COT drives resistance to RAF inhibition through MAP kinase pathway reactivation

Drug-resistance mechanism in melanoma Clinical trials in melanoma patients carrying B-RAF gene mutations have shown promising results with the B-RAF kinase inhibitor PLX4032, but many patients go on to become resistant. Two papers now uncover possible mechanisms for this resistance. Nazarian et al . report that melanomas can acquire resistance due to mutations of N-RAS or increased expression of PDGFRβ , and Johannessen et al . report resistance due to upregulation of MAP3K8/COT. Each of these mechanisms seems to apply to some patients in the recent PLX4032 trial, yet surprisingly, no secondary B-RAF mutations were observed. Recent data from early clinical trials in melanoma patients carrying mutations in the B-RAF gene have shown promising results with the B-RAF kinase inhibitor PLX4032; however, many patients eventually develop resistance to this treatment. Two papers now uncover possible mechanisms of resistance to PLX4032. One paper shows that upregulation of MAP3K8 (which encodes COT) can confer resistance of melanoma cells to B-RAF inhibitors, whereas another paper found that melanomas can acquire resistance due to mutations of N-RAS or increased expression of PDGFRβ. Each of these resistance mechanisms seems to apply to at least some patients on recent PLX4032 trial, whereas, surprisingly, so far no secondary B-RAF mutations have been observed. Oncogenic mutations in the serine/threonine kinase B-RAF (also known as BRAF) are found in 50–70% of malignant melanomas 1 . Pre-clinical studies have demonstrated that the B-RAF(V600E) mutation predicts a dependency on the mitogen-activated protein kinase (MAPK) signalling cascade in melanoma 2 , 3 , 4 , 5 , 6 —an observation that has been validated by the success of RAF and MEK inhibitors in clinical trials 7 , 8 , 9 . However, clinical responses to targeted anticancer therapeutics are frequently confounded by de novo or acquired resistance 10 , 11 , 12 . Identification of resistance mechanisms in a manner that elucidates alternative ‘druggable’ targets may inform effective long-term treatment strategies 13 . Here we expressed ∼600 kinase and kinase-related open reading frames (ORFs) in parallel to interrogate resistance to a selective RAF kinase inhibitor. We identified MAP3K8 (the gene encoding COT/Tpl2) as a MAPK pathway agonist that drives resistance to RAF inhibition in B-RAF(V600E) cell lines. COT activates ERK primarily through MEK-dependent mechanisms that do not require RAF signalling. Moreover, CO