A heterogeneous drug tolerant persister state in BRAF-mutant melanoma is characterized by ion channel dysregulation and susceptibility to ferroptosis

There is increasing interest in cancer cell subpopulations that can withstanding treatment via non-genetic mechanisms, such as tumor cell plasticity and adaptation. These cell populations may be comprised of cells with diverse phenotypes, e.g., quiescent or slow cycling. Such populations have been broadly termed "drug-tolerant persisters" (DTPs) and may be responsible for minimal residual disease following anticancer treatment and acquired resistance. Understanding molecular mechanisms that drive emergence of DTPs could lead to new strategies to improve therapeutic outcomes. Recently, we reported that BRAF-mutant melanoma cells under prolonged BRAF inhibition enter a DTP state with balanced cell death and division, which we termed "idling." Here, we apply single cell barcoding to show that idling DTP populations emerge via cell state transitions, rather than selection of a few pre-existing drug-tolerant clones. Within the time frame of our experiments, DTPs exhibit varying proportions of fast- and slow-cycling cells within each lineage, suggesting that entry into the DTP state is a stochastic process. Furthermore, single-cell transcriptomics and bulk epigenomics reveal common gene expression and ontology signatures in DTP lineages that are consistent with rebalancing of ion channels. Calcium flux experiments uncover a reduction of divalent cation reserves in intracellular organelles, likely leading to endoplasmic reticulum stress. Accordingly, idling DTPs are more prone to ferroptotic cell death, as indicated by increased sensitivity to inhibition of glutathione peroxidase 4 (GPX4), which prevents removal of toxic lipid peroxides. In summary, we propose that ion channel homeostasis is a central process underlying idling DTP emergence in BRAF-mutated melanoma. Future studies will investigate translational aspects of this insight. ### Competing Interest Statement The authors have declared no competing interest.