Temporal and spatial regulatory functions of the V(D)J recombinase

RAG suppresses group 2 innate lymphoid cells

Antigen specificity is the central trait distinguishing adaptive from innate immune function. Assembly of antigen-specific T cell and B cell receptors occurs through V(D)J recombination mediated by the Recombinase Activating Gene endonucleases RAG1 and RAG2 (collectively called RAG). In the absence of RAG, mature T and B cells do not develop and thus RAG is critically associated with adaptive immune function. In addition to adaptive T helper 2 (Th2) cells, group 2 innate lymphoid cells (ILC2s) contribute to type 2 immune responses by producing cytokines like Interleukin-5 (IL-5) and IL-13. Although it has been reported that RAG expression modulates the function of innate natural killer (NK) cells, whether other innate immune cells such as ILC2s are affected by RAG remains unclear. We find that in RAG-deficient mice, ILC2 populations expand and produce increased IL-5 and IL-13 at steady state and contribute to increased inflammation in atopic dermatitis (AD)-like disease. Further, we show that RAG modulates ILC2 function in a cell-intrinsic manner independent of the absence or presence of adaptive T and B lymphocytes. Lastly, employing multiomic single cell analyses of RAG1 lineage-traced cells, we identify key transcriptional and epigenomic ILC2 functional programs that are suppressed by a history of RAG expression. Collectively, our data reveal a novel role for RAG in modulating innate type 2 immunity through suppression of ILC2s.

When DNA-damage responses meet innate and adaptive immunity

When cells proliferate, stress on DNA replication or exposure to endogenous or external insults frequently results in DNA damage. DNA-Damage Response (DDR) networks are complex signaling pathways used by multicellular organisms to prevent DNA damage. Depending on the type of broken DNA, the various pathways, Base-Excision Repair (BER), Nucleotide Excision Repair (NER), Mismatch Repair (MMR), Homologous Recombination (HR), Non-Homologous End-Joining (NHEJ), Interstrand Crosslink (ICL) repair, and other direct repair pathways, can be activated separately or in combination to repair DNA damage. To preserve homeostasis, innate and adaptive immune responses are effective defenses against endogenous mutation or invasion by external pathogens. It is interesting to note that new research keeps showing how closely DDR components and the immune system are related. DDR and immunological response are linked by immune effectors such as the cyclic GMP-AMP synthase (cGAS)-Stimulator of Interferon Genes (STING) pathway. These effectors act as sensors of DNA damage-caused immune response. Furthermore, DDR components themselves function in immune responses to trigger the generation of inflammatory cytokines in a cascade or even trigger programmed cell death. Defective DDR components are known to disrupt genomic stability and compromise immunological responses, aggravating immune imbalance and leading to serious diseases such as cancer and autoimmune disorders. This study examines the most recent developments in the interaction between DDR elements and immunological responses. The DDR network's immune modulators' dual roles may offer new perspectives on treating infectious disorders linked to DNA damage, including cancer, and on the development of target immunotherapy.

The role of chromatin loop extrusion in antibody diversification

Cohesin mediates chromatin loop formation across the genome by extruding chromatin between convergently oriented CTCF-binding elements. Recent studies indicate that cohesin-mediated loop extrusion in developing B cells presents immunoglobulin heavy chain (Igh) variable (V), diversity (D) and joining (J) gene segments to RAG endonuclease through a process referred to as RAG chromatin scanning. RAG initiates V(D)J recombinational joining of these gene segments to generate the large number of different Igh variable region exons that are required for immune responses to diverse pathogens. Antigen-activated mature B cells also use chromatin loop extrusion to mediate the synapsis, breakage and end joining of switch regions flanking Igh constant region exons during class-switch recombination, which allows for the expression of different antibody constant region isotypes that optimize the functions of antigen-specific antibodies to eliminate pathogens. Here, we review recent advances in our understanding of chromatin loop extrusion during V(D)J recombination and class-switch recombination at the Igh locus.

How transcription factors drive choice of the T cell fate

Recent evidence has elucidated how multipotent blood progenitors transform their identities in the thymus and undergo commitment to become T cells. Together with environmental signals, a core group of transcription factors have essential roles in this process by directly activating and repressing specific genes. Many of these transcription factors also function in later T cell development, but control different genes. Here, we review how these transcription factors work to change the activities of specific genomic loci during early intrathymic development to establish T cell lineage identity. We introduce the key regulators and highlight newly emergent insights into the rules that govern their actions. Whole-genome deep sequencing-based analysis has revealed unexpectedly rich relationships between inherited epigenetic states, transcription factor-DNA binding affinity thresholds and influences of given transcription factors on the activities of other factors in the same cells. Together, these mechanisms determine T cell identity and make the lineage choice irreversible.

Repair of G1 induced DNA double-strand breaks in S-G2/M by alternative NHEJ

The alternative non-homologous end-joining (NHEJ) pathway promotes DNA double-strand break (DSB) repair in cells deficient for NHEJ or homologous recombination, suggesting that it operates at all stages of the cell cycle. Here, we use an approach in which DNA breaks can be induced in G1 cells and their repair tracked, enabling us to show that joining of DSBs is not functional in G1-arrested XRCC4-deficient cells. Cell cycle entry into S-G2/M restores DSB repair by Pol θ-dependent and PARP1-independent alternative NHEJ with repair products bearing kilo-base long DNA end resection, micro-homologies and chromosome translocations. We identify a synthetic lethal interaction between XRCC4 and Pol θ under conditions of G1 DSBs, associated with accumulation of unresolved DNA ends in S-G2/M. Collectively, our results support the conclusion that the repair of G1 DSBs progressing to S-G2/M by alternative NHEJ drives genomic instability and represent an attractive target for future DNA repair-based cancer therapies.

Depending on the cell cycle stage, cells can repair their genome via different pathways. Here the authors reveal mechanistic insights into repair of double strand breaks induced during G1 in an error-prone manner by Pol θ-dependent and PARP1-independent alt NHEJ during the SG2/M phases of the cell cycle

Intra-Vκ Cluster Recombination Shapes the Ig Kappa Locus Repertoire

During V(D)J recombination, RAG proteins introduce DNA double-strand breaks (DSBs) at recombination signal sequences (RSSs) that contain either 12- or 23-nt spacer regions. Coordinated 12/23 cleavage predicts that DSBs at variable (V) gene segments should equal the level of breakage at joining (J) segments. Contrary to this, here we report abundant RAG-dependent DSBs at multiple Vκ gene segments independent of V-J rearrangement. We find that a large fraction of Vκ gene segments are flanked not only by a bone-fide 12 spacer but also an overlapping, 23-spacer flipped RSS. These compatible pairs of RSSs mediate recombination and deletion inside the Vκ cluster even in the complete absence of Jκ gene segments and support a V(D)J recombination center (RC) independent of the conventional Jκ-centered RC. We propose an improved model of Vκ-Jκ repertoire formation by incorporating these surprisingly frequent, evolutionarily conserved intra-Vκ cluster recombination events.

Cell circuits and niches controlling B cell development

Studies over the last decade uncovered overlapping niches for hematopoietic stem cells (HSCs), multipotent progenitor cells, common lymphoid progenitors, and early B cell progenitors. HSC and lymphoid niches are predominantly composed by mesenchymal progenitor cells (MPCs) and by a small subset of endothelial cells. Niche cells create specialized microenvironments through the concomitant production of short-range acting cell-fate determining cytokines such as interleukin (IL)-7 and stem cell factor and the potent chemoattractant C-X-C motif chemokine ligand 12. This type of cellular organization allows for the cross-talk between hematopoietic stem and progenitor cells with niche cells, such that niche cell activity can be regulated by the quality and quantity of hematopoietic progenitors being produced. For example, preleukemic B cell progenitors and preB acute lymphoblastic leukemias interact directly with MPCs, and downregulate IL-7 expression and the production of non-leukemic lymphoid cells. In this review, we discuss a novel model of B cell development that is centered on cellular circuits formed between B cell progenitors and lymphopoietic niches.

Cell circuits between B cell progenitors and IL-7+ mesenchymal progenitor cells control B cell development

B cell development is characterized by well-defined transitions. Fistonich et al. demonstrate that two distinct cell circuits formed between proB, preB, and IL-7⁺ cells regulate the size and quality of B cell progenitors and control B cell development.

B cell progenitors require paracrine signals such as interleukin-7 (IL-7) provided by bone marrow stromal cells for proliferation and survival. Yet, how B cells regulate access to these signals in vivo remains unclear. Here we show that proB and IL-7⁺ cells form a cell circuit wired by IL-7R signaling, which controls CXCR4 and focal adhesion kinase (FAK) expression and restricts proB cell movement due to increased adhesion to IL-7⁺CXCL12^Hi cells. PreBCR signaling breaks this circuit by switching the preB cell behavior into a fast-moving and lower-adhesion state via increased CXCR4 and reduced FAK/α4β1 expression. This behavioral change reduces preB cell exposure to IL-7, thereby attenuating IL-7R signaling in vivo. Remarkably, IL-7 production is downregulated by signals provided by preB cells with unrepaired double-stranded DNA breaks and by preB acute lymphoblastic leukemic cells. Combined, these studies revealed that distinct cell circuits control the quality and homeostasis of B cell progenitors.

Corrected and Republished from: BCL11A Is a Critical Component of a Transcriptional Network That Activates RAG Expression and V(D)J Recombination

Recombination activating gene 1 (RAG1) and RAG2 are critical enzymes for initiating variable-diversity-joining [V(D)J] segment recombination, an essential process for antigen receptor expression and lymphocyte development. The BCL11A transcription factor is required for B cell and plasmacytoid dendritic cell (pDC) development, but its molecular function(s) in early B cell fate specification and commitment is unknown. We show here that the major B cell isoform, BCL11A-XL, binds directly to the RAG1 promoter as well as directly to regulatory regions of transcription factors previously implicated in both B cell and pDC development to activate RAG1 and RAG2 gene transcription in pro- and pre-B cells. We employed BCL11A overexpression with recombination substrates to demonstrate direct consequences of BCL11A/RAG modulation on V(D)J recombination. We conclude that BCL11A is a critical component of a transcriptional network that regulates B cell fate by controlling V(D)J recombination.

iRAGu: A Novel Inducible and Reversible Mouse Model for Ubiquitous Recombinase Activity

Developing lymphocytes express the recombination activating genes (RAGs) 1 and 2 products that form a site specific recombinase complex (RAG), introducing double strand DNA breaks (DSBs) at recombination signal sequences (RSSs) flanking the V, D, and J gene segments in the antigen receptor loci. The subsequent steps in the reaction consist in the ligation of DSBs by ubiquitous enzymes of the non-homologous end joining DNA repair pathway. This mutagenesis process is responsible for the generation of the very large clonal diversity of T and B lymphocytes, itself allowing the recognition of a virtually open-ended antigenic universe. Sequences resembling RSS are found at high frequency all over the genome, and involved in RAG mediated illegitimate recombination and translocations. Hence, natural and induced ectopic activity of RAG is a threat to the genome only recently underscored. Here, we report and characterize a novel mouse transgenic system for which ubiquitous expression of the recombinase is inducible. In this system, the RAG1 protein is constitutively expressed and functional, while the RAG2 protein, coupled to the estrogen receptor, becomes functionally active upon 4-hydroxytamoxifen (TAM) administration. We describe two transgenic lines. The first one, when introgressed into an endogenous Rag2^−/− genetic background is faithfully recapitulating lymphocyte development, repertoire dynamics and cryptic rearrangements, in a TAM-dependent manner. In this model, deprivation of TAM is followed by lymphocyte development arrest, evidencing the reversibility of the system. The second transgenic line is leaky, as the transgenes promote lymphocyte differentiation in absence of TAM treatment. Upon TAM-induction defects in lymphocytes composition and global health reveals the deleterious effect of uncontrolled RAG activity. Overall, this novel transgenic model provides a tool where RAG activity can be specifically manipulated to assess the dynamics of lymphocyte differentiation and the challenges imposed by the recombinase on the vertebrate genome.

Riches in RAGs: Revealing the V(D)J Recombinase through High-Resolution Structures

Development of the adaptive immune system is dependent on V(D)J recombination, which forms functional antigen receptor genes through rearrangement of component gene segments. The V(D)J recombinase, comprising recombination-activating proteins RAG1 and RAG2, guides the initial DNA cleavage events to the recombination signal sequence (RSS), which flanks each gene segment. Although the enzymatic steps for RAG-mediated endonucleolytic activity were established over two decades ago, only recently have high-resolution structural studies of the catalytically active core regions of the RAG proteins shed light on conformational requirements for the reaction. While outstanding questions remain, we have a clearer picture of how RAG proteins function in generating the diverse repertoires of antigen receptors, the underlying foundation of the adaptive immune system.

PAXX and XLF DNA repair factors are functionally redundant in joining DNA breaks in a G1-arrested progenitor B-cell line

Classical nonhomologous end joining (C-NHEJ) is a major mammalian DNA double-strand break (DSB) repair pathway. Core C-NHEJ factors, such as XRCC4, are required for joining DSB intermediates of the G1 phase-specific V(D)J recombination reaction in progenitor lymphocytes. Core factors also contribute to joining DSBs in cycling mature B-lineage cells, including DSBs generated during antibody class switch recombination (CSR) and DSBs generated by ionizing radiation. The XRCC4-like-factor (XLF) C-NHEJ protein is dispensable for V(D)J recombination in normal cells, but because of functional redundancy, it is absolutely required for this process in cells deficient for the ataxia telangiectasia-mutated (ATM) DSB response factor. The recently identified paralogue of XRCC4 and XLF (PAXX) factor has homology to these two proteins and variably contributes to ionizing radiation-induced DSB repair in human and chicken cells. We now report that PAXX is dispensable for joining V(D)J recombination DSBs in G1-arrested mouse pro-B-cell lines, dispensable for joining CSR-associated DSBs in a cycling mouse B-cell line, and dispensable for normal ionizing radiation resistance in both G1-arrested and cycling pro-B lines. However, we find that combined deficiency for PAXX and XLF in G1-arrested pro-B lines abrogates DSB joining during V(D)J recombination and sensitizes the cells to ionizing radiation exposure. Thus, PAXX provides core C-NHEJ factor-associated functions in the absence of XLF and vice versa in G1-arrested pro-B-cell lines. Finally, we also find that PAXX deficiency has no impact on V(D)J recombination DSB joining in ATM-deficient pro-B lines. We discuss implications of these findings with respect to potential PAXX and XLF functions in C-NHEJ.

Orientation-specific RAG activity in chromosomal loop domains contributes to Tcrd V(D)J recombination during T cell development

T cell antigen receptor δ (Tcrd) variable region exons are assembled by RAG-initiated V(D)J recombination events in developing γδ thymocytes. Here, we use linear amplification-mediated high-throughput genome-wide translocation sequencing (LAM-HTGTS) to map hundreds of thousands of RAG-initiated Tcrd D segment (Trdd1 and Trdd2) rearrangements in CD4(-)CD8(-) double-negative thymocyte progenitors differentiated in vitro from bone marrow-derived hematopoietic stem cells. We find that Trdd2 joins directly to Trdv, Trdd1, and Trdj segments, whereas Trdd1 joining is ordered with joining to Trdd2, a prerequisite for further rearrangement. We also find frequent, previously unappreciated, Trdd1 and Trdd2 rearrangements that inactivate Tcrd, including sequential rearrangements from V(D)J recombination signal sequence fusions. Moreover, we find dozens of RAG off-target sequences that are generated via RAG tracking both upstream and downstream from the Trdd2 recombination center across the Tcrd loop domain that is bounded by the upstream INT1-2 and downstream TEA elements. Disruption of the upstream INT1-2 boundary of this loop domain allows spreading of RAG on- and off-target activity to the proximal Trdv domain and, correspondingly, shifts the Tcrd V(D)J recombination landscape by leading to predominant V(D)J joining to a proximal Trdv3 pseudogene that lies just upstream of the normal boundary.

The cohesin complex prevents the end-joining of distant DNA double-strand ends in S phase: Consequences on genome stability maintenance

DNA double-strand break (DSB) repair is essential for genome stability maintenance, but the joining of distant DNA double strand ends (DSEs) inevitably leads to genome rearrangements. Therefore, DSB repair should be tightly controlled to secure genome stability while allowing genetic variability. Tethering of the proximal ends of a 2-ended DSB limits their mobility, protecting thus against their joining with a distant DSE. However, replication stress generates DSBs with only one DSE, on which tethering is impossible. Consistently, we demonstrated that the joining of 2 DSBs only 3.2 kb apart is repressed in the S, but not the G1, phase, revealing an additional mechanism limiting DNA ends mobility in S phase. The cohesin complex, by maintaining the 2 sister chromatids linked, limits DSEs mobility and thus represses the joining of distant DSEs, while allowing that of adjacent DSEs. At the genome scale, the cohesin complex protects against deletions, inversions, translocations and chromosome fusion.

TFIIS-Dependent Non-coding Transcription Regulates Developmental Genome Rearrangements

Because of their nuclear dimorphism, ciliates provide a unique opportunity to study the role of non-coding RNAs (ncRNAs) in the communication between germline and somatic lineages. In these unicellular eukaryotes, a new somatic nucleus develops at each sexual cycle from a copy of the zygotic (germline) nucleus, while the old somatic nucleus degenerates. In the ciliate Paramecium tetraurelia, the genome is massively rearranged during this process through the reproducible elimination of repeated sequences and the precise excision of over 45,000 short, single-copy Internal Eliminated Sequences (IESs). Different types of ncRNAs resulting from genome-wide transcription were shown to be involved in the epigenetic regulation of genome rearrangements. To understand how ncRNAs are produced from the entire genome, we have focused on a homolog of the TFIIS elongation factor, which regulates RNA polymerase II transcriptional pausing. Six TFIIS-paralogs, representing four distinct families, can be found in P. tetraurelia genome. Using RNA interference, we showed that TFIIS4, which encodes a development-specific TFIIS protein, is essential for the formation of a functional somatic genome. Molecular analyses and high-throughput DNA sequencing upon TFIIS4 RNAi demonstrated that TFIIS4 is involved in all kinds of genome rearrangements, including excision of ~48% of IESs. Localization of a GFP-TFIIS4 fusion revealed that TFIIS4 appears specifically in the new somatic nucleus at an early developmental stage, before IES excision. RT-PCR experiments showed that TFIIS4 is necessary for the synthesis of IES-containing non-coding transcripts. We propose that these IES+ transcripts originate from the developing somatic nucleus and serve as pairing substrates for germline-specific short RNAs that target elimination of their homologous sequences. Our study, therefore, connects the onset of zygotic non coding transcription to the control of genome plasticity in Paramecium, and establishes for the first time a specific role of TFIIS in non-coding transcription in eukaryotes.

Paramecium tetraurelia provides an excellent model for studying the mechanisms involved in the production of non-coding transcripts and their mode of action. Different types of non-coding RNAs (ncRNAs) were shown to be implicated in the programmed DNA elimination process that occurs in this organism. At each sexual cycle, during development of the somatic nucleus from the germline nucleus, the genome is massively rearranged through the reproducible elimination of germline-specific sequences including thousands of short, single copy, non-coding Internal Eliminated Sequences (IES). Here, we demonstrate, using RNA interference, that the TFIIS4 gene encoding a development-specific homolog of RNA polymerase II elongation factor TFIIS, is indispensable for ncRNA synthesis in the new somatic nucleus. TFIIS4 depletion impairs the assembly of a functional somatic genome and affects excision of a large fraction of IESs, which leads to strong lethality in the sexual progeny. We propose that TFIIS4-dependent ncRNAs provide an important component of the molecular machinery that is responsible for developmental genome remodeling in Paramecium.

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.

Etiology and treatment of hematological neoplasms: stochastic mathematical models

Leukemias are driven by stemlike cancer cells (SLCC), whose initiation, growth, response to treatment, and posttreatment behavior are often "stochastic", i.e., differ substantially even among very similar patients for reasons not observable with present techniques. We review the probabilistic mathematical methods used to analyze stochastics and give two specific examples. The first example concerns a treatment protocol, e.g., for acute myeloid leukemia (AML), where intermittent cytotoxic drug dosing (e.g., once each weekday) is used with intent to cure. We argue mathematically that, if independent SLCC are growing stochastically during prolonged treatment, then, other things being equal, front-loading doses are more effective for tumor eradication than back loading. We also argue that the interacting SLCC dynamics during treatment is often best modeled by considering SLCC in microenvironmental niches, with SLCC-SLCC interactions occurring only among SLCC within the same niche, and we present a stochastic dynamics formalism, involving "Poissonization," applicable in such situations. Interactions at a distance due to partial control of total cell numbers are also considered. The second half of this chapter concerns chromosomal aberrations, lesions known to cause some leukemias. A specific example is the induction of a Philadelphia chromosome by ionizing radiation, subsequent development of chronic myeloid leukemia (CML), CML treatment, and treatment outcome. This time evolution involves a coordinated sequence of > 10 steps, each stochastic in its own way, at the subatomic, molecular, macromolecular, cellular, tissue, and population scales, with corresponding time scales ranging from picoseconds to decades. We discuss models of these steps and progress in integrating models across scales.

Histone methylation and V(D)J recombination

V(D)J recombination is the process by which the diversity of antigen receptor genes is generated and is also indispensable for lymphocyte development. This recombination event occurs in a cell lineage- and stage-specific manner, and is carefully controlled by chromatin structure and ordered histone modifications. The recombinationally active V(D)J loci are associated with hypermethylation at lysine4 of histone H3 and hyperacetylation of histones H3/H4. The recombination activating gene 1 (RAG1) and RAG2 complex initiates recombination by introducing double-strand DNA breaks at recombination signal sequences (RSS) adjacent to each coding sequence. To be recognized by the RAG complex, RSS sites must be within an open chromatin context. In addition, the RAG complex specifically recognizes hypermethylated H3K4 through its plant homeodomain (PHD) finger in the RAG2 C terminus, which stimulates RAG catalytic activity via that interaction. In this review, we describe how histone methylation controls V(D)J recombination and discuss its potential role in lymphoid malignancy by mistargeting the RAG complex.

Reprint of "Functional overlaps between XLF and the ATM-dependent DNA double strand break response"

Developing B and T lymphocytes generate programmed DNA double strand breaks (DSBs) during the V(D)J recombination process that assembles exons that encode the antigen-binding variable regions of antibodies. In addition, mature B lymphocytes generate programmed DSBs during the immunoglobulin heavy chain (IgH) class switch recombination (CSR) process that allows expression of different antibody heavy chain constant regions that provide different effector functions. During both V(D)J recombination and CSR, DSB intermediates are sensed by the ATM-dependent DSB response (DSBR) pathway, which also contributes to their joining via classical non-homologous end-joining (C-NHEJ). The precise nature of the interplay between the DSBR and C-NHEJ pathways in the context of DSB repair via C-NHEJ remains under investigation. Recent studies have shown that the XLF C-NHEJ factor has functional redundancy with several members of the ATM-dependent DSBR pathway in C-NHEJ, highlighting unappreciated major roles for both XLF as well as the DSBR in V(D)J recombination, CSR and C-NHEJ in general. In this review, we discuss current knowledge of the mechanisms that contribute to the repair of DSBs generated during B lymphocyte development and activation with a focus on potential functionally redundant roles of XLF and ATM-dependent DSBR factors.

Alternative end-joining pathway(s): bricolage at DNA breaks

To cope with DNA double strand break (DSB) genotoxicity, cells have evolved two main repair pathways: homologous recombination which uses homologous DNA sequences as repair templates, and non-homologous Ku-dependent end-joining involving direct sealing of DSB ends by DNA ligase IV (Lig4). During the last two decades a third player most commonly named alternative end-joining (A-EJ) has emerged, which is defined as any Ku- or Lig4-independent end-joining process. A-EJ increasingly appears as a highly error-prone bricolage on DSBs and despite expanding exploration, it still escapes full characterization. In the present review, we discuss the mechanism and regulation of A-EJ as well as its biological relevance under physiological and pathological situations, with a particular emphasis on chromosomal instability and cancer. Whether or not it is a genuine DSB repair pathway, A-EJ is emerging as an important cellular process and understanding A-EJ will certainly be a major challenge for the coming years.

Functional overlaps between XLF and the ATM-dependent DNA double strand break response

Developing B and T lymphocytes generate programmed DNA double strand breaks (DSBs) during the V(D)J recombination process that assembles exons that encode the antigen-binding variable regions of antibodies. In addition, mature B lymphocytes generate programmed DSBs during the immunoglobulin heavy chain (IgH) class switch recombination (CSR) process that allows expression of different antibody heavy chain constant regions that provide different effector functions. During both V(D)J recombination and CSR, DSB intermediates are sensed by the ATM-dependent DSB response (DSBR) pathway, which also contributes to their joining via classical non-homologous end-joining (C-NHEJ). The precise nature of the interplay between the DSBR and C-NHEJ pathways in the context of DSB repair via C-NHEJ remains under investigation. Recent studies have shown that the XLF C-NHEJ factor has functional redundancy with several members of the ATM-dependent DSBR pathway in C-NHEJ, highlighting unappreciated major roles for both XLF as well as the DSBR in V(D)J recombination, CSR and C-NHEJ in general. In this review, we discuss current knowledge of the mechanisms that contribute to the repair of DSBs generated during B lymphocyte development and activation with a focus on potential functionally redundant roles of XLF and ATM-dependent DSBR factors.

Mechanisms of programmed DNA lesions and genomic instability in the immune system

Chromosomal translocations involving antigen receptor loci are common in lymphoid malignancies. Translocations require DNA double-strand breaks (DSBs) at two chromosomal sites, their physical juxtaposition, and their fusion by end-joining. Ability of lymphocytes to generate diverse repertoires of antigen receptors and effector antibodies derives from programmed genomic alterations that produce DSBs. We discuss these lymphocyte-specific processes, with a focus on mechanisms that provide requisite DSB target specificity and mechanisms that suppress DSB translocation. We also discuss recent work that provides new insights into DSB repair pathways and the influences of three-dimensional genome organization on physiological processes and cancer genomes.

VprBP binds full-length RAG1 and is required for B-cell development and V(D)J recombination fidelity

The N-terminus of full-length RAG1, though dispensable for RAG1/2 cleavage activity, is required for efficient V(D)J recombination. This region supports RING E3 ubiquitin ligase activity in vitro, but whether full-length RAG1 functions as a single subunit or a multi-subunit E3 ligase in vivo is unclear. We show the multi-subunit cullin RING E3 ligase complex VprBP/DDB1/Cul4A/Roc1 associates with full-length RAG1 through VprBP. This complex is assembled into RAG protein-DNA complexes, and supports in-vitro ubiquitylation activity that is insensitive to RAG1 RING domain mutations. Conditional B lineage-specific VprBP disruption arrests B-cell development at the pro-B-to-pre-B cell transition, but this block is bypassed by expressing rearranged immunoglobulin transgenes. Mice with a conditional VprBP disruption show modest reduction of D-J(H) rearrangement, whereas V(H)-DJ(H) and V(κ)-J(κ) rearrangements are severely impaired. D-J(H) coding joints from VprBP-insufficent mice show longer junctional nucleotide insertions and a higher mutation frequency in D and J segments than normal. These data suggest full-length RAG1 recruits a cullin RING E3 ligase complex to ubiquitylate an unknown protein(s) to limit error-prone repair during V(D)J recombination.

RAG-2 unleashed: lymphocytes beware

The programmed degradation of the RAG-2 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.