Bernasconi-Elias, PaulaHu, TiancenJenkins, DavidFirestone, BrantGans, SaraKurth, EstherCapodieci, PaolaDeplazes-Lauber, JoellePetropoulos, KonstantinThiel, PhillipPonsel, DirkChoi, Sung HeeLeMotte, PeterLondon, AnneGoetcshkes, MargaretNolin, ErinJones, Michael D.Slocum, KellyKluk, Michael J.Weinstock, David M.Christodoulou, AlexandraWeinberg, OlgaJaehrling, JanEttenberg, Seth A.Buckler, AlanBlacklow, StephenAster, JonFryer, Christy J.2016-12-022016Bernasconi-Elias, P., T. Hu, D. Jenkins, B. Firestone, S. Gans, E. Kurth, P. Capodieci, et al. 2016. “Characterization of activating mutations of NOTCH3 in T cell acute lymphoblastic leukemia and anti-leukemic activity of NOTCH3 inhibitory antibodies.” Oncogene 35 (47): 6077-6086. doi:10.1038/onc.2016.133. http://dx.doi.org/10.1038/onc.2016.133.0950-9232http://nrs.harvard.edu/urn-3:HUL.InstRepos:29625977Notch receptors have been implicated as oncogenic drivers in several cancers, the most notable example being NOTCH1 in T-cell acute lymphoblastic leukemia (T-ALL). To characterize the role of activated NOTCH3 in cancer, we generated an antibody that detects the neo-epitope created upon gamma-secretase cleavage of NOTCH3 to release its intracellular domain (ICD3), and sequenced the negative regulatory region (NRR) and PEST domain coding regions of NOTCH3 in a panel of cell lines. We also characterize NOTCH3 tumor-associated mutations that result in activation of signaling and report new inhibitory antibodies. We determined the structural basis for receptor inhibition by obtaining the first co-crystal structure of a NOTCH3 antibody with the NRR protein and defined two distinct epitopes for NRR antibodies. The antibodies exhibit potent anti-leukemic activity in cell lines and tumor xenografts harboring NOTCH3 activating mutations. Screening of primary T-ALL samples reveals that two of 40 tumors examined show active NOTCH3 signaling. We also identified evidence of NOTCH3 activation in 12 of 24 patient-derived orthotopic xenograft models, two of which exhibit activation of NOTCH3 without activation of NOTCH1. Our studies provide additional insights into NOTCH3 activation and offer a path forward for identification of cancers that are likely to respond to therapy with NOTCH3 selective inhibitory antibodies.en-USNotch signalingNOTCH3T-ALLinhibitory antibodyJournal Article2016-12-0210.1038/onc.2016.133