T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematologic cancer that accounts for 10% to 15% of pediatric and 25% of adult ALL cases. Glucocorticoids play a central role in the treatment of T-ALL due to their capacity to induce growth arrest and apoptosis in lymphoid progenitor cells. The development of glucocortioid resistance in leukemia patients is an important clinical problem and a significant contributor to therapeutic failure. Despite numerous studies addressing the mechanisms that mediate the response of lymphoid tumor cells to glucocorticoid therapy the mechanism of glucocorticoid resistance remain incompletely understood. Activating mutations in NOTCH1 are present in over 50% of T-ALL cases and aberrant NOTCH1 signaling can transform T-cell progenitors. Importantly, proteolytic cleavage by the gamma-secretase complex is required for ligand-mediated activation of wild type NOTCH1 and for aberrant NOTCH1 signaling in T-ALLs harboring activating mutations in the NOTCH1 gene and, small molecule gamma-secretase inhibitors can effectively block the function of oncogenic NOTCH1in T-ALL. Dr. Ferrando’s laboratory has recently shown that inhibition of NOTCH1 signaling with GSIs can reverse glucocorticoid resistance in T-ALL. Reversal of glucocorticoid resistance in this model is mediated by HES1 downregulation, increased glucocorticoid receptor autoupregualtion and improved glucocorticoid-induced upregulation of BIM, a critical proapototic factor, in glucocorticoid-induced programmed cell death. The identification of NOTCH1-HES1-glucocorticoid receptor regulatory axis in glucocorticoid resistant T-ALL serves as proof of principle for the main goal of this project, which aims to identify novel genes and pathways controlling glucocorticoid resistance in T-ALL. As second phase of this project, the previously identified pathways will be exploited to develop novel targeted therapies for the treatment of glucocorticoid resistant T-ALL.