Functional Characterization of the Novel T599I-VKSRdel BRAF Mutation in a Follicular Variant Papillary Thyroid Carcinoma

Context: Mutations in BRAF are rare in the follicular variant of papillary thyroid carcinoma (FV-PTC). Objective: We identified and functionally characterized a novel T599I-VKSR(600-603) del BRAF mutation in a FV-PTC patient. We analyzed in vitro the effects of this novel mutation in comparison with other thyroid cancer-associated mutations. Design: Expression vectors for the BRAF mutants were generated and their in vitro kinase activity, signaling along the MAPK pathway, and capability of stimulating transcription from an AP1-responsive reporter evaluated. Results: BRAF kinase and signaling were increased to a similar extent by the T599I-VKSR (600-603) del, V600E, and K601E mutations. Instead, the G474R, a mutation previously found in a FV-PTC, knocked down the BRAF kinase and its intracellular signaling. Some cancer-associated low-activity BRAF mutants stimulate the MAPK cascade via CRAF; however, the G474R protein lacked also this property. Conclusion: The T599I-VKSR(600-603) del is a novel gain-of-function mutation that targets BRAF in FV-PTC. Moreover, G474R is the first example of a mutation knocking down enzymatic BRAF activity in a FV-PTC. These findings underscore the importance of functional studies to characterize the role of BRAF mutations associated with thyroid cancer. (J Clin Endocrinol Metab 93: 4398-4402, 2008)