Rho-mediated MRTF and YAP1 activation contributes to BRAF inhibitor resistance in Sox9High/Sox10Low melanoma cells.

Over half of cutaneous melanoma tumors have BRAFV600E/K mutations. Acquired resistance to BRAF inhibitors (BRAFi) remains a major hurdle in attaining durable therapeutic responses. In this study we demonstrate that approximately 50-60% of melanoma cell lines with acquired vemurafenib resistance activate the RhoA family signaling pathway. RhoA High BRAFi-resistant cells are sensitive to the combination of ROCK inhibitors and vemurafenib. Further, these RhoA High cells have u003e100-fold increase in Sox9 expression and u003e100-fold decrease in Sox10 expression. Two transcriptional co-activators downstream of RhoA, MRTF and YAP1, are activated in Sox9 High /Sox10 Low BRAFi-resistant cells. Pharmacological inhibition of these transcriptional mechanisms re-sensitizes the cells to vemurafenib. Analysis of human BRAFi-resistant tumors reveals that many resistant tumors show gene expression signatures consistent with increased RhoA activation or activation of the transcriptional co-activators MRTF and YAP1. A subset of melanoma tumors in the TCGA dataset with low Sox10 expression also have elevated RhoA, MRTF, and YAP1 activation signatures. Taken together, these results support the concept of targeting RhoA-regulated gene transcription pathways as a promising approach for treating or preventing BRAFi-resistance in melanoma.