Skip to main content
NCBI home page
NIHPA Author Manuscripts logoLink to NIHPA Author Manuscripts
. Author manuscript; available in PMC: 2015 Mar 1.
Published in final edited form as: Trends Immunol. 2013 Dec 10;35(3):131–137. doi: 10.1016/j.it.2013.11.002

BACH2-BCL6 balance regulates selection at the pre-BCR checkpoint

Srividya Swaminathan 1, Cihangir Duy 2, Markus Müschen 1
PMCID: PMC3943645  NIHMSID: NIHMS541090  PMID: 24332591

Abstract

At the pre-B cell receptor (pre-BCR) checkpoint, developing pre-B cells are selected for successful rearrangement of VH-DJH gene segments and expression of a pre-BCR. Reduced stringency at this checkpoint may obstruct the B cell repertoire with non-functional B cell clones. Earlier studies described that activation of BCL6 by a functional pre-BCR mediates positive selection of pre-B cells that have passed the checkpoint. This concept is now further elaborated by the recent finding that the BACH2 transcription factor induces negative selection and opposes BCL6-function prior to the pre-BCR checkpoint. Here we discuss the antagonism between BCL6 and BACH2 during early B cell development, as well as its implications in both repertoire selection and counter-selection of pre-malignant clones for leukemia-suppression.

Keywords: BACH2, BCL6, negative selection, pre-B cell receptor checkpoint, leukemogenesis

Pre-B cell receptor checkpoint as a safeguard against leukemogenesis

Early B cell development is a tightly regulated process with numerous checkpoints in place to control the quality of cells generated at every stage. These checkpoints ensure that early B cells do not go astray and undergo malignant transformation. Alterations in key checkpoint control signaling molecules is deleterious to a pre-B cell and increases the probability of leukemia initiation [1,2].

The first instance of checkpoint control signaling during B cell development occurs when multi-lineage progenitors commit to B-lineage by activating a complex cell signaling network composed of E2A, EBF, PAX5 and FoxO1 [3–9]. The master regulators of B cell differentiation, namely E2a and Ebf, activate the expression of Pax5 (Paired Box Protein 5) and the Rag recombinases [6] to trigger DNA rearrangement (VH-DJH recombination) at the immunoglobulin heavy chain locus. While the RAG1/RAG2 enzymes execute the rearrangement, PAX5 remodels the chromatin at the heavy chain locus to increase the proximity of the different V segments to the rearranged D-J joint [10]. By regulating V-DJ recombination in the above described manner, PAX5 both serves as a checkpoint itself and, also activates subsequent checkpoints during early B cell development.

The μ heavy chain of the pre-B cell receptor represents the checkpoint subsequent to PAX5 activation [1,2]. At this checkpoint, two opposing processes occur- one serves to eliminate deleterious pre-B cells with non-functional VH-DJH rearrangements (negative selection), while the other allows the survival and proliferation of pre-B cells carrying functional heavy chain rearrangements [2]. A disruption of the balance between the two processes leads to leukemic transformation of the pre-B cell.

Previous studies highlighted the requirement of BCL6 in the survival of early B cells that emerge following successful completion of the pre-B cell receptor checkpoint [11]. Further investigations revealed that this BCL6-mediated survival is essential to trigger the subsequent step in early B cell development, namely, immunoglobulin light chain recombination [11]. Recently, BACH2, a protein directly transactivated by PAX5 [12,13], was shown to be required for the negative selection of pre-B cells that had failed to productively rearrange their immunoglobulin heavy chains [14]. Here we review how BACH2 and BCL6 play central but opposing roles during early B cell differentiation, and thereby, maintain balance between negative selection and survival at the pre-B cell receptor checkpoint. In addition, we examine the disruption of this balance during leukemic transformation of a pre-B cell. Finally, we discuss how this knowledge can be exploited to predict the clinical outcome of patients with pre-B acute lymphoblastic leukemia (pre-B ALL), so as to design improved treatment strategies.

Diverse functions of BCL6 and BACH2 in the immune system

Transcriptional regulation by BCL6

BCL6 (B cell lymphoma/leukemia gene 6) was identified on the basis of its frequent translocations involving the 3q27 region in B cell non-Hodgkin’s lymphoma [15–18] (B-NHL). BCL6 is a member of the ZBTB (zinc finger and BTB) family that is comprised of an N-terminal BTB/POZ (Broad-complex, Tramtrack, Bric-a-brac/Poxvirus and zinc fingers) domain and Krüppel-type zinc fingers at the C-terminus. The zinc finger domain of BCL6 is required for DNA binding at its consensus sequence TTCCT(A/C)GAA [19,20]. Initial studies using reporter assays characterized BCL6 as a transcriptional repressor [21–24].

The mechanism of BCL6-mediated gene repression requires the physical interaction of corepressor proteins such as BCOR (BCL6-interacting corepressor), NCOR (nuclear receptor corepressor) or NCOR2 (SMRT) with BCL6 at target loci [25–28]. A second repressor domain (often called RD2 domain) has been found in the middle region of BCL6 that functions independently of the BTB domain by recruiting other co-repressors such as NuRD (nucleosome remodeling and deacetylase) and CtBP [29,30] (C-terminal binding protein). An integrative approach that combines ChIP-on-chip data with gene expression profiles revealed that recruitment of BCL6 to gene control regions does not necessarily cause gene repression, as only a fraction of the bound genes are repressed [31]. Regulatory variation might depend on distinct corepressor complexes at specific promoter sequences and lineage-specific mechanisms [32,33].

BCL6 is a pre-requisite for the germinal center reaction

Early studies with BCL6-deficient mice demonstrated that BCL6 is indispensable for the formation of germinal centers (GC) and, therefore, for the production of high-affinity antibodies [34–36]. This is attributed to the function of BCL6 in mediating lineage commitment of CD4+ T cells to T follicular helper (TFH) cells, which are required for GC formation, and to the negative regulatory effect of BCL6 on DNA damage response genes in GC B cells [37–41]. Bcl6 also acts as a switch in lymphoid cell fate decision by repressing the expression of Blimp-1 (Prdm1), a transcription factor that promotes plasma cell differentiation of GC B cells and blocks the TFH differentiation of CD4+ T cells [39] [42,43]. It is of interest to note that in mice in which Bcl6 is targeted in all tissues (germline knockout), as well as mice with targeted deletion of Bcl6 exclusively in the B-lymphoid compartment, exhibit a normal phenotype - in terms of absolute B cell numbers - at every stage of development [35] [44]. Bcl6 deficient mice, however, show highly restricted clonality despite the presence of a normal sized B cell pool [11]. Consistent with previous findings, conditional Bcl6 ablation has been shown to result in a normal sized post GC pool [44]. It will be of interest to determine if the post-GC memory B cells in the Bcl6 conditional knockout model developed by Takemori and colleagues show a pattern of clonal restriction similar to that observed in Bcl6 deficient early B cells. Additionally, it would be interesting to test whether conditional ablation of Bcl6 in this model affects survival of pre-B cells at the time of Ig light chain gene rearrangement in a manner similar to that demonstrated by Duy et al. [11] in the Bcl6 deficient mice.

BCL6 in mature B cell malignancies and inflammation

Deregulation of BCL6 is primarily associated with the pathogenesis of B cell lymphomas. Constitutive expression of BCL6 is required for survival of lymphoma cell lines and induces formation of diffuse large B cell lymphoma (DLBCL) in transgenic mice [37] [45–48]. Translocations of BCL6 that block p53-dependent apoptosis occur in 20 to 40 percent of diffuse large B cell lymphoma patients (DLBCLs) [37] [49,50].

BCL6 deficiency has also been associated with the development of autoimmune disease, with a recent study pointing to an important role for BCL6 in the function of regulatory T cells ((Tregs) [51]. Bcl6-deficient mice present severe myocarditis and pulmonary vasculitis. Sawant et al showed that Tregs deficient in BCL6 are unable to prevent Th2-type inflammatory allergic responses in vivo. These symptoms were attributed to the requirement of Bcl6 for the repression of the Th2 program in Tregs, comprised of Gata3, cmaf, Ccr8, and the Th2 cytokines Il4, Il5, and Il13. BCL6 therefore helps maintain immune homeostasis and keeps inflammatory responses at bay by repressing both Th2 cell differentiation and inhibiting Th2 cytokine expression in Tregs [51].

Transcriptional regulation by BACH2

Another B-lymphoid transcription factor that is critical for somatic hypermutation and class switch recombination is BACH2. BTB And CNC Homology 1 Basic Leucine Zipper Transcription Factor 2, abbreviated as BACH2, was identified from a yeast 2-hybrid screen for MafK [52]. BACH2 maps to chromosome 6q15 [53], and has a telomere to centromere transcriptional orientation [54]. As the name suggests, BACH2 possesses 2 domains: BTB and bZip. The BTB domain, located at the amino terminus is required for protein-protein interaction [52]. By virtue of its bZip domain, which is rich in basic amino acids like arginine and lysine, BACH2 mediates direct DNA binding [55]. The bZip domain of BACH2 also possesses the nuclear localization signal [56], which is essential for BACH2 to function as a transcription factor.

BACH2 binds to the TPA (12 O-Tetra decanoylphorbol-13-acetate) response element (TRE) on the DNA. In addition to TRE, BACH2 also binds to the related MARE (MAF Response Element) and ARE (Antioxidant Response Element) as homodimers or in combination with MAF proteins [55], and results in transcriptional repression. TRE, MARE and ARE elements share the same consensus sequence (TGAG/CTCA) and bind proteins belonging to the MAF family [55]. Although widely characterized as a repressor, BACH2 can activate transcription at selected loci. One such situation occurs when BACH2 binds to MAZR to activate fgf4 transcription [57].

BACH2 is indispensable for germinal center reaction

A comparison of stage specific expression of BACH2 during B cell development revealed that it was present at every stage except in plasma cells [58]. Further studies conducted to understand the reason behind the loss of BACH2 expression at the plasma cell stage led to the identification of the crucial role played by BACH2 in germinal center formation after antigen encounter [59]. Experiments carried out in a Bach2 deficient mouse model revealed that Bach2 delays the process of plasma cell differentiation by direct transcriptional repression of plasma cell factor Blimp1 [59–63]. Pax5, a crucial regulator of B cell differentiation, activates Bach2 and Aid and initiates the germinal center reaction. The Pax5-Bach2-Bcl6 axis delays plasma cell differentiation and induces the production of high affinity antibodies by germinal center B cells upon immunization [59]. More recently, it has been observed that antigen exposure lowers Bach2 expression levels in class-switched IgG1 memory B cells thereby facilitating their differentiation into plasma cells [64].

BACH2 in the pathogenesis of immunological malignancies and autoimmunity

As observed in the case of BCL6, deregulated BACH2 expression has been associated with lymphoid malignancies. Loss of heterozygosity of BACH2 has been reported to occur at a frequency of 20 percent in human B cell lymphomas [53]. Deletions of 6q15 that include the BACH2 locus occur in approximately 30 percent of pre-B ALL cases [65]. In a number of leukemias and lymphomas, disruption of wildtype BACH2 expression has been attributed to viral integrations [66–69]. Insertional mutagenesis studies using experimental murine retroviral models such as MLV (Murine Leukemia Virus) demonstrate that the BACH2 locus is a hotspot for viral integrations and transpositions [67,68]. Additionally, there is evidence to suggest that viral integrations at the BACH2 locus can also occur in human disease. For example, Epstein Barr Virus (EBV) was found inserted into the BACH2 locus in the human derived Burkitt Lymphoma cell line Raji [66]. Deletions and translocations of chromosome 6q spanning the BACH2 locus occur frequently in high-grade B cell lymphomas [14] [54] [70]. In multiple studies, high BACH2 level has been identified as an independent prognostic factor deciding favorable clinical outcome in DLBCL patients [71,72].

Although largely restricted to the B-lymphoid lineage within the hematopoetic system, BACH2 expression has been recently identified to be crucial for the development of Tregs and the maintenance of immune homeostasis. Although Bach2-deficient mice appear normal at birth, they subsequently develop progressive inflammation and autoimmune disease, similar to that observed in Bcl6-deficient mice. By stabilizing both Treg development through modulation of Foxp3 expression [73], and, by also repressing the differentiation of T cells to its effector lineages (Th1, Th2 and Th17), Bach2 constrains excessive immune activation [73,74]. These biochemical findings are backed by the presence of genetic polymorphisms in the BACH2 gene in multiple human autoimmune disorders, thus further underscoring the importance of BACH2 in immune tolerance.

BACH2 and BCL6 induce opposite outcomes during genotoxic stress

Rapidly proliferating GC B cells undergo a high level of cellular stress and DNA damage that is caused by somatic hypermutation and DNA double-strand break (DSB) events from class-switch recombination [75,76]. BCL6 allows these physiological processes to occur by repressing the expression of DNA damage response and checkpoint genes (CHEK1, ARF, CDKN1A, TP53) as well as the DNA damage sensor ATR. This allows GC B cell proliferation and survival even in the presence of genotoxic stress [37] [38] [42] [77] [78].

In contrast to BCL6, binding of BACH2 to its consensus sequence downregulates antioxidant response genes and triggers a program of oxidative stress induced cell death [55] [56] [79–81]. The majority of BACH2 protein is present in the cytoplasm in the absence of oxidative stress. Oxidative stress triggers nuclear localization of BACH2 by activating its nuclear localization signal and blocking its cytoplasmic localization [56].

BACH2 and BCL6 compete for binding to promoter at checkpoint-regulator genes

Our recent studies demonstrated that Bach2 deficient pre-B cells had lower expression of cell death inducers like Arf, p53 and Btg2. This observation raised the possibility that Bach2 acts as a transactivator of these cell cycle checkpoint genes, and triggers an apoptotic program upon genotoxic stress [14]. The concomitant expression of both BACH2 and BCL6 in the germinal center and their opposing roles on survival indicated the regulation of a set of common genes. Consistent with this hypothesis, a systematic ChIP-seq analysis revealed that 134 of the 541 BCL6- and 565 BACH2-target genes are shared. Interestingly, a number of cell cycle checkpoint genes such as TP53, ARF, GADD45A and GADD45B, that emerged from this analysis showed opposite transcriptional outcomes upon BACH2 and BCL6 binding. Although BCL6 and BACH2 bind different consensus sequences on the DNA, the binding sites for BACH2 and BCL6 on the promoters of the above mentioned genes were found to be overlapping. Unlike BCL6, BACH2 functions as a transcriptional activator at these loci, thus activating a program of cell death as opposed to survival in the case of BCL6 [14]. In light of these findings, we discuss in the following sections the interplay between BACH2 and BCL6 in early B cell development.

BACH2 and BCL6 maintain early B cell homeostasis

Gene expression changes during early human B cell development show that BACH2 expression is upregulated much earlier than that of BCL6 [82]. Bach2 is upregulated in a Pax5-dependent fashion at the multipotent progenitor (MPP) to the pro-B transition, before the completion of the heavy chain checkpoint; in contrast, Bcl6 levels remain fairly low until the completion of the pre-B cell receptor checkpoint, and increase at the pro-B to pre-B transition [11] [14] [82].

At the pre-B cell receptor checkpoint, Bach2 executes negative selection of early B cells with failed VH-DJH rearrangements. Additionally, it modulates heavy chain rearrangement by increasing the expression and activity of the RAG recombinases [14]. We propose that the above two events are interconnected: By activating the RAG recombinases, BACH2 increases the probability of a pre-B cell to undergo successful heavy chain rearrangement. Simultaneously, the BACH2-dependent rise in RAG recombinase levels would initiate a DNA damage response pathway in non-functional pre-B cells thereby aiding their negative selection. Further work will be required to test these hypotheses. BCL6 is turn is upregulated subsequent to the completion of the pre-B cell receptor checkpoint, and protects cells undergoing light chain rearrangement from the genotoxic stress caused by RAG1 and RAG2 [11].

Collectively, these observations are in agreement with the functional roles played by Bach2 and Bcl6 at the μ heavy chain checkpoint. While Bach2 is required for negative selection that occurs prior to the completion of the checkpoint [14], Bcl6 protects cells that emerge after successful completion of the checkpoint [11]. Bach2 and Bcl6 may thus represent a paradigm of how early B cell development is a perfectly equilibrated process with an interplay of several factors maintaining the integrity of the B cell repertoire (Figure 1). Any deviation from this balance could result in leukemic transformation of the pre-B cell (Figure 2).

Figure 1. BACH2 and BCL6 maintain homeostasis at the pre-B cell receptor checkpoint.

Figure 1

Open in a new tab

(a) BACH2 and BCL6 ensure high fidelity of the B cell repertoire by equilibrating two opposing processes, namely, negative selection and survival respectively, at the immunoglobulin heavy chain checkpoint. BACH2/BCL6 expression ratios remain high before the heavy chain checkpoint. This ensures that BACH2 can eliminate nonfunctional pre-B cells by negative selection, uninhibited by BCL6. Subsequently, a functional heavy chain upregulates BCL6 and tilts the balance to BCL6-mediated survival of pre-B cells emerging from the checkpoint. (b) Diagrammatic representation of the molecular mechanism behind the balance of negative selection and survival at the pre-B cell receptor checkpoint. In the absence of BCL6, BACH2 induces transactivation of ARF/TP53 to induce negative selection by apoptosis. Once negative selection is completed, BCL6 levels increase and cause displacement of BACH2 from promoters of ARF and TP53. This triggers a survival program by BCL6-mediated repression of ARF/TP53.

Figure 2. BACH2 and BCL6 maintain balance between checkpoint control and leukemic transformation of early B cells.

Figure 2

Open in a new tab

Upon challenge of early B cells with oncogenes like cMYC, BACH2 and BCL6 maintain the balance between apoptosis and leukemic transformation at the pre-B cell receptor checkpoint. BACH2 and BCL6 compete for promoters of ARF and TP53 in B cell precursor leukemia. Upon oncogene challenge, BACH2 (boldface, left) being downstream of PAX5, ensures that deleterious pre-B cells are eliminated by negative selection through activation of the ARF/TP53- mediated tumor surveillance mechanism (left, checkpoint control). If BACH2 expression is compromised (right, dashed arrow), BCL6-mediated repression of ARF/TP53 is unopposed leading to leukemia. Boldface indicates high level of a particular protein

BACH2-BCL6 equilibrium is upset during leukemic transformation

Defects in pre-BCR checkpoint control have been attributed to leukemia [1,2]. Similar to its role in DLBCL, BCL6 acts as a protooncogene in the pathogenesis of BCR-ABL1-driven ALLs. Furthermore, BCL6 enables survival of Philadelphia chromosome-positive ALL (Ph+ ALL) cells upon BCR-ABL1- kinase inhibition through repression of p53 expression [83].

In contrast, as discussed earlier, BACH2 triggers oxidative stress-induced cell death, by binding and repressing the expression of antioxidant response genes. Treatment of cells with oxidative stress-inducing chemotherapeutic agents results in nuclear accumulation of BACH2 [79] and subsequently, cell death. Studies with the BCR-ABL1 inhibitor, imatinib show that BACH2 is upregulated upon imatinib treatment in Ph+ ALL cell lines [13] [84]. BCR-ABL1 signaling lowers BACH2 expression and activity, the former by inactivation of PAX5 [13], and, the latter by preventing nuclear translocation of BACH2 [80].

We have recently shown that BACH2 is required downstream of PAX5 to prevent the accumulation of B cells with non-functional μHC rearrangements, providing an example of how defects in pre-BCR checkpoint control can result in pre-B ALL [14]. Bach2 expression halted or slowed down the process of pre-B leukemogenesis induced by oncogenes Myc and BCR-ABL1 by activating apoptosis [14]. In both normal pre-B cells and pre-B ALL, BACH2-induced activation of TP53 is opposed by BCL6. Thus, loss of BACH2 or any of its upstream activators, such as PAX5, results in unopposed BCL6-mediated suppression of TP53 at the pre-BCR checkpoint, leading to leukemia.

Concluding remarks and future perspectives

Using the pre-BCR checkpoint as an example, we have discussed how deregulated checkpoint control signaling can lead to B cell precursor ALL. B cell development is a tightly controlled process with diverse proteins maintaining the balance between negative selection and survival, and preserving the quality of the repertoire. By modulating the two opposing cellular processes at the pre-BCR checkpoint, namely apoptosis and survival, BACH2 and BCL6 ensure that the vast majority of the B cell repertoire is made up of cells carrying productive VH-DJH rearrangements.

Activation of BACH2 prior to BCL6 during early B cell development represents a critical failsafe mechanism to terminate pre-malignant pre-B cell clones before they can complete malignant transformation. BACH2 is frequently inactivated in established pre-B ALL clones or at the time of leukemia relapse [14] [85]. Consistent with their opposing role in pre-BCR checkpoint control, the ratio of BACH2 and BCL6 expression levels represent a statistically significant predictor of clinical outcome in patients with ALL [14]. While BACH2 function cannot be easily reinstated in patients with ALL carrying BACH2 deletions or somatic mutations, we propose that pharmacological inhibition of BCL6 (with BCL6 inhibitors like RI-BPI) will restore the balance of BACH2/BCL6-mediated checkpoint control and relieve BCL6-mediated transcriptional repression of ARF/TP53 (Figure 3). Treatment studies like these however need to be verified usingin vitro approaches and in vivo models before extending them to the clinic. Additionally, it would be interesting to test experimentally if a balance between BACH2 and BCL6 is required to maintain the stringency of the germinal center B cell repertoire. It would also be imperative to test if BACH2 is involved in RAG1/2-mediated receptor editing. Such an understanding would shed new light on BACH2’s role in the elimination of mature B cells with improper receptor editing. It would also be of interest to know if BACH2-BCL6 balance determines the elimination of B cells that are irreparably damaged by AID in the germinal center during somatic hypermutation and class switch recombination. Answering these questions would pave the way to better understanding the pathogenesis of Hodgkin’s B cell lymphomas that often carry crippling somatic mutations in their heavy chain loci.

Figure 3. Knowledge of relative levels of BACH2 and BCL6 in Ph+ ALL patients can be exploited to design effective treatment strategies.

Figure 3

Open in a new tab

(a) BACH2 expression levels can be lowered in Ph+ ALL patients by 6q15 deletions encompassing its locus, by somatic mutations or hypermethylation of its promoter or by loss of its upstream activator PAX5. This will lower the BACH2/BCL6 expression levels thus, leading to loss of ARF/TP53- dependent tumor surveillance. Such patients may not respond effectively to single agent (TKI) treatment. (b) A potential therapeutic strategy that may be employed to treat patients described in (a) is the combination of BCL6 inhibitors (e.g. RI-BPI) and TKIs to block BCL6 mediated repression of ARF/TP53. This treatment design will relieve BCL6-induced resistance of leukemia cells to TKIs and thereby, aid in their elimination.

Highlights.

  1. BACH2 and BCL6 regulate selection events at the pre-BCR checkpoint

  2. Regulation of negative and positive selection by BACH2 and BCL6 maintain homeostasis

  3. Pre-B cell susceptibility to leukomogenesis depends on relative BACH2 and BCL6 levels

Acknowledgments

We would like to thank A. Melinick (Weill Cornell Medical College) and K. Igarashi (Tohoku University Graduate School of Medicine) for their critical discussions and support. This work is supported by grants from the NIH/NCI through R01CA137060, R01CA139032, R01CA157644, R01CA169458 and R01CA172558 (to M.M.), Translational Research Program grants from the Leukemia and Lymphoma Society (grants 6132-09, 6097-10 and 6221-12), the William Lawrence and Blanche Hughes Foundation and a Stand Up To Cancer-American Association for Cancer Research Innovative Research Grant (IRG00909, to M.M.), the California Institute for Regenerative Medicine (CIRM; TR2-01816 to MM). M.M. is a scholar of the Leukemia and Lymphoma Society.

Footnotes

Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

References

  • 1.Hendriks RW, Middendorp S. The pre-BCR checkpoint as a cell-autonomous proliferation switch. Trends in Immunology. 2004;25:249–256. doi: 10.1016/j.it.2004.02.011. [DOI] [PubMed] [Google Scholar]
  • 2.Mårtensson I-L, et al. The pre-B cell receptor checkpoint. FEBS Lett. 2010;584:2572–2579. doi: 10.1016/j.febslet.2010.04.057. [DOI] [PubMed] [Google Scholar]
  • 3.Urbánek P, et al. Complete block of early B cell differentiation and altered patterning of the posterior midbrain in mice lacking Pax5/BSAP. Cell. 1994;79:901–912. doi: 10.1016/0092-8674(94)90079-5. [DOI] [PubMed] [Google Scholar]
  • 4.Nutt SL, et al. Commitment to the B-lymphoid lineage depends on the transcription factor Pax5. Nature. 1999;401:556–562. doi: 10.1038/44076. [DOI] [PubMed] [Google Scholar]
  • 5.Cobaleda C, et al. Pax5: the guardian of B cell identity and function. Nat Immunol. 2007;8:463–470. doi: 10.1038/ni1454. [DOI] [PubMed] [Google Scholar]
  • 6.O’Riordan M, Grosschedl R, et al. Coordinate Regulation of B Cell Differentiation by the Transcription Factors EBF and E2A. Immunity. 2004;11:21–31. doi: 10.1016/s1074-7613(00)80078-3. [DOI] [PubMed] [Google Scholar]
  • 7.Sigvardsson M, et al. Early B-Cell Factor, E2A, and Pax-5 Cooperate To Activate the Early B Cell-Specific mb-1 Promoter. Mol Cell Biol. 2002;22:8539–8551. doi: 10.1128/MCB.22.24.8539-8551.2002. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 8.Pongubala JMR, et al. Transcription factor EBF restricts alternative lineage options and promotes B cell fate commitment independently of Pax5. Nat Immunol. 2008;9:203–215. doi: 10.1038/ni1555. [DOI] [PubMed] [Google Scholar]
  • 9.Dengler HS, et al. Distinct roles for Foxo1 at multiple stages of B cell differentiation. Nat Immunol. 2008;9:1388–1398. doi: 10.1038/ni.1667. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 10.Fuxa M, et al. Pax5 induces V-to-DJ rearrangements and locus contraction of the immunoglobulin heavy-chain gene. Genes Dev. 2004;18:411–422. doi: 10.1101/gad.291504. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 11.Duy C, et al. BCL6 is critical for the development of a diverse primary B cell repertoire. J Exp Med. 2010;207:1209–1221. doi: 10.1084/jem.20091299. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 12.Schebesta A, et al. Transcription factor Pax5 activates the chromatin of key genes involved in B cell signaling, adhesion, migration, and immune function. Immunity. 2007;27:49–63. doi: 10.1016/j.immuni.2007.05.019. [DOI] [PubMed] [Google Scholar]
  • 13.Casolari DA, et al. Transcriptional suppression of BACH2 by the Bcr-Abl oncoprotein is mediated by PAX5. Leukemia. 2013;27:409–415. doi: 10.1038/leu.2012.220. [DOI] [PubMed] [Google Scholar]
  • 14.Swaminathan S, et al. Bach2 mediates negative selection and p53-dependent tumor suppression at the pre-B cell receptor checkpoint. Nature Medicine. 2013;19:1014–1022. doi: 10.1038/nm.3247. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 15.Baron BW, et al. Identification of the gene associated with the recurring chromosomal translocations t(3;14)(q27;q32) and t(3;22)(q27;q11) in B-cell lymphomas. Proc Natl Acad Sci U S A. 1993;90:5262–5266. doi: 10.1073/pnas.90.11.5262. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 16.Deweindt C, et al. Cloning of a breakpoint cluster region at band 3q27 involved in human non-Hodgkin’s lymphoma. Genes Chromosomes Cancer. 1993;8:149–154. doi: 10.1002/gcc.2870080303. [DOI] [PubMed] [Google Scholar]
  • 17.Kerckaert JP, et al. LAZ3, a novel zinc-finger encoding gene, is disrupted by recurring chromosome 3q27 translocations in human lymphomas. Nat Genet. 1993;5:66–70. doi: 10.1038/ng0993-66. [DOI] [PubMed] [Google Scholar]
  • 18.Ye BH, et al. Alterations of a zinc finger-encoding gene, BCL-6, in diffuse large-cell lymphoma. Science. 1993;262:747–750. doi: 10.1126/science.8235596. [DOI] [PubMed] [Google Scholar]
  • 19.Dent AL, et al. Regulation of gene expression by the proto-oncogene BCL-6. Crit Rev Oncol Hematol. 2002;41:1–9. doi: 10.1016/s1040-8428(01)00164-0. [DOI] [PubMed] [Google Scholar]
  • 20.Mascle X, et al. Point mutations in BCL6 DNA-binding domain reveal distinct roles for the six zinc fingers. Biochem Biophys Res Commun. 2003;300:391–396. doi: 10.1016/s0006-291x(02)02873-5. [DOI] [PubMed] [Google Scholar]
  • 21.Kawamata N, et al. Recognition DNA sequence of a novel putative transcription factor, BCL6. Biochem Biophys Res Commun. 1994;204:366–374. doi: 10.1006/bbrc.1994.2468. [DOI] [PubMed] [Google Scholar]
  • 22.Chang CC, et al. BCL-6, a POZ/zinc-finger protein, is a sequence-specific transcriptional repressor. Proc Natl Acad Sci USA. 1996;93:6947–6952. doi: 10.1073/pnas.93.14.6947. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 23.Seyfert VL, et al. Transcriptional repression by the proto-oncogene BCL-6. Oncogene. 1996;12:2331–2342. [PubMed] [Google Scholar]
  • 24.Baron BW, et al. BCL6 can repress transcription from the human immunodeficiency virus type I promoter/enhancer region. Genes Chromosomes Cancer. 1997;19:14–21. [PubMed] [Google Scholar]
  • 25.Dhordain P, et al. Corepressor SMRT binds the BTB/POZ repressing domain of the LAZ3/BCL6 oncoprotein. Proc Natl Acad Sci U S A. 1997;94:10762–10767. doi: 10.1073/pnas.94.20.10762. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 26.Dhordain P, et al. The LAZ3(BCL-6) oncoprotein recruits a SMRT/mSIN3A/histone deacetylase containing complex to mediate transcriptional repression. Nucleic Acids Res. 1998;26:4645–4651. doi: 10.1093/nar/26.20.4645. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 27.Huynh KD, Bardwell VJ. The BCL-6 POZ domain and other POZ domains interact with the co-repressors N-CoR and SMRT. Oncogene. 1998;17:2473–2484. doi: 10.1038/sj.onc.1202197. [DOI] [PubMed] [Google Scholar]
  • 28.Huynh KD, et al. BCoR, a novel corepressor involved in BCL-6 repression. Genes Dev. 2000;14:1810–1823. [PMC free article] [PubMed] [Google Scholar]
  • 29.Fujita N, et al. MTA3 and the Mi-2/NuRD complex regulate cell fate during B lymphocyte differentiation. Cell. 2004;119:75–86. doi: 10.1016/j.cell.2004.09.014. [DOI] [PubMed] [Google Scholar]
  • 30.Mendez LM, et al. CtBP is an essential corepressor for BCL6 autoregulation. Mol Cell Biol. 2008;28:2175–2186. doi: 10.1128/MCB.01400-07. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 31.Basso K, et al. Integrated biochemical and computational approach identifies BCL6 direct target genes controlling multiple pathways in normal germinal center B cells. Blood. 2010;115:975–984. doi: 10.1182/blood-2009-06-227017. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 32.Miles RR, et al. Analysis of BCL6-interacting proteins by tandem mass spectrometry. Mol Cell Proteomics. 2005;4:1898–1909. doi: 10.1074/mcp.M500112-MCP200. [DOI] [PubMed] [Google Scholar]
  • 33.Huang C, et al. Lineage-specific functions of Bcl-6 in immunity and inflammation are mediated by distinct biochemical mechanisms. Nat Immunol. 2013;14:380–388. doi: 10.1038/ni.2543. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 34.Dent AL, et al. Control of inflammation, cytokine expression, and germinal center formation by BCL-6. Science. 1997;276:589–592. doi: 10.1126/science.276.5312.589. [DOI] [PubMed] [Google Scholar]
  • 35.Ye BH, et al. The BCL-6 proto-oncogene controls germinal-centre formation and Th2-type inflammation. Nat Genet. 1997;16:161–170. doi: 10.1038/ng0697-161. [DOI] [PubMed] [Google Scholar]
  • 36.Fukuda T, et al. Disruption of the Bcl6 gene results in an impaired germinal center formation. J Exp Med. 1997;186:439–448. doi: 10.1084/jem.186.3.439. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 37.Phan RT, Dalla-Favera The BCL6 proto-oncogene suppresses p53 expression in germinal-centre B cells. Nature. 2004;432:635–639. doi: 10.1038/nature03147. [DOI] [PubMed] [Google Scholar]
  • 38.Ranuncolo SM, et al. Bcl-6 mediates the germinal center B cell phenotype and lymphomagenesis through transcriptional repression of the DNA-damage sensor ATR. Nat Immunol. 2007;8:705–714. doi: 10.1038/ni1478. [DOI] [PubMed] [Google Scholar]
  • 39.Johnston RJ, et al. Bcl6 and Blimp-1 are reciprocal and antagonistic regulators of T follicular helper cell differentiation. Science. 2009;325:1006–1010. doi: 10.1126/science.1175870. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 40.Nurieva RI, et al. Bcl6 mediates the development of T follicular helper cells. Science. 2009;325:1001–1005. doi: 10.1126/science.1176676. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 41.Yu D, et al. The transcriptional repressor Bcl-6 directs T follicular helper cell lineage commitment. Immunity. 2009;31:457–468. doi: 10.1016/j.immuni.2009.07.002. [DOI] [PubMed] [Google Scholar]
  • 42.Shaffer AL, et al. BCL-6 represses genes that function in lymphocyte differentiation, inflammation, and cell cycle control. Immunity. 2000;13:199–212. doi: 10.1016/s1074-7613(00)00020-0. [DOI] [PubMed] [Google Scholar]
  • 43.Tunyaplin C, et al. Direct repression of prdm1 by Bcl-6 inhibits plasmacytic differentiation. J Immunol. 2004;173:1158–1165. doi: 10.4049/jimmunol.173.2.1158. [DOI] [PubMed] [Google Scholar]
  • 44.Kaji T, et al. Distinct cellular pathways select germline-encoded and somatically mutated antibodies into immunological memory. J Exp Med. 2012;209:2079–2097. doi: 10.1084/jem.20120127. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 45.Baron BW, et al. The human BCL6 transgene promotes the development of lymphomas in the mouse. Proc Natl Acad Sci U S A. 2004;101:14198–14203. doi: 10.1073/pnas.0406138101. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 46.Polo JM, et al. Specific peptide interference reveals BCL6 transcriptional and oncogenic mechanisms in B-cell lymphoma cells. Nat Med. 2004;10:1329–1335. doi: 10.1038/nm1134. [DOI] [PubMed] [Google Scholar]
  • 47.Cattoretti G, et al. Deregulated BCL6 expression recapitulates the pathogenesis of human diffuse large B cell lymphomas in mice. Cancer Cell. 2005;7:445–455. doi: 10.1016/j.ccr.2005.03.037. [DOI] [PubMed] [Google Scholar]
  • 48.Cerchietti LC, et al. A small-molecule inhibitor of BCL6 kills DLBCL cells in vitro and in vivo. Cancer Cell. 2010;17:400–411. doi: 10.1016/j.ccr.2009.12.050. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 49.Saito M, et al. BCL6 suppression of BCL2 via Miz1 and its disruption in diffuse large B cell lymphoma. Proc Natl Acad Sci U S A. 2009;106:11294–11299. doi: 10.1073/pnas.0903854106. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 50.Ohno H, Fukuhara S, et al. Significance of rearrangement of the BCL6 gene in B-cell lymphoid neoplasms. Leuk Lymphoma. 1997;27:53–63. doi: 10.3109/10428199709068271. [DOI] [PubMed] [Google Scholar]
  • 51.Sawant DV, et al. Bcl6 controls the Th2 inflammatory activity of regulatory T cells by repressing Gata3 function. J Immunol. 2012;189:4759–4769. doi: 10.4049/jimmunol.1201794. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 52.Oyake T, et al. Bach proteins belong to a novel family of BTB-basic leucine zipper transcription factors that interact with MafK and regulate transcription through the NF-E2 site. Mol Cell Biol. 1996;16:6083–6095. doi: 10.1128/mcb.16.11.6083. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 53.Sasaki S, et al. Cloning and expression of human B cell-specific transcription factor BACH2 mapped to chromosome 6q15. Oncogene. 2000;19:3739–3749. doi: 10.1038/sj.onc.1203716. [DOI] [PubMed] [Google Scholar]
  • 54.Türkmen S, et al. A BACH2-BCL2L1 fusion gene resulting from a t(6;20)(q15;q11.2) chromosomal translocation in the lymphoma cell line BLUE-1. Genes Chromosomes Cancer. 2011;50:389–396. doi: 10.1002/gcc.20863. [DOI] [PubMed] [Google Scholar]
  • 55.Muto A, et al. Activation of Maf/AP-1 repressor Bach2 by oxidative stress promotes apoptosis and its interaction with promyelocytic leukemia nuclear bodies. J Biol Chem. 2002;277:20724–20733. doi: 10.1074/jbc.M112003200. [DOI] [PubMed] [Google Scholar]
  • 56.Hoshino H, et al. Oxidative stress abolishes leptomycin B-sensitive nuclear export of transcription repressor Bach2 that counteracts activation of Maf recognition element. J Biol Chem. 2000;275:15370–15376. doi: 10.1074/jbc.275.20.15370. [DOI] [PubMed] [Google Scholar]
  • 57.Kobayashi A, et al. A combinatorial code for gene expression generated by transcription factor Bach2 and MAZR (MAZ-related factor) through the BTB/POZ domain. Mol Cell Biol. 2000;20:1733–1746. doi: 10.1128/mcb.20.5.1733-1746.2000. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 58.Muto A, et al. Identification of Bach2 as a B-cell-specific partner for small maf proteins that negatively regulate the immunoglobulin heavy chain gene 3’ enhancer. EMBO J. 1998;17:5734–5743. doi: 10.1093/emboj/17.19.5734. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 59.Muto A, et al. The transcriptional programme of antibody class switching involves the repressor Bach2. Nature. 2004;429:566–571. doi: 10.1038/nature02596. [DOI] [PubMed] [Google Scholar]
  • 60.Ochiai K, et al. Plasmacytic transcription factor Blimp-1 is repressed by Bach2 in B cells. J Biol Chem. 2006;281:38226–38234. doi: 10.1074/jbc.M607592200. [DOI] [PubMed] [Google Scholar]
  • 61.Ochiai K, et al. Regulation of the plasma cell transcription factor Blimp-1 gene by Bach2 and Bcl6. Int Immunol. 2008;20:453–460. doi: 10.1093/intimm/dxn005. [DOI] [PubMed] [Google Scholar]
  • 62.Kallies A, Nutt SL. Bach2: plasma-cell differentiation takes a break. EMBO J. 2010;29:3896–3897. doi: 10.1038/emboj.2010.282. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 63.Muto A, et al. Bach2 represses plasma cell gene regulatory network in B cells to promote antibody class switch. EMBO J. 2010;29:4048–4061. doi: 10.1038/emboj.2010.257. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 64.Kometani K, et al. Repression of the Transcription Factor Bach2 Contributes to Predisposition of IgG1 Memory B Cells toward Plasma Cell Differentiation. Immunity. 2013;39:136–147. doi: 10.1016/j.immuni.2013.06.011. [DOI] [PubMed] [Google Scholar]
  • 65.Merup M, et al. 6q deletions in acute lymphoblastic leukemia and non-Hodgkin’s lymphomas. Blood. 1998;91:3397–3400. [PubMed] [Google Scholar]
  • 66.Takakuwa T, et al. Integration of Epstein-Barr virus into chromosome 6q15 of Burkitt lymphoma cell line (Raji) induces loss of BACH2 expression. Am J Pathol. 2004;164:967–974. doi: 10.1016/S0002-9440(10)63184-7. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 67.Liu J, et al. Identification of novel Bach2 transcripts and protein isoforms through tagging analysis of retroviral integrations in B-cell lymphomas. BMC Mol Biol. 2009;10:2. doi: 10.1186/1471-2199-10-2. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 68.Rasmussen MH, et al. Antisense transcription in gammaretroviruses as a mechanism of insertional activation of host genes. J Virol. 2010;84:3780–3788. doi: 10.1128/JVI.02088-09. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 69.Vassiliou GS, et al. Mutant nucleophosmin and cooperating pathways drive leukemia initiation and progression in mice. Nat Genet. 2011;43:470–475. doi: 10.1038/ng.796. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 70.Kobayashi S, et al. Identification of IGHCδ-BACH2 fusion transcripts resulting from cryptic chromosomal rearrangements of 14q32 with 6q15 in aggressive B-cell lymphoma/leukemia. Genes Chromosomes Cancer. 2011;50:207–216. doi: 10.1002/gcc.20845. [DOI] [PubMed] [Google Scholar]
  • 71.Sakane-Ishikawa E, et al. Prognostic significance of BACH2 expression in diffuse large B-cell lymphoma: a study of the Osaka Lymphoma Study Group. J Clin Oncol. 2005;23:8012–8017. doi: 10.1200/JCO.2005.02.1626. [DOI] [PubMed] [Google Scholar]
  • 72.Green M, et al. High levels of BACH2 associated with lower levels of BCL2 transcript abundance in t(14;18)(q21;q34) translocation positive non-Hodgkin’s lymphoma. Leuk Res. 2009;33:731–734. doi: 10.1016/j.leukres.2008.09.007. [DOI] [PubMed] [Google Scholar]
  • 73.Roychoudhuri R, et al. BACH2 represses effector programs to stabilize Treg-mediated immune homeostasis. Nature. 2013;498:506–510. doi: 10.1038/nature12199. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 74.Tsukumo SI, et al. Bach2 maintains T cells in a naive state by suppressing effector memory-related genes. Proc Natl Acad Sci USA. 2013;110:10735–10740. doi: 10.1073/pnas.1306691110. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 75.Schlissel MS, et al. Leukemia and lymphoma: a cost of doing business for adaptive immunity. Genes Dev. 2006;20:1539–1544. doi: 10.1101/gad.1446506. [DOI] [PubMed] [Google Scholar]
  • 76.Liu M, et al. Two levels of protection for the B cell genome during somatic hypermutation. Nature. 2008;451:841–845. doi: 10.1038/nature06547. [DOI] [PubMed] [Google Scholar]
  • 77.Shvarts A, et al. A senescence rescue screen identifies BCL6 as an inhibitor of anti-proliferative p19(ARF)-p53 signaling. Genes Dev. 2002;16:681–686. doi: 10.1101/gad.929302. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 78.Phan RT, et al. BCL6 interacts with the transcription factor Miz-1 to suppress the cyclin-dependent kinase inhibitor p21 and cell cycle arrest in germinal center B cells. Nat Immunol. 2005;6:054–1060. doi: 10.1038/ni1245. [DOI] [PubMed] [Google Scholar]
  • 79.Kamio T, et al. B-cell-specific transcription factor BACH2 modifies the cytotoxic effects of anticancer drugs. Blood. 2003;102:3317–3322. doi: 10.1182/blood-2002-12-3656. [DOI] [PubMed] [Google Scholar]
  • 80.Yoshida C, et al. Bcr-Abl signaling through the PI-3/S6 kinase pathway inhibits nuclear translocation of the transcription factor Bach2, which represses the antiapoptotic factor heme oxygenase-1. Blood. 2007;109:1211–1219. doi: 10.1182/blood-2005-12-040972. [DOI] [PubMed] [Google Scholar]
  • 81.Watanabe-Matsui M, et al. Heme regulates B-cell differentiation, antibody class switch, and heme oxygenase-1 expression in B cells as a ligand of Bach2. Blood. 2011;117:5438–5448. doi: 10.1182/blood-2010-07-296483. [DOI] [PubMed] [Google Scholar]
  • 82.Van Zelm MC, et al. Ig gene rearrangement steps are initiated in early human precursor B cell subsets and correlate with specific transcription factor expression. J Immunol. 2005;175:5912–5922. doi: 10.4049/jimmunol.175.9.5912. [DOI] [PubMed] [Google Scholar]
  • 83.Duy C, et al. BCL6 enables Ph+ acute lymphoblastic leukaemia cells to survive BCR-ABL1 kinase inhibition. Nature. 2011;473:384–388. doi: 10.1038/nature09883. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 84.Ono A, et al. Nuclear positioning of the BACH2 gene in BCR-ABL positive leukemic cells. Genes Chromosomes Cancer. 2007;46:67–74. doi: 10.1002/gcc.20390. [DOI] [PubMed] [Google Scholar]
  • 85.Hogan LE, et al. Integrated genomic analysis of relapsed childhood acute lymphoblastic leukemia reveals therapeutic strategies. Blood. 2011;118:5218–5226. doi: 10.1182/blood-2011-04-345595. [DOI] [PMC free article] [PubMed] [Google Scholar]

RESOURCES