Delineating the Pro-oncogenic Role of Deubiquitination in High-Risk Acute Lymphoblastic Leukemia

High-risk T-cell acute lymphoblastic leukemia (T-ALL) is the most aggressive subtype of ALL and one-fifth of pediatric patients and more than 50% of adult patients with T-ALL do not achieve a complete remission or relapse after chemotherapy, making resistance to therapy the most substantial challenge in treatment. In this proposal, we will characterize mechanisms of aberrant stabilization of the NOTCH1 oncogene via active deubiquitination and its role in inhibition of specific DNA damage response (DDR) pathways leading to chemotherapy resistance in cases of high-risk T-ALL. We have strong preliminary data demonstrating stabilization of NOTCH1 oncogene by a deubiquitinase called ubiquitin-specific peptidase 7 (USP7), in high-risk T-ALL cases. We have also shown that USP7/NOTCH1 inhibits the Ataxia Telangiectasia Mutated (ATM) kinase and DDR pathways triggered by DNA double strand breaks induced by chemotherapy, such as homologous recombination (HR) and non-homologous end joining (NHEJ). We hypothesize that USP7 is a major pro-oncogenic factor in NOTCH1-positive (NOTCH1+) high-risk T-ALL cases, as it actively deubiquitinates and stabilizes NOTCH1 and inhibits ATM and HR/NHEJ. In this application we plan to characterize a) levels of USP7, NOTCH1, ATM and DNA damage response-associated components, with a focus on HR and NHEJ, and study b) whether changes in USP7/NOTCH1 and ATM can modify DDR signatures, and c) the functional role of USP7/NOTCH1 and ATM in resistance to chemotherapy. Successful completion of our studies will lead to the characterization of a previously unexplored, cancer-promoting mechanism in high-risk T-ALL.