Epigenome-Wide CRISPR-Cas9-Based Knockout Screens on Chemoresistant Cells.

Chemotherapy resistance remains a major challenge in cancer treatment, driven by cancer cells' ability to acquire adaptive properties, rewire signaling pathways, and alter chromatin structure to evade drug-induced cytotoxicity. Because these processes rely heavily on epigenetic mechanisms that regulate chromatin organization and transcriptional plasticity, epigenetic regulators have emerged as key contributors to chemotherapy resistance. To investigate resistance to paclitaxel, one of the most widely used chemotherapeutic agents in triple-negative breast cancer (TNBC), we employed an epigenome-focused knockout library (EPIKOL), a CRISPR-Cas9-based library, designed to systematically disrupt genes involved in chromatin regulation. Chemoresistant cell lines were generated through a stepwise dose-escalation protocol that recapitulates clinically relevant drug adaptation. However, these resistant cells exhibit a multidrug-resistant (MDR) phenotype, posing significant challenges for efficient viral transduction and the selection of stable cell populations. In this study, we describe key methodological steps for achieving high-efficiency lentiviral transduction and selection, enabling the successful application of EPIKOL CRISPR screens in chemoresistant TNBC models. Following the described protocol, an epigenome-wide CRISPR screen was conducted on chemoresistant TNBC cells, and novel epigenetic regulators of chemoresistance were identified. This protocol provides a robust framework for identifying epigenetic regulators that contribute to acquired paclitaxel resistance using a CRISPR-based loss-of-function approach.