BRAF Splice Variant Resistance to RAF Inhibitor Requires Enhanced MEK Association

Abstract

Summary: Expression of aberrantly spliced BRAF V600E isoforms (BRAF V600E ΔEx) mediates resistance in 13%–30% of melanoma patients progressing on RAF inhibitors. BRAF V600E ΔEx confers resistance, in part, through enhanced dimerization. Here, we uncoupled BRAF V600E ΔEx dimerization from maintenance of MEK-ERK1/2 signaling. Furthermore, we show BRAF V600E ΔEx association with its substrate, MEK, is enhanced and required for RAF inhibitor resistance. RAF inhibitor treatment increased phosphorylation at serine 729 (S729) in BRAF V600E ΔEx. Mutation of S729 to a non-phosphorylatable residue reduced BRAF V600E ΔEx-MEK interaction, reduced dimerization or oligomerization, and increased RAF inhibitor sensitivity. Conversely, mutation of the BRAF dimerization domain elicited partial effects on MEK association and RAF inhibitor sensitivity. Our data implicate BRAF S729 in resistance to RAF inhibitor and underscore the importance of substrate association with BRAF V600E ΔEx. These findings may provide opportunities to target resistance driven by aberrantly spliced forms of BRAF V600E. : BRAF splice variants represent a common resistance mechanism to FDA-approved RAF inhibitors in melanoma. Through co-IP and functional studies, Vido et al. demonstrate that the phospho-binding site serine 729 mediates enhanced association between splice variants and their substrate, MEK, that is required for resistance to RAF inhibitors. Keywords: BRAF, MEK, melanoma, resistance, serine 729