Treatment of BRAF-mutant melanoma with combined dabrafenib and trametinib, which 
target RAF and the downstream MAP-ERK kinase (MEK)1 and MEK2 kinases, 
respectively, improves progression-free survival and response rates compared 
with dabrafenib monotherapy. Mechanisms of clinical resistance to combined 
RAF/MEK inhibition are unknown. We performed whole-exome sequencing (WES) and 
whole-transcriptome sequencing (RNA-seq) on pretreatment and drug-resistant 
tumors from five patients with acquired resistance to dabrafenib/trametinib. In 
three of these patients, we identified additional mitogen-activated protein 
kinase (MAPK) pathway alterations in the resistant tumor that were not detected 
in the pretreatment tumor, including a novel activating mutation in MEK2 
(MEK2(Q60P)). MEK2(Q60P) conferred resistance to combined RAF/MEK inhibition in 
vitro, but remained sensitive to inhibition of the downstream kinase 
extracellular signal-regulated kinase (ERK). The continued MAPK signaling-based 
resistance identified in these patients suggests that alternative dosing of 
current agents, more potent RAF/MEK inhibitors, and/or inhibition of the 
downstream kinase ERK may be needed for durable control of BRAF-mutant melanoma.