Desmin-related cardiomyopathies caused by mutations in the DES gene are characterized by cytoskeletal disorganization and impaired cardiomyocyte function. Viral infections, particularly with Coxsackievirus B3 (CVB3), have been implicated as environmental triggers for cardiac decompensation. However, the interaction between desmin mutations and viral infection has never been explored. Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) from a healthy donor (control-CMs) and from patients carrying DESS46Y, DESD214−E245del, or DESP419H mutations (DESmut-CMs) were infected with the cardiovirulent enterovirus CVB3/28. Structural changes were assessed by immunofluorescence for sarcomeric proteins and desmin. Contractile function was evaluated through video-based motion tracking. Viral replication, protein expression and antiviral responses were measured via plaque assays, immunostaining, and qPCR. Coxsackievirus and Adenovirus Receptor (CAR) and cell-surface vimentin expression were quantified post-infection. DESmut-CMs exhibited baseline sarcomeric disorganization and desmin aggregation, which were further aggravated by CVB3/28 infection in a mutation-specific manner. CVB3/28 significantly reduced spontaneous contractility in control-CMs, DESS46Y, and DESP419H-CMs, with minimal effect in DESD214−E245del-CMs. Infected DESmut-CMs showed enhanced viral replication, increased VP1 expression and elevated virion release. This was accompanied by a stunted IFN-β response, reduced APOBEC3A expression, and infection-induced upregulation of viral receptors CAR and cell-surface vimentin. CVB3/28 infection compromises the structural integrity and contractile function of cardiomyocytes and exerts a more severe effect in cells harboring DES mutations. These findings underscore a pathogenic synergy between genetic cytoskeletal defects and viral infection, revealing a mechanistic basis for the heightened vulnerability of patients carrying mutation in DES gene to virus-induced cardiac decompensation. CVB3/28 infection disrupts cardiomyocyte structure and impairs contractility, with more severe effects in cells carrying DES mutations. By enhancing viral replication and weakening antiviral defenses, DES mutations act synergistically with CVB3/28 infection to increase the risk of cardiac dysfunction.

Hypoxia is a hallmark of the tumour microenvironment, driving metabolic reprogramming, immune activation, and genome instability. Here, we showed that cobalt chloride (CoCl2), a hypoxia-mimicking agent, potently induces the expression of the DNA cytidine deaminase APOBEC3A (A3A) in THP-1, a human monocytic cell line. A3A upregulation occurred in a dose-dependent manner, independently of type I interferon signalling, and was accompanied by increased double-strand DNA breaks. Transcriptomic profiling revealed broad hypoxia-driven reprogramming, characterized by activation of the stress response and downregulation of mitochondrial signalling pathways. Mechanistically, cobalt chloride induced mitochondrial dysfunction, metabolic reprogramming, and cytosolic release of mitochondrial DNA (mtDNA). Cytosolic mtDNA was transcribed by RNA polymerase III into immunostimulatory RNA, which activated the RIG-I/TRAF6/NF-κB signalling cascade to drive A3A expression. Inhibition or knockdown of RNA polymerase III markedly reduced both A3A levels and DNA damage, highlighting the central role of this pathway. All together, our findings reveal a novel interferon-independent signalling route through which hypoxia-induced mitochondrial stress activates A3A, directly linking metabolic dysfunction to genome instability. This mechanism involves mitochondrial perturbation as a key driver of APOBEC3-mediated mutagenesis in hypoxic tumours and other diseases associated with mitochondrial stress.

Introduction Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection has been associated with cardiovascular complications, including cardiac failure, arrhythmias, and acute coronary syndromes, particularly among hospitalized patients. Elevated levels of both troponin and cell-free mitochondrial DNA in the bloodstream have been identified as biomarkers, correlating with the severity of the disease. These markers suggest that SARS-CoV-2 may induce damage to both cardiac cells and mitochondria. Given the crucial role of mitochondria in maintaining the physiological functions of cardiomyocytes, we aim to investigate the impact of SARS-CoV-2 on both cardiac cells and their mitochondrial function. Objective We aim to explore whether SARS-CoV-2 can directly infect cardiomyocytes and subsequently examine the extent of mitochondrial network disruptions. Finally, we will investigate the downstream consequences of these mitochondrial alterations, particularly metabolic processes and innate immune responses, which are closely tied to SARS-CoV-2 infection. Method We used human pluripotent stem cell-derived cardiomyocytes (HPSC-CMs) as a model to study SARS-CoV-2 infection. Infection was assessed through microscopy and viral RNA detection. We also examined changes in cellular, nuclear, and mitochondrial morphology following infection using advanced imaging techniques. Additionally, we measured alterations in RNA expression of inflammatory markers and mitochondrial genes. Finally, we used a non-cardiac cell line to confirm the ability of SARS-CoV-2 to induce mitochondrial disruptions and to investigate the viral and cellular protein factors involved in this process. Results Following SARS-CoV-2 infection, we confirmed viral genome presence in HPSC-CMs and observed Troponin+ and Spike+ cells. We showed that infection led to an increase of inflammation markers and a morphological perturbation of the HPSC-CMs. Both cardiac and non-cardiac cell line showed signs of mitochondrial network perturbation. We identify ORF9b, an antagonist of innate immune response, as a viral candidate for this perturbation. Conclusion These findings confirm that SARS-CoV-2 can directly infect cardiomyocytes, triggering an inflammatory response. The virus targets mitochondria, which play a dual role as both the primary energy source and a reservoir of compounds utilized by the virus for replication, as well as a platform for initiating innate immune responses. This interaction may provide a key mechanism underlying the cardiac injuries observed during SARS-CoV-2 infection.

Mitochondria play a vital role in cellular metabolism, energy production, and immune signaling, making them key targets for viral manipulation. Viruses exploit mitochondrial functions to enhance replication and evade immune responses. They also disrupt mitochondrial dynamics by altering fission/fusion balance and modulating mitophagy, which is essential for mitochondrial quality control. Additionally, they reprogram mitochondrial metabolism, affecting pathways such as oxidative phosphorylation and glycolysis to support replication. Viruses regulate apoptosis, either inhibiting or activating mitochondria-mediated apoptosis to prolong host cell survival or facilitate viral spread. Viral infections also induce oxidative stress through reactive oxygen species generation, affecting cellular integrity. Furthermore, viruses manipulate mitochondrial antiviral immunity by degrading mitochondrial antiviral signaling protein and triggering the release of mitochondrial DNA, modulating immune responses. Understanding these interactions offers valuable insights into viral pathogenesis and presents therapeutic opportunities. Targeting mitochondrial dysfunction and enhancing antiviral immunity could provide new strategies to mitigate viral damage and enhance cellular resilience.

Background Restriction factors are host cell proteins that limit virus replication and form part of the intrinsic immune response, acting as a first line of defense. Hepatitis B virus (HBV) does not escape this rule and TREX1, a host restriction enzyme, plays a key role in inhibiting HBV replication.Methods TREX1-expressing constructs were generated and modified by site-directed mutagenesis. The location and activity of these constructs were analyzed by immunofluorescence and fluorescence-activated cell sorting. HepaD38 cells were transfected or transduced with TREX1 constructs with or without cobalt chloride-mimicked hypoxia and HBV replication was quantified by quantitative polymerase chain reaction.Results TREX1 was identified as a restriction factor that suppresses HBV replication. Furthermore, TREX1 expression was enhanced under cobalt chloride-induced hypoxia, leading to a 2-fold reduction in HBV replication. Analysis of 36 HBV-infected patients with hepatocellular carcinoma revealed that TREX1 expression was inversely correlated to the HBV viral load and HBV covalently closed episomal circular DNA (cccDNA).Conclusions Current treatments fail to eliminate HBV genomic reservoirs, which persist as cccDNA. It would be therapeutically relevant to study whether HBV nucleocapsid recycling containing TREX1 enzyme could be released into the nucleus and degrade the viral and nuclear DNA of infected cells. Under cobalt chloride-induced hypoxia, TREX1 was upregulated, enhancing its intrinsic immunity role by inhibiting hepatitis B virus (HBV) replication. Data revealed an inverse correlation between TREX1 expression, HBV viral load, and cccDNA levels in patients, highlighting the critical function of TREX1 in restricting viral replication.

Perturbation of the deoxyribonucleotide triphosphate (dNTP) pool is recognized for contributing to the mutagenic processes involved in oncogenesis. The RAS gene family encodes well characterized oncoproteins whose structure and function are among the most frequently altered in several cancers. In this work, we show that fluctuation of the dNTP pool induces CG->TA mutations across the whole genome, including RAS gene at codons for glycine 12 and 13, known hotspots in cancers. Cell culture addition of the ribonucleotide reductase inhibitor thymidine increases the mutation frequency in nuclear DNA and leads to disruption of mitochondrial metabolism. Interestingly, this effect is counteracted by the addition of deoxycytidine. Finally, screening for the loss of hydrogen bonds detecting CG->TA transition in RAS gene of 135 patients with colorectal cancer confirmed the clinical relevance of this process. All together, these data demonstrate that fluctuation of intracellular dNTP pool alters the nuclear DNA and mitochondrial metabolism.

Background Beyond the observed alterations in cellular structure and mitochondria, the mechanisms linking rare genetic mutations to the development of heart failure in patients affected by desmin mutations remain unclear due in part, to the lack of relevant human cardiomyocyte models. Methods To shed light on the role of mitochondria in these mechanisms, we investigated cardiomyocytes derived from human induced pluripotent stem cells carrying the heterozygous DES E439K mutation that were either isolated from a patient or generated by gene editing. To increase physiological relevance, cardiomyocytes were either cultured on an anisotropic micropatterned surface to obtain elongated and aligned cardiomyocytes, or as a cardiac spheroid to create a micro-tissue. Moreover, when applicable, results from cardiomyocytes were confirmed with heart biopsies of suddenly died patient of the same family harboring DES E439K mutation, and post-mortem heart samples from five control healthy donors. Results The heterozygous DES E439K mutation leads to dramatic changes in the overall cytoarchitecture of cardiomyocytes, including cell size and morphology. Most importantly, mutant cardiomyocytes display altered mitochondrial architecture, mitochondrial respiratory capacity and metabolic activity reminiscent of defects observed in patient’s heart tissue. Finally, to challenge the pathological mechanism, we transferred normal mitochondria inside the mutant cardiomyocytes and demonstrated that this treatment was able to restore mitochondrial and contractile functions of cardiomyocytes. Conclusions This work highlights the deleterious effects of DES E439K mutation, demonstrates the crucial role of mitochondrial abnormalities in the pathophysiology of desmin-related cardiomyopathy, and opens up new potential therapeutic perspectives for this disease.

Human adenoviruses (HAdVs) are a large family of DNA viruses counting more than a hundred strains divided into seven species (A to G). HAdVs induce respiratory tract infections, gastroenteritis and conjunctivitis. APOBEC3B is a cytidine deaminase that restricts several DNA viruses. APOBEC3B is also implicated in numerous cancers where it is responsible for the introduction of clustered mutations into the cellular genome. In this study, we demonstrate that APOBEC3B is an adenovirus restriction factor acting through a deaminase-dependent mechanism. APOBEC3B introduces C-to-T clustered mutations into the adenovirus genome. APOBEC3B reduces the propagation of adenoviruses by limiting viral genome replication, progression to late phase, and production of infectious virions. APOBEC3B restriction efficiency varies between adenoviral strains, the A12 strain being more sensitive to APOBEC3B than the B3 or C2 strains. In A12-infected cells, APOBEC3B clusters in the viral replication centers. Importantly, we show that adenovirus infection leads to a reduction of the quantity and/or enzymatic activity of the APOBEC3B protein depending on the strains. The A12 strain seems less able to resist APOBEC3B than the B3 or C2 strains, a characteristic which could explain the strong depletion of the APOBEC3-targeted motifs in the A12 genome. These findings suggest that adenoviruses evolved different mechanisms to antagonize APOBEC3B. Elucidating these mechanisms could benefit the design of cancer treatments. This study also identifies adenoviruses as triggers of the APOBEC3B-mediated innate response. The involvement of certain adenoviral strains in the genesis of the APOBEC3 mutational signature observed in tumors deserves further study.

On May 6, 2022, a powerful outbreak of monkeypox virus (MPXV) had been reported outside of Africa, with many continuing new cases being reported around the world. Analysis of mutations among the two different lineages present in the 2021 and 2022 outbreaks revealed the presence of G->A mutations occurring in the 5’GpA context, indicative of APOBEC3 cytosine deaminase activity. By using a sensitive PCR (3D-PCR) method allowing differential amplification of AT-rich DNA, we demonstrate that G->A hypermutated MPXV genomes can be recovered experimentally from APOBEC3 transfection followed by MPXV infection. Here, among the 7 human APOBEC3 cytidine deaminases (A3A-A3C, A3DE, A3F-A3H), only APOBEC3F was capable of extensively deaminating cytidine residues in MPXV genomes. Hyperedited genomes were also recovered in ~42% of analyzed patients, indicating that editing is part of the natural cycle of MPXV infection. Moreover, we demonstrate that substantial repair of these mutations occurs. Upon selection, corrected G->A mutations escaping drift loss contribute to the MPXV evolution observed in the current epidemics. Thus, stochastic or transient overexpression of APOBEC3F gene exposes the MPXV genome to a broad spectrum of mutations that may be modeling the mutational landscape after multiple cycles of viral replication.

Viruses have evolved countless mechanisms to subvert and impair the host innate immune response. Measles virus (MeV), an enveloped, non-segmented, negative-strand RNA virus, alters the interferon response through different mechanisms, yet no viral protein has been described as directly targeting mitochondria. Among the crucial mitochondrial enzymes, 5'-aminolevulinate synthase (ALAS) is an enzyme that catalyzes the first step in heme biosynthesis, generating 5'-aminolevulinate from glycine and succinyl-CoA. In this work, we demonstrate that MeV impairs the mitochondrial network through the V protein, which antagonizes the mitochondrial enzyme ALAS1 and sequesters it to the cytosol. This re-localization of ALAS1 leads to a decrease in mitochondrial volume and impairment of its metabolic potential, a phenomenon not observed in MeV deficient for the V gene. This perturbation of the mitochondrial dynamics demonstrated both in culture and in infected IFNAR-/- hCD46 transgenic mice, causes the release of mitochondrial double-stranded DNA (mtDNA) in the cytosol. By performing subcellular fractionation post infection, we demonstrate that the most significant source of DNA in the cytosol is of mitochondrial origin. Released mtDNA is then recognized and transcribed by the DNA-dependent RNA polymerase III. The resulting double-stranded RNA intermediates will be captured by RIG-I, ultimately initiating type I interferon production. Deep sequencing analysis of cytosolic mtDNA editing divulged an APOBEC3A signature, primarily analyzed in the 5'TpCpG context. Finally, in a negative feedback loop, APOBEC3A an interferon inducible enzyme will orchestrate the catabolism of mitochondrial DNA, decrease cellular inflammation, and dampen the innate immune response.

We analyzed the level of APOBEC3-induced MPXV editing in infected cells and in patients. We demonstrated that G->A hypermutated MPXV genomes can be recovered from APOBEC3F transfection followed by MPXV infection. Hyperedited genomes were also recovered in similar to 42% of patients. Background On May 6, 2022, a powerful outbreak of monkeypox virus (MPXV) had been reported outside of Africa, with many continuing new cases being reported around the world. Analysis of mutations among the 2 different lineages present in the 2021 and 2022 outbreaks revealed the presence of G->A mutations occurring in the 5 ' GpA context, indicative of APOBEC3 cytidine deaminase activity. Methods By using a sensitive polymerase chain reaction (differential DNA denaturation PCR) method allowing differential amplification of AT-rich DNA, we analyzed the level of APOBEC3-induced MPXV editing in infected cells and in patients. Results We demonstrate that G->A hypermutated MPXV genomes can be recovered experimentally from APOBEC3 transfection followed by MPXV infection. Here, among the 7 human APOBEC3 cytidine deaminases (A3A-A3C, A3DE, A3F-A3H), only APOBEC3F was capable of extensively deaminating cytidine residues in MPXV genomes. Hyperedited genomes were also recovered in similar to 42% of analyzed patients. Moreover, we demonstrate that substantial repair of these mutations occurs. Upon selection, corrected G->A mutations escaping drift loss contribute to the MPXV evolution observed in the current epidemic. Conclusions Stochastic or transient overexpression of the APOBEC3F gene exposes the MPXV genome to a broad spectrum of mutations that may be modeling the mutational landscape after multiple cycles of viral replication.

In this study, we demonstrated in culture and in patients that HBV could be edited by APOBEC3C(S188I). This enzyme led to an enhanced editing activity in a more specific 5MODIFIER LETTER PRIMETpCpA -> 5MODIFIER LETTER PRIMETpTpA context. This constitutes a new hallmark of APOBEC3C(S188I). Single-nucleotide polymorphism in APOBEC3C (resulting in a serine to isoleucine in position 188) is present in approximately 10% of African populations and greatly enhances restriction against human immunodeficiency virus-1 and simian immunodeficiency virus by improving dimerization and DNA processivity of the enzyme. In this study, we demonstrated in culture and in infected patients that hepatitis B virus (HBV) could be edited by APOBEC3C(S188I). Using next-generation sequencing, we demonstrated that APOBEC3C(S188I) led to enhanced editing activity in 5MODIFIER LETTER PRIMETpCpA -> 5MODIFIER LETTER PRIMETpTpA context. This constitutes a new hallmark of this enzyme, which could be used to determine its impact on HBV or nuclear DNA.

Viruses have evolved a plethora of mechanisms to impair host innate immune responses. Herpes simplex virus type 1 (HSV-1), a double-stranded linear DNA virus, impairs the mitochondrial network and dynamics predominantly through the UL12.5 gene. We demonstrated that HSV-1 infection induced a remodeling of mitochondrial shape, resulting in a fragmentation of the mitochondria associated with a decrease in their volume and an increase in their sphericity. This damage leads to the release of mitochondrial DNA (mtDNA) to the cytosol. By generating a stable THP-1 cell line expressing the DNase I-mCherry fusion protein and a THP-1 cell line specifically depleted of mtDNA upon ethidium bromide treatment, we showed that cytosolic mtDNA contributes to type I interferon and APOBEC3A upregulation. This was confirmed by using an HSV-1 strain (KOS37 UL98-SPA) with a deletion of the UL12.5 gene that impaired its ability to induce mtDNA stress. Furthermore, by using an inhibitor of RNA polymerase III, we demonstrated that upon HSV-1 infection, cytosolic mtDNA enhanced type I interferon induction through the RNA polymerase III/RIG-I pathway. APOBEC3A was in turn induced by interferon. Deep sequencing analyses of cytosolic mtDNA mutations revealed an APOBEC3A signature predominantly in the 5'TpCpG context. These data demonstrate that upon HSV-1 infection, the mitochondrial network is disrupted, leading to the release of mtDNA and ultimately to its catabolism through APOBEC3-induced mutations. IMPORTANCE Herpes simplex virus 1 (HSV-1) impairs the mitochondrial network through the viral protein UL12.5. This leads to the fusion of mitochondria and simultaneous release of mitochondrial DNA (mtDNA) in a mouse model. We have shown that released mtDNA is recognized as a danger signal, capable of stimulating signaling pathways and inducing the production of proinflammatory cytokines. The expression of the human cytidine deaminase APOBEC3A is highly upregulated by interferon responses. This enzyme catalyzes the deamination of cytidine to uridine in single-stranded DNA substrates, resulting in the catabolism of edited DNA. Using human cell lines deprived of mtDNA and viral strains deficient in UL12, we demonstrated the implication of mtDNA in the production of interferon and APOBEC3A expression during viral infection. We have shown that HSV-1 induces mitochondrial network fragmentation in a human model and confirmed the implication of RNA polymerase III/RIG-I signaling in the capture of cytosolic mtDNA.

APOBEC3 (A3) enzymes are best known for their role as antiviral restriction factors and as mutagens in cancer. Although four of them, A3A, A3B, A3F and A3G, are induced by type-1-interferon (IFN-I), their role in inflammatory conditions is unknown. We thus investigated the expression of A3, and particularly A3A and A3B because of their ability to edit cellular DNA, in Systemic Lupus Erythematosus (SLE), a chronic inflammatory disease characterized by high IFN-α serum levels. In a cohort of 57 SLE patients, A3A and A3B, but also A3C and A3G, were upregulated ~ 10 to 15-fold (> 1000-fold for A3B) compared to healthy controls, particularly in patients with flares and elevated serum IFN-α levels. Hydroxychloroquine, corticosteroids and immunosuppressive treatment did not reverse A3 levels. The A3AΔ3B polymorphism, which potentiates A3A, was detected in 14.9% of patients and in 10% of controls, and was associated with higher A3A mRNA expression. A3A and A3B mRNA levels, but not A3C or A3G, were correlated positively with dsDNA breaks and negatively with lymphopenia. Exposure of SLE PBMCs to IFN-α in culture induced massive and sustained A3A levels by 4 h and led to massive cell death. Furthermore, the rs2853669 A > G polymorphism in the telomerase reverse transcriptase (TERT) promoter, which disrupts an Ets-TCF-binding site and influences certain cancers, was highly prevalent in SLE patients, possibly contributing to lymphopenia. Taken together, these findings suggest that high baseline A3A and A3B levels may contribute to cell frailty, lymphopenia and to the generation of neoantigens in SLE patients. Targeting A3 expression could be a strategy to reverse cell death and the generation of neoantigens.

The human APOBEC3A (A3A) cytidine deaminase is a powerful DNA mutator enzyme recognized as a major source of somatic mutations in tumor cell genomes. However, there is a discrepancy between APOBEC3A mRNA levels after interferon stimulation in myeloid cells and A3A detection at the protein level. To understand this difference, we investigated the expression of two novel alternative "A3Alt" proteins encoded in the +1-shifted reading frame of the APOBEC3A gene. A3Alt-L and its shorter isoform A3Alt-S appear to be transmembrane proteins targeted to the mitochondrial compartment that induce membrane depolarization and apoptosis. Thus, the APOBEC3A gene represents a new example wherein a single gene encodes two proapoptotic proteins, A3A cytidine deaminases that target the genome and A3Alt proteins that target mitochondria.

Human cells are stressed by numerous mechanisms that can lead to leakage of mitochondrial DNA (mtDNA) to the cytoplasm and ultimately apoptosis. This agonist DNA constitutes a danger to the cell and is counteracted by cytoplasmic DNases and APOBEC3 cytidine deamination of DNA. To investigate APOBEC3 editing of leaked mtDNA to the cytoplasm, we performed a PCR analysis of APOBEC3 edited cytoplasmic mtDNA (cymtDNA) at the single cell level for primary CD4 + T cells and the established P2 EBV blast cell line. Up to 17% of primary CD4 + T cells showed signs of APOBEC3 edited cymtDNA with ~50% of all mtDNA sequences showing signs of APOBEC3 editing – between 1500–5000 molecules. Although the P2 cell line showed a much lower frequency of stressed cells, the number of edited mtDNA molecules in such cells was of the same order. Addition of the genotoxic molecules, etoposide or actinomycin D increased the number of cells showing APOBEC3 edited cymtDNA to around 40%. These findings reveal a very dynamic image of the mitochondrial network, which changes considerably under stress. APOBEC3 deaminases are involved in the catabolism of mitochondrial DNA to circumvent chronic immune stimulation triggered by released mitochondrial DNA from damaged cells.

Background APOBEC1 (A1) enzymes are cytidine deaminases involved in RNA editing. In addition to this activity, a few A1 enzymes have been shown to be active on single stranded DNA. As two human ssDNA cytidine deaminases APOBEC3A (A3A), APOBEC3B (A3B) and related enzymes across the spectrum of placental mammals have been shown to introduce somatic mutations into nuclear DNA of cancer genomes, we explored the mutagenic threat of A1 cytidine deaminases to chromosomal DNA. Results Molecular cloning and expression of various A1 enzymes reveal that the cow, pig, dog, rabbit and mouse A1 have an intracellular ssDNA substrate specificity. However, among all the enzymes studied, mouse A1 appears to be singular, being able to introduce somatic mutations into nuclear DNA with a clear 5'TpC editing context, and to deaminate 5-methylcytidine substituted DNA which are characteristic features of the cancer related mammalian A3A and A3B enzymes. However, mouse A1 activity fails to elicit formation of double stranded DNA breaks, suggesting that mouse A1 possess an attenuated nuclear DNA mutator phenotype reminiscent of human A3B. Conclusions At an experimental level mouse APOBEC1 is remarkable among 12 mammalian A1 enzymes in that it represents a source of somatic mutations in mouse genome, potentially fueling oncogenesis. While the order Rodentia is bereft of A3A and A3B like enzymes it seems that APOBEC1 may well substitute for it, albeit remaining much less active. This modifies the paradigm that APOBEC3 and AID enzymes are the sole endogenous mutator enzymes giving rise to off-target editing of mammalian genomes.

Numerous human APOBEC3 cytidine deaminases have proven to be, inter alia, host cell restriction factors for retroviruses and hepadnaviruses. Although they can bind to genomic RNA and become encapsidated, they are only catalytically active on single-stranded DNA. As there are many cellular deoxyribonucleases (DNases), we hypothesized that a parallel could be struck between APOBEC3 and DNases. For human hepatitis B virus (HBV), we show that DNase I can considerably reduce the virion genome copy number from a variety of transfected or infected cells. DNASE1 is overexpressed and encapsidated in HBV particles in vitro in hypoxic environments and in vivo in cirrhotic patient livers as well as in the serum of infected patients. The use of CoCl 2 and dimethyloxalylglycine, mimetic agents used to induce hypoxia by inhibiting prolyl hydroxylase enzymes that stabilize hypoxia-inducible factor (HIF)-1α, showed that the formation of HIF-1α/HIF-1β heterodimers results in the induction of DNASE1 . Indeed, transfection with HIF-1α and HIF-1β expression constructs upregulated DNASE1 . These findings suggest that human DNase I can impact HBV replication through the catabolism of the DNA genome within the capsid. The activity of DNases in general may explain in part the high frequency of empty or ‘light’ hepatitis B virions observed in vivo.