Highlights d Multiple SCLC molecular subtypes arise from a neuroendocrine cell of origin d MYC drives the NEUROD1 + and YAP1 + subtypes of SCLC in a temporal evolution d MYC directly activates NOTCH signaling to reprogram neuroendocrine fate d Multiple SCLC molecular subtypes are present within individual human tumors

Small cell lung cancer (SCLC) is a highly aggressive neuroendocrine tumor that is treated clinically as a single disease with poor outcomes. However, SCLC is recently recognized to comprise multiple molecular subsets with unique therapeutic vulnerabilities. Four distinct subtypes of SCLC have been defined based on expression of lineage-related transcription factors: ASCL1, NEUROD1, POU2F3 or YAP1. The origins of these subtypes remain unknown. We use mouse and human SCLC models with a time-series analysis of single-cell transcriptome profiling to reveal that the oncogene MYC drives the dynamic evolution of SCLC subtypes by activation of Notch signaling. MYC cooperates with Notch signaling to promote a temporal shift from an ASCL1-to-NEUROD1-to-YAP1-positive state from a neuroendocrine cell of origin, whereas MYC promotes POU2F3+ tumors from a distinct cell type. SCLC molecular subtypes are therefore not distinct, but rather represent dynamic stages of MYC-driven tumor evolution. Treatment-naive human SCLC exhibits intratumoral heterogeneity in SCLC subtypes, suggesting this dynamic evolution occurs in patient tumors. These findings demonstrate that genetics, cell of origin, and tumor cell plasticity determine SCLC subtype. Given the reported unique therapeutic vulnerabilities of each subtype, we postulate that SCLC tumors represent a “moving therapeutic target” that may require more general, combinatorial, or plasticity-directed therapeutic approaches to combat this transcriptional flexibility. We anticipate that molecular subsets of other cancer types may also represent dynamic stages of tumor evolution. Citation Format: Abbie S. Ireland, Alexi M. Micinski, David W. Kastner, Bingqian Guo, Sarah J. Wait, Kyle B. Spainhower, Christopher C. Conley, Opal S. Chen, Matthew R. Guthrie, Danny Soltero, Yi Qiao, Xiaomeng Huang, Szabolcs Tarapcsak, Siddhartha Devarakonda, Milind D. Chalishazar, Jason Gertz, Justin C. Moser, Gabor Marth, Sonam Puri, Benjamin L. Witt, Benjamin T. Spike, Trudy G. Oliver. MYC drives temporal evolution of small cell lung cancer subtypes by reprogramming neuroendocrine fate [abstract]. In: Proceedings of the AACR Virtual Special Conference on Tumor Heterogeneity: From Single Cells to Clinical Impact; 2020 Sep 17-18. Philadelphia (PA): AACR; Cancer Res 2020;80(21 Suppl):Abstract nr PO-120.