NF1 modulates microtubular repair and dictates sensitivity to maytansinoid antibody-drug conjugates in HER2-positive breast cancer

NF1 is a tumor suppressor widely mutated across several cancers. Its best characterized function is the inhibition of RAS signaling through its GTPase-Activating Protein (GAP) domain. Additional functions have been proposed but not deeply investigated, due to its large size and complex domain structure. Here, combining patient data, in vitro/in vivo models and protein biochemistry, we show that NF1 is a novel Microtubule-Associated Protein (MAP) that modulates microtubular dynamics and intra-tubular repair. NF1 loss results in mitotic defects such as prolonged mitosis, supranumerary centrosomes and chromosome missegregation, leading to low-grade aneuploidy in cell lines and patients. Loss of NF1-mediated intratubular repair creates a liability specifically targetable by maytansinoids, a class of microtubule-depolymerizing agents widely used as payloads in Antibody-Drug Conjugates such as Trastuzumab-Emtansine (T-DM1). Our results directly imply loss of intratubular repair as a driver of tumorigenesis, and NF1 as the first ADC payload-associated biomarker.