ASCL1 represses a SOX9+ neural-crest-stem-like state in small cell lung cancer

ASCL1 is a neuroendocrine-lineage-specific oncogenic driver of small cell lung cancer (SCLC), highly expressed in a significant fraction of tumors. However, ~25% of human SCLC are ASCL1-low and associated with low-neuroendocrine fate and high MYC expression. Using genetically-engineered mouse models (GEMMs), we show that alterations in Rb1/Trp53/Myc in the mouse lung induce an ASCL1+ state of SCLC in multiple cells of origin. Genetic depletion of ASCL1 in MYC-driven SCLC dramatically inhibits tumor initiation and progression to the NEUROD1+ subtype of SCLC. Surprisingly, ASCL1 loss promotes a SOX9+ mesenchymal/neural-crest-stem-like state and the emergence of bone and more rarely, cartilaginous tumors. ASCL1 is critical for expression of key lineage-related transcription factors NKX2-1, FOXA2, and INSM1, and represses regulators of Hippo, Wnt and Notch developmental pathways in vivo. ASCL1 also represses SOX9 expression in human SCLC cells, suggesting a conserved function for ASCL1. Here, we utilize single-cell RNA sequencing to capture transcriptomic signatures of RPM (Rb1/Trp53/Myc), RPR2 (Rb1/Trp53/Rbl2), and RPMA (Rb1/Trp53/Myc/Ascl1) GEMM tumors to support our conclusion that in MYC-driven SCLC, ASCL1 promotes neuroendocrine fate and represses the emergence of SOX9+ non-endodermal stem-like or mesenchymal fates. These data were integrated with 4 additional invasive RPM tumors from NCBI GEO: GSE149180. Single-cell RNA-sequencing using 10X platform on 1 RPMA tumor initiated with CMV-Cre (general promoter), 1 RPMA tumor initiated with CGRP-Cre (neuroendocrine cell-specific promoter), 1 early RPM tumor initiated with CGRP-Cre, and 1 RPR2 tumor initiated with CGRP-Cre. Tumors were digested with an enzymatic cocktail into single cell suspension and subject to 10X Chromium library preparation and Illumina sequencing targeting ~200M reads per sample.