Resistance mechanism to Notch inhibition and combination therapy in human T-cell acute lymphoblastic leukemia.

Gain-of-function mutations in NOTCH1 are among the most frequent genetic alterations in T cell acute lymphoblastic leukemia (T-ALL), highlighting the Notch signaling pathway as a promising therapeutic target for personalized medicine. Yet, a major limitation for long-term success of targeted therapy is relapse due to tumor heterogeneity or acquired resistance. Thus, we performed a genome-wide CRISPR-Cas9 screen to identify prospective resistance mechanisms to pharmacological NOTCH inhibitors and novel targeted combination therapies to efficiently combat T-ALL. Mutational loss of Phosphoinositide-3-Kinase regulatory subunit 1 (PIK3R1) causes resistance to Notch inhibition. PIK3R1 deficiency leads to increased PI3K/AKT signaling which regulates cell cycle and the spliceosome machinery, both at the transcriptional and post-translational level. Moreover, several therapeutic combinations have been identified, where simultaneous targeting of the cyclin-dependent kinases 4 and 6 (CDK4/6) and NOTCH proved to be the most efficacious in T-ALL xenotransplantation models.