Evaluation of potential mechanisms underlying genotype-phenotype correlations in multiple endocrine neoplasia type 2

Multiple endocrine neoplasia type 2 (MEN2) and RET specific modifications of the ACMG/AMP variant classification guidelines and impact on the MEN2 RET database

The Multiple Endocrine Neoplasia type 2 (MEN2) RET proto-oncogene database, originally published in 2008, is a comprehensive repository of all publicly available RET gene variations associated with MEN2 syndromes. The variant-specific genotype/phenotype information, age of earliest reported medullary thyroid carcinoma (MTC) onset, and relevant references with a brief summary of findings are cataloged. The ACMG/AMP 2015 consensus statement on variant classification was modified specifically for MEN2 syndromes and RET variants using ClinGen sequence variant interpretation working group recommendations and ClinGen expert panel manuscripts, as well as manuscripts from the American Thyroid Association Guidelines Task Force on Medullary Thyroid Carcinoma and other MEN2 RET literature. The classifications for the 166 single unique variants in the MEN2 RET database were reanalyzed using the MEN2 RET specifically modified ACMG/AMP classification guidelines (version 1). Applying these guidelines added two new variant classifications to the database (likely benign and likely pathogenic) and resulted in clinically significant classification changes (e.g., from pathogenic to uncertain) in 15.7% (26/166) of the original variants. Of those clinically significant changes, the highest percentage of changes, 46.2% (12/26), were changes from uncertain to benign or likely benign. The modified ACMG/AMP criteria with MEN2 RET specifications will optimize and standardize RET variant classifications.

Efficacy of a Novel Second-Generation Somatostatin-Dopamine Chimera (TBR-065) in Human Medullary Thyroid Cancer: A Preclinical Study

INTRODUCTION

Somatostatin and dopamine (DA) receptors have a pivotal role in controlling hormone secretion and cell proliferation in different neuroendocrine neoplasms, including medullary thyroid cancer (MTC). In the present preclinical study, we evaluated the anti-tumor activity of TBR-065 (formerly BIM-23B065), a second-generation somatostatin-DA chimera, in 2 human MTC cell lines.

METHODS

The effects of lanreotide (LAN) and TBR-065 on cell growth and proliferation, calcitonin (CT) secretion, cell cycle, apoptosis, cell migration, and tumor-induced angiogenesis have been evaluated through 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, DNA flow cytometry with propidium iodide (PI), Annexin V-FITC/PI staining, electrochemiluminescence immuno assay, wound-healing assay, and zebrafish platform, respectively.

RESULTS

TBR-065 exerted a more prominent anti-tumor activity than LAN in both MTC cell lines, as shown by inhibition of cell proliferation (maximal inhibition in TT: -50.3 and -37.6%, respectively; in MZ-CRC-1: -58.8 and -27%, respectively) and migration (in TT: -42.7 and -22.9%, respectively; in MZ-CRC-1: -75.5 and -58.2%, respectively). Only the new chimera decreased significantly the fraction of cells in S phase (TT: -33.8%; MZ-CRC-1: -18.8%) and increased cells in G2/M phase (TT: +13%; MZ-CRC-1: +30.5%). In addition, TBR-065 exerted a more prominent pro-apoptotic effect than LAN in TT cells. A concomitant decrease in CT secretion was observed after 2 days of incubation with both drugs, with a more relevant effect of TBR-065. However, neither LAN nor TBR-065 showed any effect on tumor-induced angiogenesis, as evaluated using a zebrafish/tumor xenograft model.

DISCUSSION/CONCLUSION

In MTC cell lines, a second-generation somatostatin-DA analog, TBR-065, exerts a more relevant anti-tumor activity than LAN, through modulation of cell cycle, induction of apoptosis, and reduction in migration. Further studies are required to establish whether TBR-065 has comparable potent inhibitory effects on tumor growth in vivo.

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.

A comprehensive review on MEN2B

MEN2B is a very rare autosomal dominant hereditary tumor syndrome associated with medullary thyroid carcinoma (MTC) in 100% cases, pheochromocytoma in 50% cases and multiple extra-endocrine features, many of which can be quite disabling. Only few data are available in the literature. The aim of this review is to try to give further insights into the natural history of the disease and to point out the missing evidence that would help clinicians optimize the management of such patients. MEN2B is mainly characterized by the early occurrence of MTC, which led the American Thyroid Association to recommend preventive thyroidectomy before the age of 1 year. However, as the majority of mutations are de novo, improved knowledge of the nonendocrine signs would help to lower the age of diagnosis and improve long-term outcomes. Future large-scale studies will be aimed at characterizing more in detail the main characteristics and outcomes of MEN2B.

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.

Differences in the transcriptome of medullary thyroid cancer regarding the status and type of RET gene mutations

Medullary thyroid cancer (MTC) can be caused by germline mutations of the RET proto-oncogene or occurs as a sporadic form. It is well known that RET mutations affecting the cysteine-rich region of the protein (MEN2A-like mutations) are correlated with different phenotypes than those in the kinase domain (MEN2B-like mutations). Our aim was to analyse the whole-gene expression profile of MTC with regard to the type of RET gene mutation and the cancer genetic background (hereditary vs sporadic). We studied 86 MTC samples. We demonstrated that there were no distinct differences in the gene expression profiles of hereditary and sporadic MTCs. This suggests a homogeneous nature of MTC. We also noticed that the site of the RET gene mutation slightly influenced the gene expression profile of MTC. We found a significant association between the localization of RET mutations and the expression of three genes: NNAT (suggested to be a tumour suppressor gene), CDC14B (involved in cell cycle control) and NTRK3 (tyrosine receptor kinase that undergoes rearrangement in papillary thyroid cancer). This study suggests that these genes are significantly deregulated in tumours with MEN2A-like and MEN2B-like mutations; however, further investigations are necessary to demonstrate any clinical impact of these findings.

MiR-182 promotes cancer invasion by linking RET oncogene activated NF-κB to loss of the HES1/Notch1 regulatory circuit

Background

Dominant-activating mutations in the RET proto-oncogene, a receptor tyrosine kinase, are responsible for the development of medullary thyroid carcinoma (MTC) and causative for multiple endocrine neoplasia (MEN) type 2A and 2B. These tumors are highly aggressive with a high propensity for early metastasis and chemoresistance. This attribute makes this neoplasia an excellent model for probing mechanisms underlying cancer progression.

Methods

The expression level of miR-182 was measured in MTC tumor specimens and in TT cells by real-time RT-PCR. TT cells and modified NThy-ori 3.1 that stably express RETM918T were used to investigate RET-dependent regulation of miR-182. Identification and validation of miR-182 targets and pathways was accomplished with luciferase assays, qRT-PCR, Western blotting and immunofluorescence. In vitro, overexpression and knockdown experiments were carried out to examine the impact of miR-182 and HES1 on invasion and migration.

Results

We found that miR-182 expression is significantly upregulated in MTC patient samples and tumor-derived cell lines harboring mutated RET. Inhibition of RET oncogenic signaling through a dominant-negative RET∆TK mutant in TT cells reduces miR-182, whereas overexpression of RETM918T in NThy-ori 3.1 cells increases miR-182 levels. We further show that overexpression of this miRNA in NThy.miR-182 cells promotes the invasive and migratory properties without affecting cell proliferation. MiR-182 is upregulated after RET induced NF-κB translocation into the nucleus via binding of NF-κB to the miR-182 promoter. Database analysis revealed that HES1, a repressor of the Notch pathway, is a target of miR-182, whose upregulation correlates with loss of HES1 transcription in MTC tissue samples and mutant RET cell lines. Moreover, we demonstrated that the 3′UTR of the HES1 mRNA bearing the targeting sequence for miR-182 clearly reduced luciferase reporter activity in cells expressing miR-182. Decreased expression of HES1 promotes migration by upregulating Notch1 inhibitor Deltex1 and consequent repression of Notch1.

Conclusion

We demonstrate a novel mechanism for MTC aggressiveness in which mutated RET/NF-κB-driven expression of miR-182 impedes HES1 activation in a negative feedback loop. This observation might open new possibilities to treat RET oncogene associated metastatic cancer.

Electronic supplementary material

The online version of this article (doi:10.1186/s12943-016-0563-x) contains supplementary material, which is available to authorized users.

Medullary Thyroid Carcinoma: Recent Advances Including MicroRNA Expression

Medullary thyroid carcinoma (MTC) is an uncommon neuroendocrine tumor arising from the C cells in the thyroid and accounts for about 5 % of all thyroid cancers. MTC exhibits more aggressive behavior than follicular tumors, with the majority of cases presenting with lymph node metastasis. It is particularly common among patients carrying germline RET mutations with almost 100 % penetrance. Because activating RET mutations occur in over 90 % of hereditary and 40 % of sporadic MTC, clinical trials of several RET-targeting multikinase inhibitors (MKIs) have resulted in FDA approval of vandetanib and cabozantinib for the treatment of MTC. Nevertheless, in light of significant individual differences in tumor behavior and treatment responses, there has been a persistent need for research efforts to decipher the molecular events within RET-driven or non-RET-driven tumors. Recently, the gene regulatory roles of microRNAs (miRNAs) in MTC have been studied extensively. Multiple miRNA deregulations have been discovered in MTC with potential prognostic and therapeutic implications. This review provides an overview of the basic pathology of MTC and an update on recent investigational progress.

A 6-Base Pair in Frame Germline Deletion in Exon 7 Of RET Leads to Increased RET Phosphorylation, ERK Activation, and MEN2A

CONTEXT

Multiple endocrine neoplasia type 2 (MEN2) is usually caused by missense mutations in the proto-oncogene, RET.

OBJECTIVE

This study aimed to determine the mutation underlying MEN2A in a female patient diagnosed with bilateral pheochromocytoma at age 31 years and with medullary thyroid carcinoma (MTC) 6 years later.

METHODS

Leukocyte DNA was used for exome and Sanger sequencing. Wild-type (WT) RET and mutants were expressed in HEK293 cells. Activation of MAPK/ERK and PI3K/AKT was analyzed by Western blotting and luciferase assay. The effect of RET mutants on cell proliferation was tested in a colony forming assay.

RESULTS

Exome sequencing revealed a 6-nucleotide/2-amino acid in-frame deletion in exon 7 of RET (c.1512_1517delGGAGGG, p.505_506del). In vitro expression showed that phosphorylation of the crucial tyrosine 905 was much stronger in the p.505_506del RET mutant compared with WT RET, indicating ligand-independent autophosphorylation. Furthermore, the p.505_506del RET mutant induced a strong activation of the MAPK/ERK pathway and the PI3K/AKT pathway. Consequently, the p.505_506del RET mutant cells increased HEK293 colony formation 4-fold compared with WT RET.

CONCLUSION

The finding of bilateral pheochromocytoma and MTC in our patient was highly suspicious of a RET mutation. Exome sequencing revealed a 6-base-pair deletion in exon 7 of RET, an exon not yet associated with MEN2. Increased ligand-independent phosphorylation of the p.505_506del RET mutant, increased activation of downstream pathways, and stimulation of cell proliferation demonstrated the pathogenic nature of the mutation. We therefore recommend screening the whole sequence of RET in MTC and pheochromocytoma patients with red flags for a genetic cause.

Molecular pathology of hereditary and sporadic medullary thyroid carcinomas

OBJECTIVES

Medullary thyroid carcinoma (MTC) is a relatively uncommon type of thyroid malignancy, with unique histologic features and molecular pathology. It is important to recognize, because its management, which is in part driven by the genetic basis of this disease, is different from follicular-derived thyroid tumors. The aim of this article is to briefly review the histopathologic features of MTC and then explore its molecular pathology, including the role of molecular diagnostic testing and the use of targeted therapy for advanced disease.

METHODS

A review of published literature was performed.

RESULTS

A subset of MTC cases is hereditary and due to germline mutations in the RET tyrosine kinase receptor gene. Somatic mutations in either RET or RAS are also present in most sporadic tumors.

CONCLUSIONS

Molecular genetic testing is routinely performed to identify hereditary cases. In addition, understanding the molecular basis of both hereditary and sporadic MTC has led to the development of targeted therapy with tyrosine kinase inhibitors. Although additional data are needed, tumor mutation status may affect response to targeted therapy. Therefore, it is possible that genetic testing of tumor tissue to predict treatment response, as is currently done for other cancer types, may come into practice in the future.

In silico analysis of RET variants in medullary thyroid cancer: from the computer to the bedside

OBJECTIVE

The American Thyroid Association (ATA) medullary thyroid cancer (MTC) guidelines group RET variants, in the setting of familial medullary thyroid cancer and multiple endocrine neoplasia type 2, into 4 classes of severity based on epidemiological data. The aim of this study was to determine if genotype correlates with phenotype in RET missense mutations.

STUDY DESIGN

In silico mutational tolerance prediction.

SETTING

Academic research hospital.

SUBJECTS AND METHODS

We analyzed all RET variants currently listed in the ATA guidelines for the management of MTC using 2 computer-based (in silico) mutation tolerance prediction approaches: PolyPhen-2 HumVar and PolyPhen-2 HumDiv. Our analysis also included 27 different RET single-nucleotide polymorphisms resulting in missense variants.

RESULTS

There was a statistically significant difference in the overall HumDiv score between ATA groups A and B (P = .025) and a statistically significant different HumVar score between benign polymorphisms and ATA group A (P = .023). Overall, RET variants associated with a less aggressive clinical phenotype generally had a lower Hum Div/Var score.

CONCLUSIONS

Polyphen-2 Hum Div/Var may provide additional clinical data to help distinguish benign from MEN2/familial medullary thyroid carcinoma-causing RET variants as well as less aggressive phenotypes (ATA A) from more aggressive ones (ATA B-C). In silico genetic analyses, with proper validation, may predict the phenotypic severity of RET variants, providing clinicians with a tool to aid clinical decision making in cases in which the RET variant is currently unknown or little epidemiological data are available.

RET revisited: expanding the oncogenic portfolio

The RET receptor tyrosine kinase is crucial for normal development but also contributes to pathologies that reflect both the loss and the gain of RET function. Activation of RET occurs via oncogenic mutations in familial and sporadic cancers - most notably, those of the thyroid and the lung. RET has also recently been implicated in the progression of breast and pancreatic tumours, among others, which makes it an attractive target for small-molecule kinase inhibitors as therapeutics. However, the complex roles of RET in homeostasis and survival of neural lineages and in tumour-associated inflammation might also suggest potential long-term pitfalls of broadly targeting RET.

Hirschsprung's disease and medullary thyroid carcinoma: 15-year experience with molecular genetic screening of the RET proto-oncogene

PURPOSE

Inactivating germline mutations in the RET proto-oncogene are the major genetic cause of Hirschsprung's disease (HD). In some cases, HD can be associated with medullary thyroid carcinoma (MTC) that is commonly caused by activating RET mutations.

METHODS

The retrospective and prospective genetic analyses of 157 patients with HD operated on between December 1979 and June 2011 were carried out. DNA was isolated from peripheral leukocytes. HD patients and family members were tested for RET mutations by direct sequencing and single-strand conformation polymorphism methods.

RESULTS

RET mutations were detected in 16 patients (10%). Association with MTC was found in two families, other eight families had a mutation with potentially high risk of MTC development and four novel mutations were detected. Total colonic aganglionosis was noted to have a high mutation detection rate (40%). Three patients underwent total thyroidectomy (two had clinical manifestation of MTC, one C-cell hyperplasia).

CONCLUSION

Results show the benefit of systematic RET mutation screening in HD patients in order to identify the risk of MTC in the preclinical stage of the disease. All patients should be tested for RET mutations at least in exon 10, and now additionally in exon 11 and 13, as well.

Germline RET sequence variation I852M and occult medullary thyroid cancer: harmless polymorphism or causative mutation?

OBJECTIVE

Rearranged during transfection (RET) gene analysis, widely used to identify carriers at risk of medullary thyroid cancer (MTC), occasionally uncovers novel sequence 'variants of unknown clinical significance' including RET I852M. This study aimed to clarify whether RET I852M represents a harmless polymorphism or a pathogenic mutation.

DESIGN

Clinical investigation supported by functional characterization of I852M mutant cells in vitro.

PATIENTS AND MEASUREMENTS

Genotype-phenotype correlation including five kindreds from a three-generational Caucasian I852M RET family.

RESULTS

A node-negative occult MTC was found in the 64-year-old index patient who had increased basal and stimulated peak calcitonin levels of 190 and 13 307 ng/l, respectively. Her 4-year-old grandson had no histopathological evidence of C-cell disease although his serum calcitonin levels had increased within 5 months from 3·2 to 6·3 ng/l basally and from 17·2 to 24·5 ng/l after pentagastrin stimulation. His mother and two 11- and 1·5-year-old siblings, also carrying the gene, had normal basal and stimulated calcitonin levels and hence did not undergo surgery. Functional characterization of transfected NIH3T3 cells in vitro (cell proliferation rate; cell viability; anchorage-independent cell growth; cell migration; and invasion) indicated that I852M mutant cells have transforming and migratory activities similar to American Thyroid Association (ATA) class A V804M mutants. I852M mutants demonstrated a weaker proliferative potential than fast-proliferating ATA class C C634R mutants and revealed a weaker migratory activity compared with aggressively growing ATA class D A883F mutants.

CONCLUSIONS

I852M sequence variations represent genuine RET mutations, falling into ATA class A of weakly activating RET germline mutations.

In silico and in vitro analysis of rare germline allelic variants of RET oncogene associated with medullary thyroid cancer

Germline and somatic RET oncogene mutations are found in 98% hereditary and 40% sporadic medullary thyroid carcinomas. Our aim was to analyse by in silico and in vitro assays the transforming activity of six rare RET mutations (T338I, V648I, M918V, A883T, S904F and M848T). Six known RET mutations were used as controls. The in silico analysis showed the highest score value (i.e. 65) for S904F, M848T, M918T and C634R, whereas L790F, G691S, T338I and V648I had 0 score. Intermediate score values were obtained by A883T (score=55), M918V, V804M and Y791F (score=15). The in vitro focus formation assay showed that cells transfected with S904F, M918T, M848T or C634R generated the largest number of focus formation units (FFU). Intermediate numbers of FFU were observed in cells transfected with M918V, V804M, Y791F or A883T, while cells transfected with L790F, G691S, T338I or V648I showed a number of FFU similar to control cells. A positive correlation between the in silico score and in vitro FFU was found (P=0.0005). Only cells transfected with M918T or C634R grew faster and generated higher number of colonies in soft agar than control cells. However, the cells that were transfected with V804M produced an intermediate number of colonies. In conclusion, two of the six rare RET mutations, S904F and M848T possessed a relatively high transforming activity but a low aggressiveness; the other four mutations T338I, V648I, M918V and A883T were low or non-transforming, and their ability to induce tumoural transformation might be related to particular genetic conditions.

Rap1GAP interacts with RET and suppresses GDNF-induced neurite outgrowth

Glial cell line-derived neurotrophic factor (GDNF) was originally recognized for its ability to promote survival of midbrain dopaminergic neurons, but it has since been demonstrated to be crucial for the survival and differentiation of many neuronal subpopulations, including motor neurons, sympathetic neurons, sensory neurons and enteric neurons. To identify possible effectors or regulators of GDNF signaling, we performed a yeast two-hybrid screen using the intracellular domain of RET, the common signaling receptor of the GDNF family, as bait. Using this approach, we identified Rap1GAP, a GTPase-activating protein (GAP) for Rap1, as a novel RET-binding protein. Endogenous Rap1GAP co-immunoprecipitated with RET in neural tissues, and RET and Rap1GAP were co-expressed in dopaminergic neurons of the mesencephalon. In addition, overexpression of Rap1GAP attenuated GDNF-induced neurite outgrowth, whereas suppressing the expression of endogenous Rap1GAP by RNAi enhanced neurite outgrowth. Furthermore, using co-immunoprecipitation analyses, we found that the interaction between RET and Rap1GAP was enhanced following GDNF treatment. Mutagenesis analysis revealed that Tyr981 in the intracellular domain of RET was crucial for the interaction with Rap1GAP. Moreover, we found that Rap1GAP negatively regulated GNDF-induced ERK activation and neurite outgrowth. Taken together, our results suggest the involvement of a novel interaction of RET with Rap1GAP in the regulation of GDNF-mediated neurite outgrowth.

The evolving field of tyrosine kinase inhibitors in the treatment of endocrine tumors

Activation of tyrosine kinase receptors (TKRs) and their related pathways has been associated with development of endocrine tumors. Compounds that target and inactivate the kinase function of these receptors, tyrosine kinase inhibitors (TKIs), are now being applied to the treatment of endocrine tumors. Recent clinical trials of TKIs in patients with advanced thyroid cancer, islet cell carcinoma, and carcinoid have shown promising preliminary results. Significant reductions in tumor size have been described in medullary and papillary thyroid carcinoma, although no complete responses have been reported. Case reports have described significant tumor volume reductions of malignant pheochromocytomas and paragangliomas. In addition, these compounds showed an initial tumoricidal or apoptotic response followed by long-term static effects on tumor growth. Despite the promising preliminary results, this class of therapeutic agents has a broad spectrum of adverse effects, mediated by inhibition of kinase activities in normal tissues. These adverse effects will have to be balanced with their benefit in clinical use. New strategies will have to be applied in clinical research to achieve optimal benefits. In this review, we will address the genetic alterations of TKRs, the rationale for utilizing TKIs for endocrine tumors, and current information on tumor and patient responses to specific TKIs. We will also discuss the adverse effects related to TKI treatment and the mechanisms involved. Finally, we will summarize the challenges associated with use of this class of compounds and potential solutions.

Four novel RET germline variants in exons 8 and 11 display an oncogenic potential in vitro

CONTEXT

Most germline-activating mutations of the RET proto-oncogene associated with inherited medullary thyroid cancer (MTC) are localized in exons 10, 11 and 13-15. Four novel RET variants, located in the extracellular domain (p.A510V, p.E511K and p.C531R) coded by exon 8 and in the intracellular juxtamembrane region (p.K666N) coded by exon 11, were identified on the leukocyte DNA from apparently sporadic cases.

METHODS

Plasmids carrying Ret9-wild-type (Ret9-WT), Ret9-C634R and all Ret9 variants were transfected, and the phosphorylation levels of RET and ERK were evaluated by western blot analyses. The transforming potentials were assessed by the focus formation assay.

RESULTS

The p.A510V, p.E511K and p.C531R variants were found to generate RET and ERK phosphorylation levels and to have a transforming activity higher than that of Ret9-WT variant, but lower than that of Ret9-C634R variant. Differently, the p.K666N variant, located immediately downstream of the transmembrane domain, and involving a conserved residue, displayed high kinase and transforming activities. Computational analysis predicted non-conservative alterations in the mutant proteins consistent with putative modifications of the receptor conformation.

CONCLUSIONS

The molecular analyses revealed an oncogenic potential for all the novel germline RET variants. Therefore, the prevalence of exon 8 genomic variations with an oncogenic potential may be higher than previously thought, and the analysis of this exon should be considered after the exclusion of mutations in the classical hotspots. In addition, on the basis of these functional data, it is advisable to extend the genetic screening to all the first-degree relatives of the MTC patients, and to perform a strict follow-up of familial carriers.

RET mutation Tyr791Phe: the genetic cause of different diseases derived from neural crest

Activating germline RET mutations are presented in patients with familial medullary thyroid carcinoma (FMTC) and multiple endocrine neoplasia (MEN) types 2A and 2B, whereas inactivating germline mutations in patients with Hirschsprung's disease (HSCR). The aim of this study was to evaluate genotype-phenotype correlations of the frequently discussed Tyr791Phe mutation in exon 13 of the RET proto-oncogene. Screening of three groups of patients was performed (276 families with medullary thyroid carcinoma (MTC), 122 families with HSCR, and 29 patients with pheochromocytoma). We found this mutation in 3 families with apparently sporadic MTC, 3 families with FMTC/MEN2, 1 patient with pheochromocytoma, and 3 families with HSCR. All gene mutation carriers have a silent polymorphism Leu769Leu in exon 13. In three families second germline mutations were detected: Cys620Phe (exon 10) in MEN2A family, Met918Thr (exon 16) in MEN2B family, and Ser649Leu (exon 11) in HSCR patient. Detection of the Tyr791Phe mutation in MEN2/MTC and also in HSCR families leads to the question whether this mutation has a dual character (gain-of-function as well as loss-of-function). A rare case of malignant pheochromocytoma in a patient with the Tyr791Phe mutation is presented. This study shows various clinical characteristics of the frequently discussed Tyr791Phe mutation.

High resolution array-comparative genomic hybridization profiling reveals deoxyribonucleic acid copy number alterations associated with medullary thyroid carcinoma

CONTEXT

Activating mutations in the RET protooncogene have been demonstrated in multiple endocrine neoplasia 2 and sporadic medullary thyroid carcinoma (MTC). However, the complete genetic etiology underlying MTC tumorigenesis remains unclear.

OBJECTIVE

Our objective was to define more precisely the chromosomal regions and uncover novel genes associated with MTC tumorigenesis.

DESIGN AND SETTING

In this study, we used high resolution array-based comparative genomic hybridization to define tumor-associated copy number alterations (CNA) in 30 primary MTCs: 20 sporadic tumors (50% of which harbored RET mutation), and 10 hereditary.

RESULTS

We identified 98 CNA, including 76 genomic allelic losses, two gains, and 20 copy number variations associated with MTC. Across sporadic and hereditary groups, there was a similar and overlapping pattern of predominant allelic loss. There were 29 regions containing at least 30% CNA in the 30 tumor samples. The most frequent allelic loss occurred in four loci, 7q36.1, 12p13.31, 13q12.11, and 19p13.3-11. No regions were found to be uniquely altered in the hereditary tumors. There were 21 CNA specific to sporadic MTC, with loss of 11q23.3 uniquely altered in RET negative tumors. Pathway analysis found cellular growth and proliferation as the most significant overall target, and cell death as the most significant pathway targeted in sporadic MTC.

CONCLUSIONS

Our findings underscore the importance of candidate tumor suppressor genes together with RET alterations in MTCs. Despite of RET status, all MTC might share similar oncogenetic mechanisms. Dysfunction of cell proliferation and cell death may both be involved in MTC tumorigenesis.

Characterization of the RET protooncogene transmembrane domain mutation S649L associated with nonaggressive medullary thyroid carcinoma

CONTEXT

For rare and novel RET mutations associated with hereditary medullary thyroid carcinoma (MTC), clinical and functional studies are needed to classify the RET mutation into one of the three clinical risk groups.

OBJECTIVE

We analyzed proliferative properties and clinical implications associated with the RET protooncogene transmembrane domain mutation S649L.

DESIGN

The transforming potential and mitogenic properties of S649L mutation were investigated clinically and by evaluating kinase activity, cell proliferation, and colony formation.

PATIENTS

Fifteen individuals from five kindreds were identified as carriers of a RET protooncogene mutation in exon 11 codon 649 (TCG(Ser)-->TTG(Leu)). In two out of five index patients, a second RET mutation (C634W or V804L) was detected.

RESULTS

Eight gene carriers were operated on. Histology revealed MTC and C-cell hyperplasia in three index and three screening patients respectively. In all other gene carriers (aged 41-64 years), calcitonin levels were in the normal range, and pentagastrin-stimulated calcitonin levels were <100 pg/ml. Therefore, thyroidectomy had not yet been performed. In one index patient carrying the S649L mutation, hyperparathyroidism was confirmed histologically. RET S649L-expressing NIH3T3 cells exhibited a clear increase of phosphotyrosine and proliferation rate when compared with parental NIH3T3 cells but a significantly lower kinase activity and cell growth rate when compared with RET C634R-expressing cells. When compared with RET C634R, the S649L mutant showed moderate transforming potential with small-sized colonies.

CONCLUSIONS

Our clinical and in vitro findings indicate that the transmembrane RET S649L mutation is associated with late-onset non-aggressive disease. Recommendations for prophylactic thyroidectomy should be individualized depending on stimulated calcitonin levels.

Prognostic significance of somatic RET oncogene mutations in sporadic medullary thyroid cancer: a 10-year follow-up study

BACKGROUND

Medullary thyroid carcinoma (MTC) is a well-differentiated thyroid tumor that maintains the typical features of C cells. An advanced stage and the presence of lymph node metastases at diagnosis have been demonstrated to be the most important bad prognostic factors. Somatic RET mutations have been found in 40-50% of MTCs. Although a relationship between somatic mutations and bad prognosis has been described, data are controversial and have been performed in small series with short-term follow ups. The aim of this study was to verify the prognostic value of somatic RET mutations in a large series of MTCs with a long follow up.

METHODS

We studied 100 sporadic MTC patients with a 10.2 yr mean follow-up. RET gene exons 10-11 and 13-16 were analyzed. The correlation between the presence/absence of a somatic RET mutation, clinical/pathological features, and outcome of MTC patients was evaluated.

RESULTS

A somatic RET mutation was found in 43 of 100 (43%) sporadic MTCs. The most frequent mutation (34 of 43, 79%) was M918T. RET mutation occurrence was more frequent in larger tumors (P=0.03), and in MTC with node and distant metastases (P<0.0001 and P=0.02, respectively), thus, a significant correlation was found with a more advanced stage at diagnosis (P=0.004). A worse outcome was also significantly correlated with the presence of a somatic RET mutation (P=0.002). Among all prognostic factors found to be correlated with a worse outcome, at multivariate analysis only the advanced stage at diagnosis and the presence of a RET mutation showed an independent correlation (P<0.0001 and P=0.01, respectively). Finally, the survival curves of MTC patients showed a significantly lower percentage of surviving patients in the group with RET mutations (P=0.006).

CONCLUSIONS

We demonstrated that the presence of a somatic RET mutation correlates with a worse outcome of MTC patients, not only for the highest probability to have persistence of the disease, but also for a lower survival rate in a long-term follow up. More interestingly, the presence of a somatic RET mutation correlates with the presence of lymph node metastases at diagnosis, which is a known bad prognostic factor for the definitive cure of MTC patients.

Medullary thyroid carcinoma in a 2-month-old male with multiple endocrine neoplasia 2B and symptoms of pseudo-Hirschsprung disease: a case report

A 5-week-old male patient was seen for symptoms suggestive of Hirschsprung disease (abdominal distension, failure to thrive, and explosive defecation). Rectum biopsies revealed an intestinal ganglioneuromatosis, which is usually associated with multiple endocrine neoplasia (MEN) syndrome type 2B. The ensuing molecular genetic analysis revealed a M918T mutation of the RET protooncogene, which is associated with early-onset medullary thyroid carcinoma (MTC). Therefore, total thyroidectomy and central lymphadenectomy were performed at the age of 9 weeks. Histology showed a medullary microcarcinoma. This report of MTC occurrence within the first weeks of life underlines the importance of early diagnosis and thyroidectomy in patients with MEN 2B syndrome. Because many patients with MEN 2A and B show gastrointestinal symptoms before the development of MTC, the possibility of MEN 2 should be recognized, and genetic testing for the presence of RET mutations should be included in the explorative diagnosis for megacolon.

Hereditary urological cancer syndromes

Hereditary urological cancer syndromes are rare, but it is important that they are recognized because they have important prognostic implications; prompt diagnosis can dramatically improve patient outcomes. The urologist or urological oncologist should, therefore, ascertain which tumors of the many seen in clinical practice warrant referral for the opinion of a clinical geneticist. Despite the aggressive natural history of most inherited urological cancer syndromes, organ-preserving treatments are desirable because these syndromes predispose affected patients to the formation of multifocal and metachronous tumors. Identification of the molecular mechanisms that underlie carcinogenesis in both familial and sporadic urological cancers has, in some cases, resulted in novel and specifically targeted approaches to therapy. Patients who present with early-onset or multiple tumors should be carefully investigated for the presence of a hereditary cancer syndrome, and once a diagnosis is made, appropriate screening should be instigated for family members to enable early detection of tumors both within and outside the urogenital tract.

Ras/ERK1/2-mediated STAT3 Ser727 phosphorylation by familial medullary thyroid carcinoma-associated RET mutants induces full activation of STAT3 and is required for c-fos promoter activation, cell mitogenicity, and transformation

The precise role of STAT3 Ser(727) phosphorylation in RET-mediated cell transformation and oncogenesis is not well understood. In this study, we have shown that familial medullary thyroid carcinoma (FMTC) mutants RET(Y791F) and RET(S891A) induced, in addition to Tyr(705) phosphorylation, constitutive STAT3 Ser(727) phosphorylation. Using inhibitors and dominant negative constructs, we have demonstrated that RET(Y791F) and RET(S891A) induce STAT3 Ser(727) phosphorylation via a canonical Ras/ERK1/2 pathway and that integration of the Ras/ERK1/2/ELK-1 and STAT3 pathways was required for up-regulation of the c-fos promoter by FMTC-RET. Moreover, inhibition of ERK1/2 had a more severe effect on cell proliferation and cell phenotype in HEK293 cells expressing RET(S891A) compared with control and RET(WT)-transfected cells. The transforming activity of RET(Y791F) and RET(S891A) in NIH-3T3 cells was also inhibited by U0126, indicating a role of the ERK1/2 pathway in RET-mediated transformation. To investigate the biological significance of Ras/ERK1/2-induced STAT3 Ser(727) phosphorylation for cell proliferation and transformation, N-Ras-transformed NIH-3T3 cells were employed. These cells displayed elevated levels of activated ERK1/2 and Ser(727)-phosphorylated STAT3, which were inhibited by treatment with U0126. Importantly, overexpression of STAT3, in which the Ser(727) was mutated into Ala (STAT3(S727A)), rescued the transformed phenotype of N-Ras-transformed cells. Immunohistochemistry in tumor samples from FMTC patients showed strong nuclear staining of phosphorylated ERK1/2 and Ser(727) STAT3. These data show that FMTC-RET mutants activate a Ras/ERK1/2/STAT3 Ser(727) pathway, which plays an important role in cell mitogenicity and transformation.

RET signaling in endocrine tumors: delving deeper into molecular mechanisms

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

RET and neuroendocrine tumors

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