Gatekeeper Mutations Mediate Resistance to BRAF-Targeted Therapies

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Abstract
BRAF is a serine-threonine–specific protein kinase that is mutated in 2% of human cancers. Oncogenic BRAF is a validated therapeutic target that constitutively activates mitogen-activated protein kinase kinase (MEK)–extracellular signal–regulated kinase (ERK) signaling, driving tumor cell proliferation and survival. Drugs designed to target BRAF have been developed, but it is difficult to prove that they mediate their antitumor effects by inhibiting BRAF rather than by working through off-target effects. We generated drug-resistant versions of oncogenic BRAF by mutating the gatekeeper residue. Signaling by the mutant proteins was resistant to the small-molecule inhibitor sorafenib, but sorafenib still inhibited the growth of tumors driven by the mutant protein. In contrast, both BRAF signaling and tumor growth were resistant to another RAF drug, PLX4720. These data provide unequivocal evidence that sorafenib mediates its antitumor effects in a manner that is independent of its ability to target oncogenic BRAF, whereas PLX4720 inhibits tumor growth by targeting oncogenic BRAF directly.