This application describes a five year mentored research project designed to transition the applicant to an independent scientist in the field of lung cancer research. This proposal will be conducted at Memorial Sloan Kettering Cancer Center (MSKCC) under the mentorship of Neal Rosen, MD, PhD, a world recognized leader in the field of targeted therapeutics in cancer and feedback regulation of intracellular signaling. Under the mentorship of Dr. Rosen, the principal investigator has previously described the process of tumor adaptation to RAF inhibitors in melanomas harboring a BRAF V600E mutation (Lito et al., Cancer Cell, 2012 and reviewed in Lito et al., Nature Medicine, 2013), or to MEK inhibitors in KRAS mutant cancers (Lito et al., Cancer Cell, 2014). The work in this proposal set out to investigate the mechanism of acquired resistance to RAF inhibitors in patient-derived models of BRAF V600E-mutant lung cancer. Preliminary findings identified an acquired deletion in the stromal antigen 2 gene (STAG2) after treatment with the RAF inhibitor dabrafenib. STAG2 is a component of cohesin, a protein complex that regulates the separation of sister chromatids during anaphase and plays a key role in mitosis. Based on this and other preliminary results, this proposal now aims to determine the role of ERK signaling in the regulation of mitosis in BRAF V600E-mutant lung cancer. We hypothesized that BRAF V600E regulates G2/M signaling in a STAG2 dependent manner and that STAG2 plays a role in the dependence of tumors on BRAF V600E. To test this, we will first determine if BRAF V600E regulates G2/M signaling in a panel of cell lines harboring this mutation. Then, we will determine if STAG2 is required for tumor formation by BRAF V600E in patient-derived models of lung cancer and if STAG2 modulates BRAF V600E-dependent signaling during G2/M. Finally, we will investigate if STAG2 is required for the sensitivity of BRAF V600E tumors to RAF inhibitor treatment and determine the mechanism by which this occurs. These efforts have the potential of identifying novel cellular functions regulated by BRAF V600E and/or cohesin proteins, as well as identifying rational therapeutic interventions that are impactful in the care of patients with lung cancer. Thi is an ideal project to transition the principal investigator towards an independent translational scientist because it builds on his previous experience to establish an independent research program at an institution with the available resources to conduct high impact translational research.