The 3'-flap endonuclease XPF-ERCC1 promotes alternative end joining and chromosomal translocation during B cell class switching.

Robust alternative end joining (A-EJ) in classical non-homologous end joining (c-NHEJ)-deficient murine cells features double-strand break (DSB) end resection and microhomology (MH) usage and promotes chromosomal translocation. The activities responsible for removing 3' single-strand overhangs following resection and MH annealing in A-EJ remain unclear. We show that, during class switch recombination (CSR) in mature mouse B cells, the structure-specific endonuclease complex XPF-ERCC1SLX4, although not required for normal CSR, represents a nucleotide-excision-repair-independent 3' flap removal activity for A-EJ-mediated CSR. B cells deficient in DNA ligase 4 and XPF-ERCC1 exhibit further impaired class switching, reducing joining to the resected S region DSBs without altering the MH pattern in S-S junctions. In ERCC1-deficient A-EJ cells, 3' single-stranded DNA (ssDNA) flaps that are generated predominantly in S/G2 phase of the cell cycle are susceptible to nuclease resolution. Moreover, ERCC1 promotes c-myc-IgH translocation in Lig4-/- cells. Our study reveals an important role of the flap endonuclease XPF-ERCC1 in A-EJ and oncogenic translocation in mouse B cells.

Existence of reprogrammed lymphoma stem cells in a murine ALCL-like model.

While cancer stem cells are well established in certain hematologic and solid malignancies, their existence in T cell lymphoma is unclear and the origin of disease is not fully understood. To examine the existence of lymphoma stem cells, we utilized a mouse model of anaplastic large cell lymphoma. Established NPM-ALK+ lymphomas contained heterogeneous cell populations ranging from mature T cells to undifferentiated hematopoietic stem cells. Interestingly, CD4-/CD8- double negative (DN) lymphoma cells aberrantly expressed the T cell receptor α/ß chain. Serial transplantation of sorted CD4/CD8 and DN lymphoma subpopulations identified lymphoma stem cells within the DN3/DN4 T cell population, whereas all other subpopulations failed to establish serial lymphomas. Moreover, transplanted lymphoma DN3/DN4 T cells were able to differentiate and gave rise to mature lymphoma T cells. Gene expression analyses unmasked stem-cell-like transcriptional regulation of the identified lymphoma stem cell population. Furthermore, these lymphoma stem cells are characterized by low CD30 expression levels, which might contribute to limited long-term therapeutic success in patients treated with anti-CD30-targeted therapies. In summary, our results highlight the existence of a lymphoma stem cell population in a NPM-ALK-driven CD30+ mouse model, thereby giving the opportunity to test innovative treatment strategies developed to eradicate the origin of disease.

[Role of the Nucleolus in Rearrangements of the IGH Locus].

The review summarizes the results from a series of studies focusing on the role that the nucleolus plays in maturation of the IGH locus and the choice of its partner genes in leukemia-associated translocations. The role of nuclear compartmentalization and nuclear localization of translocated oncogenes in ectopic activation of their transcription is discussed.

Complete mimicry: a case of alveolar rhabdomyosarcoma masquerading as acute leukemia.

BACKGROUND: A small number of rhabdomyosarcoma (RMS) cases involve the bone marrow. A leukemic presentation of RMS has been reported in a few case series, although almost all cases of leukemic RMS are not completely mimicking leukemia. We encountered a case with RMS cell infiltration of the bone marrow that resembled floating hematological cells. CASE PRESENTATION: We encountered a rare case of a 15-year-old boy with a 2-week history of left femoral pain. Upon admission, he was afebrile with no other symptoms. No apparent cause of femoral pain was detected on an initial examination. Laboratory findings revealed normal white blood cell (WBC) count and hemoglobin concentration, with a platelet count of 10.3 × 104/µL. WBCs included 2.0% metamyelocytes, 4.5% myelocytes, and 0.5% blasts. Lactate dehydrogenase concentration was 1299 U/L, creatine kinase was 437 U/L, and C-reactive protein was 1.25 mg/dL. Bone marrow aspiration demonstrated hypercellular marrow (nucleated cell count 1.84 × 104/µL) and 89.0% of blast-like cells of all nucleated cells. The proliferating cells were negative for myeloperoxidase and esterase, and strongly positive for CD56. Positron emission tomography exhibited extensive accumulation of 18F-fludeoxyglucose with a SUVmax of 7.09. Magnetic resonance imaging revealed T1-low intensity, gadolinium-enhanced, diffuse, and irregular lesions on his pelvis and bilateral femurs. These laboratory and imaging findings suggested hematological malignancy with diffuse bone involvement, suggestive of acute leukemia. However, the pathological diagnosis of bone marrow and basal penile muscle biopsy was alveolar RMS. Karyotype analysis of bone marrow cells revealed the characteristic translocation of t(2;13)(q35;q14). The final diagnosis was alveolar RMS with massive involvement of the bone marrow and the primary site in the perineal muscles. The tumor cells both of the primary site and bone marrow were positive for myogenin. CONCLUSIONS: A literature review found a misdiagnosed case of completely mimicking leukemic RMS as natural-killer (NK)-cell leukemia. Such a misdiagnosis can have critical consequences. We experienced a rare case of alveolar RMS with symmetrical diffuse bone marrow involvement completely masquerading as acute leukemia. The results of a surface marker study showing that the tumor cells had a near NK-cell phenotype were misleading.

Associations of Plasma Cytokine and Microbial Translocation Biomarkers With Immune Reconstitution Inflammatory Syndrome.

A nested case-cohort study was performed in participants of a clinical trial of first-line human immunodeficiency virus treatments to investigate plasma biomarkers of inflammation and microbial translocation for their association with immune reconstitution inflammatory syndrome (IRIS). Fifty-one of 1452 participants with baseline CD4 count <350 cells/µL developed IRIS. Plasma from 51 IRIS cases, including 6 stratified by preenrollment CD4 count ≤200 cells/µL, were analyzed and compared to 94 non-IRIS controls. At baseline, CXCL10, lipopolysaccharide, soluble CD14, 16S ribosomal DNA, and interferon-α2 were associated with greater risk of IRIS. Systemic inflammation through persistent monocyte activation and microbial translocation appear to be important in IRIS pathogenesis.

Aggressive B-cell lymphomas: frequency, immunophenotype, and genetics in a reference laboratory population.

Diffuse large B-cell lymphoma (DLBCL) is the most common type of lymphoma worldwide. The current World Health Organization classification includes several subtypes based on a combination of clinical, immunohistochemical, and genetic differences. Other aggressive variants of B-cell lymphomas, including Burkitt lymphoma and double-hit lymphomas are part of the differential diagnosis and often have overlapping features with DLBCL. In this study, we evaluated 760 of cases of DLBCL and other aggressive B-cell lymphomas using a relatively uniform immunohistochemical panel and genetic methods. We assessed the frequency of different subtypes and locations and documented distinctive immunophenotypic and genetic findings of these cases. Most cases in the study group were DLBCL (89%), including 38 CD5+ DLBCL, 28 T-cell/histiocyte-rich large B-cell lymphomas, and 33 Epstein-Barr virus-positive DLBCL (including 6 cases in elderly patients). The study also included 39 Burkitt lymphoma and 39 cases of double-hit lymphoma. In general, our results support the World Health Organization classification approach as well as other studies of DLBCL. In this study, we focus on specific issues of interest including cell-of-origin classification testing, comparing the Hans classifier with the tally classifier, correlation of MYC immunohistochemistry with MYC fluorescence in situ hybridization, and Epstein-Barr virus positivity in aggressive B-cell lymphomas.

Molecular pathogenesis of lymphomas of mucosa-associated lymphoid tissue--from (auto)antigen driven selection to the activation of NF-κB signaling.

Lymphomas of mucosa-associated lymphoid tissue (MALT) are typically present at sites such as the stomach, lung or urinary tract, where lymphoid tissues scatter in mucosa lamina propria, intra- or sub-epithelial cells. The infection of certain pathogens, such as Helicobacter pylori, Chlamydophila psittaci, Borrelia burgdorferi, hepatitis C virus, or certain autoantigens cause these sites to generate a germinal center called the "acquired lymphoid tissue". The molecular pathogenesis of MALT lymphoma is a multi-step process. Receptor signaling, such as the contact stimulation of B cell receptors and CD4 positive T cells mediated by CD40/CD40-ligand and T helper cell type 2 cytokines like interleukin-4, contributes to tumor cell proliferation. A number of genetic alterations have been identified in MALT lymphoma, and among them are important translocations, such as t(11;18)(q21;q21), t(1;14)(p22;q32), t(14;18)(q32;q21) and t(3;14)(p13;q32). Fusion proteins generated by these translocations share the same NF-κB signaling pathway, which is activated by the caspase activation and recruitment domain containing molecules of the membrane associated guanylate kinase family, B cell lymphoma-10 and MALT1 (CBM) protein complex. They act downstream of cell surface receptors, such as B cell receptors, T cell receptors, B cell activating factors and Toll-like receptors, and participate in the biological process of MALT lymphoma. The discovery of therapeutic drugs that exclusively inhibit the antigen receptor signaling pathway will be beneficial for the treatment of B cell lymphomas in the future.

Molecular pathogenesis of B-cell posttransplant lymphoproliferative disorder: what do we know so far?

Posttransplant lymphoproliferative disorder (PTLD) is a potentially fatal disease that arises in 2%-10% of solid organ and hematopoietic stem cell transplants and is most frequently of B-cell origin. This very heterogeneous disorder ranges from benign lymphoproliferations to malignant lymphomas, and despite the clear association with Epstein-Barr Virus (EBV) infection, its etiology is still obscure. Although a number of risk factors have been identified (EBV serostatus, graft type, and immunosuppressive regimen), it is currently not possible to predict which transplant patient will eventually develop PTLD. Genetic studies have linked translocations (involving C-MYC, IGH, BCL-2), various copy number variations, DNA mutations (PIM1, PAX5, C-MYC, RhoH/TTF), and polymorphisms in both the host (IFN-gamma, IL-10, TGF-beta, HLA) and the EBV genome to B-cell PTLD development. Furthermore, the tumor microenvironment seems to play an important role in the course of disease representing a local niche that can allow antitumor immune responses even in an immunocompromised host. Taken together, B-cell PTLD pathogenesis is very complex due to the interplay of many different (patient-dependent) factors and requires thorough molecular analysis for the development of novel tailored therapies. This review aims at giving a global overview of the currently known parameters that contribute to the development of B-cell PTLD.

Translocations in normal B cells and cancers: insights from new technical approaches.

Chromosomal translocations are recurrent genetic events that define many types of cancers. Since their first description several decades ago as defining elements in cancer cells, our understanding of the mechanisms that determine their formation as well as their implications for cancer progression and therapy has remarkably progressed. Chromosomal translocations originate from double-strand breaks (DSBs) that are brought into proximity in the nuclear space and joined inappropriately by DNA-repair pathways. The frequency and pattern of translocations are influenced by perturbations of any of these events. DSB formation is heavily determined by physiologic processes, such as the activity of RAG1/2 and AID enzymes during B-cell development or maturation, or by pathologic factors, such as ionizing radiations, ROS, or fragile sites. Cellular processes of mRNA transcription, DNA replication, and repair can influence the chromosomal territories and modify the relative position and proximity of genes inside the nucleus. DNA-repair factors contribute not only to the maintenance of genome integrity but also to translocations in normal and cancer cells. Next-generation sequencing techniques provide an unprecedented and powerful tool to approach the field of chromosomal translocations. Using specific examples, we will explain how genome-wide translocation mapping methods, such as high-throughput genomic translocation sequencing (HTGTS) and translocation-capture sequencing, combined with large-scale methods to determine nuclear proximity of genes or chromosome domains, such as 4C and Hi-C, have changed our view of the factors and the rules governing translocation formation in noncancer cells. Finally, we will review chromosomal rearrangements and newly described findings, such as chromothripsis, in cancer cells based on these novel rules on translocation formation.

Different mechanisms causing loss of mismatched human leukocyte antigens in relapsing t(6;11)(q27;q23) acute myeloid leukemia after haploidentical transplantation.

Mismatched human leukocyte antigens (HLAs) on leukemic cells can be targeted by donor T cells in HLA-mismatched/haploidentical stem cell transplantation. In two cases of acute myeloid leukemia with t(6;11)(q27;q23) abnormality presented here, flow cytometry analysis showed a lack of HLA-A unshared between recipients and donors in relapsing leukemic cells after HLA-haploidentical transplantation. However, high-resolution HLA genotyping showed that one case lacked a corresponding HLA haplotype, whereas the other preserved it. These cases suggest that leukemic cells, which lacked mismatched HLA expression, might have an advantage in selective expansion under donor T-cell immune surveillance after HLA-haploidentical transplantation. Most importantly, down-regulation of unshared HLA expression potentially occurs by genetic alterations other than loss of HLA alleles.

A human NK cell activation/inhibition threshold allows small changes in the target cell surface phenotype to dramatically alter susceptibility to NK cells.

NK cell activation is negatively regulated by the expression of target cell MHC class I molecules. We show that this relationship is nonlinear due to an NK cell activation/inhibition threshold. Ewing's sarcoma family tumor cell monolayers, which were highly susceptible to NK cells in vitro, developed a highly resistant phenotype when cultured as three-dimensional multicellular tumor spheroid structures. This suggested that tumor architecture is likely to influence the susceptibility to NK cells in vivo. Resistance of the multicellular tumor spheroid was associated with the increased expression of MHC class I molecules and greatly reduced NK cell activation, implying that a threshold of NK cell activation/inhibition had been crossed. Reducing MHC class I expression on Ewing's sarcoma family tumor monolayers did not alter their susceptibility to NK cells, whereas increased expression of MHC class I rendered them resistant and allowed the threshold point to be identified. This threshold, as defined by MHC class I expression, was predictive of the number of NK-resistant target cells within a population. A threshold permits modest changes in the target cell surface phenotype to profoundly alter the susceptibility to NK cells. Whereas this allows for the efficient detection of target cells, it also provides a route for pathogens and tumors to evade NK cell attack.

Safety of probiotic Escherichia coli strain Nissle 1917 depends on intestinal microbiota and adaptive immunity of the host.

Probiotics are viable microorganisms that are increasingly used for treatment of a variety of diseases. Occasionally, however, probiotics may have adverse clinical effects, including septicemia. Here we examined the role of the intestinal microbiota and the adaptive immune system in preventing translocation of probiotics (e.g., Escherichia coli Nissle). We challenged C57BL/6J mice raised under germfree conditions (GF-raised C57BL/6J mice) and Rag1(-/-) mice raised under germfree conditions (GF-raised Rag1(-/-) mice) and under specific-pathogen-free conditions (SPF-raised Rag1(-/-) mice) with probiotic E. coli strain Nissle 1917, strain Nissle 1917 mutants, the commensal strain E. coli mpk, or Bacteroides vulgatus mpk. Additionally, we reconstituted Rag1(-/-) mice with CD4(+) T cells. E. coli translocation and dissemination and the mortality of mice were assessed. In GF-raised Rag1(-/-) mice, but not in SPF-raised Rag1(-/-) mice or GF-raised C57BL/6J mice, oral challenge with E. coli strain Nissle 1917, but not oral challenge with E. coli mpk, resulted in translocation and dissemination. The mortality rate was significantly higher for E. coli strain Nissle 1917-challenged GF-raised Rag1(-/-) mice (100%; P < 0.001) than for E. coli strain Nissle 1917-challenged SPF-raised Rag1(-/-) mice (0%) and GF-raised C57BL/6J mice (0%). Translocation of and mortality due to strain E. coli Nissle 1917 in GF-raised Rag1(-/-) mice were prevented when mice were reconstituted with T cells prior to strain E. coli Nissle 1917 challenge, but not when mice were reconstituted with T cells after E. coli strain Nissle 1917 challenge. Cocolonization experiments revealed that E. coli mpk could not prevent translocation of strain E. coli Nissle 1917. Moreover, we demonstrated that neither lipopolysaccharide structure nor flagella play a role in E. coli strain Nissle 1917 translocation and dissemination. Our results suggest that if both the microbiota and adaptive immunity are defective, translocation across the intestinal epithelium and dissemination of the probiotic E. coli strain Nissle 1917 may occur and have potentially severe adverse effects. Future work should define the possibly related molecular factors that promote probiotic functions, fitness, and facultative pathogenicity.

Alternative end-joining catalyzes robust IgH locus deletions and translocations in the combined absence of ligase 4 and Ku70.

Class switch recombination (CSR) in B lymphocytes is initiated by introduction of multiple DNA double-strand breaks (DSBs) into switch (S) regions that flank immunoglobulin heavy chain (IgH) constant region exons. CSR is completed by joining a DSB in the donor S mu to a DSB in a downstream acceptor S region (e.g., S gamma1) by end-joining. In normal cells, many CSR junctions are mediated by classical nonhomologous end-joining (C-NHEJ), which employs the Ku70/80 complex for DSB recognition and XRCC4/DNA ligase 4 for ligation. Alternative end-joining (A-EJ) mediates CSR, at reduced levels, in the absence of C-NHEJ, even in combined absence of Ku70 and ligase 4, demonstrating an A-EJ pathway totally distinct from C-NHEJ. Multiple DSBs are introduced into S mu during CSR, with some being rejoined or joined to each other to generate internal switch deletions (ISDs). In addition, S-region DSBs can be joined to other chromosomes to generate translocations, the level of which is increased by absence of a single C-NHEJ component (e.g., XRCC4). We asked whether ISD and S-region translocations occur in the complete absence of C-NHEJ (e.g., in Ku70/ligase 4 double-deficient B cells). We found, unexpectedly, that B-cell activation for CSR generates substantial ISD in both S mu and S gamma1 and that ISD in both is greatly increased by the absence of C-NHEJ. IgH chromosomal translocations to the c-myc oncogene also are augmented in the combined absence of Ku70 and ligase 4. We discuss the implications of these findings for A-EJ in normal and abnormal DSB repair.

Translocations involving MUM1 are rare in diffuse large B-cell lymphoma.

Diffuse large B-cell lymphoma (DLBCL) comprises a diverse group of neoplasms that have recently been subdivided by gene expression profiling and immunohistochemical studies into at least 2 subgroups [germinal center (GC) type and non-GC type]. The non-GC subtype has a post-GC activated phenotype and typically expresses MUM1 by immunohistochemistry. We hypothesized that MUM1 may be dysregulated/up-regulated in these tumors by a chromosomal translocation, as is seen in many cases of plasma cell myeloma [where MUM1 is juxtaposed with the immunoglobulin heavy chain gene (IgH)]. Therefore, using a novel MUM1 break-apart probe constructed in our laboratory, we performed fluorescence in situ hybridization on 33 cases of DLBCL (17 GC type and 16 non-GC type) for a MUM1 translocation. We identified 1 case of a MUM1 translocation out of 31 cases with successful fluorescence in situ hybridization. This case was a non-GC DLBCL (1/15). We conclude that genetic abnormalities involving MUM1 are rare in DLBCL and that a mechanism of deregulation of the MUM1 protein other than by a translocation event is involved in the majority of non-GC cases.

Parp1 facilitates alternative NHEJ, whereas Parp2 suppresses IgH/c-myc translocations during immunoglobulin class switch recombination.

Immunoglobulin class switch recombination (CSR) is initiated by DNA breaks triggered by activation-induced cytidine deaminase (AID). These breaks activate DNA damage response proteins to promote appropriate repair and long-range recombination. Aberrant processing of these breaks, however, results in decreased CSR and/or increased frequency of illegitimate recombination between the immunoglobulin heavy chain locus and oncogenes like c-myc. Here, we have examined the contribution of the DNA damage sensors Parp1 and Parp2 in the resolution of AID-induced DNA breaks during CSR. We find that although Parp enzymatic activity is induced in an AID-dependent manner during CSR, neither Parp1 nor Parp2 are required for CSR. We find however, that Parp1 favors repair of switch regions through a microhomology-mediated pathway and that Parp2 actively suppresses IgH/c-myc translocations. Thus, we define Parp1 as facilitating alternative end-joining and Parp2 as a novel translocation suppressor during CSR.

Restricting activation-induced cytidine deaminase tumorigenic activity in B lymphocytes.

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

Regulation of macrophage nitric oxide production by the protein tyrosine phosphatase Src homology 2 domain phosphotyrosine phosphatase 1 (SHP-1).

Nitric oxide (NO) is a potent molecule involved in the cytotoxic effects mediated by macrophages (MØ) against microorganisms. We previously reported that Src homology 2 domain phosphotyrosine phosphatase 1 (SHP-1)-deficient cells generate a greater amount of NO than wild-type cells in response to interferon-gamma (IFN-gamma). We also reported that the Leishmania-induced MØ SHP-1 activity is needed for the survival of the parasite within phagocytes through the attenuation of NO-dependent and NO-independent mechanisms. In the present study, we investigated the role of SHP-1 in regulating key signalling molecules important in MØ NO generation. Janus tyrosine kinase 2 (JAK2), mitogen-activated extracellular signal-regulated protein kinase kinase (MEK), extracellular signal-regulated kinases 1 and 2 (Erk1/Erk2) mitogen-activated protein kinases, p38 and stress-activated mitogen-activated protein kinases/c-Jun NH(2)-terminal kinase (SAPK/JNK) were examined in immortalized bone marrow-derived MØ (BMDM) from both SHP-1-deficient motheaten mice (me-3) and their respective littermates (LM-1). The results indicated that Erk1/Erk2 and SAPK/JNK are the main kinases regulated by SHP-1 because the absence of SHP-1 caused an increase in their phosphorylation. Moreover, only Apigenin, the specific inhibitor of Erk1/Erk2, was able to block IFN-gamma-induced inducible nitric oxide synthase (iNOS) transcription and translation in me-3 cells. Transcription factor analyses revealed that in the absence of SHP-1, activator protein-1 (AP-1) was activated. The activation of AP-1, and not nuclear factor-kappaB (NF-kappaB) or signal transducer and activator of transcription-1 alpha (STAT-1 alpha), may explain the enhanced NO generation in SHP-1-deficient cells. These observations emphasize the involvement of the MAPKs Erk1/Erk2 and SAPK/JNK in NO generation via AP-1 activation. Collectively, our findings suggest that SHP-1 plays a pivotal role in the negative regulation of signalling events leading to iNOS expression and NO generation. Furthermore, our observations underline the importance of SHP-1-mediated negative regulation in maintaining NO homeostasis and thus preventing the abnormal generation of NO that can be detrimental to the host.

Immunohistochemical discrimination between the ASPL-TFE3 fusion proteins of alveolar soft part sarcoma.

Alveolar soft part sarcoma (ASPS), a rare soft tissue sarcoma, is characterized by a chromosomal translocation der(17)t(X;17)(p11;q25) resulting in the production of 2 fusion proteins encoded by regions of the genes for alveolar soft part locus (ASPL) and the transcription factor E3 (TFE3). In this study, polyclonal antibodies were generated to 25 mer peptides encompassing the junctional regions of ASPL-TFE3 type 1 and ASPL-TFE3 type 2. The specificity of the affinity purified antibodies for the synthetic peptides and recombinant expressed ASPL-TFE3 type 1 and ASPL-TFE3 type 2 proteins was evaluated by enzyme-linked immunosorbent assay and was highly fusion type specific. Immunohistochemical staining of formalin-fixed, paraffin-embedded ASPS tumors with the fusion-specific antibodies resulted in intense nuclear staining and differentiation between tumors that express the type 1 protein and tumors that express the type 2 protein. These antibodies will be useful for the differential diagnosis of type 1 and type 2 ASPS and also in the detection of the fusion proteins in biochemical and cell biologic investigations.

Antigen receptor diversification and chromosome translocations.

Double-stranded DNA breaks (DSBs) can result in chromosomal abnormalities, including deletions, translocations and aneuploidy, which can promote neoplastic transformation. DSBs arise accidentally during DNA replication and can be induced by environmental factors such as ultraviolet light or ionizing radiation, and they are generated during antigen receptor-diversification reactions in lymphocytes. Cellular pathways that maintain genomic integrity use sophisticated mechanisms that recognize and repair all DSBs regardless of their origin. Such pathways, along with DNA-damage checkpoints, ensure that either the damage is properly repaired or cells with damaged DNA are eliminated. Here we review how impaired DNA-repair or DNA-damage checkpoints can lead to genetic instability and predispose lymphocytes undergoing diversification of antigen receptor genes to malignant transformation.