RAG-mediated DNA double-strand breaks activate a cell type-specific checkpoint to inhibit pre-B cell receptor signals.
ABSTRACT: DNA double-strand breaks (DSBs) activate a canonical DNA damage response, including highly conserved cell cycle checkpoint pathways that prevent cells with DSBs from progressing through the cell cycle. In developing B cells, pre-B cell receptor (pre-BCR) signals initiate immunoglobulin light (Igl) chain gene assembly, leading to RAG-mediated DNA DSBs. The pre-BCR also promotes cell cycle entry, which could cause aberrant DSB repair and genome instability in pre-B cells. Here, we show that RAG DSBs inhibit pre-BCR signals through the ATM- and NF-?B2-dependent induction of SPIC, a hematopoietic-specific transcriptional repressor. SPIC inhibits expression of the SYK tyrosine kinase and BLNK adaptor, resulting in suppression of pre-BCR signaling. This regulatory circuit prevents the pre-BCR from inducing additional Igl chain gene rearrangements and driving pre-B cells with RAG DSBs into cycle. We propose that pre-B cells toggle between pre-BCR signals and a RAG DSB-dependent checkpoint to maintain genome stability while iteratively assembling Igl chain genes.
Project description:Pre-B cell receptor (pre-BCR) signals initiate immunoglobulin light (Igl) chain gene assembly leading to RAG-mediated DNA double-strand breaks (DSBs). These signals also promote cell cycle entry, which could cause aberrant DSB repair and genome instability in pre-B cells. Here we show that RAG DSBs inhibit pre-BCR signals through the ATM- and NF-κB2-dependent induction of SPIC, a hematopoietic-specific transcriptional repressor. SPIC inhibits expression of the SYK tyrosine kinase and BLNK adaptor to prevent the pre-BCR from inducing additional Igl chain gene rearrangements and driving pre-B cells with RAG DSBs into cycle. We propose that pre-B cells toggle between pre-BCR signals and this RAG DSB-dependent checkpoint to maintain genome stability while iteratively assembling Igl chain genes. Overall design: Three independent IL-7 cultures for each genotype (Rag1-/-:μIgH:Bcl2, Art-/-:μIgH:Bcl2 and Art-/-:Nfkb2-/-:μIgH:Bcl2) were withdrawn from IL-7 for 2 days. RNA was isolated using RNeasy (Qiagen). Gene expression profiling was performed using Illumina MouseRef-8 expression microarrays.
Project description:Pre-B cell receptor (pre-BCR) signals initiate immunoglobulin light (Igl) chain gene assembly leading to RAG-mediated DNA double-strand breaks (DSBs). These signals also promote cell cycle entry, which could cause aberrant DSB repair and genome instability in pre-B cells. Here we show that RAG DSBs inhibit pre-BCR signals through the ATM- and NF-κB2-dependent induction of SPIC, a hematopoietic-specific transcriptional repressor. SPIC inhibits expression of the SYK tyrosine kinase and BLNK adaptor to prevent the pre-BCR from inducing additional Igl chain gene rearrangements and driving pre-B cells with RAG DSBs into cycle. We propose that pre-B cells toggle between pre-BCR signals and this RAG DSB-dependent checkpoint to maintain genome stability while iteratively assembling Igl chain genes. Three independent IL-7 cultures for each genotype (Rag1-/-:μIgH:Bcl2, Art-/-:μIgH:Bcl2 and Art-/-:Nfkb2-/-:μIgH:Bcl2) were withdrawn from IL-7 for 2 days. RNA was isolated using RNeasy (Qiagen). Gene expression profiling was performed using Illumina MouseRef-8 expression microarrays.
Project description:Early B cell development is regulated by stage-specific transcription factors. PU.1, an ETS-family transcription factor, is essential for coordination of early B cell maturation and immunoglobulin gene (Ig) rearrangement. Here we show that RAG DNA double-strand breaks (DSBs) generated during Ig light chain gene (Igl) rearrangement in pre-B cells induce global changes in PU.1 chromatin binding. RAG DSBs activate a SPIC/BCLAF1 transcription factor complex that displaces PU.1 throughout the genome and regulates broad transcriptional changes. SPIC recruits BCLAF1 to gene-regulatory elements that control expression of key B cell developmental genes. The SPIC/BCLAF1 complex suppresses expression of the SYK tyrosine kinase and enforces the transition from large to small pre-B cells. These studies reveal that RAG DSBs direct genome-wide changes in ETS transcription factor activity to promote early B cell development.
Project description:The RAG1/RAG2 endonuclease (RAG) initiates the V(D)J recombination reaction that assembles immunoglobulin heavy (IgH) and light (IgL) chain variable region exons from germline gene segments to generate primary antibody repertoires. IgH V(D)J assembly occurs in progenitor (pro-) B cells followed by that of IgL in precursor (pre-) B cells. Expression of IgH ? and IgL (Ig? or Ig?) chains generates IgM, which is expressed on immature B cells as the B-cell antigen-binding receptor (BCR). Rag expression can continue in immature B cells, allowing continued Ig? V(D)J recombination that replaces the initial V?J? exon with one that generates a new specificity. This 'receptor editing' process, which can also lead to Ig? V(D)J recombination and expression, provides a mechanism whereby antigen encounter at the Rag-expressing immature B-cell stage helps shape pre-immune BCR repertoires. As the major site of postnatal B-cell development, the bone marrow is the principal location of primary immunoglobulin repertoire diversification in mice. Here we report that early B-cell development also occurs within the mouse intestinal lamina propria (LP), where the associated V(D)J recombination/receptor editing processes modulate primary LP immunoglobulin repertoires. At weanling age in normally housed mice, the LP contains a population of Rag-expressing B-lineage cells that harbour intermediates indicative of ongoing V(D)J recombination and which contain cells with pro-B, pre-B and editing phenotypes. Consistent with LP-specific receptor editing, Rag-expressing LP B-lineage cells have similar VH repertoires, but significantly different V? repertoires, compared to those of Rag2-expressing bone marrow counterparts. Moreover, colonization of germ-free mice leads to an increased ratio of Ig?-expressing versus Ig?-expressing B cells specifically in the LP. We conclude that B-cell development occurs in the intestinal mucosa, where it is regulated by extracellular signals from commensal microbes that influence gut immunoglobulin repertoires.
Project description:Mammalian cells have evolved a common DNA damage response (DDR) that sustains cellular function, maintains genomic integrity, and suppresses malignant transformation. In pre-B cells, DNA double-strand breaks (DSBs) induced at Ig? loci by the Rag1/Rag2 (RAG) endonuclease engage this DDR to modulate transcription of genes that regulate lymphocyte-specific processes. We previously reported that RAG DSBs induced at one Ig? allele signal through the ataxia telangiectasia mutated (ATM) kinase to feedback-inhibit RAG expression and RAG cleavage of the other Ig? allele. In this article, we show that DSBs induced by ionizing radiation, etoposide, or bleomycin suppress Rag1 and Rag2 mRNA levels in primary pre-B cells, pro-B cells, and pro-T cells, indicating that inhibition of Rag1 and Rag2 expression is a prevalent DSB response among immature lymphocytes. DSBs induced in pre-B cells signal rapid transcriptional repression of Rag1 and Rag2, causing downregulation of both Rag1 and Rag2 mRNA, but only Rag1 protein. This transcriptional inhibition requires the ATM kinase and the NF-?B essential modulator protein, implicating a role for ATM-mediated activation of canonical NF-?B transcription factors. Finally, we demonstrate that DSBs induced in pre-B cells by etoposide or bleomycin inhibit recombination of Ig? loci and a chromosomally integrated substrate. Our data indicate that immature lymphocytes exploit a common DDR signaling pathway to limit DSBs at multiple genomic locations within developmental stages wherein monoallelic Ag receptor locus recombination is enforced. We discuss the implications of our findings for mechanisms that orchestrate the differentiation of monospecific lymphocytes while suppressing oncogenic Ag receptor locus translocations.
Project description:Two major DNA repair pathways, nonhomologous end-joining (NHEJ) and homologous recombination (HR), repair double-stranded DNA breaks (DSBs) in all eukaryotes. Additionally, several alternative end-joining pathways (or subpathways) have been reported that characteristically use short-sequence homologies at the DNA ends to facilitate joining. How a cell chooses which DNA repair pathway to use (at any particular DSB) is a central and largely unanswered question. For one type of DSB, there is apparently no choice. DSBs mediated by the lymphocyte-specific recombination activating gene (RAG) endonuclease are repaired virtually exclusively by NHEJ. Here we demonstrate that non-RAG-mediated DSBs can be similarly forced into the NHEJ pathway by physical association with the RAG endonuclease.
Project description:DNA double-strand breaks (DSBs) arise during physiological transcription, DNA replication, and antigen receptor diversification. Mistargeting or misprocessing of DSBs can result in pathological structural variation and mutation. Here we describe a sensitive method (END-seq) to monitor DNA end resection and DSBs genome-wide at base-pair resolution in vivo. We utilized END-seq to determine the frequency and spectrum of restriction-enzyme-, zinc-finger-nuclease-, and RAG-induced DSBs. Beyond sequence preference, chromatin features dictate the repertoire of these genome-modifying enzymes. END-seq can detect at least one DSB per cell among 10,000 cells not harboring DSBs, and we estimate that up to one out of 60 cells contains off-target RAG cleavage. In addition to site-specific cleavage, we detect DSBs distributed over extended regions during immunoglobulin class-switch recombination. Thus, END-seq provides a snapshot of DNA ends genome-wide, which can be utilized for understanding genome-editing specificities and the influence of chromatin on DSB pathway choice.
Project description:DNA double-strand breaks (DSBs), selectively visualized as ?-H2AX+ foci, occur during the development of the central nervous system, including the retina, although their origin and biological significance are poorly understood. Mutant mice with DSB repair mechanism defects exhibit increased numbers of ?-H2AX+ foci, increased cell death during neural development, and alterations in axonogenesis in the embryonic retina. The aim of this study was to identify putative sources of DSBs. One of the identified DSBs sources is LINE-1 retrotransposition. While we did not detect changes in LINE-1 DNA content during the early period of cell death associated with retinal neurogenesis, retinal development was altered in mice lacking RAG-2, a component of the RAG-1,2-complex responsible for initiating somatic recombination in lymphocytes. Although ?-H2AX+ foci were less abundant in the rag2-/- mouse retina, retinal ganglion cell death was increased and axonal growth and navigation were impaired in the RAG-2 deficient mice, a phenotype shared with mutant mice with defective DNA repair mechanisms. These findings demonstrate that RAG-2 is necessary for proper retinal development, and suggest that both DSB generation and repair are genuine processes intrinsic to neural development.
Project description:Fanconi D2 (FANCD2) is monoubiquitinated on K561 (FANCD2-Ub) in response to DNA double-strand breaks (DSBs) to stimulate repair of these potentially lethal DNA lesions. FANCD2-Ub was upregulated in CD34+ chronic myeloid leukemia (CML) cells and in BCR-ABL1 kinase-positive cell lines in response to elevated levels of reactive oxygen species (ROS) and DNA cross-linking agent mitomycin C. Downregulation of FANCD2 and inhibition of FANCD2-Ub reduced the clonogenic potential of CD34+ CML cells and delayed BCR-ABL1 leukemogenesis in mice. Retarded proliferation of BCR-ABL1 positive FANCD2-/- leukemia cells could be rescued by FANCD2 expression. BCR-ABL1 positive FANCD2-/- cells accumulated more ROS-induced DSBs in comparison with BCR-ABL1 positive FANCD2+/+ cells. Antioxidants diminished the number of DSBs and enhanced proliferation of BCR-ABL1 positive FANCD2-/- cells. Expression of wild-type FANCD2 and FANCD2(S222A) phosphorylation-defective mutant (deficient in stimulation of intra-S phase checkpoint, but proficient in DSB repair), but not FANCD2(K561R) monoubiquitination-defective mutant (proficient in stimulation of intra-S phase checkpoint, but deficient in DSB repair) reduced the number of DSBs and facilitated proliferation of BCR-ABL1 positive FANCD2-/- cells. We hypothesize that FANCD2-Ub has an important role in BCR-ABL1 leukemogenesis because of its ability to facilitate the repair of numerous ROS-induced DSBs.
Project description:The choice of DNA double strand break (DSB) repair pathway is determined at the stage of DSB end resection. Resection was proposed to control the balance between the two major DSB repair pathways, homologous recombination (HR) and non-homologous end joining (NHEJ). Here, we examined the regulation of DSB repair pathway choice at two-ended DSBs following ionizing radiation (IR) in G2 phase of the cell cycle. We found that cells pre-exposed to low-dose IR preferred to undergo HR following challenge IR in G2, whereas NHEJ repair kinetics in G1 were not affected by pre-IR treatment. Consistent with the increase in HR usage, the challenge IR induced Replication protein A (RPA) foci formation and RPA phosphorylation, a marker of resection, were enhanced by pre-IR. However, neither major DNA damage signals nor the status of core NHEJ proteins, which influence the choice of repair pathway, was significantly altered in pre-IR treated cells. Moreover, the increase in usage of HR due to pre-IR exposure was prevented by treatment with ATM inhibitor during the incubation period between pre-IR and challenge IR. Taken together, the results of our study suggest that the ATM-dependent damage response after pre-IR changes the cellular environment, possibly by regulating gene expression or post-transcriptional modifications in a manner that promotes resection.