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 phosphatidylinositol-3
kinase (PI3K). Accordingly, we show that Ret-MEN2B is more active than Ret-MEN2A in associating with She 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.