ASCL1 Drives Tolerance to Osimertinib in EGFR Mutant Lung Cancer in Permissive Cellular Contexts
The majority of EGFR mutant lung adenocarcinomas respond well to EGFR tyrosine kinase inhibitors (TKIs). However, most of these responses are partial with drug tolerant residual disease remaining even at the time of maximal response. This residual disease ultimately leads to relapses which eventually develop in most patients. To investigate the cellular and molecular properties of residual tumor cells in vivo, we leveraged patient-derived models of EGFR mutant lung cancer. Subcutaneous EGFR mutant PDXs were treated with the 3rd generation TKI osimertinib until maximal tumor regression. Residual tissue inevitably harbored tumor cells that were transcriptionally, but not mutationally, distinct from bulk pre-treatment tumor. Using single cell transcriptional profiling we found evidence of cells matching the profiles of drug-tolerant cells present in the pre-treatment tumor. In one of the PDXs analyzed, osimertinib treatment caused dramatic transcriptomic changes that featured upregulation of the neuroendocrine lineage transcription factor ASCL1. Mechanistically, we found that ASCL1 confers drug tolerance by initiating an epithelial-to-mesenchymal gene expression program in permissive cellular contexts. Our study reveals fundamental insights into the biology of drug tolerance, the plasticity of cells through TKI treatment and explains why specific phenotypes are only observed in certain tumors.