APOBEC3 Editing Research Articles

Viral whole genome sequencing reveals high variations in APOBEC3 editing between HPV risk categories.

High-risk human papillomavirus (HPV) infections are responsible for cervical cancer. However, little is known about the differences between HPV types and risk categories regarding their genetic diversity and particularly APOBEC3-induced mutations - which contribute to the innate immune response to HPV. Using a capture-based next-generation sequencing, 156 HPV whole genome sequences covering 43 HPV types were generated from paired cervical and anal swabs of 30 Togolese female sex workers (FSWs) sampled in 2017. Genetic diversity and APOBEC3-induced mutations were assessed at the viral whole genome and gene levels. Thirty-four pairwise sequence comparisons covering 24 HPV types in cervical and anal swabs revealed identical infections in the two anatomical sites. Differences in genetic diversity among HPV types was observed between patients. The E6 gene was significantly less conserved in low-risk HPVs (lrHPVs) compared to high-risk HPVs (hrHPVs) (p = 0.009). APOBEC3-induced mutations were found to be more common in lrHPVs than in hrHPVs (p = 0.005), supported by our data and by using large HPV sequence collections from the GenBank database. Focusing on the most common lrHPVs 6 and 11 and hrHPVs 16 and 18, APOBEC3-induced mutations were predominantly found in the E4 and E6 genes in lrHPVs, but were almost absent in these genes in hrHPVs. The variable APOBEC3 mutational signatures could contribute to the different oncogenic potentials between HPVs. Further studies are needed to conclusively determine whether APOBEC3 editing levels are associated to the carcinogenic potential of HPVs at the type and sublineage scales.

Molecular Basis of Tumorigenesis of Bladder Cancer and Emerging Concepts in Developing Therapeutic Targets.

Advanced urothelial carcinoma (UC) is an aggressive disease whose mutagenic processes are yet to be elucidated. Targeted therapies are urgently needed, but the road from bench to bedside is slowly progressing. In this review, we discuss urothelial carcinoma etiology, along with the most recent advances in UC candidate targeted therapies. A comprehensive database search was performed. We aimed to review the most recent updates on UC genomics and targeted therapies. Pre-clinical as well as clinical studies were included. Our review highlights the advances in understanding the molecular basis of urothelial tumorigenesis, including smoking, chemical parasitic carcinogens, inheritance, and APOBEC3 editing enzymes. We discussed how these factors contributed to the current mutational landscape of UC. Therapeutic options for UC are still very limited. However, several promising therapeutic approaches are in development to leverage our knowledge of molecular targets, such as targeting fibroblast growth factor receptors (FGFR), DNA damage repair pathways, and HER2. Blindly testing targeted therapies based on other cancer data is not sufficient. UC-specific biomarkers are needed to precisely use the appropriate drug for the appropriate population. More efforts to understand UC biology and evolution are urgently needed.

APOBEC3 deaminase editing in mpox virus as evidence for sustained human transmission since at least 2016.

Historically, mpox has been characterized as an endemic zoonotic disease that transmits through contact with the reservoir rodent host in West and Central Africa. However, in May 2022, human cases of mpox were detected spreading internationally beyond countries with known endemic reservoirs. When the first cases from 2022 were sequenced, they shared 42 nucleotide differences from the closest mpox virus (MPXV) previously sampled. Nearly all these mutations are characteristic of the action of APOBEC3 deaminases, host enzymes with antiviral function. Assuming APOBEC3 editing is characteristic of human MPXV infection, we developed a dual-process phylogenetic molecular clock that-inferring a rate of ~6 APOBEC3 mutations per year-estimates that MPXV has been circulating in humans since 2016. These observations of sustained MPXV transmission present a fundamental shift to the perceived paradigm of MPXV epidemiology as a zoonosis and highlight the need for revising public health messaging around MPXV as well as outbreak management and control.

An APOBEC3 Mutational Signature in the Genomes of Human-Infecting Orthopoxviruses.

The ongoing worldwide monkeypox outbreak is caused by viral lineages (globally referred to as hMPXV1) that are related to but distinct from clade IIb MPXV viruses transmitted within Nigeria. Analysis of the genetic differences has indicated that APOBEC-mediated editing might be responsible for the unexpectedly high number of mutations observed in hMPXV1 genomes. Here, using 1,624 publicly available hMPXV1 sequences, we analyzed the mutations that accrued between 2017 and the emergence of the current predominant variant (B.1), as well as those that that have been accumulating during the 2022 outbreak. We confirmed an overwhelming prevalence of C-to-T and G-to-A mutations, with a sequence context (5'-TC-3') consistent with the preferences of several human APOBEC3 enzymes. We also found that mutations preferentially occur in highly expressed viral genes, although no transcriptional asymmetry was observed. A comparison of the mutation spectrum and context was also performed against the human-specific variola virus (VARV) and the zoonotic cowpox virus (CPXV), as well as fowlpox virus (FWPV). The results indicated that in VARV genomes, C-to-T and G-to-A changes were more common than the opposite substitutions, although the effect was less marked than for hMPXV1. Conversely, no preference toward C-to-T and G-to-A changes was observed in CPXV and FWPV. Consistently, the sequence context of C-to-T changes confirmed a preference for a T in the -1 position for VARV, but not for CPXV or FWPV. Overall, our results strongly support the view that, irrespective of the transmission route, orthopoxviruses infecting humans are edited by the host APOBEC3 enzymes. IMPORTANCE Analysis of the viral lineages responsible for the 2022 monkeypox outbreak suggested that APOBEC enzymes are driving hMPXV1 evolution. Using 1,624 public sequences, we analyzed the mutations that accumulated between 2017 and the emergence of the predominant variant and those that characterize the last outbreak. We found that the mutation spectrum of hMPXV1 has been dominated by TC-to-TT and GA-to-AA changes, consistent with the editing activity of human APOBEC3 proteins. We also found that mutations preferentially affect highly expressed viral genes, possibly because transcription exposes single-stranded DNA (ssDNA), a target of APOBEC3 editing. Notably, analysis of the human-specific variola virus (VARV) and the zoonotic cowpox virus (CPXV) indicated that in VARV genomes, TC-to-TT and GA-to-AA changes are likewise extremely frequent. Conversely, no preference toward TC-to-TT and GA-to-AA changes is observed in CPXV. These results suggest that APOBEC3 proteins have an impact on the evolution of different human-infecting orthopoxviruses.

Monkeypox virus (MPXV) genomics: A mutational and phylogenomic analyses of B.1 lineages.

The recent increase in monkeypox (MPX) cases has attracted attention of public health authorities due to its quick spread and transmission across non-endemic regions. This outbreak, unlike previous ones, displays different epidemiological features and transmission dynamics, which appear to be largely influenced by the newly divergent MPX lineages (B.1). Yet, the genomic characteristics driving the high dispersal and diversification of these lineages remain largely unknown. Herein, we sought to explore and characterize the genomic features and phylogenetic diversity of the B.1 lineages through a comparative genomic analysis inclusive of 1900 high quality complete MPXV genomes. Our analyses indicate that the current MPXV-2022 outbreak encompasses thirteen derived lineages with ten unique non-synonymous mutations in several genes linked to immune evasion, virulence factors and host recognition. Such mutations may translate in the rapid evolution and diversification of current MPXV lineages. Moreover, our analyses uncovered signals of genomic modifications suggestive of immune-modulatory enzymatic activity, such as APOBEC3 editing, which, as previously suggested could have favored evolutionary trends leading to the rapid spread of MPXV into non-endemic countries. Genomic surveillance continues to play a major role in unveiling the genomic signatures signaling potential adaptation of this emerging MPXV lineage and how it will continue to impact public health in the near future.

Multiple lineages of monkeypox virus detected in the United States, 2021-2022.

Monkeypox is a viral zoonotic disease endemic in Central and West Africa. In May 2022, dozens of non-endemic countries reported hundreds of monkeypox cases, most with no epidemiological link to Africa. We identified two lineages of monkeypox virus (MPXV) among two 2021 and seven 2022 US monkeypox cases: the major 2022 outbreak variant called B.1 and a minor contemporaneously sampled variant called A.2. Analyses of mutations among these two variants revealed an extreme preference for GA-to-AA mutations indicative of human APOBEC3 cytosine deaminase activity among Clade IIb MPXV (previously West African, Nigeria) sampled since 2017. Such mutations were not enriched within other MPXV clades. These findings suggest that APOBEC3 editing may be a recurrent and a dominant driver of MPXV evolution within the current outbreak.

Genotoxic stress increases cytoplasmic mitochondrial DNA editing by human APOBEC3 mutator enzymes at a single cell level

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.

Abstract 4249: Evidence of APOBEC3 editing in the HPV16 genome

Abstract Human papillomavirus (HPV) is a very common sexually transmitted infection, however only a small proportion of women progress to cervical precancer or cancer. HPV16 is the most carcinogenic type, causing more than half of the cervical cancer globally. The HPV16 genome is 7,906base-pairs coding for 8 genes (E6, E7, E1, E2, E4, E5, L2, L1) and one upstream regulatory region (URR). Within HPV16 there are 4 main lineages (A, B, C, D) that are strongly associated with disease risk. Human APOBEC3A (hA3A) cytidine deaminases have been shown to have antiviral effects. The APOBEC mutational process results in a C to T base change at specific motifs (5’ [C/T]•C>T•W 3’). Previous studies established that there was evidence of APOBEC3 editing on a small number of HPV16 samples. It’s unknown how these mutations are related to infection clearance or the long-term accumulation of genomic mutations that contribute to HPV-associated cancers. We conducted detailed analyses to comprehensively evaluate APOBEC3 editing on HPV16 genomes using HPV16 whole-genome sequencing data from 3,215 HPV16-infected women in the NCI-HPV Persistence and Progression (PaP) cohort. Cases were defined as women with cervical precancer (CIN3, N = 1,093) or cancer (N = 109) and controls were women with no histologic evidence of precancer or cancer (<CIN2, N = 1,107). HPV16 DNA was extracted from banked specimens and whole-genome sequenced using a high-throughput assay. We evaluated all rare variants, defined as having a minor allele frequency of <1%, for matching an APOBEC3-associated variant. An APOBEC3-associated variant was defined as a variant having one of the eight possible motifs (5’ [C/T]•C>T•W 3’) out of 96 potential motifs of 3 base-pairs. Using logistic regression, we compared the number of APOBEC3-associated variants in cases and controls, among HPV16 variant lineages, and among genome regions of the virus. We discovered that there is evidence of APOBEC3 editing throughout the HPV16 genome. Specifically, we observed that women with precancer or cancer had less APOBEC3-associated variants compared to the controls (OR = 0.84, p-value = 0.06). We further showed that women with an HPV16 A lineage infection have more APOBEC3-associated variants compared to those with a non-A lineage infection (OR = 1.35, p-value = 0.02). After controlling for the number of APOBEC3 vulnerable loci, we observed that the L1 (OR = 0.23, p-value = 0.04) and E7 (OR = 0.29, p-value = 0.07) genes have less APOBEC3 footprints overall, and particularly in the cases compared to the controls in these regions, compared to the viral non-coding upstream regulatory region (URR). Overall, we found that APOBEC3 is not affecting the HPV16 genome in a uniform way, and instead, it appears to be targeting specific regions which could suggest antiviral activity. Importantly, we determined that APOBEC3-associated variants are less prevalent in cases which could be related to disease progression in these individuals. Further evaluation is underway. Citation Format: Yanzi Xiao, Bin Zhu, Meredith Yeager, Michael Cullen, Joseph Boland, Nicolas Wentzensen, Tina Raine-Bennett, Zigui Chen, Kai Yu, Qi Yang, Mia Steinberg, David Roberson, Sara Bass, Laurie Burdette, Thomas Lorey, Philip Castle, Robert Burk, Mark Schiffman, Lisa Mirabello. Evidence of APOBEC3 editing in the HPV16 genome [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4249. doi:10.1158/1538-7445.AM2017-4249

Human APOBEC3 induced mutation of human immunodeficiency virus type-1 contributes to adaptation and evolution in natural infection.

Human APOBEC3 proteins are cytidine deaminases that contribute broadly to innate immunity through the control of exogenous retrovirus replication and endogenous retroelement retrotransposition. As an intrinsic antiretroviral defense mechanism, APOBEC3 proteins induce extensive guanosine-to-adenosine (G-to-A) mutagenesis and inhibit synthesis of nascent human immunodeficiency virus-type 1 (HIV-1) cDNA. Human APOBEC3 proteins have additionally been proposed to induce infrequent, potentially non-lethal G-to-A mutations that make subtle contributions to sequence diversification of the viral genome and adaptation though acquisition of beneficial mutations. Using single-cycle HIV-1 infections in culture and highly parallel DNA sequencing, we defined trinucleotide contexts of the edited sites for APOBEC3D, APOBEC3F, APOBEC3G, and APOBEC3H. We then compared these APOBEC3 editing contexts with the patterns of G-to-A mutations in HIV-1 DNA in cells obtained sequentially from ten patients with primary HIV-1 infection. Viral substitutions were highest in the preferred trinucleotide contexts of the edited sites for the APOBEC3 deaminases. Consistent with the effects of immune selection, amino acid changes accumulated at the APOBEC3 editing contexts located within human leukocyte antigen (HLA)-appropriate epitopes that are known or predicted to enable peptide binding. Thus, APOBEC3 activity may induce mutations that influence the genetic diversity and adaptation of the HIV-1 population in natural infection.

Differential impact of APOBEC3-driven mutagenesis on HIV evolution in diverse anatomical compartments

Previous studies on HIV quasispecies have revealed HIV compartmentalization in various tissues within an infected individual. Such HIV variation is a result of a combination of factors including high replication and mutation rates, recombination, and APOBEC3-host selective pressure. To evaluate the differential impact of APOBEC3 editing in HIV-1 compartments, we analyzed the level of G-to-A hypermutation in HIV-1 protease and reverse transcriptase sequences among 30 HAART-treated patients for whom peripheral blood mononuclear cells and body tissues or fluids [cerebral spinal fluid (CSF), rectal tissue, or renal tissue] were collected on the same day. APOBEC3-mediated hypermutation was identified in 36% (11/30) of participants in at least one viral reservoir. HIV hypermutated sequences were often observed in viral sanctuaries (total n = 10; CSF, n = 6; renal tissue, n = 1; rectal tissue n = 3) compared with peripheral blood (total n = 4). Accordingly, APOBEC3 editing generated more G-to-A drug resistance mutations in sanctuaries: three patients' CSF (i.e. G73S in protease; M184I, M230I in reverse transcriptase) and two other patients' rectal tissues (M184I, M230I in reverse transcriptase) while such mutations were absent from paired peripheral blood mononuclear cells. APOBEC3-induced mutations observed in peripheral blood underestimate the overall proportion of hypermutated viruses in anatomical compartments. The resulting mutations may favor escape to antiretrovirals in these compartments in conjunction with a lower penetration of drugs in some sanctuaries. On the other side, because hypermutated sequences often harbor inactivating mutations, our results suggest that accumulation of defective viruses may be more dominant in sanctuaries than in peripheral blood of patients on effective HAART.

Latency, tropism and genetic variation of Simian Foamy Virus in blood and saliva from infected Humans

Simian foamy viruses (SFV) are widespread retroviruses among non-human primates (NHP). SFV actively replicate in the oral cavity of NHP and can be transmitted to humans through NHP bites, in whom they establish a persistent infection. We aimed to study three major properties of these zoonotic retroviruses: replicative status, tropism and variability. In 14 hunters from Cameroon previously shown to be infected with a gorilla SFV strain, viral DNA could be detected by quantitative polymerase chain reaction in most samples of peripheral blood mononuclear cells (PBMCs) and saliva. The SFV DNA levels were 7.1± 6.0 SFV DNA copies/10 5 cells in PBMCs and 2.4±4.3 SFV DNA copies/10 5 cells in saliva. In contrast, no SFV RNA was detected by qRT-PCR in either PBMCs or saliva. PBMCs populations (T4, T8, B, NK lymphocytes and monocytes) were sorted with magnetic beads before quantification of SFV DNA. Our preliminary results showed the presence of SFV DNA in all PBMCs populations at different levels. We finally assessed the viral diversity in vivo. Although intra-individual SFV genetic variation was low (<0,5%) we detected some viral diversity in 3 out of 9 individuals. In one subject, genetic variation might be associated with coinfection with 2 SFV strains, while in the two other subjects, variations seemed to derive from APOBEC3 editing with a high rate of G-to-A substitutions. Our study demonstrates that SFV infection is mostly latent in PBMCs and in saliva. Such a scenario may explain the putative lack of secondary human-to-human transmissions of SFV.

E138K and M184I mutations in HIV-1 reverse transcriptase coemerge as a result of APOBEC3 editing in the absence of drug exposure

Recent clinical trials with rilpivirine combined with emtricitabine and tenofovir revealed that patients failing treatment, frequently, harbored viruses encoding resistance-associated mutations in the HIV-1 reverse transcriptase at position E138K and M184I. We show here that APOBEC3 proteins play a role in the emergence of these drug resistance mutations. We used a Vif mutant that has suboptimal activity against APOBEC3 to assess the in-vitro frequency of APOBEC3-induced resistance mutations in reverse transcriptase. To assess the degree of in-vivo G-to-A viral hypermutation, a large amount of data of HIV-1 RT proviral sequences from peripheral blood mononuclear cells (PBMCs) recovered from infected patients under HAART was analyzed. In-vitro replication experiments in cell lines with and without APOBEC3 expression suggest that APOBEC3-driven mutagenesis contributes to the generation of both M184I and E138K within HIV proviral repository in the absence of drug exposure. Additionally, analysis of 601 patients PBMCs sequences revealed that the copresence of mutations E138K and M184I were never detected in nonhypermutated sequences, whereas these mutations were found at a high frequency (24%) in the context of APOBEC3 editing and in the absence of exposure to etravirine-rilpivirine. We demonstrate using in-vitro experiments and analyzing patients PBMCs sequences that M184I and E138K resistance-associated mutations may pre-exist in proviral reservoir at a high frequency prior to drug exposure, as a result of APOBEC3 editing. Thus, incomplete neutralization of one or more APOBEC3 proteins may favor viral escape to rilpivirine-emtricitabine.

HIV-1 genome is often defective in PBMCs and rectal tissues after long-term HAART as a result of APOBEC3 editing and correlates with the size of reservoirs

Precise characterization of viruses present in reservoirs in long-term pretreated patients will be a major issue to consider in the context of viral eradication. We assessed the frequency of defective viruses present in cellular reservoirs. Peripheral blood mononuclear cells (PBMCs) and rectal biopsy samples were compared between five patients on successful long-term highly active antiretroviral therapy (HAART) (>7 years without blips) and five untreated patients. Molecular cloning and sequencing of the reverse transcriptase region were used to detect the presence of and quantify in-frame stop codons in HIV quasi-species. The relationship between the size of the reservoir and the frequency of defective genomes was assessed. Defective genomes were systematically detected in all patients on long-term HAART in both compartments (PBMCs and rectal tissues), with a higher level of defective genomes per sample compared with PBMCs of untreated patients. A high level of defective genomes was correlated with a small size of HIV proviral DNA. Regarding the nucleotide context, guanine (G) to adenine (A) substitution at tryptophan positions was responsible for the appearance of 89% of all in-frame stop codons in the context of G-to-A hypermutation, likely reflecting APOBEC3 footprints on the viral genome. We propose a scenario whereby defective genomes accumulate during HAART treatment, eventually reaching a viral extinction threshold. In the context of viral eradication, measurement of the relative amounts of defective and non-defective viruses (by molecular cloning and ultradeep sequencing) should be used as a new criterion for eradicating HIV.

Massive APOBEC3 Editing of Hepatitis B Viral DNA in Cirrhosis

DNA viruses, retroviruses and hepadnaviruses, such as hepatitis B virus (HBV), are vulnerable to genetic editing of single stranded DNA by host cell APOBEC3 (A3) cytidine deaminases. At least three A3 genes are up regulated by interferon-α in human hepatocytes while ectopic expression of activation induced deaminase (AICDA), an A3 paralog, has been noted in a variety of chronic inflammatory syndromes including hepatitis C virus infection. Yet virtually all studies of HBV editing have confined themselves to analyses of virions from culture supernatants or serum where the frequency of edited genomes is generally low (≤10−2). We decided to look at the nature and frequency of HBV editing in cirrhotic samples taken during removal of a primary hepatocellular carcinoma. Forty-one cirrhotic tissue samples (10 alcoholic, 10 HBV+, 11 HBV+HCV+ and 10 HCV+) as well as 4 normal livers were studied. Compared to normal liver, 5/7 APOBEC3 genes were significantly up regulated in the order: HCV±HBV>HBV>alcoholic cirrhosis. A3C and A3D were up regulated for all groups while the interferon inducible A3G was over expressed in virus associated cirrhosis, as was AICDA in ∼50% of these HBV/HCV samples. While AICDA can indeed edit HBV DNA ex vivo, A3G is the dominant deaminase in vivo with up to 35% of HBV genomes being edited. Despite these highly deleterious mutant spectra, a small fraction of genomes survive and contribute to loss of HBeAg antigenemia and possibly HBsAg immune escape. In conclusion, the cytokine storm associated with chronic inflammatory responses to HBV and HCV clearly up regulates a number of A3 genes with A3G clearly being a major restriction factor for HBV. Although the mutant spectrum resulting from A3 editing is highly deleterious, a very small part, notably the lightly edited genomes, might help the virus evolve and even escape immune responses.

Mutational pressure makes HIV1 gp120 linear B-cell epitopes shorter and may lead to their disappearance

We showed that nucleotide sequences coding for linear B-cell epitopes of human immunodeficiency virus type 1 (HIV1) gp120 protein are enriched with codons containing cytosine and guanine in their first and second codon positions. Guanine and cytosine are the most mutable nucleotides in HIV1 genes (due to APOBEC3 and APOBEC1 editing of viral DNA and RNA, respectively, as well as due to reverse transcriptase preference to incorporate 8-oxo-G against C). We introduced all the possible G to A, C to U, C to A and G to U single nonsynonymous nucleotide mutations in gp120 coding region from the HIV1 reference strain. The BepiPred algorithm ( www.cbs.dtu.dk/services/BepiPred) was used for the linear B-cell epitopes predictions. Results of this “in-silico directed mutagenesis” showed that: (i) single nonsynonymous G to A transitions will cause partial or complete destruction of linear epitopes in 18% of 229 possible cases; (ii) single nonsynonymous C to U transitions will cause partial or complete destruction of linear epitopes in 58% of 142 possible cases; (iii) single nonsynonymous C to A transversions will cause partial or complete destruction of linear epitopes in 28% of 184 possible cases; (iv) single nonsynonymous G to U transversions will cause partial or complete destruction of linear epitopes in 37% of 240 possible cases. Moreover, single transition of C to U direction leading to amino acid replacement inside an epitope will cause partial or complete destruction of this epitope at a probability of 98%.

Hepatitis B: modern concepts in pathogenesis ??? APOBEC3 cytidine deaminases as effectors in innate immunity against the hepatitis B virus

APOBEC3 editing enzymes inhibit retroviruses by cytidine deamination in minus-strand cDNA, leading to G to A hypermutated proviruses, and by less well characterized inhibition of retroviral replication independently of catalysis. This review focuses on the effects of APOBEC3 enzymes on the pararetrovirus hepatitis B virus. The cytidine deaminases APOBEC3B, APOBEC3C, APOBEC3F and APOBEC3G deaminate cytidine residues in hepatitis-B-virus minus-strand cDNA, resulting in G to A hypermutated genomes in the serum of hepatitis-B-virus-infected patients. APOBEC3B, APOBEC3F and APOBEC3G directly inhibit hepatitis-B-virus reverse transcription independently of deaminase activity. In human liver, APOBEC3B, APOBEC3F and APOBEC3G are expressed to low levels, but in human primary hepatocytes stimulated with interferon-alpha, APOBEC3G is induced to levels sufficient for hepatitis-B-virus inhibition. APOBEC3B inhibits hepatitis-B-virus gene transcription, and APOBEC3B and APOBEC3G preferentially mutate the hepatitis-B-virus x gene leading to the truncated hepatitis-B-virus x variants in hepatitis-B-virus-associated liver cancer. The interferon-inducible APOBEC3G and the other APOBEC3s restrict hepatitis B virus by cytidine deamination in hepatitis-B-virus minus-strand cDNA and by direct inhibition of hepatitis-B-virus reverse transcriptase. The nuclear localized APOBEC3B is implicated in liver cancer development. To what extent these enzymes contribute to noncytolytic clearance of hepatitis B virus in vivo remains to be defined, yet the APOBEC3 cytidine deaminases are likely to play a role in these processes.

Interferon‐inducible expression of APOBEC3 editing enzymes in human hepatocytes and inhibition of hepatitis B virus replication†

Hypermutations in hepatitis B virus (HBV) DNA by APOBEC3 cytidine deaminases have been detected in vitro and in vivo, and APOBEC3G (A3G) and APOBEC3F (A3F) have been shown to inhibit the replication of HBV in vitro, but the presumably low or even absent hepatic expression of these enzymes has raised the question as to their physiological impact on HBV replication. We show that normal human liver expresses the mRNAs of APOBEC3B (A3B), APOBEC3C (A3C), A3F, and A3G. In primary human hepatocytes, interferon alpha (IFN-alpha) stimulated the expression of these cytidine deaminases up to 14-fold, and the mRNAs of A3G, A3F, and A3B reached expression levels of 10%, 3%, and 3%, respectively, relative to GAPDH mRNA abundance. On transfection, the full-length protein A3B(L) inhibited HBV replication in vitro as efficiently as A3G or A3F, whereas the truncated splice variant A3B(S) and A3C had no effect. A3B(L) and A3B(S) were detected predominantly in the nucleus of uninfected cells; however, in HBV-expressing cells both proteins were found also in the cytoplasm and were associated with HBV viral particles, similarly to A3G and A3F. Moreover, A3G, A3F, and A3B(L), but not A3B(S), induced extensive G-to-A hypermutations in a fraction of the replicated HBV genomes. In conclusion, the editing enzymes A3B(L), A3F, and most markedly A3G, which are expressed in liver and up-regulated by IFN-alpha in hepatocytes, are candidates to contribute to the noncytolytic clearance of HBV.