Mechanisms of melanoma resistance to treatment with BRAF and MEK inhibitors

Abstract

Several mechanisms of resistance to inhibition of BRAF activity in melanoma cells have been described so far. Genetic studies have shown that mutations in MEK1 kinase (MAP kinase kinase), which result in constitutive activation of ERK kinase, result in resistance to treatment. Another mechanism of the acquired BRAF inhibition resistance is the accumulation of activating mutations in the NRAS oncogene, which drives the activation of CRAF. This in turn leads to a permanent activation of the signal transduction to MEK and ERK. Another important mechanism of resistance is the formation of variants of the BRAF V600E gene splicing, including variants that lack exons 4 to 8 containing the RAS-binding domain. The presence of the p61 BRAF V600E variant leads to the constitutive ERK signal, which is resistant to RAF inhibition. In addition, treatment resistance is affected by hyperactivation of tyrosine kinase receptors such as platelet-derived factor receptor β (PDFRβ), insulin-like growth factor 1 receptor (IGF-1R) and erythropoietin-producing hepatocellular receptors (EPH) – leading to the induction of the 3-phosphoinositol kinase pathway (PI3K) in patients treated with BRAF or MEK inhibitors. Another interesting path of BRAFi/MEKi resistance is over-expression of the epidermal growth factor receptor (EGFR) through ne­gative feedback in patients treated with BRAF inhibitors (BRAFi) – EGFR is not normally expressed in untreated melanomas.