Molecular mechanisms of RET receptor-mediated oncogenesis in multiple endocrine neoplasia 2

Abstract

Multiple endocrine neoplasia type 2 is an inherited cancer syndrome characterized by tumors of thyroid and adrenal tissues. Germline mutations of the REarranged during Transfection (RET) proto-oncogene, leading to its unregulated activation, are the underlying cause of this disease. Multiple endocrine neoplasia type 2 has been a model in clinical cancer genetics, demonstrating how knowledge of the genetic basis can shape the diagnosis and treatment of the disease. Here, we discuss the nature and effects of the most common recurrent mutations of RET found in multiple endocrine neoplasia type 2. Current understanding of the molecular mechanisms of RET mutations and how they alter the structure and function of the RET protein leading to its aberrant activation, and the effects on RET localization and signaling are described.

Figure 1

The RET receptor: structure, activation, and oncogenic mutations. (A) Schematic diagram depicting RET tyrosine kinase receptor domains and the location of recurrent oncogenic mutations. The RET protein has a large extracellular domain containing a cysteine-rich region and a series of cadherin homology domains, a transmembrane domain, and an intracellular tyrosine kinase domain. The positions of the most common mutations found in patients with multiple endocrine neoplasia type 2 (MEN 2) are shown. (B) Mechanism of RET activation. Wild-type RET activation requires the dimerization of RET, mediated through formation of a multicomponent complex. RET is activated by binding both a soluble ligand (glial cell-line-derived neurotrophic factor; GDNF) and a non-signaling extracellular co-receptor (GDNF family receptor; GFRα). Upon activation of RET, phosphorylation of multiple intracellular tyrosines leads to stimulation of downstream signaling pathways.

Figure 2

Molecular mechanisms of pathogenic RET activation. Schematic diagrams showing mechanisms of RET activation in the presence of various multiple endocrine neoplasia type 2 (MEN 2) mutations. (A) Substitutions of extracellular cysteines lead to formation of intermolecular disulfide bonds and to constitutive RET dimerization and activation. (B) The MEN 2B mutation, M918T (star) in the kinase domain, leads to a conformational change with multiple effects including an increase in RET kinase activity and activation of receptors in either dimeric or monomeric form. (C) Intracellular kinase domain mutations asterisk implicated in familial medullary thyroid carcinoma (FMTC) (e.g. residues 791 and 891), permit activation of monomeric RET, allowing for a partially active conformation.

Figure 3

Structure of the RET tyrosine kinase domain. Ribbon diagrams of the intracellular regions of activated RET, in two orientations, showing the positions of key functional features of the kinase: the ATP binding pocket; the activation or autoinhibitory loop; and the substrate binding pocket. Two orientations of the model, displaying the autoinhibitory/substrate binding face (left) and the ATP-binding face (right), are shown. Amino acid residues that are mutated in patients with multiple endocrine neoplasia type 2 (MEN 2) are represented in the stick form. The three-dimensional representation was based on the crystal structure of the phosphorylated (activated) RET tyrosine kinase domain (residues 709–990).