Highlights
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RAC1P29S activates PAK, AKT, and the SRF/MRTF transcription program in melanocytes
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RAC1P29S induces a melanocytic to mesenchymal transition through SRF/MRTF and PAK
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RAC1P29S cooperates with BRAF mutation or NF1 deletion to promote melanomagenesis
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RAC1P29S induces resistance to BRAF inhibitors through SRF/MRTF
Summary
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.
