Abstract LB-259: EZH2 inhibitors abrogate upregulation of H3K27me3 by CDK9 inhibitors and potentiate its antitumor activity in DLBCL
Authors: Xie Shao , Wei Fan , Sun Yiming , Gao Yinglei , Ding Jian , Chen Yi 
Published: August 2018

Abstract
Despite recent improvements in treatment, a significant fraction of patients with diffuse large B-cell lymphoma (DLBCL) still fail therapy. Therefore, new treatments are still needed to advance the cure rate. Cyclin-dependent kinase 9 (CDK9) constitutes positive transcription elongation factor b, which is responsible for the transcription elongation of most of the protein-coding genes through the phosphorylation of the carboxyl-terminal domain of RNA polymerase II at the serine 2. Research indicates that CDK9 is involved in lymphocyte differentiation and activation, and shows an aberrant expression in DLBCL. In addition, Mcl-1 expression, which can be downregulated by CDK9 inhibition, was found to be overexpressed in a significant fraction of DLBCL, contributing to therapy resistance. All these suggest that CDK9 inhibition seems to be a potential therapeutic strategy for DLBCL. LS-007 is a novel CDK9 inhibitor, which displays potential activity against leukemia and ovarian cancer. So, firstly, the therapeutic potential of LS-007 in DLBCL was assessed. We found that LS-007 triggered apoptosis and dramatically repressed DLBCL cell growth associated with lost Mcl-1 and XIAP proteins owing to CDK9 inhibition in vitro and in vivo. However, we also found that CDK9 inhibition specifically elevated the trimethylation of H3K27 (H3K27me3) by mass spectrometry and western blot. Methylation of lysine 27 on H3 is a modification associated with gene repression, and plays a critical role in regulating the expression of genes that determine the balance between cell differentiation and proliferation. This modification is generated by the polycomb repressive complex 2, composed of the SET domain-containing histone methyltransferase EZH2, and accessory proteins EED, SUZ12, and RbAp48. However, the methylation can be removed by the histone demethylases UTX and JMJD3. Accumulating studies imply that H3K27me3 is closely engaged in the development of DLBCL, especially germinal center B cell subtype11. And reducing H3K27me3 level exhibits significant anti-proliferation activity against DLBCL. Considering the important role of the H3K27me3 in tumor progression, we thought that it may restrain CDK9 inhibitors' anti-tumor potency against DLBCL and other solid tumor types. Subsequently, the synergistic effect of the combination therapy of CDK9 inhibitors with EZH2 inhibitors was investigated. As expected, EZH2 inhibitors reversed the upregulation of H3K27me3, improved apoptosis and DNA damage compared to CDK9 inhibitors alone, and synergistically inhibited DLBCL growth in vitro and in vivo. By expanding our studies to other solid tumors with this combination therapy, we also got an impressive synergistic effect. Further, we speculated that CDK9 inhibition reduced the expression of JMJD3/UTX more strongly than that of EZH2, leading to H3K27me3 elevation. To summarize, we found that LS-007 might represent a new therapeutic agent for DLBCL and provide a rational basis for the application of CDK9 inhibitors in combination with EZH2 inhibitors in clinical trials. We know that EZH2 inhibitors only display potent activity in very limited tumor types; however, our data also provide new possibilities for the application of such inhibitors.
Citation Format: Shao Xie, Fan Wei, Yiming Sun, Yinglei Gao, Jian Ding, Yi Chen. EZH2 inhibitors abrogate upregulation of H3K27me3 by CDK9 inhibitors and potentiate its antitumor activity in DLBCL [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-259.