Identifying resistance mechanisms and combination therapies in Notch-driven triple negative breast cancer

Triple negative breast cancer (TNBC) is known to be the most aggressive type of breast cancer (BC) and is primarily treated with chemotherapy. Nonetheless, new therapeutic options are urgently needed. Advances in next-generation sequencing have identified alterations in various biomarkers, including NOTCH receptors in a subset of TNBC patients. These findings highlight Notch signaling as a promising therapeutic target, with inhibitors already available. Resistance following chemo- or targeted monotherapies, associated with tumor cell plasticity and intra-tumoral heterogeneity, is a major factor contributing to the failure of these therapies in solid tumors. However, the molecular mechanisms behind therapy-induced tumor cell plasticity and associated resistance remain largely unknown. Using a genome-wide CRISPR-Cas9 screen, we investigated escape mechanisms in Notch-driven TNBC treated with targeted therapy and identified the pluripotency-associated transcription factor SOX2 as a target of resistance to Notch inhibition. Furthermore, we described a novel reciprocal inhibitory feedback mechanism between Notch signaling and SOX2 and identified the molecular mechanisms underlying this relationship. This interaction influences divergent cell states, epithelial to mesenchymal transition (EMT), cancer stem cell (CSC) features, and resistance to targeted therapy. Additionally, we evaluated both monotherapy and drug combinations in Notch-inhibitor sensitive and resistant TNBC xenografts. Moreover, we assessed second-line treatments to overcome resistance in these resistant xenografts. We were able to identify distinct treatment options that effectively control tumor growth and reduce metastatic burden, depending on the expression of NOTCH1 and SOX2 in these tumors. Overall, our findings reveal a new molecular relationship between SOX2 and NOTCH1, offering insights into drug resistance and tumor cell plasticity in Notch-driven TNBC. This work opens avenues for combination and second-line treatments to avoid or overcome therapeutic resistance in TNBC patients stratified based on NOTCH and SOX2.