MYC drives temporal evolution of small cell lung cancer subtypes by reprogramming neuroendocrine fate [Bulk RNA-seq of RPM and RPR2 tumors]

Distinct SCLC molecular subtypes have been defined based on expression of lineage-related transcription factors: ASCL1, NEUROD1, POU2F3 or YAP1, but their origins remain unknown. We perform bulk RNA-sequencing on SCLC tumors from RPM and Rb1/Trp53/Rbl2 (RPR2) GEMMs, initiated by CGRP-Cre, to complement time-series analysis of single-cell transcriptome profiling and reveal that MYC drives the dynamic evolution of SCLC subtypes. MYC promotes a temporal shift from an ASCL1-to-NEUROD1-to-YAP1+ state from a neuroendocrine cell of origin. MYC activates Notch signaling to dedifferentiate tumor cells to non-neuroendocrine fates. Sequenced RPM tumors driven by MycT58A, in comparison to RPR2 tumors associated with high Mycl, have increased intratumoral subtype heterogeneity by bulk-seq, single-cell RNA seq, and IHC compared to RPR2 tumors. In RPM tumors with high MYC, tumors are able to proceed to non-NE subtypes resembling the NEUROD1+ and YAP1+ human SCLC subtypes. These findings support our overall conclusions that genetics, cell of origin, and tumor cell plasticity determine SCLC subtype. Overall design: Bulk RNA-sequencing on 10 invasive tumors from the RPM GEMM and 5 invasive tumors from the RPR2 GEMM all infected by CGRP-Cre to study heterogeneity within RPM tumors and gene expression across models.