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Matti L Gild, Iñigo Landa, Mabel Ryder, Ronald A Ghossein, Jeffrey A Knauf, and James A Fagin

Inhibitors of RET, a tyrosine kinase receptor encoded by a gene that is frequently mutated in medullary thyroid cancer, have emerged as promising novel therapies for the disease. Rapalogs and other mammalian target of rapamycin (mTOR) inhibitors are effective agents in patients with gastroenteropancreatic neuroendocrine tumors, which share lineage properties with medullary thyroid carcinomas. The objective of this study was to investigate the contribution of mTOR activity to RET-induced signaling and cell growth and to establish whether growth suppression is enhanced by co-targeting RET and mTOR kinase activities. Treatment of the RET mutant cell lines TT, TPC-1, and MZ-CRC-1 with AST487, a RET kinase inhibitor, suppressed growth and showed profound and sustained inhibition of mTOR signaling, which was recapitulated by siRNA-mediated RET knockdown. Inhibition of mTOR with INK128, a dual mTORC1 and mTORC2 kinase inhibitor, also resulted in marked growth suppression to levels similar to those seen with RET blockade. Moreover, combined treatment with AST487 and INK128 at low concentrations suppressed growth and induced apoptosis. These data establish mTOR as a key mediator of RET-mediated cell growth in thyroid cancer cells and provide a rationale for combinatorial treatments in thyroid cancers with oncogenic RET mutations.

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Viktor Johanson, Håkan Ahlman, Peter Bernhardt, Svante Jansson, Lars Kölby, Fredrik Persson, Göran Stenman, Christina Swärd, Bo Wängberg, Mats Stridsberg, and Ola Nilsson

Hereditary medullary thyroid carcinoma (MTC) is caused by germline mutations in the RET proto-oncogene, resulting in constitutive activation of the RET tyrosine kinase. A substantial proportion of sporadic MTCs also have RET mutations, making the RET tyrosine kinase a potential therapeutic target in MTC. We have established a transplantable MTC in nude mice from a sporadic human MTC carrying a RET C634R mutation. Transplanted tumors had an exponential growth rate with an approximate doubling time of about 3 weeks, and expressed a neuroendocrine phenotype characteristic of MTC, e.g., expression of calcitonin, chromogranin A (CgA), synaptophysin, synaptic vesicle protein 2 (SV2), vesicular monoamine transporter-1 and -2, carcinoembryonic antigen, cytokeratin 8/18, epithelial cadherin, and neural cell adhesion molecule. Plasma calcitonin and CgA levels were elevated in tumor-bearing mice and correlated with tumor size. Cytogenetic analysis, including spectral karyotyping, confirmed the human origin of the xenografted tumors and demonstrated an abnormal, near triploid karyotype. Treatment of tumor-bearing nude mice with the tyrosine kinase inhibitor ZD6474, which specifically inhibits RET, epidermal growth factor receptor (EGFR), and vascular endothelium growth factor receptor (VEGFR) tyrosine kinases, resulted in a dose-dependent inhibition of tumor growth. Oral ZD6474 given once daily (250 mg/kg, 5 days/week) reduced tumor volume to 11% when compared with controls after 4 weeks. Our results show that this transplantable MTC, designated GOT2, represents a novel and useful model for studies of MTC and RET tyrosine kinase-dependent tumor growth.

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Sara Molatore, Andrea Kügler, Martin Irmler, Tobias Wiedemann, Frauke Neff, Annette Feuchtinger, Johannes Beckers, Mercedes Robledo, Federico Roncaroli, and Natalia S Pellegata

Rats affected by the MENX syndrome spontaneously develop multiple neuroendocrine tumors (NETs) including adrenal, pituitary and thyroid gland neoplasms. MENX was initially reported to be inherited as a recessive trait and affected rats were found to be homozygous for the predisposing Cdkn1b mutation encoding p27. We here report that heterozygous MENX-mutant rats (p27+/mut) develop the same spectrum of NETs seen in the homozygous (p27mut/mut) animals but with slower progression. Consequently, p27+/mut rats have a significantly shorter lifespan compared with their wild-type (p27+/+) littermates. In the tumors of p27+/mut rats, the wild-type Cdkn1b allele is neither lost nor silenced, implying that p27 is haploinsufficient for tumor suppression in this model. Transcriptome profiling of rat adrenal (pheochromocytoma) and pituitary tumors having different p27 dosages revealed a tissue-specific, dose-dependent effect of p27 on gene expression. In p27+/mut rats, thyroid neoplasms progress to invasive and metastatic medullary thyroid carcinomas (MTCs) accompanied by increased calcitonin levels, as in humans. Comparison of expression signatures of late-stage vs early-stage MTCs from p27+/mut rats identified genes potentially involved in tumor aggressiveness. The expression of a subset of these genes was evaluated in human MTCs and found to be associated with aggressive RET-M918T-positive tumors. Altogether, p27 haploinsufficiency in MENX rats uncovered a novel, representative model of invasive and metastatic MTC exploitable for translational studies of this often aggressive and incurable cancer.

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Elizabeth Grubbs, Daniel Halperin, Steven G Waguespack, and Robert F Gagel

The multiple endocrine neoplasia (MEN) workshops had their beginnings at Queen’s University in Kingston, Ontario in June 1984. This initial meeting brought clinicians and scientists together to focus on mapping the gene for multiple endocrine neoplasia type 2 (MEN2). These efforts culminated in the identification of the RET protooncogene as the causative gene a decade later. Over the next 35 years there were a total of 16 international workshops focused on the several MEN syndromes. Importantly, these workshops were instrumental in efforts to define the molecular basis for multiple endocrine neoplasia type 1 (MEN1), MEN2, von Hippel-Lindau disease (VHL), Carney Complex, hereditary pheochromocytoma and hyperparathyroidism. In this same spirit some 150 scientists and clinicians met at MD Anderson Cancer Center, 27–29 March 2019, for the 16th International Workshop on Multiple Endocrine Neoplasia (MEN2019). Appropriate to its location in a cancer centre, the workshop focused on important issues in the causation and treatment of malignant aspects of the MEN syndromes: medullary thyroid carcinoma, pancreatic neuroendocrine tumours, malignant pheochromocytoma and parathyroid carcinoma. Workshops at the meeting focused on a better understanding of how the identified molecular defects in these genetic syndromes lead to transformation, how to apply targeted kinase inhibitors and immunotherapy to treat these tumours and important clinical management issues. This issue of Endocrine-Related Cancer describes these discussions and recommendations.

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Aguirre A de Cubas, L Javier Leandro-García, Francesca Schiavi, Veronika Mancikova, Iñaki Comino-Méndez, Lucía Inglada-Pérez, Manuel Perez-Martinez, Nuria Ibarz, Pilar Ximénez-Embún, Elena López-Jiménez, Agnieszka Maliszewska, Rocío Letón, Álvaro Gómez Graña, Carmen Bernal, Cristina Álvarez-Escolá, Cristina Rodríguez-Antona, Giuseppe Opocher, Javier Muñoz, Diego Megias, Alberto Cascón, and Mercedes Robledo

Pheochromocytomas (PCCs) and paragangliomas (PGLs) are rare neuroendocrine neoplasias of neural crest origin that can be part of several inherited syndromes. Although their mRNA profiles are known to depend on genetic background, a number of questions related to tumor biology and clinical behavior remain unanswered. As microRNAs (miRNAs) are key players in the modulation of gene expression, their comprehensive analysis could resolve some of these issues. Through characterization of miRNA profiles in 69 frozen tumors with germline mutations in the genes SDHD, SDHB, VHL, RET, NF1, TMEM127, and MAX, we identified miRNA signatures specific to, as well as common among, the genetic groups of PCCs/PGLs. miRNA expression profiles were validated in an independent series of 30 composed of VHL-, SDHB-, SDHD-, and RET-related formalin-fixed paraffin-embedded PCC/PGL samples using quantitative real-time PCR. Upregulation of miR-210 in VHL- and SDHB-related PCCs/PGLs was verified, while miR-137 and miR-382 were confirmed as generally upregulated in PCCs/PGLs (except in MAX-related tumors). Also, we confirmed overexpression of miR-133b as VHL-specific miRNAs, miR-488 and miR-885-5p as RET-specific miRNAs, and miR-183 and miR-96 as SDHB-specific miRNAs. To determine the potential roles miRNAs play in PCC/PGL pathogenesis, we performed bioinformatic integration and pathway analysis using matched mRNA profiling data that indicated a common enrichment of pathways associated with neuronal and neuroendocrine-like differentiation. We demonstrated that miR-183 and/or miR-96 impede NGF-induced differentiation in PC12 cells. Finally, global proteomic analysis in SDHB and MAX tumors allowed us to determine that miRNA regulation occurs primarily through mRNA degradation in PCCs/PGLs, which partially confirmed our miRNA–mRNA integration results.

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Lisa D Berman-Booty and Karen E Knudsen

Prostate cancer remains the second leading cause of cancer death in men in the USA and most western countries. Prostatic acinar adenocarcinoma is the most commonly diagnosed form of prostate cancer. Small-cell neuroendocrine carcinoma is less frequently identified at the time of initial diagnosis, but this highly aggressive form of prostate cancer is increasingly observed in patients who have failed first- and second-line hormone therapy. Thus, developing and exploring models of neuroendocrine prostate cancer (NePC) are of increasing importance. This review examines the relevant xenograft tumor and genetically engineered mouse models of NePC, with the aim of addressing salient features and clinical relevance.

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Sara Redaelli, Ivan Plaza-Menacho, and Luca Mologni

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

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D Engelmann, D Koczan, P Ricken, U Rimpler, J Pahnke, Z Li, and B M Pützer

Activating mutations in the Ret proto-oncogene are responsible for occurrence of multiple endocrine neoplasia (MEN) type 2A and 2B, and familial medullary thyroid carcinoma (FMTC). A striking genotype–phenotype correlation between the mutated RET codon and clinical manifestation implies that tumorigenesis is conditioned by the type of mutation. We investigated gene expression profiles between and within distinct MEN2 subtypes through whole-genome microarray analysis in tumors induced by NIH-3T3 cells transformed with defined RET-MEN2A (C609Y, C634R), MEN2B, (A883F, M918T), and FMTC (Y791F) mutations. Expression profiling identified a statistically significant modification of 1494 genes, 628 down- and 866 upregulated in MEN2B compared with MEN2A/FMTC tumors. By contrast, no obvious alterations were observed among individual MEN2B and MEN2A type mutations, or between MEN2A and FMTC. Functional clustering of differential genes revealed RET-MEN2B specific upregulation of genes associated with novel growth and survival pathways. Intriguingly, RET-MEN2A/FMTC-specific tumors were characterized by a considerable number of genes involved in the host antitumor immune response via stimulation of natural killer/T-cell proliferation, migration, and cytotoxicity, which were completely absent in RET-MEN2B related cancers. QPCR on tumors versus cultured NIH-RET cell lines demonstrated that they are largely attributed to the host innate immune system, whereas expression of CX3CL1 involved in leukocyte recruitment is exclusively RET-MEN2A/FMTC tumor cell dependent. In correlation, massive inflammatory infiltrates were apparent only in tumors carrying MEN type 2A/FMTC mutations, suggesting that RET-MEN2B receptors specifically counteract immune infiltration by preventing chemokine expression, which may contribute to the different clinical outcome of both subtypes.

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Eva-Maria Duerr, Yusuke Mizukami, Aylwin Ng, Ramnik J Xavier, Hirotoshi Kikuchi, Vikram Deshpande, Andrew L Warshaw , Jonathan Glickman, Matthew H Kulke, and Daniel C Chung

Current classifications of human gastroenteropancreatic neuroendocrine tumors (NETs) are inconsistent and based upon histopathologic but not molecular features. We sought to compare a molecular classification with the World Health Organization (WHO) histologic classification, identify genes that may be important for tumor progression, and determine whether gastrointestinal NETs (GI-NETs) differ in their molecular profile from pancreatic NETs (PNETs). DNA microarray analysis was performed to identify differentially expressed genes in PNETs and GI-NETs. Confirmation of expression levels was obtained by quantitative real-time PCR. Immunoblotting and mutational analysis were performed for selected genes. Hierarchical clustering of 19 PNETs revealed a ‘benign’ and ‘malignant’ cluster that corresponded well with the WHO categories of well-differentiated endocrine tumor (WDET) and well-differentiated endocrine carcinoma (WDEC) respectively. FEV, adenylate cyclase 2 (ADCY2), nuclear receptor subfamily 4, group A, member 2 (NR4A2), and growth arrest and DNA-damage-inducible, beta (GADD45b) were the most highly up-regulated genes in the malignant group of PNETs. Platelet-derived growth factor receptor (PDGFR) was expressed in both WDETs and WDECs, and phosphorylation of PDGFR-β was observed in 83% of all PNETs. Malignant ileal GI-NETs exhibited a distinctive gene expression profile, and extracellular matrix protein 1 (ECM), vesicular monoamine member 1 (VMAT1), galectin 4 (LGALS4), and RET Proto-oncogene (RET) were highly up-regulated genes. Gene expression profiles reflect the current WHO classification and can distinguish benign from malignant PNETs and also PNETs from GI-NETs. This suggests that molecular profiling may enhance tumor classification schemes. Potential gene targets have also been identified, and PDGFR and RET are candidates that may represent novel therapeutic targets.

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Iván Plaza-Menacho

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.