Pheochromocytomas in Nf1 knockout mice express a neural progenitor gene expression profile

Animal and Cell Culture Models of PPGLs - Achievements and Limitations

Research on rare tumors heavily relies on suitable models for basic and translational research. Paragangliomas (PPGL) are rare neuroendocrine tumors (NET), developing from adrenal (pheochromocytoma, PCC) or extra-adrenal (PGL) chromaffin cells, with an annual incidence of 2-8 cases per million. While most PPGL cases exhibit slow growth and are primarily treated with surgery, limited systemic treatment options are available for unresectable or metastatic tumors. Scarcity of appropriate models has hindered PPGL research, preventing the translation of omics knowledge into drug and therapy development. Human PPGL cell lines are not available, and few animal models accurately replicate the disease's genetic and phenotypic characteristics. This review provides an overview of laboratory models for PPGLs, spanning cellular, tissue, organ, and organism levels. We discuss their features, advantages, and potential contributions to diagnostics and therapeutics. Interestingly, it appears that in the PPGL field, disease models already successfully implemented in other cancers have not been fully explored.

TMEM127 suppresses tumor development by promoting RET ubiquitination, positioning, and degradation

The TMEM127 gene encodes a transmembrane protein of poorly known function that is mutated in pheochromocytomas, neural crest-derived tumors of adrenomedullary cells. Here, we report that, at single-nucleus resolution, TMEM127-mutant tumors share precursor cells and transcription regulatory elements with pheochromocytomas carrying mutations of the tyrosine kinase receptor RET. Additionally, TMEM127-mutant pheochromocytomas, human cells, and mouse knockout models of TMEM127 accumulate RET and increase its signaling. TMEM127 contributes to RET cellular positioning, trafficking, and lysosome-mediated degradation. Mechanistically, TMEM127 binds to RET and recruits the NEDD4 E3 ubiquitin ligase for RET ubiquitination and degradation via TMEM127 C-terminal PxxY motifs. Lastly, increased cell proliferation and tumor burden after TMEM127 loss can be reversed by selective RET inhibitors in vitro and in vivo. Our results define TMEM127 as a component of the ubiquitin system and identify aberrant RET stabilization as a likely mechanism through which TMEM127 loss-of-function mutations cause pheochromocytoma.

Domain landscapes of somatic NF1 mutations in pheochromocytoma and paraganglioma

Pheochromocytoma and paraganglioma (PPGL), are rare neuroendocrine tumors arising from the adrenal medulla and extra-adrenal paraganglia, respectively. Up to about 60% are explained by germline or somatic mutations in one of the major known susceptibility genes e.g., inNF1,RET,VHL, SDHx,MAXandHRAS. Targeted Next Generation Sequencing was performed in 14 sporadic tumors using a panel including 26 susceptibility genes to characterize the mutation profile. A total of 6 germline and 8 somatic variants were identified. The most frequent somatic mutations were found in NF1(36%), four have not been reported earlier in PCC or PGL. Gene expression profile analysis showed that NF1 mutated tumors are classified into RTK3 subtype, cluster 2, with a high expression of genes associated with chromaffin cell differentiation, and into a RTK2 subtype, cluster 2, as well with overexpression of genes associated with cortisol biosynthesis. On the other hand, by analyzing the entire probe set on the array and TCGA data, ALDOC was found as the most significantly down regulated gene in NF1-mutated tumors compared to NF1-wild-type. Differential gene expression analysis showed a significant difference between Nt - and Ct-NF1 domains in mutated tumors probably engaging different cellular pathways. Notably, we had a metastatic PCC with a Ct-NF1 frameshift mutation and when performing protein docking analysis, Ct-NF1 showed an interaction with Nt-FAK suggesting their involvement in cell adhesion and cell growth. These results show that depending on the location of the NF1-mutation different pathways are activated in PPGLs. Further studies are required to clarify their clinical significance.

Mouse models in endocrine tumors

Endocrine and neuroendocrine tumors comprise a highly heterogeneous group of neoplasms that can arise from (neuro)endocrine cells, either from endocrine glands or from the widespread diffuse neuroendocrine system, and, consequently, are widely distributed throughout the body. Due to their diversity, heterogeneity and limited incidence, studying in detail the molecular and genetic alterations that underlie their development and progression is still a highly elusive task. This, in turn, hinders the discovery of novel therapeutic options for these tumors. To circumvent these limitations, numerous mouse models of endocrine and neuroendocrine tumors have been developed, characterized and used in pre-clinical, co-clinical (implemented in mouse models and patients simultaneously) and post-clinical studies, for they represent powerful and necessary tools in basic and translational tumor biology research. Indeed, different in vivo mouse models, including cell line-based xenografts (CDXs), patient-derived xenografts (PDXs) and genetically engineered mouse models (GEMs), have been used to delineate the development, progression and behavior of human tumors. Results gained with these in vivo models have facilitated the clinical application in patients of diverse breakthrough discoveries made in this field. Herein, we review the generation, characterization and translatability of the most prominent mouse models of endocrine and neuroendocrine tumors reported to date, as well as the most relevant clinical implications obtained for each endocrine and neuroendocrine tumor type.

Molecular markers of paragangliomas/pheochromocytomas

Paragangliomas/pheochromocytomas comprise rare tumors that arise from the extra-adrenal paraganglia, with an incidence of about 2 to 8 per million people each year. Approximately 40% of cases are due to genetic mutations in at least one out of more than 30 causative genes. About 25-30% of pheochromocytomas/paragangliomas develop under the conditions of a hereditary tumor syndrome a third of which are caused by mutations in the VHL gene. Together, the gene mutations in this disorder have implicated multiple processes including signaling pathways, translation initiation, hypoxia regulation, protein synthesis, differentiation, survival, proliferation, and cell growth. The present review contemplates the mutations associated with the development of pheochromocytomas/paragangliomas and their potential to serve as specific markers of these tumors and their progression. These data will improve our understanding of the pathogenesis of these tumors and likely reveal certain features that may be useful for early diagnostics, malignancy prognostics, and the determination of new targets for disease therapeutics.

Adrenal cortical and chromaffin stem cells: Is there a common progeny related to stress adaptation?

The adrenal gland is a highly plastic organ with the capacity to adapt the body homeostasis to different physiological needs. The existence of stem-like cells in the adrenal cortex has been revealed in many studies. Recently, we identified and characterized in mice a pool of glia-like multipotent Nestin-expressing progenitor cells, which contributes to the plasticity of the adrenal medulla. In addition, we found that these Nestin progenitors are actively involved in the stress response by giving rise to chromaffin cells. Interestingly, we also observed a Nestin-GFP-positive cell population located under the adrenal capsule and scattered through the cortex. In this article, we discuss the possibility of a common progenitor giving rise to subpopulations of cells both in the adrenal cortex and medulla, the isolation and characterization of this progenitor as well as its clinical potential in transplantation therapies and in pathophysiology.

Adrenomedullary progenitor cells: Isolation and characterization of a multi-potent progenitor cell population

The adrenal is a highly plastic organ with the ability to adjust to physiological needs by adapting hormone production but also by generating and regenerating both adrenocortical and adrenomedullary tissue. It is now apparent that many adult tissues maintain stem and progenitor cells that contribute to their maintenance and adaptation. Research from the last years has proven the existence of stem and progenitor cells also in the adult adrenal medulla throughout life. These cells maintain some neural crest properties and have the potential to differentiate to the endocrine and neural lineages. In this article, we discuss the evidence for the existence of adrenomedullary multi potent progenitor cells, their isolation and characterization, their differentiation potential as well as their clinical potential in transplantation therapies but also in pathophysiology.

Pheochromocytomas and Paragangliomas: Clinical and Genetic Approaches

Pheochromocytomas (PCCs) and paragangliomas (PGLs) are neuroendocrine tumors derived from the chromaffin tissue. Diagnosis of these tumors is extremely important as they are linked to the hypertension syndrome with great cardiovascular morbidity and mortality. A great majority of PCCs and PGLs are sporadic and benign tumors; however, the classic idea of 10% exception of these features is changing. The description of new genes linked to familial forms of PCC/PGLs, such as succinate dehydrogenase (SDH) complex subunits, KIF1Bβ, EGLN1, TMEM127, and MAX, added to the well-known PCC familial syndrome (MEN2, VHL, and neurofibromatosis type 1) presents new challenges for diagnosis. In this review, we discuss the diversity of clinical and genetic approaches to this syndrome as well the diverse criteria that should guide genetic investigation.

Current views on cell metabolism in SDHx-related pheochromocytoma and paraganglioma

Warburg's metabolic hypothesis is based on the assumption that a cancer cell's respiration must be under attack, leading to its damage, in order to obtain increased glycolysis. Although this may not apply to all cancers, there is some evidence proving that primarily abnormally functioning mitochondrial complexes are indeed related to cancer development. Thus, mutations in complex II (succinate dehydrogenase (SDH)) lead to the formation of pheochromocytoma (PHEO)/paraganglioma (PGL). Mutations in one of the SDH genes (SDHx mutations) lead to succinate accumulation associated with very low fumarate levels, increased glutaminolysis, the generation of reactive oxygen species, and pseudohypoxia. This results in significant changes in signaling pathways (many of them dependent on the stabilization of hypoxia-inducible factor), including oxidative phosphorylation, glycolysis, specific expression profiles, as well as genomic instability and increased mutability resulting in tumor development. Although there is currently no very effective therapy for SDHx-related metastatic PHEOs/PGLs, targeting their fundamental metabolic abnormalities may provide a unique opportunity for the development of novel and more effective forms of therapy for these tumors.

Pheochromocytoma and paraganglioma pathogenesis: learning from genetic heterogeneity

The neuroendocrine tumours pheochromocytomas and paragangliomas carry the highest degree of heritability in human neoplasms, enabling genetic alterations to be traced to clinical phenotypes through their transmission in families. Mutations in more than a dozen distinct susceptibility genes have implicated multiple pathways in these tumours, offering insights into kinase downstream signalling interactions and hypoxia regulation, and uncovering links between metabolism, epigenetic remodelling and cell growth. These advances extend to co-occurring tumours, including renal, thyroid and gastrointestinal malignancies. Hereditary pheochromocytomas and paragangliomas are powerful models for recognizing cancer driver events, which can be harnessed for diagnostic purposes and for guiding the future development of targeted therapies.

Implication of tumor stem-like cells in the tumorigenesis of sporadic paraganglioma

It is commonly believed that paragangliomas are rare tumors arising from the neural crest-derived chromaffin cells. Although it has been speculated that paraganglioma is related to stem cell origin, there has been lack of direct evidence demonstrating the presence of (neural) stem cells in these tumor tissues. In this study, we found a subgroup of human paraganglioma from ten clinical samples displayed definitive markers of CD133 and/or nestin, the fundamental features of neural stem cell capable of self-renewal and differentiation. A panel of lineage-specific markers was also manifest in some of these tumors, consistent with the hierarchical and heterogeneous nature of these tumors. These observations strongly suggest that at least some forms of paraganglioma maintain tumor stem-like cells (TSCs) that potentially contribute to the histologic complexity of human paraganglioma. Finally, we found that the genomic DNA structure becomes highly unstable in tumor cells of paraganglioma, indicating the loss of tight control of genomic surveillance system be an important transitory event from normal multi-potent tissue stem cells to TSCs.

Genetics of pheochromocytoma and paraganglioma syndromes: new advances and future treatment options

PURPOSE OF REVIEW

To summarize the recent advances in the genetics of pheochromocytoma and paraganglioma (PHEO/PGL), focusing on the new susceptibility genes and dividing PHEOs/PGLs into two groups based on their transcription profile.

RECENT FINDINGS

Recently, TMEM127, MYC-associated factor X, and hypoxia-inducible factor (HIF) 2α have been described in the pathogenesis of PHEOs/PGLs. Thus, now about 30-40% of these tumors are linked to the germline mutations, which also include mutations in the VHL, RET, NF1, SDHx, and SDHAF2 genes. Furthermore, PHEOs/PGLs have been divided into two groups, cluster 1 (SDHx/VHL) and cluster 2 (RET/NF1), based on the transcription profile revealed by genome-wide expression microarray analysis.

SUMMARY

PHEOs/PGLs are the most inherited tumors among (neuro)endocrine tumors. Future approaches in genetics, including whole-genome sequencing, will allow the discovery of additional PHEO/PGL susceptibility genes. The current division of PHEOs/PGLs into cluster 1 and 2 provides us with additional knowledge related to the pathogenesis of these tumors, including the introduction of new treatment options for patients with metastatic PHEOs/PGLs. New discoveries related to the role of the HIF-1/HIF-2α genes in the pathogenesis of almost all inherited PHEOs/PGLs may call for a new regrouping of these tumors and discoveries of new treatment targets.

Combined blockade of signalling pathways shows marked anti-tumour potential in phaeochromocytoma cell lines

Currently, there is no completely effective therapy available for metastatic phaeochromocytomas (PCCs) and paragangliomas. In this study, we explore new molecular targeted therapies for these tumours, using one more benign (mouse phaeochromocytoma cell (MPC)) and one more malignant (mouse tumour tissue (MTT)) mouse PCC cell line - both generated from heterozygous neurofibromin 1 knockout mice. Several PCC-promoting gene mutations have been associated with aberrant activation of PI3K/AKT, mTORC1 and RAS/RAF/ERK signalling. We therefore investigated different agents that interfere specifically with these pathways, including antagonism of the IGF1 receptor by NVP-AEW541. We found that NVP-AEW541 significantly reduced MPC and MTT cell viability at relatively high doses but led to a compensatory up-regulation of ERK and mTORC1 signalling at suboptimal doses while PI3K/AKT inhibition remained stable. We subsequently investigated the effect of the dual PI3K/mTORC1/2 inhibitor NVP-BEZ235, which led to a significant decrease of MPC and MTT cell viability at doses below 50 nM but again increased ERK signalling. Accordingly, we next examined the combination of NVP-BEZ235 with the established agent lovastatin, as this has been described to inhibit ERK signalling. Lovastatin alone significantly reduced MPC and MTT cell viability at therapeutically relevant doses and inhibited both ERK and AKT signalling, but increased mTORC1/p70S6K signalling. Combination treatment with NVP-BEZ235 and lovastatin showed a significant additive effect in MPC and MTT cells and resulted in inhibition of both AKT and mTORC1/p70S6K signalling without ERK up-regulation. Simultaneous inhibition of PI3K/AKT, mTORC1/2 and ERK signalling suggests a novel therapeutic approach for malignant PCCs.

Somatic NF1 inactivation is a frequent event in sporadic pheochromocytoma

Germline mutations in the RET, SDHA, SDHAF2, SDHB, SDHC, SDHD, MAX, TMEM127, NF1 or VHL genes are identified in about 30% of patients with pheochromocytoma or paraganglioma and somatic mutations in RET, VHL or MAX genes are reported in 17% of sporadic tumors. In the present study, using mutation screening of the NF1 gene, mapping of chromosome aberrations by single nucleotide polymorphism (SNP) array, microarray-based expression profiling and immunohistochemistry (IHC), we addressed the implication of NF1 somatic alterations in pheochromocytomas and paragangliomas. We studied 53 sporadic tumors, selected because of their classification with RET/NF1/TMEM127-related tumors by genome wide expression studies, as well as a second set of 11 independent tumors selected on their low individual levels of NF1 expression evaluated by microarray. Direct sequencing of the NF1 gene in tumor DNA identified the presence of an inactivating NF1 somatic mutation in 41% (25/61) of analyzed sporadic tumors, associated with loss of the wild-type allele in 84% (21/25) of cases. Gene expression signature of NF1-related tumors highlighted the downregulation of NF1 and the major overexpression of SOX9. Among the second set of 11 tumors, two sporadic tumors carried somatic mutations in NF1 as well as in another susceptibility gene. These new findings suggest that NF1 loss of function is a frequent event in the tumorigenesis of sporadic pheochromocytoma and strengthen the new concept of molecular-based targeted therapy for pheochromocytoma or paraganglioma.

Warburg effect's manifestation in aggressive pheochromocytomas and paragangliomas: insights from a mouse cell model applied to human tumor tissue

A glycolytic profile unifies a group of pheochromocytomas and paragangliomas (PHEOs/PGLs) with distinct underlying gene defects, including von Hippel-Lindau (VHL) and succinate dehydrogenase B (SDHB) mutations. Nevertheless, their tumor aggressiveness is distinct: PHEOs/PGLs metastasize rarely in VHL-, but frequently in SDHB-patients. To date, the molecular mechanisms causing the more aggressive phenotype in SDHB-PHEOs/PGLs remain largely unknown. Recently, however, an excellent model to study aggressive PHEOs (mouse tumor tissue (MTT) cells) has been developed from mouse PHEO cells (MPC). We employed this model for a proteomics based approach to identify changes characteristic for tumor aggressiveness, which we then explored in a homogeneous set of human SDHB- and VHL-PHEOs/PGLs. The increase of glucose transporter 1 in VHL, and of hexokinase 2 in VHL and SDHB, confirmed their glycolytic profile. In agreement with the cell model and in support of decoupling of glycolysis, the Krebs cycle and oxidative phosphorylation (OXPHOS), SDHB tumors showed increased lactate dehydrogenase levels. In SDHB-PGLs OXPHOS complex activity was increased at complex III and, as expected, decreased at complex II. Moreover, protein and mRNA expression of all tested OXPHOS-related genes were higher in SDHB- than in VHL-derived tumors. Although there was no direct evidence for increased reactive oxygen species production, elevated superoxide dismutase 2 expression may reflect elevated oxidative stress in SDHB-derived PHEOs/PGLs. For the first time, we show that despite dysfunction in complex II and evidence for a glycolytic phenotype, the Warburg effect does not seem to fully apply to SDHB-PHEOs/PGLs with respect to decreased OXPHOS. In addition, we present evidence for increased LDHA and SOD2 expression in SDHB-PHEOs/PGLs, proteins that have been proposed as promising therapeutic targets in other cancers. This study provides new insight into pathogenic mechanisms in aggressive human PHEOs/PGLs, which may lead to identifying new diagnostic and prognostic markers in the near future.

Zebrafish neurofibromatosis type 1 genes have redundant functions in tumorigenesis and embryonic development

Neurofibromatosis type 1 (NF1) is a common, dominantly inherited genetic disorder that results from mutations in the neurofibromin 1 (NF1) gene. Affected individuals demonstrate abnormalities in neural-crest-derived tissues that include hyperpigmented skin lesions and benign peripheral nerve sheath tumors. NF1 patients also have a predisposition to malignancies including juvenile myelomonocytic leukemia (JMML), optic glioma, glioblastoma, schwannoma and malignant peripheral nerve sheath tumors (MPNSTs). In an effort to better define the molecular and cellular determinants of NF1 disease pathogenesis in vivo, we employed targeted mutagenesis strategies to generate zebrafish harboring stable germline mutations in nf1a and nf1b, orthologues of NF1. Animals homozygous for loss-of-function alleles of nf1a or nf1b alone are phenotypically normal and viable. Homozygous loss of both alleles in combination generates larval phenotypes that resemble aspects of the human disease and results in larval lethality between 7 and 10 days post fertilization. nf1-null larvae demonstrate significant central and peripheral nervous system defects. These include aberrant proliferation and differentiation of oligodendrocyte progenitor cells (OPCs), dysmorphic myelin sheaths and hyperplasia of Schwann cells. Loss of nf1 contributes to tumorigenesis as demonstrated by an accelerated onset and increased penetrance of high-grade gliomas and MPNSTs in adult nf1a^+/−; nf1b^−/−; p53^e7/e7 animals. nf1-null larvae also demonstrate significant motor and learning defects. Importantly, we identify and quantitatively analyze a novel melanophore phenotype in nf1-null larvae, providing the first animal model of the pathognomonic pigmentation lesions of NF1. Together, these findings support a role for nf1a and nf1b as potent tumor suppressor genes that also function in the development of both central and peripheral glial cells as well as melanophores in zebrafish.

Differentiation of chromaffin progenitor cells to dopaminergic neurons

The differentiation of dopamine-producing neurons from chromaffin progenitors might represent a new valuable source for replacement therapies in Parkinson's disease. However, characterization of their differentiation potential is an important prerequisite for efficient engraftment. Based on our previous studies on isolation and characterization of chromaffin progenitors from adult adrenals, this study investigates their potential to produce dopaminergic neurons and means to enhance their dopaminergic differentiation. Chromaffin progenitors grown in sphere culture showed an increased expression of nestin and Mash1, indicating an increase of the progenitor subset. Proneurogenic culture conditions induced the differentiation into neurons positive for neural markers β-III-tubulin, MAP2, and TH accompanied by a decrease of Mash1 and nestin. Furthermore, Notch2 expression decreased concomitantly with a downregulation of downstream effectors Hes1 and Hes5 responsible for self-renewal and proliferation maintenance of progenitor cells. Chromaffin progenitor-derived neurons secreted dopamine upon stimulation by potassium. Strikingly, treatment of differentiating cells with retinoic and ascorbic acid resulted in a twofold increase of dopamine secretion while norepinephrine and epinephrine were decreased. Initiation of dopamine synthesis and neural maturation is controlled by Pitx3 and Nurr1. Both Pitx3 and Nurr1 were identified in differentiating chromaffin progenitors. Along with the gained dopaminergic function, electrophysiology revealed features of mature neurons, such as sodium channels and the capability to fire multiple action potentials. In summary, this study elucidates the capacity of chromaffin progenitor cells to generate functional dopaminergic neurons, indicating their potential use in cell replacement therapies.

Murine models and cell lines for the investigation of pheochromocytoma: applications for future therapies?

Pheochromocytomas (PCCs) are slow-growing neuroendocrine tumors arising from adrenal chromaffin cells. Tumors arising from extra-adrenal chromaffin cells are called paragangliomas. Metastases can occur up to approximately 60% or even more in specific subgroups of patients. There are still no well-established and clinically accepted "metastatic" markers available to determine whether a primary tumor is or will become malignant. Surgical resection is the most common treatment for non-metastatic PCCs, but no standard treatment/regimen is available for metastatic PCC. To investigate what kind of therapies are suitable for the treatment of metastatic PCC, animal models or cell lines are very useful. Over the last two decades, various mouse and rat models have been created presenting with PCC, which include models presenting tumors that are to a certain degree biochemically and/or molecularly similar to human PCC, and develop metastases. To be able to investigate which chemotherapeutic options could be useful for the treatment of metastatic PCC, cell lines such as mouse pheochromocytoma (MPC) and mouse tumor tissue (MTT) cells have been recently introduced and they both showed metastatic behavior. It appears these MPC and MTT cells are biochemically and molecularly similar to some human PCCs, are easily visualized by different imaging techniques, and respond to different therapies. These studies also indicate that some mouse models and both mouse PCC cell lines are suitable for testing new therapies for metastatic PCC.

Signaling pathways in pheochromocytomas and paragangliomas: prospects for future therapies

There is currently no completely effective therapy available for metastatic pheochromocytomas or paragangliomas. Increasing understanding of the germline and somatic mutations leading to pheochromocytoma and paraganglioma development has revealed crucial insights into the molecular pathology of these tumors. A detailed understanding of the molecular pathway alterations giving rise to pheochromocytomas and paragangliomas should allow for the exploration and development of new effective molecular-targeted therapy options for this rare but frequently fatal malignancy. Molecular analysis has shown that pheochromocytoma/paraganglioma-promoting gene mutations can be divided into two major groups-clusters 1 and 2-following two different routes to tumorigenesis. Cluster 1 mutations are associated with pseudohypoxia and aberrant VEGF signaling while cluster 2 mutations are associated with abnormal activation of kinase signaling pathways such as PI3 kinase/AKT, RAS/RAF/ERK, and mTORC1/p70S6K suggesting relevant targets for novel molecular-targeted therapy approaches which will be discussed in detail in this chapter.

Pheochromocytomas and paragangliomas: assessment of malignant potential

Pheochromocytomas and paragangliomas (PPGLs) are rare catecholamine-secreting tumors which arise from the adrenal glands or sympathetic neuronal tissue. Malignant transformation of these tumors occurs in a significant proportion and may therefore lower overall survival rates. In patients with PPGLs it is impossible to identify malignant disease without the presence of metastatic disease, something which can occur as long as 20 years after initial surgery. Early identification of malignant disease would necessitate a more aggressive treatment approach, something which may result in better disease outcome. We have therefore reviewed possible predictors of malignancy and current developments in order to help clinicians to swiftly assess malignant potential in patients with PPGLs. Currently, there is no absolute marker which can objectively reflect malignant potential. Tumor size is the most reliable predictor and should therefore be used as the baseline characteristic. The combination of various clinical markers (extra-adrenal disease and post-operative hypertension), biochemical markers (high dopamine, high norepinephrine and epinephrine to total catecholamine ratio) and/or histological markers (SNAIL, microRNAs and/or microarray results) can raise or lower the suspicion of malignancy. Furthermore, we discuss how clinical markers may affect biochemical results linked to malignancy, how biochemical results may distinguish hereditary syndromes, the role of imaging in determining malignant potential and tumor detection, and recent results of proposed histological markers.

The receptor tyrosine kinase Alk controls neurofibromin functions in Drosophila growth and learning

Anaplastic Lymphoma Kinase (Alk) is a Receptor Tyrosine Kinase (RTK) activated in several cancers, but with largely unknown physiological functions. We report two unexpected roles for the Drosophila ortholog dAlk, in body size determination and associative learning. Remarkably, reducing neuronal dAlk activity increased body size and enhanced associative learning, suggesting that its activation is inhibitory in both processes. Consistently, dAlk activation reduced body size and caused learning deficits resembling phenotypes of null mutations in dNf1, the Ras GTPase Activating Protein-encoding conserved ortholog of the Neurofibromatosis type 1 (NF1) disease gene. We show that dAlk and dNf1 co-localize extensively and interact functionally in the nervous system. Importantly, genetic or pharmacological inhibition of dAlk rescued the reduced body size, adult learning deficits, and Extracellular-Regulated-Kinase (ERK) overactivation dNf1 mutant phenotypes. These results identify dAlk as an upstream activator of dNf1-regulated Ras signaling responsible for several dNf1 defects, and they implicate human Alk as a potential therapeutic target in NF1.

Neurofibromatosis-1 (NF1) syndrome is a common (1/3,000 births) genetic disorder affecting multiple organ systems, including the nervous system. Its clinical features include short stature, learning disabilities, and several types of benign and malignant tumors. NF1 is caused by mutations that inactivate the NF1 gene, a crucial negative regulator of Ras signaling. Although unregulated Ras signaling is a hallmark of NF1, the specific Ras signaling pathways responsible for disease development remain largely unknown. The Drosophila and human Nf1 genes are highly conserved; and, as in patients, mutant flies are smaller than usual and present deficient learning. Here, we identified the Drosophila Receptor Tyrosine Kinase dAlk as a negative regulator of organismal growth and olfactory learning. We show that excessive dAlk activation results in growth and learning defects similar to those of Nf1 mutants. Genetic suppression studies and pharmacological inhibition indicate dAlk as a critical upstream activator of Nf1-regulated neuronal Ras/ERK signals that contribute to size determination and learning. Importantly, our results strongly suggest that Alk represents a novel, highly specific, and promising therapeutic target in human NF1.

Neuroendocrine neoplasms of the gut and pancreas: new insights

Neuroendocrine neoplasms arise in almost every organ of the body and are variably defined according to the site of origin. This Review focuses on neuroendocrine neoplasms of the digestive tract and pancreas. The 2010 WHO classification of tumors of the digestive system introduces grading and staging tools for neuroendocrine neoplasms. A carcinoid is now defined as a grade 1 or 2 neuroendocrine tumor and grade 3, small-cell or large-cell carcinomas are defined as neuroendocrine carcinoma. Epidemiological data show a worldwide increase in the prevalence and incidence of gastroentero-pancreatic neuroendocrine tumors in the past few decades, which is probably due to improved methods of detection of these tumors. The current diagnostic procedures and treatment options for neuroendocrine neoplasms are defined and summarized in the Review, although evidence-based data are lacking. Surgery remains the treatment mainstay and somatostatin analogues the basis for both diagnosis and therapy as the only 'theranostic' tool. Emerging compounds including chemotherapeutic agents, small molecules and biological therapies may provide new hope for patients.

Minireview: the busy road to pheochromocytomas and paragangliomas has a new member, TMEM127

Characterization of the entire spectrum of cancer-associated genetic disruptions is an overarching goal of contemporary and future oncology and can inform on patient diagnosis, treatment, and surveillance. Hereditary endocrine tumors, by having the potential to reveal the cancer's primary molecular defect, have been especially informative in this realm. Within this group, pheochromocytomas and paragangliomas, neural crest-derived, catecholamine-secreting tumors have come to represent true conduits for gene discovery. About one-third of pheochromocytomas and paragangliomas are now known to result from germline mutations in one of at least eight genes that belong to a variety of functional classes. Greater understanding of the molecular signals transduced by these genes and their respective mutants has advanced our understanding of kinase signaling pathways, hypoxia regulation, and the link between metabolic disruptions and cell growth. A new susceptibility gene without homology to other functional classes has been recently identified and encodes for a three-spanner transmembrane protein, transmembrane protein 127 (TMEM127). Initial insights from in vitro and patient data suggest that this candidate tumor suppressor is linked to the endosomal system and the mechanistic target of rapamycin [formerly mammalian target of rapamycin (mTOR)] pathway, and that mutation carriers often have clinical features that are typically associated with sporadic forms of pheochromocytoma. Functional characterization of transmembrane protein 127 (TMEM127) and discovery of additional pheochromocytoma/paraganglioma susceptibility genes is likely to shed light on our understanding of these tumors and extend these insights to other cancers.

Chromaffin progenitor cells from the adrenal medulla

Chromaffin cells of the adrenal medulla are neural crest-derived cells of the sympathoadrenal lineage. Different lines of evidence suggest the existence of a subpopulation of proliferation-competent progenitor cells even in the adult state. The identification of sympathoadrenal progenitors in the adrenal would greatly enhance the understanding of adrenal physiology and their potential role in adrenal pathogenesis. Isolation and differentiation of these progenitor cells in culture would provide a tool to understand their development in vitro. Furthermore, due to the close relation to sympathetic neurons, these cells might provide an expandable source of cells for cell therapy in the treatment of neurodegenerative diseases. We therefore aim to establish protocols for the efficient isolation, enrichment and differentiation of chromaffin progenitor cells to dopaminergic neurons in culture.

Pheochromocytoma in rats with multiple endocrine neoplasia (MENX) shares gene expression patterns with human pheochromocytoma

Pheochromocytomas are rare neoplasias of neural crest origin arising from chromaffin cells of the adrenal medulla and sympathetic ganglia (extra-adrenal pheochromocytoma). Pheochromocytoma that develop in rats homozygous for a loss-of-function mutation in p27Kip1 (MENX syndrome) show a clear progression from hyperplasia to tumor, offering the possibility to gain insight into tumor pathobiology. We compared the gene-expression signatures of both adrenomedullary hyperplasia and pheochromocytoma with normal rat adrenal medulla. Hyperplasia and tumor show very similar transcriptome profiles, indicating early determination of the tumorigenic signature. Overrepresentation of developmentally regulated neural genes was a feature of the rat lesions. Quantitative RT-PCR validated the up-regulation of 11 genes, including some involved in neural development: Cdkn2a, Cdkn2c, Neurod1, Gal, Bmp7, and Phox2a. Overexpression of these genes precedes histological changes in affected adrenal glands. Their presence at early stages of tumorigenesis indicates they are not acquired during progression and may be a result of the lack of functional p27Kip1. Adrenal and extra-adrenal pheochromocytoma development clearly follows diverged molecular pathways in MENX rats. To correlate these findings to human pheochromocytoma, we studied nine genes overexpressed in the rat lesions in 46 sporadic and familial human pheochromocytomas. The expression of GAL, DGKH, BMP7, PHOX2A, L1CAM, TCTE1, EBF3, SOX4, and HASH1 was up-regulated, although with different frequencies. Immunohistochemical staining detected high L1CAM expression selectively in 27 human pheochromocytomas but not in 140 nonchromaffin neuroendocrine tumors. These studies reveal clues to the molecular pathways involved in rat and human pheochromocytoma and identify previously unexplored biomarkers for clinical use.

Plexiform neurofibroma genesis: questions of Nf1 gene dose and hyperactive mast cells

PURPOSE OF REVIEW

Tumorigenic cells can co-opt normal functions of nonmalignant hematopoietic cells, promoting tumor progression. Recent mouse and human studies indicate that mast cells underpin inflammation in the plexiform neurofibroma microenvironment of neurofibromatosis type 1. In this model, Nf1 homozygous-deficient Schwann cells recruit hyperactive mast cells, promoting tumorigenesis. Here, we discuss the importance of Nf1 gene dosage, delineate hematopoietic contributions to the plexiform neurofibroma microenvironment, and highlight applications to human treatment.

RECENT FINDINGS

Previous studies found that plexiform neurofibroma formation in a mouse model requires biallelic loss of Nf1 in Schwann cells and an Nf1 heterozygous cellular background. Now, transplantation and pharmacological experiments have indicated that tumor formation specifically requires Nf1 heterozygosity of c-kit-dependent bone marrow.

SUMMARY

Neurofibromatosis type 1 results from autosomal dominant mutations of the NF1 tumor suppressor gene. Although unpredictable second-hit mutations in the remaining NF1 allele precede local manifestations such as tumor formation, human and mouse data indicate that NF1/Nf1 gene haploinsufficiency modulates cellular physiology and disease pathogeneses. In particular, Nf1 haplo insufficient mast cells demonstrate multiple gain-in-functions, and mast cells permeate neurofibroma tissue. Transplantation experiments have shown that these aberrant mast cells critically underpin the tumor microenvironment. Using these findings, clinicians have medically treated a patient with a debilitating plexiform neurofibroma.

Tyrosine hydroxylase, chromogranin A, and steroidogenic acute regulator as markers for successful separation of human adrenal medulla

Progress in high throughput "-omic" techniques now allows the simultaneous measurement of expression levels of thousands of genes and promises the improved understanding of the molecular biology of diseases such as cancer. Detection of the dysfunction of molecular pathways in diseases requires healthy control tissue. This is difficult to obtain from pheochromocytomas (PHEOs), rare chromaffin tumors derived from adrenal medulla. The two options for obtaining adrenal tissue are: (1) whole organ removal post-mortem or during radical nephrectomy; (2) removal during PHEO surgery. Access to high quality normal adrenal tissue is limited. Removal of whole adrenals during nephrectomy is rare, because of improved surgical techniques. For adrenals removed post-mortem, the lag time to proper organ perfusion causes uncontrolled tissue degradation. Adjacent normal adrenal tissue can almost never be obtained from resected PHEOs, because they often replace the entire medulla or are well-encapsulated. If a margin of normal adrenal is attached to a resected PHEO, it seldom contains any medulla. The clean separation of medulla and cortex is further complicated, because their border is convoluted, and because adult adrenal consists of approximately 90% cortex. Thus, the quality of separation has to be evaluated with specific medullary and cortical markers. We describe the successful dissection of highly pure, medullary tissue from adrenals snap-frozen upon resection during radical nephrectomy or after brain death. Separation quality has been verified by quantitative reverse transcription with polymerase chain reaction for the medullary enzymes, tyrosine hydroxylase, and chromogranin A, and for the cortical enzyme, steroidogenic acute regulator.

Integrative genomic analyses of neurofibromatosis tumours identify SOX9 as a biomarker and survival gene

Understanding the biological pathways critical for common neurofibromatosis type 1 (NF1) peripheral nerve tumours is essential, as there is a lack of tumour biomarkers, prognostic factors and therapeutics. We used gene expression profiling to define transcriptional changes between primary normal Schwann cells (n = 10), NF1-derived primary benign neurofibroma Schwann cells (NFSCs) (n = 22), malignant peripheral nerve sheath tumour (MPNST) cell lines (n = 13), benign neurofibromas (NF) (n = 26) and MPNST (n = 6). Dermal and plexiform NFs were indistinguishable. A prominent theme in the analysis was aberrant differentiation. NFs repressed gene programs normally active in Schwann cell precursors and immature Schwann cells. MPNST signatures strongly differed; genes up-regulated in sarcomas were significantly enriched for genes activated in neural crest cells. We validated the differential expression of 82 genes including the neural crest transcription factor SOX9 and SOX9 predicted targets. SOX9 immunoreactivity was robust in NF and MPSNT tissue sections and targeting SOX9 – strongly expressed in NF1-related tumours – caused MPNST cell death. SOX9 is a biomarker of NF and MPNST, and possibly a therapeutic target in NF1.

Isolation of neural crest derived chromaffin progenitors from adult adrenal medulla

Chromaffin cells of the adrenal medulla are neural crest-derived cells of the sympathoadrenal lineage. Unlike the closely-related sympathetic neurons, a subpopulation of proliferation-competent cells exists even in the adult. Here, we describe the isolation, expansion, and in vitro characterization of proliferation-competent progenitor cells from the bovine adrenal medulla. Similar to neurospheres, these cells, when prevented from adherence to the culture dish, grew in spheres, which we named chromospheres. These chromospheres were devoid of mRNA specific for smooth muscle cells (MYH11) or endothelial cells (PECAM1). During sphere formation, markers for differentiated chromaffin cells, such as phenylethanolamine-N-methyl transferase, were downregulated while neural progenitor markers nestin, vimentin, musashi 1, and nerve growth factor receptor, as well as markers of neural crest progenitor cells such as Sox1 and Sox9, were upregulated. Clonal analysis and bromo-2'-deoxyuridine-incorporation analysis demonstrated the self-renewing capacity of chromosphere cells. Differentiation protocols using NGF and BMP4 or dexamethasone induced neuronal or endocrine differentiation, respectively. Electrophysiological analyses of neural cells derived from chromospheres revealed functional properties of mature nerve cells, such as tetrodotoxin-sensitive sodium channels and action potentials. Our study provides evidence that proliferation and differentiation competent chromaffin progenitor cells can be isolated from adult adrenal medulla and that these cells might harbor the potential for the treatment of neurodegenerative diseases, such as Parkinson's disease.

The Yin and Yang of Sox proteins: Activation and repression in development and disease

The general view of development consists of the acquisition of committed/differentiated phenotypes following a period of self-renewal and progenitor expansion. Lineage specification and progression are phenomena of antagonistic events, silencing tissue-specific gene expression in precursors to allow self-renewal and multipotentiality, and subsequently suppressing proliferation and embryonic gene expression to promote the restricted expression of tissue-specific genes during maturation. The high mobility group-containing Sox family of transcription factors constitutes one of the earliest classes of genes to be expressed during embryonic development. These proteins not only are indispensable for progenitor cell specification but also are critical for terminal differentiation of multiple cell types in a wide variety of lineages. Sox transcription factors are now known to induce or repress progenitor cell characteristics and cell proliferation or to activate the expression of tissue-specific genes. Sox proteins fulfill their diverse functions in developmental regulation by distinct molecular mechanisms. Not surprisingly, in addition to DNA binding and bending, Sox transcription factors also interact with different protein partners to function as coactivators or corepressors of downstream target genes. Here we seek to provide an overview of the current knowledge of Sox gene functional mechanisms, in an effort to understand their roles in both development and pathology.

The tumor stem cell concept-implications for endocrine tumors?

The cancer stem cell hypothesis has recently evolved from an increasing body of evidence suggesting that in some cancers a small population of tumor cells with stem cell-like properties represents a critical component that dictates the malignant behavior of a given tumor. These observations challenge classical cancer biology and its theory, that tumor growth is mainly based on genomic alterations followed by modulation of cell cycle pathways, which finally result in uncontrolled clonal proliferation. Over the last few years, much progress in the field of tumor stem cells has been achieved in non-endocrine malignancies. In this review, we summarize the existing evidence regarding the tumor stem cell concept for tumor pathophysiology in general and highlight current models that have the potential to further impact research on endocrine tumors.

Review: the role of neural crest cells in the endocrine system

The neural crest is a pluripotent population of cells that arises at the junction of the neural tube and the dorsal ectoderm. These highly migratory cells form diverse derivatives including neurons and glia of the sensory, sympathetic, and enteric nervous systems, melanocytes, and the bones, cartilage, and connective tissues of the face. The neural crest has long been associated with the endocrine system, although not always correctly. According to current understanding, neural crest cells give rise to the chromaffin cells of the adrenal medulla, chief cells of the extra-adrenal paraganglia, and thyroid C cells. The endocrine tumors that correspond to these cell types are pheochromocytomas, extra-adrenal paragangliomas, and medullary thyroid carcinomas. Although controversies concerning embryological origin appear to have mostly been resolved, questions persist concerning the pathobiology of each tumor type and its basis in neural crest embryology. Here we present a brief history of the work on neural crest development, both in general and in application to the endocrine system. In particular, we present findings related to the plasticity and pluripotency of neural crest cells as well as a discussion of several different neural crest tumors in the endocrine system.

Genetic markers for the diagnosis and prognosis of pheochromocytoma

The last 5 years have witnessed important advances in understanding the mechanisms of tumorigenesis of chromaffin cells. Large-scale microarray analyses of pheochromocytomas have identified two distinct gene-expression profiles encompassing all hereditary and sporadic tumors. Gene-expression profiling of benign and malignant pheochromocytomas is providing a better understanding of the mechanisms of metastasis. Such studies hold promise for the development of new prognostic markers for early detection of malignant pheochromocytoma and for the identification of novel targets for therapeutic intervention.