The RET proto‐oncogene: A molecular therapeutic target in thyroid cancer

Abstract

The RET proto-oncogene is responsible for the development of several human inherited and non-inherited diseases. Germline point mutations were identified in multiple endocrine neoplasia types 2A and 2B, and familial medullary thyroid carcinoma. More than 10 rearranged forms of RET, referred to as RET/PTC 1-9, ELKS/RET and RFP/RET, have been cloned from sporadic and radiation-associated papillary thyroid carcinomas. These mutations induced oncogenic activation of RET tyrosine kinase by different mechanisms. I n 1985, the RET (REarranged during transfection) gene was identified as a novel oncogene activated by DNA rearrangement.(1) The resulting chimeric oncogene encoded a fusion protein, consisting of an amino-terminal half with a putative zinc finger motif, and a carboxyl-terminal half with a tyrosine kinase domain.(2) This fusion resulted from recombination between two unlinked human DNA segments, which occurred during the transfection process.The name RET (RET proto-oncogene) has been retained to designate the gene coding for the tyrosine kinase.(3) The RET proto-oncogene encodes a receptor tyrosine kinase with four cadherin-related motifs and a cysteine-rich region in the extracellular domain, and its ligands are glial cell line-derived neurotrophic factor (GDNF) and related molecules, including neurturin (NRTN), artemin (ARTN) and persephin (PSPN). RET activation by these neurotrophic factors is mediated through a unique multicomponent receptor system, consisting of glycosylphosphatidylinositol-anchored coreceptor (GFRα1-4) as a ligandbinding component and RET tyrosine kinase as a signaling component.(4 -6) Gene knock-out studies have revealed that GDNF/RET signal transduction is essential for the development of the kidney and the enteric nervous system. (7)(8)(9)(10) RET mutations are responsible for the development of several human diseases, including multiple endocrine neoplasia 2A and 2B (MEN 2A and MEN 2B), familial medullary thyroid carcinoma (FMTC), papillary thyroid carcinoma (PTC) and Hirschsprung's disease (HSCR).(11) MEN 2A and MEN 2B share the clinical features of medullary thyroid carcinoma (MTC) and pheochromocytoma, and FMTC is characterized by MTC alone. MEN 2A, MEN 2B and FMTC have been identified to be caused by gain-of-function germline mutations of RET, but somatic rearrangements of RET also lead to intrinsic tyrosine kinase activation in sporadic and radiation-associated PTC. In contrast, loss-of-function mutations of RET are responsible for the development of HSCR, which is a congenital malformation associated with the absence of enteric neurons. (11) Recently, useful therapeutic options for the treatment of neoplastic diseases have been developed, including the use of tyrosine kinase inhibitors, monoclonal antibodies or gene therapy. RET has also emerged as a potent target in some preclinical approaches to the treatment of RET-associated cancer. This artile discusses the mechanisms involved in oncogenic RET signaling and the potential of RET as a therapeutic target. Activation mechanis...

RET ligand-induced internalization and its consequences for downstream signaling

“…The RET proto-oncogene encodes a transmembrane glycoprotein belonging to the receptor tyrosine kinase (RTK) family. RET has been shown to activate cell signaling pathways involved in proliferation, differentiation and migration (Kodama et al, 2005). RET is comprised of an extracellular region which binds its ligands, a transmembrane region and a cytoplasmic kinase domain that is responsible for autophosphorylating intracellular tyrosine residues that interact with adaptor proteins involved in downstream cell signaling.…”

RET(MEN 2B) is active in the endoplasmic reticulum before reaching the cell surface

“…RET has been shown to activate many different signalling pathways, which are dependent on different tyrosine residues in the RET intracellular domain. The activation of AKT and extracellular signal-regulated kinase depends on the phosphorylation of tyrosines 1062 and 1096 in RET, while the activation of signal transducer and activator of transcription 3 (STAT3) may depend on tyrosines 752 and 928 (Kodama et al, 2005). Here, we show that the expression of the 150 kDa precursor form of RET MEN 2B/S32L clearly induces the phosphorylation of not only AKT but also ERK and STAT3 (Figure 4).…”

“…Several attempts have been made to develop methods that would inhibit the activity of oncogenic RET. These methods are based on competitive inhibition using the soluble RET extracellular domain (Cerchia et al, 2003), blocking with inhibitory extracellular antibodies, chemical inhibition or gene transfer (reviewed by Kodama et al, 2005). Part of these inhibitory approaches affect RET from the extracellular side, whereas part of them affect RET from the intracellular side.…”

“…As RET is the signal transducer of four different ligand/co-receptor complexes, it has important physiological functions in different tissues. In spite of its broad expression pattern and vitally important functions during development (Airaksinen and Saarma, 2002), the oncogenic mutant forms of RET cause quite limited clinical symptoms -activating mutations in RET are only responsible for hereditary endocrine cancer syndromes (Kodama et al, 2005). This paradox has not yet been explained, and is likely due to some still uncharacterized activation or inactivation mechanisms of RET.…”

“…Although only a very weak downstream signalling could previously be detected from the RET MEN 2B precursor in the ER, our results are fully in line with the well-documented signalling capacity of RET PTC , which is another oncogenic form of RET. Patients with this type of cancer have a chromosomal re-arrangement in which the intracellular kinase domain of RET in its N-terminus is fused to a soluble protein, which spontaneously forms cytoplasmic dimers and thereby induces an activation of the RET kinase domain (Kodama et al, 2005). RET PTC indeed activates STAT3, ERK and AKT (Hwang et al, 2003;Knauf et al, 2003;Miyagi et al, 2004), although it is a cytoplasmic protein that does not reach the cell surface.…”