SOX9 drives a stem-like transcriptional state and platinum resistance in high-grade serous ovarian cancer.

Chemotherapy resistance remains a formidable challenge to the treatment of high-grade serous ovarian cancer (HGSOC). The drug-tolerant cells may originate from a small population of inherently resistant cancer stem cells (CSCs) in primary tumors. In contrast, sufficient evidence suggests that drug tolerance can also be transiently acquired by nonstem cancer cells. Regardless of the route, key regulators of this plastic process are poorly understood. Here, we utilized multiomics, tumor microarrays, and epigenetic modulation to demonstrate that SOX9 is a key chemo-induced driver of chemoresistance in HGSOC. Epigenetic upregulation of SOX9 was sufficient to induce chemoresistance in multiple HGSOC lines. Moreover, this upregulation induced the formation of a stem-like subpopulation and significant chemoresistance in vivo. Mechanistically, SOX9 increased transcriptional divergence, reprogramming the transcriptional state of naive cells into a stem-like state. Supporting this, we identified a rare cluster of SOX9-expressing cells in primary tumors that were highly enriched for CSCs and chemoresistance-associated stress gene modules. Notably, single-cell analysis showed that chemo treatment results in rapid population-level induction of SOX9 that enriches for a stem-like transcriptional state. Altogether, these findings implicate SOX9 as a critical regulator of early steps of transcriptional reprogramming that lead to chemoresistance through a CSC-like state in HGSOC.