Highlights • BCL6 maintains a delicate balance between survival and apoptosis in germinal center (GC) B cells. • BCL6 maintains promoters and enhancers in a poised state in GC and lymphoma cells. • ...BCL6 mechanism of action varies among cell types in the immune system. • BCL6 mechanism-specific targeting kills lymphomas without harming other tissues.
Summary
One of the unusual features of germinal center (GC) B cells is that they manifest many hallmarks of cancer cells. Accordingly, most B‐cell neoplasms originate from the GC reaction, and ...characteristically display abundant point mutations, structural genomic lesions, and clonal diversity from the genetic and epigenetic standpoints. The dominant biological theme of GC‐derived lymphomas is mutation of genes involved in epigenetic regulation and immune receptor signaling, which come into play at critical transitional stages of the GC reaction. Hence, mechanistic studies of these mutations reveal fundamental insight into the biology of the normal and malignant GC B cell. The BCL6 transcription factor plays a central role in establishing the GC phenotype in B cells, and most lymphomas are dependent on BCL6 to maintain survival, proliferation, and perhaps immune evasion. Many lymphoma mutations have the commonality of enhancing the oncogenic functions of BCL6, or overcoming some of its tumor suppressive effects. Herein, we discuss how unique features of the GC reaction create vulnerabilities that select for particular lymphoma mutations. We examine the interplay between epigenetic programming, metabolism, signaling, and immune regulatory mechanisms in lymphoma, and discuss how these are leading to novel precision therapy strategies to treat lymphoma patients.
Activated B-cell (ABC)-diffuse large B-cell lymphomas (DLBCLs) are clinically aggressive and phenotypically complex malignancies, whose transformation mechanisms remain unclear. Partially ...differentiated antigen-secreting cells (plasmablasts) have long been regarded as cells-of-origin for these tumors, despite lack of definitive experimental evidence. Recent DLBCL reclassification based on mutational landscapes identified MCD/C5 tumors as specific ABC-DLBCLs with unfavorable clinical outcome, activating mutations in the signaling adaptors MYD88 and CD79B, and immune evasion through mutation of antigen-presenting genes. MCD/C5s manifest prominent extranodal dissemination and similarities with primary extranodal lymphomas (PENLs). In this regard, recent studies on TBL1XR1, a gene recurrently mutated in MCD/C5s and PENLs, suggest that aberrant memory B cells (MBs), and not plasmablasts, are the true cells-of-origin for these tumors. Moreover, transcriptional and phenotypic profiling suggests that MCD/C5s, as a class, represent bona fide MB tumors. Based on emerging findings we propose herein a generalized stepwise model for MCD/C5 and PENLs pathogenesis, whereby acquisition of founder mutations in activated B cells favors the development of aberrant MBs prone to avoid plasmacytic differentiation on recall and undergo systemic dissemination. Cyclic reactivation of these MBs through persistent antigen exposure favors their clonal expansion and accumulation of mutations, which further facilitate their activation. As a result, MB-like clonal precursors become trapped in an oscillatory state of semipermanent activation and phenotypic sway that facilitates ulterior transformation and accounts for the extranodal clinical presentation and biology of these tumors. In addition, we discuss diagnostic and therapeutic implications of a MB cell-of-origin for these lymphomas.
In a Perspective, Venturutti and Melnick propose a novel origin for activated B-cell–type diffuse large B-cell lymphomas (ABC-DLBCLs). They present evidence that ABC-DLBCLs arise from aberrant memory B cells that carry founder mutations that block plasmacytic maturation and, under the influence of recurrent antigen exposure, undergo cyclical reactivation that facilitates both extranodal localization and progressive transformation.
We present a highly sensitive and selective chemical labeling and capture approach for genome-wide profiling of 5-hydroxylmethylcytosine (5hmC) using DNA isolated from ∼1,000 cells (nano-hmC-Seal). ...Using this technology, we assessed 5hmC occupancy and dynamics across different stages of hematopoietic differentiation. Nano-hmC-Seal profiling of purified Tet2-mutant acute myeloid leukemia (AML) murine stem cells allowed us to identify leukemia-specific, differentially hydroxymethylated regions that harbor known and candidate disease-specific target genes with differential 5hmC peaks compared to normal stem cells. The change of 5hmC patterns in AML strongly correlates with differential gene expression, demonstrating the importance of dynamic alterations of 5hmC in regulating transcription in AML. Together, covalent 5hmC labeling offers an effective approach to study and detect DNA methylation dynamics in in vivo disease models and in limited clinical samples.
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•Chemical labeling enables highly sensitive profiling of 5hmC using limited input DNA•Genome-wide 5hmC distributions obtained for hematopoietic stem cell differentiation•5hmC changes correlate with differential gene expression in an AML model•Redistributions of 5hmC in the Tet2-deficient leukemia stem cells
Han et al. developed a highly sensitive and robust 5hmC sequencing approach using genomic DNA isolated from ∼1,000 cells. Using their approach, 5hmC maps across different stages of mouse hematopoietic differentiation were obtained and insights into roles of 5hmC in tumor-initiating cells in an AML mouse model were gained.
Since the publication of the Revised European-American Classification of mature lymphoid neoplasms in 1994, subsequent updates of the classification of mature lymphoid neoplasms have been generated ...through iterative international efforts to achieve broad consensus among hematopathologists, geneticists, molecular scientists, and clinicians. Significant progress in the characterization of malignancies of the immune system in the last years, with many new insights provided by genomic studies, have led to the current proposal. We have followed the same process that was successfully used for the 3rd and 4th editions of the WHO classification of hematological neoplasms. The definition, recommended studies, and criteria for the diagnosis of many entities have been extensively refined. Some categories considered provisional are now upgraded to definite entities. Terminology of some diseases has been revised to adapt nomenclature to the current knowledge of their biology, but these modifications have been restricted to well-justified situations. Major findings from recent genomic studies have impacted the conceptual framework and diagnostic criteria for many disease entities. These changes will have an impact on optimal clinical management. The conclusions of this work are summarized in this report as the proposed International Consensus Classification (ICC) of mature lymphoid, histiocytic, and dendritic cell tumors.
To understand the health impact of long-duration spaceflight, one identical twin astronaut was monitored before, during, and after a 1-year mission onboard the International Space Station; his twin ...served as a genetically matched ground control. Longitudinal assessments identified spaceflight-specific changes, including decreased body mass, telomere elongation, genome instability, carotid artery distension and increased intima-media thickness, altered ocular structure, transcriptional and metabolic changes, DNA methylation changes in immune and oxidative stress-related pathways, gastrointestinal microbiota alterations, and some cognitive decline postflight. Although average telomere length, global gene expression, and microbiome changes returned to near preflight levels within 6 months after return to Earth, increased numbers of short telomeres were observed and expression of some genes was still disrupted. These multiomic, molecular, physiological, and behavioral datasets provide a valuable roadmap of the putative health risks for future human spaceflight.
During the humoral immune response, B cells undergo a dramatic change in phenotype to enable antibody affinity maturation in germinal centers (GCs). Using genome-wide chromosomal conformation capture ...(Hi-C), we found that GC B cells undergo massive reorganization of the genomic architecture that encodes the GC B cell transcriptome. Coordinate expression of genes that specify the GC B cell phenotype—most prominently BCL6—was achieved through a multilayered chromatin reorganization process involving (1) increased promoter connectivity, (2) formation of enhancer networks, (3) 5′ to 3′ gene looping, and (4) merging of gene neighborhoods that share active epigenetic marks. BCL6 was an anchor point for the formation of GC-specific gene and enhancer loops on chromosome 3. Deletion of a GC-specific, highly interactive locus control region upstream of Bcl6 abrogated GC formation in mice. Thus, large-scale and multi-tiered genomic three-dimensional reorganization is required for coordinate expression of phenotype-driving gene sets that determine the unique characteristics of GC B cells.
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•GC B cells feature massive unpacking of the arms of long chromosomes•The GC transcriptome is coordinated through GC-specific topologic reorganization•The BCL6 locus is a major hotspot for multilayered topological shifts in GC B cells•A GC B cell de novo highly interactive LCR specifies the GC phenotype
The formation of germinal center B cells is crucial to the production of high-affinity antibodies during the humoral immune response. Here, Bunting et al. provide evidence for multilayered reorganization of genomic architecture centered around BCL6 and an upstream locus control region, coordinating the transcriptional reprograming during the transition between naive and germinal center B cells.
somatic mutations occur in ∼10% of diffuse large B-cell lymphomas (DLBCL) but are of unknown significance. Herein, we show that TET2 is required for the humoral immune response and is a DLBCL tumor ...suppressor. TET2 loss of function disrupts transit of B cells through germinal centers (GC), causing GC hyperplasia, impaired class switch recombination, blockade of plasma cell differentiation, and a preneoplastic phenotype. TET2 loss was linked to focal loss of enhancer hydroxymethylation and transcriptional repression of genes that mediate GC exit, such as PRDM1. Notably, these enhancers and genes are also repressed in
-mutant DLBCLs. Accordingly,
mutation in patients yields a
-mutant gene-expression signature,
and
mutations are generally mutually exclusive, and hydroxymethylation loss caused by TET2 deficiency impairs enhancer H3K27 acetylation. Hence, TET2 plays a critical role in the GC reaction, and its loss of function results in lymphomagenesis through failure to activate genes linked to GC exit signals. SIGNIFICANCE: We show that TET2 is required for exit of the GC, B-cell differentiation, and is a tumor suppressor for mature B cells. Loss of TET2 phenocopies
somatic mutation. These results advocate for sequencing
in patients with lymphoma and for the testing of epigenetic therapies to treat these tumors.
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BCL6 was initially discovered as an oncogene in B-cell lymphomas, where it drives the malignant phenotype by repressing proliferation and DNA damage checkpoints and blocking B-cell terminal ...differentiation. BCL6 mediates its effects by binding to hundreds of target genes and then repressing these genes by recruiting several different chromatin-modifying corepressor complexes. Structural characterization of BCL6-corepressor complexes suggested that BCL6 might be a druggable target. Accordingly, a number of compounds have been designed to bind to BCL6 and block corepressor recruitment. These compounds, based on peptide or small-molecule scaffolds, can potently block BCL6 repression of target genes and kill lymphoma cells. In the case of diffuse large B-cell lymphomas (DLBCL), BCL6 inhibitors are equally effective in suppressing both the germinal center B-cell (GCB)- and the more aggressive activated B-cell (ABC)-DLBCL subtypes, both of which require BCL6 to maintain their survival. In addition, BCL6 is implicated in an expanding scope of hematologic and solid tumors. These include, but are not limited to, B-acute lymphoblastic leukemia, chronic myeloid leukemia, breast cancer, and non-small cell lung cancer. BCL6 inhibitors have been shown to exert potent effects against these tumor types. Moreover, mechanism-based combinations of BCL6 inhibitors with other agents have yielded synergistic and often quite dramatic activity. Hence, there is a compelling case to accelerate the development of BCL6-targeted therapies for translation to the clinical setting.
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PURPOSE OF REVIEWPerturbation of the epigenome is emerging as a central driving force in the pathogenesis of diffuse large B-cell lymphomas (DLBCL) and follicular lymphoma. The purpose of this review ...is to explain how alteration of different layers of the epigenome contributes to the biology and clinical features of these tumors.
RECENT FINDINGSKey new findings implicate DNA methylation heterogeneity as a core feature of DLBCL. Epigenetic diversity is linked to unfavorable clinical outcomes, clonal selection at relapse, and is driven at least in part because of the actions of activation-induced cytosine deaminase, which is a unique feature of B-cell lymphomas. Somatic mutations in histone modifier genes drive lymphomagenesis through the establishment of aberrant gene-specific histone modification signatures. For example, EZH2 somatic mutations drive silencing of bivalent gene promoters through histone 3 lysine 27 trimethylation, whereas KMT2D (MLL2) mutations disrupt specific sets of enhancers through depletion of histone 3 lysine 4 mono and dimethylation (H3K4me1/me2).
SUMMARYAppreciation of the epigenome in determining lymphoma clonal heterogeneity and in driving lymphoma phenotypes through altered promoter and enhancer histone modification profiles is leading to a paradigm shift in how we understand and design therapies for DLBCL and follicular lymphoma.
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