BRAF^V600E mutation is associated with preferential sensitivity to mitogen-activated protein kinase kinase inhibition in thyroid cancer cell lines

Context: Mutually exclusive mutations of RET, RAS, or BRAF are present in about 70% of papillary thyroid carcinomas, whereas only the latter two are seen in poorly differentiated and anaplastic cancers. Although the signal output common to these oncoproteins is ERK, a recent report showed that only BRAF mutations consistently predicted responsiveness to MAPK kinase (MEK) inhibitors. Objectives: Here we investigated whether sensitivity to MEK inhibition was determined by oncogene status in 13 human thyroid cancer cell lines: four with BRAF mutations, four RAS, one RET/ PTC1, and four wild type. Results: Growth of BRAF (+) cells was inhibited by the MEK antagonist PD0325901 with an IC ₅₀ of less than 5nM. By contrast, RAS, RET/PTC1, or wild-type cells had IC₅₀ of 4 nM to greater than 1000nM. Sensitivity was not predicted by coexisting mutations in PIK3CA or by PTEN status. Similar effects were obtained with the MEK inhibitor AZD6244. PD0325901 induced a sustained G1/S arrest in BRAF (+) but not BRAF (-) lines. PD0325901 was equipotent at inhibiting pERK1/2 after 2 h, regardless of genetic background, but pERK rebounded at 24 h in most lines. MEK inhibitor resistance was associated with partial refractoriness of pERK to further inhibition by the compounds. AZD6244 was more potent at inhibiting growth of NPA (BRAF +) than Cal62 (KRAS +) xenografts. Conclusion: Thyroid cancers with BRAF mutation are preferentially sensitive to MEK inhibitors, whereas tumors with other MEK-ERK effector pathway gene mutations have variable responses, either because they are only partially dependent on ERK and/or because feedback responses elicit partial refractoriness to MEK inhibition.