The APOBEC3 cytidine deaminases: an innate defensive network opposing exogenous retroviruses and endogenous retroelements

APOBEC3B Activity Is Prevalent in Urothelial Carcinoma Cells and Only Slightly Affected by LINE-1 Expression

The most common mutational signature in urothelial carcinoma (UC), the most common type of urinary bladder cancer is assumed to be caused by the misdirected activity of APOBEC3 (A3) cytidine deaminases, especially A3A or A3B, which are known to normally restrict the propagation of exogenous viruses and endogenous retroelements such as LINE-1 (L1). The involvement of A3 proteins in urothelial carcinogenesis is unexpected because, to date, UC is thought to be caused by chemical carcinogens rather than viral activity. Therefore, we explored the relationship between A3 expression and L1 activity, which is generally upregulated in UC. We found that UC cell lines highly express A3B and in some cases A3G, but not A3A, and exhibit corresponding cytidine deamination activity in vitro. While we observed evidence suggesting that L1 expression has a weak positive effect on A3B and A3G expression and A3B promoter activity, neither efficient siRNA-mediated knockdown nor overexpression of functional L1 elements affected catalytic activity of A3 proteins consistently. However, L1 knockdown diminished proliferation of a UC cell line exhibiting robust endogenous L1 expression, but had little impact on a cell line with low L1 expression levels. Our results indicate that UC cells express A3B at levels exceeding A3A levels by far, making A3B the prime candidate for causing genomic mutations. Our data provide evidence that L1 activation constitutes only a minor and negligible factor involved in induction or upregulation of endogenous A3 expression in UC.

Transposable element dysregulation in systemic lupus erythematosus and regulation by histone conformation and Hsp90

Systemic lupus erythematosus (SLE) represents an autoimmune disease in which activation of the type I interferon pathway leads to dysregulation of tolerance and the generation of autoantibodies directed against nuclear constituents. The mechanisms driving the activation of the interferon pathway in SLE have been the subject of intense investigation but are still incompletely understood. Transposable elements represent an enormous source of RNA that could potentially stimulate the cell intrinsic RNA-recognition pathway, leading to upregulation of interferons. We used RNA-seq to define transposable element families and subfamilies in three cell types in SLE and found diverse effects on transposable element expression in the three cell types and even within a given family of transposable elements. When potential mechanisms were examined, we found that Hsp90 inhibition could drive increased expression of multiple type of transposable elements. Both direct inhibition and the delivery of a heat shock itself, which redirects heat shock regulators (including Hsp90) off of basal expression promoters and onto heat shock-responsive promoters, led to increased transposable element expression. This effect was amplified by the concurrent delivery of a histone deacetylase inhibitor. We conclude that transposable elements are dysregulated in SLE and there are tissue-specific effects and locus-specific effects. The magnitude of RNAs attributable to transposable elements makes their dysregulation of critical interest in SLE where transposable element RNA complexed with proteins has been shown to drive interferon expression.

Prospects in Innate Immune Responses as Potential Control Strategies against Non-Primate Lentiviruses

Lentiviruses are infectious agents of a number of animal species, including sheep, goats, horses, monkeys, cows, and cats, in addition to humans. As in the human case, the host immune response fails to control the establishment of chronic persistent infection that finally leads to a specific disease development. Despite intensive research on the development of lentivirus vaccines, it is still not clear which immune responses can protect against infection. Viral mutations resulting in escape from T-cell or antibody-mediated responses are the basis of the immune failure to control the infection. The innate immune response provides the first line of defense against viral infections in an antigen-independent manner. Antiviral innate responses are conducted by dendritic cells, macrophages, and natural killer cells, often targeted by lentiviruses, and intrinsic antiviral mechanisms exerted by all cells. Intrinsic responses depend on the recognition of the viral pathogen-associated molecular patterns (PAMPs) by pathogen recognition receptors (PRRs), and the signaling cascades leading to an antiviral state by inducing the expression of antiviral proteins, including restriction factors. This review describes the latest advances on innate immunity related to the infection by animal lentiviruses, centered on small ruminant lentiviruses (SRLV), equine infectious anemia virus (EIAV), and feline (FIV) and bovine immunodeficiency viruses (BIV), specifically focusing on the antiviral role of the major restriction factors described thus far.

To ERV Is Human: A Phenotype-Wide Scan Linking Polymorphic Human Endogenous Retrovirus-K Insertions to Complex Phenotypes

Approximately 8% of the human genome is comprised of endogenous retroviral insertions (ERVs) originating from historic retroviral integration into germ cells. The function of ERVs as regulators of gene expression is well established. Less well studied are insertional polymorphisms of ERVs and their contribution to the heritability of complex phenotypes. The most recent integration of ERV, HERV-K, is expressed in a range of complex human conditions from cancer to neurologic diseases. Using an in-house computational pipeline and whole-genome sequencing data from the diverse 1,000 Genomes Phase 3 population (n = 2,504), we identified 46 polymorphic HERV-K insertions that are tagged by adjacent single nucleotide polymorphisms (SNPs). To test the potential role of polymorphic HERV-K in the heritability of complex diseases, existing databases were queried for enrichment of established relationships between the HERV-K insertion-associated SNPs (hiSNPs), and tissue specific gene expression and disease phenotypes. Overall, hiSNPs for the 46 polymorphic HERV-K sites were statistically enriched (p < 1.0E^-16) for eQTLs across 44 human tissues. Fifteen of the 46 HERV-K insertions had hiSNPs annotated in the EMBL-EBI GWAS Catalog and cumulatively associated with >100 phenotypes. Experimental factor ontology enrichment analysis suggests that polymorphic HERV-K specifically contribute to neurologic and immunologic disease phenotypes, including traits related to intra cranial volume (FDR 2.00E-09), Parkinson's disease (FDR 1.80E-09), and autoimmune diseases (FDR 1.80E-09). These results provide strong candidates for context-specific study of polymorphic HERV-K insertions in disease-related traits, serving as a roadmap for future studies of the heritability of complex disease.

Cancer Risks Linked to the Bad Luck Hypothesis and Epigenomic Mutational Signatures

Exposure to pathogen infection, and occupational and environmental agents, contributes to induction of most types of cancer through different mechanisms. Cancer is defined and characterized by accumulation of mutations and epimutations that lead to changes in the cellular genome and epigenome. According to a recent Bad Luck Hypothesis, random error mutations during DNA replication in a small population of stem cells may be implicated in two-thirds of variation of cancer risk in 25 organs and tissues. What determines stem cell vulnerability and risk of malignancy across the spectrum of organs, such as the brain, bone marrow, skeletal muscles, skin, and liver? Have stem cells pooled in particular tissues or organs evolved some critical ability to deal with DNA damage in the presence of extrinsic environmental factors? This paper describes how the complex replication and repair DNA systems control mutational events. In addition, recent advances on cancer epigenomic signatures and epigenetic mechanisms are discussed, which will guide future investigation of the origin of cancer initiating cells in tissue and organs in a clinical setting.

Crystal structure of the catalytic domain of HIV-1 restriction factor APOBEC3G in complex with ssDNA

The human APOBEC3G protein is a cytidine deaminase that generates cytidine to deoxy-uridine mutations in single-stranded DNA (ssDNA), and capable of restricting replication of HIV-1 by generating mutations in viral genome. The mechanism by which APOBEC3G specifically deaminates 5′-CC motifs has remained elusive since structural studies have been hampered due to apparently weak ssDNA binding of the catalytic domain of APOBEC3G. We overcame the problem by generating a highly active variant with higher ssDNA affinity. Here, we present the crystal structure of this variant complexed with a ssDNA substrate at 1.86 Å resolution. This structure reveals atomic-level interactions by which APOBEC3G recognizes a functionally-relevant 5′-TCCCA sequence. This complex also reveals a key role of W211 in substrate recognition, implicating a similar recognition in activation-induced cytidine deaminase (AID) with a conserved tryptophan.

APOBEC3G (A3G) is a single-stranded DNA (ssDNA) cytidine deaminase that restricts HIV-1. Here the authors provide molecular insights into A3G substrate recognition by determining the 1.86 Å resolution crystal structure of its catalytic domain bound to ssDNA.

Diversity in the Cow Ultralong CDR H3 Antibody Repertoire

Typical antibodies found in humans and mice usually have short CDR H3s and generally flat binding surfaces. However, cows possess a subset of antibodies with ultralong CDR H3s that can range up to 70 amino acids and form a unique “stalk and knob” structure, with the knob protruding far out of the antibody surface, where it has the potential to bind antigens with concave epitopes. Activation-induced cytidine deaminase (AID) has a proven role in diversifying antibody repertoires in humoral immunity, and it has been found to induce somatic hypermutation in bovine immunoglobulin genes both before and after contact with antigen. Due to limited use of variable and diversity genes in the V(D)J recombination events that produce ultralong CDR H3 antibodies in cows, the diversity in the bovine ultralong antibody repertoire has been proposed to rely on AID-induced mutations targeted to the IGHD8-2 gene that encodes the entire knob region. In this review, we discuss the genetics, structures, and diversity of bovine ultralong antibodies, as well as the role of AID in creating a diverse antibody repertoire.

Jembrana disease virus Vif antagonizes the inhibition of bovine APOBEC3 proteins through ubiquitin-mediate protein degradation

Viral infectivity factor (Vif) encoded by lentiviruses is essential for viral replication and escaping from antiviral activity of host defensive factors APOBEC3. Jembrana disease virus (JDV) causes an acute disease syndrome with approximately 20% case fatality rate in Bali cattle. However, the interplay mechanism between JDV Vif and Bos taurus APOBEC3 (btA3) is poorly understood. In this study, we determined that JDV Vif recruits ElonginB, ElonginC(ELOB/C), Cul2 and RBX1 but without the need of CBF-β to form E3 ubiquitin ligase and induces the degradation of btA3 proteins. Further investigation identified BC-box (T149LQ151) motif required for ELOB/C binding, Cul2 box (Y167xxxxV/X172) and a zinc-binding motif (H95-C113-H115-C133) required for Cul2 binding in JDV Vif. The precise mechanism of JDV Vif overcoming the antiviral activity of btA3 proteins is helpful for the application of the broad spectrum antiviral drug targeting conserved functional domains of various species Vif proteins in the future.

Viral subversion of APOBEC3s: Lessons for anti-tumor immunity and tumor immunotherapy

APOBEC3s (A3) are endogenous DNA-editing enzymes that are expressed in immune cells including T lymphocytes. A3s target and mutate the genomes of retroviruses that infect immune tissues such as the human immunodeficiency virus (HIV). Therefore, A3s were classically defined as host anti-viral innate immune factors. In contrast, we and others showed that A3s can also benefit the virus by mediating escape from adaptive immune recognition and drugs. Crucially, whether A3-mediated mutations help or hinder HIV, is not up to chance. Rather, the virus has evolved multiple mechanisms to actively and maximally subvert A3 activity. More recently, extensive A3 mutational footprints in tumor genomes have been observed in many different cancers. This suggests a role for A3s in cancer initiation and progression. On the other hand, multiple anti-tumor activities of A3s have also come to light, including impact on immune checkpoint molecules and possible generation of tumor neo-antigens. Here, we review the studies that reshaped the view of A3s from anti-viral innate immune agents to host factors exploited by HIV to escape from immune recognition. Viruses and tumors share many attributes, including rapid evolution and adeptness at exploiting mutations. Given this parallel, we then discuss the pro- and anti-tumor roles of A3s, and suggest that lessons learned from studying A3s in the context of anti-viral immunity can be applied to tumor immunotherapy.

Mouse APOBEC3 expression in NIH 3T3 cells mediates hypermutation of AKV murine leukemia virus

Mouse APOBEC3 (mA3) is a cytidine deaminase that can act on the single-stranded DNA reverse transcripts of retroviruses resulting in G→A hypermutation of proviral DNA. Many mA3 studies have used NIH 3T3 cells assuming that endogenous mA3 production was negligible. We developed a monoclonal antibody specific for mA3 that reveals detectable mA3 in NIH 3T3 cells and we demonstrate that AKV released from the cells undergoes G→A hypermutation. Inactivation of the mA3 gene abolished the deamination confirming that AKV hypermutation was mediated by mA3. The G→A mutations in AKV viral transcripts deviated from a normal distribution with all the mutations contained within only 20% of the transcripts. Single cell analyses revealed that the expression of mA3 in NIH 3T3 cells was limited to 20% of the cells, which likely accounted for the abnormal distribution of mutations. Endogenous NIH 3T3 mA3 was found to restrict AKV replication.

Post-transcriptional regulation of LINE-1 retrotransposition by AID/APOBEC and ADAR deaminases

Long interspersed element-1 (LINE-1 or L1) retrotransposons represent the only functional family of autonomous transposable elements in humans and formed 17% of our genome. Even though most of the human L1 sequences are inactive, a limited number of copies per individual retain the ability to mobilize by a process termed retrotransposition. The ongoing L1 retrotransposition may result in insertional mutagenesis that could lead to negative consequences such as genetic disease and cancer. For this reason, cells have evolved several mechanisms of defense to restrict L1 activity. Among them, a critical role for cellular deaminases [activation-induced deaminase (AID)/apolipoprotein B mRNA-editing catalytic polypeptide-like (APOBEC) and adenosine deaminases that act on RNA (ADAR) enzymes] has emerged. The majority of the AID/APOBEC family of proteins are responsible for the deamination of cytosine to uracil (C-to-U editing) within DNA and RNA targets. The ADARs convert adenosine bases to inosines (A-to-I editing) within double-stranded RNA (dsRNA) targets. This review will discuss the current understanding of the regulation of LINE-1 retrotransposition mediated by these enzymes.

Detection of APOBEC3 Proteins and Catalytic Activity in Urothelial Carcinoma

Members of the APOBEC3 (A3) family of enzymes were shown to act in an oncogenic manner in several cancer types. Immunodetection of APOBEC3A (A3A), APOBEC3B (A3B), and APOBEC3G (A3G) proteins is particularly challenging due to the large sequence homology of these proteins and limited availability of antibodies. Here we combine independent immunoblotting with an in vitro activity assay technique, to detect and categorize specific A3s expressed in urothelial bladder cancer and other cancer cells.

Stem-loop structure preference for site-specific RNA editing by APOBEC3A and APOBEC3G

APOBEC3A and APOBEC3G cytidine deaminases inhibit viruses and endogenous retrotransposons. We recently demonstrated the novel cellular C-to-U RNA editing function of APOBEC3A and APOBEC3G. Both enzymes deaminate single-stranded DNAs at multiple TC or CC nucleotide sequences, but edit only a select set of RNAs, often at a single TC or CC nucleotide sequence. To examine the specific site preference for APOBEC3A and -3G-mediated RNA editing, we performed mutagenesis studies of the endogenous cellular RNA substrates of both proteins. We demonstrate that both enzymes prefer RNA substrates that have a predicted stem-loop with the reactive C at the 3′-end of the loop. The size of the loop, the nucleotides immediately 5′ to the target cytosine and stability of the stem have a major impact on the level of RNA editing. Our findings show that both sequence and secondary structure are preferred for RNA editing by APOBEC3A and -3G, and suggest an explanation for substrate and site-specificity of RNA editing by APOBEC3A and -3G enzymes.

Detection of the LINE-1 retrotransposon RNA-binding protein ORF1p in different anatomical regions of the human brain

Background

Recent reports indicate that retrotransposons – a type of mobile DNA – can contribute to neuronal genetic diversity in mammals. Retrotransposons are genetic elements that mobilize via an RNA intermediate by a “copy-and-paste” mechanism termed retrotransposition. Long Interspersed Element-1 (LINE-1 or L1) is the only active autonomous retrotransposon in humans and its activity is responsible for ~ 30% of genomic mass. Historically, L1 retrotransposition was thought to be restricted to the germline; however, new data indicate L1 s are active in somatic tissue with certain regions of the brain being highly permissive. The functional implications of L1 insertional activity in the brain and how host cells regulate it are incomplete. While deep sequencing and qPCR analysis have shown that L1 copy number is much higher in certain parts of the human brain, direct in vivo studies regarding detection of L1-encoded proteins is lacking due to ineffective reagents.

Results

Using a polyclonal antibody we generated against the RNA-binding (RRM) domain of L1 ORF1p, we observe widespread ORF1p expression in post-mortem human brain samples including the hippocampus which has known elevated rates of retrotransposition. In addition, we find that two brains from different individuals of different ages display very different expression of ORF1p, especially in the frontal cortex.

Conclusions

We hypothesize that discordance of ORF1p expression in parts of the brain reported to display elevated levels of retrotransposition may suggest the existence of factors mediating post-translational regulation of L1 activity in the human brain. Furthermore, this antibody reagent will be useful as a complementary means to confirm findings related to retrotransposon biology and activity in the brain and other tissues in vivo.

Electronic supplementary material

The online version of this article (10.1186/s13100-017-0101-4) contains supplementary material, which is available to authorized users.

p53 controls expression of the DNA deaminase APOBEC3B to limit its potential mutagenic activity in cancer cells

Cancer genome sequencing has implicated the cytosine deaminase activity of apolipoprotein B mRNA editing enzyme catalytic polypeptide-like (APOBEC) genes as an important source of mutations in diverse cancers, with APOBEC3B (A3B) expression especially correlated with such cancer mutations. To better understand the processes directing A3B over-expression in cancer, and possible therapeutic avenues for targeting A3B, we have investigated the regulation of A3B gene expression. Here, we show that A3B expression is inversely related to p53 status in different cancer types and demonstrate that this is due to a direct and pivotal role for p53 in repressing A3B expression. This occurs through the induction of p21 (CDKN1A) and the recruitment of the repressive DREAM complex to the A3B gene promoter, such that loss of p53 through mutation, or human papilloma virus-mediated inhibition, prevents recruitment of the complex, thereby causing elevated A3B expression and cytosine deaminase activity in cancer cells. As p53 is frequently mutated in cancer, our findings provide a mechanism by which p53 loss can promote cancer mutagenesis.

Structural determinants of APOBEC3B non-catalytic domain for molecular assembly and catalytic regulation

The catalytic activity of human cytidine deaminase APOBEC3B (A3B) has been correlated with kataegic mutational patterns within multiple cancer types. The molecular basis of how the N-terminal non-catalytic CD1 regulates the catalytic activity and consequently, biological function of A3B remains relatively unknown. Here, we report the crystal structure of a soluble human A3B-CD1 variant and delineate several structural elements of CD1 involved in molecular assembly, nucleic acid interactions and catalytic regulation of A3B. We show that (i) A3B expressed in human cells exists in hypoactive high-molecular-weight (HMW) complexes, which can be activated without apparent dissociation into low-molecular-weight (LMW) species after RNase A treatment. (ii) Multiple surface hydrophobic residues of CD1 mediate the HMW complex assembly and affect the catalytic activity, including one tryptophan residue W127 that likely acts through regulating nucleic acid binding. (iii) One of the highly positively charged surfaces on CD1 is involved in RNA-dependent attenuation of A3B catalysis. (iv) Surface hydrophobic residues of CD1 are involved in heterogeneous nuclear ribonucleoproteins (hnRNPs) binding to A3B. The structural and biochemical insights described here suggest that unique structural features on CD1 regulate the molecular assembly and catalytic activity of A3B through distinct mechanisms.

Genetic alterations in adult T-cell leukemia/lymphoma

Adult T‐cell leukemia/lymphoma (ATL) is a peripheral T‐cell neoplasm with a dismal prognosis. It is caused by human T‐cell leukemia virus type‐1 (HTLV‐1) retrovirus. A long latency period from HTLV‐1 infection to ATL onset suggests that not only HTLV‐1 proteins, such as Tax and HBZ, but also additional genetic and/or epigenetic events are required for ATL development. Although many studies have demonstrated the biological functions of viral genes, alterations of cellular genes associated with ATL have not been fully investigated. Recently, a large‐scale integrated genetic analysis revealed the entire landscape of somatic aberrations in ATL. This neoplasm is characterized by frequent gain‐of‐function alterations in components of the T‐cell receptor/NF‐κB signaling pathway, including activating mutations in the PLCG1,PRKCB,CARD11 and VAV1 genes, and CTLA4‐CD28 and ICOS‐CD28 fusions. Importantly, molecules associated with immune surveillance, such as HLA‐A/B,CD58 and FAS, are affected recurrently. Among them, one notable lesion occurs as frequent structural variations that truncate the PD‐L1 3′‐untranslated region, leading to its overexpression. Other genetic targets include transcription factors (IRF4,IKZF2, and GATA3) and chemokine receptors (CCR4,CCR7 and GPR183), which are functionally relevant in normal T cells. A substantial proportion of ATL cases show widespread accumulation of repressive epigenetic changes, such as trimethylation of histone H3 lysine 27 and DNA hypermethylation of CpG islands, which coordinately modulate multiple pathways, including Cys2‐His2 zinc finger genes involved in silencing retroelements. Here we review the current understanding of the genetic/epigenetic aberrations in ATL, focusing on their relevance in its molecular pathogenesis.

Pathways of DNA Demethylation

The regulation of the genome relies on the epigenome to instruct, define and restrict the activities of growth and development. Among the cohort of epigenetic instructions, DNA methylation is perhaps the best understood. In most mammals, cycles of the addition and removal of DNA methylation constitute phases of reprogramming when the developing embryo must negotiate lineage defining and developmental commitment events. In these instances, the DNA methylation instruction is often removed, thereby allowing a change in permission for future development and a return to a more plastic and pluripotent state. Because of this, the germ line, upon demethylation, can give rise to gametes that are fully functional across generations and poised for totipotency. This return to a less differentiated state can also be achieved experimentally. The loss of DNA methylation constitutes one of the significant barriers to induced pluripotency and is a prerequisite for the generation of iPS cells. Taking fully differentiated cells, such as skin cells, and turning back the developmental clock heralded a technological breakthrough discovery in 2006 (Takahashi and Yamanaka 2006) with unprecedented promise in regenerative medicine. In this chapter, the mechanistic possibilities for DNA demethylation will be described in the context of natural and experimentally induced epigenetic reprogramming. The balance of the maintenance of this heritable mark together with its timely removal is essential for lifelong health and may be a key in our understanding of ageing.

APOBEC3B, a molecular driver of mutagenesis in human cancers

Human cancers results in large part from the accumulation of multiple mutations. The progression of premalignant cells is an evolutionary process in which mutations provide the fundamental driving force for genetic diversity. The increased mutation rate in premalignant cells allows selection for increased proliferation and survival and ultimately leads to invasion, metastasis, recurrence, and therapeutic resistance. Therefore, it is important to understand the molecular determinants of the mutational processes. Recent genome-wide sequencing data showed that apolipoprotein B mRNA editing catalytic polypeptide-like 3B (APOBEC3B) is a key molecular driver inducing mutations in multiple human cancers. APOBEC3B, a DNA cytosine deaminase, is overexpressed in a wide spectrum of human cancers. Its overexpression and aberrant activation lead to unexpected clusters of mutations in the majority of cancers. This phenomenon of clustered mutations, termed kataegis (from the Greek word for showers), forms unique mutation signatures. In this review, we will discuss the biological function of APOBEC3B, its tumorigenic role in promoting mutational processes in cancer development and the clinical potential to develop novel therapeutics by targeting APOBEC3B.

Crystal structure of APOBEC3A bound to single-stranded DNA reveals structural basis for cytidine deamination and specificity

Nucleic acid editing enzymes are essential components of the immune system that lethally mutate viral pathogens and somatically mutate immunoglobulins, and contribute to the diversification and lethality of cancers. Among these enzymes are the seven human APOBEC3 deoxycytidine deaminases, each with unique target sequence specificity and subcellular localization. While the enzymology and biological consequences have been extensively studied, the mechanism by which APOBEC3s recognize and edit DNA remains elusive. Here we present the crystal structure of a complex of a cytidine deaminase with ssDNA bound in the active site at 2.2 Å. This structure not only visualizes the active site poised for catalysis of APOBEC3A, but pinpoints the residues that confer specificity towards CC/TC motifs. The APOBEC3A-ssDNA complex defines the 5'-3' directionality and subtle conformational changes that clench the ssDNA within the binding groove, revealing the architecture and mechanism of ssDNA recognition that is likely conserved among all polynucleotide deaminases, thereby opening the door for the design of mechanistic-based therapeutics.

Intersection of diverse neuronal genomes and neuropsychiatric disease: The Brain Somatic Mosaicism Network

Neuropsychiatric disorders have a complex genetic architecture. Human genetic population-based studies have identified numerous heritable sequence and structural genomic variants associated with susceptibility to neuropsychiatric disease. However, these germline variants do not fully account for disease risk. During brain development, progenitor cells undergo billions of cell divisions to generate the ~80 billion neurons in the brain. The failure to accurately repair DNA damage arising during replication, transcription, and cellular metabolism amid this dramatic cellular expansion can lead to somatic mutations. Somatic mutations that alter subsets of neuronal transcriptomes and proteomes can, in turn, affect cell proliferation and survival and lead to neurodevelopmental disorders. The long life span of individual neurons and the direct relationship between neural circuits and behavior suggest that somatic mutations in small populations of neurons can significantly affect individual neurodevelopment. The Brain Somatic Mosaicism Network has been founded to study somatic mosaicism both in neurotypical human brains and in the context of complex neuropsychiatric disorders.

Evidence for APOBEC3B mRNA and protein expression in oral squamous cell carcinomas

It has been demonstrated that APOBEC3B possesses cytidine deaminase activity, which is likely to result in C-to-T signature mutations. Increased expression of the APOBEC3B gene has been shown to correlate with higher incidence of such mutations in various cancer types, such as breast, bladder, lung, and head and neck carcinomas. In the current study, we used in silico methods, immunohistochemistry and qRT-PCR to detect the presence of APOBEC3B signature mutations and examine the levels and patterns of APOBEC3B expression in oral squamous cell carcinomas (OSCCs). Using the Cancer Genome Atlas (TCGA) database, we have found a high incidence of C-to-T transitions in head and neck squamous cell carcinomas (HNSCCs), of which OSCCs constitute the largest subgroup. Additionally, we compared APOBEC3B expression, at both mRNA and protein level, between OSCCs and non-cancerous samples. APOBEC3B was detected in both groups, but nuclear localization was consistent only in normal oral cells. APOBEC3B mRNA levels were clearly higher in OSCCs than in controls. These results suggest that while in normal oral cells APOBEC3B has an important nuclear function to fulfill, this activity may be hindered in a subgroup of tumor cells, due to the more prominent localization of the enzyme in the cytoplasm.

The potential role of retroviruses in autoimmunity

In the last 20 years research in Immunology underwent fundamental changes. Most importantly, the identification of the key role of innate immune pattern recognition receptors (PRRs) that recognize evolutionarily conserved molecular patterns on infectious pathogens. This results in priming of innate immune cells, which in turn activate and direct the adaptive immune response. Progress in innate immune recognition instigated the current working hypothesis, that recognition of endogenous ligands by PRRs results in innate immune cell activation (autoinflammation) or activation of adaptive cells, with self-reactive antigen receptors (autoimmunity). In particular, nucleic acid-sensing innate immune receptors seem to be prime candidates for a mechanistic understanding of autoreactive activation of the immune system. However, it remains uncertain what the actual source of nucleic acid ligands is and what other signals are needed to drive activation of autoreactive innate immune cells and break self-tolerance of the adaptive immune system. Here, I will review our present understanding about whether the infection with exogenous retroviruses or the reactivation of endogenous retroviruses might play an etiological role in certain autoimmune conditions of humans and murine experimental models.

Clusters of Multiple Mutations: Incidence and Molecular Mechanisms

It has been long understood that mutation distribution is not completely random across genomic space and in time. Indeed, recent surprising discoveries identified multiple simultaneous mutations occurring in tiny regions within chromosomes while the rest of the genome remains relatively mutation-free. Mechanistic elucidation of these phenomena, called mutation showers, mutation clusters, or kataegis, in parallel with findings of abundant clustered mutagenesis in cancer genomes, is ongoing. So far, the combination of factors most important for clustered mutagenesis is the induction of DNA lesions within unusually long and persistent single-strand DNA intermediates. In addition to being a fascinating phenomenon, clustered mutagenesis also became an indispensable tool for identifying a previously unrecognized major source of mutation in cancer, APOBEC cytidine deaminases. Future research on clustered mutagenesis may shed light onto important mechanistic details of genome maintenance, with potentially profound implications for human health.

Chemical proteomic map of dimethyl fumarate-sensitive cysteines in primary human T cells

Dimethyl fumarate (DMF) is an electrophilic drug that is used to treat autoimmune conditions, including multiple sclerosis and psoriasis. The mechanism of action of DMF is unclear but may involve the covalent modification of proteins or DMF serving as a prodrug that is converted to monomethyl fumarate (MMF). We found that DMF, but not MMF, blocked the activation of primary human and mouse T cells. Using a quantitative, site-specific chemical proteomic platform, we determined the DMF sensitivity of >2400 cysteine residues in human T cells. Cysteines sensitive to DMF, but not MMF, were identified in several proteins with established biochemical or genetic links to T cell function, including protein kinase Cθ (PKCθ). DMF blocked the association of PKCθ with the costimulatory receptor CD28 by perturbing a CXXC motif in the C2 domain of this kinase. Mutation of these DMF-sensitive cysteines also impaired PKCθ-CD28 interactions and T cell activation, designating the C2 domain of PKCθ as a key functional, electrophile-sensing module important for T cell biology.

Contribution of type W human endogenous retroviruses to the human genome: characterization of HERV-W proviral insertions and processed pseudogenes

Background

Human endogenous retroviruses (HERVs) are ancient sequences integrated in the germ line cells and vertically transmitted through the offspring constituting about 8 % of our genome. In time, HERVs accumulated mutations that compromised their coding capacity. A prominent exception is HERV-W locus 7q21.2, producing a functional Env protein (Syncytin-1) coopted for placental syncytiotrophoblast formation. While expression of HERV-W sequences has been investigated for their correlation to disease, an exhaustive description of the group composition and characteristics is still not available and current HERV-W group information derive from studies published a few years ago that, of course, used the rough assemblies of the human genome available at that time. This hampers the comparison and correlation with current human genome assemblies.

Results

In the present work we identified and described in detail the distribution and genetic composition of 213 HERV-W elements. The bioinformatics analysis led to the characterization of several previously unreported features and provided a phylogenetic classification of two main subgroups with different age and structural characteristics. New facts on HERV-W genomic context of insertion and co-localization with sequences putatively involved in disease development are also reported.

Conclusions

The present work is a detailed overview of the HERV-W contribution to the human genome and provides a robust genetic background useful to clarify HERV-W role in pathologies with poorly understood etiology, representing, to our knowledge, the most complete and exhaustive HERV-W dataset up to date.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-016-0301-x) contains supplementary material, which is available to authorized users.

APOBEC3B deletion impacts on susceptibility to acquire HIV-1 and its advancement among individuals in western India

APOBEC3B deletion polymorphism has been associated with risk of HIV-1 acquisition and its progression. Therefore, we aimed to investigate the association of APOBEC3B ins/del polymorphism with risk of acquisition of HIV-1 and its progression. In the present case-control study, we enrolled a total of 150 HIV-infected individuals and 150 healthy controls. Polymorphism for APOBEC3B gene was genotyped by PCR. APOBEC3B ID, DD genotypes, and D allele were associated with higher risk of acquisition of HIV-1 (p = 0.004, OR = 4.96; p = 0.03, OR = 3.55; and p = 0.004; OR = 1.60). The individuals with ID genotypes and combined genotype ID+DD of APOBEC3B in the presence of tobacco and alcohol showed the higher risk of advancement of HIV disease; however, risk could not reach statistical significance (OR = 1.14, 95% CI: 0.59-2.18; OR = 1.33, 95% CI: 0.83-2.15 and OR = 1.44, 95% CI: 0.77-2.69; OR = 1.50, 95% CI: 0.94-2.40). Individuals in advanced HIV disease stage and ID genotype and combined genotype ID + DD of APOBEC3B were more likely to be associated with advanced HIV disease stage but risk could not reach significant (OR = 1.50, 95% CI: 0.94-2.40; OR = 1.27, 95% CI: 0.88-1.84). Individuals with ID and DD genotype of APOBEC3B had influence on susceptibility to acquisition of HIV-1. This suggests that APOBEC3B deletion may attenuate innate cellular immunity against HIV-1 and thus confer the host persistence for HIV infection.

Restricting retrotransposons: a review

Retrotransposons have generated about 40 % of the human genome. This review examines the strategies the cell has evolved to coexist with these genomic "parasites", focussing on the non-long terminal repeat retrotransposons of humans and mice. Some of the restriction factors for retrotransposition, including the APOBECs, MOV10, RNASEL, SAMHD1, TREX1, and ZAP, also limit replication of retroviruses, including HIV, and are part of the intrinsic immune system of the cell. Many of these proteins act in the cytoplasm to degrade retroelement RNA or inhibit its translation. Some factors act in the nucleus and involve DNA repair enzymes or epigenetic processes of DNA methylation and histone modification. RISC and piRNA pathway proteins protect the germline. Retrotransposon control is relaxed in some cell types, such as neurons in the brain, stem cells, and in certain types of disease and cancer, with implications for human health and disease. This review also considers potential pitfalls in interpreting retrotransposon-related data, as well as issues to consider for future research.

Immune responses to endogenous retroelements: taking the bad with the good

The ultimate form of parasitism and evasion of host immunity is for the parasite genome to enter the germ line of the host species. Retroviruses have invaded the host germ line on the grandest scale, and this is evident in the extraordinary abundance of endogenous retroelements in the genome of all vertebrate species that have been studied. Many of these endogenous retroelements have retained viral characteristics; some also the capacity to replicate and, consequently, the potential to trigger host innate and adaptive immune responses. However, although retroelements are mainly recognized for their pathogenic potential, recent evidence suggests that this 'enemy within' may also have beneficial roles in tuning host immune reactivity. In this Review, we discuss how the immune system recognizes and is shaped by endogenous retroelements.

Alpha-1-antitrypsin interacts with gp41 to block HIV-1 entry into CD4+ T lymphocytes

Background

Study of a clinic case reveals that alpha-1-antitrypsin (AAT) deficiency is related to CD4+ T cell count decline and AIDS progression, suggesting that AAT might be an endogenous inhibitor of HIV/AIDS. Previous study shows that AAT inhibits HIV-1 replication in infected host cells and the C-terminus fragment of AAT, VIRIP, interferes with HIV-1 infection. However, it is still unclear whether and how intact AAT inhibits HIV-1 infection. It is also unknown what the mechanism of AAT is and which critical step(s) are involved.

Results

In the present study, the C-terminus of AAT (C) was synthesized. C terminus-truncated AAT (ΔAAT) was also prepared by digesting AAT with metalloproteinase. Primary CD4+ T cells were then co-cultured with HIV-1 with the presence or absence of AAT/C/ΔAAT to detect cis-infection of HIV-1. The interaction between AAT/C/ΔAAT and gp120/gp41 was also measured. Meanwhile, HIV-1 reverse transcriptase activity and viral DNA integration were also detected in these lymphocytes. The results demonstrated that AAT and C, not ΔAAT, inhibited HIV-1 entry by directly interacting with gp41. Meanwhile, AAT, C and ΔAAT could not directly interfere with the steps of viral RNA reverse transcription and viral DNA integration.

Conclusion

AAT inhibits HIV-1 entry by directly interacting with gp41 through its C-terminus and thereby inhibits HIV-1 infection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-016-0751-2) contains supplementary material, which is available to authorized users.

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.

APOBEC4 Enhances the Replication of HIV-1

APOBEC4 (A4) is a member of the AID/APOBEC family of cytidine deaminases. In this study we found a high mRNA expression of A4 in human testis. In contrast, there were only low levels of A4 mRNA detectable in 293T, HeLa, Jurkat or A3.01 cells. Ectopic expression of A4 in HeLa cells resulted in mostly cytoplasmic localization of the protein. To test whether A4 has antiviral activity similar to that of proteins of the APOBEC3 (A3) subfamily, A4 was co-expressed in 293T cells with wild type HIV-1 and HIV-1 luciferase reporter viruses. We found that A4 did not inhibit the replication of HIV-1 but instead enhanced the production of HIV-1 in a dose-dependent manner and seemed to act on the viral LTR. A4 did not show detectable cytidine deamination activity in vitro and weakly interacted with single-stranded DNA. The presence of A4 in virus producer cells enhanced HIV-1 replication by transiently transfected A4 or stably expressed A4 in HIV-susceptible cells. APOBEC4 was capable of similarly enhancing transcription from a broad spectrum of promoters, regardless of whether they were viral or mammalian. We hypothesize that A4 may have a natural role in modulating host promoters or endogenous LTR promoters.

Nuclear Magnetic Resonance Structure of the APOBEC3B Catalytic Domain: Structural Basis for Substrate Binding and DNA Deaminase Activity

Human APOBEC3B (A3B) is a member of the APOBEC3 (A3) family of cytidine deaminases, which function as DNA mutators and restrict viral pathogens and endogenous retrotransposons. Recently, A3B was identified as a major source of genetic heterogeneity in several human cancers. Here, we determined the solution nuclear magnetic resonance structure of the catalytically active C-terminal domain (CTD) of A3B and performed detailed analyses of its deaminase activity. The core of the structure comprises a central five-stranded β-sheet with six surrounding helices, common to all A3 proteins. The structural fold is most similar to that of A3A and A3G-CTD, with the most prominent difference being found in loop 1. The catalytic activity of A3B-CTD is ∼15-fold lower than that of A3A, although both exhibit a similar pH dependence. Interestingly, A3B-CTD with an A3A loop 1 substitution had significantly increased deaminase activity, while a single-residue change (H29R) in A3A loop 1 reduced A3A activity to the level seen with A3B-CTD. This establishes that loop 1 plays an important role in A3-catalyzed deamination by precisely positioning the deamination-targeted C into the active site. Overall, our data provide important insights into the determinants of the activities of individual A3 proteins and facilitate understanding of their biological function.

Clustered mutations in hominid genome evolution are consistent with APOBEC3G enzymatic activity

The gradual accumulation of mutations by any of a number of mutational processes is a major driving force of divergence and evolution. Here, we investigate a potentially novel mutational process that is based on the activity of members of the AID/APOBEC family of deaminases. This gene family has been recently shown to introduce—in multiple types of cancer—enzyme-induced clusters of co-occurring somatic mutations caused by cytosine deamination. Going beyond somatic mutations, we hypothesized that APOBEC3—following its rapid expansion in primates—can introduce unique germline mutation clusters that can play a role in primate evolution. In this study, we tested this hypothesis by performing a comprehensive comparative genomic screen for APOBEC3-induced mutagenesis patterns across different hominids. We detected thousands of mutation clusters introduced along primate evolution which exhibit features that strongly fit the known patterns of APOBEC3G mutagenesis. These results suggest that APOBEC3G-induced mutations have contributed to the evolution of all genomes we studied. This is the first indication of site-directed, enzyme-induced genome evolution, which played a role in the evolution of both modern and archaic humans. This novel mutational mechanism exhibits several unique features, such as its higher tendency to mutate transcribed regions and regulatory elements and its ability to generate clusters of concurrent point mutations that all occur in a single generation. Our discovery demonstrates the exaptation of an anti-viral mechanism as a new source of genomic variation in hominids with a strong potential for functional consequences.

Increased expression with differential subcellular location of cytidine deaminase APOBEC3G in human CD4(+) T-cell activation and dendritic cell maturation

APOBEC3G (apolipoprotein B mRNA editing enzyme catalytic polypeptide-like 3G; A3G) is an innate defense protein showing activity against retroviruses and retrotransposons. Activated CD4(+) T cells are highly permissive for HIV-1 replication, whereas resting CD4(+) T cells are refractory. Dendritic cells (DCs), especially mature DCs, are also refractory. We investigated whether these differences could be related to a differential A3G expression and/or subcellular distribution. We found that A3G mRNA and protein expression is very low in resting CD4(+) T cells and immature DCs, but increases strongly following T-cell activation and DC maturation. The Apo-7 anti-A3G monoclonal antibody (mAb), which was specifically developed, confirmed these differences at the protein level and disclosed that A3G is mainly cytoplasmic in resting CD4(+) T cells and immature DCs. Nevertheless, A3G translocates to the nucleus in activated-proliferating CD4(+) T cells, yet remaining cytoplasmic in matured DCs, a finding confirmed by immunoblotting analysis of cytoplasmic and nuclear fractions. Apo-7 mAb was able to immunoprecipitate endogenous A3G allowing to detect complexes with numerous proteins in activated-proliferating but not in resting CD4(+) T cells. The results show for the first time the nuclear translocation of A3G in activated-proliferating CD4(+) T cells.

The Influence of LINE-1 and SINE Retrotransposons on Mammalian Genomes

Transposable elements have had a profound impact on the structure and function of mammalian genomes. The retrotransposon Long INterspersed Element-1 (LINE-1 or L1), by virtue of its replicative mobilization mechanism, comprises ∼17% of the human genome. Although the vast majority of human LINE-1 sequences are inactive molecular fossils, an estimated 80-100 copies per individual retain the ability to mobilize by a process termed retrotransposition. Indeed, LINE-1 is the only active, autonomous retrotransposon in humans and its retrotransposition continues to generate both intra-individual and inter-individual genetic diversity. Here, we briefly review the types of transposable elements that reside in mammalian genomes. We will focus our discussion on LINE-1 retrotransposons and the non-autonomous Short INterspersed Elements (SINEs) that rely on the proteins encoded by LINE-1 for their mobilization. We review cases where LINE-1-mediated retrotransposition events have resulted in genetic disease and discuss how the characterization of these mutagenic insertions led to the identification of retrotransposition-competent LINE-1s in the human and mouse genomes. We then discuss how the integration of molecular genetic, biochemical, and modern genomic technologies have yielded insight into the mechanism of LINE-1 retrotransposition, the impact of LINE-1-mediated retrotransposition events on mammalian genomes, and the host cellular mechanisms that protect the genome from unabated LINE-1-mediated retrotransposition events. Throughout this review, we highlight unanswered questions in LINE-1 biology that provide exciting opportunities for future research. Clearly, much has been learned about LINE-1 and SINE biology since the publication of Mobile DNA II thirteen years ago. Future studies should continue to yield exciting discoveries about how these retrotransposons contribute to genetic diversity in mammalian genomes.

Role of APOBEC3F Gene Variation in HIV-1 Disease Progression and Pneumocystis Pneumonia

Human APOBEC3 cytidine deaminases are intrinsic resistance factors to HIV-1. However, HIV-1 encodes a viral infectivity factor (Vif) that degrades APOBEC3 proteins. In vitro APOBEC3F (A3F) anti-HIV-1 activity is weaker than A3G but is partially resistant to Vif degradation unlike A3G. It is unknown whether A3F protein affects HIV-1 disease in vivo. To assess the effect of A3F gene on host susceptibility to HIV- acquisition and disease progression, we performed a genetic association study in six well-characterized HIV-1 natural cohorts. A common six-Single Nucleotide Polymorphism (SNP) haplotype of A3F tagged by a codon-changing variant (p. I231V, with allele (V) frequency of 48% in European Americans) was associated with significantly lower set-point viral load and slower rate of progression to AIDS (Relative Hazards (RH) = 0.71, 95% CI: 0.56, 0.91) and delayed development of pneumocystis pneumonia (PCP) (RH = 0.53, 95% CI: 0.37-0.76). A validation study in the International Collaboration for the Genomics of HIV (ICGH) showed a consistent association with lower set-point viral load. An in vitro assay revealed that the A3F I231V variant may influence Vif mediated A3F degradation. Our results provide genetic epidemiological evidence that A3F modulates HIV-1/AIDS disease progression.

Strand-biased cytosine deamination at the replication fork causes cytosine to thymine mutations in Escherichia coli

The rate of cytosine deamination is much higher in single-stranded DNA (ssDNA) than in double-stranded DNA, and copying the resulting uracils causes C to T mutations. To study this phenomenon, the catalytic domain of APOBEC3G (A3G-CTD), an ssDNA-specific cytosine deaminase, was expressed in an Escherichia coli strain defective in uracil repair (ung mutant), and the mutations that accumulated over thousands of generations were determined by whole-genome sequencing. C:G to T:A transitions dominated, with significantly more cytosines mutated to thymine in the lagging-strand template (LGST) than in the leading-strand template (LDST). This strand bias was present in both repair-defective and repair-proficient cells and was strongest and highly significant in cells expressing A3G-CTD. These results show that the LGST is accessible to cellular cytosine deaminating agents, explains the well-known GC skew in microbial genomes, and suggests the APOBEC3 family of mutators may target the LGST in the human genome.

The dynamic genome: transposons and environmental adaptation in the nervous system

Classically thought as genomic clutter, the functional significance of transposable elements (TEs) has only recently become a focus of attention in neuroscience. Increasingly, studies have demonstrated that the brain seems to have more retrotransposition and TE transcription relative to other somatic tissues, suggesting a unique role for TEs in the central nervous system. TE expression and transposition also appear to vary by brain region and change in response to environmental stimuli such as stress. TEs appear to serve a number of adaptive roles in the nervous system. The regulation of TE expression by steroid, epigenetic and other mechanisms in interplay with the environment represents a significant and novel avenue to understanding both normal brain function and disease.

Exosomes: From Functions in Host-Pathogen Interactions and Immunity to Diagnostic and Therapeutic Opportunities

Since their first description in the 1980s, exosomes, small endosomal-derived extracellular vesicles, have been involved in innate and adaptive immunity through modulating immune responses and mediating antigen presentation. Increasing evidence has reported the role of exosomes in host-pathogen interactions and particularly in the activation of antimicrobial immune responses. The growing interest concerning exosomes in infectious diseases, their accessibility in various body fluids, and their capacity to convey a rich content (e.g., proteins, lipids, and nucleic acids) to distant recipient cells led the scientific community to consider the use of exosomes as potential new diagnostic and therapeutic tools. In this review, we summarize current understandings of exosome biogenesis and their composition and highlight the function of exosomes as immunomodulators in pathological states such as in infectious disorders. The potential of using exosomes as diagnostic and therapeutic tools is also discussed.

Inhibition of APOBEC3G activity impedes double-stranded DNA repair

The cellular cytidine deaminase APOBEC3G (A3G) was first described as an anti-HIV-1 restriction factor, acting by directly deaminating reverse transcripts of the viral genome. HIV-1 Vif neutralizes the activity of A3G, primarily by mediating degradation of A3G to establish effective infection in host target cells. Lymphoma cells, which express high amounts of A3G, can restrict Vif-deficient HIV-1. Interestingly, these cells are more stable in the face of treatments that result in double-stranded DNA damage, such as ionizing radiation and chemotherapies. Previously, we showed that the Vif-derived peptide (Vif25-39) efficiently inhibits A3G deamination, and increases the sensitivity of lymphoma cells to ionizing radiation. In the current study, we show that additional peptides derived from Vif, A3G, and APOBEC3F, which contain the LYYF motif, inhibit deamination activity. Each residue in the Vif25-39 sequence moderately contributes to the inhibitory effect, whereas replacing a single residue in the LYYF motif completely abrogates inhibition of deamination. Treatment of A3G-expressing lymphoma cells exposed to ionizing radiation with the new inhibitory peptides reduces double-strand break repair after irradiation. Incubation of cultured irradiated lymphoma cells with peptides that inhibit double-strand break repair halts their propagation. These results suggest that A3G may be a potential therapeutic target that is amenable to peptide and peptidomimetic inhibition.

A method to avoid errors associated with the analysis of hypermutated viral sequences by alignment-based methods

The human genome encodes for a family of editing enzymes known as APOBEC3 (apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like3). They induce context dependent G-to-A changes, referred to as "hypermutation", in the genome of viruses such as HIV, SIV, HBV and endogenous retroviruses. Hypermutation is characterized by aligning affected sequences to a reference sequence. We show that indels (insertions/deletions) in the sequences lead to an incorrect assignment of APOBEC3 targeted and non-target sites. This can result in an incorrect identification of hypermutated sequences and erroneous biological inferences made based on hypermutation analysis.

The external domains of the HIV-1 envelope are a mutational cold spot

In RNA viruses, mutations occur fast and have large fitness effects. While this affords remarkable adaptability, it can also endanger viral survival due to the accumulation of deleterious mutations. How RNA viruses reconcile these two opposed facets of mutation is still unknown. Here we show that, in human immunodeficiency virus (HIV-1), spontaneous mutations are not randomly located along the viral genome. We find that the viral mutation rate experiences a threefold reduction in the region encoding the most external domains of the viral envelope, which are strongly targeted by neutralizing antibodies. This contrasts with the hypermutation mechanisms deployed by other, more slowly mutating pathogens such as DNA viruses and bacteria, in response to immune pressure. We show that downregulation of the mutation rate in HIV-1 is exerted by the template RNA through changes in sequence context and secondary structure, which control the activity of apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3 (A3)-mediated cytidine deamination and the fidelity of the viral reverse transcriptase.

Mutations allow RNA virus to adapt fast but also entail fitness costs. Geller et al. show that, in HIV-1, mutations occur three times less often in the most external domains of the envelope, and that this is due to changes in RNA sequence context and structure, which control viral and host-encoded mutational mechanisms.

Integrated molecular analysis of adult T cell leukemia/lymphoma

Adult T cell leukemia/lymphoma (ATL) is a peripheral T cell neoplasm of largely unknown genetic basis, associated with human T cell leukemia virus type-1 (HTLV-1) infection. Here we describe an integrated molecular study in which we performed whole-genome, exome, transcriptome and targeted resequencing, as well as array-based copy number and methylation analyses, in a total of 426 ATL cases. The identified alterations overlap significantly with the HTLV-1 Tax interactome and are highly enriched for T cell receptor-NF-κB signaling, T cell trafficking and other T cell-related pathways as well as immunosurveillance. Other notable features include a predominance of activating mutations (in PLCG1, PRKCB, CARD11, VAV1, IRF4, FYN, CCR4 and CCR7) and gene fusions (CTLA4-CD28 and ICOS-CD28). We also discovered frequent intragenic deletions involving IKZF2, CARD11 and TP73 and mutations in GATA3, HNRNPA2B1, GPR183, CSNK2A1, CSNK2B and CSNK1A1. Our findings not only provide unique insights into key molecules in T cell signaling but will also guide the development of new diagnostics and therapeutics in this intractable tumor.

The developmental control of transposable elements and the evolution of higher species

Transposable elements (TEs) account for at least 50% of the human genome. They constitute essential motors of evolution through their ability to modify genomic architecture, mutate genes and regulate gene expression. Accordingly, TEs are subject to tight epigenetic control during the earliest phases of embryonic development via histone and DNA methylation. Key to this process is recognition by sequence-specific RNA- and protein-based repressors. Collectively, these mediators are responsible for silencing a very broad range of TEs in an evolutionarily dynamic fashion. As a consequence, mobile elements and their controllers exert a marked influence on transcriptional networks in embryonic stem cells and a variety of adult tissues. The emerging picture is not that of a simple arms race but rather of a massive and sophisticated enterprise of TE domestication for the evolutionary benefit of the host.

Endogenous Retroviruses in the Genomics Era

Endogenous retroviruses comprise millions of discrete genetic loci distributed within the genomes of extant vertebrates. These sequences, which are clearly related to exogenous retroviruses, represent retroviral infections of the deep past, and their abundance suggests that retroviruses were a near-constant presence throughout the evolutionary history of modern vertebrates. Endogenous retroviruses contribute in myriad ways to the evolution of host genomes, as mutagens and as sources of genetic novelty (both coding and regulatory) to be acted upon by the twin engines of random genetic drift and natural selection. Importantly, the richness and complexity of endogenous retrovirus data can be used to understand how viruses spread and adapt on evolutionary timescales by combining population genetics and evolutionary theory with a detailed understanding of retrovirus biology (gleaned from the study of extant retroviruses). In addition to revealing the impact of viruses on organismal evolution, such studies can help us better understand, by looking back in time, how life-history traits, as well as ecological and geological events, influence the movement of viruses within and between populations.

Recent Insights into the Control of Human Papillomavirus (HPV) Genome Stability, Loss, and Degradation

Most human papillomavirus (HPV) antiviral strategies have focused upon inhibiting viral DNA replication, but it is increasingly apparent that viral DNA levels can be chemically controlled by approaches that promote its instability. HPVs and other DNA viruses have a tenuous relationship with their hosts. They must replicate and hide from the DNA damage response (DDR) and innate immune systems, which serve to protect cells from foreign or “non-self” DNA, and yet they draft these same systems to support their life cycles. DNA binding antiviral agents promoting massive viral DNA instability and elimination are reviewed. Mechanistic studies of these agents have identified genetic antiviral enhancers and repressors, antiviral sensitizers, and host cell elements that protect and stabilize HPV genomes. Viral DNA degradation appears to be an important means of controlling HPV DNA levels in some cases, but the underlying mechanisms remain poorly understood. These findings may prove useful not only for understanding viral DNA persistence but only in devising future antiviral strategies.

Incomplete APOBEC3G/F Neutralization by HIV-1 Vif Mutants Facilitates the Genetic Evolution from CCR5 to CXCR4 Usage

Incomplete APOBEC3G/F neutralization by a defective HIV-1Vif protein can promote genetic diversification by inducing G-to-A mutations in the HIV-1 genome. The HIV-1 Env V3 loop, critical for coreceptor usage, contains several putative APOBEC3G/F target sites. Here, we determined if APOBEC3G/F, in the presence of Vif-defective HIV-1 virus, can induce G-to-A mutations at V3 positions critical to modulation of CXCR4 usage. Peripheral blood mononuclear cells (PBMC) and monocyte-derived macrophages (MDM) from 2 HIV-1-negative donors were infected with CCR5-using 81.A-VifWT virus (i.e., with wild-type [WT] Vif protein), 81.A-VifE45G, or 81.A-VifK22E (known to incompletely/partially neutralize APOBEC3G/F). The rate of G-toA mutations was zero or extremely low in 81.A-VifWT- and 81.A-VifE45G-infected PBMC from both donors. Conversely, G-to-A enrichment was detected in 81.A-VifK22E-infected PBMC (prevalence ranging from 2.18% at 7 days postinfection [dpi] to 3.07% at 21 dpi in donor 1 and from 10.49% at 7 dpi to 8.69% at 21 dpi in donor 2). A similar scenario was found in MDM. G-to-A mutations occurred at 8 V3 positions, resulting in nonsynonymous amino acid substitutions. Of them, G24E and E25K strongly correlated with phenotypically/genotypically defined CXCR4-using viruses (P = 0.04 and 5.5e-7, respectively) and increased the CXCR4 N-terminal binding affinity for V3 (WT, -40.1 kcal/mol; G24E, -510 kcal/mol; E25K, -522 kcal/mol). The analysis of paired V3 and Vif DNA sequences from 84 HIV-1-infected patients showed that the presence of a Vif-defective virus correlated with CXCR4 usage in proviral DNA (P = 0.04). In conclusion, incomplete APOBEC3G/F neutralization by a single Vif amino acid substitution seeds a CXCR4-using proviral reservoir. This can have implications for the success of CCR5 antagonist-based therapy, as well as for the risk of disease progression.

DNA cytosine and methylcytosine deamination by APOBEC3B: enhancing methylcytosine deamination by engineering APOBEC3B

APOBEC (apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like) is a family of enzymes that deaminates cytosine (C) to uracil (U) on nucleic acid. APOBEC3B (A3B) functions in innate immunity against intrinsic and invading retroelements and viruses. A3B can also induce genomic DNA mutations to cause cancer. A3B contains two cytosine deaminase domains (CD1, CD2), and there are conflicting reports about whether both domains are active. Here we demonstrate that only CD2 of A3B (A3BCD2) has C deamination activity. We also reveal that both A3B and A3BCD2 can deaminate methylcytosine (mC). Guided by structural and functional analysis, we successfully engineered A3BCD2 to gain over two orders of magnitude higher activity for mC deamination. Important determinants that contribute to the activity and selectivity for mC deamination have been identified, which reveals that multiple elements, rather than single ones, contribute to the mC deamination activity and selectivity in A3BCD2 and possibly other APOBECs.

The virus-induced protein APOBEC3G inhibits anoikis by activation of Akt kinase in pancreatic cancer cells

Pancreatic cancer is one of the more common cancers with a poor prognosis. Some varieties of cancer are related to virus infection. As a virus-induced protein, APOBEC3G (A3G) presents extensive anti-virus ability, but the role of A3G in pancreatic cancer was previously unknown. The expression of A3G in pancreatic cancer was examined using TaqMan real-time qPCR, immunohistochemical and immunofluorescent staining. Subsequently, the role of A3G in pancreatic cancer was evaluated in vivo using the tumor xenograft model. Anoikis was detected by colony formation assay and flow cytometry in vitro. The Akt kinase activity and target protein PTEN were examined by co-immunoprecipitation and immunoblot. The virus-induced protein A3G was significantly up-regulated in pancreatic cancer, and the up-regulation of A3G promoted xenograft tumor formation. A3G inactivated PTEN by binding to the C2 tensin-type and PDZ domains, thereby inducing anoikis resistance through Akt activation. Our results demonstrate that the up-regulation of A3G in pancreatic cancer cells induces anoikis resistance, and they provide novel insight into the mechanism by which A3G affects the malignant behavior of pancreatic cancer cells.