, the virus may depend upon the host function much more than does the host itself. Knowing which host functions regulate the different steps of a virus' life cycle, can lead to new antiviral targets and help in developing novel treatment strategies, in addition to improving a fundamental understanding of viral pathogenesis. Therefore, in this review we will discuss known host factors which influence key steps of HBV life cycle, and further elucidate therapeutic interventions targeting host-HBV interactions.

Evaluation of drug resistance mutations in patients with chronic hepatitis B

Mutations occurring in viral polymerase gene of hepatitis B virus (HBV) due to the use of nucleos(t)id analogs reduce the activity of the drugs by causing antiviral resistance. In this study, it was aimed to evaluate mutations responsible for drug resistance and drug resistance mutation rates in patients followed up by the diagnosis of chronic hepatitis B (CHB). A total of 318 CHB patients were included in the study. HBV mutations were detected using the INNO-LiPA commercial kit based on the reverse hybridization principle. Drug resistance mutation was detected in 46.86% (149/318) of the patients. The rates of drug resistance were found 36.79% (117/318) for lamivudine resistance, 12.58% (40/318) for entecavir (ETV), and 7.86% (25/318) for adefovir. In 10 patients, the possible tenofovir (TDF) resistance (3.14%) was found. Single-drug and double-drug resistances were detected in 34.59% and in 11.01% of the patients, respectively. Triple drug resistance was detected in only 1.26% of the patients. Unlike various studies in Turkey and in other countries, remarkable resistance to ETV and TDF were found in this study. The high rate of the probable TDF resistance was striking, with 3.14%.

Interferon signaling during Hepatitis B Virus (HBV) infection and HBV-associated hepatocellular carcinoma

Chronic Hepatitis B Virus (HBV) infection is linked to hepatocellular carcinoma (HCC) pathogenesis. The World Health Organization estimates that globally 257 million people are chronic HBV carriers at risk of developing liver cancer. Current therapies for prevention and treatment of HCC are inadequate. Although interferon-based treatment strategies hold great promise for combating chronic infection and HCC, many patients do not respond to the IFN-based drugs for reasons not completely understood. Interferon signaling plays key roles in activation of innate and adaptive immunity. However, HBV has evolved various mechanisms to suppress IFN signaling. In this review, we present the basics about HBV infection and interferon signaling. Next, we discuss mechanisms through which HBV downregulates the function -activity and transcription- of the transcription factor STAT1 during acute and chronic infection. STAT1 is activated in response to all types (I/II/III) of interferon signaling and is essential in mediating all types (I/II/III) of interferon responses. Lastly, we discuss emerging evidence from different human cancers linking loss of interferon signaling to aggressive cancer and cancer stem cells. Whether the same occurs during HBV-associated hepatocarcinogenesis is discussed and currently under investigation.

Hepatitis B virus infection

Hepatitis B virus (HBV) is a hepatotropic virus that can establish a persistent and chronic infection in humans through immune anergy. Currently, 3.5% of the global population is chronically infected with HBV, although the incidence of HBV infections is decreasing owing to vaccination and, to a lesser extent, the use of antiviral therapy to reduce the viral load of chronically infected individuals. The course of chronic HBV infection typically comprises different clinical phases, each of which potentially lasts for decades. Well-defined and verified serum and liver biopsy diagnostic markers enable the assessment of disease severity, viral replication status, patient risk stratification and treatment decisions. Current therapy includes antiviral agents that directly act on viral replication and immunomodulators, such as interferon therapy. Antiviral agents for HBV include reverse transcriptase inhibitors, which are nucleoside or nucleotide analogues that can profoundly suppress HBV replication but require long-term maintenance therapy. Novel compounds are being actively investigated to achieve the goal of HBV surface antigen seroclearance (functional cure), a serological state that is associated with a higher remission rate (thus, no viral rebound) after treatment cessation and a lower rate of cirrhosis and hepatocellular carcinoma. This Primer addresses several aspects of HBV infection, including epidemiology, immune pathophysiology, diagnosis, prevention and management.

Towards HBV curative therapies

Tremendous progress has been made over the last 2 decades to discover and develop approaches to control hepatitis B virus (HBV) infections and to prevent the development of hepatocellular carcinoma using various interferons and small molecules as antiviral agents. However, none of these agents have significant impact on eliminating HBV from infected cells. Currently the emphasis is on silencing or eliminating cccDNA, which could lead to a cure for HBV. Various approaches are being developed including the development of capsid effectors, CRISPR/Cas9, TALENS, siRNA, entry and secretion inhibitors, as well as immunological approaches. It is very likely that a combination of these modalities will need to be employed to successfully eliminate HBV or prevent virus rebound on discontinuation of therapy. In the next 5 years clinical data will emerge which will provide insight on the safety and feasibility of these approaches and if they can be applied to eradicate HBV infections globally. In this review, we summarize current treatments and we highlight and examine recent therapeutic strategies that are currently being evaluated at the preclinical and clinical stage.

Advanced Strategies for Eliminating the cccDNA of HBV

Persistent hepatitis B virus (HBV) infection of hepatocytes is associated with a covalently closed circular DNA (cccDNA) episome. Although serologic hepatitis B surface antigen tests are negative, the presence of cccDNA is obviously increased in HBeAg-positive patients compared with that in HBeAg-negative patients, inactive carriers and patients. Moreover, trace cccDNA levels can also be found in the liver cells of patients with resolved hepatitis B infections. Therefore, clearance of cccDNA in hepatocytes could be an effective cure for HBV. In this review, we summarize the strategies that have been employed to eliminate cccDNA in recent years and discuss the future development of treatments for chronic hepatitis B.

The role of host DNA ligases in hepadnavirus covalently closed circular DNA formation

Hepadnavirus covalently closed circular (ccc) DNA is the bona fide viral transcription template, which plays a pivotal role in viral infection and persistence. Upon infection, the non-replicative cccDNA is converted from the incoming and de novo synthesized viral genomic relaxed circular (rc) DNA, presumably through employment of the host cell’s DNA repair mechanisms in the nucleus. The conversion of rcDNA into cccDNA requires preparation of the extremities at the nick/gap regions of rcDNA for strand ligation. After screening 107 cellular DNA repair genes, we herein report that the cellular DNA ligase (LIG) 1 and 3 play a critical role in cccDNA formation. Ligase inhibitors or functional knock down/out of LIG1/3 significantly reduced cccDNA production in an in vitro cccDNA formation assay, and in cccDNA-producing cells without direct effect on viral core DNA replication. In addition, transcomplementation of LIG1/3 in the corresponding knock-out or knock-down cells was able to restore cccDNA formation. Furthermore, LIG4, a component in non-homologous end joining DNA repair apparatus, was found to be responsible for cccDNA formation from the viral double stranded linear (dsl) DNA, but not rcDNA. In conclusion, we demonstrate that hepadnaviruses utilize the whole spectrum of host DNA ligases for cccDNA formation, which sheds light on a coherent molecular pathway of cccDNA biosynthesis, as well as the development of novel antiviral strategies for treatment of hepatitis B.

Hepadnavirus cccDNA is the persistent form of viral genome, and in terms of human hepatitis B virus (HBV), cccDNA is the basis for viral rebound after the cessation of therapy, as well as the elusiveness of a cure with current medications. Therefore, the elucidation of molecular mechanism of cccDNA formation will aid HBV research at both basic and medical levels. In this study, we screened a total of 107 cellular DNA repair genes and identified DNA ligase 1 and 3 as key factors for cccDNA formation from viral relaxed (open) circular DNA. In addition, we found that the cellular DNA ligase 4 is responsible for converting viral double-stranded linear DNA into cccDNA. Our study further confirmed the involvement of host DNA repair machinery in cccDNA formation, and may reveal new antiviral targets for treatment of hepatitis B in future.

Recent advances in the study of hepatitis B virus covalently closed circular DNA

Chronic hepatitis B infection is caused by hepatitis B virus (HBV) and a total cure is yet to be achieved. The viral covalently closed circular DNA (cccDNA) is the key to establish a persistent infection within hepatocytes. Current antiviral strategies have no effect on the pre-existing cccDNA reservoir. Therefore, the study of the molecular mechanism of cccDNA formation is becoming a major focus of HBV research. This review summarizes the current advances in cccDNA molecular biology and the latest studies on the elimination or inactivation of cccDNA, including three major areas: (1) epigenetic regulation of cccDNA by HBV X protein, (2) immune-mediated degradation, and (3) genome-editing nucleases. All these aspects provide clues on how to finally attain a cure for chronic hepatitis B infection.

Hepatocytic expression of human sodium-taurocholate cotransporting polypeptide enables hepatitis B virus infection of macaques

Hepatitis B virus (HBV) is a major global health concern, and the development of curative therapeutics is urgently needed. Such efforts are impeded by the lack of a physiologically relevant, pre-clinical animal model of HBV infection. Here, we report that expression of the HBV entry receptor, human sodium-taurocholate cotransporting polypeptide (hNTCP), on macaque primary hepatocytes facilitates HBV infection in vitro, where all replicative intermediates including covalently closed circular DNA (cccDNA) are present. Furthermore, viral vector-mediated expression of hNTCP on hepatocytes in vivo renders rhesus macaques permissive to HBV infection. These in vivo macaque HBV infections are characterized by longitudinal HBV DNA in serum, and detection of HBV DNA, RNA, and HBV core antigen (HBcAg) in hepatocytes. Together, these results show that expressing hNTCP on macaque hepatocytes renders them susceptible to HBV infection, thereby establishing a physiologically relevant model of HBV infection to study immune clearance and test therapeutic and curative approaches.

Establishment of a fluorescent in situ hybridization assay for imaging hepatitis B virus nucleic acids in cell culture models

While chronic hepatitis B remains a global public health problem, the detailed spatiotemporal dynamics of the key molecular events leading to the multiplication and egress of hepatitis B virus (HBV) are still largely unclear. Previously, we developed a chromogenic in situ hybridization assay for detection of HBV RNA, DNA and covalently closed circular DNA in clinical liver biopsies. Here, we report the establishment of a fluorescent in situ hybridization method for the visualization of HBV RNA, HBV core particle DNA and intranuclear DNA in a tetracycline-inducible HBV replication system (HepAD38) and a de novo infection system (HepG2-NTCP). Using 3D-STORM (three-dimensional stochastic optical reconstruction microscopy), we were able to obtain images of HBV RNA and DNA with improved spatial resolution allowing in-depth analyses of key virological events within complex subcellular compartments. Taken together, these techniques should facilitate a deeper understanding of the molecular events of the HBV life cycle and shed new light on the intricate mechanisms of virus-host interactions.

Hepatitis B Virus and DNA Damage Response: Interactions and Consequences for the Infection

Hepatitis B virus (HBV) is a major etiologic agent of acute and chronic hepatitis, and end-stage liver disease. Establishment of HBV infection, progression to persistency and pathogenesis are determined by viral and cellular factors, some of which remain still undefined. Key steps of HBV life cycle e.g., transformation of genomic viral DNA into transcriptionally active episomal DNA (cccDNA) or transcription of viral mRNAs from cccDNA, take place in the nucleus of infected cells and strongly depend on enzymatic activities provided by cellular proteins. In this regard, DNA damage response (DDR) pathways and some DDR proteins are being recognized as important factors regulating the infection. On one hand, HBV highjacks specific DDR proteins to successfully complete some of the steps of its life cycle. On the other hand, HBV subverts DDR pathways to presumably create a cellular environment that favours its replication. Direct consequences of these interactions are: HBV DNA integration into host chromosomal DNA, and accumulation of mutations in host chromosomal DNA that could eventually trigger carcinogenic processes, which would explain in part the incidence of hepatocellular carcinoma in chronically infected patients. Unravelling the interactions that HBV establishes with DDR pathways might help identify new molecular targets for therapeutic intervention.

The Role of cccDNA in HBV Maintenance

Chronic hepatitis B virus (HBV) infection continues to be a major health burden worldwide; it can cause various degrees of liver damage and is strongly associated with the development of liver cirrhosis and hepatocellular carcinoma. The molecular mechanisms determining HBV persistence are not fully understood, but these appear to be multifactorial and the unique replication strategy employed by HBV enables its maintenance in infected hepatocytes. Both the stability of the HBV genome, which forms a stable minichromosome, the covalently closed circular DNA (cccDNA) in the hepatocyte nucleus, and the inability of the immune system to resolve chronic HBV infection are believed to be key mechanisms of HBV chronicity. Since a true cure of HBV requires clearance of intranuclear cccDNA from infected hepatocytes, understanding the mechanisms involved in cccDNA biogenesis, regulation and stability is mandatory to achieve HBV eradication. This review will summarize the state of knowledge on these mechanisms including the impact of current treatments on the cccDNA stability and activity. We will focus on events challenging cccDNA persistence in dividing hepatocytes.