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

Phase separation of SPIN1 through its IDR facilitates histone methylation readout and tumorigenesis

Spindlin1 (SPIN1) is a unique multivalent histone modification reader that plays a role in ribosomal RNA transcription, chromosome segregation, and tumorigenesis. However, the function of the extended N-terminal region of SPIN1 remains unclear. Here, we demonstrated that SPIN1 can form phase-separated and liquid-like condensates both in vitro and in vivo through its N-terminal intrinsically disordered region (IDR). The phase separation of SPIN1 recruits the histone methyltransferase MLL1 to the same condensates and enriches the H3K4 methylation marks. This process also facilitates the binding of SPIN1 to H3K4me3 and activates tumorigenesis-related genes. Moreover, SPIN1-IDR enhances the genome-wide chromatin binding of SPIN1 and facilitates its localization to genes associated with the MAPK signaling pathway. These findings provide new insights into the biological function of the IDR in regulating SPIN1 activity and reveal a previously unrecognized role of SPIN1-IDR in histone methylation readout. Our study uncovers the crucial role of appropriate biophysical properties of SPIN1 in facilitating gene expression and links phase separation to tumorigenesis, which provides a new perspective for understanding the function of SPIN1.

REThinking the role of the RET oncogene in breast cancer

The REarranged during Transfection (RET) receptor tyrosine kinase plays a crucial role in the development of various anatomical structures during embryogenesis and it is involved in many physiological cellular processes. This protein is also associated with the initiation of various cancer types, such as thyroid cancer, non-small cell lung cancer, and multiple endocrine neoplasms. In breast cancer, and especially in the estrogen receptor-positive (ER+) subtype, the activity of RET is of notable importance. Indeed, RET seems to be involved in tumor progression, resistance to therapies, and cellular proliferation. Nevertheless, the ways RET alterations could impact the prognosis of breast cancer and its response to treatment remain only partially elucidated. Several inhibitors of RET kinase have been developed thus far, with various degrees of selectivity toward RET inhibition. These molecules showed notable efficacy in the treatment of RET-driven tumors, including some breast cancer cases. Despite these encouraging results, further investigation is needed to fully understand the potential role RET inhibition in breast cancer. This review aims to recapitulate the existing evidence about the role of RET oncogene in breast cancer, from its pathogenic and potentially prognostic role, to the clinical applications of RET inhibitors.

Modeling RET-Rearranged Non-Small Cell Lung Cancer (NSCLC): Generation of Lung Progenitor Cells (LPCs) from Patient-Derived Induced Pluripotent Stem Cells (iPSCs)

REarranged during Transfection (RET) oncogenic rearrangements can occur in 1-2% of lung adenocarcinomas. While RET-driven NSCLC models have been developed using various approaches, no model based on patient-derived induced pluripotent stem cells (iPSCs) has yet been described. Patient-derived iPSCs hold great promise for disease modeling and drug screening. However, generating iPSCs with specific oncogenic drivers, like RET rearrangements, presents challenges due to reprogramming efficiency and genotypic variability within tumors. To address this issue, we aimed to generate lung progenitor cells (LPCs) from patient-derived iPSCs carrying the mutation RETC634Y, commonly associated with medullary thyroid carcinoma. Additionally, we established a RETC634Y knock-in iPSC model to validate the effect of this oncogenic mutation during LPC differentiation. We successfully generated LPCs from RETC634Y iPSCs using a 16-day protocol and detected an overexpression of cancer-associated markers as compared to control iPSCs. Transcriptomic analysis revealed a distinct signature of NSCLC tumor repression, suggesting a lung multilineage lung dedifferentiation, along with an upregulated signature associated with RETC634Y mutation, potentially linked to poor NSCLC prognosis. These findings were validated using the RETC634Y knock-in iPSC model, highlighting key cancerous targets such as PROM2 and C1QTNF6, known to be associated with poor prognostic outcomes. Furthermore, the LPCs derived from RETC634Y iPSCs exhibited a positive response to the RET inhibitor pralsetinib, evidenced by the downregulation of the cancer markers. This study provides a novel patient-derived off-the-shelf iPSC model of RET-driven NSCLC, paving the way for exploring the molecular mechanisms involved in RET-driven NSCLC to study disease progression and to uncover potential therapeutic targets.

A network map of GDNF/RET signaling pathway in physiological and pathological conditions

Glial cell line-derived neurotrophic factor (GDNF) signals through a multi-component receptor system predominantly consisting of glycosyl-phosphatidylinositol-anchored GDNF family receptor alpha-1 (GFRα1) and the Rearranged during transfection (RET) receptor tyrosine kinase. GDNF/RET signaling is vital to the central and peripheral nervous system, kidney morphogenesis, and spermatogenesis. In addition, the dysregulation of the GDNF/RET signaling has been implicated in the pathogenesis of cancers. Despite the extensive research on GDNF/RET signaling, a molecular network of reactions induced by GDNF reported across the published literature. However, a comprehensive GDNF/RET pathway resource is currently unavailable. We describe an integrated signaling pathway reaction map of GDNF/RET consisting of 1151 molecular reactions. These include information pertaining to 52 molecular association events, 70 enzyme catalysis events, 36 activation/inhibition events, 22 translocation events, 856 gene regulation events, and 115 protein-level expression events induced by GDNF in diverse cell types. We developed a comprehensive GDNF/RET signaling network map based on these molecular reactions. The pathway map was made accessible through WikiPathways database ( https://www.wikipathways.org/index.php/Pathway:WP5143 ). Biocuration and development of gene regulatory network map of GDNF/RET signaling pathway.

Vepafestinib is a pharmacologically advanced RET-selective inhibitor with high CNS penetration and inhibitory activity against RET solvent front mutations

RET receptor tyrosine kinase is activated in various cancers (lung, thyroid, colon and pancreatic, among others) through oncogenic fusions or gain-of-function single-nucleotide variants. Small-molecule RET kinase inhibitors became standard-of-care therapy for advanced malignancies driven by RET. The therapeutic benefit of RET inhibitors is limited, however, by acquired mutations in the drug target as well as brain metastasis, presumably due to inadequate brain penetration. Here, we perform preclinical characterization of vepafestinib (TAS0953/HM06), a next-generation RET inhibitor with a unique binding mode. We demonstrate that vepafestinib has best-in-class selectivity against RET, while exerting activity against commonly reported on-target resistance mutations (variants in RETL730, RETV804 and RETG810), and shows superior pharmacokinetic properties in the brain when compared to currently approved RET drugs. We further show that these properties translate into improved tumor control in an intracranial model of RET-driven cancer. Our results underscore the clinical potential of vepafestinib in treating RET-driven cancers.

RET-Altered Cancers-A Tumor-Agnostic Review of Biology, Diagnosis and Targeted Therapy Activity

RET alterations, such as fusions or mutations, drive the growth of multiple tumor types. These alterations are found in canonical (lung and thyroid) and non-canonical (e.g., gastrointestinal, breast, gynecological, genitourinary, histiocytic) cancers. RET alterations are best identified via comprehensive next-generation sequencing, preferably with DNA and RNA interrogation for fusions. Targeted therapies for RET-dependent cancers have evolved from older multikinase inhibitors to selective inhibitors of RET such as selpercatinib and pralsetinib. Prospective basket trials and retrospective reports have demonstrated the activity of these drugs in a wide variety of RET-altered cancers, notably those with RET fusions. This paved the way for the first tumor-agnostic selective RET inhibitor US FDA approval in 2022. Acquired resistance to RET kinase inhibitors can take the form of acquired resistance mutations (e.g., RET G810X) or bypass alterations.

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.

Genomic and epigenomic profile of thyroid cancer

Thyroid cancer is the most common malignancy of the endocrine system, and its incidence has been steadily increasing. Advances in sequencing have allowed analysis of the entire cancer genome, and has provided new information on the genetic lesions and modifications responsible for the onset, progression, dedifferentiation and metastasis of thyroid carcinomas. Moreover, integrated genomics has advanced our understanding of the development of cancer and its behavior, and has facilitated the identification of new genetic mutations and molecular pathways. The functional analysis of epigenetic modifications, such as DNA methylation, histone acetylation and non-coding RNAs, have contributed to define new regulatory mechanisms that control cell malignancy in thyroid cancer, especially aggressive forms. Here we review the most recent advances in genomics and epigenomics of thyroid cancer, which have resulted in a new classification and interpretation of the initiation and progression of thyroid tumors, providing new tools and opportunities for further investigation and for the clinical development of new treatment strategies.

Glaucoma-TrEl: A web-based interactive database to build evidence-based hypotheses on the role of trace elements in glaucoma

Objective

Glaucoma is a chronic neurological disease that is associated with high intraocular pressure (IOP), causes gradual damage to retinal ganglion cells, and often culminates in vision loss. Recent research suggests that glaucoma is a complex multifactorial disease in which multiple interlinked genes and pathways play a role during onset and development. Also, differential availability of trace elements seems to play a role in glaucoma pathophysiology, although their mechanism of action is unknown. The aim of this work is to disseminate a web-based repository on interactions between trace elements and protein-coding genes linked to glaucoma pathophysiology.

Results

In this study, we present Glaucoma-TrEl, a web database containing information about interactions between trace elements and protein-coding genes that are linked to glaucoma. In the database, we include interactions between 437 unique genes and eight trace elements. Our analysis found a large number of interactions between trace elements and protein-coding genes mutated or linked to the pathophysiology of glaucoma. We associated genes interacting with multiple trace elements to pathways known to play a role in glaucoma. The web-based platform provides an easy-to-use and interactive tool, which serves as an information hub facilitating future research work on trace elements in glaucoma.

Atropisomerism in the Pharmaceutically Relevant Realm

Atropisomerism is a conformational chirality that occurs when there is hindered rotation about a σ-bond. While atropisomerism is exemplified by biaryls, it is observed in many other pharmaceutically relevant scaffolds including heterobiaryls, benzamides, diarylamines, and anilides. As bond rotation leads to racemization, atropisomers span the gamut of stereochemical stability. LaPlante has classified atropisomers based on their half-life of racemization at 37 °C: class 1 (t_1/2 < 60 s), class 2 (60 s < t_1/2 < 4.5 years), and class 3 (t_1/2 > 4.5 years). In general, class-3 atropisomers are considered to be suitable for drug development. There are currently four FDA-approved drugs that exist as stable atropisomers, and many others are in clinical trials or have recently appeared in the drug discovery literature. Class-1 atropisomers are more prevalent, with ∼30% of recent FDA-approved small molecules possessing at least one class-1 axis. While class-1 atropisomers do not possess the requisite stereochemical stability to meet the classical definition of atropisomerism, they often bind a given target in a specific set of chiral conformations.Over the past decade, our laboratory has embarked on a research program aimed at leveraging atropisomerism as a design feature to improve the target selectivity of promiscuous lead compounds. Our studies initially focused on introducing class-3 atropisomerism into promiscuous kinase inhibitors, resulting in a proof of principle in which the different atropisomers of a compound can have different selectivity profiles with potentially improved target selectivity. This inspired a careful analysis of the binding conformations of diverse ligands bound to different target proteins, resulting in the realization that the sampled dihedral conformations about a prospective atropisomeric axis played a key role in target binding and that preorganizing the prospective atropisomeric axis into a desired target's preferred conformational range can lead to large gains in target selectivity.As atropisomerism is becoming more prevalent in modern drug discovery, there is an increasing need for strategies for atropisomerically pure samples of pharmaceutical compounds. This has led us and other groups to develop catalytic atroposelective methodologies toward pharmaceutically privileged scaffolds. Our laboratory has contributed examples of atroposelective methodologies toward heterobiaryl systems while also exploring the chirality of less-studied atropisomers such as diarylamines and related scaffolds.This Account will detail recent encounters with atropisomerism in medicinal chemistry and how atropisomerism has transitioned from a "lurking menace" into a leverageable design strategy in order to modulate various properties of biologically active small molecules. This Account will also discuss recent advances in atroposelective synthesis, with a focus on methodologies toward pharmaceutically privileged scaffolds. We predict that a better understanding of the effects of conformational restriction about a prospective atropisomeric axis on target binding will empower chemists to rapidly "program" the selectivity of a lead molecule toward a desired target.

The kinase PLK1 promotes the development of <i>Kras</i>/<i>Tp53</i>-mutant lung adenocarcinoma through transcriptional activation of the receptor RET

Increased abundance of polo-like kinase 1 (PLK1) is observed in various tumor types, particularly in lung adenocarcinoma (LUAD). Here, we found that PLK1 accelerated the progression of LUAD through a mechanism that was independent of its role in mediating mitotic cell division. Analysis of human tumor databases revealed that increased PLK1 abundance in LUAD correlated with mutations in KRAS and p53, with tumor stage, and with reduced survival in patients. In a mouse model of KRAS^G12D-driven, p53-deficient LUAD, PLK1 overexpression increased tumor burden, decreased tumor cell differentiation, and reduced animal survival. PLK1 overexpression in cultured cells and mice indirectly increased the expression of the gene encoding the receptor tyrosine kinase RET by phosphorylating the transcription factor TTF-1. Signaling by RET and mutant KRAS in these tumors converged to activate the mitogen-activated protein kinase (MAPK) pathway. Pharmacological inhibition of the MAPK pathway kinase MEK combined with inhibition of either RET or PLK1 markedly suppressed tumor growth. Our findings show that PLK1 can amplify MAPK signaling and reveal a potential target for stemming progression in lung cancers with high PLK1 abundance.

LIBRETTO-531: a phase III study of selpercatinib in multikinase inhibitor-naïve RET-mutant medullary thyroid cancer

Selpercatinib is a first-in-class, highly selective and potent, central nervous system-active RET kinase inhibitor. In the phase I/II trial, selpercatinib demonstrated clinically meaningful antitumor activity with manageable toxicity in heavily pre-treated and treatment-naive patients with RET-mutant medullary thyroid cancer (MTC). LIBRETTO-531 (NCT04211337) is a multicenter, open-label, randomized, controlled, phase III trial comparing selpercatinib to cabozantinib or vandetanib in patients with advanced/metastatic RET-mutant MTC. The primary objective is to compare progression-free survival (per RECIST 1.1) by blinded independent central review of patients with progressive, advanced, multikinase inhibitor-naive, RET-mutant MTC treated with selpercatinib versus cabozantinib or vandetanib. Key secondary objectives are to compare other efficacy outcomes (per RECIST 1.1) and tolerability of selpercatinib versus cabozantinib or vandetanib.

MANAGEMENT OF ENDOCRINE DISEASE: Medullary thyroid cancer: from molecular biology and therapeutic pitfalls to future targeted treatment perspectives

During the last decades, knowledge of the molecular biology in medullary thyroid carcinoma (MTC) and specifically on the role of rearranged during transfection (RET)-activating mutations in tumorigenesis has led to the evolution of novel targeted therapies, mainly tyrosine kinase inhibitors (TKIs). Vandetanib and cabozantinib have been approved for the management of metastatic progressive MTC. Two novel, highly selective RET inhibitors, selpercatinib and pralsetinib, have recently been approved for the treatment of RET-mutant MTCs and RET-fusion differentiated thyroid cancer. The administration of targeted therapies in MTC patients has changed the therapeutic strategies; however, in the majority of cases, there are no real data showing an improvement of prognosis by TKIs in MTC. Drug resistance remains the main reason for treatment failure. Thus, the understanding of the molecular landscape of tumorigenesis and the mechanisms underlying resistance to targeted therapies is of paramount importance for the further development of more efficient therapies for MTC. The present review focuses on the molecular pathways implicated in MTC tumorigenesis, the approved targeted therapies, the tumoral escape mechanisms, as well as the future perspectives for targeted therapy.

Comprehensive immunohistochemical analysis of RET, BCAR1, and BCAR3 expression in patients with Luminal A and B breast cancer subtypes

PURPOSE

Breast cancer (BC) is considered a heterogeneous disease composed of distinct subtypes with diverse clinical outcomes. Luminal subtype tumors have the best prognosis, and patients benefit from endocrine therapy. However, resistance to endocrine therapies in BC is an obstacle to successful treatment, and novel biomarkers are needed to understand and overcome this mechanism. The RET, BCAR1, and BCAR3 genes may be associated with BC progression and endocrine resistance.

METHODS

Aiming to evaluate the expression profile and prognostic value of RET, BCAR1, and BCAR3, we performed immunohistochemistry on tissue microarrays (TMAs) containing a cohort of 361 Luminal subtype BC.

RESULTS

Low expression levels of these three proteins were predominantly observed. BCAR1 expression was correlated with nuclear grade (p = 0.057), and BCAR3 expression was correlated with lymph node status (p = 0.011) and response to hormonal therapy (p = 0.021). Further, low expression of either BCAR1 or BCAR3 was significantly associated with poor prognosis (p = 0.005; p = 0.042). Pairwise analysis showed that patients with tumors with low BCAR1/low BCAR3 expression had a poorer overall survival (p = 0.013), and the low BCAR3 expression had the worst prognosis with RET high expression stratifying these patients into two different groups. Regarding the response to hormonal therapy, non-responder patients presented lower expression of RET in comparison to the responder group (p = 0.035). Additionally, the low BCAR1 expression patients had poorer outcomes than BCAR1 high (p = 0.015).

CONCLUSION

Our findings suggest RET, BCAR1, and BCAR3 as potential candidate markers for endocrine therapy resistance in Luminal BC.

Investigating natural compounds against oncogenic RET tyrosine kinase using pharmacoinformatic approaches for cancer therapeutics

Rearranged during transfection (RET) tyrosine kinase is a transmembrane receptor tyrosine kinase regulating vital aspects of cellular proliferation, differentiation, and survival. An outstanding challenge in designing protein kinase inhibitors is due to the development of drug resistance. The “gain of function” mutations in the RET gate-keeper residue, Val804, confers resistance to the majority of known RET inhibitors, including vandetanib. To curtail this resistance, researchers developed selpercatinib (LOXO-292) against the RET gate-keeper mutant forms – V804M and V804L. In the present in silico investigation, a receptor–ligand pharmacophore model was generated to identify small molecule inhibitors effective for wild-type (WT) as well as mutant RET kinase variants. The generated model was employed to screen 144 766 natural products (NPs) available in the ZINC database and the retrieved NPs were filtered for their drug-likeness. The resulting 2696 drug-like NPs were subjected to molecular docking with the RET WT kinase domain and a total of 27 molecules displayed better dock scores than the reference inhibitors – vandetanib and selpercatinib. From 27 NPs, an aggregate of 12 compounds demonstrated better binding free energy (BFE) scores than the reference inhibitors, towards RET. Thus, the 12 NPs were also subjected to docking, simulation, and BFE estimation towards the constructed gate-keeper RET mutant structures. The BFE calculations revealed 3 hits with better BFE scores than the reference inhibitors towards WT, V804M, and V804L RET variants. Thus, the scaffolds of hit compounds presented in this study could act as potent RET inhibitors and further provide insights for drug optimization targeting aberrant activation of RET signaling, specifically the mutation of gate-keeper residue – Val804.

Identification of natural product inhibitors against rearranged during transfection (RET) tyrosine kinase as cancer therapeutics using combination of in silico techniques.

Tyrosine kinase inhibitor prodrug-loaded liposomes for controlled release at tumor microenvironment

Tyrosine kinase inhibitors (TKIs) represent one of the most advanced class of therapeutics for cancer treatment. Most of them are also cytochrome P450 (CYP) inhibitors and/or substrates thereof. Accordingly, their efficacy and/or toxicity can be affected by CYP-mediated metabolism and by metabolism-derived drug-drug interactions. In order to enhance the therapeutic performance of these drugs, we developed a prodrug (Pro962) of our TKI TK962 specifically designed for liposome loading and pH-controlled release in the tumor. A cholesterol moiety was linked to TK962 through pH-sensitive hydrazone bond for anchoring to the liposome phospholipid bilayer to prevent leakage of the prodrug from the nanocarrier. Bioactivity studies performed on isolated target kinases showed that the prodrug maintains only partial activity against them and the release of TK962 is required. Biopharmaceutical studies carried out with prodrug loaded liposomes showed that the prodrug was firmly associated with the vesicles and the drug release was prevented under blood-mimicking conditions. Conversely, conventional liposome loaded with TK962 readily released the drug. Flow cytometric studies showed that liposomes efficiently provided for intracellular prodrug delivery. The use of the hydrazone linker yielded a pH-controlled drug release, which resulted in about 50% drug release at pH 4 and 5 in 2 h. Prodrug, prodrug loaded liposomes and active lead compound have been tested against cancer cell lines in either 2D or 3D models. The liposome formulation showed higher cytotoxicity than the unformulated lead TK962 in both 2D and 3D models. The stability of prodrug, prodrug loaded liposomes and active lead compound in human serum and against human, mouse, and rat microsomes was also assessed, demonstrating that liposome formulations impair the metabolic reactions and protect the loaded compounds from catabolism. The results suggest that the liposomal formulation of pH releasable TKI prodrugs is a promising strategy to improve the metabolic stability, intracellular cancer cell delivery and release, and in turn the efficacy of this class of anticancer drugs.

Hallmarks of RET and Co-occuring Genomic Alterations in RET-aberrant Cancers

Activating receptor-tyrosine kinase rearranged during transfection (RET) mutations and fusions are potent drivers of oncogenesis. The recent FDA approvals of highly potent and selective RET inhibitors, selpercatinib and pralsetinib, has altered the therapeutic management of RET aberrant tumors. There is ample evidence of the role of RET signaling in certain cancers. RET aberrations as fusions or mutations occur in multiple cancers, however, there is considerable phenotypic diversity. There is emerging data on the lack of responsiveness of immunotherapy in RET-altered cancers. Herein, we review the registrational data from the selective RET-inhibitor trials, and comprehensively explore RET alterations in pan-cancer adult malignancies and their co-alterations. These co-occuring alterations may define the future of RET inhibition from specific selective targeting to customized combination therapies as data are rapidly emerging on both on-target and off-target acquired resistance mechanisms. Fascinatingly, oncogenic RET fusions have been reported to mediate resistance to EGFR inhibition and KRAS^G12C inhibition.

Radiotherapy and immunotherapy converge on elimination of tumor-promoting erythroid progenitor cells through adaptive immunity

Tumor-induced CD45^-Ter119⁺CD71⁺ erythroid progenitor cells, termed "Ter cells," promote tumor progression by secreting artemin (ARTN), a neurotrophic peptide that activates REarranged during Transfection (RET) signaling. We demonstrate that both local tumor ionizing radiation (IR) and anti-programmed death ligand 1 (PD-L1) treatment decreased tumor-induced Ter cell abundance in the mouse spleen and ARTN secretion outside the irradiation field in an interferon- and CD8⁺ T cell-dependent manner. Recombinant erythropoietin promoted resistance to radiotherapy or anti-PD-L1 therapies by restoring Ter cell numbers and serum ARTN concentration. Blockade of ARTN or potential ARTN signaling partners, or depletion of Ter cells augmented the antitumor effects of both IR and anti-PD-L1 therapies in mice. Analysis of samples from patients who received radioimmunotherapy demonstrated that IR-mediated reduction of Ter cells, ARTN, and GFRα3, an ARTN signaling partner, were each associated with tumor regression. Patients with melanoma who received immunotherapy exhibited favorable outcomes associated with decreased expression of GFRα3. These findings demonstrate an out-of-field, or "abscopal," effect mediated by adaptive immunity, which is induced during local tumor irradiation. This effect, in turn, governs the therapeutic effects of radiation and immunotherapy. Therefore, our results identify multiple targets to potentially improve outcomes after radiotherapy and immunotherapy.

Multiple endocrine neoplasia 2A with RET mutation p.Cys611Tyr: A case report

RATIONALE

Multiple endocrine neoplasia 2A (MEN2A) is a rare autosomal-dominant genetic syndrome, frequently misdiagnosed or neglected clinically, resulting in delayed therapy to patients.

PATIENT CONCERNS

A 47-year-old Chinese male patient underwent laparoscopic right adrenal tumorectomy, and postoperative pathology confirmed the tumor as pheochromocytoma (PHEO). He was readmitted to the department of endocrinology and metabolism due to constant increase in carcinoembryonic antigen (CEA) at 5 months after the operation.

DIAGNOSIS

The patient was confirmed with medullary thyroid carcinoma (MTC), multiple neck lymph node metastasis, and pituitary microadenoma. The p.Cys611Tyr (c.1832G>A, C611Y) mutation was detected. Therefore, he was diagnosed with MEN2A.

INTERVENTIONS

He underwent total thyroidectomy. The gene-sequencing analysis of his family was conducted, and the C611Y mutation was detected in his daughter.

OUTCOMES

The level of carcinoembryonic antigen decreased significantly after thyroidectomy in this patient. Long-term follow-up management was conducted. Elevated serum calcitonin and bilateral thyroid nodules were found in his 13-year-old daughter. Thus, MEN2A was highly suspected and she was suggested to undergo total thyroidectomy.

CONCLUSION

Patients with MEN2A should be screened regularly and managed by a multidisciplinary team.

Ultrasonic Characteristics of Medullary Thyroid Carcinoma: Differential From Papillary Thyroid Carcinoma and Benign Thyroid Nodule

ABSTRACT

The aim of this study was to evaluate the differences in ultrasonic features of patients with medullary thyroid carcinoma (MTC), papillary thyroid carcinoma (PTC), and benign thyroid nodules. This study included 53 cases of MTC, 151 cases of PTC, and 200 cases of benign thyroid nodule which were pathologically confirmed. There were no significant differences in sex and thyroid gland involvement among the MTC, PTC, and benign thyroid nodule groups. The age among the 3 groups was statistically different (P = 0.002). The TNM stage of MTC was significantly higher than that of PTC (P < 0.001). Compared with PTC, the lesion size, shape, margin, echogenicity, internal nodule component, and blood flow were significantly different in MTC (P < 0.001, P < 0.001, P < 0.001, P < 0.001, P = 0.045, P < 0.001). However, there were no differences in the form of calcification and cervical lymph node involvement between the 2 groups (P = 0.671, P = 0.128). Except for the lesion size and shape (P = 0.068, P = 0.444), MTC group have significant differences in the grade of Thyroid Imaging Reporting and Data System, lesion margin, echogenicity, internal nodule component, calcification, cervical lymph node, and blood flow compared with benign thyroid nodule group (P < 0.001, P = 0.014, P = 0.032, P < 0.001, P < 0.001, P < 0.001). Our data indicate that ultrasound have important value in preoperative diagnosis of MTC. The ultrasonic features of MTC include relatively large nodules, aspect ratio less than 1, smooth edge, solid hypoechoic, microcalcification, and rich blood flow. It is necessary to combine multiple ultrasonic features for the differential diagnosis of MTC, PTC, and benign thyroid nodules.

Targeted therapies for RET-fusion cancer: Dilemmas and breakthrough

Genomic profiling has revolutionized treatment options for patients with oncogene-driven cancers, such as epidermal growth factor receptor (EGFR) mutant carcinoma. Rearranged during transfection (RET) rearrangement, as one of the main activated oncogenes, has been well studied and found to be involved in the malignant behavior of carcinogenesis, resulting in acquired resistance to EGFR tyrosine kinase inhibitors and inducing an intrinsic resistance to immunotherapy. Thus, targeted therapies have been investigated against RET arrangement cancers, including several multi-kinase inhibitors and selective RET inhibitors. However, modest efficacy, a relatively high rate of toxicity, and poor effectiveness against brain metastasis are common limitations of multi-targeted novel molecular inhibitors. A promising prospect was shown recently in selective RET inhibitors in several ongoing clinical trials. In this review, we reviewed the concurrent dilemmas of targeted therapies against RET arrangement cancer from preclinical and clinical studies and proposed several clinical considerations for clinical practice prospectively.

Small Molecules and Peptides Targeting Glial Cell Line-Derived Neurotrophic Factor Receptors for the Treatment of Neurodegeneration

Glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) are able to promote the survival of multiple neuronal populations in the body and, therefore, hold considerable promise for disease-modifying treatments of diseases and conditions caused by neurodegeneration. Available data reveal the potential of GFLs for the therapy of Parkinson's disease, neuropathic pain and diseases caused by retinal degeneration but, also, amyotrophic lateral sclerosis and, possibly, Alzheimer's disease. Despite promising data collected in preclinical models, clinical translation of GFLs is yet to be conducted. The main reasons for the limited success of GFLs clinical development are the poor pharmacological characteristics of GFL proteins, such as the inability of GFLs to cross tissue barriers, poor diffusion in tissues, biphasic dose-response and activation of several receptors in the organism in different cell types, along with ethical limitations on patients' selection in clinical trials. The development of small molecules selectively targeting particular GFL receptors with improved pharmacokinetic properties can overcome many of the difficulties and limitations associated with the clinical use of GFL proteins. The current review lists several strategies to target the GFL receptor complex with drug-like molecules, discusses their advantages, provides an overview of available chemical scaffolds and peptides able to activate GFL receptors and describes the effects of these molecules in cultured cells and animal models.

Systems-level differential gene expression analysis reveals new genetic variants of oral cancer

Oral cancer (OC) ranked as eleventh malignancy worldwide, with the increasing incidence among young patients. Limited understanding of complications in cancer progression, its development system, and their interactions are major restrictions towards the progress of optimal and effective treatment strategies. The system-level approach has been designed to explore genetic complexity of the disease and to identify novel oral cancer related genes to detect genomic alterations at molecular level, through cDNA differential analysis. We analyzed 21 oral cancer-related cDNA datasets and listed 30 differentially expressed genes (DEGs). Among 30, we found 6 significant DEGs including CYP1A1, CYP1B1, ADCY2, C7, SERPINB5, and ANAPC13 and studied their functional role in OC. Our genomic and interactive analysis showed significant enrichment of xenobiotics metabolism, p53 signaling pathway and microRNA pathways, towards OC progression and development. We used human proteomic data for post-translational modifications to interpret disease mutations and inter-individual genetic variations. The mutational analysis revealed the sequence predicted disordered region of 14%, 12.5%, 10.5% for ADCY2, CYP1B1, and C7 respectively. The MiRNA target prediction showed functional molecular annotation including specific miRNA-targets hsa-miR-4282, hsa-miR-2052, hsa-miR-216a-3p, for CYP1B1, C7, and ADCY2 respectively associated with oral cancer. We constructed the system level network and found important gene signatures. The drug-gene interaction of OC source genes with seven FDA approved OC drugs help to design or identify new drug target or establishing novel biomedical linkages regarding disease pathophysiology. This investigation demonstrates the importance of system genetics for identifying 6 OC genes (CYP1A1, CYP1B1, ADCY2, C7, SERPINB5, and ANAPC13) as potential drugs targets. Our integrative network-based system-level approach would help to find the genetic variants of OC that can accelerate drug discovery outcomes to develop a better understanding regarding treatment strategies for many cancer types.

Receptor Tyrosine Kinases in Osteosarcoma Treatment: Which Is the Key Target?

Recent clinical trials have shown several multi-target tyrosine kinase inhibitors (TKIs) to be effective in the treatment of osteosarcoma. However, these TKIs have a number of targets, and it is yet unclear which of these targets has a key role in osteosarcoma treatment. In this review, we first summarize the TKIs that were studied in clinical trials registered on ClinicalTrials.gov. Further, we compare and discuss the targets of these TKIs. We found that TKIs with promising therapeutic effect for osteosarcoma include apatinib, cabozantinib, lenvatinib, regorafenib, and sorafenib. The key targets for osteosarcoma treatment may include VEGFRs and RET. The receptor tyrosine kinases (RTKs) MET, IGF-1R, AXL, PDGFRs, KIT, and FGFRs might be relevant but unimportant targets for osteosarcoma treatment. Inhibition of one type of RTK for the treatment of osteosarcoma is not effective. It is necessary to inhibit several relevant RTKs simultaneously to achieve a breakthrough in osteosarcoma treatment. This review provides comprehensive information on TKI targets relevant in osteosarcoma treatment, and it will be useful for further research in this field.

Intracellular RET signaling pathways activated by GDNF

Activation of REarranged during Transfection (RET) proto-oncogene is responsible for various human cancers such as papillary and medullary thyroid carcinomas and non-small cell lung carcinomas. RET activation in these tumors is caused by point mutations or gene rearrangements, resulting in constitutive activation of RET tyrosine kinase. Physiologically, RET is activated by glial cell line-derived neurotrophic factor (GDNF) ligands that bind to coreceptor GDNF family receptor alphas (GFRαs), leading to RET dimerization. GDNF-GFRα1-RET signaling plays crucial roles in the development of the enteric nervous system, kidney and lower urinary tract as well as in spermatogenesis. Intracellular tyrosine phosphorylation in RET and recruitment of adaptor proteins to phosphotyrosines are essential for various biological functions. Significance of intracellular RET signaling pathways activated by GDNF is discussed and summarized in this review.

Plasma membrane localization of the GFL receptor components: a nexus for receptor crosstalk

The glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) comprise a group of four homologous and potent growth factors that includes GDNF, neurturin (NRTN), artemin (ARTN), and persephin (PSPN). The survival, growth, and mitotic activities of the GFLs are conveyed by a single receptor tyrosine kinase, Ret. The GFLs do not bind directly to Ret in order to activate it, and instead bind with high affinity to glycerophosphatidylinositol (GPI)-anchored coreceptors called the GDNF family receptor-αs (GFRαs). Several mechanisms have recently been identified that influence the trafficking of Ret and GFRαs in and out of the plasma membrane, thereby affecting their availability for ligand binding, as well as their levels by targeting to degradative pathways. This review describes these mechanisms and their powerful effects on GFL signaling and function. We also describe the recent discovery that p75 and Ret form a signaling complex, also regulated by plasma membrane shuttling, that either enhances GFL survival signals or p75 pro-apoptotic signals, dependent on the cellular context.

Roles of the RET Proto-oncogene in Cancer and Development

RET (REarranged during Transfection)is activated by DNA rearrangement of the 3' fragment of the receptor tyrosine kinase gene, namely, RET proto-oncogene, with the 5' fragment of various genes with putative dimerization domains, such as a coiled coil domain, that are necessary for constitutive activation. RET rearrangements have been detected in a variety of human cancers, including thyroid, lung, colorectal, breast, and salivary gland cancers. Moreover, point mutations in RET are responsible for multiple endocrine neoplasia types 2A and 2B, which can develop into medullary thyroid cancer and pheochromocytoma. Substantial effort is currently being exerted in developing RET kinase inhibitors. RET is also responsible for Hirschsprung's disease, a developmental abnormality in the enteric nervous system. Gene knockout studies have demonstrated that RET plays essential roles in the development of the enteric nervous system and kidney as well as in spermatogenesis. Studies regarding RET continue to provide fascinating challenges in the fields of cancer research, neuroscience, and developmental biology.

RET Gene Fusions in Malignancies of the Thyroid and Other Tissues

Following the identification of the BCR-ABL1 (Breakpoint Cluster Region-ABelson murine Leukemia) fusion in chronic myelogenous leukemia, gene fusions generating chimeric oncoproteins have been recognized as common genomic structural variations in human malignancies. This is, in particular, a frequent mechanism in the oncogenic conversion of protein kinases. Gene fusion was the first mechanism identified for the oncogenic activation of the receptor tyrosine kinase RET (REarranged during Transfection), initially discovered in papillary thyroid carcinoma (PTC). More recently, the advent of highly sensitive massive parallel (next generation sequencing, NGS) sequencing of tumor DNA or cell-free (cfDNA) circulating tumor DNA, allowed for the detection of RET fusions in many other solid and hematopoietic malignancies. This review summarizes the role of RET fusions in the pathogenesis of human cancer.

Ret Receptor Has Distinct Alterations and Functions in Breast Cancer

Ret receptor tyrosine kinase is a proto-oncogene that participates in development of various cancers. Several independent studies have recently identified Ret as a key player in breast cancer. Although Ret overexpression and function have been under investigation, mainly in estrogen receptor positive breast cancer, a more comprehensive analysis of the impact of recurring Ret alterations in breast cancer is needed. This review consolidates the current knowledge of Ret alterations and their potential effects in breast cancer. We discuss and integrate data on Ret changes in different breast cancer subtypes and potential function in progression, as well as the participation of distinct Ret network signaling partners in these processes. We propose that it will be essential to define a shared molecular feature of tumors with alteration in Ret receptor, be this at the genetic level or via overexpression in order to design effective therapies to target the Ret pathway. Here we review experimental evidence from basic research and pre-clinical studies concentrating on Ret alterations as potential biomarkers for recurrence, and we discuss the possibility that targeting the Ret pathway might in the future become a treatment for breast cancer.

Targeted Therapy For RET-Rearranged Non-Small Cell Lung Cancer: Clinical Development And Future Directions

Approximately 1-2% of unselected patients with Non-small Cell Lung Cancer (NSCLC) harbor RET rearrangements resulting in enhanced cell survival and proliferation. The initial treatment strategy for RET rearranged NSCLC has been multi-target tyrosine kinase inhibition. With overall response rates (ORR) of 16-53% and a median progression-free survival (PFS) of 4.5-7.3 months these outcomes are clearly inferior to the efficacy outcomes of selective tyrosine kinase inhibitors (TKI) in other oncogene-addicted NSCLC. Additionally, multi-kinase inhibition in RET-driven NSCLC patients showed concerning rates of high-grade toxicity, mainly induced by anti-VEGFR-kinase activity. Novel selective RET inhibitors like BLU-667, LOXO-292 and RXDX-105 have been recently investigated in early phase clinical trials showing promising efficacy with a manageable toxicity profile.

Targeting RET-rearranged non-small-cell lung cancer: future prospects

Non-small-cell lung cancer (NSCLC) patients with mutated or rearranged oncogene drivers can be treated with upfront selective inhibitors achieving higher response rates and longer survival than chemotherapy. The RET gene can undergo chromosomal rearrangements in 1%–2% of all NSCLC patients, involving various upstream fusion partners such as KIF5B, CCDC6, NCOA4, and TRIM33. Many multikinase inhibitors are active against rearranged RET. Cabozantinib, vandetanib, sunitinib, lenvatinib, and nintedanib achieved tumor responses in about 30% of these patients in retrospective studies. Prospective phase II trials investigated the activity and toxicity of cabozantinib, vandetanib, sorafenib, and lenvatinib, and did not reach significantly higher response rates. VEGFR and EGFR inhibition represented the main ways of developing off-target toxicity. An intrinsic resistance emerged according to the type of RET fusion partners, as KIF5B-RET fusion is the most resistant. Also acquired mutations in rearranged RET oncogene developed as resistance to these multikinase inhibitors. Interestingly, RET fusions have been found as a resistance mechanism to EGFR-TKIs in EGFR-mutant NSCLC patients. The combination of EGFR and RET inhibition can overcome this resistance. The limitations in terms of activity and tolerability of the various multikinase inhibitors prompted the investigation of new highly selective RET inhibitors, such as RXDX-105, BLU-667, and LOXO-292. Some data emerged about intracranial antitumor activity of BLU-667 and LOXO-292. If these novel drugs will achieve high activity in RET rearranged NSCLC, also these oncogene-addicted tumors can undergo a significant survival improvement.

Silencing of COPB2 inhibits the proliferation of gastric cancer cells and induces apoptosis via suppression of the RTK signaling pathway

Emerging studies have reported that coatomer protein complex subunit β2 (COPB2) is overexpressed in several types of malignant tumor; however, to the best of our knowledge, no studies regarding COPB2 in gastric cancer have been published thus far. Therefore, the present study aimed to determine the significance and function of COPB2 in gastric cancer. COPB2 expression in gastric cancer cell lines was measured using reverse transcription-quantitative polymerase chain reaction (RT-qPCR) analysis. In addition, lentivirus-short hairpin RNA (shRNA) COPB2 (Lv-shCOPB2) was generated and used to infect BGC-823 cells to analyze the effects of COPB2 on the cancerous phenotype. The effects of shRNA-mediated COPB2 knockdown on cell proliferation were detected using MTT, 5-bromo-2-deoxyuridine and colony formation assays. In addition, the effects of COPB2 knockdown on apoptosis were analyzed by flow cytometry. Nude mice and fluorescence imaging were used to characterize the regulation of tumor growth in vivo, and qPCR and immunohistochemistry were subsequently conducted to analyze COPB2 expression in xenograft tumor tissues. Furthermore, a receptor tyrosine kinase (RTK) signaling pathway antibody array was used to explore the relevant molecular mechanisms underlying the effects of COPB2 knockdown. The results revealed that COPB2 mRNA was abundantly overexpressed in gastric cancer cell lines, whereas knockdown of COPB2 significantly inhibited cell growth and colony formation ability, and led to increased cell apoptosis in vitro. The tumorigenicity assay revealed that knockdown of COPB2 reduced tumor growth in nude mice, and fluorescence imaging indicated that the total radiant efficiency of mice in the Lv-shCOPB2-infected group was markedly reduced compared with the mice in the Lv-shRNA control-infected group in vivo. The antibody array assay revealed that the levels of phosphorylation in 23 target RTKs were significantly reduced: In conclusion, COPB2 was highly expressed in gastric cancer cell lines, and knockdown suppressed colony formation and promoted cell apoptosis via inhibiting the RTK signaling and its downstream signaling cascade molecules. Therefore, COPB2 may present a valuable target for gene silencing strategy in gastric cancer.

Exploring the Potential of RET Kinase Inhibition for Irritable Bowel Syndrome: A Preclinical Investigation in Rodent Models of Colonic Hypersensitivity

Abdominal pain represents a significant complaint in patients with irritable bowel syndrome (IBS). While the etiology of IBS is incompletely understood, prior exposure to gastrointestinal inflammation or psychologic stress is frequently associated with the development of symptoms. Inflammation or stress-induced expression of growth factors or cytokines may contribute to the pathophysiology of IBS. Here, we aimed to investigate the therapeutic potential of inhibiting the receptor of glial cell line-derived neurotrophic factor, rearranged during transfection (RET), in experimental models of inflammation and stress-induced visceral hypersensitivity resembling IBS sequelae. In RET-cyan fluorescent protein [(CFP) Ret^CFP/+] mice, thoracic and lumbosacral dorsal root ganglia were shown to express RET, which colocalized with calcitonin gene-related peptide. To understand the role of RET in visceral nociception, we employed GSK3179106 as a potent, selective, and gut-restricted RET kinase inhibitor. Colonic hyperalgesia, quantified as exaggerated visceromotor response to graded pressures (0-60 mm Hg) of isobaric colorectal distension (CRD), was produced in multiple rat models induced 1) by colonic irritation, 2) following acute colonic inflammation, 3) by adulthood stress, and 4) by early life stress. In all the rat models, RET inhibition with GSK3179106 attenuated the number of abdominal contractions induced by CRD. Our findings identify a role for RET in visceral nociception. Inhibition of RET kinase with a potent, selective, and gut-restricted small molecule may represent a novel therapeutic strategy for the treatment of IBS through the attenuation of post-inflammatory and stress-induced visceral hypersensitivity.

Evolution of oncogenic signatures of mutation hotspots in tyrosine kinases supports the atavistic hypothesis of cancer

Cancer has been shown as an evolutionary process emerging hallmarks that are reminiscent of unicellular organisms. Since cancer is mostly driven by somatic mutations, especially by oncogenic hotspot mutations, we proposed a molecular atavism of cancer caused by gain-of-function mutations in oncogenes. As tyrosine kinase (TK) family contains the largest subgroup of oncogenes with hotspot mutations, we traced the most predominant mutation hotspots of TK oncogenes across phylogeny with the domain information and adjacent sequences integrated as onco-signatures. We detected 9 out of 17 TK oncogenes with onco-homologs possessing an onco-signature, which could be divided into two classes by whether their onco-homologs existed in mammals or not. In Class I we identified mammalian onco-homologs assuming oncogenic functions with onco-signatures always intact in cancer, such as HCK and LYN. In Class II with no bona fide mammalian onco-homologs, Pyk2, a protist onco-homolog with an onco-signature of BRAF was found assuming oncogenic-like functions. Onco-signatures in both classes root deep in the primitive system. Together, these evidences supported our proposal that cancer can be driven by reverse evolution of oncogenes through gain-of-function mutations. And also for the first time, we provided the specific targets for experimental verification of the atavistic hypothesis of cancer.

Preclinical evaluation of a GFRA1 targeted antibody-drug conjugate in breast cancer

Despite recent advances in treatment, breast cancer remains the second-most common cause of cancer death among American women. A greater understanding of the molecular characteristics of breast tumors could ultimately lead to improved tumor-targeted treatment options, particularly for subsets of breast cancer patients with unmet needs. Using an unbiased genomics approach to uncover membrane-localized tumor-associated antigens (TAAs), we have identified glial cell line derived neurotrophic factor (GDNF) family receptor α 1 (GFRA1) as a breast cancer TAA. Immunohistochemistry (IHC) revealed that GFRA1 displays a limited normal tissue expression profile coupled with overexpression in specific breast cancer subsets. The cell surface localization as determined by fluorescence-activated cell sorting (FACS) and the rapid internalization kinetics of GFRA1 makes it an ideal target for therapeutic exploitation as an antibody-drug conjugate (ADC). Here, we describe the development of a pyrrolobenzodiazepine (PBD)-armed, GFRA1-targeted ADC that demonstrates cytotoxicity in GFRA1-positive cell lines and patient-derived xenograft (PDX) models. The safety profile of the rat cross-reactive GFRA1-PBD was assessed in a rat toxicology study to find transient cellularity reductions in the bone marrow and peripheral blood, consistent with known off-target effects of PBD ADC's. These studies reveal no evidence of on-target toxicity and support further evaluation of GFRA1-PBD in GFRA1-positive tumors.

Role of artemin in non-small cell lung cancer

Background

In this study, we investigated the role of artemin, a member of the glial cell‐derived neurotrophic factor of ligands, in the malignant phenotype of lung cancer.

Methods

Artemin expression was examined in various types of lung cancer and normal lung tissues, as well as in lung cancer cell lines by immunohistochemistry and semi‐quantitative PCR. Functional studies were performed using artemin overexpression or knockdown vectors in lung cancer cell lines. Methyl thiazolyl tetrazolium, flow cytometry, wound healing, and transwell assays were conducted to evaluate the contribution of artemin on tumor cell proliferation, migration, and invasion.

Results

Artemin is broadly expressed in lung cancer tissues, and is associated with tumor staging. Overexpression of artemin in NL9980 large cell lung cancer cells increased proliferating cells and enhanced migrating capability in wound healing and transwell assays, as well as demonstrating enhanced invasion capability. Silencing artemin in LTEP‐α‐2 adenocarcinoma cell lines decreased cellular proliferation, migration, and invasion capabilities.

Conclusion

Artemin could promote the proliferation and invasiveness of lung cancer cells in vitro and therefore could be a new potential target to combat lung cancer.

EGFR mediates activation of RET in lung adenocarcinoma with neuroendocrine differentiation characterized by ASCL1 expression

Achaete-scute homolog 1 (ASCL1) is a neuroendocrine transcription factor specifically expressed in 10-20% of lung adenocarcinomas (AD) with neuroendocrine (NE) differentiation (NED). ASCL1 functions as an upstream regulator of the RET oncogene in AD with high ASCL1 expression (A⁺AD). RET is a receptor tyrosine kinase with two main human isoforms; RET9 (short) and RET51 (long). We found that elevated expression of RET51 associated mRNA was highly predictive of poor survival in stage-1 A⁺AD (p=0.0057). Functional studies highlighted the role of RET in promoting invasive properties of A⁺AD cells. Further, A⁺AD cells demonstrated close to 10 fold more sensitivity to epidermal growth factor receptor (EGFR) inhibitors, including gefitinib, than AD cells with low ASCL1 expression. Treatment with EGF robustly induced phosphorylation of RET at Tyr-905 in A⁺AD cells with wild type EGFR. This phosphorylation was blocked by gefitinib and by siRNA-EGFR. Immunoprecipitation experiments found EGFR in a complex with RET in the presence of EGF and suggested that RET51 was the predominant RET isoform in the complex. In the microarray datasets of stage-1 and all stages of A⁺AD, high levels of EGFR and RET RNA were significantly associated with poor overall survival (p < 0.01 in both analyses). These results implicate EGFR as a key regulator of RET activation in A⁺AD and suggest that EGFR inhibitors may be therapeutic in patients with A⁺AD tumors even in the absence of an EGFR or RET mutation.

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.

RET-mediated modulation of tumor microenvironment and immune response in multiple endocrine neoplasia type 2 (MEN2)

Medullary thyroid carcinomas (MTC) arise from thyroid parafollicular, calcitonin-producing C-cells and can occur either as sporadic or as hereditary diseases in the context of familial syndromes, including multiple endocrine neoplasia 2A (MEN2A), multiple endocrine neoplasia 2B (MEN2B) and familial MTC (FMTC). In a large fraction of sporadic cases, and virtually in all inherited cases of MTC, activating point mutations of the RET proto-oncogene are found. RET encodes for a receptor tyrosine kinase protein endowed with transforming potential on thyroid parafollicular cells. As in other cancer types, microenvironmental factors play a critical role in MTC. Tumor-associated extracellular matrix, stromal cells and immune cells interact and influence the behavior of cancer cells both in a tumor-promoting and in a tumor-suppressing manner. Several studies have shown that, besides the neoplastic transformation of thyroid C-cells, a profound modification of tumor microenvironment has been associated to the RET FMTC/MEN2-associated oncoproteins. They influence the surrounding stroma, activating cancer-associated fibroblasts (CAFs), promoting cancer-associated inflammation and suppressing anti-cancer immune response. These mechanisms might be exploited to develop innovative anti-cancer therapies and novel prognostic tools in the context of familial, RET-associated MTC.

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.

Molecular genetics of syndromic and non-syndromic forms of parathyroid carcinoma

Parathyroid carcinoma (PC) may occur as part of a complex hereditary syndrome or an isolated (i.e., non-syndromic) non-hereditary (i.e., sporadic) endocrinopathy. Studies of hereditary and syndromic forms of PC, which include the hyperparathyroidism-jaw tumor syndrome (HPT-JT), multiple endocrine neoplasia types 1 and 2 (MEN1 and MEN2), and familial isolated primary hyperparathyroidism (FIHP), have revealed some genetic mechanisms underlying PC. Thus, cell division cycle 73 (CDC73) germline mutations cause HPT-JT, and CDC73 mutations occur in 70% of sporadic PC, but in only ∼2% of parathyroid adenomas. Moreover, CDC73 germline mutations occur in 20%-40% of patients with sporadic PC and may reveal unrecognized HPT-JT. This indicates that CDC73 mutations are major driver mutations in the etiology of PCs. However, there is no genotype-phenotype correlation and some CDC73 mutations (e.g., c.679_680insAG) have been reported in patients with sporadic PC, HPT-JT, or FIHP. Other genes involved in sporadic PC include germline MEN1 and rearranged during transfection (RET) mutations and somatic alterations of the retinoblastoma 1 (RB1) and tumor protein P53 (TP53) genes, as well as epigenetic modifications including DNA methylation and histone modifications, and microRNA misregulation. This review summarizes the genetics and epigenetics of the familial syndromic and non-syndromic (sporadic) forms of PC.

Mechanisms that drive inflammatory tumor microenvironment, tumor heterogeneity, and metastatic progression

Treatment of cancer metastasis has been largely ineffective. It is paramount to understand the mechanisms underlying the metastatic process, of which the tumor microenvironment is an indispensable participant. What are the critical cellular and molecular players at the primary tumor site where metastatic cascade initiates? How is tumor-associated inflammation regulated? How do altered vasculatures contribute to metastasis? What is the dynamic nature or heterogeneity of primary tumors and what are the challenges to catch a moving target? This review summarizes recent progress, mechanistic understanding, and options for metastasis-targeted therapy.

Targeting RET-driven cancers: lessons from evolving preclinical and clinical landscapes

The gene encoding the receptor-tyrosine kinase RET was first discovered more than three decades ago, and activating RET rearrangements and mutations have since been identified as actionable drivers of oncogenesis. Several multikinase inhibitors with activity against RET have been explored in the clinic, and confirmed responses to targeted therapy with these agents have been observed in patients with RET-rearranged lung cancers or RET-mutant thyroid cancers. Nevertheless, response rates to RET-directed therapy are modest compared with those achieved using targeted therapies matched to other oncogenic drivers of solid tumours, such as sensitizing EGFR or BRAF^V600E mutations, or ALK or ROS1 rearrangements. To date, no RET-directed targeted therapeutic has received regulatory approval for the treatment of molecularly defined populations of patients with RET-mutant or RET-rearranged solid tumours. In this Review, we discuss how emerging data have informed the debate over whether the limited success of multikinase inhibitors with activity against RET can be attributed to the tractability of RET as a drug target or to the lack, until 2017, of highly specific inhibitors of this oncoprotein in the clinic. We emphasize that novel approaches to targeting RET-dependent tumours are necessary to improve the clinical efficacy of single-agent multikinase inhibition and, thus, hasten approvals of RET-directed targeted therapies.

Discovery of wt RET and V804M RET Inhibitors: From Hit to Lead

Oncogenic activation of RET kinase has been found in several neoplastic diseases, like medullary thyroid carcinoma, multiple endocrine neoplasia, papillary thyroid carcinoma, and non-small-cell lung cancer. Currently approved RET inhibitors were not originally designed to be RET inhibitors, and their potency against RET kinase has not been optimized. Hence, novel compounds able to inhibit both wild-type RET (^wt RET) and its mutants (e.g., ^V804M RET) are needed. Herein we present the development and the preliminary evaluation of a new sub-micromolar ^wt RET/^V804M RET inhibitor, N-(2-fluoro-5-trifluoromethylphenyl)-N'-{4'-[(2''-benzamido)pyridin-4''-ylamino]phenyl}urea (69), endowed with a 4-anilinopyridine structure, starting from our previously identified 4-anilinopyrimidine hit compound. Profiling against a panel of kinases indicated 69 as a multi cKIT/^wt RET/^V804M RET inhibitor.

The molecular basis for RET tyrosine-kinase inhibitors in thyroid cancer

RET receptor tyrosine kinase acts as a mutated oncogenic driver in several human malignancies and it is over-expressed in other cancers. Small molecule compounds with RET tyrosine kinase inhibitory activity are being investigated for the targeted treatment of these malignancies. Multi-targeted compounds with RET inhibitory concentration in the nanomolar range have entered clinical practice. This review summarizes mechanisms of RET oncogenic activity and properties of new compounds that, at the preclinical stage, have demonstrated promising anti-RET activity.

The RET E616Q Variant is a Gain of Function Mutation Present in a Family with Features of Multiple Endocrine Neoplasia 2A

The REarranged during Transfection (RET) proto-oncogene is a receptor tyrosine kinase involved in growth and differentiation during embryogenesis and maintenance of the urogenital and nervous systems in mammals. Distinct mutations across hotspot RET exons can cause Multiple Endocrine Neoplasia Type 2A (MEN2A) characterised by development of medullary thyroid cancer (MTC), phaeochromocytoma (PCC) and primary hyperparathyroidism (PHPT), with a strong correlation between genotype and phenotype. Here, we report a 42-year-old man presented in the clinic with a unilateral PCC, with subsequent investigations revealing a nodular and cystic thyroid gland. He proceeded to thyroidectomy, which showed bilateral C-cell hyperplasia (CCH) without evidence of MTC. His brother had neonatal Hirschsprung disease (HSCR). Genetic testing revealed the presence of a heterozygous variant of unknown significance (VUS) in the cysteine-rich region of exon 10 in the RET gene (c.1846G>C, p.E616Q), in both affected siblings and their unaffected mother. Exon 10 RET mutations are known to be associated with HSCR and MEN2. Variants in the cysteine-rich region of the RET gene, outside of the key cysteine residues, may contribute to the development of MEN2 in a less aggressive manner, with a lower penetrance of MTC. Currently, a VUS in RET cannot be used to inform clinical management and direct future care. Analysis of RET^E616Q reveals a gain of function mutant phenotype for this variant, which has not previously been reported, indicating that this VUS should be considered at risk for future clinical management.