BRAF Fusions Define a Distinct Molecular Subset of Melanomas with Potential Sensitivity to MEK Inhibition

Purpose: Recurrent "driver" mutations at specific loci in BRAF, NRAS, KIT, GNAQ, and GNA11 define clinically relevant molecular subsets of melanoma, but more than 30% are "pan-negative" for these recurrent mutations. We sought to identify additional potential drivers in "pan-negative" melanoma. Experimental Design: Using a targeted next-generation sequencing (NGS) assay (FoundationOne (TM)) and targeted RNA sequencing, we identified a novel PAPSS1-BRAF fusion in a "pan-negative" melanoma. We then analyzed NGS data from 51 additional melanomas genotyped by FoundationOne (TM), as well as melanoma RNA, whole-genome and whole-exome sequencing data in The CancerGenome Atlas (TCGA), to determine the potential frequency of BRAF fusions in melanoma. We characterized the signaling properties of confirmed molecular alterations by ectopic expression of engineered cDNAs in 293H cells. Results: Activation of the mitogen-activated protein kinase (MAPK) pathway in cells by ectopic expression of PAPSS1-BRAF was abrogated by mitogen-activated protein kinase kinase (MEK) inhibition but not by BRAF inhibition. NGS data analysis of 51 additional melanomas revealed a second BRAF fusion (TRIM24-BRAF) in a "pan-negative" sample; MAPK signaling induced by TRIM24-BRAF was also MEK inhibitor sensitive. Through mining TCGA skin cutaneous melanoma dataset, we further identified two potential BRAF fusions in another 49 "pan-negative" cases. Conclusions: BRAF fusions define a new molecular subset of melanoma, potentially comprising 4% to 8% of "pan-negative" cases. Their presence may explain an unexpected clinical response to MEK inhibitor therapy or assist in selecting patients for MEK-directed therapy.