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Increased in Vivo Phosphorylation of Ret Tyrosine 1062 Is a Potential Pathogenetic Mechanism of Multiple Endocrine Neoplasia Type 2B¹

Centro di Endocrinologia ed Oncologia Sperimentale del CNR, c/o Dipartimento di Biologia e Patologia Cellulare e Molecolare [D. S., R. M. M., C. M., R. V., G. V., M. S.], and Dipartimento di Endocrinologia ed Oncologia Molecolare e Clinica, Facoltà di Medicina e Chirurgia [G. F.], Università di Napoli "Federico II," Naples, Italy; and Dipartimento di Medicina Sperimentale e Clinica, Facoltà di Medicina e Chirurgia di Catanzaro, Università di Catanzaro, Catanzaro [A. F], Italy

Mutations of the Ret receptor tyrosine kinase are responsible for inheritance of multiple endocrine neoplasia (MEN2A and MEN2B) and familial medullary thyroid carcinoma syndromes. Although several familial medullary thyroid carcinoma and most MEN2A mutations involve substitutions of extracellular cysteine residues, in most MEN2B cases there is a methionine-to-threonine substitution at position 918 (M918T) of the Ret kinase domain. The mechanism by which the MEN2B mutation converts Ret into a potent oncogene is poorly understood. Both MEN2A and MEN2B oncoproteins exert constitutive activation of the kinase. However, the highly aggressive MEN2B phenotype is not supported by higher levels of Ret-MEN2B kinase activity compared with Ret-MEN2A. It has been proposed that Ret-MEN2B is more than just an activated Ret kinase and that the M918T mutation, by targeting the kinase domain of Ret, might alter Ret substrate specificity, thus affecting Ret autophosphorylation sites and the ability of Ret to phosphorylate intracellular substrates. We show that the Ret-MEN2B mutation causes specific potentiated phosphorylation of tyrosine 1062 (Y1062) compared with Ret-MEN2A. Phosphorylated Y1062 is part of a Ret multiple effector docking site that mediates recruitment of the Shc adapter and of phosphatidyl-inositol-3 kinase (PI3K). Accordingly, we show that Ret-MEN2B is more active than Ret-MEN2A in associating with Shc and in causing constitutive activation of the Ras/mitogen-activated protein kinase and PI3K/Akt cascades. We conclude that the MEN2B mutation specifically potentiates the ability of Ret to autophosphorylate Y1062 and consequently to couple to the Ras/mitogen-activated protein kinase and the PI3K/Akt pathways. The more efficient triggering of these pathways may account for the difference between MEN2A and MEN2B syndromes.

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