miR-181b negatively regulates activation-induced cytidine deaminase in B cells

MicroRNA regulation of molecular networks mapped by global microRNA, mRNA, and protein expression in activated T lymphocytes

MicroRNAs (miRNAs) regulate specific immune mechanisms, but their genome-wide regulation of T lymphocyte activation is largely unknown. We performed a multidimensional functional genomics analysis to integrate genome-wide differential mRNA, miRNA, and protein expression as a function of human T lymphocyte activation and time. We surveyed expression of 420 human miRNAs in parallel with genome-wide mRNA expression. We identified a unique signature of 71 differentially expressed miRNAs, 57 of which were previously not known as regulators of immune activation. The majority of miRNAs are upregulated, mRNA expression of these target genes is downregulated, and this is a function of binding multiple miRNAs (combinatorial targeting). Our data reveal that consideration of this complex signature, rather than single miRNAs, is necessary to construct a full picture of miRNA-mediated regulation. Molecular network mapping of miRNA targets revealed the regulation of activation-induced immune signaling. In contrast, pathways populated by genes that are not miRNA targets are enriched for metabolism and biosynthesis. Finally, we specifically validated miR-155 (known) and miR-221 (novel in T lymphocytes) using locked nucleic acid inhibitors. Inhibition of these two highly upregulated miRNAs in CD4(+) T cells was shown to increase proliferation by removing suppression of four target genes linked to proliferation and survival. Thus, multiple lines of evidence link top functional networks directly to T lymphocyte immunity, underlining the value of mapping global gene, protein, and miRNA expression.

MicroRNA control of lymphocyte differentiation and function

MicroRNAs (miRNAs) are a class of endogenous, non-coding regulatory RNAs that control gene regulation by guiding silencing protein complexes to mRNA in a sequence-dependent manner. In this way miRNAs are able to repress gene expression post-transcriptionally by affecting mRNA stability or translation. These ubiquitous molecules play central roles in a wide range of biological processes, including cell proliferation, differentiation and apoptosis. Within the context of the immune system, genetic studies have identified distinct roles for specific miRNAs in gene regulation during development, activation and maturation. Conversely, dysregulation of miRNA expression has been specifically correlated with cancer. This review outlines our current understanding of miRNA function in lymphocytes as it impacts expression of protein-coding genes in the context of proper development, as well as oncogenesis.

MicroRNAs in multiple sclerosis and experimental autoimmune encephalomyelitis

MicroRNA (miRNA) are small non-coding RNA molecules about 21-25 nucleotides long. They control gene regulation by translational repression and cleavage. Several studies have shown that many miRNA are associated with the etiology of different diseases. Recent developments in diverse miRNA profiling platforms like microarray and quantitative real-time PCR may enable the identification of specific miRNA as novel diagnostic and predictive markers for various diseases. MiRNAs could even be used as therapeutic drug targets. Multiple sclerosis (MS) is a chronic autoimmune disease affecting the central nervous system. Dysregulated immune system processes result in demyelination of neurons and consequently, electrical impulses that travel along the nerves are disrupted resulting in the impairment of organs. In the past three years, there has been an increased interest in establishing miRNA-based biomarkers for MS. So far, there are six studies on miRNA expression in MS patients in which first miRNAs were discovered as potential disease markers. For instance, one study showed that blood levels of miR-145 can discriminate MS patients from healthy controls, and another showed that active lesions in the brain are characterized by a strong up-regulation of miR-155. Studies on experimental autoimmune encephalomyelitis (EAE), the animal model of MS, further support the significance of miRNA as e.g. mice with miR-155 deletion are highly resistant to EAE. Such investigations help to understand the molecular processes involved in the disease. The identification of miRNA markers that are associated with type of MS, individual disease activity or clinical progression under treatment may open new avenues for early diagnosis and optimized therapy of MS.

Complex regulation and function of activation-induced cytidine deaminase

Activation-induced cytidine deaminase (AID) instigates mutations and DNA breaks in Ig genes that undergo somatic hypermutation and class switch recombination during B cell activation in response to immunization and infection. This review discusses how AID expression and activity are regulated, including recent discoveries of AID-interacting proteins that might recruit AID to Ig genes, and allow it to target both DNA strands. Also discussed is the accumulating evidence that AID binds to, mutates, and creates breaks at numerous non-Ig sites in the genome, which initiates cell transformation and malignancies.

MicroRNA expression profiling identifies activated B cell status in chronic lymphocytic leukemia cells

Chronic lymphocytic leukemia (CLL) is thought to be a disease of resting lymphocytes. However, recent data suggest that CLL cells may more closely resemble activated B cells. Using microRNA (miRNA) expression profiling of highly-enriched CLL cells from 38 patients and 9 untransformed B cells from normal donors before acute CpG activation and 5 matched B cells after acute CpG activation, we demonstrate an activated B cell status for CLL. Gene set enrichment analysis (GSEA) identified statistically-significant similarities in miRNA expression between activated B cells and CLL cells including upregulation of miR-34a, miR-155, and miR-342-3p and downregulation of miR-103, miR-181a and miR-181b. Additionally, decreased levels of two CLL signature miRNAs miR-29c and miR-223 are associated with ZAP70(+) and IgV(H) unmutated status and with shorter time to first therapy. These data indicate an activated B cell status for CLL cells and suggest that the direction of change of individual miRNAs may predict clinical course in CLL.

Mechanisms of Yin Yang 1 in oncogenesis: the importance of indirect effects

Yin Yang 1 (YY1) is a ubiquitously expressed transcription factor that performs numerous functions including transcriptional regulation, cell growth control, apoptosis, large-scale chromosomal dynamics, and X-chromosome inactivation. YY1 clearly is able to control cell functions, including proliferation, by acting as a transcription factor either to activate or repress specific genes. Based on its ability to regulate cell growth control genes, it has been argued that YY1 can function as an oncogene that initiates oncogenesis. Although this is an attractive hypothesis, no reports indicate that YY1 can acutely transform cells in culture or form tumors within animals when overexpressed. Thus, it remains unclear whether YY1 is a "classic" oncogene. However, YY1 controls many diverse cell functions, and these functions may provide clues to its role in oncogenesis. We propose that in many cases YY1 may function in oncogenesis and disease progression through "indirect" effects by virtue of its role in either recruiting Polycomb group proteins to DNA, regulating mutator protein accumulation, controlling large-scale chromosomal dynamics or genomic integrity. Disruption of these functions may causally initiate cancer or may contribute to disease progression. Targeting YY1 functions provides possible avenues for clinical intervention.

Role of activation-induced cytidine deaminase in inflammation-associated cancer development

Human cancer is a genetic disease resulting from the stepwise accumulation of genetic alterations in various tumor-related genes. Normal mutation rates, however, cannot account for the abundant genetic changes accumulated in tumor cells, suggesting that certain molecular mechanisms underlie such a large number of genetic alterations. Activation-induced cytidine deaminase (AID), a nucleotide-editing enzyme that triggers DNA alterations and double-strand DNA breaks in the immunoglobulin gene, has been identified in activated B lymphocytes. Recent studies revealed that AID-mediated genotoxic effects target not only immunoglobulin genes but also a variety of other genes in both B lymphocytes and non-lymphoid cells. Consistent with the finding that several transcription factors including nuclear factor-κB (NF-κB) mediate AID expression in B cells, proinflammatory cytokine stimulation of several types of gastrointestinal epithelial cells, such as gastric, colonic, hepatic, and biliary epithelium, induces aberrant AID expression through the NF-κB signaling pathway. In vivo studies revealed that constitutive AID expression promotes the tumorigenic pathway by enhancing the susceptibility to mutagenesis in a variety of epithelial organs. The activity of AID as a genome mutator provides a new avenue for studies aimed at understanding mutagenesis mechanisms during carcinogenesis.

Human activated CD4(+) T lymphocytes increase IL-2 expression by downregulating microRNA-181c

MicroRNAs, a large family of small regulatory RNAs, are posttranscriptional gene regulators that bind mRNA in a sequence-specific manner, thereby controlling diverse aspects of cell function, including immune reaction. In this study, we screened and identified a group of differentially expressed miRNAs in naive and activated CD4(+) T cells. Among the miRNAs studied, miR-181c was proven to have the potential to regulate CD4(+) T cell activation. miR-181c was downregulated in the process of CD4(+) T cell activation, and transfection of miR-181c mimics partially repressed the activation of both Jurkat cells and human peripheral blood mononuclear cells (PBMC) CD4(+) T cells. We further showed that miR-181c can bind to the IL-2 3' UTR and repress its expression by inhibiting translation. Moreover, miR-181c mimics reduced activated CD4(+) T cell proliferation. Taken together, our results show that miR-181c serves as a negative regulator that modulates the activation of CD4(+) T cells.

MicroRNAs: key components of immune regulation

The regulation of gene expression at the posttranscriptional level has revealed important control levels for genes important to the immune system. MicroRNAs (miRNAs) are small RNAs that regulate gene expression by inhibiting protein translation or by degrading the mRNA transcript. A single miRNA can potentially regulate the expression of multiple genes and the proteins encoded. MiRNA can influence molecular signaling pathways and regulate many biological processes including immune function. Although the role of miRNAs in development and oncogenesis has been well characterized, their role in the immune system has only begun to emerge. During the past few years, many miRNAs have been found to be important in the development, differentiation, survival, and function of B and T lymphocytes, dendritic cells, macrophages, and other immune cell types. We discuss here recent findings revealing important roles for miRNA in immunity and how miRNAs can regulate innate and adaptive immune responses.

Amino-terminal phosphorylation of activation-induced cytidine deaminase suppresses c-myc/IgH translocation

Activation-induced cytidine deaminase (AID) is a mutator enzyme that initiates class switch recombination and somatic hypermutation of immunoglobulin genes (Ig) in B lymphocytes. However, AID also produces off-target DNA damage, including mutations in oncogenes and double-stranded breaks that can serve as substrates for oncogenic chromosomal translocations. AID is strictly regulated by a number of mechanisms, including phosphorylation at serine 38 and threonine 140, which increase activity. Here we show that phosphorylation can also suppress AID activity in vivo. Serine 3 is a novel phospho-acceptor which, when mutated to alanine, leads to increased class switching and c-myc/IgH translocations without affecting AID levels or catalytic activity. Conversely, increasing AID phosphorylation specifically on serine 3 by interfering with serine/threonine protein phosphatase 2A (PP2A) leads to decreased class switching. We conclude that AID activity and its oncogenic potential can be downregulated by phosphorylation of serine 3 and that this process is controlled by PP2A.

MicroRNAs prevent the generation of autoreactive antibodies

MicroRNAs have been shown to be critical for a number of aspects of immune system regulation and function. Here, we have examined the role of microRNAs in terminal B cell differentiation by analyzing Cd19-Cre(ki/+) Dicer1(fl/fl) mice. We found that in the absence of Dicer, the transitional and marginal zone (MZ) B cell compartments were overrepresented and follicular (FO) B cell generation was impaired. microRNA analysis revealed that miR185, a microRNA overexpressed in FO cells, dampened B cell receptor (BCR) signaling through Bruton tyrosine kinase downregulation. Dicer-deficient B cells had a skewed BCR repertoire with hallmarks of autoreactivity, which correlated with high titers of autoreactive antibodies in serum and autoimmune features in females. Together, our results reveal a crucial role for microRNAs in late B cell differentiation and in the establishment of B cell tolerance.

Regulation of activation-induced deaminase stability and antibody gene diversification by Hsp90

Activation-induced deaminase (AID) is the mutator enzyme that initiates somatic hypermutation and isotype switching of the antibody genes in B lymphocytes. Undesired byproducts of AID function are oncogenic mutations. AID expression levels seem to correlate with the extent of its physiological and pathological functions. In this study, we identify AID as a novel Hsp90 (heat shock protein 90 kD) client. We find that cytoplasmic AID is in a dynamic equilibrium regulated by Hsp90. Hsp90 stabilizes cytoplasmic AID, as specific Hsp90 inhibition leads to cytoplasmic polyubiquitination and proteasomal degradation of AID. Consequently, Hsp90 inhibition results in a proportional reduction in antibody gene diversification and off-target mutation. This evolutionarily conserved regulatory mechanism determines the functional steady-state levels of AID in normal B cells and B cell lymphoma lines. Thus, Hsp90 assists AID-mediated antibody diversification by stabilizing AID. Hsp90 inhibition provides the first pharmacological means to down-regulate AID expression and activity, which could be relevant for therapy of some lymphomas and leukemias.

MicroRNA expression changes during zebrafish development induced by perfluorooctane sulfonate

Perfluorooctane sulfonate (PFOS), a kind of widely distributed environmentally organic compound, has been found to cause developmental toxicity. Although microRNAs (miRNAs) play an important role in many metabolic tasks, whether and how they are involved in the process of PFOS-induced toxicity is largely unknown. To address this problem, PFOS-induced changes in miRNAs and target gene expression in zebrafish embryos, and the potential mechanism of PFOS-induced toxic action were studied in this research. Zebrafish embryos were exposed to 1 µg ml(-1) PFOS or DMSO control from 6 h post-fertilization (hpf) to 24 or 120 hpf. Subsequently, RNA was isolated from the embryo pool and the expression profiles of 219 known zebrafish miRNAs were analyzed using microarray. Finally, quantitative real-time polymerase chain reaction was used to validate several miRNAs expression of microarray data. The analysis revealed that PFOS exposure induced significant changes in miRNA expression profiles. A total of 39 and 81 miRNAs showed significantly altered expression patterns after PFOS exposure 24 and 120 hpf. Of the changed miRNAs, 20 were significantly up-regulated and 19 were significantly down-regulated (p < 0.01) at 24 hpf, whereas 41 were significantly up-regulated and 40 were significantly down-regulated (p < 0.01) at 120 hpf. These miRNAs were involved in development, apoptosis and cell signal pathway, cell cycle progression and proliferation, oncogenesis, adipose metabolism and hormone secretion, whereas there is still little functional information available for 32 miRNAs. Our results demonstrate that PFOS exposure alters the expression of a suite of miRNAs and may induce developmental toxicity.

Induction and assessment of class switch recombination in purified murine B cells

Humoral immunity is the branch of the immune system maintained by B cells and mediated through the secretion of antibodies. Upon B cell activation, the immunoglobulin locus undergoes a series of genetic modifications to alter the binding capacity and effector function of secreted antibodies. This process is highlighted by a genomic recombination event known as class switch recombination (CSR) in which the default IgM antibody isotype is substituted for one of IgG, IgA, or IgE. Each isotype possesses distinct effector functions thereby making CSR crucial to the maintenance of immunity. Diversification of the immunoglobulin locus is mediated by the enzyme activation-induced cytidine deaminase (AID). A schematic video describing this process in detail is available online (http://video.med.utoronto.ca/videoprojects/immunology/aam.html). AID's activity and the CSR pathway are commonly studied in the assessment of B cell function and humoral immunity in mice. The protocol outlined in this report presents a method of B cell isolation from murine spleens and subsequent stimulation with bacterial lipopolysaccharide (LPS) to induce class switching to IgG3 (for other antibody isotypes see Table 1). In addition, the fluorescent cell staining dye Carboxyfluorescein succinimidyl ester (CFSE) is used to monitor cell division of stimulated cells, a process crucial to isotype switching. The regulation of AID and the mechanism by which CSR occurs are still unclear and thus in vitro class switch assays provide a reliable method for testing these processes in various mouse models. These assays have been previously used in the context of gene deficiency using knockout mice. Furthermore, in vitro switching of B cells can be preceded by viral transduction to modulate gene expression by RNA knockdown or transgene expression. The data from these types of experiments have impacted our understanding of AID activity, resolution of the CSR reaction, and antibody-mediated immunity in the mouse.

Helicobacter pylori-induced activation-induced cytidine deaminase expression and carcinogenesis

Tumorigenesis is a multistep process in which the accumulation of genetic alterations drives the transformation of normal cells into malignant derivatives. Activation-induced cytidine deaminase (AID) contributes to immune system diversity by inducing somatic hypermutations and class-switch recombinations of human immunoglobulin genes. The mutagenic activity of AID, however, can also induce genetic changes in various genes and may lead to the development of cancer. Helicobacter pylori, a class 1 carcinogen for human gastric cancer, affects AID expression by two different mechanisms, introduction of bacterial virulence factors into host cells and induction of inflammatory responses, thereby contributing to the accumulation of mutations in tumor-related genes. Aberrant AID activity may therefore be a novel link between infection and carcinogenesis.

MicroRNAs as biomarkers in rheumatic diseases

MicroRNAs (miRNAs) are endogenous, noncoding, single-stranded RNAs of 19-25 nucleotides in length. They regulate gene expression and are important in a wide range of physiological and pathological processes. MiRNAs are attractive as potential biomarkers because their expression pattern is reflective of underlying pathophysiologic processes and they are specific to various disease states. Moreover, miRNAs can be detected in a variety of sources, including tissue, blood and body fluids; they are reasonably stable and appear to be resistant to differences in sample handling, which increases their appeal as practical biomarkers. The clinical utility of miRNAs as diagnostic or prognostic biomarkers has been demonstrated in various malignancies and a few nonmalignant diseases. There is accumulating evidence that miRNAs have an important role in systemic rheumatic diseases and that various diseases or different stages of the same disease are associated with distinct miRNA expression profiles. Preliminary data suggest that miRNAs are promising as candidate biomarkers of diagnosis, prognosis, disease activity and severity in autoimmune diseases. MiRNAs identified as potential biomarkers in pilot studies should be validated in larger studies designed specifically for biomarker validation.

The mechanisms regulating the subcellular localization of AID

Activation induced deaminase (AID) is a unique enzyme that directly introduces mutations in the immunoglobulin genes to generate antibody diversity during the humoral immune response. Since this mutator enzyme poses a measurable risk of off-target mutation, which can be deleterious or transforming for a cell, several regulatory mechanisms exist to control its activity. At least three of these mechanisms affect AID subcellular localization. It was recently found that AID is actively imported into the nucleus, most likely through importin-α/β recognizing a structural nuclear localization signal. However, AID is largely excluded from the nucleus in steady state thanks to two mechanisms. In addition to nuclear export through the exportin CRM1, a mechanism retaining AID in the cytoplasm exists. Cytoplasmic retention hinders the passive diffusion of AID into the nucleus playing an important role in the nuclear exclusion of AID. Subcellular localization of AID also determines its stability. The regulation of the nuclear fraction of AID by these many mechanisms has functional implications for antibody diversification.

microRNAs: critical regulators in Th17 cells and players in diseases

microRNAs are a novel group of small, conserved, non-coding RNA molecules that are present in all species. These molecules post-transcriptionally regulate gene expression by targeting mRNAs for degradation or by repressing the translation of the mRNAs. A good understanding of miRNA-mediated gene regulation is critical to gain a comprehensive view of many physiological processes and disease states. Emerging evidence demonstrates that miRNAs play an important role in the differentiation and function of the adaptive immune system. This review provides an overview of the diverse functions of miRNAs in modulating immune responses and in immune cell development, particularly the development of Th17 cells, and explores the involvement of miRNAs in several autoimmune diseases including multiple sclerosis (MS), rheumatoid arthritis (RA), inflammatory bowel disease (IBD) and diabetes.

The role of microRNA in rheumatoid arthritis and other autoimmune diseases

MicroRNAs (miRNAs) represent a class of non-coding RNA molecules playing pivotal roles in cellular and developmental processes. miRNAs modulate the expression of multiple target genes at the post-transcriptional level and are predicted to affect up to one-third of all human protein-encoding genes. Recently, miRNA involvement in the adaptive and innate immune systems has been recognized. Rheumatoid arthritis serves an example of a chronic inflammatory disorder in which miRNAs modulate the inflammatory process in the joints, with the potential to serve as biomarkers for both the inflammatory process and the potential for therapeutic response. This review discusses the investigations that led to miRNA discovery, miRNA biogenesis and mode of action, and the diverse roles of miRNAs in modulating the immune and inflammatory responses. We conclude with a discussion of the implications of miRNA biology in rheumatoid arthritis and other autoimmune disorders.

microRNA-181b targets MLK2 in HL-60 cells

microRNAs (miRNAs) play critical roles in many different cellular processes, including metabolism, apoptosis, differentiation and development. We showed miR-181b to be highly expressed in acute myeloid leukemia (AML). Furthermore, miR-181b contributed to proliferation of AML cells by targeting MLK2. Our results demonstrated that miR-181b plays an important role in the biology of AML and may be useful in developing therapies targeting miRNAs.

AID and somatic hypermutation

In response to an assault by foreign organisms, peripheral B cells can change their antibody affinity and isotype by somatically mutating their genomic DNA. The ability of a cell to modify its DNA is exceptional in light of the potential consequences of genetic alterations to cause human disease and cancer. Thus, as expected, this mechanism of antibody diversity is tightly regulated and coordinated through one protein, activation-induced deaminase (AID). AID produces diversity by converting cytosine to uracil within the immunoglobulin loci. The deoxyuracil residue is mutagenic when paired with deoxyguanosine, since it mimics thymidine during DNA replication. Additionally, B cells can manipulate the DNA repair pathways so that deoxyuracils are not faithfully repaired. Therefore, an intricate balance exists which is regulated at multiple stages to promote mutation of immunoglobulin genes, while retaining integrity of the rest of the genome. Here we discuss and summarize the current understanding of how AID functions to cause somatic hypermutation.

MicroRNAs of the immune system: roles in inflammation and cancer

MicroRNAs (miRNAs) are small noncoding RNAs that regulate gene expression by binding to complementary target mRNAs and either promoting their decay or inhibiting their translation. Most eukaryotic genomes studied encode miRNAs, which are processed from longer noncoding transcripts through pathways conserved from fungi to plants to animals. miRNAs are now understood to be key mediators of developmental transitions in a number of model organisms. With respect to the immune system, miRNAs affect all facets of immune system development, from hematopoiesis to activation in response to infection during both the innate and the adaptive immune response. At the same time, miRNA dysregulation is a central event in the development and pathophysiology of a number of cancers of the immune system. Here we will discuss our current understanding of this general regulatory mechanism, focusing on its involvement in inflammation and in oncogenesis.

MicroRNA activity in B lymphocytes

Gene expression regulation by miRNAs has been reported to control key aspects of B cell differentiation and function (Chen et al., Science 303:83-86, 2004; Xiao et al., Cell 131:146-159, 2007; O'Carroll et al., Genes Dev. 21:1999-2004, 2007; Koralov et al. Cell 132:860-874, 2008; Rodriguez et al., Science 316:608-611, 2007; Costinean et al., Proc Natl Acad Sci USA 103:7024-7029, 2006; Thai et al., Science 316:604-608, 2007; Vigorito et al., Immunity 27:847-859, 2007; Dorsett et al., Immunity 28:630-638, 2008; Teng et al., Immunity 28:621-629, 2008; de Yebenes et al., J Exp Med 205:2199-2206, 2008; He et al., Nature 435:828-833, 2005; Ventura et al. Cell 132:875-886, 2008; Xiao et al., Nat Immunol 9:405-414, 2008). In this chapter, we describe the methodology used to perform a functional screening of a miRNA library to identify miRNAs relevant for mature B cell function in primary mouse B cells. These procedures include the construction of a miRNA library and the expression of individual miRNA clones in spleen B cells, as well as the description of functional assays used to determine the impact of miRNA expression on several aspects of B cell function, such as proliferation, apoptosis, and class switch recombination.

A coming-of-age story: activation-induced cytidine deaminase turns 10

The discovery and characterization of activation-induced cytidine deaminase (AID) 10 years ago provided the basis for a mechanistic understanding of secondary antibody diversification and the subsequent generation and maintenance of cellular memory in B lymphocytes, which signified a major advance in the field of B cell immunology. Here we celebrate and review the triumphs in the mission to understand the mechanisms through which AID influences antibody diversification, as well as the implications of AID function on human physiology. We also take time to point out important ongoing controversies and outstanding questions in the field and highlight key experiments and techniques that hold the potential to elucidate the remaining mysteries surrounding this vital protein.

The emerging role of microRNAs in immune cell development and differentiation

MicroRNAs (miRNAs) are small RNA strands (20-25 nucleotides) that regulate gene expression by translational repression as well as by messenger RNA degradation. This review will examine the application and function of miRNAs in immune cell development and differentiation.

Differential micro RNA expression in PBMC from multiple sclerosis patients

Differences in gene expression patterns have been documented not only in Multiple Sclerosis patients versus healthy controls but also in the relapse of the disease. Recently a new gene expression modulator has been identified: the microRNA or miRNA. The aim of this work is to analyze the possible role of miRNAs in multiple sclerosis, focusing on the relapse stage. We have analyzed the expression patterns of 364 miRNAs in PBMC obtained from multiple sclerosis patients in relapse status, in remission status and healthy controls. The expression patterns of the miRNAs with significantly different expression were validated in an independent set of samples. In order to determine the effect of the miRNAs, the expression of some predicted target genes of these were studied by qPCR. Gene interaction networks were constructed in order to obtain a co-expression and multivariate view of the experimental data. The data analysis and later validation reveal that two miRNAs (hsa-miR-18b and hsa-miR-599) may be relevant at the time of relapse and that another miRNA (hsa-miR-96) may be involved in remission. The genes targeted by hsa-miR-96 are involved in immunological pathways as Interleukin signaling and in other pathways as wnt signaling. This work highlights the importance of miRNA expression in the molecular mechanisms implicated in the disease. Moreover, the proposed involvement of these small molecules in multiple sclerosis opens up a new therapeutic approach to explore and highlight some candidate biomarker targets in MS.

AID upmutants isolated using a high-throughput screen highlight the immunity/cancer balance limiting DNA deaminase activity

DNA deaminases underpin pathways in antibody diversification (AID) and anti-viral immunity (APOBEC3s). Here we show how a high-throughput bacterial papillation assay can be used to screen for AID mutants with increased catalytic activity. The upmutations focus on a small number of residues, some highlighting regions implicated in AID's substrate interaction. Many of the upmutations bring the sequence of AID closer to that of APOBEC3s. AID upmutants can yield increased antibody diversification, raising the possibility that modification of AID's specific activity might be used to regulate antibody diversification in vivo. However, upmutation of AID also led to an increased frequency of chromosomal translocations, suggesting that AID's specific activity may have been limited by the risk of genomic instability.

Restricting activation-induced cytidine deaminase tumorigenic activity in B lymphocytes

DNA breaks play an essential role in germinal centre B cells as intermediates to immunoglobulin class switching, a recombination process initiated by activation-induced cytidine deaminase (AID). Immunoglobulin gene hypermutation is likewise catalysed by AID but is believed to occur via single-strand DNA breaks. When improperly repaired, AID-mediated lesions can promote chromosomal translocations (CTs) that juxtapose the immunoglobulin loci to heterologous genomic sites, including oncogenes. Two of the most studied translocations are the t(8;14) and T(12;15), which deregulate cMyc in human Burkitt's lymphomas and mouse plasmacytomas, respectively. While a complete understanding of the aetiology of such translocations is lacking, recent studies using diverse mouse models have shed light on two important issues: (1) the extent to which non-specific or AID-mediated DNA lesions promote CTs, and (2) the safeguard mechanisms that B cells employ to prevent AID tumorigenic activity. Here we review these advances and discuss the usage of pristane-induced mouse plasmacytomas as a tool to investigate the origin of Igh-cMyc translocations and B-cell tumorigenesis.

miRNA profiling of B-cell subsets: specific miRNA profile for germinal center B cells with variation between centroblasts and centrocytes

MicroRNAs (miRNAs) are an important class of small RNAs that regulate gene expression at the post-transcriptional level. It has become evident that miRNAs are involved in hematopoiesis, and that deregulation of miRNAs may give rise to hematopoietic malignancies. The aim of our study was to establish miRNA profiles of naïve, germinal center (GC) and memory B cells, and validate their expression patterns in normal lymphoid tissues. Quantitative (q) RT-PCR profiling revealed that several miRNAs were elevated in GC B cells, including miR-17-5p, miR-106a and miR-181b. One of the most abundant miRNAs in all three B-cell subsets analyzed was miR-150, with a more than 10-fold lower level in GC B cell as compared with the other two subsets. miRNA in situ hybridization (ISH) in tonsil tissue sections confirmed the findings from the profiling work. Interestingly, gradual decrease of miR-17-5p, miR-106a and miR-181b staining intensity from the dark to the light zone was observed in GC. A strong cytoplasmic staining of miR-150 was observed in a minority of the centroblasts in the dark zone of the GC. Inverse staining pattern of miR-150 against c-Myb and Survivin was observed in tonsil tissue sections, suggesting possible targeting of these genes by miR-150. In line with this, the experimental induction of miR-150 lead to reduced c-Myb, Survivin and Foxp1 expression levels in the Burkitt's lymphoma cell line, DG75. In conclusion, miRNA profiles of naïve, GC and memory B cells were established and validated by miRNA ISH. Within the GC cells, a marked difference was observed between the light and the dark zone.

A backup role of DNA polymerase kappa in Ig gene hypermutation only takes place in the complete absence of DNA polymerase eta

Patients with the variant form of xeroderma pigmentosum (XPV) syndrome have a genetic deficiency in DNA polymerase (Pol) eta, and display accordingly an increased skin sensitivity to UV light, as well as an altered mutation pattern of their Ig V genes in memory B cells, alteration that consists in a reduced mutagenesis at A/T bases. We previously suggested that another polymerase with a different mutation signature, Pol kappa, is used as backup for Ig gene hypermutation in both humans and mice in cases of complete Pol eta deficiency, a proposition supported in this study by the analysis of Pol eta x Pol kappa double-deficient mice. We also describe a new XPV case, in which a splice site mutation of the first noncoding exon results in a decreased mRNA expression, a mRNA that otherwise encodes a normal Pol eta protein. Whereas the Pol eta mRNA level observed in patient's fibroblasts is one-twentieth the value of healthy controls, it is only reduced to one-fourth of the normal level in activated B cells. Memory B cells from this patient showed a 50% reduction in A/T mutations, with a spectrum that still displays a strict Pol eta signature. Pol eta thus appears as a dominant enzyme in hypermutation, its presence precluding the use of a substitute enzyme even in conditions of reduced availability. Such a dominant behavior may explain the lack of Pol kappa signature in Ig gene mutations of some XPV patients previously described, for whom residual Pol eta activity might exist.

Active nuclear import and cytoplasmic retention of activation-induced deaminase

The enzyme activation-induced deaminase (AID) triggers antibody diversification in B cells by catalyzing deamination and consequently mutation of immunoglobulin genes. To minimize off-target deamination, AID is restrained by several regulatory mechanisms including nuclear exclusion, thought to be mediated exclusively by active nuclear export. Here we identify two other mechanisms involved in controlling AID subcellular localization. AID is unable to passively diffuse into the nucleus, despite its small size, and its nuclear entry requires active import mediated by a conformational nuclear localization signal. We also identify in its C terminus a determinant for AID cytoplasmic retention, which hampers diffusion to the nucleus, competes with nuclear import and is crucial for maintaining the predominantly cytoplasmic localization of AID in steady-state conditions. Blocking nuclear import alters the balance between these processes in favor of cytoplasmic retention, resulting in reduced isotype class switching.

MicroRNA in autoimmunity and autoimmune diseases

MicroRNAs (miRNAs) are small conserved non-coding RNA molecules that post-transcriptionally regulate gene expression by targeting the 3' untranslated region (UTR) of specific messenger RNAs (mRNAs) for degradation or translational repression. miRNA-mediated gene regulation is critical for normal cellular functions such as the cell cycle, differentiation, and apoptosis, and as much as one-third of human mRNAs may be miRNA targets. Emerging evidence has demonstrated that miRNAs play a vital role in the regulation of immunological functions and the prevention of autoimmunity. Here we review the many newly discovered roles of miRNA regulation in immune functions and in the development of autoimmunity and autoimmune disease. Specifically, we discuss the involvement of miRNA regulation in innate and adaptive immune responses, immune cell development, T regulatory cell stability and function, and differential miRNA expression in rheumatoid arthritis and systemic lupus erythematosus.

The stability of AID and its function in class-switching are critically sensitive to the identity of its nuclear-export sequence

The carboxyterminal region of activation-induced deaminase (AID) is required for its function in Ig class switch recombination (CSR) and also contains a nuclear-export sequence (NES). Here, based on an extensive fine-structure mutation analysis of the AID NES, as well as from AID chimeras bearing heterologous NESs, we show that while a functional NES is indeed essential for CSR, it is not sufficient. The precise nature of the NES is critical both for AID stabilization and CSR function: minor changes in the NES can perturb stabilization and CSR without jeopardizing nuclear export. The results indicate that the AID NES fulfills a function beyond simply providing a signal for nuclear export and suggest the possibility that the quality of exportin-binding may be critical to the stabilization of AID and its activity in CSR.

MicroRNA control in the immune system: basic principles

MicroRNA (miRNA) control has emerged as a critical regulatory principle in the mammalian immune system. Genetic ablation of the miRNA machinery, as well as loss or deregulation of certain individual miRNAs, severely compromises immune development and response and can lead to immune disorders like autoimmunity and cancer. Although individual miRNAs modulate protein output from hundreds of target genes, they may impact physiological processes by regulating the concentrations of just a few key cellular proteins that may be components of a single or of functionally interrelated pathways in a given cellular context.

Haploinsufficiency of activation-induced deaminase for antibody diversification and chromosome translocations both in vitro and in vivo

The humoral immune response critically relies on the secondary diversification of antibodies. This diversification takes places through somatic remodelling of the antibody genes by two molecular mechanisms, Class Switch Recombination (CSR) and Somatic Hypermutation (SHM). The enzyme Activation Induced Cytidine Deaminase (AID) initiates both SHM and CSR by deaminating cytosine residues on the DNA of immunoglobulin genes. While crucial for immunity, AID-catalysed deamination is also the triggering event for the generation of lymphomagenic chromosome translocations. To address whether restricting the levels of AID expression in vivo contributes to the regulation of its function, we analysed mice harbouring a single copy of the AID gene (AID(+/-)). AID(+/-) mice express roughly 50% of normal AID levels, and display a mild hyperplasia, reminiscent of AID deficient mice and humans. Moreover, we found that AID(+/-) cells have an impaired competence for CSR and SHM, which indicates that AID gene dose is limiting for its physiologic function. We next evaluated the impact of AID reduction in AID(+/-) mice on the generation of chromosome translocations. Our results show that the frequency of AID-promoted c-myc/IgH translocations is reduced in AID(+/-) mice, both in vivo and in vitro. Therefore, AID is haploinsufficient for antibody diversification and chromosome translocations. These findings suggest that limiting the physiologic levels of AID expression can be a regulatory mechanism that ensures an optimal balance between immune proficiency and genome integrity.