Modern therapy of advanced melanoma offers effective targeted therapeutic options in the form of BRAF plus MEK inhibition for patients with BRAF V600 mutations.,For patients lacking these mutations, checkpoint inhibition remains the only first-line choice for treatment of metastatic disease.,However, approximately half of patients do not respond to immunotherapy, requiring effective options for a second-line treatment.,Advances in genetic profiling have found other possible target molecules, especially a wide array of rare non-V600 BRAF mutations which may respond to available targeted therapy.,More information on the characteristics of such mutants is needed to further assess the efficacy of targeted therapies in the metastatic and adjuvant setting of advanced melanoma.,Thus, it may be helpful to classify known BRAF mutations by their kinase activation status and dependence on alternative signaling pathways.,While BRAF V600 mutations appear to have an overall more prominent role of kinase activity for tumor growth, non-V600 BRAF mutations show great differences in kinase activation and, hence, response to BRAF plus MEK inhibition.,When BRAF-mutated melanomas rely on additional signaling molecules such as RAS for tumor growth, greater benefit may be expected from MEK inhibition than BRAF inhibition.,In other cases, mutations of c-kit or NRAS may serve as important pharmacological targets in advanced melanoma.,However, since benefit from currently available targeted therapies for non-V600 mutants is usually inferior regarding response and long-term outcome, checkpoint inhibitors remain the standard recommended first-line therapy for these patients.,Herein, we review the current clinical data for characteristics and response to targeted therapy of melanomas lacking a V600 BRAF mutation.
Activated RAS promotes dimerization of members of the RAF kinase family1-3.,ATP-competitive RAF inhibitors activate ERK signaling4-7 by transactivating RAF dimers4.,In melanomas with mutant BRAF(V600E), levels of RAS activation are low and these drugs bind to BRAF(V600E) monomers and inhibit their activity.,This tumor-specific inhibition of ERK signaling results in a broad therapeutic index and RAF inhibitors have remarkable clinical activity in patients with melanomas that harbor mutant BRAF(V600E)8.,However, resistance invariably develops.,Here, we identify a novel resistance mechanism.,We find that a subset of cells resistant to vemurafenib (PLX4032, RG7204) express a 61kd variant form of BRAF(V600E) that lacks exons 4-8, a region that encompasses the RAS-binding domain. p61BRAF(V600E) exhibits enhanced dimerization in cells with low levels of RAS activation, as compared to full length BRAF(V600E).,In cells in which p61BRAF(V600E) is expressed endogenously or ectopically, ERK signaling is resistant to the RAF inhibitor.,Moreover, a mutation that abolishes the dimerization of p61BRAF(V600E) restores its sensitivity to vemurafenib.,Finally, we identified BRAF(V600E) splicing variants lacking the RAS-binding domain in the tumors of six of 19 patients with acquired resistance to vemurafenib.,These data support the model that inhibition of ERK signaling by RAF inhibitors is dependent on levels of RAS-GTP too low to support RAF dimerization and identify a novel mechanism of acquired resistance in patients: expression of splicing isoforms of BRAF(V600E) that dimerize in a RAS-independent manner.