Breaking bad in the germinal center: how deregulation of BCL6 contributes to lymphomagenesis

Abstract

The B cell lymphoma 6 (BCL6) transcriptional repressor is a master regulator of the germinal center (GC) B cell program, required for their unique proliferative and stress tolerant phenotype. Most B cell lymphomas arise from GC B cells and are dependent on the continued or deregulated expression of BCL6 to maintain their survival. The actions of BCL6 in B cells involve formation of distinct chromatin modifying complexes that silence specific promoter and enhancer networks, respectively. The same biochemical mechanisms are maintained in malignant lymphoma cells. Targeted inhibition of these BCL6 functions has emerged as the basis for rational design of lymphoma therapies and combinatorial regimens. In this review, we summarize recent advances on BCL6 mechanisms of action and the deregulation of its target gene networks in lymphoma.

Keywords: BCL6; epigenetic regulation; gene enhancers; lymphomagenesis; transcription factor targeted therapy; transcriptional repression.

Figure 1

BCL6 maintains a precarious balance between normal and malignant phenotypes in GC B cells. (A) During the GC response, B cells undergo somatic hypermutation and rapid proliferation that leads to accumulation of genotoxic stress. Attenuated DNA damage sensing in these cells allows their survival and upon completion of clonal selection GC B cells either differentiate to plasmacytesor die through apoptosis. However, the burden of attenuated DNA damage sensing and error prone division can drive the cells towards lymphomagenesis. (B) In normal B cells, BCL6 maintains a delicate balance between survival and apoptosis. This is achieved by simultaneously repressing tumor suppressors and oncogenes. (C) Promoter mutations or translocations of BCL6-repressed oncogenes often characterize lymphoma cells, suggesting that loss of the BCL6 tumor suppressor function can tip the balance towards malignant transformation. Abbreviations:BCL6, B cell lymphoma 6; GC, germinal center.

Figure 2

Repression of BCL6 target gene networks enable certain hallmarks associated with transformation. To sustain proliferation and survival during affinity maturation, BCL6 represses key genes involved in sensing DNA damage, error prone division, and cell cycle checkpoints. BCL6 also represses EP300, which negatively regulates HSP90. In turn, HSP90 sustains BCL6 expression forming a feed-forward loop, causing GC B cells to adapt to and become dependent on stress signaling. To maintain the GC B cell phenotype and prevent termination of the GC transcriptional program, BCL6 suppresses certain aspects of BCR and CD40 signaling pathways that together with TLR signaling converge to activate NF-κB and promote GC exit. Moreover, by repressing terminal differentiation genes, such as PRDM1, BCL6 can maintain the GC phenotype and prevent premature terminal differentiation into memory or plasma cells. Critical BCL6 target genes are recurrently mutated in lymphoma (yellow), thus escaping BCL6 control. Green arrows indicate activation and red lines repression. Abbreviations: BCL6, B cell lymphoma 6; BCR, B cell receptor; GC, germinal center; TLR, Toll-like receptor; NF-κB, nuclear factor-κB; PRDM1, PR domain containing 1, with ZNF domain; EP300, E1A binding protein p300; HSP90, heat shock protein 90. Note: symbols denoting both gene and protein are not italicized. Symbols referring to genes only are italicized.

Figure 3

Mechanisms of BCL6 transcript and protein regulation in normal GC B cells versus lymphoma cells. (A) Normal B cells. BCL6 transcript and protein levels are highly upregulated upon B cell activation. Transcription factors activated by extracellular signals such as IRF8 and MEF2B induce BCL6 transcription. BCL6 expression is normally shut down after affinity maturation is accomplished to allow B cell differentiation. At this point, BCL6 repression is mediated by IRF4. BCL6 represses its own expression by binding to its own promoter and might also regulate an upstream LCR. It is plausible that looping of the LCR to the BCL6 promoter could regulate BCL6 expression in normal GC B cells. HSP90and AUF1 stabilize BCL6 transcripts. Furthermore, eIF4e augments BCL6 mRNA nuclear export and translation. BCL6 protein is also stabilized by the HSP90 chaperone, which might be induced by replicative and other sources of stress. At the post-transcriptional level, the BCL6 protein is acetylated and inactivated by P300/CREBBP. Moreover, BCL6 is targeted for ubiquitination and proteasomal degradation by ubiquitin ligase complex SCF (SKP1CUL1F-box protein) that contains the FBXO11. (B) BCL6 is constitutively expressed and deregulated in B cell lymphomas. Sustained BCL6 transcript levels can result from BCL6 promoter mutations or translocations (red asterisks) that abolish DNA binding of negative regulators of BCL6, such as IRF4 and BCL6 itself. Alternatively, hypermethylation (black pegs) of BCL6 regulatory regions may abrogate CTCF-mediated BCL6 downregulation. Somatic mutations affecting P300/CREBBP result in failure to acetylate and inactivate BCL6 protein. FBXO11 mutations result in failure to ubiquitinate and degrade BCL6 (red x). Small molecule inhibitors (gold star) of factors that maintain BCL6 transcript and/or protein stability (such as HSP90 and BRD4) could block BCL6 expression and exhibit anti-lymphoma activity. Abbreviations: BCL6, B cell lymphoma 6; LCR, locus control region; GC, germinal center; Ac, acetylation; P, phosphorylation; Ub, ubiquitination; HSP90, heat shock protein 90; IRF8, interferon regulatory factor 8; MEF2B, myocyte enhancer factor 2B; AUF1, AU-rich element RNA-binding protein 1; eIF4e, eukaryotic translation initiation factor 4E; CREBBP, CREB-binding protein; FBXO11, F-box only protein 11; CTCF, CCCTC-binding factor; BRD4, bromodomain-containing protein 4.

Figure 4

BCL6 mediates its GC B cell functions through distinct biochemical mechanisms. GCs form in the secondary lymphoid organs by activated B cells. After forming long interactions with T cells within the interfollicular regions, activated B cells upregulate BCL6 and enter the follicles where they rapidly proliferate while undergoing somatic hypermutation. These large mitotically active B cells are called centroblasts and are located in the dark zone of the GC Centroblasts then differentiate into centrocytes and migrate to the light zone of the GC where they test the affinity of their Ig receptors with help from GC T_FH cells and FDCs. Centrocytes with high affinity BCRs are selected to differentiate into plasmacytes or memory B cells but those with low affinity die through apoptosis. Recent findings suggest that the BCL6 BTB domain is essential for sustaining GC B cell proliferation. BCL6 BTB mutations disrupting the ability of BCL6 to recruit SMRT/NCOR and BCOR corepressors impair GC formation and affinity maturation. BCL6 mediates transcriptional repression through two biochemically distinct mechanisms acting at key B cell promoters and enhancers. At promoters, BCL6 dimers can simultaneously recruit PRC1-like BCOR complexes and HDAC3-containing SMRT/NCOR complexes to effectively repress transcription in a repressed chromatin environment. At enhancers, BCL6 selectively recruits HDAC3-containing SMRT/NCOR complexes to functionally inactivate these elements through H3K27 deacetylation. In this model, interaction of GC B and GC T_FH cells in the light zone activates CD40 signaling in centrocytes and causes ERK-mediated phosphorylation of SMRT, which leads to cytoplasmatic localization. Eviction of the SMRT corepressor from the BCL6 complexes leads to reactivation of BCL6-targeted gene programs during clonal selection. Switching these networks ‘on’ might be reversible allowing cycling back to the dark zone for additional rounds of affinity maturation. Abbreviations: BCL6, B cell lymphoma 6; BCR, B cell receptor; GC, germinal center; T_FH, follicular B helper T cells; BTB, broad complex/tramtrack/bric-a-brac; FDC, follicular dendritic cell; TSS, transcription start site; SMRT, silencing mediator of retinoid and thyroid receptor; NCOR, nuclear receptor corepressor; BCOR, BCL6 corepressor; PRC1, polycomb repressor complex 1; HDAC3, histone deacetylase 3.