Preexisting MEK1 Exon 3 Mutations in V600E/KBRAF Melanomas Do Not Confer Resistance to BRAF Inhibitors

BRAF inhibitors (BRAFi) induce antitumor responses in nearly 60% of patients with advanced (V600E/K)BRAF melanomas. Somatic activating MEK1 mutations are thought to be rare in melanomas, but their potential concurrence with (V600E/K)BRAF may be selected for by BRAFi. We sequenced MEK1/2 exon 3 in melanomas at baseline and upon disease progression. Of 31 baseline (V600E/K)BRAF melanomas, 5 (16%) carried concurrent somatic BRAF/MEK1 activating mutations. Three of 5 patients with BRAF/MEK1 double-mutant baseline melanomas showed objective tumor responses, consistent with the overall 60% frequency. No MEK1 mutation was found in disease progression melanomas, except when it was already identified at baseline. MEK1-mutant expression in (V600E/K)BRAF melanoma cell lines resulted in no significant alterations in p-ERK1/2 levels or growth-inhibitory sensitivities to BRAFi, MEK1/2 inhibitor (MEKi), or their combination. Thus, activating MEK1 exon 3 mutations identified herein and concurrent with (V600E/K)BRAF do not cause BRAFi resistance in melanoma. Cancer Discov; 2(5); 414-24. (c) 2012 AACR. SIGNIFICANCE: As BRAF inhibitors gain widespread use for treatment of advanced melanoma, biomarkers for drug sensitivity or resistance are urgently needed. We identify here concurrent activating mutations in BRAF and MEK1 in melanomas and show that the presence of a downstream mutation in MEK1 does not necessarily make BRAF-mutant melanomas resistant to BRAF inhibitors.