Polymorphic APOBEC3 modulates chronic hepatitis B in Moroccan population

APOBEC3, TRIM5α, and BST2 polymorphisms in healthy individuals of various populations with special references to its impact on HIV transmission

AIDS restriction genes (ARGs) like APOBEC3, TRIM5α, and BST2 can act as immunological detectors of the innate protective mechanism of the body. ARGs influence the course of viral pathogenesis and progression of the disease. The infection caused by different viruses including HIV activates the innate immune receptors leading to production of proinflammatory cytokines, interferons and signals that recruit and activate cells involved in the process of inflammation following induction of adaptive immunity. Differential expression of genes involved in viral infection decide the fate and subsequent susceptibility to infection and its clinical outcome. Nevertheless, comprehensive reports on the incidence of genetic polymorphism of APOBEC3s, TRIM5α, and BST-2 in the general population and its association with pathological conditions have not been described well. Therefore, the occurrence of APOBEC3, TRIM5α, and BST2 polymorphism in healthy individuals and its impact on HIV transmission was analyzed. We conducted an extensive search using the several databases including, EMBASE, PubMed (Medline), and Google Scholar. APOBEC3-D, -F, -G, and -H out of the seven human APOBEC3s, help in the control of viral infection. Amongst various restriction factors, TRIM5α and BST-2 also restrict the viral infection followed by the development of the disease. In the current review, a brief account of the polymorphism in the APOBEC3G, TRIM5α, and BST2 genes are explored among different populations along with the interaction of APOBEC3G with Vif protein. Furthermore, this review specifically focus on ARGs polymorphism (APOBEC3G, TRIM5α, and BST2) associated with HIV transmission.

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.

Human APOBEC3 Variations and Viral Infection

Human APOBEC3 (apolipoprotein B mRNA-editing catalytic polypeptide-like 3) enzymes are capable of inhibiting a wide range of endogenous and exogenous viruses using deaminase and deaminase-independent mechanisms. These enzymes are essential components of our innate immune system, as evidenced by (a) their strong positive selection and expansion in primates, (b) the evolution of viral counter-defense mechanisms, such as proteasomal degradation mediated by HIV Vif, and (c) hypermutation and inactivation of a large number of integrated HIV-1 proviruses. Numerous APOBEC3 single nucleotide polymorphisms, haplotypes, and splice variants have been identified in humans. Several of these variants have been reported to be associated with differential antiviral immunity. This review focuses on the current knowledge in the field about these natural variations and their roles in infectious diseases.

APOBEC: A molecular driver in cervical cancer pathogenesis

Cervical cancer is one of the foremost common cancers in women. Human papillomavirus (HPV) infection remains a major risk factor of cervical cancer. In addition, numerous other genetic and epigenetic factors also are involved in the underlying pathogenesis of cervical cancer. Recently, it has been reported that apolipoprotein B mRNA editing enzyme catalytic polypeptide like (APOBEC), DNA-editing protein plays an important role in the molecular pathogenesis of cancer. Particularly, the APOBEC3 family was shown to induce tumor mutations by aberrant DNA editing mechanism. In general, APOBEC3 enzymes play a pivotal role in the deamination of cytidine to uridine in DNA and RNA to control diverse biological processes such as regulation of protein expression, innate immunity, and embryonic development. Innate antiviral activity of the APOBEC3 family members restrict retroviruses, endogenous retro-element, and DNA viruses including the HPV that is the leading risk factor for cervical cancer. This review briefly describes the pathogenesis of cervical cancer and discusses in detail the recent findings on the role of APOBEC in the molecular pathogenesis of cervical cancer.

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APOBEC enzymes deaminate cytidine to uridine and control diverse biological processes including viral restriction.

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APOBEC3, DNA/RNA-editing enzyme plays an important role in the molecular pathogenesis of cervical cancer.

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APOBEC3-mediated DNA editing leads to the accumulation of somatic mutations in tumors and HPV genome.

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Deregulation of APOBEC3 family genes cause genomic instability and result in drug resistance, and immune-evasion in tumors.

Correlation of APOBEC3G Polymorphism with Human Papillomavirus (HPV) Persistent Infection and Progression of Cervical Lesions

BACKGROUND We studied the effect of APOBEC3G on persistent human papillomavirus (HPV) infection and the correlation between APOBEC3G polymorphism and HPV persistent infection and cervical disease progression in Uygur women in China. MATERIAL AND METHODS From January 2015 to December 2017, we enrolled 529 Uygur ethnic group patients with HPV infection. SIHA cells were transfected with APOBEC3G. Real-time quantitative polymerase chain reaction (RT-qPCR) and Western blot analysis were used to detect mRNA and protein expression levels of APOBEC3G and HPV E6 and p53. Exon 3 of APOBEC3G was sequenced by first-generation sequencing. RESULTS The mRNA and protein expression levels of APOBEC3G in the cervical cancer group were significantly higher than in the cervical intraepithelial neoplasia (CIN) group (p<0.05). The mRNA and protein expression levels of APOBEC3G in the CIN group were significantly higher than in the non-cervical lesions group (p<0.05). The mRNA and protein expression levels of HPV E6 in SIHA cells transfected with APOBEC3G were significantly lower than in the control group and the no-load group (p<0.05), and the mRNA and protein expression levels of p53 were significantly higher than in the control group and the no-load group (p<0.05). There was a polymorphic locus rs5757465 on exon 3 of APOBEC3G in Uygur women, and this rare CC type was a risk factor for cervical lesions and cervical cancer (OR=3.714, 95%CI: 1.916-7.202, p<0.05). CONCLUSIONS APOBEC3G is involved in continuous HPV infection, cervical prelesions, and the development of cervical cancer, and the rare genotype (CC) of APOBEC3G may be one of the factors causing cervical lesions in Uygur women who have HPV infection.

Different antiviral activities of natural APOBEC3C, APOBEC3G, and APOBEC3H variants against hepatitis B virus

Some APOBEC3 family members have antiviral activity against retroviruses and DNA viruses. Hepatitis B virus (HBV) is a DNA virus that is the major causative factor of severe liver diseases such as cirrhosis and hepatocellular carcinoma. To determine whether APOBEC3 variants in humans have different anti-HBV activities, we evaluated natural variants of APOBEC3C, APOBEC3G, and APOBEC3H using an HBV-replicating cell culture model. Our data demonstrate that the APOBEC3C variant S188I had increased restriction activity and hypermutation frequency against HBV DNA. In contrast, the APOBEC3G variant H186R did not alter the anti-HBV and hypermutation activities. Among APOBEC3H polymorphisms (hap I-VII) and splicing variants (SV-200, SV-183, SV-182, and SV-154), hap II SV-183 showed the strongest restriction activity. These data suggest that the genetic variations in APOBEC3 genes may affect the efficiency of HBV elimination in humans.

Correlation of Apolipoprotein B mRNA-editing Enzyme, Catalytic Polypeptide- like 3G Genetic Variant rs8177832 with HIV-1 Predisposition in Pakistani Population

BACKGROUND

Human immunodeficiency virus (HIV) infection is a global health burden which ultimately results in acquired immune deficiency syndrome (AIDS). There are multiple host factors which are capable of limiting HIV-1 replication. One of the most important host factors which inhibit HIV-1 DNA synthesis is the apolipoprotein B mRNA-editing enzyme, catalytic polypeptide- like 3G (APOBEC3G). Any genetic variation of this important host factor may influence the host susceptibility to viral infection.

OBJECTIVE

The aim of the current study was to evaluate any correlation of APOBEC3G genetic variation rs8177832 with HIV-1 infection.

METHODS

The study involved 142 healthy control and 100 HIV-1 infected subjects. The genetic variation rs8177832 of all studied subjects was determined by allele-specific polymerase chain reaction (AS-PCR).

RESULTS

The results showed that the distribution of rs8177832 genotypes AA, AG and GG in healthy subjects and HIV-1 subjects was; 42.253%, 42.957%, 14.788% and 66%, 27%, 7% respectively. Statistical analyses of data showed that there was a significant variation in rs8177832 genotype AA in healthy control and HIV-1 infected subjects (42.257% vs 66%; p-value<0.001).

CONCLUSION

Thus it was concluded that APOBEC3G rs8177832 AA genotype contributes in genetic predisposition to HIV-1 infection in Pakistani population.

APOBEC3B lysine residues are dispensable for DNA cytosine deamination, HIV-1 restriction, and nuclear localization

The APOBEC3 DNA cytosine deaminase family comprises a fundamental arm of the innate immune response and is best known for retrovirus restriction. Several APOBEC3 enzymes restrict HIV-1 and related retroviruses by deaminating viral cDNA cytosines to uracils compromising viral genomes. Human APOBEC3B (A3B) shows strong virus restriction activities in a variety of experimental systems, and is subjected to tight post-translational regulation evidenced by cell-specific HIV-1 restriction activity and active nuclear import. Here we ask whether lysines and/or lysine post-translational modifications are required for these A3B activities. A lysine-free derivative of human A3B was constructed and shown to be indistinguishable from the wild-type enzyme in DNA cytosine deamination, HIV-1 restriction, and nuclear localization activities. However, lysine loss did render the protein resistant to degradation by SIV Vif. Taken together, we conclude that lysine side chains and modifications thereof are unlikely to be central to A3B function or regulation in human cells.

The 30 kb deletion in the APOBEC3 cluster decreases APOBEC3A and APOBEC3B expression and creates a transcriptionally active hybrid gene but does not associate with breast cancer in the European population

APOBEC3B, in addition to other members of the APOBEC3 gene family, has recently been intensively studied due to its identification as a gene whose activation in cancer is responsible for a specific pattern of massively occurring somatic mutations. It was recently shown that a common large deletion in the APOBEC3 cluster (the APOBEC3B deletion) may increase the risk of breast cancer. However, conflicting evidence regarding this association was also reported. In the first step of our study, using different approaches, including an in-house designed multiplex ligation-dependent probe amplification assay, we analyzed the structure of the deletion and showed that although the breakpoints are located in highly homologous regions, which may generate recurrent occurrence of similar but not identical deletions, there is no sign of deletion heterogeneity. This knowledge allowed us to distinguish transcripts of all affected genes, including the highly homologous canonical APOBEC3A and APOBEC3B, and the hybrid APOBEC3A/APOBEC3B gene. We unambiguously confirmed the presence of the hybrid transcript and showed that the APOBEC3B deletion negatively correlates with APOBEC3A and APOBEC3B expression and positively correlates with APOBEC3A/APOBEC3B expression, whose mRNA level is >10-fold and >1500-fold lower than the level of APOBEC3A and APOBEC3B, respectively. In the next step, we performed a large-scale association study in three different cohorts (2972 cases and 3682 controls) and showed no association of the deletion with breast cancer, familial breast cancer or ovarian cancer. Further, we conducted a meta-analysis that confirmed the lack of the association of the deletion with breast cancer in non-Asian populations.

Lack of Ser267Phe variant of sodium taurocholate cotransporting polypeptide among Moroccans regardless of hepatitis B virus infection status

Background

The sodium taurocholate co-transporting polypeptide, encoded by SLC10A1, was identified as a functional receptor for hepatitis B virus (HBV). The objective of this study was to determine if there was an association of the Ser267Phe variant (rs2296651) with HBV infection status in Moroccan patients.

Methods

Using a TaqMan 5’ allelic discrimination assay, the Ser267Phe variant was genotyped in 286 chronic hepatitis B patients, 135 individuals with spontaneous clearance from HBV infection and 109 healthy controls negative for hepatitis B serological markers.

Results

In this cohort, we detected only wild-type genotype (S267S) in all groups. This polymorphism was not associated with the HBV infection status in Moroccan patients.

Conclusions

The S267F variant is absent among Moroccans regardless of chronic HBV infection status.

Association between polymorphisms of the APOBEC3G gene and chronic hepatitis B viral infection and hepatitis B virus-related hepatocellular carcinoma

AIM

To determine the relationship between five A3G gene single nucleotide polymorphisms and the incidence of hepatitis B virus (HBV) infection and hepatocellular carcinoma (HCC).

METHODS

This association study was designed as a retrospective study, including 657 patients with chronic HBV infection (CHB) and 299 healthy controls. All subjects were ethnic Han Chinese. Chronic HBV-infected patients recruited between 2012 and 2015 at The First Hospital of Jilin University (Changchun) were further classified into HBV-related HCC patients (n = 287) and non-HCC patients (n = 370). Frequency matching by age and sex was performed for each group. Human genomic DNA was extracted from whole blood. Gene polymorphisms were identified using a mass spectroscopic method.

RESULTS

There were no significant differences between the genotype and allele frequencies of the rs7291971, rs5757465 and rs5757463 A3G gene polymorphisms, and risk of CHB and HBV-related HCC. The AG genotype and G allele for rs8177832 were significantly related to a decreased risk of CHB (OR = 0.67, 95%CI: 0.47-0.96; OR = 0.69, 95%CI: 0.50-0.95, respectively) and HCC (OR = 0.53, 95%CI: 0.34-0.84; OR = 0.58, 95%CI: 0.39-0.87, respectively). A significant relationship was found between rs2011861 computed tomography, TT genotypes and increased risk of HCC (OR = 1.69, 95%CI: 1.02-2.80; OR = 1.82, 95%CI: 1.08-3.06, respectively). Haplotype analyses showed three protective and four risk haplotypes for HCC. Also, one protective haplotype was found against CHB.

CONCLUSION

This study indicates that the A3G rs8177832 polymorphism is associated with a decreased risk of CHB infection and HCC, while the rs2011861 polymorphism is associated with an increased risk of HCC.

HBV/HIV co-infection and APOBEC3G polymorphisms in a population from Burkina Faso

BACKGROUND

Apolipoprotein B mRNA editing enzyme catalytic polypeptide-like 3G (APOBEC3G) is a potent host defense factor, which interferes with HIV-1 and HBV. Our study had three objectives, to screen a population of HIV-1 infected and uninfected patients in Burkina Faso for HBV, to screen the population for APOBEC3G variants rs6001417, rs8177832, and rs35228531 previously described, and to analyze the effect of these three variants and their haplotypes on HIV-1/HBV co-infection in Burkina Faso.

METHODS

HBV detection was performed on samples from HIV-1 infected and uninfected subjects using rapid detection tests and real-time PCR. APOBEC3 genotyping was done by the TaqMan allelic discrimination method. Fisher Exact test, Odds ratio (OR), confidence intervals (CI) at 95 %, Linkage disequilibrium (LD) summary statistics and haplotype frequencies were calculated.

RESULTS

The prevalence of HBV was 56.7 % among HIV-1 positive patients of our study while it was about 12.8 % among HIV-1 seronegative subjects. Genotype E was the genotype of HBV present in our hepatitis B positive samples. Minor allele frequencies of rs6001417, rs8177832, and rs35228531 were higher in seronegative subjects. The T minor allele of variant rs35228531 was protective against HIV-1/HBV co-infection with OR = 0.61, 95 % CI (0.42-0.90), p = 0.013. There was also an association between the GGT haplotype and protection against HIV-1/HBV co-infection, OR = 0.57, 95 % CI (0.33-0.99), p = 0.050. The other haplotypes present in the population were not statistically significant. There minor allele T of the rs35228531 was protective against HIV mono-infection OR = 0.53, 95 % CI (0.3-0.93), P = 0.030. But there was no effect of protection against HBV mono-infection.

CONCLUSION

APOBEC3G through its variants rs6001417, rs8177832, and rs35228531, in this study interferes with HIV-1/HBV co-infection could be due the HIV-1 mono-infection in a population from Burkina Faso.

Analysis of APOBEC3A/3B germline deletion polymorphism in breast, cervical and oral cancers from South India and its impact on miRNA regulation

Breast cancer and cervical cancer are the leading causes of death in women worldwide as well as in India, whilst oral cancer is the top most common cancer among Asian especially in Indian men in terms of both incidence and mortality rate. Genetic factors determining the predisposition to cancer are being explored to identify the signature genetic variations associated with these cancers. Recently, a germline deletion polymorphism in APOBEC3 gene cluster which completely deletes APOBEC3B coding region has been studied for its association with cancer risk. We screened the germline deletion polymorphism in 409 cancer patients (224 breast cancer, 88 cervical cancer and 97 oral cancer samples), 478 controls and 239 cervical cancer tissue DNAs of South Indian origin. The results suggest that the APOBEC3A/3B deletion polymorphism is not significantly associated with cancer risk in our study population (OR 0.739, 95 % CI, p value 0.91457). Considering the viral restriction property of APOBEC3s, we also screened cervical cancer tissue DNAs for the human papilloma virus infection. We observed a gradual increase in the frequency of HPV16 infection from AA/BB cases (66.86 %) to AA/-- cases (71.43) which signifies the impact of this deletion polymorphism in HPV infection. In addition, we performed in silico analysis to understand the effect of this polymorphism on miRNA regulation of the APOBEC3A/3B fusion transcript. Only 8 APOBEC3B targeting miRNAs were observed to regulate the fusion transcript of which miR-34b-3p and miR-138-5p were found to be frequently downregulated in cancers suggesting miRNA-mediated deregulation of APOBEC3A expression in cancer patients harbouring this particular deletion polymorphism.

Coding region variant 186H/R in Exon 4 of APOBEC3G among individuals of Western India

The allelic variations in the AIDS restriction genes have been associated with the acquisition of HIV-1 and its progression. The distribution of antiviral gene variants significantly differs between populations. Therefore, we aimed to evaluate the distribution of variant allele of 186H/R in exon4 of APOBEC3G between HIV infected individuals and healthy controls among western Indian.In the present cross-sectional study, we enrolled a total of 153 HIV-infected patients confirmed and 156 unrelated healthy individuals. Polymorphism for 186H/R in exon4 of APOBEC3G gene was genotyped by PCR-RFLP. With the frequency of 186HR heterozygous genotype of APOBEC3G was found to be 13% in healthy controls and none in HIV infected cases. The frequency of 186HH common genotype of APOBEC3G was observed higher in HIV infected individuals compared with healthy controls (100% vs 91.7%). The variant genotype 186RR in APOBEC3G was not found in both the groups. The frequency of 186R allele of APOBEC3G was found 4.16% in healthy controls and nil in HIV-infected cases. The frequency of 186H allele of APOBEC3G was found to be higher in HIV-infected cases compared with healthy controls (100% vs 95.83%). The frequency of 186R allele in exon4 of APOBEC3G was found to be 4.16% in healthy controls. This observation differs from the previous report published from North India stating the absence of 186R allele of APOBEC3G in the North Indian individuals. The variant 186H/R in exon4 of APOBEC3G was neither associated with risk of acquisition of HIV-1 nor its progression.

Role of the single deaminase domain APOBEC3A in virus restriction, retrotransposition, DNA damage and cancer

The apolipoprotein mRNA editing enzyme catalytic polypeptide-like 3 (APOBEC3; A3) proteins are a family of seven cytidine deaminases (A3A, A3B, A3C, A3D, A3F, A3G and A3H) that restrict certain viral infections. These innate defence factors are best known for their ability to restrict the replication of human immunodeficiency virus type 1 (HIV-1) lacking a functional Vif protein (HIV-1Δvif) through the deamination of cytidine residues to uridines during reverse transcription, ultimately leading to lethal G → A changes in the viral genome. The best studied of the A3 proteins has been APOBEC3G because of its potent activity against HIV-1Δvif. However, one member of this family, A3A, has biological properties that make it unique among the A3 proteins. In this review, we will focus on the structural and phylogenetic features of the human and non-human primate A3A proteins, their role in the restriction of retroviruses and other viruses, and current findings on other biological properties affected by this protein.

Associations between activation-induced cytidine deaminase/apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like cytidine deaminase expression, hepatitis B virus (HBV) replication and HBV-associated liver disease (Review)

The hepatitis B virus (HBV) infection is a major risk factor in the development of chronic hepatitis (CH) and hepa-tocellular carcinoma (HCC). The activation-induced cytidine deaminase (AID)/apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like (APOBEC) family of cytidine deaminases is significant in innate immunity, as it restricts numerous viruses, including HBV, through hypermutation-dependent and -independent mechanisms. It is important to induce covalently closed circular (ccc)DNA degradation by interferon-α without causing side effects in the infected host cell. Furthermore, organisms possess multiple mechanisms to regulate the expression of AID/APOBECs, control their enzymatic activity and restrict their access to DNA or RNA substrates. Therefore, the AID/APOBECs present promising targets for preventing and treating viral infections. In addition, gene polymorphisms of the AID/APOBEC family may alter host susceptibility to HBV acquisition and CH disease progression. Through G-to-A hypermutation, AID/APOBECs also edit HBV DNA and facilitate the mutation of HBV DNA, which may assist the virus to evolve and potentially escape from the immune responses. The AID/APOBEC family and their associated editing patterns may also exert oncogenic activity. Understanding the effects of cytidine deaminases in CH virus-induced hepatocarcinogenesis may aid with developing efficient prophylactic and therapeutic strategies against HCC.

The APOBEC3B deletion polymorphism is associated with prevalence of hepatitis B virus, hepatitis C virus, Torque Teno virus, and Toxoplasma gondii co-infection among HIV-infected individuals

BACKGROUND

Data regarding the influence of the APOBEC3B deletion on infectious diseases remain limited and shown discrepancies.

OBJECTIVES

To characterize the APOBEC3B deletion polymorphism status and its association with prevalence of co-infection with blood-borne pathogens in Indonesian HIV-infected individuals.

MATERIALS AND METHODS

A total of 597 HIV-positive blood samples were tested for the hepatitis B virus (HBV), hepatitis C virus (HCV), Torque Teno virus (TTV), GB virus-C (GBV-C), and Toxoplasma gondii. Nucleic acid was extracted from plasma samples and used for the molecular detection of HIV RNA, HBV DNA, HCV RNA, TTV DNA, and GBV-C RNA, whereas HBsAg, anti-HCV, IgM and IgG anti-T. gondii were detected through serological testing. The APOBEC3B deletion polymorphism was genotyped by polymerase chain reaction (PCR).

RESULTS

The deletion genotype was associated with HCV viremia (p<0.001) as well as elevated IgG anti-T. gondii (adjusted OR [aOR]=3.4). The deletion genotype was also associated with decreased levels of HBsAg (aOR=0.03), and anti-HCV (aOR=0.1). D/D was frequently found in HIV-infected individuals with CD4+T cells<14% (aOR=5.8). The intact genotype was associated with a reduced likelihood of a CD4+T cell count<200 cells/μL (aOR=0.2) but a higher prevalence of TTV co-infection (aOR=8.6).

CONCLUSIONS

The APOBEC3B deletion polymorphism was found to be associated with HBV, HCV, TTV, and T. gondii co-infection in Indonesian HIV-infected individuals.

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.

Recent insights into hepatitis B virus-host interactions

Hepatitis B virus (HBV) poses a threat to global public health mainly because of complications of HBV-related chronic liver disease. HBV exhibits a narrow host range, replicating primarily in hepatocytes by a still poorly understood mechanism. For the generation of progeny virions, HBV depends on interactions with specific host factors through its life cycle. Revealing and characterizing these interactions are keys to identifying novel antiviral targets, and to developing specific treatment strategies for HBV patients. In this review, recent insights into the HBV-host interactions, especially on virus entry, intracellular trafficking, genome transcription and replication, budding and release, and even cellular restriction factors were reviewed.

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 role of cytidine deaminases on innate immune responses against human viral infections

The APOBEC family of proteins comprises deaminase enzymes that edit DNA and/or RNA sequences. The APOBEC3 subgroup plays an important role on the innate immune system, acting on host defense against exogenous viruses and endogenous retroelements. The role of APOBEC3 proteins in the inhibition of viral infection was firstly described for HIV-1. However, in the past few years many studies have also shown evidence of APOBEC3 action on other viruses associated with human diseases, including HTLV, HCV, HBV, HPV, HSV-1, and EBV. APOBEC3 inhibits these viruses through a series of editing-dependent and independent mechanisms. Many viruses have evolved mechanisms to counteract APOBEC effects, and strategies that enhance APOBEC3 activity constitute a new approach for antiviral drug development. On the other hand, novel evidence that editing by APOBEC3 constitutes a source for viral genetic diversification and evolution has emerged. Furthermore, a possible role in cancer development has been shown for these host enzymes. Therefore, understanding the role of deaminases on the immune response against infectious agents, as well as their role in human disease, has become pivotal. This review summarizes the state-of-the-art knowledge of the impact of APOBEC enzymes on human viruses of distinct families and harboring disparate replication strategies.