Genome-wide loss-of-function CRISPR/Cas9 screen of Triple Negative Breast Cancer GSI-resistant MB157R cells treated with GSI or vehicle.

Cell plasticity of Triple negative breast cancer (TNBC) contributes to tumor heterogeneity and is one of the major reasons for the limited success of chemo- or combination therapies in the clinics. The molecular mechanisms of therapy-induced tumor cell plasticity and associated resistance to chemo or targeted therapies are largely unknown. Using a genome wide CRISPR-Cas9 screen we investigated the escape mechanisms of Notch driven TNBC, when treated with targeted therapy. We describe molecularly a reciprocal inhibitory feedback mechanism between Notch signaling and the pluripotency associated transcription factor SOX2, which shapes divergent cell states, EMT, cancer stem cell features and associates with therapeutic response and escape to targeted therapy. Moreover, we performed and assessed monotherapy and drug combination treatments in Notch-inhibitor sensitive and resistant TNBC xenotransplant and identified combination and second line treatment options which were able to induce tumor control and reduce metastatic burden. Overall design: Comparative analysis of MB157R cells, stably expressing Cas9, infected with the human GeCKO v2 CRISPR libraries and treated with either vehicle or GSI for 14 days.