PROJECT SUMMARY / ABSTRACT The goal of this project is to identify novel molecular determinants of small-cell lung carcinoma (SCLC) preneoplastic development. Relatively little is known about the molecular mechanisms that initiate preneoplasia in this highly aggressive, widely metastatic, and lethal lung cancer. Almost all patients with SCLC are, or were, heavy smokers. Loss-of-function mutations in genes encoding the tumor suppressors TP53 and Rb1 occur in almost all SCLC tumors. Moreover, TP53 and Rb1 have been shown recently to constrain the self-renewal of pulmonary neuroendocrine cells (PNECs), which are a cell of origin for SCLC. Despite the documentation of these and other genetic alterations essential to the molecular pathogenesis of SCLC, the identification of effective therapeutic targets has been limited. Detection of key vulnerabilities unique to SCLC would be a major advance toward eradicating deaths from lung cancer. One potential contributor to the preneoplastic leading to SCLC development is the molecular scaffold Kinase Suppressor of Ras 2 (KSR2). Our analysis reveals that the molecular scaffold KSR2 is undetectable in normal lung tissue but is robustly expressed in PNECs and SCLC cell lines, predominantly those of the most common Achaete-scute complex homolog 1 (ASCL1) subtype. The relevance of this correlation to the development and progression of preneoplastic SCLC lesions is implied by previous observations that (1) ChIP-seq analysis revealed KSR2 as a transcriptional target of ASCL1 in SCLC, and (2) that ASCL1 is essential for the development of normal lung neuroendocrine cells and for the tumor-initiating capacity within SCLC. Similarly, KSR2 knockdown markedly suppresses clonogenicity and self-renewal in highly tumorigenic SCLC subpopulations in vitro and in vivo. These data suggest the hypothesis that KSR2 is essential for self-renewal and long-term propagation of a foundational neuroendocrine population essential to SCLC formation, which will be tested by 1) determining the role of KSR2 in PNEC and SCLC TPC self-renewal and SCLC tumor formation and 2) defining the KSR2- mediated signaling pathways that support SCLC TPC self-renewal.