A NOVEL, RAS-INDEPENDENT ROLE FOR NF1 IN MICROTUBULE DYNAMICS AND DAMAGE REPAIR DICTATES SENSITIVITY TO T-DM1 IN HER2 POSITIVE BREAST CANCER.

Neurofibromatosis 1 (NF1) is among the first tumor suppressor genes to be cloned. Loss-of-function (LoF) mutations in NF1 are at the basis of the onco-developmental syndrome Neurofibromatosis type 1 and are frequently found at the somatic level across multiple tumors. In breast cancer, NF1 LoF has been implicated in resistance to both endocrine and anti-HER2 agents. This has been attributed to the best characterized activity of NF1, its GTPase-activating protein (GAP) function that attenuates RAS signaling. NF1 encodes Neurofibromin (NF1), a large multifunctional protein consisting of 2,818 amino acids. Because of its large size and difficulty in cloning, NF1 structure has resisted characterization for several years and only recently high-resolution reconstructions by Cryo-EM have been obtained. In early studies, NF1 was found to co-purify with tubulin and to co-localize with microtubules (MTs); two regions in NF1 have been proposed to mediate this interaction. The physiological relevance of this interaction remains uncertain, and NF1 is not generally included among bona fide Microtubule-Associated Proteins (MAPs). Our understanding of microtubule dynamics has evolved in recent years. It is now recognized that dynamic changes do not occur only at the MT end (“dynamic instability”) but also within the lattice, where the mechanical stress induced by motors or other sources generates discontinuities that are continuously repaired by intra-lattice tubulin incorporation and redistribution of the tip-binding protein EB1. To date, only two MAPs, CLASP and SSNA1 have been found to recognize and modulate such MT damage. None of these proteins is commonly mutated in cancer, so the relevance of MT repair for tumorigenesis remains uncertain, despite the established relevance of MT-dependent functions such as mitosis. Here, combining in vitro/in vivo models and protein biochemistry, we show that NF1 loss sensitizes cancer cells to T-DM1, the first approved Antibody-drug conjugate (ADC) in breast cancer, through a novel, RAS-independent function on microtubular dynamics and repair. NF1 exhibits all biochemical properties of a bona fide MAP and specifically enhances intratubular repair. These results define NF1 as a key regulatory factor for microtubule repair and the first ADC payload-associated predictive biomarker identified to date.