Structure and physiology of the RET receptor tyrosine kinase

The rapidly changing field of predictive biomarkers of non-small cell lung cancer

Lung cancer is a leading cause of cancer-related death worldwide in both men and women, however mortality in the US and EU are recently declining in parallel with the gradual cut of smoking prevalence. Consequently, the relative frequency of adenocarcinoma increased while that of squamous and small cell carcinomas declined. During the last two decades a plethora of targeted drug therapies have appeared for the treatment of metastasizing non-small cell lung carcinomas (NSCLC). Personalized oncology aims to precisely match patients to treatments with the highest potential of success. Extensive research is done to introduce biomarkers which can predict the effectiveness of a specific targeted therapeutic approach. The EGFR signaling pathway includes several sufficient targets for the treatment of human cancers including NSCLC. Lung adenocarcinoma may harbor both activating and resistance mutations of the EGFR gene, and further, mutations of KRAS and BRAF oncogenes. Less frequent but targetable genetic alterations include ALK, ROS1, RET gene rearrangements, and various alterations of MET proto-oncogene. In addition, the importance of anti-tumor immunity and of tumor microenvironment has become evident recently. Accumulation of mutations generally trigger tumor specific immune defense, but immune protection may be upregulated as an aggressive feature. The blockade of immune checkpoints results in potential reactivation of tumor cell killing and induces significant tumor regression in various tumor types, such as lung carcinoma. Therapeutic responses to anti PD1-PD-L1 treatment may correlate with the expression of PD-L1 by tumor cells. Due to the wide range of diagnostic and predictive features in lung cancer a plenty of tests are required from a single small biopsy or cytology specimen, which is challenged by major issues of sample quantity and quality. Thus, the efficacy of biomarker testing should be warranted by standardized policy and optimal material usage. In this review we aim to discuss major targeted therapy-related biomarkers in NSCLC and testing possibilities comprehensively.

CCDC6-RET fusion protein regulates Ras/MAPK signaling through the fusion- GRB2-SHC1 signal niche

Rearranged during transfection (RET) rearrangement oncoprotein-mediated Ras/MAPK signaling cascade is constitutively activated in cancers. Here, we demonstrate a unique signal niche. The niche is a ternary complex based on the chimeric RET liquid-liquid phase separation. The complex comprises the rearranged kinase (RET fusion); the adaptor (GRB2), and the effector (SHC1). Together, they orchestrate the Ras/MAPK signal cascade, which is dependent on tyrosine kinase. CCDC6-RET fusion undergoes LLPS requiring its kinase domain and its fusion partner. The CCDC6-RET fusion LLPS promotes the autophosphorylation of RET fusion, with enhanced kinase activity, which is necessary for the formation of the signaling niche. Within the signal niche, the interactions among the constituent components are reinforced, and the signal transduction efficiency is amplified. The specific RET fusion-related signal niche elucidates the mechanism of the constitutive activation of the Ras/MAPK signaling pathway. Beyond just focusing on RET fusion itself, exploration of the ternary complex potentially unveils a promising avenue for devising therapeutic strategies aimed at treating RET fusion-driven diseases.

Targeting RET alterations in non-small cell lung cancer

Rearranged during transfection (RET) alterations, which lead to aberrant activation of the RET proto-oncogene, have been identified in various cancers. In non-small cell lung cancer (NSCLC), RET mutations often manifest as RET fusion genes and are observed in 1-2 % of patients with NSCLC. In recent years, selective RET inhibitors such as selpercatinib and pralsetinib, approved by the Food and Drug Administration (FDA) in 2020, have been part of the revolutionary changes in the treatment landscape for non-small cell lung cancer. While first-generation RET inhibitors have become part of the standard of care for RET-fusion positive NSCLC, a new challenge has emerged: acquired resistance to RET inhibitors. RET resistance is a complex phenomenon that can manifest as either on-target or off-target resistance. Numerous studies have been conducted to identify the mechanisms behind this resistance. This review provides an overview of the biology of RET in NSCLC, methods of RET testing, and a comprehensive analysis of the clinical outcomes associated with multikinase and selective RET inhibitors for NSCLC. Additionally, we will explore future perspectives for RET fusion-positive NSCLC, including ongoing trials and the challenges involved in overcoming resistance to RET inhibitors.

Architecture and regulation of a GDNF-GFRα1 synaptic adhesion assembly

Glial-cell line derived neurotrophic factor (GDNF) bound to its co-receptor GFRα1 stimulates the RET receptor tyrosine kinase, promoting neuronal survival and neuroprotection. The GDNF-GFRα1 complex also supports synaptic cell adhesion independently of RET. Here, we describe the structure of a decameric GDNF-GFRα1 assembly determined by crystallography and electron microscopy, revealing two GFRα1 pentamers bridged by five GDNF dimers. We reconsitituted the assembly between adhering liposomes and used cryo-electron tomography to visualize how the complex fulfils its membrane adhesion function. The GFRα1:GFRα1 pentameric interface was further validated both in vitro by native PAGE and in cellulo by cell-clustering and dendritic spine assays. Finally, we provide biochemical and cell-based evidence that RET and heparan sulfate cooperate to prevent assembly of the adhesion complex by competing for the adhesion interface. Our results provide a mechanistic framework to understand GDNF-driven cell adhesion, its relationship to trophic signalling, and the central role played by GFRα1.

Targeting the RET tyrosine kinase in neuroblastoma: A review and application of a novel selective drug design strategy

The RET (REarranged during Transfection) gene, which encodes for a transmembrane receptor tyrosine kinase, is an established oncogene associated with the etiology and progression of multiple types of cancer. Oncogenic RET mutations and rearrangements resulting in gene fusions have been identified in many adult cancers, including medullary and papillary thyroid cancers, lung adenocarcinomas, colon and breast cancers, and many others. While genetic RET aberrations are much less common in pediatric solid tumors, increased RET expression has been shown to be associated with poor prognosis in children with solid tumors such as neuroblastoma, prompting an interest in RET inhibition as a form of therapy for these children. A number of kinase inhibitors currently in use for patients with cancer have RET inhibitory activity, but these inhibitors also display activity against other kinases, resulting in unwanted side effects and limiting their safety and efficacy. Recent efforts have been focused on developing more specific RET inhibitors, but due to high levels of conservation between kinase binding pockets, specificity remains a drug design challenge. Here, we review the background of RET as a potential therapeutic target in neuroblastoma tumors and the results of recent preclinical studies and clinical trials evaluating the safety and efficacy of RET inhibition in adults and children. We also present a novel approach to drug discovery leveraging the chemical phenomenon of atropisomerism to develop specific RET inhibitors and present preliminary data demonstrating the efficacy of a novel RET inhibitor against neuroblastoma tumor cells.

Design and Synthesis of Novel Thieno[3,2-c]quinoline Compounds with Antiproliferative Activity on RET-Dependent Medullary Thyroid Cancer Cells

RET kinase gain-of-function mutations represent the main cause of the high aggressiveness and invasiveness of medullary thyroid cancer (MTC). The selective inhibition of the RET kinase is a suitable strategy for the treatment of this endocrine neoplasia. Herein, we performed an innovative ligand-based virtual screening protocol using the DRUDITonline web service, focusing on the RET kinase as a biological target. In this process, thieno[3,2-c]quinolines 6a-e and 7a-e were proposed as new potential RET inhibitors. The selected compounds were synthetized by appropriate synthetic strategies, and in vitro evaluation of antiproliferative properties conducted on the particularly aggressive MTC cell line TT(C634R) identified compounds 6a-d as promising anticancer agents, with IC50 values in the micromolar range. Further structure-based computational studies revealed a significant capability of the most active compounds to the complex RET tyrosine kinase domain. The interesting antiproliferative results supported by in silico predictions suggest that these compounds may represent a starting point for the development of a new series of small heterocyclic molecules for the treatment of MTC.

Recent advances in nitrogen-containing heterocyclic compounds as receptor tyrosine kinase inhibitors for the treatment of cancer: Biological activity and structural activity relationship

Erratic cell proliferation is the initial symptom of cancer, which can eventually metastasize to other organs. Before cancer becomes metastatic, its spread is triggered by pro-angiogenic factors including vascular endothelial growth factor receptor (VEGFR), epidermal growth factor receptor (EGFR), Platelet-derived growth factor receptor (PDGFR), fibroblast growth factor receptor (FGFR) and Platelet Factor (PF4), all of which are part of receptor tyrosine kinase (RTK) family. Receptor tyrosine kinases (RTKs) are cell-surface proteins and aresignaling enzymes that transfer ATP-phosphate to tyrosine residue substrates. Important biological processes like proliferation, differentiation, motility, and cell-cycle regulation are all possessedby these proteins. Unusual RTK expression is typically associated with cell growth abnormalities, which is linked to tumor acquisition, angiogenesis, and cancer progression. In addition to the already available medications, numerous other heterocyclic are being studied for their potential action against a variety of cancers. In the fight against cancer, in particular, these heterocycles have been used for their dynamic core scaffold and their inherent adaptability. In this review article, we have compiled last five years research work including nitrogen containing heterocycles that have targeted RTK. Herein, the SAR and activity of various compounds containing diverse heterocyclic (pyrimidine, indole, pyridine, pyrazole, benzimidazole, and pyrrole) scaffolds are discussed, and they may prove useful in the future for designing new leads against RTKs. Our focus in this manuscript is to comprehensively review the latest research on the biological activity and structural activity relationship of nitrogen compounds as RTK inhibitors. We believe that this may be an important contribution to the field, as it can help guide future research efforts and facilitate the development of more effective cancer therapies.

From Circulating Tumor Cells to Mirna: New Challenges in the Diagnosis and Prognosis of Medullary Thyroid Cancer

Medullary thyroid carcinoma (MTC) is a malignant tumor that arises from parafollicular C cells, which are responsible for producing calcitonin. The majority (75%) of MTC cases are sporadic forms, while the remaining (25%) have a hereditary component. In these hereditary cases, MTC can occur in conjunction with other endocrine disorders (i.e., pheochromocytoma) or as an isolated condition known as familial medullary thyroid carcinoma. The primary genetic mutation associated with the development of MTC, regardless of its hereditary or sporadic nature, is a point mutation in the RET gene. Evaluation of serum calcitonin levels represents the most reliable and sensitive marker for both the initial diagnosis and the postsurgical monitoring of MTC. Unfortunately, most patients do not achieve normalization of postsurgical serum calcitonin (CT) levels after surgery. Therefore, there is a need to find new biomarkers to be used with serum CT in order to increase test sensitivity and specificity. In this review, we summarize the literature from 2010 to 2023 to review the role of circulating tumor cells, cell-free DNA, and miRNA and their application in diagnosis, outcome of MTC, and response to treatments.

Optogenetic dissection of RET signaling reveals robust activation of ERK and enhanced filopodia-like protrusions of regenerating axons

RET (REarranged during Transfection) is a receptor tyrosine kinase that transduces various external stimuli into biological functions, such as survival and differentiation, in neurons. In the current study, we developed an optogenetic tool for modulating RET signaling, termed optoRET, combining the cytosolic region of human RET with a blue-light-inducible homo-oligomerizing protein. By varying the duration of photoactivation, we were able to dynamically modulate RET signaling. Activation of optoRET recruited Grb2 (growth factor receptor-bound protein 2) and stimulated AKT and ERK (extracellular signal-regulated kinase) in cultured neurons, evoking robust and efficient ERK activation. By locally activating the distal part of the neuron, we were able to retrogradely transduce the AKT and ERK signal to the soma and trigger formation of filopodia-like F-actin structures at stimulated regions through Cdc42 (cell division control 42) activation. Importantly, we successfully modulated RET signaling in dopaminergic neurons of the substantia nigra in the mouse brain. Collectively, optoRET has the potential to be developed as a future therapeutic intervention, modulating RET downstream signaling with light.

RET signaling in breast cancer therapeutic resistance and metastasis

RET, a single-pass receptor tyrosine kinase encoded on human chromosome 10, is well known to the field of developmental biology for its role in the ontogenesis of the central and enteric nervous systems and the kidney. In adults, RET alterations have been characterized as drivers of non-small cell lung cancer and multiple neuroendocrine neoplasms. In breast cancer, RET signaling networks have been shown to influence diverse functions including tumor development, metastasis, and therapeutic resistance. While RET is known to drive the development and progression of multiple solid tumors, therapeutic agents selectively targeting RET are relatively new, though multiple multi-kinase inhibitors have shown promise as RET inhibitors in the past; further, RET has been historically neglected as a potential therapeutic co-target in endocrine-refractory breast cancers despite mounting evidence for a key pathologic role and repeated description of a bi-directional relationship with the estrogen receptor, the principal driver of most breast tumors. Additionally, the recent discovery of RET enrichment in breast cancer brain metastases suggests a role for RET inhibition specific to advanced disease. This review assesses the status of research on RET in breast cancer and evaluates the therapeutic potential of RET-selective kinase inhibitors across major breast cancer subtypes.

RET rearrangement-positive pancreatic cancer has remarkable response to pralsetinib: a case report

Patients with metastatic pancreatic cancer have limited treatment options and a dismal prognosis. While RET fusion is rare (0.6%) in pancreatic cancer, the efficacy of RET-targeted treatment in patients with TRIM33-RET fusion has not been previously reported. Herein, we presented a case of a 68-year-old man with pancreatic cancer harboring TRIM33-RET fusion who responded remarkably to pralsetinib despite being intolerant to chemotherapy. To our knowledge, this is the first report on the clinical value of a single TRIM33-RET fusion in pancreatic cancer, which may benefit from the targeted therapy.

FOXA2 and STAT5A regulate oncogenic activity of KIF5B-RET fusion

KIF5B-RET gene rearrangement occurs in ~1% of lung adenocarcinomas. Recently, targeted agents that inhibit RET phosphorylation have been evaluated in several clinical studies; however, little is known about the role of this gene fusion in driving lung cancer. Immunohistochemistry was used to evaluate the expression of the FOXA2 protein in tumor tissues of patients with lung adenocarcinoma. KIF5B-RET fusion cells proliferated in a cohesive form and grew tightly packed with variable-sized colonies. The expression of RET and its downstream signaling molecules, including p-BRAF, p-ERK, and p-AKT, increased. In KIF5B-RET fusion cells, the intracellular expression of p-ERK was higher in the cytoplasm than in the nucleus. Two transcription factors, STAT5A and FOXA2, exhibiting significantly different expressions at the mRNA level, were finally selected. p-STAT5A was highly expressed in the nucleus and cytoplasm, whereas the expression of the FOXA2 protein was lower; however, it was much higher in the nucleus than in the cytoplasm. Compared with the expression of FOXA2 in the RET rearrangement-wild NSCLC (45.0%), high expression (3+) were observed in most RET rearrangement NSCLCs (94.4%). Meanwhile, KIF5B-RET fusion cells began to increase belatedly from day 7 and only doubled on day 9 in 2D cell culture. However, tumors in mice injected with KIF5B-RET fusion cells began to rapidly increase from day 26. In cell cycle analyses, the KIF5B-RET fusion cells in G0/G1 were increased on day 4 (50.3 ± 2.6%) compared with the empty cells (39.3 ± 5.2%; P = 0.096). Cyclin D1 and E2 expressions were reduced, whereas CDK2 expression slightly increased. pRb and p21 expression was diminished compared with the empty cells, TGF-β1 mRNA was highly expressed, and the proteins were accumulated mostly in the nucleus. Twist mRNA and protein expression was increased, whereas Snail mRNA and protein expression was decreased. Particularly, in KIF5B-RET fusion cells treated with FOXA2 siRNA, the expression of TGF-β 1 mRNA was remarkably reduced but Twist1 and Snail mRNA were increased. Our data suggest that cell proliferation and invasiveness in KIF5B-RET fusion cells are regulated by the upregulation of STAT5A and FOXA2 through the continuous activation of multiple RET downstream signal cascades, including the ERK and AKT signaling pathways. We found that TGF-β1 mRNA, where significant increments were observed in KIF5B-RET fusion cells, is regulated at the transcriptional level by FOXA2.

Clinically relevant fusion oncogenes: detection and practical implications

Mechanistically, chimeric genes result from DNA rearrangements and include parts of preexisting normal genes combined at the genomic junction site. Some rearranged genes encode pathological proteins with altered molecular functions. Those which can aberrantly promote carcinogenesis are called fusion oncogenes. Their formation is not a rare event in human cancers, and many of them were documented in numerous study reports and in specific databases. They may have various molecular peculiarities like increased stability of an oncogenic part, self-activation of tyrosine kinase receptor moiety, and altered transcriptional regulation activities. Currently, tens of low molecular mass inhibitors are approved in cancers as the drugs targeting receptor tyrosine kinase (RTK) oncogenic fusion proteins, that is, including ALK, ABL, EGFR, FGFR1-3, NTRK1-3, MET, RET, ROS1 moieties. Therein, the presence of the respective RTK fusion in the cancer genome is the diagnostic biomarker for drug prescription. However, identification of such fusion oncogenes is challenging as the breakpoint may arise in multiple sites within the gene, and the exact fusion partner is generally unknown. There is no gold standard method for RTK fusion detection, and many alternative experimental techniques are employed nowadays to solve this issue. Among them, RNA-seq-based methods offer an advantage of unbiased high-throughput analysis of only transcribed RTK fusion genes, and of simultaneous finding both fusion partners in a single RNA-seq read. Here we focus on current knowledge of biology and clinical aspects of RTK fusion genes, related databases, and laboratory detection methods.

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).

Cryo-electron Microscopic Analysis of Single-Pass Transmembrane Receptors

Single-pass transmembrane receptors (SPTMRs) represent a diverse group of integral membrane proteins that are involved in many essential cellular processes, including signal transduction, cell adhesion, and transmembrane transport of materials. Dysregulation of the SPTMRs is linked with many human diseases. Despite extensive efforts in past decades, the mechanisms of action of the SPTMRs remain incompletely understood. One major hurdle is the lack of structures of the full-length SPTMRs in different functional states. Such structural information is difficult to obtain by traditional structural biology methods such as X-ray crystallography and nuclear magnetic resonance (NMR). The recent rapid development of single-particle cryo-electron microscopy (cryo-EM) has led to an exponential surge in the number of high-resolution structures of integral membrane proteins, including SPTMRs. Cryo-EM structures of SPTMRs solved in the past few years have tremendously improved our understanding of how SPTMRs function. In this review, we will highlight these progresses in the structural studies of SPTMRs by single-particle cryo-EM, analyze important structural details of each protein involved, and discuss their implications on the underlying mechanisms. Finally, we also briefly discuss remaining challenges and exciting opportunities in the field.

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.

Somatic RET Indels in Sporadic Medullary Thyroid Cancer: Prevalence and Response to Selpercatinib

CONTEXT

Although the majority of RET alterations are single nucleotide variants (SNV), small deletions and/or insertions have been reported at variable prevalence. No information about the efficacy of RET-specific inhibitors in patients harboring RET indels has been provided.

OBJECTIVE

We present an update on the prevalence of RET indels in medullary thyroid cancer (MTC) and describe the efficacy of selpercatinib in patients with advanced MTC with RET indels.

METHODS

The MTC tissues of 287 patients were analyzed using an Ion S5 targeted sequencing. The functional role of the reported indels have been evaluated by MutationTaster. Clinical and pathological data of MTC patients harboring a RET indel were collected and analyzed. Two patients with a RET indel were treated with selpercatinib.

RESULTS

Among 178 RET-positive cases, 147 (82.6%) harbored a SNV and 31 (17.4%) a RET in-frame indel. Nine indels were not previously reported and were found to be disease causing by MutationTaster. Patients harboring an indel were found to have an aggressive disease and 2 of them were treated with selpercatinib, experiencing a good response to the treatment.

CONCLUSION

These data show that RET indels are not infrequent and correlate with an aggressive disease. Two RET indel-positive patients showed a partial response to the treatment with a highly selective RET inhibitor; thus, these RET indels can be considered actionable mutations. In order to not miss these alterations, the analysis of the full gene is recommended.

RET kinase inhibitors for RET-altered thyroid cancers

Precision oncology has opened a new era in cancer treatment focused on targeting specific cellular pathways directly involved in tumorigenesis. The REarrangement during Transfection (RET) proto-oncogene is involved in the pathogenesis of various thyroid cancer subtypes. Mutations in RET give rise to both hereditary and sporadic medullary thyroid cancer (MTC). RET fusions are found in follicular cell-derived thyroid cancers (papillary, poorly differentiated, and anaplastic). Hence, drugs that block the RET tyrosine kinase receptor have been explored in the management of locally advanced or metastatic thyroid cancer. The multikinase inhibitors (MKIs) with nonselective RET inhibition are sorafenib, lenvatinib, vandetanib, cabozantinib, and sunitinib. Although the efficacy of these drugs varies, a major issue is the lack of specificity resulting in a higher rate of drug-related toxicities, leading to dose reduction, interruption, or discontinuation. Moreover, MKIs are subject to drug resistance by RET Val804 residue gatekeeper mutations. In phase I/II clinical studies, the highly selective first-generation RET inhibitors, selpercatinib and pralsetinib, demonstrate high efficacy in controlling disease even in the presence of gatekeeper mutations combined with greater tolerability. However, resistance mechanisms such as RET solvent front mutations (SFMs) have evolved in some patients, giving the need to develop the selective second-generation RET inhibitors. Although the approval of selpercatinib and pralsetinib in 2020 has profoundly benefited patients with RET-altered thyroid cancer, further research into optimal treatment strategies, mechanisms of drug resistance, long-term consequences of potent RET-inhibition, and development of more effective agents against emergent mutations are much needed.

Reawakening GDNF's regenerative past in mice and humans

The ability of an animal to regenerate lost tissue and body parts has obviously life-saving implications. Understanding how this ability became restricted or active in specific animal lineages will help us understand our own regeneration. According to phylogenic analysis, the glial cell line-derived neurotrophic factor (GDNF) signaling pathway, but not other family members, is conserved in axolotls, a salamander with remarkable regenerative capacity. Furthermore, comparing the pro-regenerative Spiny mouse to its less regenerative descendant, the House mouse, revealed that the GDNF signaling pathway, but not other family members, was induced in regenerating Spiny mice. According to GDNF receptor expression analysis, GDNF may promote hair follicle neogenesis – an important feature of skin regeneration – by determining the fate of dermal fibroblasts as part of new hair follicles. These findings support the idea that GDNF treatment will promote skin regeneration in humans by demonstrating the GDNF signaling pathway's ancestral and cellular nature.

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In pro-regenerative axolotls, the GDNF-GFR□1 signaling system is conserved.

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In pro-regenerative Spiny mice, the GDNF-GFR□1 signaling system is activated.

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In mice, GDNF targets upper-regeneration-competent dermal fibroblasts.

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GDNF-GFR□1 activation may promote skin regeneration in mice and humans.

Disparate phenotypes in two unfavorable pregnancies due to maternal mosaicism of a novel RET gene mutation

Mutations in RET have been found in multiple diseases including isolated and associated congenital anomalies. Here, we report a case presented with disparate phenotypes in each pregnancy but caused by the same novel mutation. Whole-exome sequencing (WES) was performed on the proband/abortion product-parental trio and a novel missense variant in RET (chr10:43615610C>G; c.2689C>G; p.Arg897Gly) was identified. The mother was a low-level somatic carrier of this new mutation, with 17.3% in blood, 19.1% in oralmucous membrane, and 15.7% in urine by droplet digital polymerase chain reaction (dd PCR). Our finding not only broadens the mutation spectrum of RET but also gives supportive genetic counseling and timely guidance on fertility choices.

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.

Aberrant RET expression impacts on normal mammary gland post-lactation transition enhancing cancer potential

RET is a receptor tyrosine kinase with oncogenic potential in the mammary epithelium. Several receptors with oncogenic activity in the breast are known to participate in specific developmental stages. We found that RET is differentially expressed during mouse mammary gland development: RET is present in lactation and its expression dramatically decreases in involution, the period during which the lactating gland returns to a quiescent state after weaning. Based on epidemiological and pre-clinical findings, involution has been described as tumor promoting. Using the Ret/MTB doxycycline-inducible mouse transgenic system, we show that sustained expression of RET in the mammary epithelium during the post-lactation transition to involution is accompanied by alterations in tissue remodeling and an enhancement of cancer potential. Following constitutive Ret expression, we observed a significant increase in neoplastic lesions in the post-involuting versus the virgin mammary gland. Furthermore, we show that abnormal RET overexpression during lactation promotes factors that prime involution, including premature activation of Stat3 signaling and, using RNA sequencing, an acute-phase inflammatory signature. Our results demonstrate that RET overexpression negatively affects the normal post-lactation transition.

Summary: We show that RET activation stimulates Stat3 signaling in mammary epithelial cell culture and in vivo during post-lactation transition, demonstrating that the RET receptor participates in the post-lactation transition priming tumorigenesis.

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.

RET receptor signaling: Function in development, metabolic disease, and cancer

The RET proto-oncogene encodes a receptor tyrosine kinase whose alterations are responsible for various human cancers and developmental disorders, including thyroid cancer, non-small cell lung cancer, multiple endocrine neoplasia type 2, and Hirschsprung's disease. RET receptors are physiologically activated by glial cell line-derived neurotrophic factor (GDNF) family ligands that bind to the coreceptor GDNF family receptor α (GFRα). Signaling via the GDNF/GFRα1/RET ternary complex plays crucial roles in the development of the enteric nervous system, kidneys, and urinary tract, as well as in the self-renewal of spermatogonial stem cells. In addition, another ligand, growth differentiation factor-15 (GDF15), has been shown to bind to GFRα-like and activate RET, regulating body weight. GDF15 is a stress response cytokine, and its elevated serum levels affect metabolism and anorexia-cachexia syndrome. Moreover, recent development of RET-specific kinase inhibitors contributed significantly to progress in the treatment of patients with RET-altered cancer. This review focuses on the broad roles of RET in development, metabolic diseases, and cancer.

Therapeutic potential of glial cell line-derived neurotrophic factor and cell reprogramming for hippocampal-related neurological disorders

Hippocampus serves as a pivotal role in cognitive and emotional processes, as well as in the regulation of the hypothalamus-pituitary axis. It is known to undergo mild neurodegenerative changes during normal aging and severe atrophy in Alzheimer’s disease. Furthermore, dysregulation in the hippocampal function leads to epilepsy and mood disorders. In the first section, we summarized the most salient knowledge on the role of glial cell-line-derived neurotrophic factor and its receptors focused on aging, cognition and neurodegenerative and hippocampal-related neurological diseases mentioned above. In the second section, we reviewed the therapeutic approaches, particularly gene therapy, using glial cell-line-derived neurotrophic factor or its gene, as a key molecule in the development of neurological disorders. In the third section, we pointed at the potential of regenerative medicine, as an emerging and less explored strategy for the treatment of hippocampal disorders. We briefly reviewed the use of partial reprogramming to restore brain functions, non-neuronal cell reprogramming to generate neural stem cells, and neural progenitor cells as source-specific neuronal types to be implanted in animal models of specific neurodegenerative disorders.

Cerebral Cortical Activity During Academic Stress Amongst Undergraduate Medical Students at Kampala International University (Uganda)

BACKGROUND

Stress among medical students is related to their academic lifespan; however, information on brain health among medical students from developing countries continues to be scarce. The objective of this study was to establish perceived academic stress levels, assess the ability to cope with stress, and investigate its effects on the visual reaction time (VRT), audio reaction time (ART), and tactile reaction time (TRT) in the somatosensory cortex among medical students of Uganda.

METHODS

This was a cross-sectional study conducted among preclinical (n = 88) and clinical (n = 96) undergraduate medical students at Kampala International University Western Campus. A standard Perceived Stress Scale (PSS) was used to categorize stress into low, moderate, and severe while the ability to cope with stress was categorized into below average, average, above average, and superior stresscoper (SS). Data on reaction time were acquired through VRT, ART, and TRT using the catch-a-ruler experiment, and this was analyzed using SPSS version 20.

RESULTS

This study shows that preclinical students are more stressed than clinical students (PSS prevalence for low stress = preclinical; clinical: 40, 60%). Moderate stress was 48.4 and 51.6% while high perceived stress was 75 and 25% among preclinical and clinical students. Among male and female students in preclinical years, higher TRT and VRT were found in clinical students showing that stress affects the tactile and visual cortical areas in the brain, although the VRT scores were only significantly (P = 0.0123) poor in male students than female students in biomedical sciences. Also, highly stressed individuals had higher TRT and ART and low VRT. SS had high VRT and ART and low TRT in preclinical students, demonstrating the importance of the visual cortex in stress plasticity. Multiple regression showed a close relationship between PSS, ability to cope with stress, age, and educational level (P < 0.05), demonstrating the importance of social and psychological support, especially in the biomedical sciences.

CONCLUSION

Preclinical students suffer more from stress and are poorer SS than clinical students. This strongly impairs their cortical regions in the brain, thus affecting their academic productivity.

[Advances in the Treatment of RET Fusion-positive Advanced Non-small Cell Lung Cancer]

Rearranged during transfection (RET) fusions are found in 0.7% to 2% of non-small cell lung cancer (NSCLC). Fusions between RET gene and other domains represent the distinct biological and clinicopathological subtypes of NSCLC. Recent years have witnessed the remarkable advancement of RET fusion-positive advanced NSCLC therapy. Conventional chemotherapy produced moderate clinical benefits. Prior to the introduction of targeted therapy or in the context of unavailability, platinum-based systemic regimens are initial therapy options. Immunotherapy predicted minimal response in the presence of RET fusions while currently available data have been scarce, and the single-agent immunotherapy or in combination with chemotherapy regimens are not recommended as initial systemic therapy in this population. The repurpose of multi-target kinase inhibitors in patients with RET fusion-positive NSCLC showed encouraging therapeutic activity, with only cabozantinib and vandetanib being recommended as initial or subsequent options under certain circumstances. However, there are still unmet clinical needs. Pralsetinib and selpercatinib have been developed as tyrosine kinase inhibitors (TKI) selectively targeting RET variation of fusions or mutations, and both agents significantly improved the prognosis of patients with RET fusion-positive NSCLC. Pralsetinib and selpercatinib have been established as preferred first-line therapy or subsequent therapy options. As observed with other TKIs treatment, resistance has also been associated with RET targeted inhibition, and the acquired resistance eventually affect the long-term therapeutic effectiveness, leading to limited subsequent treatment options. Therefore, it is essential to identify resistance mechanisms to TKI in RET fusion-positive advanced NSCLC to help reveal and establish new strategies to overcome resistance. Here, we review the advances in the treatment of RET fusion-positive advanced NSCLC.
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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.

The correlation between the expression of ATF4 and procalcitonin combined with the detection of RET mutation and the pathological stage and clinical prognosis of medullary thyroid carcinoma

To explore the correlation between the activating transcription factor 4 (ATF4) and procalcitonin (PCT) expressions combined with RET mutation and the pathological staging and clinical prognosis of sporadic medullary thyroid carcinoma (SMTC). Fifty cases (tumor tissue) of SMTC diagnosed by clinicopathology were collected and the patients with nodular goiter were selected as normal control. The RET mutation site was analyzed by detection kit and expressions of PCT and ATF4 in SMTC were analyzed by Western blot and immunohistochemistry. Multiple linear regression was used to analyze the correlation of risk factors (PCT or ATF4 expression, RET mutation, tumor differentiation, SMTC stage, lymphatic metastasis) for 5-year recurrence and survival of SMTC. The ATF4 and PCT expressions were significantly decreased and increased, respectively, with the increase of the SMTC stage. The most frequent mutation of RET gene in cancer tissue was M 22458A in exon 16. The ATF4 and PCT expressions, as well as RET mutation, were significantly associated with a 5-year recurrence, while the ATF4 expression was significantly related to better 5-year survival. ATF4 and PCT expressions combined with RET mutation are related to the clinical prognosis of SMTC and can predict SMTC staging.

P-Glycoprotein (ABCB1/MDR1) and BCRP (ABCG2) Limit Brain Accumulation and Cytochrome P450-3A (CYP3A) Restricts Oral Exposure of the RET Inhibitor Selpercatinib (RETEVMO)

Selpercatinib is a targeted, FDA-approved, oral, small-molecule inhibitor for the treatment of rearranged during transfection (RET) proto-oncogene mutation-positive cancer. Using genetically modified mouse models, we investigated the roles of the multidrug efflux transporters ABCB1 and ABCG2, the OATP1A/1B uptake transporters, and the drug-metabolizing CYP3A complex in selpercatinib pharmacokinetics. Selpercatinib was efficiently transported by hABCB1 and mAbcg2, but not hABCG2, and was not a substrate of human OATP1A2, -1B1 or -1B3 in vitro. In vivo, brain and testis penetration were increased by 3.0- and 2.7-fold in Abcb1a/1b^-/- mice and by 6.2- and 6.4-fold in Abcb1a/1b;Abcg2^-/- mice, respectively. Oatp1a/1b deficiency did not alter selpercatinib pharmacokinetics. The ABCB1/ABCG2 inhibitor elacridar boosted selpercatinib brain penetration in wild-type mice to the levels seen in Abcb1a/1b;Abcg2^-/- mice. Cyp3a^-/- mice showed a 1.4-fold higher plasma AUC_0–4h than wild-type mice, which was then 1.6-fold decreased upon transgenic overexpression of human CYP3A4 in liver and intestine. In summary, ABCG2, and especially ABCB1, limit brain and testis penetration of selpercatinib. Elacridar coadministration could mostly reverse these effects, without causing acute toxicity. CYP3A-mediated metabolism can limit selpercatinib oral exposure and hence its tissue concentrations. These insights may be useful in the further clinical development of selpercatinib.

ABCB1 and ABCG2, but not CYP3A4 limit oral availability and brain accumulation of the RET inhibitor pralsetinib

BACKGROUND AND PURPOSE

Pralsetinib is an FDA-approved oral small-molecule inhibitor for treatment of rearranged during transfection (RET) proto-oncogene fusion-positive non-small cell lung cancer. We investigated how the efflux transporters ABCB1 and ABCG2, the SLCO1A/1B uptake transporters and the drug-metabolizing enzyme CYP3A influence pralsetinib pharmacokinetics.

EXPERIMENTAL APPROACH

In vitro, transepithelial pralsetinib transport was assessed. In vivo, pralsetinib (10 mg/kg) was administered orally to relevant genetically modified mouse models. Pralsetinib concentrations in cell medium, plasma samples and organ homogenates were measured using liquid chromatography-tandem mass spectrometry.

KEY RESULTS

Pralsetinib was efficiently transported by human (h)ABCB1 and mouse (m)Abcg2, but not hACBG2. In vivo, mAbcb1a/1b markedly and mAbcg2 slightly limited pralsetinib brain penetration (6.3-and 1.8-fold, respectively). Testis distribution showed similar results. Abcb1a/1b;Abcg2-/- mice showed 1.5-fold higher plasma exposure, 23-fold increased brain penetration, and 4-fold reduced recovery of pralsetinib in the small intestinal content. mSlco1a/1b deficiency did not affect pralsetinib oral availability or tissue exposure. Oral coadministration of the ABCB1/ABCG2 inhibitor elacridar boosted pralsetinib plasma exposure (1.3-fold) and brain penetration (19.6-fold) in wild-type mice. Additionally, pralsetinib was a modest substrate of mCYP3A, but not of hCYP3A4, which did not noticeably restrict the oral availability or tissue distribution of pralsetinib.

CONCLUSIONS AND IMPLICATIONS

SLCO1A/1B and CYP3A4 are unlikely to affect the pharmacokinetics of pralsetinib, but ABCG2 and especially ABCB1 markedly limit its brain and testis penetration, as well as oral availability. These effects are mostly reversed by oral coadministration of the ABCB1/ABCG2 inhibitor elacridar. These insights may be useful in the further clinical development of pralsetinib.

Molecular dynamics study of enhanced autophosphorylation by S904F mutation of the RET kinase domain

The aberrant kinase activity of RET (rearranged during transfection), a transmembrane tyrosine kinase, is associated with human cancer. A point mutation caused by the replacement of solvent-front hydrophilic S904, located on the activation loop (A-loop), with a bulky hydrophobic phenylalanine residue can induce resistance to the type I kinase inhibitor vandetanib. A possible mechanism of this drug resistance is the release of a cis-autoinhibited conformation of RET for autophosphorylation, which activates RET kinase. Because the association between S904F mutation and enhanced autophosphorylation is unclear, we conducted molecular modeling analysis to compare unphosphorylated apo wild-type and S904F mutant structures. The structural compactness of the A-loop promoted ATP binding. When the A-loop is extended, the αC helix moves toward the glycine-rich loop, resulting in the protrusion of F735. The extruded F735 connects with E734 and R912 and constrains the ATP pocket entrance. Contrarily, a contracted A-loop pulls the αC helix away from the glycine-rich loop, burying F734 and making the ATP pocket accessible. The mutated F904 stabilizes the contracted A-loop and releases the autoinhibited conformation of RET, thereby facilitating autophosphorylation. We also simulated two ATP-bound systems. The binding free energies of ATP, estimated through the molecular mechanics with a generalized Born and surface area solvation approach, revealed that the S904F mutant was bound more tightly than was the wild type with the ATP. The findings support the premise of autophosphorylation promotion in the S904F mutant.

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.

A two-site flexible clamp mechanism for RET-GDNF-GFRα1 assembly reveals both conformational adaptation and strict geometric spacing

RET receptor tyrosine kinase plays vital developmental and neuroprotective roles in metazoans. GDNF family ligands (GFLs) when bound to cognate GFRα co-receptors recognize and activate RET stimulating its cytoplasmic kinase function. The principles for RET ligand-co-receptor recognition are incompletely understood. Here, we report a crystal structure of the cadherin-like module (CLD1-4) from zebrafish RET revealing interdomain flexibility between CLD2 and CLD3. Comparison with a cryo-electron microscopy structure of a ligand-engaged zebrafish RETECD-GDNF-GFRα1a complex indicates conformational changes within a clade-specific CLD3 loop adjacent to the co-receptor. Our observations indicate that RET is a molecular clamp with a flexible calcium-dependent arm that adapts to different GFRα co-receptors, while its rigid arm recognizes a GFL dimer to align both membrane-proximal cysteine-rich domains. We also visualize linear arrays of RETECD-GDNF-GFRα1a suggesting that a conserved contact stabilizes higher-order species. Our study reveals that ligand-co-receptor recognition by RET involves both receptor plasticity and strict spacing of receptor dimers by GFL ligands.

Literature review: enteric nervous system development, genetic and epigenetic regulation in the etiology of Hirschsprung's disease

Hirschsprung's disease (HSCR) is a developmental disorder of the enteric nervous system (ENS) derived from neural crest cells (NCCs), which affects their migration, proliferation, differentiation, or preservation in the digestive tract, resulting in aganglionosis in the distal intestine. The regulation of both NCCs and the surrounding environment involves various genes, signaling pathways, transcription factors, and morphogens. Therefore, changes in gene expression during the development of the ENS may contribute to the pathogenesis of HSCR.

This review discusses several mechanisms involved in the development of ENS, confirming that deviant genetic and epigenetic patterns, such as DNA methylation, histone modification, and microRNA (miRNA) regulation, can contribute to the development of neurocristopathy. Specifically, the epigenetic regulation of miRNA expression and its relationship to cellular interactions and gene activation through various major pathways in Hirschsprung's disease will be discussed.

Expression of the Body-Weight Signaling Players: GDF15, GFRAL and RET and their clinical relevance in Gastric Cancer

The existence, the functional role and clinical relevance of GDF15 and its signaling through a GFRAL/RET-dependent complex in gastric cancer (GC) and other human tumors remain to be elucidated, despite the widespread recognition of obesity as an important cancer-predisposing factor. Therefore, we aimed to analyze the expression levels of GDF15, GFRAL and RET in GC tissues in relation to each other and clinicopathological features, including patient survival, in order to establish a potential implication of the body-weight signaling pathway in the pathology and clinical outcome of GC. Protein expression was examined by immunohistochemistry on tissue microarrays containing 104 and 30 consecutive GC and normal gastric mucosa samples, whereas gene expression data for The Cancer Genome Atlas cohort of 413 GC patients were obtained from public sources. We found that the protein expression of GDF15, GFRAL and RET was significantly elevated and positively correlated in our set of GC tissues, which was reflected in their tendency to be overexpressed in low-grade and intermediate-grade tumors rather than high-grade ones. No other relationships between the expression status of the examined proteins and clinicopathological characteristics of GC patients were found. Through in silico data analysis, we showed that high GDF15 expression was associated with better overall survival (OS) of GC patients, whereas the opposite was true for high levels of GFRAL or RET. Specifically, GFRAL and RET emerged as independent prognostic factors associated with poor OS. Furthermore, high combined expression of the three markers: GDF15+GFRAL+RET was significantly associated with reduced OS, and it was an independent prognostic factor of borderline significance in terms of OS, when adjusted for covariates. If validated in large-scale studies, the individual and combined expression of GDF15, GFRAL and RET may provide significant clinical implications for the prognosis prediction of GC patients.

The importance of the RET gene in thyroid cancer and therapeutic implications

Since the discovery of the RET receptor tyrosine kinase in 1985, alterations of this protein have been found in diverse thyroid cancer subtypes. RET gene rearrangements are observed in papillary thyroid carcinoma, which result in RET fusion products. By contrast, single amino acid substitutions and small insertions and/or deletions are typical of hereditary and sporadic medullary thyroid carcinoma. RET rearrangements and mutations of extracellular cysteines facilitate dimerization and kinase activation, whereas mutations in the RET kinase coding domain drive dimerization-independent kinase activation. Thus, RET kinase inhibition is an attractive therapeutic target in patients with RET alterations. This approach was initially achieved using multikinase inhibitors, which affect multiple deregulated pathways that include RET kinase. In clinical practice, use of multikinase inhibitors in patients with advanced thyroid cancer resulted in therapeutic efficacy, which was associated with frequent and sometimes severe adverse effects. However, remarkable progress has been achieved with the identification of novel potent and selective RET kinase inhibitors for the treatment of advanced thyroid cancer. Although expanded clinical validation in future trials is needed, the sustained antitumoural activity and the improved safety profile of these novel compounds is opening a new exciting era in precision oncology for RET-driven cancers.

Chasing the Target: New Phenomena of Resistance to Novel Selective RET Inhibitors in Lung Cancer. Updated Evidence and Future Perspectives

Simple Summary

REarranged during Transfection (RET) is an emerging target for several types of cancer, including non-small cell lung cancer (NSCLC). The recent U.S. FDA approval of pralsetinib and selpercatinib raises issues regarding the emergence of secondary mutations and amplifications involved in parallel signaling pathways and receptors, liable for resistance mechanisms. The aim of this review is to explore recent knowledge on RET resistance in NSCLC in pre-clinic and in clinical settings and accordingly, the state-of-the-art in new drugs or combination of drugs development.

Abstract

The potent, RET-selective tyrosine kinase inhibitors (TKIs) pralsetinib and selpercatinib, are effective against the RET V804L/M gatekeeper mutants, however, adaptive mutations that cause resistance at the solvent front RET G810 residue have been found, pointing to the need for the development of the next-generation of RET-specific TKIs. Also, as seen in EGFR- and ALK-driven NSCLC, the rising of the co-occurring amplifications of KRAS and MET could represent other escaping mechanisms from direct inhibition. In this review, we summarize actual knowledge on RET fusions, focusing on those involved in NSCLC, the results of main clinical trials of approved RET-inhibition drugs, with particular attention on recent published results of selective TKIs, and finally, pre-clinical evidence regarding resistance mechanisms and suggestion on hypothetical and feasible drugs combinations and strategies viable in the near future.

Genomics and Epigenomics of Medullary Thyroid Carcinoma: From Sporadic Disease to Familial Manifestations

Our understanding of the genomics and epigenomics of medullary thyroid carcinoma (MTC) has advanced since the initial recognition of RET as a driver of MTC tumorigenesis in familial MTC. We now have insight into the frequency and prognostic significance of specific RET mutations in sporadic MTC. For example, the most common RET mutation in sporadic MTC is the RET Met918Thr mutation, the same mutation that underlies MEN2B and a poor prognosticator. This mutation is relatively infrequent in medullary thyroid microcarcinomas but is over-represented in advanced-stage disease. RAS mutations are detected in 70% of sporadic, RET wild-type MTC. Although next-generation and whole-exome sequencing studies have shown that tumors that are wild-type for RET and RAS mutations essentially lack other recurrent mutations, additional pathways and epigenetic alterations have been implicated in MTC tumorigenesis. Increased insight into the clinical course of patients with familial MTC with specific RET mutations has guided treatment recommendations for these patients. Finally, an understanding of the genomics has informed treatment for patients with advanced MTC. In this review, we will examine the genomics and epigenomics of sporadic and familial MTC, along with the prognostic significance of molecular alterations, management of patients with germline RET mutations, and treatment strategies for MTC patients.

Glial Cell Line-Derived Neurotrophic Factor and Focal Ischemic Stroke

Focal ischemic stroke (FIS) is a leading cause of human debilitation and death. Following the onset of a FIS, the brain experiences a series of spatiotemporal changes which are exemplified in different pathological processes. One prominent feature of FIS is the development of reactive astrogliosis and glial scar formation in the peri-infarct region (PIR). During the subacute phase, astrocytes in PIR are activated, referred to as reactive astrocytes (RAs), exhibit changes in morphology (hypotrophy), show an increased proliferation capacity, and altered gene expression profile, a phenomenon known as reactive astrogliosis. Subsequently, the morphology of RAs remains stable, and proliferation starts to decline together with the formation of glial scars. Reactive astrogliosis and glial scar formation eventually cause substantial tissue remodeling and changes in permanent structure around the PIR. Glial cell line-derived neurotrophic factor (GDNF) was originally isolated from a rat glioma cell-line and regarded as a potent survival neurotrophic factor. Under normal conditions, GDNF is expressed in neurons but is upregulated in RAs after FIS. This review briefly describes properties of GDNF, its receptor-mediated signaling pathways, as well as recent studies regarding the role of RAs-derived GDNF in neuronal protection and brain recovery. These results provide evidence suggesting an important role of RA-derived GDNF in intrinsic brain repair and recovery after FIS, and thus targeting GDNF in RAs may be effective for stroke therapy.

Interplay of BDNF and GDNF in the Mature Spinal Somatosensory System and Its Potential Therapeutic Relevance

The growth factors BDNF and GDNF are gaining more and more attention as modulators of synaptic transmission in the mature central nervous system (CNS). The two molecules undergo a regulated secretion in neurons and may be anterogradely transported to terminals where they can positively or negatively modulate fast synaptic transmission. There is today a wide consensus on the role of BDNF as a pro-nociceptive modulator, as the neurotrophin has an important part in the initiation and maintenance of inflammatory, chronic, and/or neuropathic pain at the peripheral and central level. At the spinal level, BDNF intervenes in the regulation of chloride equilibrium potential, decreases the excitatory synaptic drive to inhibitory neurons, with complex changes in GABAergic/glycinergic synaptic transmission, and increases excitatory transmission in the superficial dorsal horn. Differently from BDNF, the role of GDNF still remains to be unraveled in full. This review resumes the current literature on the interplay between BDNF and GDNF in the regulation of nociceptive neurotransmission in the superficial dorsal horn of the spinal cord. We will first discuss the circuitries involved in such a regulation, as well as the reciprocal interactions between the two factors in nociceptive pathways. The development of small molecules specifically targeting BDNF, GDNF and/or downstream effectors is opening new perspectives for investigating these neurotrophic factors as modulators of nociceptive transmission and chronic pain. Therefore, we will finally consider the molecules of (potential) pharmacological relevance for tackling normal and pathological pain.

Expression and purification of the extracellular domain of wild-type humanRET and the dimeric oncogenic mutant C634R

The receptor tyrosine kinase RET is essential in a variety of cellular processes. RET gain-of-function is strongly associated with several cancers, notably multiple endocrine neoplasia type 2A (MEN 2A), while RET loss-of-function causes Hirschsprung's disease and Parkinson's disease. To investigate the activation mechanism of RET as well as to enable drug development, over-expressed recombinant protein is needed for in vitro functional and structural studies. By comparing insect and mammalian cells expression of the RET extracellular domain (RET^ECD), we showed that the expression yields of RET^ECD using both systems were comparable, but mammalian cells produced monomeric functional RET^ECD, whereas RET^ECD expressed in insect cells was non-functional and multimeric. This was most likely due to incorrect disulfide formation. By fusing an Fc tag to the C-terminus of RET^ECD, we were able to produce, in HEK293T cells, dimeric oncogenic RET^ECD (C634R) for the first time. The protein remained dimeric even after cleavage of the tag via the cysteine disulfide, as in full-length RET in the context of MEN 2A and related pathologies. Our work thus provides valuable tools for functional and structural studies of the RET signaling system and its oncogenic activation mechanisms.

Glial cell line-derived neurotrophic factors (GFLs) and small molecules targeting RET receptor for the treatment of pain and Parkinson's disease

Rearranged during transfection (RET), in complex with glial cell line-derived (GDNF) family receptor alpha (GFRα), is the canonical signaling receptor for GDNF family ligands (GFLs) expressed in both central and peripheral parts of the nervous system and also in non-neuronal tissues. RET-dependent signaling elicited by GFLs has an important role in the development, maintenance and survival of dopamine and sensory neurons. Both Parkinson's disease and neuropathic pain are devastating disorders without an available cure, and at the moment are only treated symptomatically. GFLs have been studied extensively in animal models of Parkinson's disease and neuropathic pain with remarkable outcomes. However, clinical trials with recombinant or viral vector-encoded GFL proteins have produced inconclusive results. GFL proteins are not drug-like; they have poor pharmacokinetic properties and activate multiple receptors. Targeting RET and/or GFRα with small molecules may resolve the problems associated with using GFLs as drugs and can result in the development of therapeutics for disease-modifying treatments against Parkinson's disease and neuropathic pain.

RET isoforms contribute differentially to invasive processes in pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is a therapeutically challenging disease with poor survival rates, owing to late diagnosis and early dissemination. These tumors frequently undergo perineural invasion, spreading along nerves regionally and to distant sites. The RET receptor tyrosine kinase is implicated in increased aggressiveness, local invasion, and metastasis in multiple cancers, including PDAC. RET mediates directional motility and invasion towards sources of its neurotrophic factor ligands, suggesting that it may enhance perineural invasion of tumor cells towards nerves. RET is expressed as two main isoforms, RET9 and RET51, which differ in their protein interactions and oncogenic potentials, however, the contributions of RET isoforms to neural invasion have not been investigated. In this study, we generated total RET and isoform-specific knockdown PDAC cell lines and assessed the contributions of RET isoforms to PDAC invasive spread. Our data show that RET activity induces cell polarization and actin remodeling through activation of CDC42 and RHOA GTPases to promote directional motility in PDAC cells. Further, we show that RET interacts with the adaptor protein TKS5 to induce invadopodia formation, enhance matrix degradation and promote tumor cell invasion through a SRC and GRB2-dependent mechanism. Finally, we show that RET51 is the predominant isoform contributing to these RET-mediated invasive processes in PDAC. Together, our work suggests that RET expression in pancreatic cancers may enhance tumor aggressiveness by promoting perineural invasion, and that RET expression may be a valuable marker of invasiveness, and a potential therapeutic target in the treatment of these cancers.

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.

Regulation of GDNF expression in Sertoli cells

Sertoli cells regulate male germ cell proliferation and differentiation and are a critical component of the spermatogonial stem cell (SSC) niche, where homeostasis is maintained by the interplay of several signaling pathways and growth factors. These factors are secreted by Sertoli cells located within the seminiferous epithelium, and by interstitial cells residing between the seminiferous tubules. Sertoli cells and peritubular myoid cells produce glial cell line-derived neurotrophic factor (GDNF), which binds to the RET/GFRA1 receptor complex at the surface of undifferentiated spermatogonia. GDNF is known for its ability to drive SSC self-renewal and proliferation of their direct cell progeny. Even though the effects of GDNF are well studied, our understanding of the regulation its expression is still limited. The purpose of this review is to discuss how GDNF expression in Sertoli cells is modulated within the niche, and how these mechanisms impact germ cell homeostasis.