Despite the increasing number of effective anti-cancer therapies, successful treatment is limited by the development of drug resistance.,While the contribution of genetic factors to drug resistance is undeniable, little is known about how drug-sensitive cells first evade drug action to proliferate in drug.,Here we track the responses of thousands of single melanoma cells to BRAF inhibitors and show that a subset of cells escapes drug via non-genetic mechanisms within the first three days of treatment.,Cells that escape drug rely on ATF4 stress signalling to cycle periodically in drug, experience DNA replication defects leading to DNA damage, and yet out-proliferate other cells over extended treatment.,Together, our work reveals just how rapidly melanoma cells can adapt to drug treatment, generating a mutagenesis-prone subpopulation that expands over time.,BRAF inhibitors are used to treat late-stage melanoma patients harbouring BRAF mutations.,Here the authors track the responses of single melanoma cells to BRAF inhibitors and show that a subset of cells rapidly escapes drug via non-genetic mechanisms and incurs DNA damage.
RAC1 P29 is the third most commonly mutated codon in human cutaneous melanoma, after BRAF V600 and NRAS Q61.,Here, we study the role of RAC1P29S in melanoma development and reveal that RAC1P29S activates PAK, AKT, and a gene expression program initiated by the SRF/MRTF transcriptional pathway, which results in a melanocytic to mesenchymal phenotypic switch.,Mice with ubiquitous expression of RAC1P29S from the endogenous locus develop lymphoma.,When expressed only in melanocytes, RAC1P29S cooperates with oncogenic BRAF or with NF1-loss to promote tumorigenesis.,RAC1P29S also drives resistance to BRAF inhibitors, which is reversed by SRF/MRTF inhibitors.,These findings establish RAC1P29S as a promoter of melanoma initiation and mediator of therapy resistance, while identifying SRF/MRTF as a potential therapeutic target.,•RAC1P29S activates PAK, AKT, and the SRF/MRTF transcription program in melanocytes•RAC1P29S induces a melanocytic to mesenchymal transition through SRF/MRTF and PAK•RAC1P29S cooperates with BRAF mutation or NF1 deletion to promote melanomagenesis•RAC1P29S induces resistance to BRAF inhibitors through SRF/MRTF,RAC1P29S activates PAK, AKT, and the SRF/MRTF transcription program in melanocytes,RAC1P29S induces a melanocytic to mesenchymal transition through SRF/MRTF and PAK,RAC1P29S cooperates with BRAF mutation or NF1 deletion to promote melanomagenesis,RAC1P29S induces resistance to BRAF inhibitors through SRF/MRTF,RAC1P29S is a common mutation in human cutaneous melanoma.,Lionarons et al. show that RAC1P29S induces a melanocytic to mesenchymal switch via an SRF/MRTF-mediated gene expression program, cooperates with BRAF in melanomagenesis, and drives BRAF inhibitor resistance, which is reversed by SRF/MRTF inhibition.