Oncogenic RET receptors display different autophosphorylation sites and substrate binding specificities

Thyroid C-Cell Biology and Oncogenic Transformation

The thyroid parafollicular cell, or commonly named "C-cell," functions in serum calcium homeostasis. Elevations in serum calcium trigger release of calcitonin from the C-cell, which in turn functions to inhibit absorption of calcium by the intestine, resorption of bone by the osteoclast, and reabsorption of calcium by renal tubular cells. Oncogenic transformation of the thyroid C-cell is thought to progress through a hyperplastic process prior to malignancy with increasing levels of serum calcitonin serving as a biomarker for tumor burden. The discovery that Multiple Endocrine Neoplasia, type 2 is caused by activating mutations of the RET gene serves to highlight the RET-RAS-MAPK signaling pathway in both initiation and progression of medullary thyroid carcinoma. Thyroid C-cells are known to express RET at high levels relative to most cell types, therefore aberrant activation of this receptor is targeted primarily to the C-cell, providing one possible cause of tissue-specific oncogenesis. The role of RET signaling in normal C-cell function is unknown though calcitonin gene transcription appears to be sensitive to RET activation. Beyond RET the modeling of oncogenesis in animals and screening of human tumors for candidate gene mutations has uncovered mutation of RAS family members and inactivation of RB1 regulatory pathway as potential mediators of C-cell transformation. More recently, the integration of multiple biological layers of omics studies has uncovered new pathways of oncogenesis. A growing understanding of how RET interacts with these pathways, both in normal C-cell function and during oncogenic transformation, will help in the development of novel molecular targeted therapies.

Non-Small-Cell Lung Cancers (NSCLCs) Harboring RET Gene Fusion, from Their Discovery to the Advent of New Selective Potent RET Inhibitors: "Shadows and Fogs"

RET fusions are relatively rare in Non-Small-Cell Lung Cancers (NSCLCs), being around 1-2% of all NSCLCs. They share the same clinical features as the other fusion-driven NSCLC patients, as follows: younger age, adenocarcinoma histology, low exposure to tobacco, and high risk of spreading to the brain. Chemotherapy and immunotherapy have a low impact on the prognosis of these patients. Multitargeted RET inhibitors have shown modest activity jeopardized by high toxicity. New potent and selective RET inhibitors (RET-Is) (pralsetinib and selpercatinib) have achieved a higher efficacy minimizing the known toxicities of the multitargeted agents. This review will describe the sensitivity of immune-checkpoint inhibitors (ICIs) in RET fusion + NSCLC patients, as well their experiences with the 'old' multi-targeted RET inhibitors. This review will focus on the advent of new potent and selective RET-Is. We will describe their efficacy as well as the main mechanisms of resistance to them. We will further proceed to deal with the new drugs and strategies proposed to overcome the resistance to RET-Is. In the last section, we will also focus on the safety profile of RET-Is, dealing with the main toxicities as well as the rare but severe adverse events.

Therapeutic advances of targeting receptor tyrosine kinases in cancer

Receptor tyrosine kinases (RTKs), a category of transmembrane receptors, have gained significant clinical attention in oncology due to their central role in cancer pathogenesis. Genetic alterations, including mutations, amplifications, and overexpression of certain RTKs, are critical in creating environments conducive to tumor development. Following their discovery, extensive research has revealed how RTK dysregulation contributes to oncogenesis, with many cancer subtypes showing dependency on aberrant RTK signaling for their proliferation, survival and progression. These findings paved the way for targeted therapies that aim to inhibit crucial biological pathways in cancer. As a result, RTKs have emerged as primary targets in anticancer therapeutic development. Over the past two decades, this has led to the synthesis and clinical validation of numerous small molecule tyrosine kinase inhibitors (TKIs), now effectively utilized in treating various cancer types. In this manuscript we aim to provide a comprehensive understanding of the RTKs in the context of cancer. We explored the various alterations and overexpression of specific receptors across different malignancies, with special attention dedicated to the examination of current RTK inhibitors, highlighting their role as potential targeted therapies. By integrating the latest research findings and clinical evidence, we seek to elucidate the pivotal role of RTKs in cancer biology and the therapeutic efficacy of RTK inhibition with promising treatment outcomes.

Personalized Medicine in Medullary Thyroid Carcinoma: A Broad Review of Emerging Treatments

Medullary thyroid carcinoma (MTC) arises from parafollicular cells in the thyroid gland, and although rare, it represents an aggressive type of thyroid cancer. MTC is recognized for its low mutational burden, with point mutations in RET or RAS genes being the most common oncogenic events. MTC can be resistant to cytotoxic chemotherapy, and multitarget kinase inhibitors (MKIs) have been considered a treatment option. They act by inhibiting the activities of specific tyrosine kinase receptors involved in tumor growth and angiogenesis. Several tyrosine kinase inhibitors are approved in the treatment of advanced MTC, including vandetanib and cabozantinib. However, due to the significant number of adverse events, debatable efficiency and resistance, there is a need for novel RET-specific TKIs. Newer RET-specific TKIs are expected to overcome previous limitations and improve patient outcomes. Herein, we aim to review MTC signaling pathways, the most recent options for treatment and the applications for personalized medicine.

RET aberrant cancers and RET inhibitor therapies: Current state-of-the-art and future perspectives

Precision oncology informed by genomic information has evolved in leaps and bounds over the last decade. Although non-small cell lung cancer (NSCLC) has moved to center-stage as the poster child of precision oncology, multiple targetable genomic alterations have been identified in various cancer types. RET alterations occur in roughly 2% of all human cancers. The role of RET as oncogenic driver was initially identified in 1985 after the discovery that transfection with human lymphoma DNA transforms NIH-3T3 fibroblasts. Germline RET mutations are causative of multiple endocrine neoplasia type 2 syndrome, and RET fusions are found in 10-20% of papillary thyroid cases and are detected in most patients with advanced sporadic medullary thyroid cancer. RET fusions are oncogenic drivers in 2% of Non-small cell lung cancer. Rapid translation and regulatory approval of selective RET inhibitors, selpercatinib and pralsetinib, have opened up the field of RET precision oncology. This review provides an update on RET precision oncology from bench to bedside and back. We explore the impact of selective RET inhibitor in patients with advanced NSCLC, thyroid cancer, and other cancers in a tissue-agnostic fashion, resistance mechanisms, and future directions.

RET Proto-Oncogene-Not Such an Obvious Starting Point in Cancer Therapy

Mutations and fusions of RET (rearranged during transfection) gene are detected in a few common types of tumors including thyroid or non-small cells lung cancers. Multiple kinase inhibitors (MKIs) do not show spectacular effectiveness in patients with RET-altered tumors. Hence, recently, two novel RET-specific inhibitors were registered in the US and in Europe. Selpercatinib and pralsetinib showed high efficacy in clinical trials, with fewer adverse effects, in comparison to previously used MKIs. However, the effectiveness of these new drugs may be reduced by the emergence of resistance mutations in RET gene and activation of different activating signaling pathways. This review presents the function of the normal RET receptor, types of molecular disturbances of the RET gene in patients with various cancers, methods of detecting these abnormalities, and the effectiveness of modern anticancer therapies (ranging from immunotherapies, through MKIs, to RET-specific inhibitors).

RET signaling pathway and RET inhibitors in human cancer

Rearranged during transfection (RET) receptor tyrosine kinase was first identified over thirty years ago as a novel transforming gene. Since its discovery and subsequent pathway characterization, RET alterations have been identified in numerous cancer types and are most prevalent in thyroid carcinomas and non-small cell lung cancer (NSCLC). In other tumor types such as breast cancer and salivary gland carcinomas, RET alterations can be found at lower frequencies. Aberrant RET activity is associated with poor prognosis of thyroid and lung carcinoma patients, and is strongly correlated with increased risk of distant metastases. RET aberrations encompass a variety of genomic or proteomic alterations, most of which confer constitutive activation of RET. Activating RET alterations, such as point mutations or gene fusions, enhance activity of signaling pathways downstream of RET, namely PI3K/AKT, RAS/RAF, MAPK, and PLCγ pathways, to promote cell proliferation, growth, and survival. Given the important role that mutant RET plays in metastatic cancers, significant efforts have been made in developing inhibitors against RET kinase activity. These efforts have led to FDA approval of Selpercatinib and Pralsetinib for NSCLC, as well as, additional selective RET inhibitors in preclinical and clinical testing. This review covers the current biological understanding of RET signaling, the impact of RET hyperactivity on tumor progression in multiple tumor types, and RET inhibitors with promising preclinical and clinical efficacy.

Predictive molecular pathology in metastatic thyroid cancer: the role of RET fusions

BACKGROUND

Rearranged during transfection (RET) gene fusions are detected in 10-20% of thyroid cancer patients. Recently, RET fusion-positive metastatic thyroid cancers have attracted much attention owing to the FDA approval of two highly selective anti-RET tyrosine kinase inhibitors, namely, selpercatinib, and pralsetinib.

AREAS COVERED

This review summarizes the available evidence on the biological and predictive role of RET gene fusions in thyroid carcinoma patients and the latest screening assays currently used to detect these genomic alterations in histological and cytological specimens.

EXPERT OPINION

Management of advanced thyroid carcinoma has significantly evolved over the last decade thanks to the approval of three multikinase inhibitors, i.e. sorafenib, lenvatinib, cabozantinib, and of two selective RET-tyrosine inhibitors, i.e. selpercatinib and pralsetinib. In this setting, the detection of RET-fusions in advanced thyroid cancer specimens through the use of next-generation sequencing has become a commonly used strategy in clinical practice to select the best treatment options.

Discovery of N-Trisubstituted Pyrimidine Derivatives as Type I RET and RET Gatekeeper Mutant Inhibitors with a Novel Kinase Binding Pose

Mutations of the rearranged during transfection (RET) kinase are frequently reported in cancer, which make it as an attractive therapeutic target. Herein, we discovered a series of N-trisubstituted pyrimidine derivatives as potent inhibitors for both wild-type (wt) RET and RETV804M, which is a resistant mutant for several FDA-approved inhibitors. The X-ray structure of a representative inhibitor with RET revealed that the compound binds in a unique pose that bifurcates beneath the P-loop and confirmed the compound as a type I inhibitor. Through the structure-activity relationship (SAR) study, compound 20 was identified as a lead compound, showing potent inhibition of both RET and RETV804M. Additionally, compound 20 displayed potent antiproliferative activity of CCDC6-RET-driven LC-2/ad cells. Analysis of RET phosphorylation indicated that biological activity was mediated by RET inhibition. Collectively, N-trisubstituted pyrimidine derivatives could serve as scaffolds for the discovery and development of potent inhibitors of type I RET and its gatekeeper mutant for the treatment of RET-driven cancers.

OLFM4-RET fusion is an oncogenic driver in small intestine adenocarcinoma

Small intestine adenocarcinoma is a rare intestinal malignancy with distinct clinical, pathological, and molecular characteristics. Recently, a fusion of the intestinal stem-cell marker olfactomedin 4 (OLFM4) and the proto-oncogene RET has been identified in a small intestine adenocarcinoma patient. Here we investigated the biological effects of OLFM4-RET fusion and whether it can initiate tumorigenesis in small intestine. OLFM4 expression was found to be frequently lost or reduced in human small intestine adenocarcinoma, and its downregulation correlated with high tumor grade and advanced tumor stage. Expression of OLFM4-RET fusion-induced cellular transformation in HEK293 cells and blocked RET-induced inhibition of colony growth in HuTu 80 small intestine adenocarcinoma cells. Further, expression of OLFM4-RET activated the RAS-RAF-MAPK and STAT3 cell signaling pathways in both HEK293 cells and HuTu 80 cells. OLFM4-RET expression in HEK293 cells upregulated multiple families of genes related to carcinogenesis, cancer progression, and metastasis. Targeted expression of OLFM4-RET in the small intestine led to the development of hyperplasia, adenoma, or adenocarcinoma in transgenic mice. Our study suggests that OLFM4-RET is an oncogenic driver of small intestine tumorigenesis. Therefore, the small intestine adenocarcinoma patients with OLFM4-RET fusion may benefit from treatment with RET kinase inhibitor.

Precision therapy for RET-altered cancers with RET inhibitors

Rearranged during transfection (RET) is involved in the physiological development of some organ systems. Activating RET alterations via either gene fusions or point mutations are potent oncogenic drivers in non-small cell lung cancer, thyroid cancer, and in multiple diverse cancers. RET-altered cancers were initially treated with multikinase inhibitors (MKIs). The efficacy of MKIs was modest at the expense of notable toxicities from their off-target activity. Recently, highly potent and RET-specific inhibitors selpercatinib and pralsetinib were successfully translated to the clinic and FDA approved. We summarize the current state-of-the-art therapeutics with preclinical and clinical insights of these novel RET inhibitors, acquired resistance mechanisms, and future outlooks.

Targeting Rearranged during Transfection in Cancer: A Perspective on Small-Molecule Inhibitors and Their Clinical Development

Rearranged during transfection (RET) is a receptor tyrosine kinase essential for the normal development and maturation of a diverse range of tissues. Aberrant RET signaling in cancers, due to RET mutations, gene fusions, and overexpression, results in the activation of downstream pathways promoting survival, growth, and metastasis. Pharmacological manipulation of RET is effective in treating RET-driven cancers, and efforts toward developing RET-specific therapies have increased over the last 5 years. In 2020, RET-selective inhibitors pralsetinib and selpercatinib achieved clinical approval, which marked the first approvals for kinase inhibitors specifically developed to target the RET oncoprotein. This Perspective discusses current development and clinical applications for RET precision medicine by providing an overview of the incremental improvement of kinase inhibitors for use in RET-driven malignancies.

State-of-the-Art Strategies for Targeting RET-Dependent Cancers

Activating receptor tyrosine kinase RET (rarranged during transfection) gene alterations have been identified as oncogenic in multiple malignancies. RET gene rearrangements retaining the kinase domain are oncogenic drivers in papillary thyroid cancer, non-small-cell lung cancer, and multiple other cancers. Activating RET mutations are associated with different phenotypes of multiple endocrine neoplasia type 2 as well as sporadic medullary thyroid cancer. RET is thus an attractive therapeutic target in patients with oncogenic RET alterations. Multikinase inhibitors with RET inhibitor activity, such as cabozantinib and vandetanib, have been explored in the clinic for tumors with activating RET gene alterations with modest clinical efficacy. As a result of the nonselective nature of these multikinase inhibitors, patients had off-target adverse effects, such as hypertension, rash, and diarrhea. This resulted in a narrow therapeutic index of these drugs, limiting ability to dose for clinically effective RET inhibition. In contrast, the recent discovery and clinical validation of highly potent selective RET inhibitors (pralsetinib, selpercatinib) demonstrating improved efficacy and a more favorable toxicity profile are poised to alter the landscape of RET-dependent cancers. These drugs appear to have broad activity across tumors with activating RET alterations. The mechanisms of resistance to these next-generation highly selective RET inhibitors is an area of active research. This review summarizes the current understanding of RET alterations and the state-of-the-art treatment strategies in RET-dependent cancers.

Structure and function of RET in multiple endocrine neoplasia type 2

It has been twenty-five years since the discovery of oncogenic germline RET mutations as the cause of multiple endocrine neoplasia type 2 (MEN2). Intensive work over the last two and a half decades on RET genetics, signaling and cell biology has provided the current bases for the genotype-phenotype and functional correlations within this cancer syndrome. On the contrary, the structural and molecular basis for RET tyrosine kinase domain activation and oncogenic deregulation has remained largely elusive. Recent studies with a strong crystallographic and biochemical focus have started to elucidate key insights into such molecular and atomic details revealing unexpected and private mechanisms of actions and molecular determinants not previously envisioned. This review focuses on the structure and function of the RET receptor, and in particular, on what a more detailed view of the protein itself and what the current structural and molecular information tell us about the genotype and phenotype relationships in the cancer syndrome MEN2.

Novel targeted therapeutics for MEN2

The rearranged during transfection (RET) proto-oncogene was recognized as the multiple endocrine neoplasia type 2 (MEN2) causing gene in 1993. Since then, much effort has been put into a clear understanding of its oncogenic signaling, its biochemical function and ways to block its aberrant activation in MEN2 and related cancers. Several small molecules have been designed, developed or redirected as RET inhibitors for the treatment of MEN2 and sporadic MTC. However, current drugs are mostly active against several other kinases, as they were not originally developed for RET. This limits efficacy and poses safety issues. Therefore, there is still much to do to improve targeted MEN2 treatments. New, more potent and selective molecules, or combinatorial strategies may lead to more effective therapies in the near future. Here, we review the rationale for RET targeting in MEN2, the use of currently available drugs and novel preclinical and clinical RET inhibitor candidates.

Role of RET protein-tyrosine kinase inhibitors in the treatment RET-driven thyroid and lung cancers

RET is a transmembrane receptor protein-tyrosine kinase that is required for the development of the nervous system and several other tissues. The mechanism of activation of RET by its glial-cell derived neurotrophic factor (GDNF) ligands differs from that of all other receptor protein-tyrosine kinases owing to the requirement for additional GDNF family receptor-α (GFRα) co-receptors (GFRα1/2/3/4). RET point mutations have been reported in multiple endocrine neoplasia (MEN2A, MEN2B) and medullary thyroid carcinoma. In contrast, RET fusion proteins have been reported in papillary thyroid and non-small cell lung adenocarcinomas. More than a dozen fusion partners of RET have been described in papillary thyroid carcinomas, most frequently CCDC6-RET and NCOA4-RET. RET-fusion proteins, commonly KIF5B-RET, have also been found in non-small cell lung cancer (NSCLC). Several drugs targeting RET have been approved by the FDA for the treatment of cancer: (i) cabozantinib and vandetanib for medullary thyroid carcinomas and (ii) lenvatinib and sorafenib for differentiated thyroid cancers. In addition, alectinib and sunitinib are approved for the treatment of other neoplasms. Each of these drugs is a multikinase inhibitor that has activity against RET. Previous X-ray studies indicated that vandetanib binds within the ATP-binding pocket and forms a hydrogen bond with A807 within the RET hinge and it makes hydrophobic contact with L881 of the catalytic spine which occurs in the floor of the adenine-binding pocket. Our molecular modeling studies indicate that the other antagonists bind in a similar fashion. All of these antagonists bind to the active conformation of RET and are therefore classified as type I inhibitors. The drugs also make variable contacts with other residues of the regulatory and catalytic spines. None of these drugs was designed to bind preferentially to RET and it is hypothesized that RET-specific antagonists might produce even better clinical outcomes. Currently the number of new cases of neoplasms bearing RET mutations or RET-fusion proteins is estimated to be about 10,000 per year in the United States. This is about the same as the incidence of chronic myelogenous leukemia for which imatinib and second and third generation BCR-Abl non-receptor protein-tyrosine kinase antagonists have proven clinically efficacious and which are commercially successful. These findings warrant the continued development of specific antagonists targeting RET-driven neoplasms.

HSP90 inhibition blocks ERBB3 and RET phosphorylation in myxoid/round cell liposarcoma and causes massive cell death in vitro and in vivo

Myxoid sarcoma (MLS) is one of the most common types of malignant soft tissue tumors. MLS is characterized by the FUS-DDIT3 or EWSR1-DDIT3 fusion oncogenes that encode abnormal transcription factors. The receptor tyrosine kinase (RTK) encoding RET was previously identified as a putative downstream target gene to FUS-DDIT3 and here we show that cultured MLS cells expressed phosphorylated RET together with its ligand Persephin. Treatment with RET specific kinase inhibitor Vandetanib failed to reduce RET phosphorylation and inhibit cell growth, suggesting that other RTKs may phosphorylate RET. A screening pointed out EGFR and ERBB3 as the strongest expressed phosphorylated RTKs in MLS cells. We show that ERBB3 formed nuclear and cytoplasmic complexes with RET and both RTKs were previously reported to form complexes with EGFR. The formation of RTK hetero complexes could explain the observed Vandetanib resistence in MLS. EGFR and ERBB3 are clients of HSP90 that help complex formation and RTK activation. Treatment of cultured MLS cells with HSP90 inhibitor 17-DMAG, caused loss of RET and ERBB3 phosphorylation and lead to rapid cell death. Treatment of MLS xenograft carrying Nude mice resulted in massive necrosis, rupture of capillaries and hemorrhages in tumor tissues. We conclude that complex formation between RET and other RTKs may cause RTK inhibitor resistance. HSP90 inhibitors can overcome this resistance and are thus promising drugs for treatment of MLS/RCLS.

Pediatric Medullary Thyroid Carcinoma

Medullary thyroid carcinoma (MTC), which originates from thyroid parafollicular C cells, accounts for 3 to 5% of thyroid malignancies. MTC occurs either sporadically or in an inherited autosomal dominant manner. Hereditary MTC occurs as a familial MTC or as a part of multiple endocrine neoplasia (MEN) type 2A and B syndromes. A strong genotype-phenotype correlation has been observed between hereditary MTC and germ-line "gain of function" mutations of the RET proto-oncogene. Most cases of pediatric MTC are hereditary whereas sporadic MTC is rare in children and is usually diagnosed in adults. Therefore, MTC in children is most often diagnosed in the course of a familial genetic investigation. The standard treatment of MTC mainly requires surgery involving total thyroidectomy and central neck node dissection before extrathyroidal extension occurs. To prevent MTC development in hereditary syndromes, prophylactic thyroidectomy is performed in presymptomatic patients. An appropriate age at which the surgery should take place is determined based upon the data from genotyping, serum calcitonin measurements, and ultrasonography. For the treatment of advanced MTC cases, the broad spectrum receptor tyrosine kinase inhibitors vandetanib and cabozantinib, which also inhibit RET, are used although they are not always effective.

Thyroid C-Cell Biology and Oncogenic Transformation

The thyroid parafollicular cell, or commonly named "C-cell," functions in serum calcium homeostasis. Elevations in serum calcium trigger release of calcitonin from the C-cell, which in turn functions to inhibit absorption of calcium by the intestine, resorption of bone by the osteoclast, and reabsorption of calcium by renal tubular cells. Oncogenic transformation of the thyroid C-cell is thought to progress through a hyperplastic process prior to malignancy with increasing levels of serum calcitonin serving as a biomarker for tumor burden. The discovery that multiple endocrine neoplasia type 2 is caused by activating mutations of the RET gene serves to highlight the RET-RAS-MAPK signaling pathway in both initiation and progression of medullary thyroid carcinoma (MTC). Thyroid C-cells are known to express RET at high levels relative to most cell types; therefore, aberrant activation of this receptor is targeted primarily to the C-cell, providing one possible cause of tissue-specific oncogenesis. The role of RET signaling in normal C-cell function is unknown though calcitonin gene transcription appears to be sensitive to RET activation. Beyond RET, the modeling of oncogenesis in animals and screening of human tumors for candidate gene mutations have uncovered mutation of RAS family members and inactivation of Rb1 regulatory pathway as potential mediators of C-cell transformation. A growing understanding of how RET interacts with these pathways, both in normal C-cell function and during oncogenic transformation, will help in the development of novel molecular-targeted therapies.

Mechanisms of RET signaling in cancer: current and future implications for targeted therapy

De-regulation of RET signaling by oncogenic mutation, gene rearrangement, overexpression or transcriptional up-regulation is implicated in several human cancers of neuroendocrine and epithelial origin (thyroid, breast, lung). Understanding how RET signaling mechanisms associated with these oncogenic events are deregulated, and their impact in the biological processes driving tumor formation and progression, as well as response to treatment, will be crucial to find and develop better targeted therapeutic strategies. In this review we emphasie the distinct mechanisms of RET signaling in cancer and summarise current knowledge on small molecule inhibitors targeting the tyrosine kinase domain of RET as therapeutic drugs in RET-positive cancers.

Central role of RET in thyroid cancer

RET (rearranged during transfection) is a receptor tyrosine kinase involved in the development of neural crest derived cell lineages, kidney, and male germ cells. Different human cancers, including papillary and medullary thyroid carcinomas, lung adenocarcinomas, and myeloproliferative disorders display gain-of-function mutations in RET. Accordingly, RET protein has become a promising molecular target for cancer treatment.

RET inhibition: implications in cancer therapy

INTRODUCTION

The RET gene encodes a receptor tyrosine kinase essential for ontogenesis of the enteric nervous system and kidney. Following identification of RET, it was found that somatic rearrangements of this gene, conventionally designated as RET/PTC, are frequently present in papillary thyroid carcinoma. Subsequently, activating germ line point mutations of RET were identified as being responsible for the hereditary medullary thyroid carcinoma syndromes MEN2A, MEN2B and FMTC. RET rearrangements have recently been identified in a small fraction of lung adenocarcinomas.

AREA COVERED

The authors review the current field concerning the RET gene and protein, its involvement in cancer and the preclinical and clinical studies which highlight its role as a potentially important therapeutic target for several cancers.

EXPERT OPINION

Many multitargeted inhibitors which crossreact with RET have been developed and investigated in clinical trials targeting many cancer indications. In particular, VEGFR/PDGFR inhibitors, widely explored as antiangiogenics, have been intensively studied in thyroid carcinoma patients. Notwithstanding the efficacy observed with such agents, their common clinical activity in thyroid carcinoma is of short duration and includes frequent and severe side effects, limiting their therapeutic action. These findings are discussed and the need for improved, more specific RET-targeting drugs is highlighted.

Structure and physiology of the RET receptor tyrosine kinase

The identification of the ret oncogene by Masahide Takahashi and Geoffrey Cooper in 1985 was both serendipitous and paradigmatic ( Takahashi et al. 1985). By transfecting total DNA from a human lymphoma into mouse NIH3T3 cells, they obtained one clone, which in secondary transformants yielded more than 100-fold improvement in transformation efficiency. Subsequent investigations revealed that the ret oncogene was not present as such in the primary lymphoma, but was derived by DNA rearrangement during transfection from normal human sequences of the ret locus. At the time, activation by DNA rearrangement had not been previously described for a transforming gene with the NIH3T3 transfection assay. The discovery of ret opened a field of study that has had a profound impact in cancer research, developmental biology, and neuroscience, and that continues to yield surprises and important insights to this day.

Multiple functional effects of RET kinase domain sequence variants in Hirschsprung disease

The REarranged during Transfection (RET) gene encodes a receptor tyrosine kinase required for maturation of the enteric nervous system. RET sequence variants occur in the congenital abnormality Hirschsprung disease (HSCR), characterized by absence of ganglia in the intestinal tract. Although HSCR-RET variants are predicted to inactivate RET, the molecular mechanisms of these events are not well characterized. Using structure-based models of RET, we predicted the molecular consequences of 23 HSCR-associated missense variants and how they lead to receptor dysfunction. We validated our predictions in biochemical and cell-based assays to explore mutational effects on RET protein functions. We found a minority of HSCR-RET variants abrogated RET kinase function, while the remaining mutants were phosphorylated and transduced intracellular signals. HSCR-RET sequence variants also impacted on maturation, stability, and degradation of RET proteins. We showed that each variant conferred a unique combination of effects that together impaired RET protein activity. However, all tested variants impaired RET-mediated cellular functions, including cell transformation and migration. Our data indicate that the molecular mechanisms of impaired RET function in HSCR are highly variable. Although a subset of variants cause loss of RET kinase activity and downstream signaling, enzymatic inactivation is not the sole mechanism at play in HSCR.

Celecoxib, a cyclooxygenase-2 inhibitor, potentiates the chemotherapic effect of vinorelbine in the medullary thyroid cancer TT cell line

We analyzed the in vitro effects of celecoxib, a COX-2 inhibitor, and determined if celecoxib can sensitize a human MTC-derived cell line (TT) to chemotherapeutics. We found that celecoxib induced apoptosis in TT cells and decreased drug efflux by reducing the expression of MDR-1 mRNA, which codes for the drug efflux pump P-gp. We also observed that TT cells were 10-fold more resistant to doxorubicin than to vinorelbine, mimicking what can be observed in clinical practice. In addition, we found that the combination of celecoxib and vinorelbine, but not doxorubicin, induced a significant reduction in cell viability and a significant increase in apoptosis. In conclusion, we showed that celecoxib was able to enhance the chemotherapeutic effect of vinorelbine. A clinical trial exploring the in vivo activities of celecoxib in MTC patients who cannot benefit from available treatments would be desirable, taking into account the possible risks of cardiovascular effects of this drug.

Thyroid Cancer: Role of RET and Beyond

Specific thyroid cancer histotypes, such as papillary and medullary thyroid carcinoma, display genetic rearrangements or point mutations of the RET gene, resulting in its oncogenic conversion. The molecular mechanisms mediating RET rearrangement with other genes and the role of partner genes in tumorigenesis have been described. In addition, the RET protein has become a molecular target for medullary thyroid carcinoma treatment.

How to Treat a Signal? Current Basis for RET-Genotype-Oriented Choice of Kinase Inhibitors for the Treatment of Medullary Thyroid Cancer

The significance of RET in thyroid cancer comes from solid evidence that, when inherited, an RET activating mutation primes C-cells to transform into medullary carcinomas. Moreover, environmental exposure to radiation also induces rearranged transforming RET “isoforms” that are found in papillary thyroid cancer. The RET gene codes for a tyrosine kinase receptor that targets a diverse set of intracellular signaling pathways. The nature of RET point mutations predicts differences in the mechanisms by which the receptor becomes activated and correlates with different forms of clinical presentation, age of onset, and biological aggressiveness. A number of RET-targeting Tyrosine Kinase Inhibitors (TKIs) are currently undergoing clinical trials to evaluate their effectiveness in the treatment of thyroid cancer, and it is conceivable that the RET genotype may also influence response to these compounds. The question that now emerges is whether, in the future, the rational for treatment of refractory thyroid cancer will be based on the management of an abnormal RET signal. In this paper we address the RET-targeting TKIs and review studies about the signaling properties of distinct RET mutants as a means to predict response and design combinatorial therapies for the soon to be available TKIs.

Differential expression of c-Ret in motor neurons versus non-neuronal cells is linked to the pathogenesis of ALS

Amyotrophic lateral sclerosis (ALS) is a fatal neurological disorder characterized by selective degeneration of motor neurons throughout the central nervous systems. Non-cell autonomous damage induced by glial cells is linked to the selective susceptibility of motor neurons in ALS, but the mechanisms underlying this phenomenon are not known. We found that the expression of non-phosphorylated and phosphorylated forms (tyrosine (Tyr) residue 905, 1016, and 1062) of c-Ret, a member of the glial cell line-derived neurotrophic factor (GDNF) receptor, are altered in motor neurons of the lumbar spinal cord in ALS transgenic (G93A) mice and ALS (G93A) cell line models. Phosphorylated forms of c-Ret were colocalized with neurofilament aggregates in motor neurons of ALS mice. Consistent with the in vivo data, levels of non-phosphorylated and phosphorylated c-Ret (Tyr 905, 1016, and 1062) were decreased by oxidative stress in motor neuronal cells (NSC-34). Non-phosphorylated and phosphorylated forms of c-Ret immunoreactivity were markedly elevated in active microglia of ALS mice. Our findings suggest that constitutive oxidative stress modulates c-Ret function, thereby reducing GDNF signaling in motor neurons. Furthermore, the induction of c-Ret expression in microglia may contribute to non-cell autonomous cell death of motor neurons by available GDNF in ALS.

Protein-tyrosine phosphatase SHP2 contributes to GDNF neurotrophic activity through direct binding to phospho-Tyr687 in the RET receptor tyrosine kinase

The signaling mechanisms by which neurotrophic receptors regulate neuronal survival and axonal growth are still incompletely understood. In the receptor tyrosine kinase RET, a receptor for GDNF (glial cell line-derived neurotrophic factor), the functions of the majority of tyrosine residues that become phosphorylated are still unknown. Here we have identified the protein-tyrosine phosphatase SHP2 as a novel direct interactor of RET and the first effector known to bind to phosphorylated Tyr(687) in the juxtamembrane region of the receptor. We show that SHP2 is recruited to RET upon ligand binding in a cooperative fashion, such that both interaction with Tyr(687) and association with components of the Tyr(1062) signaling complex are required for stable recruitment of SHP2 to the receptor. SHP2 recruitment contributes to the ability of RET to activate the PI3K/AKT pathway and promote survival and neurite outgrowth in primary neurons. Furthermore, we find that activation of protein kinase A (PKA) by forskolin reduces the recruitment of SHP2 to RET and negatively affects ligand-mediated neurite outgrowth. In agreement with this, mutation of Ser(696), a known PKA phosphorylation site in RET, enhances SHP2 binding to the receptor and eliminates the effect of forskolin on ligand-induced outgrowth. Together, these findings establish SHP2 as a novel positive regulator of the neurotrophic activities of RET and reveal Tyr(687) as a critical platform for integration of RET and PKA signals. We anticipate that several other phosphotyrosines of unknown function in neuronal receptor tyrosine kinases will also support similar regulatory functions.

Organotypic specificity of key RET adaptor-docking sites in the pathogenesis of neurocristopathies and renal malformations in mice

The receptor tyrosine kinase ret protooncogene (RET) is implicated in the pathogenesis of several diseases and in several developmental defects, particularly those in neural crest-derived structures and the genitourinary system. In order to further elucidate RET-mediated mechanisms that contribute to these diseases and decipher the basis for specificity in the pleiotropic effects of RET, we characterized development of the enteric and autonomic nervous systems in mice expressing RET9 or RET51 isoforms harboring mutations in tyrosine residues that act as docking sites for the adaptors Plcgamma, Src, Shc, and Grb2. Using this approach, we found that development of the genitourinary system and the enteric and autonomic nervous systems is dependent on distinct RET-stimulated signaling pathways. Thus, mutation of RET51 at Y1062, a docking site for multiple adaptor proteins including Shc, caused distal colon aganglionosis reminiscent of Hirschsprung disease (HSCR). On the other hand, this mutation in RET9, which encodes an isoform that lacks the Grb2 docking site present in RET51, produced severe abnormalities in multiple organs. Mutations that abrogate RET-Plcgamma binding, previously shown to produce features reminiscent of congenital anomalies of kidneys or urinary tract (CAKUT) syndrome, produced only minor abnormalities in the nervous system. Abrogating RET51-Src binding produced no major defects in these systems. These studies provide insight into the basis of organotypic specificity and redundancy in RET signaling within these unique systems and in diseases such as HSCR and CAKUT.

Targeting the RET pathway in thyroid cancer

The RET (rearranged during transfection) protooncogene encodes a single pass transmembrane receptor that is expressed in cells derived from the neural crest and the urogenital tract. As part of a cell-surface complex, RET binds glial derived neurotrophic factor (GDNF) ligands in conjunction with GDNF-family alpha co-receptors (GFRalpha). Ligand-induced activation induces dimerization and tyrosine phosphorylation of the RET receptor with downstream activation of several signal transduction pathways. Activating germline RET mutations play a central role in the development of the multiple endocrine neoplasia (MEN) syndromes MEN2A, MEN2B, and familial medullary thyroid carcinoma (FMTC) and also in the development of the congenital abnormality Hirschsprung's disease. Approximately 50% of patients with sporadic MTC have somatic RET mutations, and a significant portion of papillary thyroid carcinomas result from chromosomal inversions or translocations, which activate RET (RET/PTC oncogenes). The RET protooncogene has a significant place in cancer prevention and treatment. Timely thyroidectomy in kindred members who have inherited a mutated RET allele, characteristic of MEN2A, MEN2B, or FMTC, can prevent MTC, the most common cause of death in these syndromes. Also, recently developed molecular therapeutics that target the RET pathway have shown activity in clinical trials of patients with advanced MTC, a disease for which there has been no effective therapy.

Computational modeling of structurally conserved cancer mutations in the RET and MET kinases: the impact on protein structure, dynamics, and stability

Structural and biochemical characterization of protein kinases that confer oncogene addiction and harbor a large number of disease-associated mutations, including RET and MET kinases, have provided insights into molecular mechanisms associated with the protein kinase activation in human cancer. In this article, structural modeling, molecular dynamics, and free energy simulations of a structurally conserved mutational hotspot, shared by M918T in RET and M1250T in MET kinases, are undertaken to quantify the molecular mechanism of activation and the functional role of cancer mutations in altering protein kinase structure, dynamics, and stability. The mechanistic basis of the activating RET and MET cancer mutations may be driven by an appreciable free energy destabilization of the inactive kinase state in the mutational forms. According to our results, the locally enhanced mobility of the cancer mutants and a higher conformational entropy are counterbalanced by a larger enthalpy loss and result in the decreased thermodynamic stability. The computed protein stability differences between the wild-type and cancer kinase mutants are consistent with circular dichroism spectroscopy and differential scanning calorimetry experiments. These results support the molecular mechanism of activation, which causes a detrimental imbalance in the dynamic equilibrium shifted toward the active form of the enzyme. Furthermore, computer simulations of the inhibitor binding with the oncogenic and drug-resistant RET mutations have also provided a plausible molecular rationale for the observed differences in the inhibition profiles, which is consistent with the experimental data. Finally, structural mapping of RET and MET cancer mutations and the computed protein stability changes suggest a similar mechanism of activation, whereby the cancer mutations which display the higher oncogenic activity tend to have the greatest destabilization effect on the inactive kinase structure.

RET Gly691Ser mutation is associated with primary vesicoureteral reflux in the French-Canadian population from Quebec

Primary vesicoureteral reflux (pVUR) is a common, genetically heterogeneous congenital urinary tract abnormality in children. It causes urine to flow backward from the bladder to the ureter due to a developmental defect at the vesicoureteral junction, whose formation requires rearrangement during transformation (Ret)-mediated signaling pathways. To study the genetic causes of pVUR in Quebec patients, we used a sequencing-based candidate gene approach to screen the RET gene and found that 83 out of 118 pVUR patients are carriers of the rare A allele of single nucleotide polymorphism (SNP) rs1799939:G>A that results in a Gly691Ser mutation, a statistically significant increase in allelic frequency, that is absent at six flanking RET SNPs tested. Ser691 is a predicted phosphorylation site and our analysis of transfected cells showed that the Gly691Ser Ret mutant can efficiently interact and associate with a 75-80-kD tyrosine phosphorylated cellular protein, an event not seen with wild-type Ret. This interaction and/or the steric or electric hindrance created by phospho-Ser691 may interfere with the known regulatory functions of the normally phosphorylated phospho-Tyr687 and phospho-Ser696 on the cytoskeleton actin reorganization that are responsible for cell motility and morphology, which consequently may lead to the deficiency in ureteral development observed in pVUR. Our study demonstrates that the Ret Gly691Ser mutation is associated with pVUR and may be one of the genetic causes of this condition in the French-Canadian population in Quebec.

Deciphering adaptor specificity in GFL-dependent RET-mediated proliferation and neurite outgrowth

Glial cell derived neurotrophic factor (GDNF)-dependent receptor tyrosine kinase RET activity is required for proper development of the nervous system and genitourinary tract. Loss-of-function mutations in RET are associated with enteric nervous system abnormalities (Hirschsprung disease) and renal deficits (Potter's syndrome), whereas activating mutations lead to hereditary cancer syndromes (multiple endocrine neoplasia type 2A and type 2B). RET activation is crucial for the proper regulation of a variety of cellular processes including cell migration, proliferation and neurite outgrowth. By analyzing a series of RET mutants we found that Y1062 was critical for stimulating GDNF-mediated proliferation as well as proliferation stimulated by GDNF-independent oncogenic forms of RET. Studies using small interfering RNA driven by lentivirus to knock-down expression of particular adaptor proteins that interact with RET phospho-Y1062, demonstrated that only Src-homology 2 and growth factor receptor binding protein 2 were necessary for RET-mediated proliferation by wild type and oncogenic forms of RET. Interestingly, we discovered that Y1062 was also required for GDNF-stimulated neurite outgrowth. However, small interfering RNAs to either Src-homology 2 or growth factor receptor binding protein 2 or a panel of other adaptor proteins known to interact with RET Y1062 were incapable of blocking GDNF-stimulated neurite formation, indicating that differential use of intracellular adaptors is responsible for regulating alternative RET-stimulated cellular events such as proliferation versus a differentiation response like neurite outgrowth.

Neurotrophic factor receptor RET: structure, cell biology, and inherited diseases

RET (REarranged during Transfection) is a transmembrane receptor tyrosine kinase that is activated by a complex consisting of a soluble glial cell line-derived neurotrophic factor (GDNF) family ligand (GFL) and a glycosyl phosphatidylinositol-anchored co-receptor, GDNF family receptors alpha (GFRalpha). RET signalling is crucial for the development of the enteric nervous system. RET also regulates the development of sympathetic, parasympathetic, motor, and sensory neurons, and is necessary for the postnatal maintenance of dopaminergic neurons. The effect of GFLs on sensory, motor, and dopaminergic neurons has raised clinical interest towards these ligands. Outside the nervous system, RET is crucial for development of the kidney and plays a key role in spermatogenesis. Inactivating mutations in RET cause the Hirschsprung's disease characterized by megacolon aganglionosis. In contrast, activating mutations give rise to different types of cancer, multiple endocrine neoplasia type 2A and type 2B, familial medullary thyroid carcinoma, and papillary thyroid carcinoma. The multiple disease phenotypes correlate with differences in the molecular and cell biological functions of different oncogenic RET proteins. In this review we summarize how the different domains of the RET protein contribute to its normal function and how mutations in these domains affect the function of the receptor.

RET signaling in endocrine tumors: delving deeper into molecular mechanisms

The rearranged during transfection (RET) proto-oncogene encodes a receptor tyrosine kinase that is implicated in the development of endocrine tumors of the thyroid and adrenal glands. In humans, activating RET mutations are found in the inherited cancer syndrome multiple endocrine neoplasia 2 and in sporadic medullary and papillary thyroid carcinomas. The specific type and location of RET mutations are strongly correlated with the disease phenotype and have both diagnostic and prognostic value. Recent advances in the molecular characterization of the RET receptor and its mutants have begun to define the mechanisms underlying the transforming ability of the different RET mutant forms. This information has revealed key functional features of these mutant proteins that distinguish the different clinically recognized mutations and provide clues as to the functional origins of the phenotypes associated with specific RET mutations. The elucidation of molecular mechanisms involved in RET-mediated transformation is a key step in the development of much needed therapeutics that target RET's oncogenic properties. Recent advances have begun to provide a deeper understanding of the receptor's function, and dysfunction, in human tumors that may guide this process.

Targeted mutation of serine 697 in the Ret tyrosine kinase causes migration defect of enteric neural crest cells

The RET receptor tyrosine kinase plays a critical role in the development of the enteric nervous system (ENS) and the kidney. Upon glial-cell-line-derived neurotrophic factor (GDNF) stimulation, RET can activate a variety of intracellular signals, including the Ras/mitogen-activated protein kinase, phosphatidylinositol 3-kinase (PI3K)/AKT, and RAC1/JUN NH(2)-terminal kinase (JNK) pathways. We recently demonstrated that the RAC1/JNK pathway is regulated by serine phosphorylation at the juxtamembrane region of RET in a cAMP-dependent manner. To determine the importance of cAMP-dependent modification of the RET signal in vivo, we generated mutant mice in which serine residue 697, a putative protein kinase A (PKA) phosphorylation site, was replaced with alanine (designated S697A mice). Homozygous S697A mutant mice lacked the ENS in the distal colon, resulting from a migration defect of enteric neural crest cells (ENCCs). In vitro organ culture showed an impaired chemoattractant response of the mutant ENCCs to GDNF. JNK activation by GDNF but not ERK, AKT and SRC activation was markedly reduced in neurons derived from the mutant mice. The JNK inhibitor SP600125 and the PKA inhibitor KT5720 suppressed migration of the ENCCs in cultured guts from wild-type mice to comparable degrees. Thus, these findings indicated that cAMP-dependent modification of RET function regulates the JNK signaling responsible for proper migration of the ENCCs in the developing gut.

RET as a diagnostic and therapeutic target in sporadic and hereditary endocrine tumors

The RET gene encodes a receptor tyrosine kinase that is expressed in neural crest-derived cell lineages. The RET receptor plays a crucial role in regulating cell proliferation, migration, differentiation, and survival through embryogenesis. Activating mutations in RET lead to the development of several inherited and noninherited diseases. Germline point mutations are found in the cancer syndromes multiple endocrine neoplasia (MEN) type 2, including MEN 2A and 2B, and familial medullary thyroid carcinoma. These syndromes are autosomal dominantly inherited. The identification of mutations associated with these syndromes has led to genetic testing to identify patients at risk for MEN 2 and familial medullary thyroid carcinoma and subsequent implementation of prophylactic thyroidectomy in mutation carriers. In addition, more than 10 somatic rearrangements of RET have been identified from papillary thyroid carcinomas. These mutations, as those found in MEN 2, induce oncogenic activation of the RET tyrosine kinase domain via different mechanisms, making RET an excellent candidate for the design of molecular targeted therapy. Recently, various kinds of therapeutic approaches, such as tyrosine kinase inhibition, gene therapy with dominant negative RET mutants, monoclonal antibodies against oncogene products, and nuclease-resistant aptamers that recognize and inhibit RET have been developed. The use of these strategies in preclinical models has provided evidence that RET is indeed a potential target for selective cancer therapy. However, a clinically useful therapeutic option for treating patients with RET-associated cancer is still not available.

Critical and distinct roles for key RET tyrosine docking sites in renal development

Molecular mechanisms that lead to congenital anomalies of kidneys and the lower urinary tract (CAKUT) are poorly understood. To elucidate the molecular basis for signaling specificity of GDNF-mediated RET signaling in kidney development, we characterized mice that exclusively express either the human RET9 or RET51 isoform, or express these isoforms with individual mutations in docking tyrosines for PTB and SH2-domain-containing adaptors Src (Y981), PLCgamma (Y1015), and Shc (Y1062). Our results provide evidence for differential and isoform-specific roles of these docking sites in murine kidney development. Homozygous Ret(RET9) and Ret(RET51) mice were viable and show normally developed kidneys, indicating redundant roles of human RET isoforms in murine kidney development. In the context of the RET51 isoform, only mutation of the docking Tyr 1015 (Y1015F) resulted in severe renal anomalies. These included bilateral megaureters and multicystic kidneys that were caused by supernumerary ureteric buds that fail to separate from the wolffian duct as well as decreased branching morphogenesis. Similar kidney and ureter defects were observed in RET9(Y1015F) mice that contain the Y1015F mutation in the RET9 isoform. Interestingly, loss of RET9(Y1062)-mediated AKT/MAPK activation resulted in renal agenesis or kidney rudiments, whereas mutation of this residue in RET51 had no obvious effect on AKT/MAPK activity and renal development. These results reveal novel roles of key RET-dependent signaling pathways in embryonic kidney development and provide murine models and new insights into the molecular basis for CAKUT.

Differential effects of glial cell line-derived neurotrophic factor and neurturin in RET/GFRalpha1-expressing cells

The c-ret protooncogene, RET, encodes a receptor tyrosine kinase. RET is activated by members of the glial cell line-derived neurotrophic factor (GDNF) family of ligands, which include GDNF, neurturin, artemin, and persephin. The ligands bind RET through GDNF family receptor alpha, termed GFRalpha1-4. Despite the importance of RET signaling in the development of the enteric nervous system and the kidney, the differential signaling mechanisms between RET ligands are poorly established. It has been suggested that signal specificity is achieved through binding of the ligand to its preferred GFRalpha. To compare the signaling profiles of GDNF and neurturin, we have identified a cell line, NG108-15, which endogenously expresses RET and GFRalpha1 but not GFRalpha2-4. Immunoblot data showed that GDNF caused a transient activation, whereas neurturin caused a sustained activation, of both p44/p42 MAP kinases and PLCgamma. Under serum starvation, NG108-15 cells differentiate and form neurites. Neurturin but not GDNF stimulated neurite outgrowth, which could be blocked by the selective PLC inhibitor U73122. On the other hand, GDNF but not neurturin promoted cell survival, and this could be blocked by the p44/p42 MAP kinase inhibitor PD98059. Our findings not only show the differential signaling of GDNF and neurturin but also suggest that this can be achieved through binding to the same GFRalpha subtype, leading to distinct biological responses.

RET and neuroendocrine tumors

The RET proto-oncogene encodes a receptor tyrosine kinase that is a main component of the signaling pathway activated by the glial cell line-derived neurotrophic factor family ligands. Gene targeting studies revealed that signaling through RET plays a crucial role in neuronal and renal organogenesis. It is well-known that germline mutations in RET lead to the human inherited diseases, multiple endocrine neoplasia type 2 (MEN 2) and Hirschsprung's disease, and that somatic rearrangements of RET cause papillary thyroid carcinoma. Due to marked advances in understanding of the molecular mechanisms of the development of MEN 2, a consensus on MEN 2 management associated with RET status is being reached and currently put into general use as a guideline. In this review, we summarize progress in the study of RET from bench to bedside, focusing on pathophysiology of neuroendocrine tumors.

RET tyrosine kinase signaling in development and cancer

The variety of diseases caused by mutations in RET receptor tyrosine kinase provides a classic example of phenotypic heterogeneity. Gain-of-function mutations of RET are associated with human cancer. Gene rearrangements juxtaposing the tyrosine kinase domain to heterologous gene partners have been found in sporadic papillary carcinomas of the thyroid (PTC). These rearrangements generate chimeric RET/PTC oncogenes. In the germline, point mutations of RET are responsible for multiple endocrine neoplasia type 2 (MEN 2A and 2B) and familial medullary thyroid carcinoma (FMTC). Both MEN 2 mutations and PTC gene rearrangements potentiate the intrinsic tyrosine kinase activity of RET and, ultimately, activate the RET downstream targets. Loss-of-function mutations of RET cause Hirschsprung's disease (HSCR) or colonic aganglionosis. A deeper understanding of the molecular signaling of normal versus abnormal RET activity in cancer will enable the development of potential new treatments for patients with sporadic and inherited thyroid cancer or MEN 2 syndrome. We now review the role and mechanisms of RET signaling in development and carcinogenesis.

Inducible dimerization of RET reveals a specific AKT deregulation in oncogenic signaling

Dominant-activating mutations in the RET (rearranged during transfection) proto-oncogene, a receptor tyrosine kinase, are causally associated with the development of multiple endocrine neoplasia type 2A (MEN2A) syndrome. Such oncogenic RET mutations induce its ligand-independent constitutive activation, but whether it spreads identical signaling to ligand-induced signaling is uncertain. To address this question, we designed a cellular model in which RET can be activated either by its natural ligand, or alternatively, by controlled dimerization of the protein that mimics MEN2A dimerization. We have shown that controlled dimerization leaves proximal RET signaling intact but impacts substantially on the tuning of the distal AKT kinase activation (delayed and sustained). In marked contrast, distal activation of ERK remained unaffected. We further demonstrated that specific temporal adjustment of ligand-induced AKT activation is dependent upon a lipid-based cholesterol-sensitive environment, and this control step is bypassed by MEN2A RET mutants. Therefore, these studies revealed that MEN2A mutations propagate previously unappreciated subtle differences in signaling pathways and unravel a role for lipid rafts in the temporal regulation of AKT activation.

A Drosophila model of multiple endocrine neoplasia type 2

Dominant mutations in the Ret receptor tyrosine kinase lead to the familial cancer syndrome multiple endocrine neoplasia type 2 (MEN2). Mammalian tissue culture studies suggest that RetMEN2 mutations significantly alter Ret-signaling properties, but the precise mechanisms by which RetMEN2 promotes tumorigenesis remain poorly understood. To determine the signal transduction pathways required for RetMEN2 activity, we analyzed analogous mutations in the Drosophila Ret ortholog dRet. Overexpressed dRetMEN2 isoforms targeted to the developing retina led to aberrant cell proliferation, inappropriate cell fate specification, and excessive Ras pathway activation. Genetic analysis indicated that dRetMEN2 acts through the Ras-ERK, Src, and Jun kinase pathways. A genetic screen for mutations that dominantly suppress or enhance dRetMEN2 phenotypes identified new genes that are required for the phenotypic outcomes of dRetMEN2 activity. Finally, we identified human orthologs for many of these genes and examined their status in human tumors. Two of these loci showed loss of heterozygosity (LOH) within both sporadic and MEN2-associated pheochromocytomas, suggesting that they may contribute to Ret-dependent oncogenesis.

RET-familial medullary thyroid carcinoma mutants Y791F and S891A activate a Src/JAK/STAT3 pathway, independent of glial cell line-derived neurotrophic factor

The RET proto-oncogene encodes a receptor tyrosine kinase whose dysfunction plays a crucial role in the development of several neural crest disorders. Distinct activating RET mutations cause multiple endocrine neoplasia type 2A (MEN2A), type 2B (MEN2B), and familial medullary thyroid carcinoma (FMTC). Despite clear correlations between the mutations found in these cancer syndromes and their phenotypes, the molecular mechanisms connecting the mutated receptor to the different disease phenotypes are far from completely understood. Luciferase reporter assays in combination with immunoprecipitations, and Western and immunohistochemistry analyses were done in order to characterize the signaling properties of two FMTC-associated RET mutations, Y791F and S891A, respectively, both affecting the tyrosine kinase domain of the receptor. We show that these RET-FMTC mutants are monomeric receptors which are autophosphorylated and activated independently of glial cell line-derived neurotrophic factor. Moreover, we show that the dysfunctional signaling properties of these mutants, when compared with wild-type RET, involve constitutive activation of signal transducers and activators of transcription 3 (STAT3). Furthermore, we show that STAT3 activation is mediated by a signaling pathway involving Src, JAK1, and JAK2, differing from STAT3 activation promoted by RET(C634R) which was previously found to be independent of Src and JAKs. Three-dimensional modeling of the RET catalytic domain suggested that the structural changes promoted by the respective amino acids substitutions lead to a more accessible substrate and ATP-binding monomeric conformation. Finally, immunohistochemical analysis of FMTC tumor samples support the in vitro data, because nuclear localized, Y705-phosphorylated STAT3, as well as a high degree of RET expression at the plasma membrane was observed.