Abstract B85: EZH2 plays a critical role in B-cell maturation and in non-Hodgkin's lymphoma: Interplay between EZH2 function and B-cell activation

Abstract

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.

Epigenetic Modulators

“…For example, in the mammalian B-cell life cycle, an unnatural trimethylated state of H3K27 arrests the germinal center in the dark zone, preventing differentiation and resulting in aberrant proliferation as opposed to transition to the light zone and either further B-cell maturation, if needed, or alternative apoptosis in a healthy immune system [151]. Preclinically, it has been observed that treatment with EZH2 inhibitors leads to enrichment of B-cell maturation gene sets, suggesting that EZH2 inhibition could result in a cancer cell differentiation event that would lead, in time, to selective apoptosis [152]. One of the advantages of a selective EZH2 inhibitor is that only cells with a genetic addiction to EZH2 activity should be affected by global reduction in H3K27me3 which could enable dosing of therapeutics up to efficacious exposures without deleterious side effects from general cellular toxicity.…”

Epigenetic Medicinal Chemistry

“…Some studies of combinations between modulators of CMPs of distinct modalities have, however, been reported; most notably, preclinical and clinical studies of combinations of HDAC inhibitors and DNA hypomethylating agents 4 and preclinical studies of DNA hypomethylating agents in combination with inhibitors of the PMTs DOT1L 5 and EZH2 6 have revealed striking synergies in reducing tumor burden. Drug synergy resulting from combining CMP modulators of differing modalities seems to make intuitive sense.…”

“…7 Likewise, studies in B-cell lymphoma models have demonstrated synergy between the EZH2 inhibitor tazemetostat and modulators of several distinct biochemical pathways. 6 Synergy was seen with B-cell signal transduction kinase inhibitors, BCl2 and BCl6 antagonists, 6 and, perhaps most surprisingly, corticosteroid agonists of the glucocorticoid receptor. 8 While many of these biochemical pathways ultimately lead to changes in gene transcription, the synergies seen between these various pathways and CMP modulators reveal hitherto unrecognized biological connections that may aid in our understanding of pathogenesis and that may be exploited for greater clinical activity.…”

“…First, the vast majority of preclinical and clinical studies of CMP modulators have focused on these compounds as monotherapies or in combination with established, nontargeted chemotherapies; these studies have shown varying levels of activity in specific therapeutic indications (almost always oncology). Some studies of combinations between modulators of CMPs of distinct modalities have, however, been reported; most notably, preclinical and clinical studies of combinations of HDAC inhibitors and DNA hypomethylating agents 4 and preclinical studies of DNA hypomethylating agents in combination with inhibitors of the PMTs DOT1L 5 and EZH2 6 have revealed striking synergies in reducing tumor burden. Drug synergy resulting from combining CMP modulators of differing modalities seems to make intuitive Equally intriguing are recent reports of robust synergy resulting from combinations of CMP modulators with compounds targeting completely orthogonal biochemical pathways in cells.…”

“…For example, surprising synergy was observed in MLL-rearranged leukemia models by combining the DOT1L inhibitor pinometostat with selective inhibitors of the signal transduction kinase MEK . Likewise, studies in B-cell lymphoma models have demonstrated synergy between the EZH2 inhibitor tazemetostat and modulators of several distinct biochemical pathways . Synergy was seen with B-cell signal transduction kinase inhibitors, BCl2 and BCl6 antagonists, and, perhaps most surprisingly, corticosteroid agonists of the glucocorticoid receptor .…”