Epigenetic Silencing Mechanisms In The Humoral Immune Response And Lymphomagenesis

A hallmark of diffuse large B cell lymphoma (DLBCL) pathogenesis is the perturbation of epigenetic mechanisms. Germinal center (GC) B cells, from which DLBCL originates, are characterized by a specialized phenotype enabling rapid proliferation, sustained replicative potential, and tolerance to DNA damage. This GC phenotype facilitates affinity maturation by supporting clonal expansion with concurrent mutations and rearrangements of the B cell receptor gene. We found that cytosine methylation patterns in GC B cells involve relative loss of methylation and acquisition of methylation heterogeneity. We found these effects to be largely mediated by AICDA, the enzyme responsible for somatic hypermutation of immunoglobulin loci. In DLBCL, expression of AICDA leads to increased epigenetic heterogeneity, a feature linked with poor clinical outcome. The GC phenotype is also mediated in part by histone modifications. We found that the histone methyltransferase EZH2 is required for GC formation and promotes the GC phenotype by silencing proliferation checkpoint and differentiation genes via repressive H3K27me3 modification of their promoters. Notably, we found that key regulatory loci implicated in GC exit are modified by EZH2 to establish GC-specific bivalent chromatin domains. We found that repression of these GC-specific bivalent domains requires cooperation of EZH2 with the BCL6 transcriptional repressor and a noncanonical PRC1 complex. Somatic mutations enhancing the activity of EZH2 or BCL6 can “lock in” certain oncogenic features of GC B cells, resulting in malignant transformation

The BCL6 RD2 Domain Governs Commitment of Activated B Cells to Form Germinal Centers

To understand how the Bcl6 transcriptional repressor functions in the immune system, we disrupted its RD2 repression domain in mice. Bcl6RD2MUT mice exhibit a complete loss of germinal center (GC) formation but retain normal extrafollicular responses. Bcl6RD2MUT antigen-engaged B cells migrate to the interfollicular zone and interact with cognate T helper cells. However, these cells fail to complete early GC-commitment differentiation and coalesce as nascent GC aggregates. Bcl6 directly binds and represses trafficking receptors S1pr1 and Gpr183 by recruiting Hdac2 through the RD2 domain. Deregulation of these genes impairs B cell migration and may contribute to GC failure in Bcl6RD2MUT mice. The development of functional GC-TFH cells was partially impaired in Bcl6RD2MUT mice. In contrast to Bcl6−/− mice, Bcl6RD2MUT animals experience no inflammatory disease or macrophage deregulation. These results reveal an essential role for RD2 repression in early GC commitment and striking biochemical specificity in Bcl6 control of humoral and innate immune-cell phenotypes

DNA Methylation Dynamics of Germinal Center B Cells Are Mediated by AID

Changes in DNA methylation are required for the formation of germinal centers (GCs), but the mechanisms of such changes are poorly understood. Activation-induced cytidine deaminase (AID) has been recently implicated in DNA demethylation through its deaminase activity coupled with DNA repair. We investigated the epigenetic function of AID in vivo in germinal center B cells (GCBs) isolated from wild-type (WT) and AID-deficient (Aicda−/−) mice. We determined that the transit of B cells through the GC is associated with marked locus-specific loss of methylation and increased methylation diversity, both of which are lost in Aicda−/− animals. Differentially methylated cytosines (DMCs) between GCBs and naive B cells (NBs) are enriched in genes that are targeted for somatic hypermutation (SHM) by AID, and these genes form networks required for B cell development and proliferation. Finally, we observed significant conservation of AID-dependent epigenetic reprogramming between mouse and human B cells

CTCF Haploinsufficiency Destabilizes DNA Methylation and Predisposes to Cancer

Epigenetic alterations, particularly in DNA methylation, are ubiquitous in cancer, yet the molecular origins and the consequences of these alterations are poorly understood. CTCF, a DNA-binding protein that regulates higher-order chromatin organization, is frequently altered by hemizygous deletion or mutation in human cancer. To date, a causal role for CTCF in cancer has not been established. Here, we show that Ctcf hemizygous knockout mice are markedly susceptible to spontaneous, radiation-, and chemically induced cancer in a broad range of tissues. Ctcf+/− tumors are characterized by increased aggressiveness, including invasion, metastatic dissemination, and mixed epithelial/mesenchymal differentiation. Molecular analysis of Ctcf+/− tumors indicates that Ctcf is haploinsufficient for tumor suppression. Tissues with hemizygous loss of CTCF exhibit increased variability in CpG methylation genome wide. These findings establish CTCF as a prominent tumor-suppressor gene and point to CTCF-mediated epigenetic stability as a major barrier to neoplastic progression

Mutant EZH2 Induces a Pre-malignant Lymphoma Niche by Reprogramming the Immune Response

International audienceFollicular lymphomas (FLs) are slow-growing, indolent tumors containing extensive follicular dendritic cell (FDC) networks and recurrent EZH2 gain-of-function mutations. Paradoxically, FLs originate from highly proliferative germinal center (GC) B cells with proliferation strictly dependent on interactions with T follicular helper cells. Herein, we show that EZH2 mutations initiate FL by attenuating GC B cell requirement for T cell help and driving slow expansion of GC centrocytes that become enmeshed with and dependent on FDCs. By impairing T cell help, mutant EZH2 prevents induction of proliferative MYC programs. Thus, EZH2 mutation fosters malignant transformation by epigenetically reprograming B cells to form an aberrant immunological niche that reflects characteristic features of human FLs, explaining how indolent tumors arise from GC B cells