Abstract 4672: Novel epigenetic approach to relapsed mantle cell lymphoma based on dual inhibition of EZH1/EZH2
Authors: Fujita Shuhei , Kagiyama Yuki , Honma Daisuke , Adachi Nobuaki , Araki Kazushi , Kitabayashi Issay 
Published: July 2017

Mantle cell lymphoma (MCL) is a well-defined and aggressive type of B cell non-Hodgkin’s lymphoma that is genetically characterized by the t(11;14)(q13;q32) chromosomal translocation, which results in constitutive overexpression of CYCLIN D1. Although newly developed drugs such as ibrutinib show promising clinical outcomes, relapsed MCL often acquires drug resistance, which is a critical obstacle to treatment. Alternative approaches to overcoming the drug resistance of relapsed MCL are urgently needed. PRC1 and 2 are important epigenetic regulators that maintain the stemness of embryonic and hematopoietic stem cells. EZH1 and 2 are catalytic components of PRC2, which trimethylates histone H3 at lysine 27 (H3K27) to repress transcription of target genes. Mutation and overexpression of EZH1/2 are associated with cancers, including hematopoietic malignancies. Here, we used a novel dual inhibitor of EZH1/2 to show that inhibiting EZH1/2 is a promising therapeutic strategy for MCL. First, we developed a xenograft (PDX) mouse model using cells from a heavily pretreated and relapsed MCL patient, and then orally administered an inhibitor of EZH1/2, called OR-S1. OR-S1 strongly impaired proliferation of the patient-derived tumors and did not cause any serious side effects. Additionally, an in vitro assay using MCL cell lines showed that OR-S1 inhibited the growth of MCL cells, and that the effect was much more significant than that using the single EZH2 inhibitor (GSK126). The IC50 of OR-S1 was about one tenth that of GSK126. These results strongly suggest that dual inhibition of EZH1/2 could be a promising therapeutic strategy for relapsed MCL. Next, to investigate the effect induced by dual inhibition of EZH1/2, we conducted further analyses of the MCL cell lines. Cells exposed to OR-S1 showed cell cycle arrest (G1 arrest) along with a dose-dependent reduction in phospho-Rb and cell differentiation, coupled with increased cell surface expression of hCD138. We then used RNA-seq analysis of MCL cell lines to compare OR-S1-treated cells with vehicle-treated cells and found that cell cycle-related signaling was significantly affected and that a cyclin-dependent kinase inhibitor, CDKN1C (TP57), was one of the genes most markedly upregulated by OR-S1. ChIP qPCR of MCL cell lines showed that the CDKN1C locus was strongly marked by H3K27 trimethylation, and that OR-S1 induced a significant reduction in the level of this histone marker. Furthermore, administration of OR-S1 alone to PDX mice induced increased expression of CDKN1C (as in the in vitro assay). Thus, dual inhibition of EZH1/2 in MCL induces expression of CDKN1C, which in turn causes cell cycle arrest and reduced growth of MCL. Taken together, these results strongly suggested that dual inhibition of EZH1 and EZH2 is a promising therapeutic strategy for MCL, illustrating the potential of novel epigenetic approaches to overcoming drug resistance of relapsed MCL.

Citation Format: Shuhei Fujita, Yuki Kagiyama, Daisuke Honma, Nobuaki Adachi, Kazushi Araki, Issay Kitabayashi. Novel epigenetic approach to relapsed mantle cell lymphoma based on dual inhibition of EZH1/EZH2 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4672. doi:10.1158/1538-7445.AM2017-4672