Secondary immunoglobulin (Ig) gene diversification relies on activation-induced cytidine deaminase (AID) to create U:G mismatches that are subsequently fixed by mutagenic repair pathways. AID activity is focused to Ig loci by cis-regulatory DNA sequences named targeting elements. Here we show that in contrast to prevailing thought in the field, the targeting elements in the chicken IGL locus are distinct from classical transcriptional enhancers. These mutational enhancer elements (MEEs) are required over and above transcription to recruit AID-mediated mutagenesis to Ig loci. We identified a small 222 bp fragment in the chicken IGL locus that enhances mutagenesis without boosting transcription, and this sequence represents a key component of a MEE. Lastly, MEEs are evolutionarily conserved amongst birds, both in sequence and function, and contain several highly conserved sequence modules that are likely involved in recruiting trans-acting targeting factors. We propose that MEEs represent a novel class of cis-regulatory elements whose function is to control genomic integrity.
Activation-induced cytidine deaminase (AID) is a key enzyme for antibody-mediated immune responses. Antibodies are encoded by the immunoglobulin genes and AID acts as a transcription-dependent DNA mutator on these genes to improve antibody affinity and effector functions. An emerging theme in field is that many transcribed genes are potential targets of AID, presenting an obvious danger to genomic integrity. Thus there are mechanisms in place to ensure that mutagenic outcomes of AID activity are specifically restricted to the immunoglobulin loci. Cis-regulatory targeting elements mediate this effect and their mode of action is likely a combination of immunoglobulin gene specific activation of AID and a perversion of faithful DNA repair towards error-prone outcomes.
The Recombination Activating Genes 1 and 2 (Rag1 and Rag2) encode the key enzyme that is required for the generation of the highly diversified antigen receptor repertoire central to adaptive immunity. The longstanding model proposed that this gene pair was acquired by horizontal gene transfer to explain its abrupt appearance in the vertebrate lineage. The analyses of the enormous amount of sequence data created by many genome sequencing projects now provides the basis for a more refined model as to how this unique gene pair evolved from a selfish DNA transposon into a sophisticated DNA recombinase essential for immunity.
RAG1; RAG2; transposase; evolution; adaptive immunity
V(D)J recombination is a somatic gene rearrangement process that assembles antigen receptor genes from individual segments during lymphocyte development. The access of the RAG1/RAG2 recombinase to these gene segments is regulated at the level of chromatin modifications, in particular histone tail modifications. Trimethylation of lysine 4 in histone H3 (H3K4me3) correlates with actively recombining gene elements, and this mark is recognized and interpreted by the plant homeodomain (PHD) of RAG2. Here we report that the PHD domain of the only known invertebrate homolog of RAG2, the SpRAG2L protein of the purple sea urchin (Strongylocentrotus purpuratus) also binds to methylated histones, but with a unique preference for H3K4me2. While the cognate substrate for the sea urchin RAG1L/RAG2L complex remains elusive, the affinity for histone tails and the nuclear localization of ectopically expressed SpRAG2L strongly support the model that this enzyme complex exerts its activity on DNA in the context of chromatin.
V(D)J recombination; SpRAG1L; SpRAG2L; RAG2; PHD domain; Strongylocentrotus purpuratus; H3K4 methylation; chromatin
SIRT6 is a histone deacetylase that has been proposed as a potential therapeutic target for metabolic disorders and the prevention of age-associated diseases. Thus the identification of compounds that modulate SIRT6 activity could be of great therapeutic importance. We have previously reported on the identification of quercetin and vitexin as SIRT6 inhibitors, using SIRT6-coated magnetic beads. In this study, we have immobilized SIRT6 onto the surface of an open tubular capillary and characterized the quercetin binding site using frontal displacement chromatography. Structurally related flavonoids were tested for their activity on SIRT6, including apigenin, naringenin, luteolin and kaempferol. In addition to obtaining their binding activity using frontal affinity chromatographic techniques, we also ranked the compounds based on their ability to displace quercetin. The data suggest that a single displacement curve is representative of the enzymatic activity of the tested ligand. In addition, using the inhibition data obtained in this study, we developed a preliminary pharmacophore model that confirmed the experimental data.
frontal displacement chromatography; pharmacophore model; luteolin; Histone deacetylation assay
The programmed degradation of the RAG2 enzyme upon entry to S phase restricts V(D)J recombination to the G0–G1 phase of the cell cycle. In this issue of Immunity Zhang et al. (2011) show that this is critical to prevent lymphoma formation.
Somatic hypermutation (SHM) of immunoglobulin genes is initiated by activation-induced cytidine deaminase (AID) in activated B cells. This process is strictly dependent on transcription. Hence, cis-acting transcriptional control elements have been proposed to target SHM to immunoglobulin loci. The Mus musculus Igκ locus is regulated by the intronic enhancer (iE/MAR) and the 3′ enhancer (3′E), and multiple studies using transgenic and knock-out approaches in mice and cell lines have reported somewhat contradictory results about the function of these enhancers in AID-mediated sequence diversification. Here we show that the M. musculus iE/MAR and 3′E elements are active solely as transcriptional enhancer when placed in the context of the IGL locus in Gallus gallus DT40 cells, but they are very inefficient in targeting AID-mediated mutation events to this locus. This suggests that either key components of the cis-regulatory targeting elements reside outside the murine Igκ transcriptional enhancer sequences, or that the targeting of AID activity to Ig loci occurs by largely species-specific mechanisms.
Adaptive immunity is mediated through numerous genetic and cellular processes that generate favourable somatic variants of antigen-binding receptors under evolutionary selection pressure by pathogens and other factors. Advances in our understanding of immunity in mammals and other model organisms are revealing the underlying basis and complexity of this remarkable system. Although the evolution of adaptive immunity has been considered to occur by acquisition of novel molecular capabilities, an increasing amount of information from new model systems suggest that co-option and redirection of preexisting systems are the major source of innovation. We combine evidence from a wide range of organisms to obtain an integrated view of the origins and patterns of divergence in adaptive immunity.
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.
Somatic hypermutation (SHM) and gene conversion (GCV) are closely related processes that increase the diversity the primary immunoglobulin repertoire. In both processes the activation-induced cytidine deaminase (AID) converts cytosine residues to uracils within the DNA of the immunoglobulin (Ig) genes in a transcription-dependent manner, and subsequent error-prone repair processes lead to changes in the antigen recognition site of the encoded receptors. This activity is specifically recruited to the Ig loci by unknown mechanisms. Our analyses of the chicken B-cell line DT40, and derivatives thereof, now revealed that even the closest neighbors of the Ig light chain (IgL) gene are protected from AID activity, albeit being transcribed and thus acting as potential targets of AID. Our findings are in support of a model in which cis-acting DNA boundary elements restrict AID activity to the IgL locus and guard the genome in the vicinity of the IgL gene from deleterious mutations.
Somatic Hypermutation; Gene Conversion; Immunoglobulin; DT40
The beyond 12/23 (B12/23) rule ensures inclusion of a Dβ gene segment in the assembled T-cell receptor (TCR) β variable region exon and is manifest by a failure of direct Vβ-to-Jβ gene segment joining. The restriction is enforced during the DNA cleavage step of V(D)J recombination by the recombination-activating gene 1 and 2 (RAG1/2) proteins and the recombination signal sequences (RSSs) flanking the TCRβ gene segments. Nothing is known about the step(s) at which DNA cleavage is defective or how TCRβ locus sequences contribute to these defects. To address this, we examined the steps of DNA cleavage by the RAG proteins using TCRβ locus V, D, and J RSS oligonucleotide substrates. The results demonstrate that the B12/23 rule is enforced through slow nicking of Jβ substrates and to some extent through poor synapsis of Vβ and Jβ substrates. Nicking is controlled largely by the coding flank and, unexpectedly, the RSS spacer, while synapsis is controlled primarily by the RSS nonamer. The results demonstrate that different Jβ substrates are crippled at different steps of cleavage by distinct combinations of defects in the various DNA elements and strongly suggest that the DNA nicking step of V(D)J recombination can be rate limiting in vivo.
It is thought that gene conversion (GCV) and somatic hypermutation (SHM) of immunoglobulin (Ig) genes occur in two steps: the generation of uracils in DNA by activation-induced cytidine deaminase, followed by their subsequent repair by various DNA repair pathways to generate sequence-diversified products. It is not known how either of the two steps is targeted specifically to Ig loci. Because of the tight link between transcription and SHM, we have investigated the role of endogenous Ig light chain (IgL) transcriptional control elements in GCV/SHM in the chicken B cell line DT40. Promoter substitution experiments led to identification of a strong RNA polymerase II promoter incapable of supporting efficient GCV/SHM. This surprising finding indicates that high levels of transcription are not sufficient for robust GCV/SHM in Ig loci. Deletion of the IgL enhancer in a context in which high-level transcription was not compromised showed that the enhancer is not necessary for GCV/SHM. Our results indicate that cis-acting elements are important for Ig gene diversification, and we propose that targeting specificity is achieved through the combined action of several Ig locus elements that include the promoter.
During lymphocyte development, V(D)J recombination assembles antigen receptor genes from component V, D, and J gene segments. These gene segments are flanked by a recombination signal sequence (RSS), which serves as the binding site for the recombination machinery. The murine Jβ2.6 gene segment is a recombinationally inactive pseudogene, but examination of its RSS reveals no obvious reason for its failure to recombine. Mutagenesis of the Jβ2.6 RSS demonstrates that the sequences of the heptamer, nonamer, and spacer are all important. Strikingly, changes solely in the spacer sequence can result in dramatic differences in the level of recombination. The subsequent analysis of a library of more than 4,000 spacer variants revealed that spacer residues of particular functional importance are correlated with their degree of conservation. Biochemical assays indicate distinct cooperation between the spacer and heptamer/nonamer along each step of the reaction pathway. The results suggest that the spacer serves not only to ensure the appropriate distance between the heptamer and nonamer but also regulates RSS activity by providing additional RAG:RSS interaction surfaces. We conclude that while RSSs are defined by a “digital” requirement for absolutely conserved nucleotides, the quality of RSS function is determined in an “analog” manner by numerous complex interactions between the RAG proteins and the less-well conserved nucleotides in the heptamer, the nonamer, and, importantly, the spacer. Those modulatory effects are accurately predicted by a new computational algorithm for “RSS information content.” The interplay between such binary and multiplicative modes of interactions provides a general model for analyzing protein–DNA interactions in various biological systems.
Spacers not only ensure that the distance between the nonamer and heptamer is correct but they also regulate recombination activity by providing protein-binding sites along the DNA sequences that affect recombination