Activated RAS promotes dimerization of members of the RAF kinase family1,2,3. ATP-competitive RAF inhibitors activate ERK signalling4,5,6,7 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 tumour-specific inhibition of ERK signalling results in a broad therapeutic index and RAF inhibitors have remarkable clinical activity in patients with melanomas that harbour mutant BRAF(V600E)8. However, resistance invariably develops. Here, we identify a new resistance mechanism. We find that a subset of cells resistant to vemurafenib (PLX4032, RG7204) express a 61-kDa variant form of BRAF(V600E), p61BRAF(V600E), which lacks exons 4–8, a region that encompasses the RAS-binding domain. p61BRAF(V600E) shows 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 signalling 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 tumours of six of nineteen patients with acquired resistance to vemurafenib. These data support the model that inhibition of ERK signalling 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.
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We are grateful to M. Baccarini and C. Pritchard for the RAF knockout mouse embryonic fibroblasts. We would like to thank K. Shokat, C. Zhang, S. Chandarlapaty, C. Pratilas and K. Robzyk for useful discussions, C. Liu for her technical expertise and T. J. Riley. We also thank D. Hucks for technical assistance, P. L. Lyle for scoring the tumour sections, and T. Chodon for assistance with tumour procurement. The ITR is supported by the Vanderbilt-Ingram Cancer Center and the T. J. Martell Foundation. This work has been funded by the National Institutes of Health (NIH; R.S.L., N.R., D.B.S.), the Beene Foundation (D.B.S.), the Melanoma Research Alliance (K.T.F., J.A.S. P.B.C., R.S.L., N.R., D.B.S.) and the STARR Foundation (N.R., D.B.S.). R.S.L. was supported by the Burroughs Wellcome Fund, American Skin Association, Joint Center for Translational Medicine, Sidney Kimmel Foundation, and Stand Up to Cancer. T.M. was supported in part by the Intramural Research Program of the NIH, NCI, Center for Cancer Research and P.B.C. was supported in part by the Danny Federici Melanoma Fund. P.I.P. was supported by T32 CACA062948-15.
J.A.S., P.B.C., N.R. and D.B.S. have participated on an advisory board for Roche. J.A.S. and P.B.C. have received clinical trial research funding from Roche. P.B.C. has received research support and served on an advisory board for Genentech. A.R. has received honorarium serving on the advisory boards of Roche-Genentech. N.R. and D.B.S. have received research funding from Astra-Zeneca.
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Poulikakos, P., Persaud, Y., Janakiraman, M. et al. RAF inhibitor resistance is mediated by dimerization of aberrantly spliced BRAF(V600E). Nature 480, 387–390 (2011). https://doi.org/10.1038/nature10662
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