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Inducible knock-out of BCL6 in lymphoma cells results in tumor stasis

ABSTRACT: Diffuse large B-cell lymphoma (DLBCL) is the most common type of non-Hodgkin lymphomas worldwide and is characterized by a high diversity of genetic and molecular alterations. Chromosomal translocations and mutations leading to deregulated expression of the transcriptional repressor BCL6 occur in a significant fraction of DLBCL patients. An oncogenic role of BCL6 in the initiation of DLBCL has been shown as the constitutive expression of BCL6 in mice recapitulates the pathogenesis of human DLBCL. However, the role of BCL6 in tumor maintenance remains poorly investigated due to the absence of suitable genetic models and limitations of pharmacological inhibitors. Here, we have utilized tetracycline-inducible CRISPR/Cas9 mutagenesis to study the consequences of BCL6 deletion in established DLBCL models in culture and in vivo. We show that BCL6 knock-out in SU-DHL-4 cells in vitro results in an anti-proliferative response 4-7 days after Cas9 induction that was characterized by cell cycle (G1) arrest. Conditional BCL6 deletion in established DLBCL tumors in vivo induced a significant tumor growth inhibition with initial tumor stasis followed by slow tumor growth kinetics. Our findings support a role of BCL6 in the maintenance of lymphoma growth and showcase the utility of inducible CRISPR/Cas9 systems for probing oncogene addiction.

ORGANISM(S): Homo sapiens

PROVIDER: GSE127266 | GEO | 2020/03/23

REPOSITORIES: GEO

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Project description:We studied transcriptional changes by Affymetrix human microarrays in DLBCL cell lines as a result of treatment with GSK126, a potent, highly-selective, SAM-competitive, small molecule inhibitor of EZH2 In eukaryotes, epigenetic post-translational modification of histones is critical for regulation of chromatin structure and gene expression. EZH2 is the catalytic subunit of the Polycomb Repressive Complex 2 (PRC2) and is responsible for repressing target gene expression through methylation of histone H3 on lysine 27 (H3K27). Over-expression of EZH2 is implicated in tumorigenesis and correlates with poor prognosis in multiple tumor types. Recent reports have identified somatic heterozygous mutations of Y641 and A677 residues within the catalytic SET domain of EZH2 in diffuse large B-cell lymphoma (DLBCL) and follicular lymphoma (FL). The Y641 residue is the most frequently mutated residue, with 22% of GCB (Germinal Cell B-cell) DLBCL and FL harboring mutations at this site. These lymphomas exhibit increased H3K27 tri-methylation (H3K27me3) due to altered substrate preferences of the mutant enzymes. However, it is unknown whether direct inhibition of EZH2 methyltransferase activity alone will be effective in treating lymphomas carrying activating EZH2 mutations. Herein, we demonstrate that GSK126, a potent, highly-selective, SAM-competitive, small molecule inhibitor of EZH2 methyltransferase activity, decreases global H3K27me3 levels and reactivates silenced PRC2 target genes. GSK126 effectively inhibits the proliferation of EZH2 mutant DLBCL cell lines and dramatically inhibits the growth of EZH2 mutant DLBCL xenografts in mice. Together, these data demonstrate that pharmacological inhibition of EZH2 activity may provide a promising treatment for EZH2 mutant lymphoma. 10 DLBCL cell lines (7 mutant and 3 wild type EZH2), that were differentially sensitive to GSK126 in proliferation assays, were treated for 72 hours, in duplicate (n=2), with either DMSO (vehicle) or 500nM of GSK126, a potent selective EZH2 inhibitor. EZH2 mutant cell lines are Pfeiffer, KARPAS-422, WSU-DLCL2, SU-DHL-10, SU-DHL-6, DB and SU-DHL-4. EZH2 wildtype cell lines are HT, OCI-LY-19 and Toledo.

Project description:MYC translocations are the biologic hallmark of Burkitt lymphomas but also occur in other mature B-cell lymphomas. If accompanied by chromosomal breaks targeting the BCL2 and/or BCL6 oncogenes, these MYC translocation-positive (MYC+) lymphomas are called double-hit lymphomas (DHLs); otherwise, the term single-hit lymphoma (SHL) is applied. In order to characterize the biologic features of these MYC+ lymphomas other than Burkitt lymphomas, we explored, after exclusion of molecular Burkitt lymphoma (mBL) as defined by gene expression profiling (GEP), the molecular, pathological and clinical aspects of 80 MYC translocation (MYC+) lymphomas (31 SHL, 26 BCL2+/MYC+, 14 BCL6+/MYC+, 6 BCL2+/BCL6+/MYC+ and 3 MYC+ lymphomas with unknown BCL6 status). Comparison of SHL and DHL revealed no difference in frequency of MYC partner (IG/non-IG), genomic complexity or MYC expression and no differences in GEP. DHL showed a more frequent GCB-like GEP and higher IGH and MYC mutation rates. GEP revealed 130 differentially expressed genes between BCL6+/MYC+ and BCL2+/MYC+ DHL. BCL2+/MYC+ DHL showed a more frequent GCB-like GEP. Analysis of all lymphomas according to MYC partner (IG/non-IG) revealed no substantial differences. In contrast to mBL and lymphomas without MYC break, SHL and DHL patients had similar poor outcome. Our data suggest that after excluding mBL, MYC+ lymphomas could be biologically widely lumped without further need for subclassification. 32 diffuse large B-cell lymphoma samples were hybridized to HG-U133A Affymetrix GeneChips. In addition, this study contains 30 already published samples, which contribute to GSE4475 (Hummel et al. 2006 (PMID 16760442)), as well as 18 already published samples from GSE22470 (Salaverria et al. 2011 (PMID 21487109)). No re-normalisation of the published samples was performed. The complete dataset representing: (1) the 32 diffuse large B-cell lymphoma Samples, (2) the 30 Samples from GSE4475 and (3) the 18 Samples from GSE22470, is linked below as a supplementary file

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Inducible knock-out of BCL6 in lymphoma cells results in tumor stasis

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