MEK1 mutations confer resistance to MEK and B-RAF inhibition

Proc Natl Acad Sci U S A. 2009 Dec 1;106(48):20411-6. doi: 10.1073/pnas.0905833106. Epub 2009 Nov 13.


Genetic alterations that activate the mitogen-activated protein kinase (MAP kinase) pathway occur commonly in cancer. For example, the majority of melanomas harbor mutations in the BRAF oncogene, which are predicted to confer enhanced sensitivity to pharmacologic MAP kinase inhibition (e.g., RAF or MEK inhibitors). We investigated the clinical relevance of MEK dependency in melanoma by massively parallel sequencing of resistant clones generated from a MEK1 random mutagenesis screen in vitro, as well as tumors obtained from relapsed patients following treatment with AZD6244, an allosteric MEK inhibitor. Most mutations conferring resistance to MEK inhibition in vitro populated the allosteric drug binding pocket or alpha-helix C and showed robust ( approximately 100-fold) resistance to allosteric MEK inhibition. Other mutations affected MEK1 codons located within or abutting the N-terminal negative regulatory helix (helix A), which also undergo gain-of-function germline mutations in cardio-facio-cutaneous (CFC) syndrome. One such mutation, MEK1(P124L), was identified in a resistant metastatic focus that emerged in a melanoma patient treated with AZD6244. Both MEK1(P124L) and MEK1(Q56P), which disrupts helix A, conferred cross-resistance to PLX4720, a selective B-RAF inhibitor. However, exposing BRAF-mutant melanoma cells to AZD6244 and PLX4720 in combination prevented emergence of resistant clones. These results affirm the importance of MEK dependency in BRAF-mutant melanoma and suggest novel mechanisms of resistance to MEK and B-RAF inhibitors that may have important clinical implications.

Publication types

  • Comparative Study
  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Base Sequence
  • Benzimidazoles / pharmacology*
  • Cell Line, Tumor
  • Drug Resistance, Neoplasm / genetics*
  • Humans
  • MAP Kinase Kinase 1 / antagonists & inhibitors
  • MAP Kinase Kinase 1 / genetics*
  • MAP Kinase Kinase 1 / metabolism
  • Melanoma / genetics*
  • Molecular Sequence Data
  • Mutagenesis
  • Mutation, Missense / genetics
  • Protein Binding / genetics
  • Protein Conformation*
  • Proto-Oncogene Proteins B-raf / genetics*
  • Sequence Analysis, DNA
  • Tumor Stem Cell Assay


  • AZD 6244
  • Benzimidazoles
  • BRAF protein, human
  • Proto-Oncogene Proteins B-raf
  • MAP Kinase Kinase 1
  • MAP2K1 protein, human