Abstract 4430: Loss of the histone methyltransferase EZH2 induces chemoresistance in acute myeloid leukemia (AML)

Resistance to chemotherapy and subsequent relapse is the most challenging issue in the treatment of patients with Acute Myeloid Leukemia (AML). However, the underlying mechanisms still remain incompletely understood. Here we report that loss of the histone methyltransferase EZH2 and subsequent reduction of H3K27 trimethylation contribute to chemoresistance in AML. In Myelodysplastic Syndrome (MDS) and Myeloproliferative Neoplasms (MPN) EZH2 is often inactivated due to mutations which is associated with poor prognosis. By use of quantitative PCR and immunohistochemistry we show that a decrease of EZH2 mRNA and protein also correlated with a poor prognosis of AML patients indicating a tumor suppressor role of EZH2 in AML. EZH2 is located on chromosome 7q36.1 and it is not yet fully clear whether EZH2 expression is affected in MDS and AML patients with del(7)/del(7q) who are largely refractory to chemotherapy and have a poor prognosis. We found EZH2 levels to be reduced in del(7)/del(7q) AML patients as determined by Western Blot. Notably, the reduction of EZH2 protein levels via treatment with H3K27 methyltransferase inhibitors or lentiviral knockdown was sufficient to induce chemoresistance of Normal Karyotype (NK)- AML blasts and cell lines in vitro and in a xenograft mouse model. Furthermore, we observed that EZH2 loss occurred during the acquisition of drug resistance in a Tyrosine Kinase Inhibitor- and Cytarabine (AraC)- resistant AML cell line. Pharmacological inhibition of CDK1 and treatment with the proteasome inhibitor Bortezomib, respectively, increased EZH2 protein and restored drug sensitivity. Functionally, the loss of EZH2 directly induced upregulation of HOX genes, suggesting a stem-cell-like signature to be associated with the resistance phenotype, which could be reverted by Bortezomib treatment. To evaluate the potential of Bortezomib to affect EZH2 levels in patient blasts we treated primary NK-AML blasts collected at diagnosis ex vivo with Bortezomib. In almost all samples Bortezomib treatment induced cytotoxic effects. In 5 out of 10 patients the EZH2 protein level could be increased by Bortezomib treatment. We furthermore examined the sensitivity of patient samples to AraC, Bortezomib or combined treatment. Notably, for those patients with increased EZH2 levels after Bortezomib exposure we found a significantly decreased cell survival for the combined treatment compared to single-agent treatment. Our data strongly suggest that restoration of EZH2 protein levels e.g. via proteasome inhibitors and thereby restoration of EZH2 function might be a novel promising approach to increase therapy response in AML. Citation Format: Stefanie Gollner, Shuchi Agrawal-Singh, Tino Schenk, Hans-Ulrich Klein, Christian Rohde, Tim Sauer, Mads Lerdrup, Sigal Tavor, Friedrich Stolzel, Gerhard Ehninger, Gabriele Kohler, Martin Dugas, Arthur Zelent, Christian Thiede, Wolfgang E. Berdel, Klaus Hansen, Carsten Muller-Tidow. Loss of the histone methyltransferase EZH2 induces chemoresistance in acute myeloid leukemia (AML). [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4430. doi:10.1158/1538-7445.AM2015-4430