Inducible knock-out of BCL6 in lymphoma cells results in tumor stasis (original) (raw)
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Blood, 2009
The BCL6 transcriptional repressor is required for development of germinal center (GC) B cells and when expressed constitutively causes diffuse large B-cell lymphomas (DLBCLs). We examined genome-wide BCL6 promoter binding in GC B cells versus DLBCLs to better understand its function in these settings. BCL6 bound to both distinct and common sets of functionally related gene in normal GC cells versus DLBCL cells. Certain BCL6 target genes were preferentially repressed in GC B cells, but not DLBCL cells. Several such genes have prominent oncogenic functions, such as BCL2, MYC, BMI1, EIF4E, JUNB, and CCND1. BCL6 and BCL2 expression was negatively correlated in primary DLBCLs except in the presence of BCL2 translocations. The specific BCL6 inhibitor retro-inverso BCL6 peptidomimetic inhibitor-induced expression of BCL2 and other oncogenes, consistent with direct repression effects by BCL6. These data are consistent with a model whereby BCL6 can directly silence oncogenes in GC B cells a...
Blood, 2003
The BCL6 proto-oncogene encodes a transcriptional repressor whose expression is deregulated by chromosomal translocations in approximately 40% of diffuse large B-cell lymphomas (DLBCLs). The BCL6 regulatory sequences are also targeted by somatic hypermutation in germinal center (GC) B cells and in a fraction of all GC-derived lymphomas. However, the functional consequences of these mutations are unknown. Here we report that a subset of mutations specifically associated with DLBCL causes deregulated BCL6 transcription. These mutations affect 2 adjacent BCL6 binding sites located within the first noncoding exon of the gene, and they prevent BCL6 from binding its own promoter, thereby disrupting its negative autoregulatory circuit. These alterations were found in approximately 16% of DLBCLs devoid of chromosomal translocations involving the BCL6 locus, but they were not found in normal GC B cells. This study establishes a novel mechanism for BCL6 deregulation and reveals a broader invo...
BCL6 represses CHEK1 and suppresses DNA damage pathways in normal and malignant B-cells
Blood Cells, Molecules, and Diseases, 2008
BCL6 is a transcriptional repressor protein that is expressed in a developmentally regulated fashion during B-cell maturation. Specifically, BCL6 is required for formation of germinal centers in response to T-cell dependent antigen activation. Germinal center B-cells feature the ability to tolerate rapid proliferation and simultaneous genetic recombination. Genetic lesions that cause constitutive expression of BCL6 are commonly associated with diffuse large B-cell lymphomas (DLBCL). Recent studies show that BCL6 contributes to the germinal center phenotype by directly repressing genes involved in sensing or responding to DNA damage including ATR, TP53 and CDKN1A. The CHEK1 protein is activated through phosphorylation by the ATR kinase domain in response to DNA damage. Activated CHEK1 can phosphorylate and modulate the activity a number of proteins including p53, providing a link between ATR sensing of DNA damage and p53 checkpoint activity. Herein we show that BCL6 can directly bind to a DNA consensus element in the CHEK1 promoter and repress its expression in normal and malignant B-cells. DLBCL cells can be killed by a specific BCL6 peptide inhibitor (BPI) that interferes with corepressor binding to the BCL6 BTB domain. BPI could reactivate CHEK1 in DLBCL cells, suggesting that its induction might contribute to BPI anti-lymphoma effects. Therefore, BCL6 can suppress multiple genes involved in a common pathway sensing, transducing and responding to genotoxic stress.
Nature Immunology, 2007
Antibody specificity and diversity is generated in B cells during germinal center maturation through clonal expansion while they undergo class-switch recombination and somatic hypermutation. Here we demonstrate that the transcriptional repressor Bcl-6 mediates this phenotype by directly repressing ATR in centroblasts and lymphoma cells. ATR is critical in replication and DNA damage-sensing checkpoints. Bcl-6 allowed B cells to evade ATR-mediated checkpoints and attenuated the response of the B cells to exogenous DNA damage. Repression of ATR was necessary and sufficient for those Bcl-6 activities. CD40 signaling 'rescued' B cells from those effects by disrupting the Bcl-6 transcription-repression complex on the promoter of the gene encoding ATR. Our data demonstrate a transcriptional regulatory loop whereby Bcl-6 mediates the centroblast phenotype through transient silencing of ATR.
Proceedings of the National Academy of Sciences, 2007
Diffuse large B cell lymphomas (DLBCLs) often express BCL6, a transcriptional repressor required for the formation of normal germinal centers. In a subset of DLBCLs, BCL6 is deregulated by chromosomal translocations or aberrant somatic hypermutation; in other tumors, BCL6 expression may simply reflect germinal center lineage. DLBCLs dependent on BCL6-regulated pathways should exhibit differential regulation of BCL6 target genes. Genomic array ChIP-on-chip was used to identify the cohort of direct BCL6 target genes. This set of genes was enriched in modulators of transcription, chromatin structure, protein ubiquitylation, cell cycle, and DNA damage responses. In primary DLBCLs classified on the basis of gene expression profiles, these BCL6 target genes were clearly differentially regulated in ''BCR'' tumors, a subset of DLBCLs with increased BCL6 expression and more frequent BCL6 translocations.
Molecular Pathogenesis of Non-Hodgkin's Lymphoma: the Role of Bcl-6
Leukemia & Lymphoma, 2003
Non-Hodgkin's lymphomas (NHL) form a heterogeneous group of diseases, with diffuse large B-cell lymphoma (DLBCL) comprising the largest subgroup. The commonest chromosomal translocations found in DLBCL are those affecting band 3q27. In 35% of DLBCL cases, as well as in a small fraction of follicular lymphomas, the normal transcriptional regulation of Bcl-6 is disrupted by these chromosomal translocations. In addition, about three-quarters of cases of DLBCL display multiple somatic mutations in the 5' non-coding region of Bcl-6, which occur independently of chromosomal translocations and appear to be due to the IgV-associated somatic hypermutation process. Bcl-6 is a 95-kD nuclear phosphoprotein belonging to the BTB/POZ (bric-a-brac, tramtrack, broad complex/ Pox virus zinc finger) zinc finger family of transcription factors. It has been suggested that Bcl-6 is important in the repression of genes involved in the control of lymphocyte activation, differentiation, and apoptosis within the germinal center, and that its down-regulation is necessary for normal B-cells to exit the germinal center. Bcl-6 remains constitutively expressed in a substantial proportion of B-cell lymphomas. Recently, acetylation has been identified as a mode for down-regulating Bcl-6 activity by inhibition of the ability of Bcl-6 to recruit complexes containing histone deacetylases (HDAC). The pharmacologic inhibition of two recently identified deacetylation pathways, HDAC-and silent information regulator (SIR)-2-dependent deacetylation, results in the accumulation of inactive acetylated Bcl-6 and thus in cell cycle arrest and apoptosis in B-cell lymphoma cells. These results reveal a new method of regulating Bcl-6, with the potential for therapeutic exploitation. These studies also indicate a novel mechanism by which acetylation promotes transcription, not only by modifying histones and activating transcriptional activators, but also by inhibiting transcriptional repressors.
Immunity, 2000
expressed in plasma cells (Falini et al., 2000). These BCL-6-negative GC B cells include cells with plas-BCL-6, a transcriptional repressor frequently translomacytic morphology and may represent cells that are cated in lymphomas, regulates germinal center B cell terminally differentiating as they exit the GC (Falini et differentiation and inflammation. DNA microarray screenal., 2000). Thus, plasmacytic differentiation, both preing identified genes repressed by BCL-6, including and post-GC, occurs only in the absence of BCL-6 exmany lymphocyte activation genes, suggesting that pression. BCL-6 modulates B cell receptor signals. BCL-6 re-Roughly one-sixth of all B cell non-Hodgkin's lymphopression of two chemokine genes, MIP-1␣ and IP-10, mas have translocations of the BCL-6 gene, making may also attenuate inflammatory responses. Blimp-1, BCL-6 one of the most frequently translocated genes another BCL-6 target, is important for plasmacytic difin these cancers (reviewed in Dalla-Favera et al., 1999; ferentiation. Since BCL-6 expression is silenced in Staudt et al., 1999). An attractive hypothesis is that plasma cells, repression of blimp-1 by BCL-6 may con-BCL-6 translocations cause non-Hodgkin's lymphomas trol plasmacytic differentiation. Indeed, inhibition of by coopting BCL-6's regulatory functions during B cell BCL-6 function initiated changes indicative of plasdifferentiation. In keeping with this idea, the BCL-6 macytic differentiation, including decreased exprestranslocations do not disrupt the BCL-6 coding region sion of c-Myc and increased expression of the cell but invariably substitute the BCL-6 promoter with a varicycle inhibitor p27kip1. These data suggest that maligety of other promoters. Thus, BCL-6 translocations likely cause transformation of B cells by deregulating the exnant transformation by BCL-6 involves inhibition of pression of normal BCL-6 protein. differentiation and enhanced proliferation. The aim of the present study was to discover the molecular pathways regulated by BCL-6 in order to un-Introduction derstand how BCL-6 controls immune responses and promotes the formation of lymphomas. Although the The transcriptional repressor BCL-6 has emerged as a consensus DNA binding site of BCL-6 has been defined multifunctional regulator of lymphocyte differentiation (Baron et al., 1995; Seyfert et al., 1996), the genomic and immune responses (reviewed in Dalla-Favera et al., targets of BCL-6 repression remain largely unknown. 1999; Staudt et al., 1999). BCL-6 mutant mice display The BCL-6 consensus binding site resembles the "GAS" two prominent phenotypes: a failure to form germinal motif recognized by the STAT family of transcription centers during a T cell-dependent immune response factors, raising speculation that BCL-6 may repress and a fatal inflammatory disease distinguished by the some cytokine response genes (Dent et al., 1997; Gupta presence of T helper type 2 (TH2) cells (Dent et al., et al., 1999; Harris et al., 1999). To comprehensively 1997; Fukuda et al., 1997; Ye et al., 1997). Although the identify BCL-6 target genes, we used Lymphochip mimolecular mechanisms underlying these phenotypes croarrays (Alizadeh et al., 1999, 2000). BCL-6 was found are largely unknown, clues to the function of BCL-6 to repress a number of genes involved in B cell activation come from its expression pattern. Although BCL-6 and terminal differentiation, inflammation, and cell cycle mRNA can be detected in many tissues (Allman et al., regulation, providing rich insights into the roles BCL-6 1996), its protein expression is limited mainly to lymphoplays in the immune system and in human lymphomas. cytes (Cattoretti et al., 1995; Onizuka et al., 1995; Allman et al., 1996), with the highest level of BCL-6 protein Results expressed in germinal center (GC) B cells. BCL-6 protein expression in B cells is exquisitely regulated following BCL-6 Expression Systems antigen encounter. Nascent pre-GC B cells upregulate To screen for BCL-6 target genes, we designed indepen-BCL-6 protein, migrate to the follicular area, and initiate dent and complementary cellular systems in which BCL-6 GC formation (Fukuda et al., 1997). In contrast, antigenfunction could be positively or negatively modulated. specific B cells that do not upregulate BCL-6 protein Gain-of-function systems were created by introducing expression differentiate in the periarteriolar lymphoid full-length (FL) BCL-6 into cell lines lacking endogenous sheath (PALS) into plasmablasts and provide an initial BCL-6 protein expression (Figure 1A). Loss-of-function systems were created by expressing a dominant-negative form of BCL-6, consisting solely of the BCL-6 zinc * To whom correspondence should be addressed (e-mail: lstaudt@ box-l.nih.gov).
Epigenetic Programing of B-Cell Lymphoma by BCL6 and Its Genetic Deregulation
Frontiers in Cell and Developmental Biology, 2019
B cell lymphoma is a clinically heterogeneous and pathologically diverse group of diseases with a strong epigenetic component. The B cell lymphoma 6 (BCL6) gene encodes a transcription factor that is critical for normal germinal center reaction B cell development by maintaining an epigenetic and transcriptional state that is permissive for cellular proliferation and DNA damage. The activity of BCL6 can be deregulated by a variety of mechanisms and contributes to the development of B-cell lymphoma. Here we review the direct and indirect mechanisms BCL6 dysregulation in B cell lymphoma, including transcriptional and post-translational regulation of BCL6 expression and activity, and the perturbation of BCL6-regulated epigenetic programs by cooperating chromatin modifying gene mutations. We underscore the critical importance of BCL6 and its associated epigenetic programs in the development of B-cell lymphoma, and discuss avenues for the therapeutic targeting of BCL6 in this context.
Oncotarget, 2017
Current understanding of the mutation spectrum of relapsed/refractory (RR) tumors is limited. We performed whole exome sequencing (WES) on 47 diffuse large B cell lymphoma (DLBCL) tumors that persisted after R-CHOP treatment, 8 matched to primary biopsies. We compared genomic alterations from the RR cohort against two treatment-naïve DLBCL cohorts (n=112). While the overall number and types of mutations did not differ significantly, we identified frequency changes in DLBCL driver genes. The overall frequency of MYD88 mutant samples increased (12% to 19%), but we noted a decrease in p.L265P (8% to 4%) and increase in p.S219C mutations (2% to 6%). CARD11 p.D230N, PIM1 p.K115N and CD79B p.Y196C mutations were not observed in the RR cohort, although these mutations were prominent in the primary DLBCL samples. We observed an increase in BCL2 mutations (21% to 38% of samples), BCL2 amplifications (3% to 6% of samples) and CREBBP mutations (31% to 42% of samples) in the RR cohort, supporte...