Abstract 1442: Targeting ATM kinase and mTOR signaling reverses bone marrow stromal cell-mediated protection of FLT3-ITD AML from FLT3-targeted therapy

Abstract

Abstract Internal tandem duplication (ITD) mutations in FMS-like tyrosine kinase 3 (FLT3) are among the most common mutations in acute myeloid leukemia (AML) and are associated with poor prognosis. FLT3-ITD causes constitutive activation of FLT3, and the strong evidence that activated FLT3 drives leukemogenesis has led to the development of several FLT3-targeted inhibitors. However, clinical studies of FLT3-targeted inhibitors have demonstrated much more effective clearing of leukemic burden in the periphery than in the bone marrow, implicating the bone marrow microenvironment as a potential contributor to drug resistance. As previously reported, the conditioned media of human bone marrow stromal cells (CM-BMSC) protects human FLT3-ITD AML cells from the killing effect of FLT3-targeted therapy despite complete inhibition of FLT3. We further show that genetic knockdown of Ataxia Telangiectasia Mutated (ATM) in combination with FLT3 inhibition substantially reverses the protection from cell death mediated by CM-BMSC. While autophosphorylation activity of ATM is downregulated upon FLT3 inhibition in regular media, CM-BMSC prevented the loss of ATM activity. Interestingly, genetic knockdown of ATM in human FLT3-ITD AML cells impaired the activity of mTOR Complex 1, and unbiased analyses of gene expression and signaling pathways by RNA-seq analysis and Reverse Phase Protein Arrays (RPPA) showed that mTOR signaling is downregulated with FLT3 inhibition in regular media, coinciding with marked inhibition of the protein translation machinery. Both mTOR signaling and protein translation were maintained during FLT3 inhibition in the presence of CM-BMSC. Furthermore, human FLT3-ITD AML cells treated with mTOR inhibitor everolimus in combination with the FLT3 inhibitor quizartinib reversed the protection mediated by CM-BMSC, and combination therapies are currently being tested in mouse models. These data suggest that ATM kinase and mTOR signaling play a key role in bone marrow stoma-mediated protection against FLT3 targeted therapy. Findings from this research provide new insights into the mechanism of bone marrow stroma-mediated protection of FLT3-ITD AML from FLT3-targeted therapy, identifying additional candidates for combinatorial therapies designed to overcome the protective effects of bone marrow stromal cells and improve patient outcomes. Citation Format: Hae J. Park, Mark A. Gregory, Vadym Zaberezhnyy, James DeGregori. Targeting ATM kinase and mTOR signaling reverses bone marrow stromal cell-mediated protection of FLT3-ITD AML from FLT3-targeted therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1442.