ASCL2 Positive Tumor Cells Modulate the Response of Metastatic Colorectal Cancer to MAPK-Targeting Therapy

Colorectal cancer (CRC) is the second leading cause of cancer related deaths with nearly a quarter of patients presenting with metastatic disease at the time of their diagnosis. Therapeutic management of metastatic CRC continues to be a major challenge due to therapy resistance and disease heterogeneity. Due to its central role in tumorigenesis, CRC is commonly treated with MAPK pathway inhibitors (MAPKi). However, clinical trials repeatedly demonstrates that durability of benefit is short-lived in many patients. While genomic mechanisms of acquired resistance have been described, they explain a minority of patients, and further research is needed to determine the underlying biology limiting MAPKi therapy durability in CRC. In this project, we use a combination of patient-derived CRC models, clinical data, engineered models, molecular barcoding and next generation sequencing to elucidate novel mechanisms contributing to MAPKi limited durability. We show that MAPKi therapy results in the enrichment of stem-programs in patient tumors, and demonstrate ASCL2 expression increases with therapy independent of driver mutation or treatment regimen. Characterization of ASCL2+ phenotype revealed a dynamic, proliferative, stem-like cell state with increased tolerance to MAPKi therapy. Additionally, MAPKi therapy resulted in tumor cell induction of ASCL2+ phenotype resulting in enrichment of ASCL2+ cells. Finally, depletion of the ASCL2+ population improved efficacy and response durability of MAPKi therapy in CRC models. Altogether, this work identifies a novel mechanism of induced tumor cell plasticity which ultimately limits the response durability of MAPKi therapy in CRC. Although several questions remain, these findings will help guide future optimization of MAPKi regimens to prolong therapeutic responses and improve outcomes for patients with metastatic CRC.