Mitochondrial complex II and genomic imprinting in inheritance of paraganglioma tumors

Back to Biochemistry: Evaluation for and Prognostic Significance of SDH Mutations in Paragangliomas and Pheochromocytomas

There is increasing recognition of the high prevalence of hereditary predisposition syndromes in patients diagnosed with paraganglioma/pheochromocytoma. It is widely acknowledged that germline pathogenic alterations of the succinate dehydrogenase complex genes (SDHA, SDHB, SDHC, SDHD, SDHAF2) contribute to the pathogenesis of most of these tumors. Herein, we have provided an update on the biology and diagnosis of succinate dehydrogenase-deficient paraganglioma/pheochromocytoma, including the molecular biology of the succinate dehydrogenase complex, mechanisms and consequences of inactivation of this complex, the prevalence of pathogenic alterations, and patterns of inheritance.

Paraganglioma of the Head and Neck: A Review

OBJECTIVE

To review the epidemiology, presentation, diagnosis, and management of head and neck paragangliomas.

METHODS

A literature review of english language papers with focus on most current literature.

RESULTS

Paragangliomas (PGLs) are a group of neuroendocrine tumors that arise in the parasympathetic or sympathetic ganglia. Head and neck PGLs (HNPGLs) comprise 65% to 70% of all PGLs and account for 0.6% of all head and neck cancers. The majority of HNPGLs are benign, and 6% to 19% of all HNPGLs develop metastasis outside the tumor site and significantly compromise survival. PGLs can have a familial etiology with germline sequence variations in different susceptibility genes, with the gene encoding succinate dehydrogenase being the most common sequence variation, or they can arise from somatic sequence variations or fusion genes. Workup includes biochemical testing to rule out secretory components, although it is rare in HNPGLs. In addition, imaging modalities, such as computed tomography and magnetic resonance imaging, help in monitoring in surgical planning. Functional imaging with DOTATATE-positron emission tomography, 18F-fluorodeoxyglucose, or 18F-fluorohydroxyphenylalanine may be necessary to rule out sites of metastases. The management of HNPGLs is complex depending on pathology, location, and aggressiveness of the tumor. Treatment ranges from observation to resection to systemic treatment. Similarly, the prognosis ranges from a normal life expectancy to a 5-year survival of 11.8% in patients with distant metastasis.

CONCLUSION

Our review is a comprehensive summary of the incidence, mortality, pathogenesis, presentation, workup and management of HNPGLs.

The SDHD:p.H102R Variant Is Frequent in Russian Patients with Head and Neck Paragangliomas and Associated with Loss of 11p15.5 Region and Hypermethylation of H19-DMR

Head and neck paragangliomas (HNPGLs) are rare neuroendocrine neoplasms derived from the parasympathetic paraganglia of the head and neck. At least 30% of HNPGLs are linked to germline mutations, predominantly in SDHx genes. In this study, we analyzed an extended cohort of Russian patients with HNPGLs using whole-exome sequencing and found a highly frequent missense variant p.H102R in the SDHD gene. We determined this variant in 34% of the SDHD mutation carriers. This variant was associated with somatic loss of the gene wild-type allele. Data from the B allele frequency method and microsatellite and microdeletion analysis indicated evident LOH at the 11p15.5 region and potential loss of the whole of chromosome 11. We found hypermethylation of H19-DMR in all tumors, whereas differential methylation of KvDMR was mostly retained. These findings do not support the paternal transmission of SDHD:p.H102R but are in agreement with the Hensen model. Using targeted sequencing, we also studied the variant frequency in a control cohort; we found SDHD:p.H102R in 1.9% of cases, allowing us to classify this variant as pathogenic. The immunohistochemistry of SDHB showed that the SDHD:p.H102R mutation, even in combination with wild-type allele loss, does not always lead to SDH deficiency. The obtained results demonstrate the frequent variant associated with HNPGLs in a Russian population and support its pathogenicity. Our findings help with understanding the mechanism of tumorigenesis and are also important for the development of cost-effective genetic screening programs.

The Pathologic and Molecular Genetic Landscape of the Hereditary Renal Cancer Predisposition Syndromes

It is estimated that 5-8% of renal tumors are hereditary in nature with many inherited as autosomal dominant. These tumors carry a unique spectrum of pathologic and molecular alterations, the knowledge of which is expanding in the recent years. Indebted to this knowledge, many advances in treatment of these tumors have been achieved. In this review, we summarize the current understanding of the genetic renal neoplasia syndromes, the clinical and pathologic presentations, their molecular pathogenesis, the advances in therapeutic implications and targeted therapy.

Management of Multiple Head and Neck Paragangliomas With Assistance of a 3-D Model

INTRODUCTION

Extirpation of multiple head and neck paragangliomas carries challenge due to close anatomic relationships with critical neurovascular bundles.

OBJECTIVES

This study aims to assess whether the application of 3-D models can assist with surgical planning and treatment of these paragangliomas, decrease surgically related morbidity and mortality.

METHODS

Fourteen patients undergoing surgical resection of multiple head and neck paragangliomas were enrolled in this study. A preoperative 3-D model was created based on radiologic data, and relevant critical anatomic relationships were preoperatively assessed and intraoperatively validated.

RESULTS

All 14 patients presented with multiple head and neck paragangliomas, including bilateral carotid body tumors (CBT, n = 9), concurrent CBT with glomus jugulare tumors (GJT, n = 4), and multiple vagal paragangliomas (n = 1). Ten patients underwent genomic analysis and all harbored succinate dehydrogenase complex subunit D (SDHD) mutations. Under guidance of the 3-D model, the internal carotid artery (ICA) was circumferentially encased by tumor on 5 of the operated sides, in 4 (80%) of which the tumor was successfully dissected out from the ICA, whereas ICA reconstruction was required on one side (20%). Following removal of CBT, anterior rerouting of the facial nerve was avoided in 3 (75%) of 4 patients during the extirpation of GJT with assistance of a 3-D model. Two patients developed permanent postoperative vocal cord paralysis. There was no vessel rupture or mortality in this study cohort.

CONCLUSION

The 3-D model is beneficial for establishment of a preoperative strategy, as well as planning and guiding the intraoperative procedure for resection of multiple head and neck paragangliomas.

Human Mitochondrial Pathologies of the Respiratory Chain and ATP Synthase: Contributions from Studies of Saccharomyces cerevisiae

The ease with which the unicellular yeast Saccharomyces cerevisiae can be manipulated genetically and biochemically has established this organism as a good model for the study of human mitochondrial diseases. The combined use of biochemical and molecular genetic tools has been instrumental in elucidating the functions of numerous yeast nuclear gene products with human homologs that affect a large number of metabolic and biological processes, including those housed in mitochondria. These include structural and catalytic subunits of enzymes and protein factors that impinge on the biogenesis of the respiratory chain. This article will review what is currently known about the genetics and clinical phenotypes of mitochondrial diseases of the respiratory chain and ATP synthase, with special emphasis on the contribution of information gained from pet mutants with mutations in nuclear genes that impair mitochondrial respiration. Our intent is to provide the yeast mitochondrial specialist with basic knowledge of human mitochondrial pathologies and the human specialist with information on how genes that directly and indirectly affect respiration were identified and characterized in yeast.

Mutation profiling in eight cases of vagal paragangliomas

Background

Vagal paragangliomas (VPGLs) belong to a group of rare head and neck neuroendocrine tumors. VPGLs arise from the vagus nerve and are less common than carotid paragangliomas. Both diagnostics and therapy of the tumors raise significant challenges. Besides, the genetic and molecular mechanisms behind VPGL pathogenesis are poorly understood.

Methods

The collection of VPGLs obtained from 8 patients of Russian population was used in the study. Exome library preparation and high-throughput sequencing of VPGLs were performed using an Illumina technology.

Results

Based on exome analysis, we identified pathogenic/likely pathogenic variants of the SDHx genes, frequently mutated in paragangliomas/pheochromocytomas. SDHB variants were found in three patients, whereas SDHD was mutated in two cases. Moreover, likely pathogenic missense variants were also detected in SDHAF3 and SDHAF4 genes encoding for assembly factors for the succinate dehydrogenase (SDH) complex. In a patient, we found a novel variant of the IDH2 gene that was predicted as pathogenic by a series of algorithms used (such as SIFT, PolyPhen2, FATHMM, MutationTaster, and LRT). Additionally, pathogenic/likely pathogenic variants were determined for several genes, including novel genes and some genes previously reported as associated with different types of tumors.

Conclusions

Results indicate a high heterogeneity among VPGLs, however, it seems that driver events in most cases are associated with mutations in the SDHx genes and SDH assembly factor-coding genes that lead to disruptions in the SDH complex.

Genetics, diagnosis, management and future directions of research of phaeochromocytoma and paraganglioma: a position statement and consensus of the Working Group on Endocrine Hypertension of the European Society of Hypertension

: Phaeochromocytoma and paraganglioma (PPGL) are chromaffin cell tumours that require timely diagnosis because of their potentially serious cardiovascular and sometimes life- threatening sequelae. Tremendous progress in biochemical testing, imaging, genetics and pathophysiological understanding of the tumours has far-reaching implications for physicians dealing with hypertension and more importantly affected patients. Because hypertension is a classical clinical clue for PPGL, physicians involved in hypertension care are those who are often the first to consider this diagnosis. However, there have been profound changes in how PPGLs are discovered; this is often now based on incidental findings of adrenal or other masses during imaging and increasingly during surveillance based on rapidly emerging new hereditary causes of PPGL. We therefore address the relevant genetic causes of PPGLs and outline how genetic testing can be incorporated within clinical care. In addition to conventional imaging (computed tomography, MRI), new functional imaging approaches are evaluated. The novel knowledge of genotype-phenotype relationships, linking distinct genetic causes of disease to clinical behaviour and biochemical phenotype, provides the rationale for patient-tailored strategies for diagnosis, follow-up and surveillance. Most appropriate preoperative evaluation and preparation of patients are reviewed, as is minimally invasive surgery. Finally, we discuss risk factors for developing metastatic disease and how they may facilitate personalised follow-up. Experts from the European Society of Hypertension have prepared this position document that summarizes the current knowledge in epidemiology, genetics, diagnosis, treatment and surveillance of PPGL.

Surveillance of succinate dehydrogenase gene mutation carriers: Insights from a nationwide cohort

OBJECTIVE

Mutations in the genes coding for succinate dehydrogenase (SDHx) are the most frequent germline alterations in pheochromocytomas and paragangliomas. Evidence for the advantages associated with presymptomatic screening for SDHx mutation carriers is scarce. This study describes a nationwide cohort of these mutation carriers and aims to compare patients with clinical manifestations of the disease and those diagnosed through genetic screening.

DESIGN

Cross-sectional study.

PATIENTS

SDHx mutation carriers (n = 118) followed through the Portuguese Oncology referral centres: 41 probands and 77 nonprobands.

MEASUREMENTS

All participants were subjected to biochemical and body imaging examinations for a complete assessment of the extent and spread of disease. Clinical data obtained this way were further analysed.

RESULTS

The mean age of this cohort was 44.5 ± 17.4 years, and more than half carried the same founder SDHB mutation. About 50.8% of the mutation carriers developed pheochromocytomas or paragangliomas. Compared to patients diagnosed through genetic screening, those diagnosed clinically were characterized by larger tumours (P < .001), more frequent metastases (P = .024), were more frequently subjected to surgery (P = .011) and radiotherapy (P = .013), and had worse outcomes, such as macroscopic positive margins (P = .034). Persistent and/or unresectable disease and disease-related mortality were also more frequent in symptomatic patients compared to those diagnosed through genetic screening (P = .014).

CONCLUSIONS

In this nationwide cohort study, a large proportion of mutation carriers were found to develop SDHx-related neoplasia. Genetic testing and subsequent follow-up resulted in the diagnosis of smaller and nonmetastatic tumours, fewer treatment procedures, fewer complications and greater number of disease-free patients.

Metabolic Drivers in Hereditary Cancer Syndromes

Cancer is a multifaceted disease in which inherited genetic variants can be important drivers of tumorigenesis. The discovery that germline mutations of metabolic genes predispose to familial forms of cancer caused a shift in our understanding of how metabolism contributes to tumorigenesis, providing evidence that metabolic alterations can be oncogenic. In this review, we focus on mitochondrial enzymes whose mutations predispose to familial cancer, and we fully appraise their involvement in cancer formation and progression. Elucidating the molecular mechanisms that orchestrate transformation in these diverse tumors may answer key biological questions about tumor formation and evolution, leading to the identification of new therapeutic targets of intervention.

Genetic, epigenetic and biochemical regulation of succinate dehydrogenase function

Succinate dehydrogenase (SDH), complex II or succinate:quinone oxidoreductase (SQR) is a crucial enzyme involved in both the tricarboxylic acid (TCA) cycle and oxidative phosphorylation (OXPHOS), the two primary metabolic pathways for generating ATP. Impaired function of SDH results in deleterious disorders from cancer to neurodegeneration. SDH function is tailored to meet the energy demands in different cell types. Thus, understanding how SDH function is regulated and how it operates in distinct cell types can support the development of therapeutic approaches against the diseases. In this article we discuss the molecular pathways which regulate SDH function and describe extra roles played by SDH in specific cell types.

Oncometabolites in cancer aggressiveness and tumour repopulation

Tumour repopulation is recognized as a crucial event in tumour relapse where therapy-sensitive dying cancer cells influence the tumour microenvironment to sustain therapy-resistant cancer cell growth. Recent studies highlight the role of the oncometabolites succinate, fumarate, and 2-hydroxyglutarate in the aggressiveness of cancer cells and in the worsening of the patient's clinical outcome. These oncometabolites can be produced and secreted by cancer and/or surrounding cells, modifying the tumour microenvironment and sustaining an invasive neoplastic phenotype. In this review, we report recent findings concerning the role in cancer development of succinate, fumarate, and 2-hydroxyglutarate and the regulation of their related enzymes succinate dehydrogenase, fumarate hydratase, and isocitrate dehydrogenase. We propose that oncometabolites are crucially involved in tumour repopulation. The study of the mechanisms underlying the relationship between oncometabolites and tumour repopulation is fundamental for identifying efficient anti-cancer therapeutic strategies and novel serum biomarkers in order to overcome cancer relapse.

The Role of Immunohistochemistry and Molecular Analysis of Succinate Dehydrogenase in the Diagnosis of Endocrine and Non-Endocrine Tumors and Related Syndromes

Succinate dehydrogenase (SDH) is an enzyme complex, composed of four protein subunits, that plays a role in both the citric acid cycle and the electron transport chain. The genes for SDHA, SDHB, SDHC, and SDHD are located in the nuclear DNA, and mutations in these genes have initially been described in paragangliomas (PGL) and pheochromocytomas (PCC), which are relatively rare tumors derived from the autonomic nervous system and the adrenal medulla, respectively. Patients with SDH mutations, that are almost exclusively in the germline, are frequently affected by multiple PGL and/or PCC. In addition, other tumors have been associated with SDH mutations as well, including gastrointestinal stromal tumors, SDH-deficient renal cell carcinoma, and pituitary adenomas. Immunohistochemistry for SDHB and SDHA has been shown to be a valuable additional tool in the histopathological analysis of these tumors, and can be considered as a surrogate marker for molecular analysis. In addition, SDHB immunohistochemistry is relevant in the decision-making whether a genetic sequence variant represents a pathogenic mutation or not. In this review, we highlight the current knowledge of the physiologic and pathologic role of the SDH enzyme complex and its involvement in endocrine and non-endocrine tumors, with an emphasis on the applicability of immunohistochemistry.

Familial paraganglioma due to a novel SDHB mutation: familial phenotypic heterogeneity and a potentially novel manifestation

Non-paraganglioma (PGL) tumors are rare manifestations of familial PGL syndromes. Primary hyperparathyroidism has not been described in PGL syndromes. We present a 36-year-old man with a history of right carotid body tumor at 24 years and an abdominal PGL at 31 years of age. At 35 years, he developed hypercalcemia (serum Ca 2.65–2.72 mmol/l), and high parathyroid hormone of 92–131 ng/l (normal range, 15–65) and a Tc99 Sestamibi scan showed a single parathyroid adenoma which was confirmed on histopathological examination of parathyroidectomy. Recently, he was diagnosed with a left glomus jugulare which has not been operated on yet. His family history is strongly positive for PGLs. Genetic testing revealed a novel SDHB mutation (p.K137E) but the phenotype and penetrance were variable in different family members.

A Novel Imprinted Gene NUWA Controls Mitochondrial Function in Early Seed Development in Arabidopsis

Imprinted genes display biased expression of paternal and maternal alleles and are only found in mammals and flowering plants. Compared to several hundred imprinted genes that are functionally characterized in mammals, very few imprinted genes were confirmed in plants and even fewer of them have been functionally investigated. Here, we report a new imprinted gene, NUWA, in plants. NUWA is an essential gene, because loss of its function resulted in reduced transmission through the female gametophyte and defective cell/nuclear proliferation in early Arabidopsis embryo and endosperm. NUWA is a maternally expressed imprinted gene, as only the maternal allele of NUWA is transcribed and translated from gametogenesis to the 16-cell globular embryo stage after fertilization, and the de novo transcription of the maternal allele of NUWA starts from the zygote stage. Different from other identified plant imprinted genes whose encoded proteins are mostly localized to the nucleus, the NUWA protein was localized to the mitochondria and was essential for mitochondria function. Our work uncovers a novel imprinted gene of a previously unidentified type, namely, a maternal-specific expressed nuclear gene with its encoded protein localizing to and controlling the function of the maternally inherited mitochondria. This reveals a unique mechanism of maternal control of the mitochondria and adds an extra layer of complexity to the regulation of nucleus-organelle coordination during early plant development.

Endocrine tumors associated with the vagus nerve

The vagus nerve (cranial nerve X) is the main nerve of the parasympathetic division of the autonomic nervous system. Vagal paragangliomas (VPGLs) are a prime example of an endocrine tumor associated with the vagus nerve. This rare, neural crest tumor constitutes the second most common site of hereditary head and neck paragangliomas (HNPGLs), most often in relation to mutations in the succinate dehydrogenase complex subunit D (SDHD) gene. The treatment paradigm for VPGL has progressively shifted from surgery to abstention or therapeutic radiation with curative-like outcomes. Parathyroid tissue and parathyroid adenoma can also be found in close association with the vagus nerve in intra or paravagal situations. Vagal parathyroid adenoma can be identified with preoperative imaging or suspected intraoperatively by experienced surgeons. Vagal parathyroid adenomas located in the neck or superior mediastinum can be removed via initial cervicotomy, while those located in the aortopulmonary window require a thoracic approach. This review particularly emphasizes the embryology, molecular genetics, and modern imaging of these tumors.

A family with pheochromocytoma-paraganglioma inherited tumour syndrome. Serial 18F-DOPA PET/CT investigations

AIM

Hereditary pheochromocytoma-paraganglioma syndromes are characterized by multiple pheochromocytomas (PCC) and paragangliomas (PGLs), inherited in an autosomal dominant manner. Early detection and removal of tumours may prevent or minimize complications related to mass effects and malignant transformation. Having confirmed the diagnosis, it is important to localize the tumours and reveal their extent preoperatively. This study aimed to introduce 18F-DOPA PET/CT as a highly sensitive non-invasive diagnostic tool for early detection of mass lesions in patients with pheochromocytoma-paraganglioma inherited tumour syndrome and to report about its impact on patient management.

PATIENTS, METHODS

We are currently supervising one of the largest documented families in Germany with genetically determined SDHD gene mutation. We performed 18F-DOPA PET/CT in order to detect tumours in asymptomatic gene carriers and enable subsequent surgical therapy.

RESULTS

In seven patients undergoing 12 18F-DOPA PET/CT scans 17 lesions have been detected. Three of these lesions, located in the head and neck region, have had no morphologic correlate in CT and one had also no morphologic correlate in MRI. Of the six histologically analyzed lesions five have been tumors (PGL or PCC) and one has been a nodular hyperplasia. This means the 18F-DOPA PET/CT scan in our study group had a sensitivity of 83%. 18F-DOPA PET/CT investigations lead to change in the management in 5/7 studied patients (70%).

CONCLUSION

The benefits of PET/CT in detection of pheochromocytoma and paraganglioma are well documented, but we are the first to use this technique for screening of a rare hereditary disease (estimated prevalence 0.3/100 000).

Additive effect of nuclear and mitochondrial mutations in a patient with mitochondrial encephalomyopathy

We describe the case of a woman in whom combination of a mitochondrial (MT-CYB) and a nuclear (SDHB) mutation was associated with clinical and metabolic features suggestive of a mitochondrial disorder. The mutations impaired overall energy metabolism in the patient's muscle and fibroblasts and increased cellular susceptibility to oxidative stress. To clarify the contribution of each mutation to the phenotype, mutant yeast strains were generated. A significant defect in strains carrying the Sdh2 mutation, either alone or in combination with the cytb variant, was observed. Our data suggest that the SDHB mutation was causative of the mitochondrial disorder in our patient with a possible cumulative contribution of the MT-CYB variant. To our knowledge, this is the first association of bi-genomic variants in the mtDNA and in a nuclear gene encoding a subunit of complex II.

Paraganglioma and phaeochromocytoma: from genetics to personalized medicine

Paragangliomas and phaeochromocytomas are neuroendocrine tumours whose pathogenesis and progression are very strongly influenced by genetics. A germline mutation in one of the susceptibility genes identified so far explains ∼40% of all cases; the remaining 60% are thought to be sporadic cases. At least one-third of these sporadic tumours contain a somatic mutation in a predisposing gene. Genetic testing, which is indicated in every patient, is guided by the clinical presentation as well as by the secretory phenotype and the immunohistochemical characterization of the tumours. The diagnosis of an inherited form drives clinical management and tumour surveillance. Different 'omics' profiling methods have provided a neat classification of these tumours in accordance with their genetic background. Transcriptomic studies have identified two main molecular pathways that underlie development of these tumours, one in which the hypoxic pathway is activated (cluster 1) and another in which the MAPK and mTOR (mammalian target of rapamycin) signalling pathways are activated (cluster 2). DNA methylation profiling has uncovered a hypermethylator phenotype in tumours related to SDHx genes (a group of genes comprising SDHA, SDHB, SDHC, SDHD and SDHAF2) and revealed that succinate acts as an oncometabolite, inhibiting 2-oxoglutarate-dependent dioxygenases, such as hypoxia-inducible factor prolyl-hydroxylases and histone and DNA demethylases. 'Omics' data have suggested new therapeutic targets for patients with a malignant tumour. In the near future, new 'omics'-based tests are likely to be transferred into clinical practice with the goal of establishing personalized medical management for affected patients.

The genetics of endocrine neoplasia

The hereditary endocrine neoplasias are characterized by the development of benign or malignant tumors in more than one endocrine tissue. These tumors typically occur at a younger age than sporadic endocrine tumors and follow an autosomal dominant pattern of inheritance. Because of the age-related penetrance and extreme phenotypic variability both within and between families, clinicians cannot always rely on the family history to make the diagnosis. Recognition of the features of a hereditary endocrine syndrome can allow for appropriate and timely risk assessment, genetic counseling and genetic testing, and identification of at-risk family members who may benefit from early and regular screening.

Current approaches and recent developments in the management of head and neck paragangliomas

Head and neck paragangliomas (HNPGLs) are rare neuroendocrine tumors belonging to the family of pheochromocytoma/paraganglioma neoplasms. Despite advances in understanding the pathogenesis of these tumors, the growth potential and clinical outcome of individual cases remains largely unpredictable. Over several decades, surgical resection has long been the treatment of choice for HNPGLs. However, increasing experience in various forms of radiosurgery has been reported to result in curative-like outcomes, even for tumors localized in the most inaccessible anatomical areas. The emergence of such new therapies challenges the traditional paradigm for the management of HNPGLs. This review will assist and guide physicians who encounter patients with such tumors, either from a diagnostic or therapeutic standpoint. This review will also particularly emphasize current and emerging knowledge in genetics, imaging, and therapeutic options as well as the health-related quality of life for patients with HNPGLs.

Genetically modeled mice with mutations in mitochondrial metabolic enzymes for the study of cancer

Mitochondrial dysfunction has long been implicated in progression of cancer. As a paradigm, the “Warburg effect,” which by means of a switch toward anaerobic metabolism enables cancer cells to proliferate in oxygen limiting conditions, is well established. Besides this metabolic transformation of tumors, it has been discovered that mutations in genes encoding mitochondrial proteins are the etiological factors in different types of cancer. This confers to mitochondrial dysfunction a causative role, rather than resultant, in tumor genesis beyond its role in tumor progression and development. Mitochondrial proteins encoded by tumor-suppressor genes are part of the succinate-dehydrogenase, the fumarate-hydratase, and the mitochondrial isocitrate-dehydrogenase enzymes, all of them participating in the Krebs cycle. The spectrum of tumors associated with mutations in these genes is becoming larger and varies between each enzyme. Several mechanisms of tumorigenesis have been proposed for the different enzymatic defects, most of them based on studies using cellular and animal models. Regarding the molecular pathways implicated in the oncogenic transformation, one of the first accepted theories was based on the constitutive expression of the hypoxia-inducible factor 1α (Hif1α) at normal oxygen tension, a theory referred to as “pseudo-hypoxic drive.” This mechanism has been linked to the three types of mutations, thus suggesting a central role in cancer. However, other alternative molecular processes, such as oxidative stress or altered chromatin remodeling, have been also proposed to play an onco-pathogenic role. In the recent years, the role of oncometabolites, a new concept emerged from biochemical studies upon these tumors, has acquired relevance as responsible for tumor formation. Nevertheless, the actual contribution of each of these mechanisms has not been definitively established. In this review, we summarize the results obtained from mouse strains genetically modified in the three different enzymes.

Gene network-based analysis identifies two potential subtypes of small intestinal neuroendocrine tumors

BACKGROUND

Tumor transcriptomes contain information of critical value to understanding the different capacities of a cell at both a physiological and pathological level. In terms of clinical relevance, they provide information regarding the cellular "toolbox" e.g., pathways associated with malignancy and metastasis or drug dependency. Exploration of this resource can therefore be leveraged as a translational tool to better manage and assess neoplastic behavior. The availability of public genome-wide expression datasets, provide an opportunity to reassess neuroendocrine tumors at a more fundamental level. We hypothesized that stringent analysis of expression profiles as well as regulatory networks of the neoplastic cell would provide novel information that facilitates further delineation of the genomic basis of small intestinal neuroendocrine tumors.

RESULTS

We re-analyzed two publically available small intestinal tumor transcriptomes using stringent quality control parameters and network-based approaches and validated expression of core secretory regulatory elements e.g., CPE, PCSK1, secretogranins, including genes involved in depolarization e.g., SCN3A, as well as transcription factors associated with neurodevelopment (NKX2-2, NeuroD1, INSM1) and glucose homeostasis (APLP1). The candidate metastasis-associated transcription factor, ST18, was highly expressed (>14-fold, p < 0.004). Genes previously associated with neoplasia, CEBPA and SDHD, were decreased in expression (-1.5 - -2, p < 0.02). Genomic interrogation indicated that intestinal tumors may consist of two different subtypes, serotonin-producing neoplasms and serotonin/substance P/tachykinin lesions. QPCR validation in an independent dataset (n = 13 neuroendocrine tumors), confirmed up-regulated expression of 87% of genes (13/15).

CONCLUSIONS

An integrated cellular transcriptomic analysis of small intestinal neuroendocrine tumors identified that they are regulated at a developmental level, have key activation of hypoxic pathways (a known regulator of malignant stem cell phenotypes) as well as activation of genes involved in apoptosis and proliferation. Further refinement of these analyses by RNAseq studies of large-scale databases will enable definition of individual master regulators and facilitate the development of novel tissue and blood-based tools to better understand diagnose and treat tumors.

Genetic testing in the clinical care of patients with pheochromocytoma and paraganglioma

PURPOSE OF REVIEW

Paraganglioma and pheochromocytoma (PGL/PCC) are tumours of neural crest origin that can present along a clinical spectrum ranging from apparently sporadic, isolated tumours to a more complex phenotype of one or multiple tumours in the context of other clinical features and family history suggestive of a defined hereditary syndrome. Genetic testing for hereditary PGL/PCC can help to confirm a genetic diagnosis for sporadic and syndromic cases. Informative genetic testing serves to clarify future risks for the patient and family members.

RECENT FINDINGS

Genetic discovery in the last decade has identified new PGL/PCC susceptibility loci. We summarize a contemporary approach adopted in our programme for genetic evaluation, testing and prospective management involving biochemical monitoring and imaging for hereditary PGL/PCC. A clinical vignette is presented to illustrate our practice.

SUMMARY

Current estimates that up to 40% of PGL/PCC are associated with germline mutations have implications for genetic testing recommendations. Prospective management of patients with defined hereditary susceptibility is based on established guidelines for well characterized syndromes. Management of tumour risk for rare syndromes, newly defined genetic associations and undefined genetic susceptibility in the setting of significant family history presents a challenge. Sustained discovery of new PGL/PCC genes underscores the need for a practice of continued genetic evaluation for patients with uninformative results. All patients with PGL/PCC should undergo genetic testing to identify potential hereditary tumour susceptibility.

Pheochromocytoma and paraganglioma syndromes: genetics and management update

Pheochromocytomas (pheos) and paragangliomas (pgls) are rare tumours of the autonomic nervous system, originating from paraganglia, which are dispersed neuroendocrine organs characterized by catecholamine and peptide-producing cells derived from the neural crest. Medical textbooks have traditionally suggested that 10% of pheos are heritable. However, the frequency of heritable pheo has been underestimated. Three syndromic conditions-Von Hippel-Lindau (vhl), multiple endocrine neoplasia type 2 (men2), and neurofibromatosis type 1 (nf1)-and three genes-subunits of the succinate dehydrogenase (SDH) complex: SDHB, SDHC, and SDHD-are established causes of hereditary pheo-pgl. In the last few years, four new genes (SDHA, SDHAF2, MAX, and TMEM127) have been found to be associated with predisposition to these tumours. The present review, illustrated by three case reports, gives an update of the genetic basis of pheo-pgl and of the parent-of-origin effect implicated in the transmission of SDHD and SDHAF2. We discuss the referral criteria that should guide the decision to offer genetic testing to affected patients. We also specify the genes that are most likely implicated-based on particular features such as malignancy, bilateralism, or childhood-onset-to help geneticists efficiently order appropriate genetic tests. Finally, we review the screening recommendations for carriers of a pheo-pgl predisposition mutation.

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.

A conditional mouse mutant in the tumor suppressor SdhD gene unveils a link between p21(WAF1/Cip1) induction and mitochondrial dysfunction

Mutations in mitochondrial complex II (MCII; succinate dehydrogenase, Sdh) genes cause familiar pheochromocytoma/paraganglioma tumors. Several mechanisms have been proposed to account for Sdh-mutation-induced tumorigenesis, the most accepted of which is based on the constitutive expression of the hypoxia-inducible factor 1α (Hif1α) at normal oxygen tension, a theory referred to as “pseudo-hypoxic drive”. Other molecular processes, such as oxidative stress, apoptosis, or chromatin remodeling have been also proposed to play a causative role. Nevertheless, the actual contribution of each of these mechanisms has not been definitively established. Moreover, the biological factors that determine the tissue-specificity of these tumors have not been identified. In this work, we made use of the inducible SDHD-ESR mouse, a conditional mutant in the SdhD gene, which encodes the small subunit of MCII, and that acts as a tumor suppressor gene in humans. The analysis of the Hif1α pathway in SDHD-ESR tissues and in two newly derived cell lines after complete SdhD loss -a requirement for hereditary paraganglioma type-1 tumor formation in humans- partially recapitulated the “pseudo-hypoxic” response and rendered inconsistent results. Therefore, we performed microarray analysis of adrenal medulla and kidney in order to identify other early gene expression changes elicited by SdhD deletion. Our results revealed that each mutant tissue displayed different variations in their gene expression profiles affecting to different biological processes. However, we found that the Cdkn1a gene was up-regulated in both tissues. This gene encodes the cyclin-dependent kinase inhibitor p21^WAF1/Cip1, a factor implicated in cell cycle, senescence, and cancer. The two SDHD-ESR cell lines also showed accumulation of this protein. This new and unprecedented evidence for a link between SdhD dysfunction and p21^WAF1/Cip1 will open new avenues for the study of the mechanisms that cause tumors in Sdh mutants. Finally, we discuss the actual role of Hif1α in tumorigenesis.

Current and future trends in the anatomical and functional imaging of head and neck paragangliomas

Head and neck paragangliomas (HNPGLs) account for approximately 3% of all paragangliomas (PGLs). Most often, HNPGLs are benign, nonsecreting, and slowly progressing. The initial physical examination and biochemical diagnosis usually adds very little to the proper diagnosis of these tumors, and, therefore, radiologists and nuclear medicine physicians play a pivotal role in providing the initial diagnosis, the locoregional staging, and the plan for detecting potential multicentric or metastatic lesions. Based on several current studies, the most accurate use of HNPGL-specific initial and subsequent imaging modalities must be guided by the knowledge of genetics and the specifically measured biochemical profile of these tumors for the proper management of these patients. Thus, this short review article presents the application of the most up-to-date anatomical and functional imaging approaches to HNPGLs tightly linked to the clinical management of these patients. Based on the most recent studies, 18F-FDOPA PET/CT has been shown to be a useful addition to anatomical imaging in the preoperative localization and molecular assessment of HNPGLs. It is estimated that the frequency of metabolically active PGLs on 18F-FDOPA PET/CT in this region is higher than 90%. For patients with hereditary PGL syndromes, (18)F-FDG-PET/CT should be reserved. Imaging of somatostatin receptors using Octreoscan or 68Ga-labeled somatostatin analogues plays an important role for selecting patients for targeted radiation therapy. This review also concludes that it is expected that in the near future, these patients will indeed benefit from new diagnostic approaches based on the identification of new targets by molecular profiling studies that will result in the development of novel PGL-specific radiopharmaceuticals.

Q-site inhibitor induced ROS production of mitochondrial complex II is attenuated by TCA cycle dicarboxylates

The impact of complex II (succinate:ubiquinone oxidoreductase) on the mitochondrial production of reactive oxygen species (ROS) has been underestimated for a long time. However, recent studies with intact mitochondria revealed that complex II can be a significant source of ROS. Using submitochondrial particles from bovine heart mitochondria as a system that allows the precise setting of substrate concentrations we could show that mammalian complex II produces ROS at subsaturating succinate concentrations in the presence of Q-site inhibitors like atpenin A5 or when a further downstream block of the respiratory chain occurred. Upon inhibition of the ubiquinone reductase activity, complex II produced about 75% hydrogen peroxide and 25% superoxide. ROS generation was attenuated by all dicarboxylates that are known to bind competitively to the substrate binding site of complex II, suggesting that the oxygen radicals are mainly generated by the unoccupied flavin site. Importantly, the ROS production induced by the Q-site inhibitor atpenin A5 was largely unaffected by the redox state of the Q pool and the activity of other respiratory chain complexes. Hence, complex II has to be considered as an independent source of mitochondrial ROS in physiology and pathophysiology.