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

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          Abstract

          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.

          Author summary

          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.

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          Most cited references45

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          Characterization of the intracellular deproteinized relaxed circular DNA of hepatitis B virus: an intermediate of covalently closed circular DNA formation.

          Covalently closed circular DNA (cccDNA) of hepatitis B virus (HBV) is formed by conversion of capsid-associated relaxed circular DNA (rcDNA) via unknown mechanisms and exists in the nucleus of the infected hepatocyte as a minichromosome that serves as the transcription template for viral RNAs. To study the molecular pathway of cccDNA formation and its regulation by viral and cellular factors, we have established a cell line that supports the replication of an envelope protein-deficient HBV genome in a tetracycline-inducible manner. Following induction of HBV replication, the cells accumulate higher levels of cccDNA as well as larger amounts of deproteinized rcDNA (DP-rcDNA) than cells that replicate wild-type HBV genomes. These results indicate that HBV envelope proteins negatively regulate cccDNA formation, and conversion of DP-rcDNA into cccDNA is a rate-limiting step of cccDNA formation in HepG2 cells. Detailed analyses reveal the following: (i) DP-rcDNA exists in both cytoplasm and nucleus; (ii) while nuclear DP-rcDNA is sensitive to DNase I digestion, a small fraction of cytoplasmic DP-rcDNA is DNase I resistant; (iii) both DNase I-sensitive and -resistant cytoplasmic DP-rcDNAs cosediment with capsids and can be immunoprecipitated with HBV core antibody; and (iv) a primer extension assay maps the 5' end of the minus strand of DP-rcDNA at the authentic end of virion rcDNA. Hence, our results favor a hypothesis that the removal of viral polymerase protein covalently linked to the 5' end of the minus-strand DNA occurs inside the capsid in the cytoplasm and most possibly via a reaction that cleaves the phosphodiester bond between the tyrosine of the polymerase and the 5' phosphoryl group of minus-strand DNA.
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            Conserved domains in DNA repair proteins and evolution of repair systems.

            A detailed analysis of protein domains involved in DNA repair was performed by comparing the sequences of the repair proteins from two well-studied model organisms, the bacterium Escherichia coli and yeast Saccharomyces cerevisiae, to the entire sets of protein sequences encoded in completely sequenced genomes of bacteria, archaea and eukaryotes. Previously uncharacterized conserved domains involved in repair were identified, namely four families of nucleases and a family of eukaryotic repair proteins related to the proliferating cell nuclear antigen. In addition, a number of previously undetected occurrences of known conserved domains were detected; for example, a modified helix-hairpin-helix nucleic acid-binding domain in archaeal and eukaryotic RecA homologs. There is a limited repertoire of conserved domains, primarily ATPases and nucleases, nucleic acid-binding domains and adaptor (protein-protein interaction) domains that comprise the repair machinery in all cells, but very few of the repair proteins are represented by orthologs with conserved domain architecture across the three superkingdoms of life. Both the external environment of an organism and the internal environment of the cell, such as the chromatin superstructure in eukaryotes, seem to have a profound effect on the layout of the repair systems. Another factor that apparently has made a major contribution to the composition of the repair machinery is horizontal gene transfer, particularly the invasion of eukaryotic genomes by organellar genes, but also a number of likely transfer events between bacteria and archaea. Several additional general trends in the evolution of repair proteins were noticed; in particular, multiple, independent fusions of helicase and nuclease domains, and independent inactivation of enzymatic domains that apparently retain adaptor or regulatory functions.
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              Hepatitis B viruses: reverse transcription a different way.

              Hepatitis B virus (HBV), the causative agent of B-type hepatitis in humans, is the type member of the Hepadnaviridae, hepatotropic DNA viruses that replicate via reverse transcription. Beyond long-established differences to retroviruses in gene expression and overall replication strategy newer work has uncovered additional distinctions in the mechanism of reverse transcription per se. These include protein-priming by the unique extra terminal protein domain of the reverse transcriptase (RT) utilizing an RNA hairpin for de novo initiation of first strand DNA synthesis, and the strict dependence of this process on cellular chaperones. Recent in vitro reconstitution systems enabled first biochemical insights into this multifactorial reaction, complemented by high resolution structural information on the RNA, though not yet the protein, level. Genetic approaches have revealed long-distance interactions in the nucleic acid templates as an important factor enabling the puzzling template switches required to produce the relaxed circular (RC) DNA found in infectious virions. Finally, the failure of even potent HBV RT inhibitors to eliminate nuclear covalently closed circular (ccc) DNA, the functional equivalent of integrated proviral DNA, has spurred a renewed interest in the mechanism of cccDNA generation. These new developments are in the focus of this review.
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                Author and article information

                Contributors
                Role: Data curationRole: Formal analysisRole: InvestigationRole: MethodologyRole: ValidationRole: Writing – original draftRole: Writing – review & editing
                Role: Formal analysisRole: InvestigationRole: MethodologyRole: ValidationRole: Writing – original draft
                Role: Formal analysisRole: InvestigationRole: MethodologyRole: ValidationRole: Writing – original draft
                Role: Formal analysisRole: InvestigationRole: ValidationRole: Writing – original draft
                Role: InvestigationRole: MethodologyRole: Writing – review & editing
                Role: InvestigationRole: Methodology
                Role: InvestigationRole: Methodology
                Role: Investigation
                Role: InvestigationRole: Methodology
                Role: InvestigationRole: MethodologyRole: Writing – review & editing
                Role: Funding acquisitionRole: Resources
                Role: ConceptualizationRole: Data curationRole: Formal analysisRole: Funding acquisitionRole: InvestigationRole: MethodologyRole: Project administrationRole: ResourcesRole: SupervisionRole: Writing – original draftRole: Writing – review & editing
                Role: Editor
                Journal
                PLoS Pathog
                PLoS Pathog
                plos
                plospath
                PLoS Pathogens
                Public Library of Science (San Francisco, CA USA )
                1553-7366
                1553-7374
                29 December 2017
                December 2017
                : 13
                : 12
                : e1006784
                Affiliations
                [1 ] Department of Microbiology and Immunology, Indiana University School of Medicine, Indianapolis, Indiana, United States of America
                [2 ] Institute for Viral Hepatitis, Key Laboratory of Molecular Biology on Infectious Diseases, Ministry of Education, the Second Affiliated Hospital of Chongqing Medical University, Chongqing, China
                University of California, San Diego, UNITED STATES
                Author notes

                The authors have declared that no competing interests exist.

                Author information
                http://orcid.org/0000-0001-6206-2723
                http://orcid.org/0000-0001-7845-7375
                http://orcid.org/0000-0002-3155-209X
                http://orcid.org/0000-0003-2225-9550
                http://orcid.org/0000-0002-7146-916X
                Article
                PPATHOGENS-D-17-01816
                10.1371/journal.ppat.1006784
                5747486
                29287110
                fa84197b-6ff6-4ed9-9eca-519f6bcba0cb
                © 2017 Long et al

                This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

                History
                : 18 August 2017
                : 1 December 2017
                Page count
                Figures: 8, Tables: 0, Pages: 26
                Funding
                Funded by: funder-id http://dx.doi.org/10.13039/100000060, National Institute of Allergy and Infectious Diseases;
                Award ID: R01AI094474
                Award Recipient :
                Funded by: funder-id http://dx.doi.org/10.13039/100000060, National Institute of Allergy and Infectious Diseases;
                Award ID: R01AI110762
                Award Recipient :
                Funded by: funder-id http://dx.doi.org/10.13039/100000060, National Institute of Allergy and Infectious Diseases;
                Award ID: R01AI123271
                Award Recipient :
                Funded by: Science & Technology Commission of China
                Award ID: 2017ZX10202203
                Award Recipient :
                This work was supported by the US National Institutes of Health Grant R01AI094474, R01AI110762 and R01AI123271 (HG), and the Major National S&T program grant 2017ZX10202203 from Science & Technology Commission of China (AH). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
                Categories
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                DNA repair
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                Biochemistry
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                Enzymology
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                Infectious disease & Microbiology
                Infectious disease & Microbiology

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