Increased in vivo phosphorylation of Ret tyrosine 1062 is a potential pathogenetic mechanism of multiple endocrine neoplasia type 2B

Mutations of the Bet receptor tyrosine kinase are responsible for inheritan ce of multiple endocrine neoplasia (MEN2A and MEN2B) and familial medullary thyroid carcinoma syndromes. Although several familial medullary thyroid c arcinoma and most MEN2A mutations involve substitutions of extracellular cy steine residues, in most MEN2B cases there is a methionine-to-threonine sub stitution at position 918 (M918T) of the Bet kinase domain. The mechanism b y which the MEN2B mutation converts Bet into a potent oncogene is poorly un derstood. Both MEN2A and MEN2B oncoproteins exert constitutive activation o f the kinase. However, the highly aggressive MEN2B phenotype is not support ed by higher levels of Ret-MEN2B kinase activity compared with Ret-MEN2A. I t has been proposed that Ret-MEN2B is more than just an activated Ret kinas e and that the M918T mutation, by targeting the kinase domain of Ret, might alter Ret substrate specificity, thus affecting Ret autophosphorylation si tes and the ability of Bet to phosphorylate intracellular substrates. We sh ow that the Ret-MEN2B mutation causes specific potentiated phosphorylation of tyrosine 1062 (Y1062) compared with Ret-MEN2A. Phosphorylated Y1062 is p art of a Ret multiple effector docking site that mediates recruitment of th e Shc adapter and of phosphatidylinositol-3 kinase (PI3K). Accordingly, we show that Ret-MEN2B is more active than Ret-MEN2A in associating with Shc a nd in causing constitutive activation of the Ras/mitogen-activated protein kinase and PI3K/Akt cascades. We conclude that the MEN2R mutation specifica lly potentiates the ability of Ret to autophosphorylate Y1062 and consequen tly to couple to the Ras/mitogen-activated protein kinase and the PI3K/Akt pathways. The more efficient triggering of these pathways may account for t he difference between MEN2A and MEN2B syndromes.