MEK1 mutations confer resistance to MEK and B-RAF inhibition

Abstract

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 ␣-helix C and showed robust (Ϸ100-fold) resistance to allosteric MEK inhibition. Other mutations affected MEK1 codons located within or abutting the Nterminal 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.BRAF ͉ drug resistance ͉ MAP kinase ͉ melanoma A pproximately one-third of all cancers harbor genetic alterations that aberrantly upregulate mitogen-activated protein kinase (MAPK)-dependent signal transduction (1). In the MAPK pathway, RAS oncoproteins activate RAF, MEK, and ERK kinases to direct key cell proliferative and survival signals. When rendered constitutively active by genetic mutation, the MAP kinase pathway is believed to confer ''oncogene dependency'' (2), an excessive reliance on its dysregulated activity for tumor viability. Therefore, protein kinases within this signaling cascade offer promising targets for novel anticancer therapeutics.In melanoma, uncontrolled MAP kinase pathway activity is nearly ubiquitous and occurs most commonly through gain-offunction mutations involving codon 600 of the B-RAF kinase (3) (BRAF V600E ; 50-70% of cases). Considerable preclinical evidence has associated the BRAF V600E mutation with heightened sensitivity to pharmacologic inhibition of RAF or MEK kinases (4, 5). Although early clinical trials of RAF and MEK inhibitors failed to show a substantial benefit (6, 7), recent phase I studies of selective RAF inhibitors have shown promising results in patients with BRAF-mutant tumors (8, 9). Thus, optimizing therapeutic efficacy while avoiding or bypassing the emergence of resistance to MAP kinase pathway inhibition will likely gain increasing importance in melanoma and other MAP kinasedriven cancers.He...

Targeting the MAPK Signaling Pathway in Cancer: Promising Preclinical Activity with the Novel Selective ERK1/2 Inhibitor BVD-523 (Ulixertinib)

“…MEK1/2 mutations, including MEK1 Q56P , have been implicated in both single-agent BRAF and combination BRAF/MEK targeting therapy-acquired resistance in patients (18,21,(34)(35)(36). Single-agent assays demonstrated that relative to the parental BRAF V600E ::MEK1 wt cells, the double-mutant BRAF V600E ::MEK1 Q56P cells displayed a markedly reduced sensitivity to the BRAF inhibitors vemurafenib and dabrafenib and the MEK inhibitor trametinib (Fig.…”

Isolation of a Novel Thioflavin S–Derived Compound That Inhibits BAG-1–Mediated Protein Interactions and Targets BRAF Inhibitor–Resistant Cell Lines

“…5,24), with recent clinical studies reporting that highly specific BRAF inhibitors are effective in the treatment of metastatic melanoma (25)(26)(27). However, initial promise has been hampered by the development of resistance (28)(29)(30), which is characterized by the reactivation of ERK1/2 (31)(32)(33) and has been attributed to various mechanisms including activating NRAS mutations (29), CRAF overexpression (34), compensatory upregulation of MAP2K kinase COT (28), activating MEK1 mutations (35), and amplification of mutant BRAF (36).…”

Intralesional Treatment of Stage III Metastatic Melanoma Patients with L19–IL2 Results in Sustained Clinical and Systemic Immunologic Responses

“…A long period with local control of disease and absence of distant metastasis implies that patients will not have to undergo toxic treatments for metastatic disease, which may accelerate the emergence of mutations and resistant cancer cells (40)(41)(42)(43)(44)(45).…”