The Expanding Role of the BCL6 Oncoprotein As a Cancer Therapeutic Target Mariano G

Published OnlineFirst November 23, 2016; DOI: 10.1158/1078-0432.CCR-16-2071

Review Clinical Cancer Research The Expanding Role of the BCL6 Oncoprotein as a Cancer Therapeutic Target Mariano G. Cardenas1, Erin Oswald1, Wenbo Yu2, Fengtian Xue2, Alexander D. MacKerell Jr2, and Ari M. Melnick1

Abstract

BCL6 was initially discovered as an oncogene in B-cell lym- are equally effective in suppressing both the germinal center B-cell phomas, where it drives the malignant phenotype by repressing (GCB)- and the more aggressive activated B-cell (ABC)-DLBCL proliferation and DNA damage checkpoints and blocking B-cell subtypes, both of which require BCL6 to maintain their survival. terminal differentiation. BCL6 mediates its effects by binding to In addition, BCL6 is implicated in an expanding scope of hema- hundreds of target genes and then repressing these genes by tologic and solid tumors. These include, but are not limited to, B- recruiting several different chromatin-modifying corepressor acute lymphoblastic leukemia, chronic myeloid leukemia, breast complexes. Structural characterization of BCL6–corepressor com- cancer, and non–small cell lung cancer. BCL6 inhibitors have been plexes suggested that BCL6 might be a druggable target. Accord- shown to exert potent effects against these tumor types. Moreover, ingly, a number of compounds have been designed to bind to mechanism-based combinations of BCL6 inhibitors with other BCL6 and block corepressor recruitment. These compounds, agents have yielded synergistic and often quite dramatic activity. based on peptide or small-molecule scaffolds, can potently block Hence, there is a compelling case to accelerate the development BCL6 repression of target genes and kill lymphoma cells. In the of BCL6-targeted therapies for translation to the clinical setting. case of diffuse large B-cell lymphomas (DLBCL), BCL6 inhibitors Clin Cancer Res; 23(4); 885–93. 2016 AACR.

Introduction formation of B cells. Indeed constitutive expression of BCL6 in GC B cells drives the development of DLBCL in mice (7–9). BCL6 (B-cell lymphoma 6) is emerging as a key oncoprotein and BCL6 also represses numerous oncogenes in GC B cells, includ- therapeutic target. BCL6 was first identified as a locus affected by ing MYC, BCL2, BMI1, CCND1, and various others (10, 11). chromosomal translocations in diffuse large B-cell lymphomas Through this function, BCL6 may mitigate its own pro-oncogenic (DLBCL; ref. 1). However, it is now known to be broadly expressed checkpoint repression effect and, thus, reduce the potential for in many lymphomas regardless of genetic lesions. Its role in malignant transformation of GC B cells. This effect is abrogated in lymphomagenesis stems from its function in the humoral immune the presence of BCL2 or MYC translocations, which drive expres- system, where upregulation of BCL6 is required for the formation sion of these oncogenes through aberrant regulatory elements. of germinal centers (GC) during the humoral immune response The presence of both MYC and/or BCL2 together with BCL6 (2–4). GCs are transient structures that form in response to antigen (regardless of translocations) is clearly deleterious. It provides stimulation. Within GCs, B cells tolerate massive proliferation and B cells with simultaneous suppression of checkpoints through the mutagenic effect of the DNA-editing enzyme AICDA to under- BCL6, along with the progrowth and survival effects of MYC go immunoglobulin affinity maturation (5). All of this is orches- and BCL6. Not surprisingly, the combination of MYC and/or trated by and dependent on BCL6, a powerful transcriptional BCL2 with BCL6 in DLBCL has been linked to unfavorable clinical repressor that silences hundreds of genes. Some of these target outcomes (12). genes control DNA damage sensing (i.e., ATR, CHEK1, TP53, ARF, In the normal immune response, BCL6 function is terminated etc.) and proliferation checkpoints (CDKN1A, CDKN1B, by the disruption of BCL6 transcriptional complexes through CDKN2A, CDKN2B, PTEN, etc.; ref. 6). BCL6 also represses genes CD40-induced ERK signaling and downregulation of BCL6 required for exit from the GC reaction and plasma cell differen- mRNA by IRF4 and PRDM1 (13–15). Termination of BCL6 func- tiation (e.g., IRF4, PRDM1; ref. 6). This ensures that GC B cells have tion is required for B cells to exit the GC reaction. Yet in DLBCLs, a sufficient time to acquire somatic hypermutation of their immu- variety of mechanisms contribute to aberrant persistence of BCL6 noglobulin genes. It is thus easy to visualize how deregulated expression. These include fusion of the BCL6 coding region to suppression of these target genes could result in malignant trans- heterologous promoters via chromosomal translocations and somatic mutation of binding sites for repressors of BCL6 expression, such as IRF4, and BCL6 itself (15, 16). Somatic mutations of 1Department of Hematology/Oncology, Weill Cornell Medicine, New York, New the BCL6 ubiquitin ligase FBXO11 can enhance the half-life of 2 York. Department of Pharmaceutical Sciences, Computer-Aided Drug Design BCL6 protein in DLBCL (17). Induction of Hsp90 activation, Center, School of Pharmacy, University of Maryland, Baltimore, Maryland. which occurs almost universally in DLBCL, forms a positive Corresponding Author: Ari M. Melnick, Weill Cornell Medicine, 413 E 69th Street, feedback loop whereby (i) HSP90 maintains BCL6 mRNA and BB-1430, New York, NY 10021. Phone: 646-962-6725; Fax: 212-746-8866; E-mail: protein stability; (ii) HSP90 enhances BCL6 repressor function by [email protected] directly forming a complex on chromatin; and (iii) BCL6 repres- doi: 10.1158/1078-0432.CCR-16-2071 sion of EP300 prevents acetylation and inactivation of HSP90, 2016 American Association for Cancer Research. thus further enhancing BCL6 protein expression (18, 19). BCL6

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expression can also be aberrantly maintained by hypermethyla- Fortunately, it is possible to avoid these adverse effects by tion of regulatory CpGs contained in the BCL6 first intron (20). targeting specific sites on BCL6 that drive its cancer functions The powerful tumorigenic activity of BCL6 and the myriad ways but leave its anti-inflammatory functions intact. Structure– that lymphoma cells maintain its activity have fueled interest in function studies show that the BCL6 BTB domain mediates the development of BCL6 inhibitors. repression by recruiting the corepressor proteins SMRT, NCOR, and BCOR to an extended groove motif that forms along the The Biological Functions of BCL6 Are BTB dimer interface (Figs. 1 and 2A and B; refs. 23, 24). BCL6 Mediated through Distinct and Specific can bind two corepressors at a time because there are grooves on either side of the dimer. The corepressors bind through an Structural Motifs 18 amino acid BCL6-binding domain (BBD) that is highly To understand BCL6 as a therapeutic target, it is first nec- conserved between SMRT and NCOR, but which is completely essary to consider how it mediates its biological actions. BCL6 differentinthecaseofBCOR(23, 24). Importantly, the BCL6 has a trimodular structure consisting of an N-terminal BTB/ lateralgrooveresiduesthatmakecontactwiththeBBDare POZ domain that mediates transcriptional repression, an unique to BCL6 and not other BTB proteins (23), which could unstructured middle region containing a second repression facilitate the development of specific inhibitors that do not domain (RD2), and a series of six C2H2 zinc fingers at the C- affect other BTB domains. Point mutation of the lateral groove terminus that bind to DNA and other proteins (6). The bio- abolishes the repressor function of the BCL6 BTB domain (23). chemical and biological functions, together with the partner The repressive mechanism of action of the BCL6 BTB domain is proteins of each of the BCL6 domains, are summarized in Fig. quite intricate. NCOR and SMRT have related structures and form 1. One approach to targeting BCL6 is to completely abrogate its similar chromatin-modifying complexes that deacetylate histones functions, for example, using molecules that could block its (25, 26), whereas BCOR is completely different and forms a zinc fingers from binding to DNA or by destroying or down- variant Polycomb PRC1 complex with multiple distinct effects regulating the entire protein using approaches such as RNAi, on chromatin (27). On one hand, BCL6 preferentially represses antisense molecules, or small molecules that target proteins for gene promoters by forming a complex with BCOR (28). However, proteolytic destruction (e.g., degronomids). Yet in the case of the BCOR complex can only stably assemble at promoters that are BCL6,suchaneffectisnotdesirable.ThisisbecauseBCL6 marked with histone 3 lysine 27 trimethylation (H3K27me3) by knockout mice, in addition to failure to form GCs, also the Polycomb PRC2 protein EZH2 (29). H3K27me3 forms a manifest a severe and lethal systemic inflammatory disease binding site for CBX8, one of the components of the BCOR driven by T cells and macrophages (21). BCL6-deficient mice complex. Hence, BCL6-mediated promoter repression in GC and die within a few weeks of birth and exhibit massive tissue lymphoma cells requires "combinatorial tethering" of the BCOR infiltration of inflammatory cells in lung, heart, and other complex through the simultaneous actions of BCL6 and EZH2 tissues (2–4). Loss of BCL6 in macrophages causes accelerated (29). On the other hand, BCL6 also represses gene enhancers, but atherosclerosis in mice (22). this effect is linked to preferential interaction with SMRT and

Transcriptional Biochemical Transcriptional repression repression DNA binding Dimerization function Protein stability

MTA3 Partner BCoR HDAC2 ? ETO proteins SMRT/NCoR NuRD ? Class II HDACs CTBP Figure 1. The biological functions of BCL6 are mediated through specific protein BTB/POZ RD2 C2H2 zinc fingers domains. The figure shows a cartoon representation of the BCL6 domain Domains structure, indicating for each one the H2N COOH biochemical function, partner proteins, and biological functions. HDAC, histone deacetylase.

Proliferation Macrophage activation Biological Survival Competition with STATs TFh functions functions DNA damage checkpoints Anti-inﬂammatory function

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AB Figure 2. Charged pocket The BTB domain of BCL6. The images represent the BCL6 BTB domain alone K47' K47 or in a complex with the SMRT-BBD or D33' D33 compound 79-6, as determined by X-ray crystallography. The BTB domain forms an obligate homodimer, with the two monomers shown in red and blue in each panel. A, Ribbon view, with arrows pointing to the charged pocket motif and the hydrophobic surface, indicating examples of I9 F11 F11' I9' speciﬁc residues that participate in these features. B, Space ﬁll representation of the BCL6 BTB Hydrophobic face Lateral groove domain in a complex with the SMRT BBD, which is the polypeptide chain shown in purple. The lateral groove is © 2016 American Association for Cancer Research indicated in the boxed area.

NCOR complexes, which include HDAC3 (28). BCL6 enhancer action is structurally characterized; (iii) it involves a unique repression is driven through this indirect recruitment of HDAC3 interface not conserved in other proteins; and (iv) targeting the to remove the activating H3K27acetyl mark. Notably, the genes BTB lateral groove will not induce deleterious inflammatory that BCL6 represses by forming promoter repression complexes effects caused by complete loss of BCL6. Targeting the BTB are mostly different from those that it represses by forming dimer interface would likely be unsuitable, as BTB domains enhancer complexes (28). aggregate and are degraded if they cannot dimerize, which Remarkably, mice expressing a mutated form of BCL6, where could unleash the inflammatory effects of BCL6 deficiency. the BTB domain cannot bind to SMRT, NCOR, and BCOR, live The BTB domain has other features, such as a charged pocked normal, healthy lives without any evidence of inflammatory motif and a hydrophobic surface for oligomerization, which syndrome. These mice are nonetheless unable to form GCs, have not been functionally characterized (Fig. 2A; ref. 23). specifically due to failure of GC B cells to proliferate and survive Protein–protein interactions have long been thought to be (30). This is because BCL6 largely represses DNA damage and "undruggable" (33) given that many protein interactions proliferation checkpoint genes through the BTB domain, which involve large surfaces that may be difficult to disrupt with may explain (as discussed below) why this domain is also impor- small molecules. However, given sufficient knowledge of the tant in other tumor types. Indeed, mutation of the BTB domain protein interface, it is proving possible to design drugs that abrogated the ability of the lymphoma oncogene EZH2Y641 to exploit vulnerable "sweet spots" (34). The first BCL6 inhibitor drive preneoplastic lymphoproliferation in vivo (29). Hence, the contained the 17 amino acids from SMRT-BBD, along with a BCL6 BTB domain is required for GC formation and lymphoma- protein transduction domain for cell penetration (35). This genesis, but not anti-inflammatory effects. Instead, the anti- BCL6 peptide inhibitor (BPI) disrupted BCL6 repression com- inflammatory effect is due to direct competition of the BCL6 zinc plexes, induced expression of BCL6 target genes, killed DLBCL fingers with STAT proteins for binding to inflammatory chemo- cells in vitro, and phenocopied the BCL6 BTB domain mutant kines in macrophages (30). Finally, the minimal BCL6 RD2 phenotype in vivo (i.e., loss of GC formation without domain consists of approximately 40 amino acids and is required inflammation; Fig. 3). Although BPI was active at low micro- for interactions with MTA3 and HDAC2 (31, 32). Mice engineered molar concentrations, it was also readily degraded by proteases to express RD2-mutant BCL6 also live normal, healthy lives and (35). The bulk of intermolecular contacts between the BBD and have no inflammation (32). They do lack GCs, although in BCL6 are confined to a 9 amino acid sequence, which was used this case, the mechanism involves partial loss of function of as the warhead for a second-generation peptide using D-amino T-follicular helper (TFH) cells and disruption of B-cell homing acids resistant to proteases (36). The peptide sequence was to the GC (32). This extraordinary cell-context biochemical inverted to maintain proper stereochemistry and included a specialization of BCL6 makes it possible to target specific func- fusogenic motif for superior uptake within cells. The resulting tions relevant to tumor cells and, at the same time, avoid dis- retro-inverso (RI)-BPI peptide retained specificity for BCL6, but rupting other critical functions. not other BTB proteins; exhibited superior stability; and did not induce immunogenicity (36). RI-BPI killed BCL6-depen- Development of Peptidomimetic BCL6 dent DLBCLs at an average concentration of 16 mmol/L, similar to BPI (Fig. 3; ref. 36). Pharmacokinetic studies in mice revealed Inhibitors peak RI-BPI intratumor concentrations after 30 minutes The most straightforward approach to targeting BCL6 is and persistence in the nuclei of lymphoma cells for >24 hours disrupting the interaction between the BCL6 BTB domain (36). RI-BPI was nontoxic to mice, even after 1 year of contin- lateral groove and its corepressors because (i) this protein uous administration (36). These peptides thus appear suitable surface mediates BCL6 oncogenic effects; (ii) the protein inter- for use in humans. An alternative peptidomimetic strategy

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Gl in In vivo GC In vivo xenograft BCL6 inhibitor Structure Kd Method qPCR Chip 50 Reference killing sensitive cells inhibition models

15.8 3 nmol/L SPR 1414· 1441 LVATVKEAGRSIHEIPREELRHTPELPL NANA NA NA NA NA 18 SMRT 1 1.4 1 μmol/L lTC

BBD-BPI (His)6-pTAT-HA tag -GLVATVKEAGRSIHEIPREEL 10.2 μmol/L EMSA Yes Yes Yes 11 3 μmol/L Yes NA 6, 21

L-BPI Fu-TAT-GRSIHEIPRG NA NA Yes Yes Yes 14 2 μmol/L NA NA 6, 21

RI-BPI TAT-GRGIEHISR-Fu NA NA Yes Yes Yes 16 3 μmol/L Yes Yes 6, 21

Apt48 gpHGPRDWCLFGgp NA NA Yes NA NA NA NA NA 30

H N O Br 79-6 S 138 31 μmol/L NMR Yes Yes Yes 408 396 μmol/L NA Yes 31 O N S

HO2C CO2H H N O CI FX1 S 7 3 μmol/L MST Yes Yes Yes 39 12 μmol/L Yes Yes 34 O N S

HO2C

HO O O O OHOHOH Rifamycin SV O ∼ 1 mmol/L NMR NA NA NA NA NA NA 39 9 NH 7 8 1 2 6 5 4 3 O 10 H O OH O

OH HO Resveratrol NA NA NA NA NA <25 μmol/L NA NA 35–37

Figure 3. Current BCL6 inhibitors. The ﬁgure depicts the structure and known activities of published BCL6 inhibitors. ChIP, chromatin immunoprecipitation; EMSA, electrophoretic mobility shift assay; GI50, growth inhibition 50%; ITC, isothermal titration calorimetry; MST, microscale thermophoresis; NA, not applicable; NMR, nuclear magnetic resonance; qPCR quantitative PCR; SPR, surface plasmon resonance.

employed aptamer screening to identify Apt48, which binds to approximately 138 mmol/L was lower than the endogenous BBD the BCL6 BTB domain in a different manner than BPI (Fig. 3; (20 mmol/L; ref. 38). A novel in silico–based fragment-based ref. 37). Apt48 promoted reexpression of BCL6 target genes and screening method called "SILCS" (39, 40) was subsequently cell-cycle arrest in lymphoma cells, suggesting interference with used to design the most potent BCL6 inhibitor to date, FX1 BCL6 function. (41). FX1 is the first reported inhibitor with higher affinity for the BCL6 BTB domain than the endogenous corepressor proteins (Kd 4 mmol/L), demonstrating the feasibility of blocking large BCL6 Small-Molecule Inhibitors protein interactions with small molecules (Fig. 3; ref. 41). SILCS The BCL6 BTB lateral groove–BBD complex was used for in silico modeling combined with NMR analysis revealed a more favorable screening of small-molecule libraries. These studies yielded a orientation of the compound within the lateral groove aromatic family of compounds with reproducible and specific BCL6 inhi- pocket. FX1 outperformed all previous BCL6 inhibitors in cell- bition. A representative compound called "79-6" was shown to based pharmacodynamic assays while maintaining specificity for bind an aromatic pocket situated in the BTB domain lateral groove BCL6. The molecule exhibited favorable pharmacokinetics in vivo (38). 79-6 induced disruption of BCL6 transcriptional complexes, and lack of toxicity similar to previous generations of BCL6 reactivation of BCL6 target genes, and selective killing of BCL6- inhibitors (41). Other small molecules with potential BCL6- dependent DLBCL cells (Fig. 3; ref. 38). Even though this small binding activity include the natural product resveratrol (42), molecule was active in animal models, its binding affinity of which can induce antiproliferative and anti-inflammatory

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activities in different types of cancer cells through various mechan- sion of BCL6 is, therefore, necessary but perhaps not in all cases isms (Fig. 3; ref. 43). Ansamycin antibiotics may also bind to sufficient to predict response to BCL6 inhibitors. Gene expression BCL6. The structure of one of the members of this family, profiles might help to identify BCL6-dependent lymphomas. rifabutin, with the BCL6 BTB domain showed binding to the Indeed, one study identified a putative BCL6 functional signature lateral groove similar to 79-6, although with a lower binding by analyzing transcriptional profiles in primary DLBCL patient affinity of approximately 1 mmol/L (Fig. 3; ref. 44). specimens. When applied to DLBCL cell lines, this signature accurately predicted which ones would be BCL6 dependent Preclinical Activity of BCL6 Inhibitors in and which ones are BCL6 independent based on their response GCB-Type DLBCLs to BCL6 inhibitor (46). Perhaps functionally relevant biomarkers such as these could aid in patient selection for BCL6 High BCL6 expression is usually associated with germinal inhibitor trials. center B (GCB)-type DLBCLs (45), which are the most obvious target disease for BCL6 inhibitors. Accordingly, daily administra- Which Lymphomas Can Be Targeted by tion of RI-BPI at a dose of 7.5 mg/kg/day reduced the growth of established BCL6-dependent GCB-DLBCL xenografts by 60% to BCL6 Inhibitors? 80% but did not affect BCL6-independent DLBCL xenografts The relevance of BCL6 is not limited to GCB-DLBCLs. In fact, (36). Activity was concentration dependent, with doses of 50 BCL6 is expressed in most activated B-cell (ABC)-type DLBCLs, mg/kg/day leading to complete regression of GCB-DLBCLs (6). In albeit at lower levels (41, 48). Indeed, BCL6 translocation and spite of its weak binding, the small molecule 79-6 concentrates to amplification is largely restricted to ABC-DLBCLs, providing high levels within lymphoma cells, which enhances its activity genetic evidence for the importance of BCL6 in these tumors (38). Administration of 50 mg/kg/day 79-6 to SCID mice induced (47). It is worth noting that BCL6 transcript or protein abun- a 65% to 70% reduction in the size of established BCL6-depen- danceisnotassociatedwiththedegreeofbiologicaldepen- dent DLBCL xenografts but had no effect on BCL6-independent dency on BCL6. Along these lines, BCL6 shRNA knockdown of DLBCLs (38). In contrast, the potent small molecule FX1 induced BCL6 was just as deleterious to ABC-DLBCL cells as to GCB- complete regression of GCB-DLBCL xenografts at 25 mg/kg but DLBCL cells (41). ABC-DLBCL cells respond to similar doses of did not suppress BCL6-independent lymphomas (41). In all of BCL6 inhibitor, such as FX1, which also suppressed the growth these studies, the various inhibitors induced BCL6 target gene of ABC-DLBCL xenografts in vivo.Mostimportantly,primary reexpression, proliferation arrest, and apoptosis in DLBCL tumor human ABC- and GCB-DLBCL cells respond equally well to xenografts. More importantly, all three compounds were active in FX1 ex vivo (41). Given that ABC-DLBCLs manifest inferior killing primary human DLBCL specimens cultured ex vivo, under- outcome, it is warranted to develop BCL6-targeted therapy for lining the potential for BCL6 inhibitors to have activity clinical these patients. Follicular lymphomas manifest features of GCB trials (36, 38, 41). cells and, thus, generally express BCL6. We have found that From the predictive biomarker perspective, it would be useful primary low-grade follicular lymphoma cells are killed by the to classify patients as having BCL6-dependent or BCL6-indepen- BCL6 inhibitor RI-BPI (E. Valls and A.M. Melnick; unpublished dent lymphomas. The simple expression of BCL6 may not be data). Burkitt lymphomas arise from GC B cells, and BL cell sufficient to indicate BCL6 dependence, as a few GCB-DLBCL cell lines are potently killed by BCL6 inhibitors in vitro and in vivo lines express BCL6 but are not affected by genetic or pharmaco- (35). Although BCL6 is not usually expressed in multiple logic BCL6 inhibition (36, 38, 41, 46). A recent study overexpres- myeloma, withdrawal of microenvironment survival signals sing BCL6 in hematopoietic cells in mice showed development of could induce BCL6 expression (49), raising the possibility that lymphomas that were BCL6 independent. These data suggest that these B-cell neoplasms could be sensitive to BCL6 inhibitors as BCL6 might sometimes act in a hit-and-run manner (47). Expres- part of combination therapy regimens.

Table 1. Therapeutic targeting of BCL6 in lymphomas and other tumors BCL6 In vivo Malignancy Description RNA BCL6 inhibition Phenotype inhibition Reference Lymphoma GCB-DLBCL Germinal center B-cell diffuse large Yes siBCL6/shBCL6/BPI/RI-BPI/ Decreased proliferation Yes 21, 29, 31, 34 B-cell lymphoma 79-6/FX1 and apoptosis ABC-DLBCL Activated B-cell diffuse large B-cell Yes siBCL6/shBCL6/BPI/ Decreased proliferation Yes 21, 29, 31, 34 lymphoma RI-BPI/79-6/FX1 and apoptosis FL Follicular lymphoma Yes RI-BPI Apoptosis NA 51

Phþ B-ALL B-cell acute lymphoblastic leukemia Yes shBCL6/RI-BPI Reduced colony formation Yes 44 with Philadelphia chromosome and delayed progression MLLr B-ALL MLL-rearranged B-cell acute Yes siBCL6/RI-BPI Apoptosis NA 43 lymphoblastic leukemia CML Chronic myeloid leukemia Yes RI-BPI/dnBCL6 Selectively eradicates CD34þ Yes 45 CD38 leukemia-forming colonies Breast cancer Breast cancer Yes siBCL6/79-6 Reduced EMT and invasion Yes 46, 47, 48 NSCLC Non–small cell lung cancer Yes FX1/shBCL6 Inhibits proliferation Yes 49 NOTE: The spectrum of tumors that are dependent on BCL6 and are suppressed by BCL6 inhibitors. Abbreviations: EMT, epithelial–mesenchymal transition; NA, not applicable.

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Therapeutic Targeting of BCL6 in Other cell lines and primary patient samples (50). Exposure to ABL tyrosine kinase inhibitors (TKI) strongly induces BCL6 expression Tumors þ in BCR-ABL B-ALLs. In this context, BCL6 functioned as a BCL6 is implicated in an expanding list of tumors. Table 1 survival feedback mechanism that enables leukemia cells to resist summarizes the spectrum of BCL6-dependent tumors, the way in TKI treatment (51). Combined treatment of primary human BCR- þ which BCL6 was inhibited, and the phenotype associated with the ABL B-ALL with TKI and RI-BPI yielded massively synergistic inhibition. For example, BCL6 is a direct transcriptional target antileukemia effects in vitro and in vivo (51). Chronic myeloid of MLL fusion proteins in 11q23-rearranged B-acute lymphoblas- leukemias (CML) are also driven by the BCR–ABL fusion protein. tic leukemias (B-ALL; ref. 50). Binding of MLL fusion protein to Similar to B-ALLs, exposure to TKI induced BCL6 in CML cells þ the BCL6 promoter was associated with hypomethylation and (52). CD34 CML leukemic stem cells (LSC) upregulate BCL6 induction of BCL6 expression. Treatment with RI-BPI reduced in response to TKI in a FOXO-dependent manner. It has been colony formation capacity and induced apoptosis in MLL fusion appreciated that CML LSCs are less responsive to TKI than bulk

AB BCL6–EZH2 combinatorial Positive HSP90 HDAC tethering of BCOR feedback inhibition inhibition

HDACs HDACs HSP90 HDACs KDM28 HSP90 HSP90 PCGF1

BCL6i BCOR RING1B CBX8

H3k27me3 BCL6 BCL6 BCL6 EZH2i BCL6 p300 p53 p300 p53 p300 p53 EZH2 SUZ12 EED BCL6 Combination inhibition therapy

Combinatorial HSP90 HDACs HSP90 HDACs targeting of BCL6 and EZH2

C BCL6 BCL6 BCL2 Survival BCL6

BCL6 Oncogene p300 p53 p300 p53 BCL6 BCL2 Antiapoptosis Switching

Combination therapy

Synergy BCL6 BCL2 Cell death

Figure 4. Strategies for rational combinatorial therapy against BCL6 in lymphomas. A, Targeting the BCL6–EZH2 combinatorial tethering mechanism. BCL6 and EZH2 cooperate to recruit the BCOR corepressor complex. BCL6 directly binds to BCOR, and EZH2 deposits the H3K27me3 mark that is bound by the CBX8 subunit of the BCOR complex. Targeting BCL6 and EZH2 together results in more thorough disabling of transcriptional repression in lymphomas and greater therapeutic effect. B, Targeting the HSP90–BCL6 positive feedback loop through which BCL6 targets, including TP53, are suppressed at the transcriptional and posttranslational levels. Disruption of this axis can be partially achieved by targeting BCL6, HSP90, or HDACs, but more complete suppression of this axis by hitting at least two of these components is highly synergistic. C, Targeting the BCL6-BCL2 oncogene switching mechanism. Targeting BCL6 results in derepression of BCL2, enabling lymphoma cells to survive by switching to a dependency on BCL2. Targeting both together prevents lymphoma cells from utilizing this escape mechanism and also is useful in cases with BCL2 translocation where BCL2 has become independent of regulation through BCL6. HDAC, histone deacetylase.

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leukemia cells, and, in many patients, the disease is not eradicated enhanced checkpoint reactivation through combination of BCL6 by TKI therapy alone (52). It is notable that CML LSCs require inhibitor and TP53 activator also yielded enhanced antitumor BCL6 for their ability to form colonies, and initiate leukemia in effects (60). Indeed, as BCL6 reactivates DNA damage check- mice. Treatment of patient-derived CML cells with RI-BPI selec- points, it is not surprising that combinations of chemotherapy þ tively induced cell-cycle arrest of CD34 LSCs, causing depletion of drugs plus BCL6 inhibitor yield enhanced activity against lym- þ CD34 CD38 LSCs, whereas CD34 subpopulations remained phomas and leukemias (41). The feedback mechanism where þ intact (52). Hence, BCL6-targeted therapy could potentially erad- upregulation of BCL6 can protect BCR-ABL neoplasms from TKIs icate CML LSCs and reduce the need for long-term TKI therapy. supports the testing of BCL6 inhibitor combination therapy in B- Angioimmunoblastic T-cell lymphomas (AITL) manifest the ALL and CML (51, 52). phenotypic hallmarks of TFH cells. BCL6 is required for the Other combination therapies could target feedback mechan- formation of TFH cells (53). AITLs strongly express BCL6, and, isms that enable cells to escape killing by BCL6 inhibitors. For hence, it has been proposed that BCL6 might be a therapeutic example, BCL6 directly binds and represses BCL2 and BCL2L1 target in this disease (53). Up to 50% of breast tumors and many (BCL-XL; Fig. 4C; refs. 10, 11). Inhibition of BCL6 induces the breast cancer cell lines contain amplification of the BCL6 locus expression of BCL2 and BCL-XL, which can help lymphoma cells (54–56). Functional studies suggest that BCL6 contributes to the survive exposure to BCL6 inhibitors, a phenomenon that has been development of breast cancer (55–58). BCL6 is important for called "oncogene switching." The combination of BCL6 inhibitors tumor maintenance as treatment of breast cancer cell lines with RI- with BH3 mimetics overcomes this effect and yields synergistic BPI or 79-6 reduced cell viability (54). BCL6 may have cell activity (61). BCL6 represses multiple NFkB-related genes, which context-specific functions in breast cancer cells, as its target genes are induced after inhibiting BCL6. The effects of NFkB are com- are only partially overlapping with those in B cells (54). BCL6 was plex, but proof-of-principle experiments combining proteasome shown to be required for survival and proliferation of non–small inhibitors with BCL6 inhibitors showed highly synergistic actions. cell lung cancer (NSCLC) cells, in part due to repression of genes BCL6 also represses STAT3 (62, 63), an oncogenic driver in tumors involved in the DNA damage response. Inhibition of BCL6 by FX1 including breast cancer, which could be induced by BCL6 inhi- kills NSCLC cells in vitro and in vivo and exerts synergistic effect bitors. Suppression of STAT activation through Jak2 or STATs with cisplatin in xenograft models (59). inhibition caused additive loss of breast cancer cell viability, perhaps in part by suppressing additional STAT protumorigenic Combination Therapies of BCL6 with Other effects (54, 63). As the functions of BCL6 in various tumor types Inhibitors are better understood, it is likely that other such mechanism- based opportunities will be developed. Given the complexity and heterogeneity of tumor cells, it is In summary, BCL6 is a suitable therapeutic target that plays unlikely that single agents will cure disease. Hence, rational a broad and critical role in many cancers. Targeting the BCL6 combination of BCL6 inhibitors with other drugs is the best way BTB domain is an effective means of disrupting its functions to achieve maximal antitumor effect. Blocking the BCL6 BTB without inducing significant off-target effects against normal domain is not expected to induce major biological side effects, tissues. Mechanism-based combination therapy of BCL6 inhibi- so it should be straightforward to combine BCL6 inhibitors with tors yields dramatic antitumor effects. Hence, we propose that other therapeutic agents (36, 38, 41). For example, the fact that development of clinical grade BCL6 compounds is a mission BCL6 and EZH2 cooperate and require each other to repress worth vigorously pursuing. transcription led to the notion of combining EZH2 and BCL6 inhibitors (29). As might be expected, this combination yielded Disclosure of Potential Conflicts of Interest enhanced reactivation of BCL6/EZH2 target genes, with corre- A.D. MacKerell Jr is a cofounder and chief scientificofficer of SilcsBio LLC and sponding cooperative effects against DLBCL cells in vitro and in vivo ex vivo is a consultant/advisory board member for BioVia. A.M. Melnick reports as well as against primary human DLBCL specimens receiving other commercial research support from Eli Lilly, GlaxoSmithKline, (29). The result was observed in EZH2-mutant and wild-type Janssen, and Roche and is a consultant/advisory board member for Boehringer DLBCL cells and is summarized in Fig. 4A (29). Ingelheim, Eli Lilly, Epizyme, and Roche. M.G. Cardenas, F. Xue, A. D. MacKerell The positive feedback loop between BCL6, Hsp90, and EP300 Jr, and A. M. Melnick are listed as coinventors on a patent on BCL6 inhibitors fl mentioned earlier was the basis to combine BCL6 inhibitor with owned by Weill Cornell Medicine. No potential con icts of interest were disclosed by the other authors. drugs against either Hsp90 or histone deacetylase (HDAC) inhibitors (18). This is because inhibition of Hsp90 impairs BCL6 repressor functions and reduces BCL6 mRNA and protein levels Grant Support (Fig. 4B). This approach yields enhanced derepression of BCL6 This work was supported by the Leukemia & Lymphoma Society Therapy target genes, such as EP300, resulting in reduced Hsp90 activity Acceleration Program. The costs of publication of this article were defrayed in part by the payment through EP300-mediated acetylation, increased TP53 expression of page charges. This article must therefore be hereby marked advertisement and function, and restored DNA damage and proliferation check- in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. points (18). Accordingly, the combination of RI-BPI with Hsp90 or HDAC inhibitor was highly synergistic in vitro and yielded more Received August 19, 2016; revised September 28, 2016; accepted September potent antilymphoma effects in vivo (18). In a similar vein, 29, 2016; published OnlineFirst November 23, 2016.

References 1. Ye BH, Lista F, Lo Coco F, Knowles DM, Offit K, Chaganti RS, et al. 2. Ye BH, Cattoretti G, Shen Q, Zhang J, Hawe N, de Waard R, et al. The BCL-6 Alterations of a zinc finger-encoding gene, BCL-6, in diffuse large-cell proto-oncogene controls germinal-centre formation and Th2-type inflam- lymphoma. Science 1993;262:747–50. mation. Nat Genet 1997;16:161–70.

www.aacrjournals.org Clin Cancer Res; 23(4) February 15, 2017 891

Cardenas et al.

3. Dent AL, Shaffer AL, Yu X, Allman D, Staudt LM. Control of inflammation, 25. Guenther MG, Lane WS, Fischle W, Verdin E, Lazar MA, Shiekhattar R. A cytokine expression, and germinal center formation by BCL-6. Science core SMRT corepressor complex containing HDAC3 and TBL1, a WD40- 1997;276:589–92. repeat protein linked to deafness. Genes Dev 2000;14:1048–57. 4. Fukuda T, Yoshida T, Okada S, Hatano M, Miki T, Ishibashi K, et al. 26. Li J, Wang J, Wang J, Nawaz Z, Liu JM, Qin J, et al. Both corepressor proteins Disruption of the Bcl6 gene results in an impaired germinal center forma- SMRT and N-CoR exist in large protein complexes containing HDAC3. tion. J Exp Med 1997;186:439–48. EMBO J 2000;19:4342–50. 5. Klein U, Dalla-Favera R. Germinal centres: role in B-cell physiology and 27. Gearhart MD, Corcoran CM, Wamstad JA, Bardwell VJ. Polycomb group malignancy. Nat Rev Immunol 2008;8:22–33. and SCF ubiquitin ligases are found in a novel BCOR complex that is 6. Hatzi K, Melnick A. Breaking bad in the germinal center: how dereg- recruited to BCL6 targets. Mol Cell Biol 2006;26:6880–9. ulation of BCL6 contributes to lymphomagenesis. Trends Mol Med 28. Hatzi K, Jiang Y, Huang C, Garrett-Bakelman F, Gearhart MD, Gianno- 2014;20:343–52. poulou EG, et al. A hybrid mechanism of action for BCL6 in B cells defined 7. Cattoretti G, Pasqualucci L, Ballon G, Tam W, Nandula SV, Shen Q, by formation of functionally distinct complexes at enhancers and promo- et al. Deregulated BCL6 expression recapitulates the pathogenesis of ters. Cell Rep 2013;4:578–88. human diffuse large B cell lymphomas in mice. Cancer Cell 2005; 29. Beguelin W, Teater M, Gearhart MD, Calvo Fernandez MT, Goldstein RL, 7:445–55. Cardenas MG, et al. EZH2 and BCL6 Cooperate to Assemble CBX8-BCOR 8. Baron BW, Anastasi J, Montag A, Huo D, Baron RM, Karrison T, et al. The complex to repress bivalent promoters, mediate germinal center formation human BCL6 transgene promotes the development of lymphomas in the and lymphomagenesis. Cancer Cell 2016;30:197–213. mouse. Proc Natl Acad Sci U S A 2004;101:14198–203. 30. Huang C, Hatzi K, Melnick A. Lineage-specific functions of Bcl-6 in 9. Kusam S, Vasanwala FH, Dent AL. Transcriptional repressor BCL-6 immor- immunity and inflammation are mediated by distinct biochemical talizes germinal center-like B cells in the absence of p53 function. Onco- mechanisms. Nat Immunol 2013;14:380–8. gene 2004;23:839–44. 31. Fujita N, Jaye DL, Geigerman C, Akyildiz A, Mooney MR, Boss JM, et al. 10. Ci W, Polo JM, Cerchietti L, Shaknovich R, Wang L, Yang SN, et al. The BCL6 MTA3 and the Mi-2/NuRD complex regulate cell fate during B lymphocyte transcriptional program features repression of multiple oncogenes in differentiation. Cell 2004;119:75–86. primary B cells and is deregulated in DLBCL. Blood 2009;113:5536–48. 32. Huang C, Gonzalez DG, Cote CM, Jiang Y, Hatzi K, Teater M, et al. The 11. Saito M, Novak U, Piovan E, Basso K, Sumazin P, Schneider C, et al. BCL6 BCL6 RD2 domain governs commitment of activated B cells to form suppression of BCL2 via Miz1 and its disruption in diffuse large B cell germinal centers. Cell Rep 2014;8:1497–508. lymphoma. Proc Natl Acad Sci U S A 2009;106:11294–9. 33. Arkin MR, Whitty A. The road less traveled: modulating signal transduction 12. Burotto M, Berkovits A, Dunleavy K. Double hit lymphoma: from biology enzymes by inhibiting their protein-protein interactions. Curr Opin Chem to therapeutic implications. Expert Rev Hematol 2016;9:669–78. Biol 2009;13:284–90. 13. Polo JM, Ci W, Licht JD, Melnick A. Reversible disruption of BCL6 34. Wells JA, McClendon CL. Reaching for high-hanging fruit in drug discovery repression complexes by CD40 signaling in normal and malignant B cells. at protein-protein interfaces. Nature 2007;450:1001–9. Blood 2008;112:644–51. 35. Polo JM, Dell'Oso T, Ranuncolo SM, Cerchietti L, Beck D, Da Silva GF, et al. 14. Ranuncolo SM, Polo JM, Dierov J, Singer M, Kuo T, Greally J, et al. Bcl-6 Specific peptide interference reveals BCL6 transcriptional and oncogenic mediates the germinal center B cell phenotype and lymphomagenesis mechanisms in B-cell lymphoma cells. Nat Med 2004;10:1329–35. through transcriptional repression of the DNA-damage sensor ATR. Nat 36. Cerchietti LC, Yang SN, Shaknovich R, Hatzi K, Polo JM, Chadburn A, et al. Immunol 2007;8:705–14. A peptomimetic inhibitor of BCL6 with potent antilymphoma effects in 15. Saito M, Gao J, Basso K, Kitagawa Y, Smith PM, Bhagat G, et al. A signaling vitro and in vivo. Blood 2009;113:3397–405. pathway mediating downregulation of BCL6 in germinal center B cells is 37. Chattopadhyay A, Tate SA, Beswick RW, Wagner SD, Ko Ferrigno P. A blocked by BCL6 gene alterations in B cell lymphoma. Cancer Cell peptide aptamer to antagonize BCL-6 function. Oncogene 2006;25: 2007;12:280–92. 2223–33. 16. Pasqualucci L, Migliazza A, Basso K, Houldsworth J, Chaganti RS, Dalla- 38. Cerchietti LC, Ghetu AF, Zhu X, Da Silva GF, Zhong S, Matthews M, et al. A Favera R. Mutations of the BCL6 proto-oncogene disrupt its negative small-molecule inhibitor of BCL6 kills DLBCL cells in vitro and in vivo. autoregulation in diffuse large B-cell lymphoma. Blood 2003;101: Cancer Cell 2010;17:400–11. 2914–23. 39. Guvench O, MacKerell ADJr. Computational fragment-based binding site 17. Duan S, Cermak L, Pagan JK, Rossi M, Martinengo C, di Celle PF, et al. identification by ligand competitive saturation. PLoS Comput Biol 2009;5: FBXO11 targets BCL6 for degradation and is inactivated in diffuse large B- e1000435. cell lymphomas. Nature 2012;481:90–3. 40. Raman EP, Yu W, Guvench O, Mackerell AD. Reproducing crystal binding 18. Cerchietti LC, Hatzi K, Caldas-Lopes E, Yang SN, Figueroa ME, Morin RD, modes of ligand functional groups using Site-Identification by Ligand et al. BCL6 repression of EP300 in human diffuse large B cell lymphoma Competitive Saturation (SILCS) simulations. J Chem Inf Model 2011; cells provides a basis for rational combinatorial therapy. J Clin Invest 51:877–96. 2010;120:4569–82. 41. Cardenas MG, Yu W, Beguelin W, Teater MR, Geng H, Goldstein RL, et al. 19. Cerchietti LC, Lopes EC, Yang SN, Hatzi K, Bunting KL, Tsikitas LA, et al. A Rationally designed BCL6 inhibitors target activated B cell diffuse large B purine scaffold Hsp90 inhibitor destabilizes BCL-6 and has specific anti- cell lymphoma. J Clin Invest 2016;126:3351–62. tumor activity in BCL-6-dependent B cell lymphomas. Nat Med 42. Faber AC, Chiles TC. Resveratrol induces apoptosis in transformed follic- 2009;15:1369–76. ular lymphoma OCI-LY8 cells: evidence for a novel mechanism involving 20. Lai AY, Fatemi M, Dhasarathy A, Malone C, Sobol SE, Geigerman inhibition of BCL6 signaling. Int J Oncol 2006;29:1561–6. C, et al. DNA methylation prevents CTCF-mediated silencing of 43. Hsieh TC, Wu JM. Resveratrol: Biological and pharmaceutical properties as the oncogene BCL6 in B cell lymphomas. J Exp Med 2010;207: anticancer molecule. Biofactors 2010;36:360–9. 1939–50. 44. Evans SE, Goult BT, Fairall L, Jamieson AG, Ko Ferrigno P, Ford R, et al. The 21. Toney LM, Cattoretti G, Graf JA, Merghoub T, Pandolfi PP, Dalla-Favera R, ansamycin antibiotic, rifamycin SV, inhibits BCL6 transcriptional repres- et al. BCL-6 regulates chemokine gene transcription in macrophages. Nat sion and forms a complex with the BCL6-BTB/POZ domain. PLoS One Immunol 2000;1:214–20. 2014;9:e90889. 22. Barish GD, Yu RT, Karunasiri MS, Becerra D, Kim J, Tseng TW, et al. The 45. Alizadeh AA, Eisen MB, Davis RE, Ma C, Lossos IS, Rosenwald A, et al. Bcl6-SMRT/NCoR cistrome represses inflammation to attenuate athero- Distinct types of diffuse large B-cell lymphoma identified by gene expres- sclerosis. Cell Metab 2012;15:554–62. sion profiling. Nature 2000;403:503–11. 23. Ahmad KF, Melnick A, Lax S, Bouchard D, Liu J, Kiang CL, et al. Mechanism 46. Polo JM, Juszczynski P, Monti S, Cerchietti L, Ye K, Greally JM, et al. of SMRT corepressor recruitment by the BCL6 BTB domain. Mol Cell Transcriptional signature with differential expression of BCL6 target genes 2003;12:1551–64. accurately identifies BCL6-dependent diffuse large B cell lymphomas. Proc 24. Ghetu AF, Corcoran CM, Cerchietti L, Bardwell VJ, Melnick A, Prive GG. Natl Acad Sci U S A 2007;104:3207–12. Structure of a BCOR corepressor peptide in complex with the BCL6 BTB 47. Green MR, Vicente-Duenas C, Romero-Camarero I, Long Liu C, Dai B, domain dimer. Mol Cell 2008;29:384–91. Gonzalez-Herrero I, et al. Transient expression of Bcl6 is sufficient for

892 Clin Cancer Res; 23(4) February 15, 2017 Clinical Cancer Research

Therapeutic Targeting of BCL6

oncogenic function and induction of mature B-cell lymphoma. Nat Com- 56. Logarajah S, Hunter P, Kraman M, Steele D, Lakhani S, Bobrow L, et al. BCL- mun 2014;5:3904. 6 is expressed in breast cancer and prevents mammary epithelial differ- 48. Iqbal J, Greiner TC, Patel K, Dave BJ, Smith L, Ji J, et al. Distinctive patterns entiation. Oncogene 2003;22:5572–8. of BCL6 molecular alterations and their functional consequences in 57. Yu JM, Sun W, Hua F, Xie J, Lin H, Zhou DD, et al. BCL6 induces EMT by different subgroups of diffuse large B-cell lymphoma. Leukemia promoting the ZEB1-mediated transcription repression of E-cadherin in 2007;21:2332–43. breast cancer cells. Cancer Lett 2015;365:190–200. 49. Hideshima T, Mitsiades C, Ikeda H, Chauhan D, Raje N, Gorgun G, et al. A 58. Wu Q, Liu X, Yan H, He YH, Ye S, Cheng XW, et al. B-cell lymphoma 6 proto-oncogene BCL6 is up-regulated in the bone marrow microenviron- protein stimulates oncogenicity of human breast cancer cells. BMC Cancer ment in multiple myeloma cells. Blood 2010;115:3772–5. 2014;14:418. 50. Geng H, Brennan S, Milne TA, Chen WY, Li Y, Hurtz C, et al. Integrative 59. Marullo R, Ahn H, Cardenas M, Melnick A, Xue F, Cerchietti L, et al. The epigenomic analysis identiﬁes biomarkers and therapeutic targets in adult transcription factor BCL6 is a rational target in non-small cell lung cancer B-acute lymphoblastic leukemia. Cancer Discov 2012;2:1004–23. (NSCLC) [abstract]. In: Proceedings of the 107th Annual Meeting of the 51. Duy C, Hurtz C, Shojaee S, Cerchietti L, Geng H, Swaminathan S, et al. American Association for Cancer Research; 2016 Apr 16–20; New Orleans, BCL6 enables Phþ acute lymphoblastic leukaemia cells to survive BCR- LA. Philadelphia (PA): AACR; 2016. Abstract nr 1271. ABL1 kinase inhibition. Nature 2011;473:384–8. 60. Cerchietti LC, Polo JM, Da Silva GF, Farinha P, Shaknovich R, Gascoyne 52. Hurtz C, Hatzi K, Cerchietti L, Braig M, Park E, Kim YM, et al. BCL6- RD, et al. Sequential transcription factor targeting for diffuse large B-cell mediated repression of p53 is critical for leukemia stem cell survival in lymphomas. Cancer Res 2008;68:3361–9. chronic myeloid leukemia. J Exp Med 2011;208:2163–74. 61. Dupont T, Yang SN, Patel J, Hatzi K, Malik A, Tam W, et al. Selective 53. Cortes JR, Palomero T. The curious origins of angioimmunoblastic T-cell targeting of BCL6 induces oncogene addiction switching to BCL2 in B-cell lymphoma. Curr Opin Hematol 2016;23:434–43. lymphoma. Oncotarget 2016;7:3520–32. 54. Walker SR, Liu S, Xiang M, Nicolais M, Hatzi K, Giannopoulou E, et al. The 62. Ding BB, Yu JJ, Yu RY, Mendez LM, Shaknovich R, Zhang Y, et al. Con- transcriptional modulator BCL6 as a molecular target for breast cancer stitutively activated STAT3 promotes cell proliferation and survival in the therapy. Oncogene 2015;34:1073–82. activated B-cell subtype of diffuse large B-cell lymphomas. Blood 55. Bos R, van Diest PJ, van der Groep P, Greijer AE, Hermsen MA, Heijnen I, 2008;111:1515–23. et al. Protein expression of B-cell lymphoma gene 6 (BCL-6) in invasive 63. Walker SR, Nelson EA, Yeh JE, Pinello L, Yuan GC, Frank DA. STAT5 breast cancer is associated with cyclin D1 and hypoxia-inducible factor- outcompetes STAT3 to regulate the expression of the oncogenic transcrip- 1alpha (HIF-1alpha). Oncogene 2003;22:8948–51. tional modulator BCL6. Mol Cell Biol 2013;33:2879–90.

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The Expanding Role of the BCL6 Oncoprotein as a Cancer Therapeutic Target

Mariano G. Cardenas, Erin Oswald, Wenbo Yu, et al.

Clin Cancer Res 2017;23:885-893. Published OnlineFirst November 23, 2016.

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