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XLSX file - 132K, Table S2: Effects of EPZ-6438 on Gene Expression in EZH2 Mutant Lymphoma Cells.

The SWI/SNF complex is a major regulator of gene expression and is increasingly thought to play an important role in human cancer, as evidenced by the high frequency of subunit mutations across virtually all cancer types. We previously reported that in preclinical models, malignant rhabdoid tumors, which are deficient in the SWI/SNF core component INI1 (SMARCB1), are selectively killed by inhibitors of the H3K27 histone methyltransferase EZH2. Given the demonstrated antagonistic activities of the SWI/SNF complex and the EZH2-containing PRC2 complex, we investigated whether additional cancers with SWI/SNF mutations are sensitive to selective EZH2 inhibition. It has been recently reported that ovarian cancers with dual loss of the redundant SWI/SNF components SMARCA4 and SMARCA2 are characteristic of a rare rhabdoid-like subtype known as small-cell carcinoma of the ovary hypercalcemic type (SCCOHT). Here, we provide evidence that a subset of commonly used ovarian carcinoma cell lines were misdiagnosed and instead were derived from a SCCOHT tumor. We also demonstrate that tazemetostat, a potent and selective EZH2 inhibitor currently in phase II clinical trials, induces potent antiproliferative and antitumor effects in SCCOHT cell lines and xenografts deficient in both SMARCA2 and SMARCA4. These results exemplify an additional class of rhabdoid-like tumors that are dependent on EZH2 activity for survival. Mol Cancer Ther; 16(5); 850–60. ©2017 AACR.

The EZH2 small-molecule inhibitor tazemetostat (EPZ-6438) is currently being evaluated in phase II clinical trials for the treatment of non-Hodgkin lymphoma (NHL). We have previously shown that EZH2 inhibitors display an antiproliferative effect in multiple preclinical models of NHL, and that models bearing gain-of-function mutations in EZH2 were consistently more sensitive to EZH2 inhibition than lymphomas with wild-type (WT) EZH2. Here, we demonstrate that cell lines bearing EZH2 mutations show a cytotoxic response, while cell lines with WT-EZH2 show a cytostatic response and only tumor growth inhibition without regression in a xenograft model. Previous work has demonstrated that cotreatment with tazemetostat and glucocorticoid receptor agonists lead to a synergistic antiproliferative effect in both mutant and wild-type backgrounds, which may provide clues to the mechanism of action of EZH2 inhibition in WT-EZH2 models. Multiple agents that inhibit the B-cell receptor pathway (e.g., ibrutinib) were found to have synergistic benefit when combined with tazemetostat in both mutant and WT-EZH2 backgrounds of diffuse large B-cell lymphomas (DLBCL). The relationship between B-cell activation and EZH2 inhibition is consistent with the proposed role of EZH2 in B-cell maturation. To further support this, we observe that cell lines treated with tazemetostat show an increase in the B-cell maturation regulator, PRDM1/BLIMP1, and gene signatures corresponding to more advanced stages of maturation. These findings suggest that EZH2 inhibition in both mutant and wild-type backgrounds leads to increased B-cell maturation and a greater dependence on B-cell activation signaling. Mol Cancer Ther; 16(11); 2586–97. ©2017 AACR.

PDF file - 811K, Fig. S1. Effects of EPZ-6438 on Gene Promoter H3K27 Methylation, Recruitment of PRC2 Components to Gene Promoters, and Cell Cycle, in WSU-DLCL2 Cells. Fig. S2: Effects of EPZ-6438 on Gene Expression in EZH2 Mutant Lymphoma Cell Lines. Fig. S3: Pharmacokinetic Profiles of EPZ-6438 in Rats and Mice. Fig. S4: EPZ-6438 Compound Levels in Plasma and WSU-DLCL2 Xenograft Tumor Homogenates from Mice Dosed for 7 or 28 Days. Fig. S5: In vivo Effects of EPZ-6438 in Lymphoma Xenograft Models. Fig. S6: Body Weights of Mice during Xenograft Efficacy Studies.

PDF file - 25K, Table S1: LCC Values for EPZ-6438 for WSU-DLCL2 Human Lymphoma Cells Dosed Either Continuously or After Compound Washout.

Table S1: Conditions and seeding densities of all cell lines used in studies; Table S2. Single agent IC50 values for agents tested for 3 days in 96-well plates as described in Materials and Methods; Figure S1. Tazemetostat dosing reduced H3K27me3 in tumor xenografts and is tolerated by SCID mice; Figure S2: CD40L has little effect on growth of DLBCL cell lines; Figure S3. SU-DHL-5 cells were treated for 4 days with TAZ (0.1 to 1 µM) followed by addition of 500 ng/mL CD40L at the indicated time points (1-60 minutes; Figure S4. An ABC-DLBCL gene signature (1) is up-regulated following tazemetostat treatment in KARPAS-422 (EZH2 mutant GCB), Farage (EZH2 WT GCB), SU-DHL-5 (EZH2 WT GCB), TMD8 (EZH2 WT ABC) and ABC vs. GCB gene signature was applied to RNAseq data from each cell line with and without tazemetostat treatment.; Figure S5: CD40L, but not IL-21 induces PRDM1 in combination with tazemetostat (TAZ); Figure S6. A CD40 responsive gene set (2) is up-regulated following tazemetostat treatment in KARPAS-422 (EZH2 mutant GCB), Farage (EZH2 WT GCB), SU-DHL-5 (EZH2 WT GCB), TMD8 (EZH2 WT ABC) and Basso_CD40¬_SIGNALING_UP gene signature was applied to RNAseq data from each cell line with and without tazemetostat treatment; Figure S7. Model for tazametostat mechanism of action in DLBCL

PDF File -1 MB, Supplemental data is to further support the statements made it the main text. Captions: Supplemental Table 1: Cell Lines used in the CRISPR pooled screen. Supplemental Table 2: Table of ovarian lines screened for tazemetostat sensitivity along with mutational and protein status for SMARCA2, SMARCA4, and ARID1A. Supplemental Table 3: p-values of COV434 SCCOHT cell line treated with tazemetostat. Supplemental Table 4: p-values of JHOC-5 clear cell ovarian cell line treated with tazemetostat. Supplemental Table 5: p-values of cell cycle analysis after tazemetostat treatment. Supplemental Table 6: p-values of apoptosis analysis after tazemetostat treatment. Supplemental Table 7: LogP scores for SWI/SNF components in ovarian lines +/- tazemetostat treatment. Supplemental Figure 1: Baseline measurement of histone 3 lysine 27 trimethyl mark and EZH2. Supplemental Figure 2a: Growth curves of SCCOHT and ovarian cancer cell lines treated with tazemetostat. Supplemental Figure 2b: Growth curves of SCCOHT and ovarian cancer cell lines treated with EPZ-007210. Supplemental Figure 3: Reduction in H3K27me3 mark after treatment. Supplemental Figure 4a: Sensitivity to PLK1 knockout as pan-essential in CRISPR pooled screen. Supplemental Figure 4b: Control sgRNAs from CRISPR pooled screen. Supplemental Figure 5: TOV112D xenograft tumor growth inhibition, tumor volume, and methyl mark. Supplemental Figure 6: body weights of mice treated with tazemetostat. Supplemental Figure 7: measured blood plasma levels of tazemetostat from xenograft mice.

Mutations within the catalytic domain of the histone methyltransferase EZH2 have been identified in subsets of patients with non-Hodgkin lymphoma (NHL). These genetic alterations are hypothesized to confer an oncogenic dependency on EZH2 enzymatic activity in these cancers. We have previously reported the discovery of EPZ005678 and EPZ-6438, potent and selective S-adenosyl-methionine-competitive small molecule inhibitors of EZH2. Although both compounds are similar with respect to their mechanism of action and selectivity, EPZ-6438 possesses superior potency and drug-like properties, including good oral bioavailability in animals. Here, we characterize the activity of EPZ-6438 in preclinical models of NHL. EPZ-6438 selectively inhibits intracellular lysine 27 of histone H3 (H3K27) methylation in a concentration- and time-dependent manner in both EZH2 wild-type and mutant lymphoma cells. Inhibition of H3K27 trimethylation (H3K27Me3) leads to selective cell killing of human lymphoma cell lines bearing EZH2 catalytic domain point mutations. Treatment of EZH2-mutant NHL xenograft-bearing mice with EPZ-6438 causes dose-dependent tumor growth inhibition, including complete and sustained tumor regressions with correlative diminution of H3K27Me3 levels in tumors and selected normal tissues. Mice dosed orally with EPZ-6438 for 28 days remained tumor free for up to 63 days after stopping compound treatment in two EZH2-mutant xenograft models. These data confirm the dependency of EZH2-mutant NHL on EZH2 activity and portend the utility of EPZ-6438 as a potential treatment for these genetically defined cancers. Mol Cancer Ther; 13(4); 842–54. ©2014 AACR.

The purpose of the study was to determine if genetic and pharmacological inhibition of terminal uridyltransferases (TUTases) ZCCHC11 and ZCCHC6 could modulate Glioblastoma (GBM) cell growth in vitro. ZCCHC11 (also known as TUT4 and TENT3A) and ZCCHC6 (also known as TUT7 and TENT3B) catalyze uridylation of diverse RNA species. Recent evidence indicates that genetic perturbation of ZCCHC11/6 expression can disrupt cell proliferation of both immortalized and patient-derived primary GBM cell lines (Kim et al., Mol Cell 2020). We confirmed that genetic inhibition of ZCCHC11/6 with siRNA or CRISPR can decrease viability of U-87 MG and A-172 GBM cell lines, and we identified DK-MG as another GBM cell line with sensitivity to ZCCHC11/6 knockdown. Herein, we report the first novel, potent, and selective inhibitor of ZCCHC11/6, TS-1. Biochemically, TS-1 can block ZCCHC11/6-mediated uridylation of an RNA substrate in vitro (IC50 = 0.65 nM for recombinant ZCCHC11 protein; IC50 = 9.6 nM for recombinant ZCCHC6 protein). ZCCHC11/6 inhibitor TS-1 could decrease cell viability in U-87 MG, A-172 and DK-MG cells (cell viability IC50s ranging between 0.7-1.5 μM) and induce apoptosis and cell cycle arrest. A cellular assay detecting uridylation of miR-191 as a marker for ZCCHC11/6 activity was developed and validated. TS-1 strongly decreased uridylation of miR-191 in U-87 MG, A-172 and DK-MG cells (IC50 ranging between 10-40 nM). In contrast, TS-2, a weakly active enantiomer of TS-1 (enzyme IC50 = 0.15 μM and 1.8 μM against ZCCHC11 and ZCCHC6, respectively), had less impact on cell viability, apoptosis, cell cycle and uridylation in cells. In conclusion, we have verified ZCCHC11/6-dependency in a set of GBM cell lines and we have also developed a first-in class potent and selective small molecule that reduces in vitro Glioblastoma cell proliferation through selective inhibition of TUTases activity. Citation Format: Robinson Triboulet, Khikmet Sadykov, Jessica L. Johnson, Andrew R. Snyder, Sarah K. Knutson, Pavan Kumar, Christopher B. Mayo, Dillon Hawley, Andrew Madanjian, Ross L. Stein, David M. Wilson, Darren M. Harvey, Shomir Ghosh, Robert M. Campbell. Selective inhibition of ZCCHC11/ZCCHC6 TUTases with genetic and pharmacological tools supports a role in glioblastoma cell growth [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1822.

The EZH2 small-molecule inhibitor tazemetostat (EPZ-6438) is currently being evaluated in phase II clinical trials for the treatment of non-Hodgkin lymphoma (NHL). We have previously shown that EZH2 inhibitors display an antiproliferative effect in multiple preclinical models of NHL, and that models bearing gain-of-function mutations in EZH2 were consistently more sensitive to EZH2 inhibition than lymphomas with wild-type (WT) EZH2. Here, we demonstrate that cell lines bearing EZH2 mutations show a cytotoxic response, while cell lines with WT-EZH2 show a cytostatic response and only tumor growth inhibition without regression in a xenograft model. Previous work has demonstrated that cotreatment with tazemetostat and glucocorticoid receptor agonists lead to a synergistic antiproliferative effect in both mutant and wild-type backgrounds, which may provide clues to the mechanism of action of EZH2 inhibition in WT-EZH2 models. Multiple agents that inhibit the B-cell receptor pathway (e.g., ibrutinib) were found to have synergistic benefit when combined with tazemetostat in both mutant and WT-EZH2 backgrounds of diffuse large B-cell lymphomas (DLBCL). The relationship between B-cell activation and EZH2 inhibition is consistent with the proposed role of EZH2 in B-cell maturation. To further support this, we observe that cell lines treated with tazemetostat show an increase in the B-cell maturation regulator, PRDM1/BLIMP1, and gene signatures corresponding to more advanced stages of maturation. These findings suggest that EZH2 inhibition in both mutant and wild-type backgrounds leads to increased B-cell maturation and a greater dependence on B-cell activation signaling. Mol Cancer Ther; 16(11); 2586–97. ©2017 AACR.

The SWI/SNF complex is a major regulator of gene expression and is increasingly thought to play an important role in human cancer, as evidenced by the high frequency of subunit mutations across virtually all cancer types. We previously reported that in preclinical models, malignant rhabdoid tumors, which are deficient in the SWI/SNF core component INI1 (SMARCB1), are selectively killed by inhibitors of the H3K27 histone methyltransferase EZH2. Given the demonstrated antagonistic activities of the SWI/SNF complex and the EZH2-containing PRC2 complex, we investigated whether additional cancers with SWI/SNF mutations are sensitive to selective EZH2 inhibition. It has been recently reported that ovarian cancers with dual loss of the redundant SWI/SNF components SMARCA4 and SMARCA2 are characteristic of a rare rhabdoid-like subtype known as small-cell carcinoma of the ovary hypercalcemic type (SCCOHT). Here, we provide evidence that a subset of commonly used ovarian carcinoma cell lines were misdiagnosed and instead were derived from a SCCOHT tumor. We also demonstrate that tazemetostat, a potent and selective EZH2 inhibitor currently in phase II clinical trials, induces potent antiproliferative and antitumor effects in SCCOHT cell lines and xenografts deficient in both SMARCA2 and SMARCA4. These results exemplify an additional class of rhabdoid-like tumors that are dependent on EZH2 activity for survival. Mol Cancer Ther; 16(5); 850–60. ©2017 AACR.

Posttranslational methylation of histones plays a critical role in gene regulation. Misregulation of histone methylation can lead to oncogenic transformation. Enhancer of Zeste homologue 2 (EZH2) methylates histone 3 at lysine 27 (H3K27) and abnormal methylation of this site is found in many cancers. Tazemetostat, an EHZ2 inhibitor in clinical development, has shown activity in both preclinical models of cancer as well as in patients with lymphoma or INI1-deficient solid tumors. Herein we report the structure-activity relationships from identification of an initial hit in a high-throughput screen through selection of tazemetostat for clinical development. The importance of several methyl groups to the potency of the inhibitors is highlighted as well as the importance of balancing pharmacokinetic properties with potency.

Inhibitors of the protein methyltransferase Enhancer of Zeste Homolog 2 (EZH2) may have significant therapeutic potential for the treatment of B cell lymphomas and other cancer indications. The ability of the scientific community to explore fully the spectrum of EZH2-associated pathobiology has been hampered by the lack of in vivo-active tool compounds for this enzyme. Here we report the discovery and characterization of EPZ011989, a potent, selective, orally bioavailable inhibitor of EZH2 with useful pharmacokinetic properties. EPZ011989 demonstrates significant tumor growth inhibition in a mouse xenograft model of human B cell lymphoma. Hence, this compound represents a powerful tool for the expanded exploration of EZH2 activity in biology.

EZH2 and EZH1 are protein methyltransferases (PMTs) responsible for histone H3, lysine 27 (H3K27) methylation. Trimethylation of H3K27 (H3K27me3) is a hallmark of many cancers, including non-Hodgkin lymphoma (NHL). Heterozygous EZH2 point mutations at Tyr641, Ala677, and Ala687 have been observed in NHL. The Tyr641 mutations enhance activity on H3K27me2 but have weak or no activity on unmethylated H3K27, whereas the Ala677 and Ala687 mutations use substrates of all methylation states effectively. It has been proposed that enzymatic coupling of the wild-type and mutant enzymes leads to the oncogenic H3K27me3 mark in mutant-bearing NHL. We show that coupling with the wild-type enzyme is needed to achieve H3K27me3 for several mutants, but that others are capable of achieving H3K27me3 on their own. All forms of PRC2 (wild-type and mutants) display kinetic signatures that are consistent with a distributive mechanism of catalysis.

The SWI/SNF complex is a major regulator of gene expression and is increasingly thought to play an important role in human cancer, as evidenced by the high frequency of subunit mutations across virtually all cancer types. We previously reported that in preclinical models, malignant rhabdoid tumors, which are deficient in the SWI/SNF core component INI1 (SMARCB1), are selectively killed by inhibitors of the H3K27 histone methyltransferase EZH2. Given the demonstrated antagonistic activities of the SWI/SNF complex and the EZH2-containing PRC2 complex, we investigated whether additional cancers with SWI/SNF mutations are sensitive to selective EZH2 inhibition. It has been recently reported that ovarian cancers with dual loss of the redundant SWI/SNF components SMARCA4 and SMARCA2 are characteristic of a rare rhabdoid-like subtype known as small-cell carcinoma of the ovary hypercalcemic type (SCCOHT). Here, we provide evidence that a subset of commonly used ovarian carcinoma cell lines were misdiagnosed and instead were derived from a SCCOHT tumor. We also demonstrate that tazemetostat, a potent and selective EZH2 inhibitor currently in phase II clinical trials, induces potent antiproliferative and antitumor effects in SCCOHT cell lines and xenografts deficient in both SMARCA2 and SMARCA4. These results exemplify an additional class of rhabdoid-like tumors that are dependent on EZH2 activity for survival.

Reply to "Uveal melanoma cells are resistant to EZH2 inhibition regardless of BAP1 status"

EZH2 catalyzes trimethylation of histone H3 lysine 27 (H3K27). Point mutations of EZH2 at Tyr641 and Ala677 occur in subpopulations of non-Hodgkin's lymphoma, where they drive H3K27 hypertrimethylation. Here we report the discovery of EPZ005687, a potent inhibitor of EZH2 (K(i) of 24 nM). EPZ005687 has greater than 500-fold selectivity against 15 other protein methyltransferases and has 50-fold selectivity against the closely related enzyme EZH1. The compound reduces H3K27 methylation in various lymphoma cells; this translates into apoptotic cell killing in heterozygous Tyr641 or Ala677 mutant cells, with minimal effects on the proliferation of wild-type cells. These data suggest that genetic alteration of EZH2 (for example, mutations at Tyr641 or Ala677) results in a critical dependency on enzymatic activity for proliferation (that is, the equivalent of oncogene addiction), thus portending the clinical use of EZH2 inhibitors for cancers in which EZH2 is genetically altered.

EZH2, the catalytic subunit of the PRC2 complex, catalyzes the mono- through trimethylation of lysine 27 on histone H3 (H3K27). Histone H3K27 trimethylation is a mechanism for suppressing transcription of specific genes that are proximal to the site of histone modification. Point mutations of the EZH2 gene (Tyr641) have been reported to be linked to subsets of human B-cell lymphoma. The mutant allele is always found associated with a wild-type allele (heterozygous) in disease cells, and the mutations were reported to ablate the enzymatic activity of the PRC2 complex for methylating an unmodified peptide substrate. Here we demonstrate that the WT enzyme displays greatest catalytic efficiency ( k cat / K ) for the zero to monomethylation reaction of H3K27 and diminished efficiency for subsequent (mono- to di- and di- to trimethylation) reactions. In stark contrast, the disease-associated Y641 mutations display very limited ability to perform the first methylation reaction, but have enhanced catalytic efficiency for the subsequent reactions, relative to the WT enzyme. These results imply that the malignant phenotype of disease requires the combined activities of a H3K27 monomethylating enzyme (PRC2 containing WT EZH2 or EZH1) together with the mutant PRC2s for augmented conversion of H3K27 to the trimethylated form. To our knowledge, this is the first example of a human disease that is dependent on the coordinated activities of normal and disease-associated mutant enzymatic function.

SummaryHdac3 is essential for efficient DNA replication and DNA damage control. Deletion of Hdac3 impaired DNA repair and greatly reduced chromatin compaction and heterochromatin content. These defects corresponded to increases in histone H3K9,K14ac; H4K5ac; and H4K12ac in late S phase of the cell cycle, and histone deposition marks were retained in quiescent Hdac3-null cells. Liver-specific deletion of Hdac3 culminated in hepatocellular carcinoma. Whereas HDAC3 expression was downregulated in only a small number of human liver cancers, the mRNA levels of the HDAC3 cofactor NCOR1 were reduced in one-third of these cases. siRNA targeting of NCOR1 and SMRT (NCOR2) increased H4K5ac and caused DNA damage, indicating that the HDAC3/NCOR/SMRT axis is critical for maintaining chromatin structure and genomic stability.

Histone deacetylases (HDAC) are enzymes that modify key residues in histones to regulate chromatin architecture, and play a vital role in cell survival, cell cycle progression, and tumorigenesis. To understand the function of Hdac3, a critical component of the N-CoR/SMRT repression complex, a conditional allele of Hdac3 was engineered. Cre-recombinase-mediated inactivation of Hdac3 led to a delay in cell cycle progression, cell-cycle dependent DNA damage, and apoptosis in mouse embryonic fibroblasts (MEFs). While no overt defects in mitosis were observed in Hdac3−/− MEFs, including normal H3Ser10 phosphorylation, DNA damage was observed in Hdac3−/−interphase cells, which appears to be associated with defective DNA double strand break repair. Moreover, we noted that Hdac3−/− MEFs were protected from DNA damage when quiescent, which may provide a mechanistic basis for the action of histone deacetylase inhibitors on cycling tumor cells.

Histone deacetylase 3 (Hdac3) is an enzymatic component of transcriptional repression complexes recruited by the nuclear hormone receptors. Inactivation of Hdac3 in cancer cell lines triggered apoptosis, and removal of Hdac3 in the germ line of mice caused embryonic lethality. Therefore, we deleted Hdac3 in the postnatal mouse liver. These mice developed hepatomegaly, which was the result of hepatocyte hypertrophy, and these morphological changes coincided with significant imbalances between carbohydrate and lipid metabolism. Loss of Hdac3 triggered changes in gene expression consistent with inactivation of repression mediated by nuclear hormone receptors. Loss of Hdac3 also increased the levels of Ppar gamma2, and treatment of these mice with a Ppar gamma antagonist partially reversed the lipid accumulation in the liver. In addition, gene expression analysis identified mammalian target of rapamycin signalling as being activated after deletion of Hdac3, and inhibition by rapamycin affected the accumulation of neutral lipids in Hdac3-null livers. Thus, Hdac3 regulates metabolism through multiple signalling pathways in the liver, and deletion of Hdac3 disrupts normal metabolic homeostasis.

The catalytic activities of covalent and ATP-dependent chromatin remodeling are central to regulating the conformational state of chromatin and the resultant transcriptional output. The enzymes that catalyze these activities are often contained within multiprotein complexes in nature. Two such multiprotein complexes, the polycomb repressive complex 2 (PRC2) methyltransferase and the SWItch/Sucrose Non-Fermentable (SWI/SNF) chromatin remodeler have been reported to act in opposition to each other during development and homeostasis. An imbalance in their activities induced by mutations/deletions in complex members (e.g. SMARCB1) has been suggested to be a pathogenic mechanism in certain human cancers. Here we show that preclinical models of synovial sarcoma—a cancer characterized by functional SMARCB1 loss via its displacement from the SWI/SNF complex through the pathognomonic SS18-SSX fusion protein—display sensitivity to pharmacologic inhibition of EZH2, the catalytic subunit of PRC2. Treatment with tazemetostat, a clinical-stage, selective and orally bioavailable small-molecule inhibitor of EZH2 enzymatic activity reverses a subset of synovial sarcoma gene expression and results in concentration-dependent cell growth inhibition and cell death specifically in SS18-SSX fusion-positive cells in vitro. Treatment of mice bearing either a cell line or two patient-derived xenograft models of synovial sarcoma leads to dose-dependent tumor growth inhibition with correlative inhibition of trimethylation levels of the EZH2-specific substrate, lysine 27 on histone H3. These data demonstrate a dependency of SS18-SSX-positive, SMARCB1-deficient synovial sarcomas on EZH2 enzymatic activity and suggests the potential utility of EZH2-targeted drugs in these genetically defined cancers.

The tumor suppressors BAP1 and ASXL1 interact to form a polycomb deubiquitinase complex that removes monoubiquitin from histone H2A lysine 119 (H2AK119Ub). However, BAP1 and ASXL1 are mutated in distinct cancer types, consistent with independent roles in regulating epigenetic state and malignant transformation. Here we demonstrate that Bap1 loss in mice results in increased trimethylated histone H3 lysine 27 (H3K27me3), elevated enhancer of zeste 2 polycomb repressive complex 2 subunit (Ezh2) expression, and enhanced repression of polycomb repressive complex 2 (PRC2) targets. These findings contrast with the reduction in H3K27me3 levels seen with Asxl1 loss. Conditional deletion of Bap1 and Ezh2 in vivo abrogates the myeloid progenitor expansion induced by Bap1 loss alone. Loss of BAP1 results in a marked decrease in H4K20 monomethylation (H4K20me1). Consistent with a role for H4K20me1 in the transcriptional regulation of EZH2, expression of SETD8-the H4K20me1 methyltransferase-reduces EZH2 expression and abrogates the proliferation of BAP1-mutant cells. Furthermore, mesothelioma cells that lack BAP1 are sensitive to EZH2 pharmacologic inhibition, suggesting a novel therapeutic approach for BAP1-mutant malignancies.

Mutations within the catalytic domain of the histone methyltransferase EZH2 have been identified in subsets of patients with non-Hodgkin lymphoma (NHL). These genetic alterations are hypothesized to confer an oncogenic dependency on EZH2 enzymatic activity in these cancers. We have previously reported the discovery of EPZ005678 and EPZ-6438, potent and selective S-adenosyl-methionine-competitive small molecule inhibitors of EZH2. Although both compounds are similar with respect to their mechanism of action and selectivity, EPZ-6438 possesses superior potency and drug-like properties, including good oral bioavailability in animals. Here, we characterize the activity of EPZ-6438 in preclinical models of NHL. EPZ-6438 selectively inhibits intracellular lysine 27 of histone H3 (H3K27) methylation in a concentration- and time-dependent manner in both EZH2 wild-type and mutant lymphoma cells. Inhibition of H3K27 trimethylation (H3K27Me3) leads to selective cell killing of human lymphoma cell lines bearing EZH2 catalytic domain point mutations. Treatment of EZH2-mutant NHL xenograft-bearing mice with EPZ-6438 causes dose-dependent tumor growth inhibition, including complete and sustained tumor regressions with correlative diminution of H3K27Me3 levels in tumors and selected normal tissues. Mice dosed orally with EPZ-6438 for 28 days remained tumor free for up to 63 days after stopping compound treatment in two EZH2-mutant xenograft models. These data confirm the dependency of EZH2-mutant NHL on EZH2 activity and portend the utility of EPZ-6438 as a potential treatment for these genetically defined cancers. Mol Cancer Ther; 13(4); 842–54. ©2014 AACR.

Inactivation of the switch/sucrose nonfermentable complex component SMARCB1 is extremely prevalent in pediatric malignant rhabdoid tumors (MRTs) or atypical teratoid rhabdoid tumors. This alteration is hypothesized to confer oncogenic dependency on EZH2 in these cancers. We report the discovery of a potent, selective, and orally bioavailable small-molecule inhibitor of EZH2 enzymatic activity, (N-((4,6-dimethyl-2-oxo-1,2-dihydropyridin-3-yl)methyl)-5-(ethyl(tetrahydro-2H-pyran-4-yl)amino)-4-methyl-4′-(morpholinomethyl)-[1,1′-biphenyl]-3-carboxamide). The compound induces apoptosis and differentiation specifically in SMARCB1 -deleted MRT cells. Treatment of xenograft-bearing mice with (N-((4,6-dimethyl-2-oxo-1,2-dihydropyridin-3-yl)methyl)-5-(ethyl(tetrahydro-2H-pyran-4-yl)amino)-4-methyl-4′-(morpholinomethyl)-[1,1′-biphenyl]-3-carboxamide) leads to dose-dependent regression of MRTs with correlative diminution of intratumoral trimethylation levels of lysine 27 on histone H3, and prevention of tumor regrowth after dosing cessation. These data demonstrate the dependency of SMARCB1 mutant MRTs on EZH2 enzymatic activity and portend the utility of EZH2-targeted drugs for the treatment of these genetically defined cancers.

Heterozygous point mutations at Y641 and A677 in the EZH2 SET domain are prevalent in about 10–24% of Non-Hodgkin lymphomas (NHL). Previous studies indicate that these are gain-of-function mutations leading to the hypertrimethylation of H3K27. These EZH2 mutations may drive the proliferation of lymphoma and make EZH2 a molecular target for patients harboring these mutations. Here, another EZH2 SET domain point mutation, A687V, occurring in about 1–2% of lymphoma patients, is also shown to be a gain-of-function mutation that greatly enhances its ability to perform dimethylation relative to wild-type EZH2 and is equally proficient at catalyzing trimethylation. We propose that A687V EZH2 also leads to hypertrimethylation of H3K27 and may thus be a driver mutation in NHL.

Histone deacetylase (Hdac)3 is a nuclear enzyme that contributes to epigenetic programming and is required for embryonic development. To determine the role of Hdac3 in bone formation, we crossed mice harboring loxP sites around exon 7 of Hdac3 with mice expressing Cre recombinase under the control of the osterix promoter. The resulting Hdac3 conditional knockout (CKO) mice were runted and had severe deficits in intramembranous and endochondral bone formation. Calvarial bones were significantly thinner and trabecular bone volume in the distal femur was decreased 75% in the Hdac3 CKO mice due to a substantial reduction in trabecular number. Hdac3-CKO mice had fewer osteoblasts and more bone marrow adipocytes as a proportion of tissue area than their wildtype or heterozygous littermates. Bone formation rates were depressed in both the cortical and trabecular regions of Hdac3 CKO femurs. Microarray analyses revealed that numerous developmental signaling pathways were affected by Hdac3-deficiency. Thus, Hdac3 depletion in osterix-expressing progenitor cells interferes with bone formation and promotes bone marrow adipocyte differentiation. These results demonstrate that Hdac3 inhibition is detrimental to skeletal health.

Introduction: Evaluating the effects of epigenetic modulators on the methylation state of histones in heterogeneous cell populations has, to this point, been limited to bulk assessment of cell lysates or purified histones. Hence, determining the differential effects of these compounds on individual cell types within a complex population has remained elusive. Methods: In order to monitor the pharmacodynamics of tazemetostat (EPZ-6438), an EZH2 inhibitor that has demonstrated clinical activity in multiple oncology indications, with greater sensitivity and specificity in phase 2 clinical trials and beyond, we have developed a chromatin flow cytometric assay that quantifies trimethylation of lysine 27 of histone H3 (H3K27me3) and total H3 levels while simultaneously allowing for immunophenotyping of discrete populations by surface marker composition in human and murine blood leukocytes. Results: Tazemetostat treatment of human lymphoma cell lines for 4 days resulted in dose-dependent reductions in H3K27me3 levels when quantified both by traditional histone western blotting and chromatin flow cytometry. Furthermore, both assays were able to discriminate between EZH2 mutant lymphoma cell lines which lack H3K27 dimethylation (H3K27me2) and wild-type lines in which the H3K27me2 is present and reduced in a dose responsive manner. To determine if the assay was capable of discriminating methylation states of specific cell populations found in whole blood from an in vivo model, we performed a 7-day study in mice treated with increasing dosages of tazemetostat (EPZ-6438). Quantification of H3K27me3 levels in blood leukocytes revealed differential responses to inhibitor treatment based on cell type. Monocytes and NK cells were found to be highly sensitive to tazemetostat, demonstrating 85% and 70% reduction in H3K27me3 respectively. B-cells were moderately sensitive to tazemetostat treatment, exhibiting a 21% reduction in H3K27me3. Granulocytes and T-cells were the least sensitive with a 7% and 3% reduction in H3K27me3 respectively. Conclusions: These results demonstrate that chromatin flow cytometry is an effective means to monitor cell type specific changes in methylation state upon compound treatment in vivo. Given the advantages of this methodology over conventional measures of histone methylation, chromatin flow cytometry will be used to monitor pharmacodynamics of H3K27me3 inhibition in whole blood leukocytes in the ongoing phase 2 and future clinical trials of tazemetostat. Citation Format: Christopher Plescia, Sarah Knutson, Natalie Warholic, Alice McDonald, Heike Keilhack, Jesse Smith, Robert Copeland, Stephen Blakemore. Chromatin flow cytometry based quantification of cell type specific alterations in histone methylation states resulting from in vitro and in vivo EZH2 inhibitor treatment. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr B133.

Introduction: The H3K27 histone methyltransferase EZH2 is the catalytic component of the polycomb repressive complex 2 (PRC2), and is amplified, overexpressed, or mutated in multiple cancer types, supporting its function as an oncogene. In addition to genetic alterations in EZH2 itself, distal genetic changes in other proteins can lead to oncogenic dependency on EZH2 activity. For example, we have previously established that cell lines and xenografts deficient in INI1 (SNF5/SMARCB1), a core component of the SWItch/Sucrose Non-Fermentable (SWI/SNF) chromatin remodeling complex, display profound sensitivity and durable regressions in the presence of the selective EZH2 inhibitor tazemetostat (EPZ-6438). Intriguingly, a complete response was observed in a patient with an INI1-negative rhabdoid tumor who participated in the tazemetostat Ph1 dose escalation study. This suggests that these tumors are addicted to dysregulated PRC2 activity, and confirms the previously proposed antagonistic relationship of SWI/SNF with PRC2, which is perturbed in INI1-deficient tumors. The loss of INI1 induces inappropriate SWI/SNF function, abrogating the repression of PRC2 activity, resulting in Polycomb target genes, such as those involved in differentiation and tumor suppression, to become aberrantly repressed. In addition to deletion of INI1, there are numerous reports describing genetic alterations in other SWI/SNF complex members. Given the oncogenic dependency of INI1-deficient tumors on PRC2 activity, we sought to investigate the sensitivity of other SWI/SNF mutated cancer types to EZH2 inhibition. Specifically, we investigated the effects of EZH2 inhibition in ovarian cancers carrying somatic mutations in the SWI/SNF complex members ARID1A and SMARCA4. Methods and results: A panel of ovarian cancer cell lines of different histologies was subjected to proliferation assays in 2-D tissue culture for 14 days in the presence of increasing concentrations of an EZH2 inhibitor. Selected cell lines were also tested in 3-D cultures, as it has been suggested in the literature that this context is necessary to observe anti-proliferative effects with EZH2 inhibitors. We found that ovarian cancer cell lines deficient in the SWI/SNF component SMARCA4 (also known as BRG1) are among the most sensitive in response to EZH2 inhibition, as demonstrated by decreased proliferation and/or morphology changes, at concentrations that are clinically achievable. In contrast, mutations in ARID1A, another SWI/SNF component, do not broadly confer sensitivity to EZH2 inhibition in ovarian cancer cell lines in either 2-D or 3-D in vitro assays. Furthermore, the effects of EZH2 inhibition on SMARCA4-negative ovarian cancer cells are context specific, since other cell types with SMARCA4 deletion, such as lung carcinoma cell lines, do not exhibit anti-proliferative affects with EZH2 inhibitor treatment. Conclusions: These data suggest that tazemetostat may have therapeutic benefit in SMARCA4-deleted ovarian cancer, such as small cell cancer of the ovary of the hypercalcemic type (SCCOHT), which shows a high degree of loss of SMARCA4 expression. Further in vitro and in vivo studies are underway to interrogate these initial results further. Citation Format: Sarah K. Knutson, Allison E. Drew, Christopher Plescia, Robert A. Copeland, Jesse J. Smith, Heike Keilhack, Scott Ribich. EZH2 inhibition leads to decreased proliferation in SMARCA4-deleted ovarian cancer cell lines. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C87.

Target selection for oncology and other indications is a critical step in the successful development of therapeutics, however it remains one of the most challenging areas of drug discovery. In fact, up to two-thirds of oncology relevant targets reported in literature have not been confirmed on follow-up studies, indicating that target validation in oncology is especially challenging. Clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 gene editing of specific loci offers an alternative method to RNA interference and complements small molecule inhibitors for determining whether or not a cell line is dependent on a specific gene product for proliferation and/or survival. Importantly, CRISPR-Cas9 may be advantageous for some studies as it offers efficient and specific gene knock-out leading to complete loss of protein function. This may be especially useful for some target classes, including epigenetic targets, which appear to require near complete loss of protein function to observe phenotypes. In our initial studies using CRISPR-Cas9 to verify the essential nature of EZH2 (Enhancer of Zest 2) expression for the proliferation of SMARCB1/SNF5/INI1 mutant malignant rhabdoid tumor cell lines, we observed that the initial reduction in proliferation was lost over time. We hypothesized that in the few cells that retain proliferative capacity at least one allele of EZH2 remains functional, and this hypothesis suggests that carrying out CRISPR-Cas9 studies for individual target genes without analyzing single cell clones could produce misleading results. To verify this, we developed a medium throughput assay to analyze 10s-100s of single cell clones for target gene disruption using CRISPR-Cas9 gene knockout, followed by a restriction digest and fluorescent undigested fragment length analysis to successfully assess EZH2 allele status. Significantly, these data support our hypothesis that retention of one functional copy of EZH2 is required for the proliferation of EZH2-dependent cell lines and that this can be rapidly assessed by our assay. Thus, the assessment of zygosity of the gene of interest can be evaluated in a medium-throughput manner in single cell clones. The assay is thereby able to unambiguously indicate whether or not a specific gene is essential for survival and/or proliferation in a given cell line, and offers a unique approach for target validation using gene editing. Importantly, this approach should be applicable to any target of interest that is expected to affect cell proliferation or survival. Such data can aid in the development of more robust cancer therapeutics by increasing confidence in target selection. Citation Format: Alexandra R. Grassian, Tim Scales, Sarah K. Knutson, Nicola J. McCarthy, Chris E. Lowe, Jon D. Moore, Robert A. Copeland, Heike Keilhack, Jesse J. Smith, Julie A. Wickendon, Scott Ribich. A medium-throughput single cell CRISPR-Cas9 assay to assess gene essentiality. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C162.

The EZH2 inhibitor tazemetostat (EPZ-6438) is emerging as a promising therapeutic agent for the treatment of non-Hodgkin's Lymphoma (NHL). A significant body of work has now demonstrated in vitro and in vivo effects of EZH2 inhibition in preclinical models of lymphoma, in addition to objective clinical responses in early human trials. While EZH2 gain-of-function mutations clearly contribute to lymphomagenesis, patients with lymphomas harboring wild-type EZH2 also show responses to tazemetostat. This suggests a broad role of EZH2 in B-cell oncogenesis. Several recent mouse model studies have demonstrated the importance of wild-type EZH2 catalytic activity in the formation of germinal centers in non-diseased lymph nodes, suggesting a central role for EZH2 in B-lymphocyte maturation. These findings indicate that the importance of EZH2 to B-cell lymphoma likely lies in its ability to regulate B-cell differentiation. To understand the relationship between B-cell maturation and sensitivity to EZH2 inhibition, we evaluated changes in maturation markers and cellular proliferation following treatment of diffuse large B-cell lymphoma (DLBCL) cell lines with tazemetostat in combination with modulators of B-cell activation. Consistent with the importance of EZH2 in the regulation of B-cell differentiation, we observed increased expression of B-cell maturation markers in DLBCL cell lines treated with single agent tazemetostat in vitro. Furthermore, we demonstrate that tazemetostat pre-treatment of subsets of DLBCL cells lines (both EZH2 mutant and wild-type) can sensitize cells to inhibitors of B-cell activation pathways, which include glucocorticoids and BTK, MAPK and PI3K pathway inhibitors. Moreover, the anti-proliferative activity of single agent tazemetostat can be diminished or delayed by co-treatment with biological stimulators of B-cell activation including B-cell receptor ligation, CD40L, LPS and BAFF. Importantly, B-cell receptor ligation and co-stimulation agents have little proliferative effects on DLBCL cell lines on their own, suggesting that the protective function of these agents is directly related to the effects of EZH2 inhibition and not a generic stimulation of proliferation. Our findings suggest that EZH2 inhibition initiates a differentiation program that enables lymphoma cells to proceed through the normal processes of B-cell selection, growth regulation and maturation. Citation Format: Danielle Johnston, Dorothy Brach, Christopher Plescia, Alison Drew, Trupti Lingaraj, Natalie Warholic, Jesse J. Smith, Robert A. Copeland, Heike Keilhack, Elayne Penebre, Sarah K. Knutson, Scott Ribich, Michael J. Thomenius, Alejandra Raimondi. EZH2 plays a critical role in B-cell maturation and in non-Hodgkin's lymphoma: Interplay between EZH2 function and B-cell activation. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr B85.

Preclinical data have suggested that small molecule inhibitors for the histone methyltransferase EZH2 represent potential new treatment modalities for Non-Hodgkin lymphomas (NHL) expressing EZH2 change of function mutations. Our group has previously reported that selective inhibition of EZH2 results in specific killing of lymphoma cells bearing EZH2 mutations in vitro and in vivo, with minimal effects on non-mutant lymphoma cells [Knutson et al. Nature Chemical Biology 2012; Keilhack et al. Blood (ASH Annual Meeting Abstracts) 2012, 120, Abstract 3712]. Since epigenetic changes have been suggested to be involved in resistance of cancer cells to many anticancer agents, we studied EPZ-6438 (or E7438), our clinical stage EZH2 inhibitor, in combination with standard of care agents for NHL, second line therapies or targeted therapies that are being explored in this indication. With continuous exposure to EPZ-6438, cell-based assays of two different EZH2 mutant cell lines demonstrated combination benefits with all components of the CHOP chemotherapy regime, second line therapies but also with several targeted therapies (for instance other epigenetic drugs, PI3K pathway or other inhibitors). These effects were not observed in an EZH2 wild type lymphoma cell line of the activated B cell type. Strong combination benefit with CHOP was also observed in two different EZH2 mutant xenograft models. For instance, in the SUDHL6 Y646N xenograft model neither EPZ-6438 nor CHOP chemotherapy alone induced a significant antitumor effect, yet their combination produced durable tumor regressions even after cessation of dosing (figure 1). Importantly, this effect was preserved when doxorubicin was omitted from the CHOP chemotherapy regime in a third study with another EZH2 mutant xenograft model. Subsequently we showed that glucocorticoid receptor agonism may be a key mechanism of the combination benefit observed with CHOP, as the antiproliferative effect of EPZ-6438 was enhanced by either prednisolone or dexamethasone alone, in several EZH2 mutant lymphoma cell lines (in vitro). Taken together these data suggest that the single agent activity of EPZ-6438 in EZH2 mutant NHL may be further enhanced and expanded through rational combination strategies. Disclosures: Johnston: Epizyme: Employment, Equity Ownership, stock options Other. Knutson:Epizyme, Inc.: Employment, Equity Ownership, Patents & Royalties, stock options Other. Warholic:Epizyme, Inc.: Employment, Equity Ownership, Patents & Royalties, stock options Other. Klaus:Epizyme, Inc.: Employment, Equity Ownership, Patents & Royalties, Stock Options Other. Wigle:Epizyme, Inc.: Employment, Equity Ownership, Patents & Royalties, stock options Other. Iwanowicz:Epizyme, Inc.: Employment, Equity Ownership, stock options Other. Littlefield:Eisai Inc.: Employment. Porter Scott:Epizyme, Inc: Employment, Equity Ownership, Patents & Royalties, Stock Options Other. Smith:Epizyme, Inc.: Employment, Equity Ownership, Stock Options Other. Moyer:Epizyme, Inc.: Employment, Equity Ownership, Stock Options Other. Copeland:Epizyme Inc. : Employment, Equity Ownership, Patents & Royalties, stock options Other; Mersana: Membership on an entity’s Board of Directors or advisory committees. Pollock:Epizyme Inc.: Employment, Equity Ownership, Patents & Royalties, Stock Options Other. Kuntz:Epizyme, Inc.: Employment, Equity Ownership, Patents & Royalties, stock options Other. Keilhack:Epizyme, Inc.: Employment, Equity Ownership, Patents & Royalties, stock options Other. Raimondi:Epizyme, Inc: Employment, Equity Ownership, Patents & Royalties, stock options Other.

Abstract 3712 The coupled enzymatic activity of wild-type and mutant EZH2 results in hyper-trimethylation of histone H3 lysine 27 (H3K27), which drives lymphomagenesis in heterozygous patients bearing the EZH2 mutations. Our group has previously reported that selective inhibition of EZH2 in cell culture results in selective killing of lymphoma cells bearing EZH2 mutations, with minimal effect on non-mutant lymphoma cells, suggesting that EZH2 enzymatic activity is a required driver of proliferation in the mutant-bearing cells [Knutson et al. (2012) Nature Chemical Biology, in press]. Through iterative medicinal chemistry we have developed a selective inhibitor of EZH2 with good pharmacological properties, E7438. E7438 binds to the enzyme in a manner competitive with S-adenosyl methionine (SAM) and a Ki for wild-type EZH2 of 2.5 ± 0.5 nM. The compound potently inhibits all known mutants of EZH2 that have been identified in non-Hodgkin lymphoma (NHL) patient samples. E7438 displays about 35-fold less activity against the closely related enzyme EZH1, and is >4500-fold selective with respect to all other protein methyltransferases tested. Lymphoma cells treated with E7438 display concentration- and time-dependent loss of H3K27 methylation with no effect on the methylation status of any other histone sites. The loss of H3K27 methylation results in selective killing of EZH2 mutant-bearing lymphoma cell lines. E7438 displays good oral bioavailability and pharmacokinetic properties. Various EZH2 mutant-bearing human lymphoma tumors were subcutaneously implanted in nude, SCID or NSG mice. Oral administration of E7438 to tumor bearing mice resulted in significant anti-tumor activity. The responses ranged from dose-dependent tumor growth inhibition to complete and sustained regressions. For example, KARPAS422 tumors in nude mice showed complete tumor elimination after 28 days of dosing, with mice remaining tumor free for up to 90 days after treatment cessation. Figure 1. E7438 causes complete and sustained tumor regression in a KARPAS422 nude mouse xenograft model of EZH2-mutated NHL. Dosing was on a BID schedule. * P< 0.05, Repeated measures ANOVA, Dunnett's post test vs. vehicle. Figure 1. E7438 causes complete and sustained tumor regression in a KARPAS422 nude mouse xenograft model of EZH2-mutated NHL. Dosing was on a BID schedule. * P< 0.05, Repeated measures ANOVA, Dunnett's post test vs. vehicle. Mice and rats tolerated E7438 administration well at doses representing high multiples of doses that show antitumor activity in mice. Activity against the EZH2 target in both species was demonstrated by dose-dependent diminution of H3K27me3 levels, assessed by ELISA, in samples of tumor, bone marrow, skin and peripheral blood mononuclear cells (PBMCs). Highly sensitive detection of existing H3K27me3 signal could also be observed with this ELISA assay in drug-naïve samples of human PBMCs. The ability to measure dose-dependent changes in H3K27me3 levels in skin and PBMCs portends the use of signal from these surrogate tissues as a non-invasive pharmacodynamics biomarker in human clinical trials. Disclosures: Keilhack: Epizyme Inc.: Employment, Equity Ownership. Yokoi:Eisai Co., Ltd.: Employment. Knutson:Epizyme Inc.: Employment, Equity Ownership. Wigle:Epizyme Inc.: Employment, Equity Ownership. Warholic:Epizyme Inc.: Employment, Equity Ownership. Kawano:Eisai Co., Ltd.: Employment. Minoshima:Eisai Co., Ltd.: Employment. Huang:Eisai Inc.: Employment. Kuznetsov:Eisai Inc.: Employment. Kumar:Eisai Inc.: Employment. Klaus:Epizyme, Inc.: Employment, Equity Ownership. Allain:Epizyme Inc.: Employment, Equity Ownership. Raimondi:Epizyme Inc.: Employment, Equity Ownership. Porter Scott:Epizyme: Employment, Equity Ownership. Chesworth:Epizyme: Employment, Equity Ownership. Moyer:Epizyme: Employment, Equity Ownership. Uenaka:Eisai Co., Ltd.: Employment. Copeland:Epizyme Inc.: Employment, Equity Ownership. Richon:Epizyme, Inc.: Employment, Equity Ownership. Pollock:Epizyme Inc.: Employment, Equity Ownership. Kuntz:Epizyme Inc.: Employment, Equity Ownership.

Abstract 707 EZH2, the catalytic subunit of the polycomb repressive complex 2 (PRC2), catalyzes the mono- through tri-methylation of lysine 27 on histone H3 (H3K27). While overexpression of EZH2 and increased H3K27 methylation have generally been associated with both hematologic malignancies and solid tumors, inactivating somatic mutations of Tyr641 (Y641F, Y641N, Y641S and Y641H) of EZH2 were recently reported to be associated with follicular lymphoma (FL) and the GCB subtype of diffuse large B-cell lymphoma (DLBCL) (Morin, Nat Genet 2010; 42: 181). In all cases, occurrence of the mutant EZH2 gene was heterozygous, and expression of both wild type and mutant alleles was detected in the mutant samples profiled by transcriptome sequencing. Further, the mutant forms of EZH2 could be incorporated into the multi-protein PRC2 complex, but the resulting complexes lacked the ability to catalyze trimethylation of an unmethylated H3K27 peptide substrate. To explore further the role of EZH2 in lymphomagenesis, we have evaluated the catalytic activity of the mutant EZH2 proteins in greater detail. Recombinant PRC2 complexes were prepared with wild type and Tyr641 mutant EZH2 forms. As previously reported, the wild type enzyme demonstrated robust activity but none of the mutant enzymes displayed significant methyltransferase activity on an unmodified H3K27 peptide. We next evaluated the activity of the enzymes using native avian erythrocyte olignucleosomes as the substrate in the reaction. In contrast to the peptide result, we found that the wild type and all of the mutant enzymes were active methyltransferases against the native nucleosome substrate. Since native nucleosome represents an admixture of the unmodified and mono-, di- and tri-methylated H3K27 we next evaluated the activity of the wild type and mutant enzymes on unmodified, and mono- and di-methylated H3K27 peptide. We demonstrate that the wild type enzyme displays greatest catalytic efficiency (kcat/K) for the zero to mono-methylation reaction of H3K27, and diminished efficiency for subsequent (mono- to di- and di- to tri-methylation) reactions. In stark contrast, the disease-associated Y641 mutants display very limited ability to perform the first methylation reaction, but have enhanced catalytic efficiency for the subsequent reactions, relative to WT-enzyme. Catalytic coupling between the mutant EZH2 species and PRC2 complexes containing either wild type EZH2 or wild type EZH1 are predicted to augment H3K27 trimethylation and thus produce the malignant phenotype associated with mutant heterozygosity. To test this prediction, the level of H3K27 methylation was evaluated in lymphoma cell lines harboring only wild type EZH2 (OCI-LY-19) or heterozygous for EZH2 Y641N (DB, KARPAS and SU-DHL-6) or EZH2 Y641F (WSU-DLCL2) by immunoblotting. As predicted by simulations, the level of H3K27 trimethylation was elevated in all of the lymphoma cell lines harboring the mutant EZH2 relative the wild type. Additionally, we observe decreased H3K27 dimethylation and monomethylation in the cells harboring the mutated EZH2 relative to wild type enzyme; these reductions in di- and monomethylation are likewise consistent with expectations based on steady state kinetic simulations. The present results imply that the malignant phenotype of follicular lymphoma and diffuse large B cell lymphoma of the GCB subtype, associated with expression of mutant forms of EZH2, results from of an overall gain-of-function with respect to formation of the trimethylated form of H3K27. These data suggest that selective, small molecule inhibitors of EZH2 enzymatic function may form a rational underpinning for molecularly targeted therapeutics against mutant-harboring lymphomas and other malignancies in which EZH2 gain-of-function is pathogenic. Disclosures: Richon: Epizyme, Inc: Employment. Sneeringer:Epizyme: Employment. Porter Scott:Epizyme, Inc: Employment. Kuntz:Epizyme, Inc: Employment. Knutson:Epizyme, Inc.: Employment. Pollock:Epizyme, Inc: Employment. Copeland:Epizyme, Inc: Employment.

Mutations at tyrosine 641 (Y641F, Y641N, Y641S and Y641H) in the SET domain of EZH2 have been identified in patients with certain subtypes of non-Hodgkin lymphoma (NHL). These mutations were shown to change the substrate specificity of EZH2 for various methylation states of lysine 27 on histone H3 (H3K27). An additional mutation at EZH2 Y641 to cysteine (Y641C) was also found in one patient with NHL and in SKM-1 cells derived from a patient with myelodisplastic syndrome (MDS). The Y641C mutation has been reported to dramatically reduce enzymatic activity. Here, we demonstrate that while the Y641C mutation ablates enzymatic activity against unmethylated and monomethylated H3K27, it is superior to wild-type in catalyzing the formation of trimethylated H3K27 from the dimethylated precursor.

Recurrent somatic loss-of-function mutations in ASXL1 (Addition of sex combs-like 1) are common genetic events in a spectrum of myeloid malignancies and these alterations demarcate patients with poor outcome. ASXL1 forms a chromatin regulatory complex with the ubiquitin hydrolase BAP1 (BRCA1 associated protein-1), a protein that has been found to be transcriptionally repressed in MDS patients. These data are consistent with BAP1 having tumor suppressive activity in MDS; however, the mechanism by which disruption of the ASXL1-BAP1 axis leads to transformation is not well understood. We conditionally deleted Bap1 in the murine hematopoietic system utilizing Mx1-Cre (hereafter referred to as Bap1 KO). One hundred percent of mice with confirmed Bap1 deletion developed a fully penetrant MDS-like disease characterized by leukocytosis, anemia, and splenomegaly. Bap1 KO mice have an expansion of the granulocyte macrophage progenitor compartment (GMP; Lin- c-Kit+ Sca1- CD34+ Fcϒ+). Given the role of BAP1 in epigenetic regulation, we investigated the effect of Bap1 loss on chromatin state and transcriptional output. We first assessed epigenetic changes in Bap1 KO mice by performing histone mass spectometry in control and Bap1 KO hematopoietic stem and progenitor cells (HSPCs, c-Kit+ enriched). Bap1 loss increased H3K27me2/3 at the expense of H3K27me0/1. We confirmed that H3K27me3 was increased in Bap1 KO bone marrow cells by completing H3K27me3 ChIP-Sequencing in HSPCs. Enumeration of H3K27me3 peaks in Bap1 KO versus control cells indicated an increase in H3K27me3 domains (Figure A). We next overlaid RNA-Sequencing from GMP sorted Bap1 KO bone marrow cells with genes marked by H3K27me3, as indicated by ChIP-Sequencing. We found that Bap1 loss resulted in a global decrease in gene expression (68% downregulated, 657/968 genes, p-adj Given the alterations in H3K27me3 in Bap1 KO mice, we investigated whether Bap1- deficient transformation could be rescued by abrogation of PRC2-mediated gene repression. We developed a genetic model with compound deletion of Bap1 and Ezh2, the catalytic component of the PRC2 complex. Co-deletion of Bap1 and Ezh2 resulted in a phenotypic rescue of Bap1 KO associated splenomegaly (spleen weights, Bap1 KO avg. 541.6 mg, Bap1/Ezh2 KO avg. 157.0 mg, p EZH2 small molecule inhibitors have proven effective in EZH2-dependent models of B cell lymphoma. To determine if Ezh2 inhibition was efficacious in the setting of Bap1 loss, we treated a cohort of Bap1 KO mice with either vehicle (NaCMC) or 500 mg/kg EPZ011989, an EZH2 inhibitor with in vivo activity. Treatment of Bap1 KO mice for 16 days resulted in significant reduction of splenomegaly (spleen weights, vehicle avg. 522.0, EPZ011989 treated avg. 216.2, p Figure 1. Figure 1. Disclosures Knutson: Epizyme, Inc: Employment. Campbell:Epizyme, Inc: Employment. Keilhack:Epizyme: Employment, Equity Ownership. Melnick:Janssen: Other: Research; ROCHE: Other: Research; Genentech: Speakers Bureau; Celgene: Consultancy; Eli Lilly: Consultancy; Epizyme: Consultancy. Armstrong:Epizyme, Inc: Consultancy. Levine:Foundation Medicine: Consultancy; CTI BioPharma: Membership on an entity's Board of Directors or advisory committees; Loxo Oncology: Membership on an entity's Board of Directors or advisory committees.