Purinergic Calcium Signaling Driven by Non-Genetic Adaptation to Braf Inhibition as a Novel Mechanism of Erk Reactivation and Drug Tolerance in Braf-Mutant Melanoma

Abstract

There is increasing evidence for cancer cell subpopulations, often termed “drug-tolerant persisters” (DTPs), that can survive drug treatments via non-genetic mechanisms and act as a reservoir from which genetic resistance mutations can emerge. Characterizing and targeting these cells with effective secondary treatments could improve patient outcomes by delaying, or even preventing, acquired drug resistance and relapse. In spite of intensive efforts, its molecular basis remains poorly understood, hampering actionable intervention. This work reports a previously unrecognized signaling mechanism supporting drug tolerance in BRAF-mutant melanoma treated with BRAF inhibitors that is driven by non-genetic adaptation of all clonal lineages via changes in chromatin structure. Its key features are cell-intrinsic intracellular Ca2+ signaling initiated by P2X7 receptors (purinergic ligand-gated cation channels), and an enhanced ability for these Ca2+ signals to reactivate ERK1/2 in the drug-tolerant state. Extracellular ATP, virtually ubiquitous in living systems, is the ligand that can initiate Ca2+ spikes via P2X7 channels. ATP is abundant in the tumor microenvironment and is released by dying cells, ironically implicating treatment-initiated cancer cell death as a source of trophic stimuli that leads to ERK reactivation and drug tolerance. Such a mechanism immediately offers an explanation of the inevitable relapse after BRAFi treatment in BRAF-mutant melanoma, and points to actionable strategies to overcome it.