EZH2-mediated epigenetic silencing in germinal center B cells contributes to proliferation and lymphomagenesis

Abstract

EZH2 is the catalytic subunit of the PRC2 Polycomb complex and mediates transcriptional repression through its histone methyltransferase activity. EZH2 is upregulated in normal germinal center (GC) B cells and is implicated in lymphomagenesis. To explore the transcriptional programs controlled by EZH2, we performed chromatin immunoprecipitation (ChIP-onchip) in GC cells and found that it binds approximately 1800 promoters, often associated with DNA sequences similar to Droso-phila Polycomb response elements. While EZH2 targets overlapped extensively between GC B cells and embryonic stem cells, we also observed a large GC-specific EZH2 regulatory program. These genes are preferentially histone 3 lysine 27-trimethylated and repressed in GC B cells and include several key cell cycle-related tumor suppressor genes. Accordingly, siRNA-mediated down-regulation of EZH2 in diffuse large B-cell lymphoma ( IntroductionPolycomb proteins (PcG) are chromatin regulators with a crucial role in establishing and maintaining epigenetic memory during development and cellular differentiation. PcG is organized into 2 main sets of protein complexes: PRC1 and PRC2. EZH2 is a subunit of PRC2, 1 and its SET domain catalyzes trimethylation of H3K27, 1-3 a histone modification associated with transcriptional silencing. H3K27me3 helps recruit PRC1 to chromatin; it is thought that PRC1 is the effector of PcG-mediated silencing and long-term epigenetic memory. [4][5][6] It has been observed that H3K27me3 and DNA methylation, a distinct epigenetic mark, are associated with different sets of genes in murine and human embryonic stem cells (hESCs); 7,8 moreover DNA methylation and H3K27me3 are mutually exclusive at the imprinted Rasgrf1 locus. 9 However, this pattern of mutual exclusion between the 2 epigenetic mechanisms appears to be disrupted in cancer cells, where many hypermethylated promoters have been shown to also be H3K27-trimethylated. 10 From a functional point of view, mice deficient in PcG complexes show developmental abnormalities and embryonic lethality. 11 Within the B-cell lineage, it was shown that EZH2 is highly expressed in lymphoid progenitors, and EZH2 deficiency induces defects in early lymphopoiesis. 12 EZH2 declines in resting B cells but is then massively up-regulated when activated B cells form germinal centers (GCs), wherein they undergo rapid proliferation and immunoglobulin affinity maturation. 13 The latter observations suggest an important role for EZH2 in GC B-cell proliferation and a possible contribution to diffuse large B-cell lymphomas (DLBCLs), which are derived from GC B cells. The potential importance of EZH2 in lymphomagenesis is further supported by the discovery of a missense mutation in the EZH2 SET domain in a sizeable fraction of DLBCLs, especially those featuring the GC B-cell gene expression signature. 14 More generally, EZH2 is overexpressed in several other types of cancer (eg, in metastatic prostate cancer, 15 breast cancer, 16 and mantle cell lymphoma 17 ).The mechanisms by which EZH2-mediated...

EZH2 Y641 mutations in follicular lymphoma

“…Addition of these factors in clonogenic assays may enhance survival of human CD34 þ hematopoietic stem progenitor cells and increase size of hematopoietic colonies, in particular, in erythroid lineage. 2 However, Ins factors as reported may stimulate proliferation of leukemic blasts.…”

“…The PRC2/EZH2 complex binds to a specific set of target genes in germinal centre (GC) centroblasts, suggesting a GC-specific EZH2 regulatory programme in normal and malignant B cells. 2 Indeed, DNA-methylation profiling studies in FL by our group and others have shown that PRC2 gene targets in stem cells are significantly overrepresented among the hypermethylated genes in these tumours, linking these two epigenetic mechanisms in FL. 3 The spectrum of cancers with EZH2 mutation is also increasing, with mutations in myeloid disorders leading to reduced or complete loss of H3K27me3 expression consistent with a tumour suppressor role for EZH2.…”

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Histone Arginine Methylation by PRMT7 Controls Germinal Center Formation via Regulating Bcl6 Transcription

“…Recently, two epigenetic regulators (EZH2 and MOZ) involved in the histone modification were shown to play roles in the epigenetic changes involved in the formation of GCs (10)(11)(12)(13). Therefore, additional histone codes encoded by various epigenetic factors involved in the process are worth defining.…”

A double blinded, placebo-controlled pilot study to examine reduction of CD34+/CD117+/CD133+ lymphoma progenitor cells and duration of remission induced by neoadjuvant valspodar in dogs with large B-cell lymphoma

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