Mutational analysis of RAG1 and RAG2 identifies three catalytic amino acids in RAG1 critical for both cleavage steps of V(D)J recombination

A Naturally Active Spy Transposon Discovered from the Insect Genome of Colletes gigas as a Promising Novel Gene Transfer Tool

Novel active DNA transposons, such as Spy transposons from the PHIS superfamily, are identified through bioinformatics in this study. The native transposases cgSpy and cvSpy displayed transposition activities of approximately 85% and 35% compared to the hyperactive piggyBac transposase (hyPB). The cgSpy transposon showed unique characteristics, including a lack of overproduction inhibition and reduced efficiency for insertion sizes between 3.1 to 8.5 kb. Integration preferences of cgSpy are found in genes and regulatory regions, making it suitable for genetic manipulation. Evaluation in T-cell engineering demonstrated that cgSpy-mediated chimeric antigen receptor (CAR) modification is comparable to the PB system, indicating its potential utility in cell therapy. This study unveils the promising application of the active native transposase, Spy, from Colletes gigas, as a valuable tool for genetic engineering, particularly in T-cell manipulation.

Ontogeny and tissue specific expression profiles of recombination activating genes (RAGs) during development in Nile tilapia, Oreochromisniloticus

Recombination activating genes (RAGs) mediates the process of rearrangement and somatic recombination (V(D)J) to generate different antibody repertoire. Studies on the expression pattern of adaptive immune genes during ontogenic development are crucial for the formulation of fish immunization strategy. In the present study, Nile tilapia was taken to explore the relative expression profile of RAG genes during their developmental stages. The developmental stages of Nile tilapia, i.e., unfertilized egg, 0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28 and 30 days post-hatch (dph) and kidney, blood, gill, liver and spleen tissues from adult fish were collected and the cDNA synthesis was carried out. Gene specific primers for RAG-1 and RAG-2 of Nile tilapia were designed and their annealing temperature (Tm) was optimized by gradient PCR. Consequently, PCR was performed to confirm the specific amplification of RAG-1 and RAG-2 genes. Quantitative real-time PCR (qRT-PCR) gene expression of RAG-1 and RAG-2 were noticed in all the developmental stages; however, a significant increase was observed after 12 dph and peaked at 24 dph, followed by a gradual decrease until 30 dph. Tissue-specific gene expression profiling revealed that the highest expression of RAG-1 and RAG-2 was observed in the kidney, followed by spleen, gill, liver and blood. The findings of the study explored the suitable timing of lymphoid maturation that could be technically used for the adoption of strategies to improve disease resistance of fish larvae for mitigating larval mortality.

Unravelling the complexities of DNA-PK activation by structure-based mutagenesis

It has been known for decades that the DNA-dependent protein kinase (DNA-PK) is only an active serine/threonine protein kinase when it is bound to a DNA double-stranded end; still, the molecular details of how this activation is achieved have remained elusive. The recent surge in structural information for DNA-PK complexes has provided valuable insights into the process of DNA end recognition by DNA-PK. A particularly intriguing feature of this kinase is a region of the protein that can transition from a seemingly structurally disordered state to a single alpha-helix that traverses down the DNA binding cradle. The DNA-PK bound DNA end of the DNA substrate engages with and appears to split around this helix which has been named the DNA End Blocking helix (DEB). Here a mutational approach is utilized to clarify the role of the DEB, and how DNA ends activate the enzyme. Our data suggest two distinct methods of kinase activation that is dependent on the DNA end chemistry. If the DNA end can split around the helix and stabilize the interaction between the DNA end and the DEB with a recently defined Helix-Hairpin-Helix (HHH) motif, the kinase forms an end-protection monomer that is active towards DNA-PK's many substrates. But if the DNA end cannot stably interact with the DEB [because of the DNA end structure, for instance hairpins, or because the DEB has been disrupted by mutation], the kinase is only partially activated, resulting in specific autophosphorylations of the DNA-PK monomer that allows nucleolytic end-processing. We posit that mutants that disrupt the capacity to stably generate the DEB/HHH DNA end-interaction are inefficient in generating the dimer complex that is requisite for NHEJ. In support of this idea, mutations that promote formation of this dimer partially rescue the severe cellular phenotypes associated with mutation of the DEB helix.

RAG genomic variation causes autoimmune diseases through specific structure-based mechanisms of enzyme dysregulation

Interpreting genetic changes observed in individual patients is a critical challenge. The array of immune deficiency syndromes is typically caused by genetic variation unique to individuals. Therefore, new approaches are needed to interpret functional variation and accelerate genomics interpretation. We constructed the first full-length structural model of human RAG recombinase across four functional states of the recombination process. We functionally tested 182 clinically observed RAG missense mutations. These experiments revealed dysfunction due to recombinase dysfunction and altered chromatin interactions. Structural modeling identified mechanical and energetic roles for each mutation. We built regression models for RAG1 (R2 = 0.91) and RAG2 (R2 = 0.97) to predict RAG activity changes. We applied our model to 711 additional RAG variants observed in population studies and identified a subset that may impair RAG function. Thus, we demonstrated a fundamental advance in the mechanistic interpretation of human genetic variations spanning from rare and undiagnosed diseases to population health.

Two distinct long-range synaptic complexes promote different aspects of end processing prior to repair of DNA breaks by non-homologous end joining

Non-homologous end joining is the major double-strand break repair (DSBR) pathway in mammals. DNA-PK is the hub and organizer of multiple steps in non-homologous end joining (NHEJ). Recent high-resolution structures show how two distinct NHEJ complexes "synapse" two DNA ends. One complex includes a DNA-PK dimer mediated by XLF, whereas a distinct DNA-PK dimer forms via a domain-swap mechanism where the C terminus of Ku80 from one DNA-PK protomer interacts with another DNA-PK protomer in trans. Remarkably, the distance between the two synapsed DNA ends in both dimers is the same (∼115 Å), which matches the distance observed in the initial description of an NHEJ long-range synaptic complex. Here, a mutational strategy is used to demonstrate distinct cellular function(s) of the two dimers: one promoting fill-in end processing, while the other promotes DNA end resection. Thus, the specific DNA-PK dimer formed (which may be impacted by DNA end structure) dictates the mechanism by which ends will be made ligatable.

Comparison of immunophenotypes between Rag2 knockout mice derived from two different sources

BACKGROUND

Recombination activating gene2 (Rag2) knockout (KO) mice are used widely in various research fields, including vaccine development, transplantation studies, and hematopoiesis research, but few studies have compared their phenotypes. This study examined whether there were differences in the immunophenotypes between Rag2 KO mice derived from different sources. In particular, the changes in the organ weight, histological structure, and subpopulation of T and B cells were compared in the spleen and thymus of C57BL/6-Rag2^em1hwl/Korl (Rag2/Korl KO) and B6.Cg-Rag2^tm1.1Cgn/J (Rag2/J KO) mice.

RESULTS

The weight of the spleen and thymus similarly decreased in the Rag2/Korl and Rag2/J KO mice compared to their wild type (WT) mice, even though the other organs were kept at the same weight. A slight difference between the Rag2/Korl and Rag2/J KO group were detected in the number of white blood cells (WBC), lymphocytes (LYM), red cell distribution width (RDW), and platelets (PLT). In addition, the white pulp of the spleen and the cortex region of the thymus decreased in both Rag2 KO mice compared to WT mice. On the other hand, significant differences in the number of CD8⁺ T and B cell subpopulations between WT and Rag2 KO mice were observed between Rag2/Korl and Rag2/J KO group, while the CD4⁺ T subpopulation was maintained similarly in both groups.

CONCLUSIONS

These results suggest that Rag2/Korl and Rag2/J KO mice exhibit similar immunophenotypes in the spleen and thymus except for the differences in the number of CD8⁺ T and B cell subpopulations.

Investigating underlying human immunity genes, implicated diseases and their relationship to COVID-19

Aim: A human immunogenetics variation study was conducted in samples collected from diverse COVID-19 populations. Materials & methods: Whole-genome and whole-exome sequencing (WGS/WES), data processing, analysis and visualization pipeline were applied to identify variants associated with genes of interest. Results: A total of 2886 mutations were found across the entire set of 13 genomes. Functional annotation of the gene variants revealed mutation type and protein change. Many variants were found to be biologically implicated in COVID-19. The involvement of these genes was also found in multiple other diseases. Conclusion: The analysis determined that ACE2, TMPRSS4, TMPRSS2, SLC6A20 and FYCOI had functional implications and TMPRSS4 was the gene most altered in virally infected patients.

The quest to establish an understanding of the genetics underlying COVID-19 is a central focus of life sciences today. COVID-19 is triggered by SARS-CoV-2, a single-stranded RNA respiratory virus. Several clinical-genomics studies have emerged positing different human gene mutations occurring due to COVID-19. A global analysis of these genes was conducted targeting major components of the immune system to identify possible variations likely to be involved in COVID-19 predisposition. Gene-variant analysis was performed on whole-genome sequencing samples collected from diverse populations. ACE2, TMPRSS4, TMPRSS2, SLC6A20 and FYCOI were found to have functional implications and TMPRSS4 may have a role in the severity of clinical manifestations of COVID-19.

OUP accepted manuscript

The Sleeping Beauty (SB) transposon system is a popular tool for genome engineering, but random integration into the genome carries a certain genotoxic risk in therapeutic applications. Here we investigate the role of amino acids H187, P247 and K248 in target site selection of the SB transposase. Structural modeling implicates these three amino acids located in positions analogous to amino acids with established functions in target site selection in retroviral integrases and transposases. Saturation mutagenesis of these residues in the SB transposase yielded variants with altered target site selection properties. Transposon integration profiling of several mutants reveals increased specificity of integrations into palindromic AT repeat target sequences in genomic regions characterized by high DNA bendability. The H187V and K248R mutants redirect integrations away from exons, transcriptional regulatory elements and nucleosomal DNA in the human genome, suggesting enhanced safety and thus utility of these SB variants in gene therapy applications.

Guardian of the Genome: An Alternative RAG/Transib Co-Evolution Hypothesis for the Origin of V(D)J Recombination

The appearance of adaptive immunity in jawed vertebrates is termed the immunological 'Big Bang' because of the short evolutionary time over which it developed. Underlying it is the recombination activating gene (RAG)-based V(D)J recombination system, which initiates the sequence diversification of the immunoglobulins and lymphocyte antigen receptors. It was convincingly argued that the RAG1 and RAG2 genes originated from a single transposon. The current dogma postulates that the V(D)J recombination system was established by the split of a primordial vertebrate immune receptor gene into V and J segments by a RAG1/2 transposon, in parallel with the domestication of the same transposable element in a separate genomic locus as the RAG recombinase. Here, based on a new interpretation of previously published data, we propose an alternative evolutionary hypothesis suggesting that two different elements, a RAG1/2 transposase and a Transib transposon invader with RSS-like terminal inverted repeats, co-evolved to work together, resulting in a functional recombination process. This hypothesis offers an alternative understanding of the acquisition of recombinase function by RAGs and the origin of the V(D)J system.

A single amino acid switch converts the Sleeping Beauty transposase into an efficient unidirectional excisionase with utility in stem cell reprogramming

The Sleeping Beauty (SB) transposon is an advanced tool for genetic engineering and a useful model to investigate cut-and-paste DNA transposition in vertebrate cells. Here, we identify novel SB transposase mutants that display efficient and canonical excision but practically unmeasurable genomic re-integration. Based on phylogenetic analyses, we establish compensating amino acid replacements that fully rescue the integration defect of these mutants, suggesting epistasis between these amino acid residues. We further show that the transposons excised by the exc⁺/int⁻ transposase mutants form extrachromosomal circles that cannot undergo a further round of transposition, thereby representing dead-end products of the excision reaction. Finally, we demonstrate the utility of the exc⁺/int⁻ transposase in cassette removal for the generation of reprogramming factor-free induced pluripotent stem cells. Lack of genomic integration and formation of transposon circles following excision is reminiscent of signal sequence removal during V(D)J recombination, and implies that cut-and-paste DNA transposition can be converted to a unidirectional process by a single amino acid change.

Structural insights into the mechanism of double strand break formation by Hermes, a hAT family eukaryotic DNA transposase

Some DNA transposons relocate from one genomic location to another using a mechanism that involves generating double-strand breaks at their transposon ends by forming hairpins on flanking DNA. The same double-strand break mode is employed by the V(D)J recombinase at signal-end/coding-end junctions during the generation of antibody diversity. How flanking hairpins are formed during DNA transposition has remained elusive. Here, we describe several co-crystal structures of the Hermes transposase bound to DNA that mimics the reaction step immediately prior to hairpin formation. Our results reveal a large DNA conformational change between the initial cleavage step and subsequent hairpin formation that changes which strand is acted upon by a single active site. We observed that two factors affect the conformational change: the complement of divalent metal ions bound by the catalytically essential DDE residues, and the identity of the –2 flanking base pair. Our data also provides a mechanistic link between the efficiency of hairpin formation (an A:T basepair is favored at the –2 position) and Hermes' strong target site preference. Furthermore, we have established that the histidine residue within a conserved C/DxxH motif present in many transposase families interacts directly with the scissile phosphate, suggesting a crucial role in catalysis.

The ESC: The Dangerous By-Product of V(D)J Recombination

V(D)J recombination generates antigen receptor diversity by mixing and matching individual variable (V), diversity (D), and joining (J) gene segments. An obligate by-product of many of these reactions is the excised signal circle (ESC), generated by excision of the DNA from between the gene segments. Initially, the ESC was believed to be inert and formed to protect the genome from reactive broken DNA ends but more recent work suggests that the ESC poses a substantial threat to genome stability. Crucially, the recombinase re-binds to the ESC, which can result in it being re-integrated back into the genome, to cause potentially oncogenic insertion events. In addition, very recently, the ESC/recombinase complex was found to catalyze breaks at recombination signal sequences (RSSs) throughout the genome, via a “cut-and-run” mechanism. Remarkably, the ESC/recombinase complex triggers these breaks at key leukemia driver genes, implying that this reaction could be a significant cause of lymphocyte genome instability. Here, we explore these alternate pathways and discuss their relative dangers to lymphocyte genome stability.

RAG1 high expression associated with IKZF1 dysfunction in adult B-cell acute lymphoblastic leukemia

The recombination mediated by recombination activating gene (RAG) is not only the dominant mutational process but also the predominant driver of oncogenic genomic rearrangement in acute lymphoblastic leukemia (ALL). It is further responsible for leukemic clonal evolution. In this study, significant RAG1 increase is observed in the subsets of B-ALL patients, and high expression of RAG1 is observed to be correlated with high proliferation markers. IKZF1-encoded protein, IKAROS, directly binds to the RAG1 promoter and regulates RAG1 expression in leukemic cells. CK2 inhibitor by increasing IKAROS activity significantly suppresses RAG1 expression in ALL in an IKAROS-dependent manner. Patients with IKZF1 deletion have significantly higher expression of RAG1 compared to that without IKZF1 deletion. CK2 inhibitor treatment also results in an increase in IKZF1 binding to the RAG1 promoter and suppression of RAG1 expression in primary ALL cells. Taken together, these results demonstrate that RAG1 high expression is associated with high proliferation markers in B-ALL. Our data for the first time proved that RAG1 expression is directly suppressed by IKAROS. Our results also reveal drive oncogenesis of B-ALL is driven by high expression of RAG1 with IKAROS dysfunction together, which have significance in an integrated prognostic model for adult ALL.

Cut-and-Run: A Distinct Mechanism by which V(D)J Recombination Causes Genome Instability

V(D)J recombination is essential to generate antigen receptor diversity but is also a potent cause of genome instability. Many chromosome alterations that result from aberrant V(D)J recombination involve breaks at single recombination signal sequences (RSSs). A long-standing question, however, is how such breaks occur. Here, we show that the genomic DNA that is excised during recombination, the excised signal circle (ESC), forms a complex with the recombinase proteins to efficiently catalyze breaks at single RSSs both in vitro and in vivo. Following cutting, the RSS is released while the ESC-recombinase complex remains intact to potentially trigger breaks at further RSSs. Consistent with this, chromosome breaks at RSSs increase markedly in the presence of the ESC. Notably, these breaks co-localize with those found in acute lymphoblastic leukemia patients and occur at key cancer driver genes. We have named this reaction “cut-and-run” and suggest that it could be a significant cause of lymphocyte genome instability.

•

A complex between the recombination by-product and RAGs triggers multiple DNA breaks

•

The breaks co-localize with chromosome breakpoints in acute lymphoblastic leukemias

•

The breaks occur at many frequently mutated genes in acute lymphoblastic leukemia

•

Cut-and-run may underpin the most common types of lymphocyte chromosome instabilities

RAG gene defects at the verge of immunodeficiency and immune dysregulation

Mutations of the recombinase activating genes (RAG) in humans underlie a broad spectrum of clinical and immunological phenotypes that reflect different degrees of impairment of T- and B-cell development and alterations of mechanisms of central and peripheral tolerance. Recent studies have shown that this phenotypic heterogeneity correlates, albeit imperfectly, with different levels of recombination activity of the mutant RAG proteins. Furthermore, studies in patients and in newly developed animal models carrying hypomorphic RAG mutations have disclosed various mechanisms underlying immune dysregulation in this condition. Careful annotation of clinical outcome and immune reconstitution in RAG-deficient patients who have received hematopoietic stem cell transplantation has shown that progress has been made in the treatment of this disease, but new approaches remain to be tested to improve stem cell engraftment and durable immune reconstitution. Finally, initial attempts have been made to treat RAG deficiency with gene therapy.

The Conserved ATM Kinase RAG2-S365 Phosphorylation Site Limits Cleavage Events in Individual Cells Independent of Any Repair Defect

Many DNA lesions associated with lymphoid malignancies are linked to off-target cleavage by the RAG1/2 recombinase. However, off-target cleavage has mostly been analyzed in the context of DNA repair defects, confounding any mechanistic understanding of cleavage deregulation. We identified a conserved SQ phosphorylation site on RAG2 365 to 366 that is involved in feedback control of RAG cleavage. Mutation of serine 365 to a non-phosphorylatable alanine permits bi-allelic and bi-locus RAG-mediated breaks in the same cell, leading to reciprocal translocations. This phenomenon is analogous to the phenotype we described for ATM kinase inactivation. Here, we establish deregulated cleavage itself as a driver of chromosomal instability without the associated repair defect. Intriguingly, a RAG2-S365E phosphomimetic rescues the deregulated cleavage of ATM inactivation, reducing the incidence of reciprocal translocations. These data support a model in which feedback control of cleavage and maintenance of genome stability involves ATM-mediated phosphorylation of RAG2.

Cracking the DNA Code for V(D)J Recombination

To initiate V(D)J recombination for generating the adaptive immune response of vertebrates, RAG1/2 recombinase cleaves DNA at a pair of recombination signal sequences, the 12- and 23-RSS. We have determined crystal and cryo-EM structures of RAG1/2 with DNA in the pre-reaction and hairpin-forming complexes up to 2.75 Å resolution. Both protein and DNA exhibit structural plasticity and undergo dramatic conformational changes. Coding-flank DNAs extensively rotate, shift, and deform for nicking and hairpin formation. Two intertwined RAG1 subunits crisscross four times between the asymmetric pair of severely bent 12/23-RSS DNAs. Location-sensitive bending of 60° and 150° in 12- and 23-RSS spacers, respectively, must occur for RAG1/2 to capture the nonamers and pair the heptamers for symmetric double-strand breakage. DNA pairing is thus sequence-context dependent and structure specific, which partly explains the "beyond 12/23" restriction. Finally, catalysis in crystallo reveals the process of DNA hairpin formation and its stabilization by interleaved base stacking.

PGBD5 promotes site-specific oncogenic mutations in human tumors

Genomic rearrangements are a hallmark of human cancers. Here, we identify the piggyBac transposable element derived 5 (PGBD5) gene as encoding an active DNA transposase expressed in the majority of childhood solid tumors, including lethal rhabdoid tumors. Using assembly-based whole-genome DNA sequencing, we found previously undefined genomic rearrangements in human rhabdoid tumors. These rearrangements involved PGBD5-specific signal (PSS) sequences at their breakpoints and recurrently inactivated tumor-suppressor genes. PGBD5 was physically associated with genomic PSS sequences that were also sufficient to mediate PGBD5-induced DNA rearrangements in rhabdoid tumor cells. Ectopic expression of PGBD5 in primary immortalized human cells was sufficient to promote cell transformation in vivo. This activity required specific catalytic residues in the PGBD5 transposase domain as well as end-joining DNA repair and induced structural rearrangements with PSS breakpoints. These results define PGBD5 as an oncogenic mutator and provide a plausible mechanism for site-specific DNA rearrangements in childhood and adult solid tumors.

PGBD5 promotes site-specific oncogenic mutations in human tumors

Genomic rearrangements are a hallmark of human cancers. Here, we identify the piggyBac transposable element derived 5 (PGBD5) gene as encoding an active DNA transposase expressed in the majority of childhood solid tumors, including lethal rhabdoid tumors. Using assembly-based whole-genome DNA sequencing, we found previously undefined genomic rearrangements in human rhabdoid tumors. These rearrangements involved PGBD5-specific signal (PSS) sequences at their breakpoints and recurrently inactivated tumor-suppressor genes. PGBD5 was physically associated with genomic PSS sequences that were also sufficient to mediate PGBD5-induced DNA rearrangements in rhabdoid tumor cells. Ectopic expression of PGBD5 in primary immortalized human cells was sufficient to promote cell transformation in vivo. This activity required specific catalytic residues in the PGBD5 transposase domain as well as end-joining DNA repair and induced structural rearrangements with PSS breakpoints. These results define PGBD5 as an oncogenic mutator and provide a plausible mechanism for site-specific DNA rearrangements in childhood and adult solid tumors.

New insights into the evolutionary origins of the recombination-activating gene proteins and V(D)J recombination

The adaptive immune system of jawed vertebrates relies on V(D)J recombination as one of the main processes to generate the diverse array of receptors necessary for the recognition of a wide range of pathogens. The DNA cleavage reaction necessary for the assembly of the antigen receptor genes from an array of potential gene segments is mediated by the recombination-activating gene proteins RAG1 and RAG2. The RAG proteins have been proposed to originate from a transposable element (TE) as they share mechanistic and structural similarities with several families of transposases and are themselves capable of mediating transposition. A number of RAG-like proteins and TEs with sequence similarity to RAG1 and RAG2 have been identified, but only recently has their function begun to be characterized, revealing mechanistic links to the vertebrate RAGs. Of particular significance is the discovery of ProtoRAG, a transposon superfamily found in the genome of the basal chordate amphioxus. ProtoRAG has many of the sequence and mechanistic features predicted for the ancestral RAG transposon and is likely to be an evolutionary relative of RAG1 and RAG2. In addition, early observations suggesting that RAG1 is able to mediate V(D)J recombination in the absence of RAG2 have been confirmed, implying independent evolutionary origins for the two RAG genes. Here, recent progress in identifying and characterizing RAG-like proteins and the TEs that encode them is summarized and a refined model for the evolution of V(D)J recombination and the RAG proteins is presented.

Human RAG mutations: biochemistry and clinical implications

The recombination-activating gene 1 (RAG1) and RAG2 proteins initiate the V(D)J recombination process, which ultimately enables the generation of T cells and B cells with a diversified repertoire of antigen-specific receptors. Mutations of the RAG genes in humans are associated with a broad spectrum of clinical phenotypes, ranging from severe combined immunodeficiency to autoimmunity. Recently, novel insights into the phenotypic diversity of this disease have been provided by resolving the crystal structure of the RAG complex, by developing novel assays to test recombination activity of the mutant RAG proteins and by characterizing the molecular and cellular basis of immune dysregulation in patients with RAG deficiency.

Sleeping Beauty Transposition

Sleeping Beauty (SB) is a synthetic transposon that was constructed based on sequences of transpositionally inactive elements isolated from fish genomes. SB is a Tc1/mariner superfamily transposon following a cut-and-paste transpositional reaction, during which the element-encoded transposase interacts with its binding sites in the terminal inverted repeats of the transposon, promotes the assembly of a synaptic complex, catalyzes excision of the element out of its donor site, and integrates the excised transposon into a new location in target DNA. SB transposition is dependent on cellular host factors. Transcriptional control of transposase expression is regulated by the HMG2L1 transcription factor. Synaptic complex assembly is promoted by the HMGB1 protein and regulated by chromatin structure. SB transposition is highly dependent on the nonhomologous end joining (NHEJ) pathway of double-strand DNA break repair that generates a transposon footprint at the excision site. Through its association with the Miz-1 transcription factor, the SB transposase downregulates cyclin D1 expression that results in a slowdown of the cell-cycle in the G1 phase, where NHEJ is preferentially active. Transposon integration occurs at TA dinucleotides in the target DNA, which are duplicated at the flanks of the integrated transposon. SB shows a random genome-wide insertion profile in mammalian cells when launched from episomal vectors and "local hopping" when launched from chromosomal donor sites. Some of the excised transposons undergo a self-destructive autointegration reaction, which can partially explain why longer elements transpose less efficiently. SB became an important molecular tool for transgenesis, insertional mutagenesis, and gene therapy.

Retroviral Integrase: Then and Now

The retroviral integrases are virally encoded, specialized recombinases that catalyze the insertion of viral DNA into the host cell's DNA, a process that is essential for virus propagation. We have learned a great deal since the existence of an integrated form of retroviral DNA (the provirus) was first proposed by Howard Temin in 1964. Initial studies focused on the genetics and biochemistry of avian and murine virus DNA integration, but the pace of discovery increased substantially with advances in technology, and an influx of investigators focused on the human immunodeficiency virus. We begin with a brief account of the scientific landscape in which some of the earliest discoveries were made, and summarize research that led to our current understanding of the biochemistry of integration. A more detailed account of recent analyses of integrase structure follows, as they have provided valuable insights into enzyme function and raised important new questions.

Genomic DNA transposition induced by human PGBD5

Transposons are mobile genetic elements that are found in nearly all organisms, including humans. Mobilization of DNA transposons by transposase enzymes can cause genomic rearrangements, but our knowledge of human genes derived from transposases is limited. In this study, we find that the protein encoded by human PGBD5, the most evolutionarily conserved transposable element-derived gene in vertebrates, can induce stereotypical cut-and-paste DNA transposition in human cells. Genomic integration activity of PGBD5 requires distinct aspartic acid residues in its transposase domain, and specific DNA sequences containing inverted terminal repeats with similarity to piggyBac transposons. DNA transposition catalyzed by PGBD5 in human cells occurs genome-wide, with precise transposon excision and preference for insertion at TTAA sites. The apparent conservation of DNA transposition activity by PGBD5 suggests that genomic remodeling contributes to its biological function.

Recruitment of RAG1 and RAG2 to Chromatinized DNA during V(D)J Recombination

V(D)J recombination is initiated by the binding of the RAG1 and RAG2 proteins to recombination signal sequences (RSSs) that consist of conserved heptamer and nonamer sequences separated by a spacer of either 12 or 23 bp. Here, we used RAG-inducible pro-B v-Abl cell lines in conjunction with chromatin immunoprecipitation to better understand the protein and RSS requirements for RAG recruitment to chromatin. Using a catalytic mutant form of RAG1 to prevent recombination, we did not observe cooperation between RAG1 and RAG2 in their recruitment to endogenous Jκ gene segments over a 48-h time course. Using retroviral recombination substrates, we found that RAG1 was recruited inefficiently to substrates lacking an RSS or containing a single RSS, better to substrates with two 12-bp RSSs (12RSSs) or two 23-bp RSSs (23RSSs), and more efficiently to a substrate with a 12/23RSS pair. RSS mutagenesis demonstrated a major role for the nonamer element in RAG1 binding, and correspondingly, a cryptic RSS consisting of a repeat of CA dinucleotides, which poorly re-creates the nonamer, was ineffective in recruiting RAG1. Our findings suggest that 12RSS-23RSS cooperation (the "12/23 rule") is important not only for regulating RAG-mediated DNA cleavage but also for the efficiency of RAG recruitment to chromatin.

Recombination-activating gene 1 and 2 (RAG1 and RAG2) in flounder (Paralichthys olivaceus)

During the development of B and T lymphocytes, Ig and TCR variable region genes are assembled from germline V, D, and J gene segments by a site-specific recombination reaction known as V(D)J recombination. The process of somatic V(D)J recombination, mediated by the recombination-activating gene (RAG) products, is the most significant characteristic of adaptive immunity in jawed vertebrates. Flounder (Paralichthys olivaceus) RAG1 and RAG2 were isolated by Genome Walker and RT-PCR, and their expression patterns were analysed by RT-PCR and in situ hybridization on sections. RAG1 spans over 7.0 kb, containing 4 exons and 3 introns, and the full-length ORF is 3207 bp, encoding a peptide of 1068 amino acids. The first exon lies in the 5'-UTR, which is an alternative exon. RAG2 full-length ORF is 1062 bp, encodes a peptide of 533 amino acids, and lacks introns in the coding region. In 6-month old flounders, the expression of RAG1 and RAG2 was essentially restricted to the pronephros (head kidney) and mesonephros (truck kidney). Additionally, both of them were mainly expressed in the thymus. These results revealed that the thymus and kidney most likely serve as the primary lymphoid tissues in the flounder.

Long-Range Regulation of V(D)J Recombination

Given their essential role in adaptive immunity, antigen receptor loci have been the focus of analysis for many years and are among a handful of the most well-studied genes in the genome. Their investigation led initially to a detailed knowledge of linear structure and characterization of regulatory elements that confer control of their rearrangement and expression. However, advances in DNA FISH and imaging combined with new molecular approaches that interrogate chromosome conformation have led to a growing appreciation that linear structure is only one aspect of gene regulation and in more recent years, the focus has switched to analyzing the impact of locus conformation and nuclear organization on control of recombination. Despite decades of work and intense effort from numerous labs, we are still left with an incomplete picture of how the assembly of antigen receptor loci is regulated. This chapter summarizes our advances to date and points to areas that need further investigation.

Regulation and Evolution of the RAG Recombinase

The modular, noncontiguous architecture of the antigen receptor genes necessitates their assembly through V(D)J recombination. This program of DNA breakage and rejoining occurs during early lymphocyte development, and depends on the RAG1 and RAG2 proteins, whose collaborative endonuclease activity targets specific DNA motifs enriched in the antigen receptor loci. This essential gene shuffling reaction requires lymphocytes to traverse several developmental stages wherein DNA breakage is tolerated, while minimizing the expense to overall genome integrity. Thus, RAG activity is subject to stringent temporal and spatial regulation. The RAG proteins themselves also contribute autoregulatory properties that coordinate their DNA cleavage activity with target chromatin structure, cell cycle status, and DNA repair pathways. Even so, lapses in regulatory restriction of RAG activity are apparent in the aberrant V(D)J recombination events that underlie many lymphomas. In this review, we discuss the current understanding of the RAG endonuclease, its widespread binding in the lymphocyte genome, its noncleavage activities that restrain its enzymatic potential, and the growing evidence of its evolution from an ancient transposase.

Novel molecular mechanism for generating NK-cell fitness and memory

The mammalian immune system has been traditionally subdivided into two compartments known as the innate and the adaptive. T cells and B cells, which rearrange their antigen-receptor genes using the RAG recombinase, comprise the adaptive arm of immunity. Meanwhile, every other white blood cell has been grouped together under the broad umbrella of innate immunity, including NK cells. NK cells are considered innate lymphocytes because of their rapid responses to stressed cells and their ability to develop without receptor gene rearrangement (i.e. in RAG-deficient mice). However, new findings implicate a critical function for RAG proteins during NK-cell ontogeny, and suggest a novel mechanism by which controlled DNA breaks during NK-cell development dictate the fitness, function, and longevity of these cells. This review highlights recent work describing how DNA break events can impact cellular differentiation and fitness in a variety of cell types and settings.

The RAG recombinase dictates functional heterogeneity and cellular fitness in natural killer cells

The emergence of recombination-activating genes (RAGs) in jawed vertebrates endowed adaptive immune cells with the ability to assemble a diverse set of antigen receptor genes. In contrast, innate lymphocytes, such as natural killer (NK) cells, are not believed to require RAGs. Here, we report that NK cells unable to express RAGs or RAG endonuclease activity during ontogeny exhibit a cell-intrinsic hyperresponsiveness but a diminished capacity to survive following virus-driven proliferation, a reduced expression of DNA damage response mediators, and defects in the repair of DNA breaks. Evidence for this novel function of RAG has also been observed in T cells and innate lymphoid cells (ILCs), revealing an unexpected role for RAG proteins beyond V(D)J recombination. We propose that DNA cleavage events mediated by RAG endow developing adaptive and innate lymphocytes with a cellular "fitness" that safeguards their persistence later in life during episodes of rapid proliferation or cellular stress.

Assembly Pathway and Characterization of the RAG1/2-DNA Paired and Signal-end Complexes

Mammalian immune receptor diversity is established via a unique restricted set of site-specific DNA rearrangements in lymphoid cells, known as V(D)J recombination. The lymphoid-specific RAG1-RAG2 protein complex (RAG1/2) initiates this process by binding to two types of recombination signal sequences (RSS), 12RSS and 23RSS, and cleaving at the boundaries of RSS and V, D, or J gene segments, which are to be assembled into immunoglobulins and T-cell receptors. Here we dissect the ordered assembly of the RAG1/2 heterotetramer with 12RSS and 23RSS DNAs. We find that RAG1/2 binds only a single 12RSS or 23RSS and reserves the second DNA-binding site specifically for the complementary RSS, to form a paired complex that reflects the known 12/23 rule of V(D)J recombination. The assembled RAG1/2 paired complex is active in the presence of Mg(2+), the physiologically relevant metal ion, in nicking and double-strand cleavage of both RSS DNAs to produce a signal-end complex. We report here the purification and initial crystallization of the RAG1/2 signal-end complex for atomic-resolution structure elucidation. Strict pairing of the 12RSS and 23RSS at the binding step, together with information from the crystal structure of RAG1/2, leads to a molecular explanation of the 12/23 rule.

Compound heterozygous mutation of Rag1 leading to Omenn syndrome

Omenn syndrome is a primary immunodeficiency disorder, featuring susceptibility to infections and autoreactive T cells and resulting from defective genomic rearrangement of genes for the T cell and B cell receptors. The most frequent etiologies are hypomorphic mutations in "non-core" regions of the Rag1 or Rag2 genes, the protein products of which are critical members of the cellular apparatus for V(D)J recombination. In this report, we describe an infant with Omenn syndrome with a previously unreported termination mutation (p.R142*) in Rag1 on one allele and a partially characterized substitution mutation (p.V779M) in a "core" region of the other Rag1 allele. Using a cellular recombination assay, we found that while the p.R142* mutation completely abolished V(D)J recombination activity, the p.V779M mutation conferred a severe, but not total, loss of V(D)J recombination activity. The recombination defect of the V779 mutant was not due to overall misfolding of Rag1, however, as this mutant supported wild-type levels of V(D)J cleavage. These findings provide insight into the role of this poorly understood region of Rag1 and support the role of Rag1 in a post-cleavage stage of recombination.

P Transposable Elements in Drosophila and other Eukaryotic Organisms

P transposable elements were discovered in Drosophila as the causative agents of a syndrome of genetic traits called hybrid dysgenesis. Hybrid dysgenesis exhibits a unique pattern of maternal inheritance linked to the germline-specific small RNA piwi-interacting (piRNA) pathway. The use of P transposable elements as vectors for gene transfer and as genetic tools revolutionized the field of Drosophila molecular genetics. P element transposons have served as a useful model to investigate mechanisms of cut-and-paste transposition in eukaryotes. Biochemical studies have revealed new and unexpected insights into how eukaryotic DNA-based transposons are mobilized. For example, the P element transposase makes unusual 17nt-3' extended double-strand DNA breaks at the transposon termini and uses guanosine triphosphate (GTP) as a cofactor to promote synapsis of the two transposon ends early in the transposition pathway. The N-terminal DNA binding domain of the P element transposase, called a THAP domain, contains a C2CH zinc-coordinating motif and is the founding member of a large family of animal-specific site-specific DNA binding proteins. Over the past decade genome sequencing efforts have revealed the presence of P element-like transposable elements or P element transposase-like genes (called THAP9) in many eukaryotic genomes, including vertebrates, such as primates including humans, zebrafish and Xenopus, as well as the human parasite Trichomonas vaginalis, the sea squirt Ciona, sea urchin and hydra. Surprisingly, the human and zebrafish P element transposase-related THAP9 genes promote transposition of the Drosophila P element transposon DNA in human and Drosophila cells, indicating that the THAP9 genes encode active P element "transposase" proteins.

Retroviral Integrase Structure and DNA Recombination Mechanism

Due to the importance of human immunodeficiency virus type 1 (HIV-1) integrase as a drug target, the biochemistry and structural aspects of retroviral DNA integration have been the focus of intensive research during the past three decades. The retroviral integrase enzyme acts on the linear double-stranded viral DNA product of reverse transcription. Integrase cleaves specific phosphodiester bonds near the viral DNA ends during the 3' processing reaction. The enzyme then uses the resulting viral DNA 3'-OH groups during strand transfer to cut chromosomal target DNA, which simultaneously joins both viral DNA ends to target DNA 5'-phosphates. Both reactions proceed via direct transesterification of scissile phosphodiester bonds by attacking nucleophiles: a water molecule for 3' processing, and the viral DNA 3'-OH for strand transfer. X-ray crystal structures of prototype foamy virus integrase-DNA complexes revealed the architectures of the key nucleoprotein complexes that form sequentially during the integration process and explained the roles of active site metal ions in catalysis. X-ray crystallography furthermore elucidated the mechanism of action of HIV-1 integrase strand transfer inhibitors, which are currently used to treat AIDS patients, and provided valuable insights into the mechanisms of viral drug resistance.

Crystal structure of the V(D)J recombinase RAG1-RAG2

V(D)J recombination in the vertebrate immune system generates a highly diverse population of immunoglobulins and T-cell receptors by combinatorial joining of segments of coding DNA. The RAG1-RAG2 protein complex initiates this site-specific recombination by cutting DNA at specific sites flanking the coding segments. Here we report the crystal structure of the mouse RAG1-RAG2 complex at 3.2 Å resolution. The 230-kilodalton RAG1-RAG2 heterotetramer is 'Y-shaped', with the amino-terminal domains of the two RAG1 chains forming an intertwined stalk. Each RAG1-RAG2 heterodimer composes one arm of the 'Y', with the active site in the middle and RAG2 at its tip. The RAG1-RAG2 structure rationalizes more than 60 mutations identified in immunodeficient patients, as well as a large body of genetic and biochemical data. The architectural similarity between RAG1 and the hairpin-forming transposases Hermes and Tn5 suggests the evolutionary conservation of these DNA rearrangements.

The architecture of the 12RSS in V(D)J recombination signal and synaptic complexes

V(D)J recombination is initiated by RAG1 and RAG2, which together with HMGB1 bind to a recombination signal sequence (12RSS or 23RSS) to form the signal complex (SC) and then capture a complementary partner RSS, yielding the paired complex (PC). Little is known regarding the structural changes that accompany the SC to PC transition or the structural features that allow RAG to distinguish its two asymmetric substrates. To address these issues, we analyzed the structure of the 12RSS in the SC and PC using fluorescence resonance energy transfer (FRET) and molecular dynamics modeling. The resulting models indicate that the 12RSS adopts a strongly bent V-shaped structure upon RAG/HMGB1 binding and reveal structural differences, particularly near the heptamer, between the 12RSS in the SC and PC. Comparison of models of the 12RSS and 23RSS in the PC reveals broadly similar shapes but a distinct number and location of DNA bends as well as a smaller central cavity for the 12RSS. These findings provide the most detailed view yet of the 12RSS in RAG–DNA complexes and highlight structural features of the RSS that might underlie activation of RAG-mediated cleavage and substrate asymmetry important for the 12/23 rule of V(D)J recombination.

The RAG2 C-terminus and ATM protect genome integrity by controlling antigen receptor gene cleavage

Tight control of antigen-receptor gene rearrangement is required to preserve genome integrity and prevent the occurrence of leukemia and lymphoma. Nonetheless, mistakes can happen, leading to the generation of aberrant rearrangements, such as Tcra/d-Igh inter-locus translocations that are a hallmark of ATM deficiency. Current evidence indicates that these translocations arise from the persistence of unrepaired breaks converging at different stages of thymocyte differentiation. Here we show that a defect in feedback control of RAG2 activity gives rise to bi-locus breaks and damage on Tcra/d and Igh in the same T cell at the same developmental stage, which provides a direct mechanism for generating these inter-locus rearrangements. Both the RAG2 C-terminus and ATM prevent bi-locus RAG-mediated cleavage through modulation of 3D conformation (higher order loops) and nuclear organization of the two loci. This limits the number of potential substrates for translocation and provides an important mechanism for protecting genome stability.

RAG2's acidic hinge restricts repair-pathway choice and promotes genomic stability

V(D)J recombination-associated DNA double-strand breaks (DSBs) are normally repaired by the high-fidelity classical nonhomologous end-joining (cNHEJ) machinery. Previous studies implicated the recombination-activating gene (RAG)/DNA postcleavage complex (PCC) in regulating pathway choice by preventing access to inappropriate repair mechanisms such as homologous recombination (HR) and alternative NHEJ (aNHEJ). Here, we report that RAG2's "acidic hinge," previously of unknown function, is critical for several key steps. Mutations that reduce the hinge's negative charge destabilize the PCC, disrupt pathway choice, permit repair of RAG-mediated DSBs by the translocation-prone aNHEJ machinery, and reduce genomic stability in developing lymphocytes. Structural predictions and experimental results support our hypothesis that reduced flexibility of the hinge underlies these outcomes. Furthermore, sequence variants present in the human population reduce the hinge's negative charge, permit aNHEJ, and diminish genomic integrity.

Metabolic sensor AMPK directly phosphorylates RAG1 protein and regulates V(D)J recombination

The ability to sense metabolic stress is critical for successful cellular adaptation. In eukaryotes, the AMP-activated protein kinase (AMPK), a highly conserved serine/threonine kinase, functions as a critical metabolic sensor. AMPK is activated by the rising ADP/ATP and AMP/ATP ratios during conditions of energy depletion and also by increasing intracellular Ca(2+). In response to metabolic stress, AMPK maintains energy homeostasis by phosphorylating and regulating proteins that are involved in many physiological processes including glucose and fatty acid metabolism, transcription, cell growth, mitochondrial biogenesis, and autophagy. Evidence is mounting that AMPK also plays a role in a number of pathways unrelated to energy metabolism. Here, we identify the recombination-activating gene 1 protein (RAG1) as a substrate of AMPK. The RAG1/RAG2 complex is a lymphoid-specific endonuclease that catalyzes specific DNA cleavage during V(D)J recombination, which is required for the assembly of the Ig and T-cell receptor genes of the immune system. AMPK directly phosphorylates RAG1 at serine 528, and the phosphorylation enhances the catalytic activity of the RAG complex, resulting in increased cleavage of oligonucleotide substrates in vitro, or increased recombination of an extrachromosomal substrate in a cellular assay. Our results suggest that V(D)J recombination can be regulated by AMPK activation, providing a potential new link between metabolic stress and development of B and T lymphocytes.

Role of recombination activating genes in the generation of antigen receptor diversity and beyond

V(D)J recombination is the process by which antibody and T-cell receptor diversity is attained. During this process, antigen receptor gene segments are cleaved and rejoined by non-homologous DNA end joining for the generation of combinatorial diversity. The major players of the initial process of cleavage are the proteins known as RAG1 (recombination activating gene 1) and RAG2. In this review, we discuss the physiological function of RAGs as a sequence-specific nuclease and its pathological role as a structure-specific nuclease. The first part of the review discusses the basic mechanism of V(D)J recombination, and the last part focuses on how the RAG complex functions as a sequence-specific and structure-specific nuclease. It also deals with the off-target cleavage of RAGs and its implications in genomic instability.

Higher-order looping and nuclear organization of Tcra facilitate targeted rag cleavage and regulated rearrangement in recombination centers

V(D)J recombination is essential for generating a diverse array of B and T cell receptors that can recognize and combat foreign antigens. As with any recombination event, tight control is essential to prevent the occurrence of genetic anomalies that drive cellular transformation. One important aspect of regulation is directed targeting of the RAG recombinase. Indeed, RAG accumulates at the 3' end of individual antigen receptor loci poised for rearrangement; however, it is not known whether focal binding is involved in regulating cleavage, and what mechanisms lead to enrichment of RAG in this region. Here, we show that monoallelic looping out of the 3' end of the T cell receptor α (Tcra) locus, coupled with transcription and increased chromatin/nuclear accessibility, is linked to focal RAG binding and ATM-mediated regulation of monoallelic cleavage on looped-out 3' regions. Our data identify higher-order loop formation as a key determinant of directed RAG targeting and the maintenance of genome stability.

Cooperative recruitment of HMGB1 during V(D)J recombination through interactions with RAG1 and DNA

During V(D)J recombination, recombination activating gene (RAG)1 and RAG2 bind and cleave recombination signal sequences (RSSs), aided by the ubiquitous DNA-binding/-bending proteins high-mobility group box protein (HMGB)1 or HMGB2. HMGB1/2 play a critical, although poorly understood, role in vitro in the assembly of functional RAG–RSS complexes, into which HMGB1/2 stably incorporate. The mechanism of HMGB1/2 recruitment is unknown, although an interaction with RAG1 has been suggested. Here, we report data demonstrating only a weak HMGB1–RAG1 interaction in the absence of DNA in several assays, including fluorescence anisotropy experiments using a novel Alexa488-labeled HMGB1 protein. Addition of DNA to RAG1 and HMGB1 in fluorescence anisotropy experiments, however, results in a substantial increase in complex formation, indicating a synergistic binding effect. Pulldown experiments confirmed these results, as HMGB1 was recruited to a RAG1–DNA complex in a RAG1 concentration-dependent manner and, interestingly, without strict RSS sequence specificity. Our finding that HMGB1 binds more tightly to a RAG1–DNA complex over RAG1 or DNA alone provides an explanation for the stable integration of this typically transient architectural protein in the V(D)J recombinase complex throughout recombination. These findings also have implications for the order of events during RAG–DNA complex assembly and for the stabilization of sequence-specific and non-specific RAG1–DNA interactions.

RAG and HMGB1 create a large bend in the 23RSS in the V(D)J recombination synaptic complexes

During V(D)J recombination, recombination activating gene proteins RAG1 and RAG2 generate DNA double strand breaks within a paired complex (PC) containing two complementary recombination signal sequences (RSSs), the 12RSS and 23RSS, which differ in the length of the spacer separating heptamer and nonamer elements. Despite the central role of the PC in V(D)J recombination, little is understood about its structure. Here, we use fluorescence resonance energy transfer to investigate the architecture of the 23RSS in the PC. Energy transfer was detected in 23RSS substrates in which the donor and acceptor fluorophores flanked the entire RSS, and was optimal under conditions that yield a cleavage-competent PC. The data are most easily explained by a dramatic bend in the 23RSS that reduces the distance between these flanking regions from >160 Å in the linear substrate to <80 Å in the PC. Analysis of multiple fluorescent substrates together with molecular dynamics modeling yielded a model in which the 23RSS adopts a U shape in the PC, with the spacer located centrally within the bend. We propose that this large bend facilitates simultaneous recognition of the heptamer and nonamer, is critical for proper positioning of the active site and contributes to the 12/23 rule.

Functional characterization of an active Rag-like transposase

The formation of diverse immunoglobulin genes results in part from Rag protein-mediated DNA double strand breaks at the edges of immunoglobulin gene segments, followed by the combinatorial reassembly of these segments. We report that a Transib transposase from the insect Helicoverpa zea is active in vitro and that its breakage and joining activities mimic those of Rag, providing strong evidence that Rag and Transib transposases were derived from a common progenitor.

Cloning and expression analysis of recombination activating genes (RAG1/2) in red snapper (Lutjanus sanguineus)

Recombination activating genes (RAG1 and RAG2), involved in the V(D)J recombination of immunoglobulin and T-cell receptor genes play a crucial role in the adaptive immune response in vertebrates. The expression of these genes was required for the proper development and maturity of lymphocytes so that they can be used as useful markers to evaluate the development of lymphoid organ. In this paper, the cDNA of RAG1 and RAG2 in red snapper, Lutjanus sanguineus were cloned by homological cloning and rapid amplification of cDNA ends (RACE) methods. Results showed the full length of RAG1 cDNA was 3944 bp, containing a 5' untranslated region (UTR) of 200 bp, a 3'-UTR of 561 bp and an open reading frame of 3183 bp encoding 1060 amino acids. Three important structural motifs, a RING/U-box domain, a RING/FYVE/PHD-type domain and a RAG Nonamer-binding domain were detected in the deduced amino acid sequence of RAG1 by InterProScan analysis. The full length of RAG2 cDNA was 2200 bp, consisting of a 141 bp 5'-UTR, a 457 bp 3'-UTR and an open reading frame of 1602 bp encoding 533 amino acids. Two important structural motifs, a Galactose oxidase/kelch, beta-propeller domain and a kelch-type beta-propeller domain were detected in the deduced amino acid sequence of RAG2 by InterProScan analysis. BLAST analysis revealed that the RAG1 and RAG2 in red snapper shared a high homology with other known RAG1 and RAG2 genes, while the greatest degree of identity was observed with Hippoglossus hippoglossus RAG1 at 82% and Takifugu rubripes RAG2 at 87%, respectively. The differential expressions of RAG1 and RAG2 in various tissues of red snapper were analyzed by fluorescent quantitative real-time PCR. The overall expression pattern of the two genes was quite similar. In healthy red snappers, the RAGs transcripts were mainly detected in thymus, following head kidney, spleen, intestine, liver and brain. After vaccinated with inactivated Vibrio alginolyticus 48 h later, the RAGs mRNA expression was significantly up-regulated in all studied tissues of red snapper. A clear time-dependent expression pattern of RAG1 and RAG2 after immunization and the expression reached the highest level at 48 h in thymus, 60 h in head kidney and spleen, respectively. These findings indicated that RAG1 and RAG2 could play an important role in the immune response to bacteria in red snapper.

Accumulation of B1-like B cells in transgenic mice over-expressing catalytically inactive RAG1 in the periphery

During their development, B lymphocytes undergo V(D)J recombination events and selection processes that, if successfully completed, produce mature B cells expressing a non-self-reactive B-cell receptor (BCR). Primary V(D)J rearrangements yield self-reactive B cells at high frequency, triggering attempts to remove, silence, or reprogramme them through deletion, anergy induction, or secondary V(D)J recombination (receptor editing), respectively. In principle, expressing a catalytically inactive V(D)J recombinase during a developmental stage in which V(D)J rearrangement is initiated may impair this process. To test this idea, we generated transgenic mice expressing a RAG1 active site mutant (dnRAG1 mice); RAG1 transcript was elevated in splenic, but not bone marrow, B cells in dnRAG1 mice relative to wild-type mice. The dnRAG1 mice accumulate splenic B cells with a B1-like phenotype that exhibit defects in B-cell activation, and are clonally diverse, yet repertoire restricted with a bias toward Jκ1 gene segment usage. The dnRAG1 mice show evidence of impaired B-cell development at the immature-to-mature transition, immunoglobulin deficiency, and poorer immune responses to thymus-independent antigens. Interestingly, dnRAG1 mice expressing the anti-dsDNA 3H9H56R heavy chain fail to accumulate splenic B1-like cells, yet retain peritoneal B1 cells. Instead, these mice show an expanded marginal zone compartment, but no difference is detected in the frequency of heavy chain gene replacement. Taken together, these data suggest a model in which dnRAG1 expression impairs secondary V(D)J recombination. As a result, selection and/or differentiation processes are altered in a way that promotes expansion of B1-like B cells in the spleen.

Chromatin topology and the regulation of antigen receptor assembly

During an organism's ontogeny and in the adult, each B and T lymphocyte generates a unique antigen receptor, thereby creating the organism's ability to respond to a vast number of different antigens. The antigen receptor loci are organized into distinct regions that contain multiple variable (V), diversity (D), and/or joining (J) and constant (C) coding elements that are scattered across large genomic regions. In this review, we discuss the epigenetic modifications that take place in the different antigen receptor loci, the chromatin structure adopted by the antigen receptor loci to allow recombination of elements separated by large genomic distances, and the relationship between epigenetics and chromatin structure and how they relate to the generation of antigen receptor diversity.

V(D)J recombination: mechanisms of initiation

V(D)J recombination assembles immunoglobulin and T cell receptor genes during lymphocyte development through a series of carefully orchestrated DNA breakage and rejoining events. DNA cleavage requires a series of protein-DNA complexes containing the RAG1 and RAG2 proteins and recombination signals that flank the recombining gene segments. In this review, we discuss recent advances in our understanding of the function and domain organization of the RAG proteins, the composition and structure of RAG-DNA complexes, and the pathways that lead to the formation of these complexes. We also consider the functional significance of RAG-mediated histone recognition and ubiquitin ligase activities, and the role played by RAG in ensuring proper repair of DNA breaks made during V(D)J recombination. Finally, we propose a model for the formation of RAG-DNA complexes that involves anchoring of RAG1 at the recombination signal nonamer and RAG2-dependent surveillance of adjoining DNA for suitable spacer and heptamer sequences.

Elucidating the domain architecture and functions of non-core RAG1: the capacity of a non-core zinc-binding domain to function in nuclear import and nucleic acid binding

Background

The repertoire of the antigen-binding receptors originates from the rearrangement of immunoglobulin and T-cell receptor genetic loci in a process known as V(D)J recombination. The initial site-specific DNA cleavage steps of this process are catalyzed by the lymphoid specific proteins RAG1 and RAG2. The majority of studies on RAG1 and RAG2 have focused on the minimal, core regions required for catalytic activity. Though not absolutely required, non-core regions of RAG1 and RAG2 have been shown to influence the efficiency and fidelity of the recombination reaction.

Results

Using a partial proteolysis approach in combination with bioinformatics analyses, we identified the domain boundaries of a structural domain that is present in the 380-residue N-terminal non-core region of RAG1. We term this domain the Central Non-core Domain (CND; residues 87-217).

Conclusions

We show how the CND alone, and in combination with other regions of non-core RAG1, functions in nuclear localization, zinc coordination, and interactions with nucleic acid. Together, these results demonstrate the multiple roles that the non-core region can play in the function of the full length protein.