APOBEC1 and APOBEC3 Cytidine Deaminases as Restriction Factors for Hepadnaviral Genomes in Non-Humans In Vivo

A tale of endurance: bats, viruses and immune dynamics

The emergence of highly zoonotic viral infections has propelled bat research forward. The viral outbreaks including Hendra virus, Nipah virus, Marburg virus, Ebola virus, Rabies virus, Middle East respiratory syndrome coronavirus, SARS-CoV and the latest SARS-CoV-2 have been epidemiologically linked to various bat species. Bats possess unique immunological characteristics that allow them to serve as a potential viral reservoir. Bats are also known to protect themselves against viruses and maintain their immunity. Therefore, there is a need for in-depth understanding into bat-virus biology to unravel the major factors contributing to the coexistence and spread of viruses.

Advancing beyond reverse transcriptase inhibitors: The new era of hepatitis B polymerase inhibitors

Hepatitis B virus (HBV) is a genetically diverse blood-borne virus responsible for chronic hepatitis B. The HBV polymerase plays a key role in viral genome replication within the human body and has been identified as a potential drug target for chronic hepatitis B therapeutics. However, available nucleotide reverse transcriptase inhibitors only target the reverse transcriptase domain of the HBV polymerase; they also pose resistance issues and require lifelong treatment that can burden patients financially. In this study, various chemical classes are reviewed that have been developed to target different domains of the HBV polymerase: Terminal protein, which plays a vital role in the formation of the viral DNA; Reverse transcriptase, which is responsible for the synthesis of the viral DNA from RNA, and; Ribonuclease H, which is responsible for degrading the RNA strand in the RNA-DNA duplex formed during the reverse transcription process. Host factors that interact with the HBV polymerase to achieve HBV replication are also reviewed; these host factors can be targeted by inhibitors to indirectly inhibit polymerase functionality. A detailed analysis of the scope and limitations of these inhibitors from a medicinal chemistry perspective is provided. The structure-activity relationship of these inhibitors and the factors that may affect their potency and selectivity are also examined. This analysis will be useful in supporting the further development of these inhibitors and in designing new inhibitors that can inhibit HBV replication more efficiently.

A Long-Running Arms Race between APOBEC1 Genes and Retroviruses in Tetrapods

Activation-induced cytidine deaminase/apolipoprotein B mRNA editing catalytic polypeptide-like (AID/APOBEC) proteins are cytosine deaminases implicated in diverse biological functions. APOBEC1 (A1) proteins have long been thought to regulate lipid metabolism, whereas the evolutionary significance of A1 proteins in antiviral defense remains largely obscure. Endogenous retroviruses (ERVs) document past retroviral infections and are ubiquitous within the vertebrate genomes. Here, we identify the A1 gene repertoire, characterize the A1-mediated mutation footprints in ERVs, and interrogate the evolutionary arms race between A1 genes and ERVs across vertebrate species. We find that A1 genes are widely present in tetrapods, recurrently amplified and lost in certain lineages, suggesting that A1 genes might have originated during the early evolution of tetrapods. A1-mediated mutation footprints can be detected in ERVs across tetrapods. Moreover, A1 genes appear to have experienced episodic positive selection in many tetrapod lineages. Taken together, we propose that a long-running arms race between A1 genes and retroviruses might have persisted throughout the evolutionary course of tetrapods. IMPORTANCE APOBEC3 (A3) genes have been thought to function in defense against retroviruses, whereas the evolutionary significance of A1 proteins in antiviral defense remains largely obscure. In this study, we identify the A1 gene repertoire, characterize the A1-mediated mutation footprints in endogenous retroviruses (ERVs), and explore the evolutionary arms race between A1 genes and ERVs across vertebrate species. We found A1 proteins originated during the early evolution of tetrapods, and detected the footprints of A1-induced hypermutations in retroviral fossils. A1 genes appear to have experienced pervasive positive selection in tetrapods. Our study indicates a long-running arms race between A1 genes and retroviruses taking place throughout the evolutionary course of tetrapods.

Roles of APOBEC3 in hepatitis B virus (HBV) infection and hepatocarcinogenesis

APOBEC3 (A3) cytidine deaminases inhibit hepatitis B virus (HBV) infection and play vital roles in maintaining a variety of biochemical processes, including the regulation of protein expression and innate immunity. Emerging evidence indicates that the deaminated deoxycytidine biochemical activity of A3 proteins in single-stranded DNA makes them a double-edged sword. These enzymes can cause cellular genetic mutations at replication forks or within transcription bubbles, depending on the physiological state of the cell and the phase of the cell cycle. Under pathological conditions, aberrant expression of A3 genes with improper deaminase activity regulation may threaten genomic stability and eventually lead to cancer development. This review attempted to summarize the antiviral activities and underlying mechanisms of A3 editing enzymes in HBV infections. Moreover, the correlations between A3 genes and hepatocarcinogenesis were also elucidated.

Tree Shrew Cells Transduced with Human CD4 and CCR5 Support Early Steps of HIV-1 Replication, but Viral Infectivity Is Restricted by APOBEC3

The host range of human immunodeficiency virus type 1 (HIV-1) is narrow. Therefore, using ordinary animal models to study HIV-1 replication, pathogenesis, and therapy is impractical. The lack of applicable animal models for HIV-1 research spurred our investigation on whether tree shrews (Tupaia belangeri chinensis), which are susceptible to many types of human viruses, can act as an animal model for HIV-1. Here, we report that tree shrew primary cells are refractory to wild-type HIV-1 but support the early replication steps of HIV-1 pseudotyped with the vesicular stomatitis virus glycoprotein envelope (VSV-G), which can bypass entry receptors. The exogenous expression of human CD4 renders the tree shrew cell line infectible to X4-tropic HIV-1IIIB, suggesting that tree shrew CXCR4 is a functional HIV-1 coreceptor. However, tree shrew cells did not produce infectious HIV-1 progeny virions, even with the human CD4 receptor. Subsequently, we identified tree shrew (ts) apolipoprotein B editing catalytic polypeptide 3 (tsAPOBEC3) proteins as active inhibitors of HIV-1 particle infectivity, with virus infectivity reduced 10- to 1,000-fold. Unlike human APOBEC3G, the tsA3Z2c-Z1b protein was not degraded by the HIV-1 viral infectivity factor (Vif) but markedly restricted HIV-1 replication through mutagenicity and reverse transcription inhibition. The pooled knockout of tsA3Z2c-Z1b partially restored the infectivity of the HIV-1 progeny. This work suggests that tsAPOBEC3 proteins serve as an additional barrier to the development of HIV-1 tree shrew models, even when virus entry is overcome by exogenous expression of human CD4. IMPORTANCE The development of animal models is critical for studying human diseases and their pathogenesis and for evaluating drug and vaccine efficacy. For improved AIDS research, the ideal animal model of HIV-1 infection should be a small laboratory mammal that closely mimics virus replication in humans. Tree shrews exhibit considerable potential as animal models for the study of human diseases and therapeutic responses. Here, we report that human CD4-expressing tree shrew cells support the early steps of HIV-1 replication and that tree shrew CXCR4 is a functional coreceptor of HIV-1. However, tree shrew cells harbor additional restrictions that lead to the production of HIV-1 virions with low infectivity. Thus, the tsAPOBEC3 proteins are partial barriers to developing tree shrews as an HIV-1 model. Our results provide insight into the genetic basis of HIV inhibition in tree shrews and build a foundation for the establishment of gene-edited tree shrew HIV-1-infected models.

A3G-induced mutations show a low prevalence and exhibit plus-strand regional distribution in hepatitis B virus DNA from patients with non-hepatocellular carcinoma (HCC) and HCC

APOBEC3G (A3G) cytidine deaminase is an innate immune restriction factor that can edit and inhibit hepatitis B virus (HBV) replication. The preferred target of A3G is deamination of the third cytosine of 5'CCC to form a mutant marker 5'CC C → K. However, the distribution of A3G-induced mutations on HBV DNA during infection is not well characterized. To provide clarity, we obtained the HBV DNA sequences from HBV infected individuals with and without hepatocellular carcinoma (HCC and non-HCC, respectively), from the NCBI database, and calculated the r values of A3G-induced 5'CC C → K mutation prevalence in HBV DNA. A3G-induced mutations were weakly prevalent and mainly distributed in the plus strand of HBV DNA (r = 1.407). The mutations on the minus strand were weaker (r = .8189). There were A3G-induced mutation regions in the 1200 to 2000 nt region of the plus strand and the 1600 to 1500 nt region of the minus strand. There was no significant difference in the r values of A3G-induced mutations in HBV DNA between the HCC and non-HCC groups. However, the rvalue of the plus strand 2400 to 2800 nt regions of HCC derived HBV DNA (r = 4.2) was significantly higher than that of the same regions of non-HCC derived HBV DNA (r = 1.21). These findings clarify the weak prevalence and preferred plus-strand distribution of A3G-induced mutations on HBV DNA from HCC and non-HCC. These findings may provide valuable clues regarding the interaction mechanism between A3G and HBV DNA and inform HCC screening.

Potential APOBEC-mediated RNA editing of the genomes of SARS-CoV-2 and other coronaviruses and its impact on their longer term evolution

Members of the APOBEC family of cytidine deaminases show antiviral activities in mammalian cells through lethal editing in the genomes of small DNA viruses, herpesviruses and retroviruses, and potentially those of RNA viruses such as coronaviruses. Consistent with the latter, APOBEC-like directional C→U transitions of genomic plus-strand RNA are greatly overrepresented in SARS-CoV-2 genome sequences of variants emerging during the COVID-19 pandemic. A C→U mutational process may leave evolutionary imprints on coronavirus genomes, including extensive homoplasy from editing and reversion at targeted sites and the occurrence of driven amino acid sequence changes in viral proteins. If sustained over longer periods, this process may account for the previously reported marked global depletion of C and excess of U bases in human seasonal coronavirus genomes. This review synthesizes the current knowledge on APOBEC evolution and function and the evidence of their role in APOBEC-mediated genome editing of SARS-CoV-2 and other coronaviruses.

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SARS-CoV-2 sequence variants contain an overabundance of C- > U transitions

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C- > U transitions are the hallmark of the activity of APOBEC cytosine deaminases

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Further work is needed to determine APOBEC's role in coronavirus evolution

Deaminase-Independent Mode of Antiretroviral Action in Human and Mouse APOBEC3 Proteins

Apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like 3 (APOBEC3) proteins (APOBEC3s) are deaminases that convert cytosines to uracils predominantly on a single-stranded DNA, and function as intrinsic restriction factors in the innate immune system to suppress replication of viruses (including retroviruses) and movement of retrotransposons. Enzymatic activity is supposed to be essential for the APOBEC3 antiviral function. However, it is not the only way that APOBEC3s exert their biological function. Since the discovery of human APOBEC3G as a restriction factor for HIV-1, the deaminase-independent mode of action has been observed. At present, it is apparent that both the deaminase-dependent and -independent pathways are tightly involved not only in combating viruses but also in human tumorigenesis. Although the deaminase-dependent pathway has been extensively characterized so far, understanding of the deaminase-independent pathway remains immature. Here, we review existing knowledge regarding the deaminase-independent antiretroviral functions of APOBEC3s and their molecular mechanisms. We also discuss the possible unidentified molecular mechanism for the deaminase-independent antiretroviral function mediated by mouse APOBEC3.

Mouse APOBEC1 cytidine deaminase can induce somatic mutations in chromosomal DNA

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.

Differential Evolution of Antiretroviral Restriction Factors in Pteropid Bats as Revealed by APOBEC3 Gene Complexity

Bats have attracted attention in recent years as important reservoirs of viruses deadly to humans and other mammals. These infections are typically nonpathogenic in bats raising questions about innate immune differences that might exist between bats and other mammals. The APOBEC3 gene family encodes antiviral DNA cytosine deaminases with important roles in the suppression of diverse viruses and genomic parasites. Here, we characterize pteropid APOBEC3 genes and show that species within the genus Pteropus possess the largest and most diverse array of APOBEC3 genes identified in any mammal reported to date. Several bat APOBEC3 proteins are antiviral as demonstrated by restriction of retroviral infectivity using HIV-1 as a model, and recombinant A3Z1 subtypes possess strong DNA deaminase activity. These genes represent the first group of antiviral restriction factors identified in bats with extensive diversification relative to homologues in other mammals.

Going to Bat(s) for Studies of Disease Tolerance

A majority of viruses that have caused recent epidemics with high lethality rates in people, are zoonoses originating from wildlife. Among them are filoviruses (e.g., Marburg, Ebola), coronaviruses (e.g., SARS, MERS), henipaviruses (e.g., Hendra, Nipah) which share the common features that they are all RNA viruses, and that a dysregulated immune response is an important contributor to the tissue damage and hence pathogenicity that results from infection in humans. Intriguingly, these viruses also all originate from bat reservoirs. Bats have been shown to have a greater mean viral richness than predicted by their phylogenetic distance from humans, their geographic range, or their presence in urban areas, suggesting other traits must explain why bats harbor a greater number of zoonotic viruses than other mammals. Bats are highly unusual among mammals in other ways as well. Not only are they the only mammals capable of powered flight, they have extraordinarily long life spans, with little detectable increases in mortality or senescence until high ages. Their physiology likely impacted their history of pathogen exposure and necessitated adaptations that may have also affected immune signaling pathways. Do our life history traits make us susceptible to generating damaging immune responses to RNA viruses or does the physiology of bats make them particularly tolerant or resistant? Understanding what immune mechanisms enable bats to coexist with RNA viruses may provide critical fundamental insights into how to achieve greater resilience in humans.

The rabbit as an orthologous small animal model for APOBEC3A oncogenesis

APOBEC3 are cytidine deaminases that convert cytidine to uridine residues. APOBEC3A and APOBEC3B enzymes able to target genomic DNA are involved in oncogenesis of a sizeable proportion of human cancers. While the APOBEC3 locus is conserved in mammals, it encodes from 1–7 genes. APOBEC3A is conserved in most mammals, although absent in pigs, cats and throughout Rodentia whereas APOBEC3B is restricted to the Primate order. Here we show that the rabbit APOBEC3 locus encodes two genes of which APOBEC3A enzyme is strictly orthologous to human APOBEC3A. The rabbit enzyme is expressed in the nucleus and the cytoplasm, it can deaminate cytidine, 5-methcytidine residues, nuclear DNA and induce double-strand DNA breaks. The rabbit APOBEC3A enzyme is negatively regulated by the rabbit TRIB3 pseudokinase protein which is guardian of genome integrity, just like its human counterpart. This indicates that the APOBEC3A/TRIB3 pair is conserved over approximately 100 million years. The rabbit APOBEC3A gene is widely expressed in rabbit tissues, unlike human APOBEC3A. These data demonstrate that rabbit could be used as a small animal model for studying APOBEC3 driven oncogenesis.

Molecular cloning and characterization of APOBEC3 family in tree shrew

The APOBEC3 family is a series antiviral factors that inhibit the replication of many viruses, such as HIV-1 and HBV. Tree shrews (Tupaia belangeri) possess great potential as an animal model for human diseases and therapeutic responses. However, the APOBEC3 family is unknown in tree shrews. Recent work has showed the presence of the APOBEC3 family in tree shrews. In this work, the cDNA sequences of five APOBEC3 members were identified in tree shrews, namely, tsAPOBEC3A, -3C, -3F, -3G and -3H. The results showed that their sequences encoded a zinc (Z)-coordinating-domain as a characteristic of APOBEC3 proteins. Phylogenetic analysis revealed that the tree shrew APOBEC3 (tsAPOBEC3) genes have occurred independently and that they are clustered with other mammalian APOBEC3 members. Transcript expression analysis indicated that tsAPOBEC3 genes are constitutively expressed, and high in immune-related tissues. tsAPOBEC3 gene expression was up-regulated in hepatocytes and PBMCs by IFN-α stimulation. Finally, tsAPOBEC3 proteins could edit both sides of DNA by inserting G→A and C→T hypermutations. Overall, the results suggest that the tsAPOBEC3 family could play a key role in defense immunity through distinct editing mechanisms. Our results provided insights into the genetic basis for the development of a tree shrew model for studying viral infection. Future studies will focus on deepening our understanding on the antiviral functions of these editing enzymes in tree shrew.

Immune Evasion Strategies during Chronic Hepatitis B and C Virus Infection

Both hepatitis B virus (HBV) and hepatitis C virus (HCV) infections are a major global healthcare problem with more than 240 million and 70 million infected, respectively. Both viruses persist within the liver and result in progressive liver disease, resulting in liver fibrosis, cirrhosis and hepatocellular carcinoma. Strikingly, this pathogenesis is largely driven by immune responses, unable to clear an established infection, rather than by the viral pathogens themselves. Even though disease progression is very similar in both infections, HBV and HCV have evolved distinct mechanisms, by which they ensure persistence within the host. Whereas HCV utilizes a cloak-and-dagger approach, disguising itself as a lipid-like particle and immediately crippling essential pattern-recognition pathways, HBV has long been considered a "stealth" virus, due to the complete absence of innate immune responses during infection. Recent developments and access to improved model systems, however, revealed that even though it is among the smallest human-tropic viruses, HBV may, in addition to evading host responses, employ subtle immune evasion mechanisms directed at ensuring viral persistence in the absence of host responses. In this review, we compare the different strategies of both viruses to ensure viral persistence by actively interfering with viral recognition and innate immune responses.

Opossum APOBEC1 is a DNA mutator with retrovirus and retroelement restriction activity

APOBEC3s (A3s) are single-stranded DNA cytosine deaminases that provide innate immune defences against retroviruses and mobile elements. A3s are specific to eutherian mammals because no direct homologs exist at the syntenic genomic locus in metatherian (marsupial) or prototherian (monotreme) mammals. However, the A3s in these species have the likely evolutionary precursors, the antibody gene deaminase AID and the RNA/DNA editing enzyme APOBEC1 (A1). Here, we used cell culture-based assays to determine whether opossum A1 restricts the infectivity of retroviruses including human immunodeficiency virus type 1 (HIV-1) and the mobility of LTR/non-LTR retrotransposons. Opossum A1 partially inhibited HIV-1, as well as simian immunodeficiency virus (SIV), murine leukemia virus (MLV), and the retrotransposon MusD. The mechanism of inhibition required catalytic activity, except for human LINE1 (L1) restriction, which was deamination-independent. These results indicate that opossum A1 functions as an innate barrier to infection by retroviruses such as HIV-1, and controls LTR/non-LTR retrotransposition in marsupials.

Hepatitis B virus X protein is capable of down-regulating protein level of host antiviral protein APOBEC3G

The apolipoprotein B mRNA editing catalytic polypeptide-like (APOBEC) family proteins bind RNA and single-stranded DNA, and create C-to-U base modifications through cytidine deaminase activity. APOBEC3G restricts human immunodeficiency virus 1 (HIV-1) infection by creating hypermutations in proviral DNA, while HIV-1-encoded vif protein antagonizes such restriction by targeting APOBEC3G for degradation. APOBEC3G also inhibits hepatitis B virus (HBV): APOBEC3G co-expression inhibits HBV replication and evidences exist indicating APOBEC3G-mediated HBV hypermutations in patients. HBV encodes a small non-structural X protein (HBx) with a recognized activating effect on HBV life cycle. In this work, we report the discovery that HBx selectively and dose-dependently decreases the protein level of co-expressed APOBEC3G in transfected Huh-7 cells. The effect was shown to take place post-translationally, but does not rely on protein degradation via proteasome or lysosome. Further work demonstrated that intracellular APOBEC3G is normally exported via exosome secretion and inhibition of exosome biogenesis causes retention of intracellular APOBEC3G. Finally, HBx co-expression specifically enhanced externalization of APOBEC3G via exosomes, resulting in decrease of intracellular APOBEC3G protein level. These data suggest the possibility that in addition to other mechanisms, HBx-mediated activation of HBV might also involve antagonizing of intracellular restriction factor APOBEC3G through promotion of its export.

Creation of chimeric human/rabbit APOBEC1 with HIV-1 restriction and DNA mutation activities

APOBEC1 (A1) proteins from lagomorphs and rodents have deaminase-dependent restriction activity against HIV-1, whereas human A1 exerts a negligible effect. To investigate these differences in the restriction of HIV-1 by A1 proteins, a series of chimeric proteins combining rabbit and human A1s was constructed. Homology models of the A1s indicated that their activities derive from functional domains that likely act in tandem through a dimeric interface. The C-terminal region containing the leucine-rich motif and the dimerization domains of rabbit A1 is important for its anti-HIV-1 activity. The A1 chimeras with strong anti-HIV-1 activity were incorporated into virions more efficiently than those without anti-HIV-1 activity, and exhibited potent DNA-mutator activity. Therefore, the C-terminal region of rabbit A1 is involved in both its packaging into the HIV-1 virion and its deamination activity against both viral cDNA and genomic RNA. This study identifies the novel molecular mechanism underlying the target specificity of A1.

DNA Editing by APOBECs: A Genomic Preserver and Transformer

Information warfare is not limited to the cyber world because it is waged within our cells as well. The unique AID (activation-induced cytidine deaminase)/APOBEC (apolipoprotein B mRNA editing enzyme, catalytic polypeptide) family comprises proteins that alter DNA sequences by converting deoxycytidines to deoxyuridines through deamination. This C-to-U DNA editing enables them to inhibit parasitic viruses and retrotransposons by disrupting their genomic content. In addition to attacking genomic invaders, APOBECs can target their host genome, which can be beneficial by initiating processes that create antibody diversity needed for the immune system or by accelerating the rate of evolution. AID can also alter gene regulation by removing epigenetic modifications from genomic DNA. However, when uncontrolled, these powerful agents of change can threaten genome stability and eventually lead to cancer.

Present and future therapies of hepatitis B: From discovery to cure

Hepatitis B virus (HBV) is a significant global pathogen, infecting more than 240 million people worldwide. While treatment for HBV has improved, HBV patients often require lifelong therapies and cure is still a challenging goal. Recent advances in technologies and pharmaceutical sciences have heralded a new horizon of innovative therapeutic approaches that are bringing us closer to the possibility of a functional cure of chronic HBV infection. In this article, we review the current state of science in HBV therapy and highlight new and exciting therapeutic strategies spurred by recent scientific advances. Some of these therapies have already entered into clinical phase, and we will likely see more of them moving along the development pipeline.

CONCLUSION

With growing interest in developing and efforts to develop more effective therapies for HBV, the challenging goal of a cure may be well within reach in the near future.

Up-regulation of the interferon-related genes in BRCA2 knockout epithelial cells

BRCA2 mutations are significantly associated with early-onset breast cancer, and the tumour-suppressing function of BRCA2 has been attributed to its involvement in homologous recombination (HR)-mediated DNA repair. In order to identify additional functions of BRCA2, we generated BRCA2-knockout HCT116 human colorectal carcinoma cells. Using genome-wide microarray analyses, we have discovered a link between the loss of BRCA2 and the up-regulation of a subset of interferon (IFN)-related genes, including APOBEC3F and APOBEC3G. The over-expression of IFN-related genes was confirmed in different human BRCA2(-/-) and mouse Brca2(-/-) tumour cell lines, and was independent of senescence and apoptosis. In isogenic wild-type BRCA2 cells, we observed over-expression of IFN-related genes after treatment with DNA-damaging agents, and following ionizing radiation. Cells with endogenous DNA damage because of defective BRCA1 or RAD51 also exhibited over-expression of IFN-related genes. Transcriptional activity of the IFN-stimulated response element (ISRE) was increased in BRCA2 knockout cells, and the expression of BRCA2 greatly decreased IFNα-stimulated ISRE reporter activity, suggesting that BRCA2 directly represses the expression of IFN-related genes through the ISRE. Finally, the colony-forming capacity of BRCA2 knockout cells was significantly reduced in the presence of either IFNβ or IFNγ, suggesting that IFNs may have potential as therapeutic agents in cancer cells with BRCA2 mutations. The GEO Accession No. for microarray analysis is GSE54830.

AID and APOBECs span the gap between innate and adaptive immunity

The activation-induced deaminase (AID)/APOBEC cytidine deaminases participate in a diversity of biological processes from the regulation of protein expression to embryonic development and host defenses. In its classical role, AID mutates germline-encoded sequences of B cell receptors, a key aspect of adaptive immunity, and APOBEC1, mutates apoprotein B pre-mRNA, yielding two isoforms important for cellular function and plasma lipid metabolism. Investigations over the last ten years have uncovered a role of the APOBEC superfamily in intrinsic immunity against viruses and innate immunity against viral infection by deamination and mutation of viral genomes. Further, discovery in the area of human immunodeficiency virus (HIV) infection revealed that the HIV viral infectivity factor protein interacts with APOBEC3G, targeting it for proteosomal degradation, overriding its antiviral function. More recently, our and others' work have uncovered that the AID and APOBEC cytidine deaminase family members have an even more direct link between activity against viral infection and induction and shaping of adaptive immunity than previously thought, including that of antigen processing for cytotoxic T lymphocyte activity and natural killer cell activation. Newly ascribed functions of these cytodine deaminases will be discussed, including their newly identified roles in adaptive immunity, epigenetic regulation, and cell differentiation. Herein this review we discuss AID and APOBEC cytodine deaminases as a link between innate and adaptive immunity uncovered by recent studies.

APOBEC3A catabolism of electroporated plasmid DNA in mouse muscle

The mouse is widely used as a model for DNA therapy and vaccination even though the efficiency of DNA delivery in higher mammals and humans is much less. The human APOBEC3 (A3) enzymes impact viral genomes by cytidine deamination, which introduces multiple uridine residues into single-stranded DNA, a process known as genetic editing. This initiates rapid DNA catabolism via a uracil DNA glycosylase dependent pathway. In tissue culture, A3A, A3C and A3B can hyperedit transfected plasmid DNA. We explored plasmid catabolism in vivo initiated by A3A, the most efficient of the human enzymes and one that is functionally conserved across most mammals. As rodents do not encode an A3A enzyme, it was possible to explore DNA degradation in the mouse model. Human A3A genetically edits co-electroporated luciferase plasmid DNA in mouse skeletal muscle that initiates DNA degradation resulting in approximately fourfold decrease in bioluminescence. Part of the degradation occurs in the nucleus as indicated by complex hyperedited DNA molecules. As human A3A is strongly upregulated by interferon α and DNA sensing pathways, it is a strong candidate enzyme for restricting plasmid DNA in higher mammals.

Reassessment of murine APOBEC1 as a retrovirus restriction factor in vivo

APOBEC1 is a cytidine deaminase involved in cholesterol metabolism that has been linked to retrovirus restriction, analogous to the evolutionarily-related APOBEC3 proteins. In particular, murine APOBEC1 was shown to inhibit Friend retrovirus (FV) in vitro, generating high levels of C-to-T and G-to-A mutations. These observations raised the possibility that FV infection might be altered in APOBEC1-null mice. To examine this question directly, we infected wild-type and APOBEC1-null mice with FV complex and evaluated acute infection levels. Surprisingly, APOBEC1-null mice exhibited similar cellular infection levels and plasma viremia relative to wild-type mice. Moreover, next-generation sequencing analyses revealed that in contrast to APOBEC3, APOBEC1 did not enhance retroviral C-to-T and G-to-A mutational frequencies in genomic DNA. Thus, APOBEC1 neither inhibited nor significantly drove the molecular evolution of FV in vivo. Our findings reinforce that not all retrovirus restriction factors characterized as potent in vitro may be functionally relevant in vivo.

Host restriction of murine gammaherpesvirus 68 replication by human APOBEC3 cytidine deaminases but not murine APOBEC3

Humans encode seven APOBEC3 (A3A-A3H) cytidine deaminase proteins that differ in their expression profiles, preferred nucleotide recognition sequence and capacity for restriction of RNA and DNA viruses. We identified APOBEC3 hotspots in numerous herpesvirus genomes. To determine the impact of host APOBEC3 on herpesvirus biology in vivo, we examined whether murine APOBEC3 (mA3) restricts murine gammaherpesvirus 68 (MHV68). Viral replication was impaired by several human APOBEC3 proteins, but not mA3, upon transfection of the viral genome. The restriction was abrogated upon mutation of the A3A and A3B active sites. Interestingly, virus restriction by A3A, A3B, A3C, and A3DE was lost if the infectious DNA was delivered by the virion. MHV68 pathogenesis, including lung replication and splenic latency, was not altered in mice lacking mA3. We infer that mA3 does not restrict wild type MHV68 and restriction by human A3s may be limited in the herpesvirus replication process.

The inhibition of hepatitis B virus by APOBEC cytidine deaminases

APOBEC3 (A3) cytidine deaminases are a family of enzymes that have been shown to inhibit the replication of HIV-1 and other retroviruses as part of the innate immune responses to virus infection. They can also hyperedit HBV DNA and inhibit HBV replication. Although A3 proteins are present at low levels in normal liver, A3 gene expression is highly stimulated by both interferon-α and interferon-γ. A3 deaminases are incorporated into nascent HBV capsids where they cleave amino groups from cytidine bases converting them to uracil in newly synthesized DNA following reverse transcription of pregenomic RNA. This modified HBV DNA is susceptible to degradation, or alternatively, numerous G-to-A nucleotide mutations are incorporated into positive-strand viral DNA disrupting coding sequences. A3 proteins in which the cytidine deaminase activity has been lost can also inhibit HBV replication, suggesting that there may be more than one way in which inhibition can occur. There is also evidence that A3 proteins might play a role in the development of hepatocellular carcinoma during chronic HBV infection.

The MOV10 helicase inhibits LINE-1 mobility

LINE-1 (long interspersed element 1) is an autonomous non-long terminal repeat retrotransposon. Its replication often causes mutation and rearrangement of host genomic DNA. Accordingly, host cells have evolved mechanisms to control LINE-1 mobility. Here, we report that a helicase named MOV10 effectively suppresses LINE-1 transposition. Mutating the helicase motifs impairs this function of MOV10, suggesting that MOV10 requires its helicase activity to suppress LINE-1 replication. Further studies show that MOV10 post-transcriptionally diminishes the level of LINE-1 RNA. The association of MOV10 with both LINE-1 RNA and ORF1 suggests that MOV10 interacts with LINE-1 RNP and consequently causes its RNA degradation. These data demonstrate collectively that MOV10 contributes to the cellular control of LINE-1 replication.

Uracil DNA glycosylase counteracts APOBEC3G-induced hypermutation of hepatitis B viral genomes: excision repair of covalently closed circular DNA

The covalently closed circular DNA (cccDNA) of the hepatitis B virus (HBV) plays an essential role in chronic hepatitis. The cellular repair system is proposed to convert cytoplasmic nucleocapsid (NC) DNA (partially double-stranded DNA) into cccDNA in the nucleus. Recently, antiviral cytidine deaminases, AID/APOBEC proteins, were shown to generate uracil residues in the NC-DNA through deamination, resulting in cytidine-to-uracil (C-to-U) hypermutation of the viral genome. We investigated whether uracil residues in hepadnavirus DNA were excised by uracil-DNA glycosylase (UNG), a host factor for base excision repair (BER). When UNG activity was inhibited by the expression of the UNG inhibitory protein (UGI), hypermutation of NC-DNA induced by either APOBEC3G or interferon treatment was enhanced in a human hepatocyte cell line. To assess the effect of UNG on the cccDNA viral intermediate, we used the duck HBV (DHBV) replication model. Sequence analyses of DHBV DNAs showed that cccDNA accumulated G-to-A or C-to-T mutations in APOBEC3G-expressing cells, and this was extensively enhanced by UNG inhibition. The cccDNA hypermutation generated many premature stop codons in the P gene. UNG inhibition also enhanced the APOBEC3G-mediated suppression of viral replication, including reduction of NC-DNA, pre-C mRNA, and secreted viral particle-associated DNA in prolonged culture. Enhancement of APOBEC3G-mediated suppression by UNG inhibition was not observed when the catalytic site of APOBEC3G was mutated. Transfection experiments of recloned cccDNAs revealed that the combination of UNG inhibition and APOBEC3G expression reduced the replication ability of cccDNA. Taken together, these data indicate that UNG excises uracil residues from the viral genome during or after cccDNA formation in the nucleus and imply that BER pathway activities decrease the antiviral effect of APOBEC3-mediated hypermutation.

APOBEC3 inhibition of mouse mammary tumor virus infection: the role of cytidine deamination versus inhibition of reverse transcription

The apolipoprotein B editing complex 3 (APOBEC3) family of proteins is a group of intrinsic antiviral factors active against a number of retroviral pathogens, including HIV in humans and mouse mammary tumor virus (MMTV) in mice. APOBEC3 restricts its viral targets through cytidine deamination of viral DNA during reverse transcription or via deaminase-independent means. Here, we used virions from the mammary tissue of MMTV-infected inbred wild-type mice with different allelic APOBEC3 variants (APOBEC3(BALB) and APOBEC3(BL/6)) and knockout mice to determine whether cytidine deamination was important for APOBEC3's anti-MMTV activity. First, using anti-murine APOBEC3 antiserum, we showed that both APOBEC3 allelic variants are packaged into the cores of milk-borne virions produced in vivo. Next, using an in vitro deamination assay, we determined that virion-packaged APOBEC3 retains its deamination activity and that allelic differences in APOBEC3 affect the sequence specificity. In spite of this in vitro activity, cytidine deamination by virion-packaged APOBEC3 of MMTV early reverse transcription DNA occurred only at low levels. Instead, the major means by which in vivo virion-packaged APOBEC3 restricted virus was through inhibition of early reverse transcription in both cell-free virions and in vitro infection assays. Moreover, the different wild-type alleles varied in their ability to inhibit this step. Our data suggest that while APOBEC3-mediated cytidine deamination of MMTV may occur, it is not the major means by which APOBEC3 restricts MMTV infection in vivo. This may reflect the long-term coexistence of MMTV and APOBEC3 in mice.

The potential role of APOBEC3G in limiting replication of hepatitis B virus

BACKGROUND AND STUDY AIMS

Recent findings introduced APOBEC3G (A3G) as a host factor that blocks viral replication. It induces G to A hypermutations in viral DNA at the step of reverse transcription and in response to interferon. This study aimed to investigate the expression of liver A3G protein in association with both replication of hepatitis B virus (HBV) and frequency of G to A mutations in BCP (basal core promoter)-PC (pre-core) region.

PATIENTS AND METHODS

Fifty-one liver biopsies of naïve chronic hepatitis B (CHB) patients enrolled for the expression of A3G were done by immunohistochemistry (IHC) standard method. The presence of HBV DNA and sequences of BCP-PC region at the time of liver biopsy was investigated in all patients.

RESULTS

Among 34 patients with detectable HBV DNA, 31 carried 1-5 G to A mutations in the BCP-PC region. IHC results showed that the expression level of A3G in CHB patients' liver was very low. Of all patients, A3G is expressed in three undetectable HBV DNA subjects and a patient with 2.24×10(4) copies ml(-1) of HBV DNA. G to A mutated residues were indicated at positions 1727, 1757 and 1896 of the HBV genome of this patient.

CONCLUSION

This study indicates that despite very low levels of both A3G in liver and the number of positive subjects, A3G has a potential role to restrict the in vivo replication of HBV.

An ancient history of gene duplications, fusions and losses in the evolution of APOBEC3 mutators in mammals

BACKGROUND

The APOBEC3 (A3) genes play a key role in innate antiviral defense in mammals by introducing directed mutations in the DNA. The human genome encodes for seven A3 genes, with multiple splice alternatives. Different A3 proteins display different substrate specificity, but the very basic question on how discerning self from non-self still remains unresolved. Further, the expression of A3 activity/ies shapes the way both viral and host genomes evolve.

RESULTS

We present here a detailed temporal analysis of the origin and expansion of the A3 repertoire in mammals. Our data support an evolutionary scenario where the genome of the mammalian ancestor encoded for at least one ancestral A3 gene, and where the genome of the ancestor of placental mammals (and possibly of the ancestor of all mammals) already encoded for an A3Z1-A3Z2-A3Z3 arrangement. Duplication events of the A3 genes have occurred independently in different lineages: humans, cats and horses. In all of them, gene duplication has resulted in changes in enzyme activity and/or substrate specificity, in a paradigmatic example of convergent adaptive evolution at the genomic level. Finally, our results show that evolutionary rates for the three A3Z1, A3Z2 and A3Z3 motifs have significantly decreased in the last 100 Mya. The analysis constitutes a textbook example of the evolution of a gene locus by duplication and sub/neofunctionalization in the context of virus-host arms race.

CONCLUSIONS

Our results provide a time framework for identifying ancestral and derived genomic arrangements in the APOBEC loci, and to date the expansion of this gene family for different lineages through time, as a response to changes in viral/retroviral/retrotransposon pressure.

Apolipoprotein B mRNA-editing, catalytic polypeptide cytidine deaminases and retroviral restriction

Apolipoprotein B (apo B) messenger RNA (mRNA)-editing, catalytic polypeptide (APOBEC) cytidine deaminases (CDAs), which can insert mutations into DNA and/or RNA as a result of their ability to deaminate cytidine (C) to uridine (U), originated from a branch of the zinc-dependent deaminase superfamily at the beginning of vertebrate radiation. The ability of mammalian CDAs encoded by the APOBEC3 genes to restrict a broad number of endogenous retroelements and exogenous retroviruses, including human immunodeficiency virus-1, is well established. Furthermore, APOBEC1 from a variety of mammalian species, which mediates the C-to-U deamination of apo B mRNA, a protein involved in lipid transport, also has a role in controlling mobile elements. A large portion of the mammalian genome is derived from ancient transposable elements. Retroelements, transported by an intracellular copy-and-paste process involving an RNA intermediate, constitute the majority of these mobile genetic elements. Endogenous retroviruses are long-terminal repeat (LTR)-type retroelements that account for approximately 10% of human and murine genomic DNA. Non-LTR members are present in extremely high copy numbers, with approximately 40% of the human and murine genomes consisting of long-interspersed nuclear element-1 (L1). These L1 elements modify mammalian genomes not only through insertions but also by the indirect replication of non-autonomous retrotransposons. As expected, vertebrate intrinsic immunity has evolved to support a balance between retroelement insertions that cause deleterious gene disruptions and those that confer beneficial genetic diversity. This review discusses the current understanding of the mechanism of action of APOBEC CDAs and their role in controlling retroviruses and retroelements.

The evolution of endogenous viral elements

Endogenous retroviruses are a common component of the eukaryotic genome, and their evolution and potential function have attracted considerable interest. More surprising was the recent discovery that eukaryotic genomes contain sequences from RNA viruses that have no DNA stage in their life cycle. Similarly, several single-stranded DNA viruses have left integrated copies in their host genomes. This review explores some major evolutionary aspects arising from the discovery of these endogenous viral elements (EVEs). In particular, the reasons for the bias toward EVEs derived from negative-sense RNA viruses are considered, as well as what they tell us about the long-term "arms races" between hosts and viruses, characterized by episodes of selection and counter-selection. Most dramatically, the presence of orthologous EVEs in divergent hosts demonstrates that some viral families have ancestries dating back almost 100 million years, and hence are far older than expected from the phylogenetic analysis of their exogenous relatives.

Lethal mutagenesis of foot-and-mouth disease virus involves shifts in sequence space

Lethal mutagenesis or virus transition into error catastrophe is an antiviral strategy that aims at extinguishing a virus by increasing the viral mutation rates during replication. The molecular basis of lethal mutagenesis is largely unknown. Previous studies showed that a critical substitution in the foot-and-mouth disease virus (FMDV) polymerase was sufficient to allow the virus to escape extinction through modulation of the transition types induced by the purine nucleoside analogue ribavirin. This substitution was not detected in mutant spectra of FMDV populations that had not replicated in the presence of ribavirin, using standard molecular cloning and nucleotide sequencing. Here we selectively amplify and analyze low-melting-temperature cDNA duplexes copied from FMDV genome populations passaged in the absence or presence of ribovirin Hypermutated genomes with high frequencies of A and U were present in both ribavirin -treated and untreated populations, but the major effect of ribavirin mutagenesis was to accelerate the occurrence of AU-rich mutant clouds during the early replication rounds of the virus. The standard FMDV quasispecies passaged in the absence of ribavirin included the salient transition-modulating, ribavirin resistance mutation, whose frequency increased in populations treated with ribavirin. Thus, even nonmutagenized FMDV quasispecies include a deep, mutationally biased portion of sequence space, in support of the view that the virus replicates close to the error threshold for maintenance of genetic information.

APOBEC1-mediated editing and attenuation of herpes simplex virus 1 DNA indicate that neurons have an antiviral role during herpes simplex encephalitis

APOBEC1 (A1) is a cytidine deaminase involved in the regulation of lipids in the small intestine. Herpes simplex virus 1 (HSV-1) is a ubiquitous pathogen that is capable of infecting neurons in the brain, causing encephalitis. Here, we show that A1 is induced during encephalitis in neurons of rats infected with HSV-1. In cells stably expressing A1, HSV-1 infection resulted in significantly reduced virus replication compared to that in control cells. Infectivity could be restored to levels comparable to those observed for control cells if A1 expression was silenced by specific A1 short hairpin RNAs (shRNA). Moreover, cytidine deaminase activity appeared to be essential for this inhibition and led to an impaired accumulation of viral mRNA transcripts and DNA copy numbers. The sequencing of viral gene UL54 DNA, extracted from infected A1-expressing cells, revealed G-to-A and C-to-T transitions, indicating that A1 associates with HSV-1 DNA. Taken together, our results demonstrate a model in which A1 induction during encephalitis in neurons may aid in thwarting HSV-1 infection.