SDH5, a gene required for flavination of succinate dehydrogenase, is mutated in paraganglioma

Succinate dehydrogenase (SDH) deficiency, Carney triad and the epigenome

In this report, we review the relationship between succinate dehydrogenase (SDH) deficiency and the epigenome, especially with regards to two clinical conditions. Carney triad (CT) is a very rare disease with synchronous or metachronous occurrence of at least three different tumor entities; gastric gastrointestinal stromal tumor (GIST), paraganglioma (PGL), and pulmonary chondroma. This condition affects mostly females and it is never inherited. Another disease that shares two of the tumor components of CT, namely GIST and PGL is the Carney-Stratakis syndrome (CSS) or dyad. CSS affects both genders during childhood and adolescence. We review herein the main clinical features and molecular mechanisms behind those two syndromes that share quite a bit of similarities, but one is non-hereditary (CT) whereas the other shows an autosomal-dominant, with incomplete penetrance, inheritance pattern (CSS). Both CT and CSS are caused by the deficiency of the succinate dehydrogenase (SDH) enzyme. The key difference between the two syndromes is the molecular mechanism that causes the SDH deficiency. Most cases of CT show down-regulation of SDH through site-specific hyper-methylation of the SDHC gene, whereas CSS cases carry inactivating germline mutations within one of the genes coding for the SDH subunits A, B, C, or D (SDHA, SDHB, SDHC, and SDHD). There is only partial overlap between the two conditions (there are a few patients with CT that have SDH subunit mutations) but both lead to increased methylation of the entire genome in the tumors associated with them. Other tumors (outside CT and CSS) that have SDH deficiency are associated with increased methylation of the entire genome, but only in CT there is site-specific methylation of the SDHC gene. These findings have implications for diagnostics and the treatment of patients with these, often metastatic tumors.

Alternative assembly of respiratory complex II connects energy stress to metabolic checkpoints

Cell growth and survival depend on a delicate balance between energy production and synthesis of metabolites. Here, we provide evidence that an alternative mitochondrial complex II (CII) assembly, designated as CII_low, serves as a checkpoint for metabolite biosynthesis under bioenergetic stress, with cells suppressing their energy utilization by modulating DNA synthesis and cell cycle progression. Depletion of CII_low leads to an imbalance in energy utilization and metabolite synthesis, as evidenced by recovery of the de novo pyrimidine pathway and unlocking cell cycle arrest from the S-phase. In vitro experiments are further corroborated by analysis of paraganglioma tissues from patients with sporadic, SDHA and SDHB mutations. These findings suggest that CII_low is a core complex inside mitochondria that provides homeostatic control of cellular metabolism depending on the availability of energy.

Mitochondrial complex II is normally composed of four subunits. Here the authors show that bioenergetic stress conditions give rise to a partially assembled variant of complex II, which shifts the anabolic pathways to less energy demanding processes.

Neurocristopathies: New insights 150 years after the neural crest discovery

The neural crest (NC) is a transient, multipotent and migratory cell population that generates an astonishingly diverse array of cell types during vertebrate development. These cells, which originate from the ectoderm in a region lateral to the neural plate in the neural fold, give rise to neurons, glia, melanocytes, chondrocytes, smooth muscle cells, odontoblasts and neuroendocrine cells, among others. Neurocristopathies (NCP) are a class of pathologies occurring in vertebrates, especially in humans that result from the abnormal specification, migration, differentiation or death of neural crest cells during embryonic development. Various pigment, skin, thyroid and hearing disorders, craniofacial and heart abnormalities, malfunctions of the digestive tract and tumors can also be considered as neurocristopathies. In this review we revisit the current classification and propose a new way to classify NCP based on the embryonic origin of the affected tissues, on recent findings regarding the molecular mechanisms that drive NC formation, and on the increased complexity of current molecular embryology techniques.

The unassembled flavoprotein subunits of human and bacterial complex II have impaired catalytic activity and generate only minor amounts of ROS

Complex II (SdhABCD) is a membrane-bound component of mitochondrial and bacterial electron transport chains, as well as of the TCA cycle. In this capacity, it catalyzes the reversible oxidation of succinate. SdhABCD contains the SDHA protein harboring a covalently bound FAD redox center and the iron-sulfur protein SDHB, containing three distinct iron-sulfur centers. When assembly of this complex is compromised, the flavoprotein SDHA may accumulate in the mitochondrial matrix or bacterial cytoplasm. Whether the unassembled SDHA has any catalytic activity, for example in succinate oxidation, fumarate reduction, reactive oxygen species (ROS) generation, or other off-pathway reactions, is not known. Therefore, here we investigated whether unassembled Escherichia coli SdhA flavoprotein, its homolog fumarate reductase (FrdA), and the human SDHA protein have succinate oxidase or fumarate reductase activity and can produce ROS. Using recombinant expression in E. coli, we found that the free flavoproteins from these divergent biological sources have inherently low catalytic activity and generate little ROS. These results suggest that the iron-sulfur protein SDHB in complex II is necessary for robust catalytic activity. Our findings are consistent with those reported for single-subunit flavoprotein homologs that are not associated with iron-sulfur or heme partner proteins.

Pheochromocytoma/Paraganglioma: A Poster Child for Cancer Metabolism

CONTEXT

Pheochromocytomas (PCCs) are tumors that are derived from the chromaffin cells of the adrenal medulla. Extra-adrenal PCCs called paragangliomas (PGLs) are derived from the sympathetic and parasympathetic chain ganglia. PCCs secrete catecholamines, which cause hypertension and have adverse cardiovascular consequences as a result of catecholamine excess. PGLs may or may not produce catecholamines depending on their genetic type and anatomical location. The most worrisome aspect of these tumors is their ability to become aggressive and metastasize; there are no known cures for metastasized PGLs.

METHODS

Original articles and reviews indexed in PubMed were identified by querying with specific PCC/PGL- and Krebs cycle pathway-related terms. Additional references were selected through the in-depth analysis of the relevant publications.

RESULTS

We primarily discuss Krebs cycle mutations that can be instrumental in helping investigators identify key biological pathways and molecules that may serve as biomarkers of or treatment targets for PCC/PGL.

CONCLUSION

The mainstay of treatment of patients with PCC/PGLs is surgical. However, the tide may be turning with the discovery of new genes associated with PCC/PGLs that may shed light on oncometabolites used by these tumors.

Metabolic implications of hypoxia and pseudohypoxia in pheochromocytoma and paraganglioma

Hypoxia is a critical driver of cancer pathogenesis, directly inducing malignant phenotypes such as epithelial-mesenchymal transition, stem cell-like characteristics and metabolic transformation. However, hypoxia-associated phenotypes are often observed in cancer in the absence of hypoxia, a phenotype known as pseudohypoxia, which is very well documented in specific tumour types, including in paraganglioma/pheochromocytoma (PPGL). Approximately 40% of the PPGL tumours carry a germ line mutation in one of a number of susceptibility genes of which those that are found in succinate dehydrogenase (SDH) or in von Hippel-Lindau (VHL) genes manifest a strong pseudohypoxic phenotype. Mutations in SDH are oncogenic, forming tumours in a select subset of tissues, but the cause for this remains elusive. Although elevated succinate levels lead to increase in hypoxia-like signalling, there are other phenotypes that are being increasingly recognised in SDH-mutated PPGL, such as DNA hypermethylation. Further, recently unveiled changes in metabolic re-wiring of SDH-deficient cells might help to decipher cancer related roles of SDH in the future. In this review, we will discuss the various implications that the malfunctioning SDH can have and its impact on cancer development.

Recontacting Patients with Updated Genetic Testing Recommendations for Medullary Thyroid Carcinoma and Pheochromocytoma or Paraganglioma

BACKGROUND

No guidelines exist regarding physicians' duty to inform former patients about novel genetic tests that may be medically beneficial. Research on the feasibility and efficacy of disseminating information and patient opinions on this topic is limited.

METHODS

Adult patients treated at our institution from 1950 to 2010 for medullary thyroid cancer, pheochromocytoma, or paraganglioma were included if their history suggested being at-risk for a hereditary syndrome but genetic risk assessment would be incomplete by current standards. A questionnaire assessing behaviors and attitudes was mailed 6 weeks after an information letter describing new genetic tests, benefits, and risks was mailed.

RESULTS

Ninety-seven of 312 (31.1%) eligible patients with an identified mailing address returned the questionnaire. After receiving the letter, 29.2% patients discussed genetic testing with their doctor, 39.3% considered pursuing genetic testing, and 8.5% underwent testing. Nearly all respondents (97%) indicated that physicians should inform patients about new developments that may improve their or their family's health, and 71% thought patients shared this responsibility. Most patients understood the letter (84%) and were pleased it was sent (84%), although 11% found it upsetting.

CONCLUSIONS

Patients believe it is important for physicians to inform them of potentially beneficial developments in genetic testing. However, physician-initiated letters to introduce new information appear inadequate alone in motivating patients to seek additional genetic counseling and testing. Further research is needed regarding optimal methods to notify former patients about new genetic tests and corresponding clinical and ethical implications.

PHGDH Defines a Metabolic Subtype in Lung Adenocarcinomas with Poor Prognosis

Molecular signatures are emerging determinants of choice of therapy for lung adenocarcinomas. An evolving therapeutic approach includes targeting metabolic dependencies in cancers. Here, using an integrative approach, we have dissected the metabolic fingerprints of lung adenocarcinomas, and we show that Phosphoglycerate dehydrogenase (PHGDH), the rate-limiting enzyme in serine biosynthesis, is highly expressed in a adenocarcinoma subset with poor prognosis. This subset harbors a gene signature for DNA replication and proliferation. Accordingly, models with high levels of PHGDH display rapid proliferation, migration, and selective channeling of serine-derived carbons to glutathione and pyrimidines, while depletion of PHGDH shows potent and selective toxicity to this subset. Differential PHGDH protein levels were defined by its degradation, and the deubiquitinating enzyme JOSD2 is a regulator of its protein stability. Our study provides evidence that a unique metabolic program is activated in a lung adenocarcinoma subset, described by PHGDH, which confers growth and survival and may have therapeutic implications.

Targeting metabolic pathways for head and neck cancers therapeutics

Cancer cells have distinctive energy metabolism pathways that support their rapid cell division. The preference for anaerobic glycolysis under the normal oxygen condition is known as the Warburg effect and has been observed in head and neck cancers. These metabolic changes are controlled by cancer-related transcription factors, such as tumor suppressor gene and hypoxia inducible factor 1α. In addition, various metabolic enzymes also actively regulate cancer-specific metabolism including the switch between aerobic and anaerobic glycolysis. For a long time, these metabolic changes in cancer cells have been considered a consequence of transformation required to maintain the high rate of tumor cell replication. However, recent studies indicate that alteration of metabolism is sufficient to initiate tumor transformation. Indeed, oncogenic mutations in the metabolic enzymes, isocitrate dehydrogenase and succinate dehydrogenase, have been increasingly found in various cancers, including head and neck cancers. In the present review, we introduce recent findings regarding the cancer metabolism, including the molecular mechanisms of how they affect cancer pathogenesis and maintenance. We also discuss the current and future perspectives on therapeutics that target metabolic pathways, with an emphasis on head and neck cancer.

The role of metabolic enzymes in mesenchymal tumors and tumor syndromes: genetics, pathology, and molecular mechanisms

The discovery of mutations in genes encoding the metabolic enzymes isocitrate dehydrogenase (IDH), succinate dehydrogenase (SDH), and fumarate hydratase (FH) has expanded our understanding not only of altered metabolic pathways but also epigenetic dysregulation in cancer. IDH1/2 mutations occur in enchondromas and chondrosarcomas in patients with the non-hereditary enchondromatosis syndromes Ollier disease and Maffucci syndrome and in sporadic tumors. IDH1/2 mutations result in excess production of the oncometabolite (D)-2-hydroxyglutarate. In contrast, SDH and FH act as tumor suppressors and genomic inactivation results in succinate and fumarate accumulation, respectively. SDH deficiency may result from germline SDHA, SDHB, SDHC, or SDHD mutations and is found in autosomal-dominant familial paraganglioma/pheochromocytoma and Carney-Stratakis syndrome, describing the combination of paraganglioma and gastrointestinal stromal tumor (GIST). In contrast, patients with the non-hereditary Carney triad, including paraganglioma, GIST, and pulmonary chondroma, usually lack germline SDH mutations and instead show epigenetic SDH complex inactivation through SDHC promoter methylation. Inactivating FH germline mutations are found in patients with hereditary leiomyomatosis and renal cell cancer (HLRCC) syndrome comprising benign cutaneous/uterine leiomyomas and renal cell carcinoma. Mutant IDH, SDH, and FH share common inhibition of α-ketoglutarate-dependent oxygenases such as the TET family of 5-methylcytosine hydroxylases preventing DNA demethylation, and Jumonji domain histone demethylases increasing histone methylation, which together inhibit cell differentiation. Ongoing studies aim to better characterize these complex alterations in cancer, the different clinical phenotypes, and variable penetrance of inherited and sporadic cancer predisposition syndromes. A better understanding of the roles of metabolic enzymes in cancer may foster the development of therapies that specifically target functional alterations in tumor cells in the future. Here, the physiologic functions of these metabolic enzymes, the mutational spectrum, and associated functional alterations will be discussed, with a focus on mesenchymal tumor predisposition syndromes.

Crystal structure of bacterial succinate:quinone oxidoreductase flavoprotein SdhA in complex with its assembly factor SdhE

Succinate:quinone oxidoreductase (SQR) functions in energy metabolism, coupling the tricarboxylic acid cycle and electron transport chain in bacteria and mitochondria. The biogenesis of flavinylated SdhA, the catalytic subunit of SQR, is assisted by a highly conserved assembly factor termed SdhE in bacteria via an unknown mechanism. By using X-ray crystallography, we have solved the structure of Escherichia coli SdhE in complex with SdhA to 2.15-Å resolution. Our structure shows that SdhE makes a direct interaction with the flavin adenine dinucleotide-linked residue His45 in SdhA and maintains the capping domain of SdhA in an "open" conformation. This displaces the catalytic residues of the succinate dehydrogenase active site by as much as 9.0 Å compared with SdhA in the assembled SQR complex. These data suggest that bacterial SdhE proteins, and their mitochondrial homologs, are assembly chaperones that constrain the conformation of SdhA to facilitate efficient flavinylation while regulating succinate dehydrogenase activity for productive biogenesis of SQR.

Exome analysis of carotid body tumor

Background

Carotid body tumor (CBT) is a form of head and neck paragangliomas (HNPGLs) arising at the bifurcation of carotid arteries. Paragangliomas are commonly associated with germline and somatic mutations involving at least one of more than thirty causative genes. However, the specific functionality of a number of these genes involved in the formation of paragangliomas has not yet been fully investigated.

Methods

Exome library preparation was carried out using Nextera® Rapid Capture Exome Kit (Illumina, USA). Sequencing was performed on NextSeq 500 System (Illumina).

Results

Exome analysis of 52 CBTs revealed potential driver mutations (PDMs) in 21 genes: ARNT, BAP1, BRAF, BRCA1, BRCA2, CDKN2A, CSDE1, FGFR3, IDH1, KIF1B, KMT2D, MEN1, RET, SDHA, SDHB, SDHC, SDHD, SETD2, TP53BP1, TP53BP2, and TP53I13. In many samples, more than one PDM was identified. There are also 41% of samples in which we did not identify any PDM; in these cases, the formation of CBT was probably caused by the cumulative effect of several not highly pathogenic mutations. Estimation of average mutation load demonstrated 6–8 mutations per megabase (Mb). Genes with the highest mutation rate were identified.

Conclusions

Exome analysis of 52 CBTs for the first time revealed the average mutation load for these tumors and also identified potential driver mutations as well as their frequencies and co-occurrence with the other PDMs.

Electronic supplementary material

The online version of this article (10.1186/s12920-018-0327-0) contains supplementary material, which is available to authorized users.

HIF-1-alpha links mitochondrial perturbation to the dynamic acquisition of breast cancer tumorigenicity

Up-regulation of hypoxia-inducible factor-1α (HIF-1α), even in normoxia, is a common feature of solid malignancies. However, the mechanisms of increased HIF-1α abundance, and its role in regulating breast cancer plasticity are not fully understood. We have previously demonstrated that dimethyl-2-ketoglutarate (DKG), a widely used cell membrane-permeable α-ketoglutarate (α-KG) analogue, transiently stabilizes HIF-1α by inhibiting prolyl hydroxylase 2. Here, we report that breast cancer tumorigenicity can be acquired through prolonged treatment with DKG. Our results indicate that, in response to prolonged DKG treatment, mitochondrial respiration becomes uncoupled, leading to the accumulation of succinate and fumarate in breast cancer cells. Further, we found that an early increase in the oxygen flux rate was accompanied by a delayed enhancement of glycolysis. Together, our results indicate that these events trigger a dynamic enrichment for cells with pluripotent/stem-like cell markers and tumorsphere-forming capacity. Moreover, DKG-mediated metabolic reprogramming results in HIF-1α induction and reductive carboxylation pathway activation. Both HIF-1α accumulation and the tumor-promoting metabolic state are required for DKG-promoted tumor repopulation capacity in vivo. Our data suggest that mitochondrial adaptation to DKG elevates the ratio of succinate or fumarate to α-KG, which in turn stabilizes HIF-1α and reprograms breast cancer cells into a stem-like state. Therefore, our results demonstrate that metabolic regulation, with succinate and/or fumarate accumulation, governs the dynamic transition of breast cancer tumorigenic states and we suggest that HIF-1α is indispensable for breast cancer tumorigenicity.

Current perspectives between metabolic syndrome and cancer

Metabolic syndrome is a cluster of risk factors that lead to cardiovascular morbidity and mortality. Recent studies linked metabolic syndrome and several types of cancer. Although metabolic syndrome may not necessarily cause cancer, it is linked to poorer cancer outcomes including increased risk of recurrence and overall mortality. This review tends to discuss the major biological and physiological alterations involved in the increase of incidence and mortality of cancer patients affected by metabolic syndrome. We focus on metabolic syndrome-associated visceral adiposity, hyperinsulinemia, hyperglycemia, insulin-like growth factor (IGF-I) pathway as well as estrogen signaling and inflammation. Several of these factors are also involved in carcinogenesis and cancer progression. A better understanding of the link between metabolic syndrome and cancer may provide new insight about oncogenesis. Moreover, prevention of metabolic syndrome - related alterations may be an important aspect in the management of cancer patients during simultaneous palliative care.

Crystal structure of an assembly intermediate of respiratory Complex II

Flavin is covalently attached to the protein scaffold in ~10% of flavoenzymes. However, the mechanism of covalent modification is unclear, due in part to challenges in stabilizing assembly intermediates. Here, we capture the structure of an assembly intermediate of the Escherichiacoli Complex II (quinol:fumarate reductase (FrdABCD)). The structure contains the E. coli FrdA subunit bound to covalent FAD and crosslinked with its assembly factor, SdhE. The structure contains two global conformational changes as compared to prior structures of the mature protein: the rotation of a domain within the FrdA subunit, and the destabilization of two large loops of the FrdA subunit, which may create a tunnel to the active site. We infer a mechanism for covalent flavinylation. As supported by spectroscopic and kinetic analyses, we suggest that SdhE shifts the conformational equilibrium of the FrdA active site to disfavor succinate/fumarate interconversion and enhance covalent flavinylation.

The mechanism for covalent flavinylation of flavoenzymes is still unclear. Here, the authors propose a mechanism based on the crystal structure of a flavinylation assembly intermediate of the E. coli respiratory Complex II comprising the E. coli FrdA subunit bound to covalent FAD and crosslinked with its assembly factor SdhE.

Lactic acidosis switches cancer cells from aerobic glycolysis back to dominant oxidative phosphorylation

While transformation of normal cells to cancer cells is accompanied with a switch from oxidative phosphorylation (OXPHOS) to aerobic glycolysis, it is interesting to ask if cancer cells can revert from Warburg effect to OXPHOS. Our previous works suggested that cancer cells reverted to OXPHOS, when they were exposed to lactic acidosis, a common factor in tumor environment. However, the conclusion cannot be drawn unless ATP output from glycolysis and OXPHOS is quantitatively determined. Here we quantitatively measured ATP generation from glycolysis and OXPHOS in 9 randomly selected cancer cell lines. Without lactic acidosis, glycolysis and OXPHOS generated 23.7% − 52.2 % and 47.8% − 76.3% of total ATP, respectively; with lactic acidosis (20 mM lactate with pH 6.7), glycolysis and OXPHOS provided 5.7% − 13.4% and 86.6% − 94.3% of total ATP. We concluded that cancer cells under lactic acidosis reverted from Warburg effect to OXPHOS phenotype.

Routine genetic screening with a multi-gene panel in patients with pheochromocytomas

PURPOSE

Several new gene mutations have been reported in recent years to be associated with a risk of familial pheochromocytoma. However, it is unclear as to whether extensive genetic testing is required in all patients.

METHODS

The clinical data of consecutive patients operated for pheochromocytoma over a decade in a tertiary referral center were reviewed. Genetic screening was performed using a 10-gene panel: RET, VHL, SDHB, SDHD, SDHA, SDHC, SDHAF2, MAX, TMEM127 and FH.

RESULTS

A total of 166 patients were analyzed: 87 of them had genetic screening performed (39 M: 44.8%, 48 F: 55.2%, age range 6-81 years, mean 45±16.8 years). In total, 22/87 (25.3%) patients had germline mutations, while 65/87 (74.7%) patients presented with apparently sporadic tumors. Germline VHL mutations were identified in 11.7% of patients, RET in 6.8% (five MEN2A/MEN2 and one MEN2B/MEN3), SDHD in 2.3%, MAX in 2.3%, SDHB in 1.1%, and TMEM127 in 1.1% of patients. At diagnosis, 15.1% of patients with unilateral non-syndromic pheochromocytoma showed germline mutations. We identified 19.7% of mutations in patients with unilateral-non-recurrent pheochromocytomas within 5 years vs. 50% in the recurrent-bilateral-metastatic group (p = 0.01). Germline mutations were more frequently seen with bilateral pheochromocytomas (p = 0.001): 80% of patients with bilateral disease had germline mutations (4 VHL, 3 RET, 1 MAX).

CONCLUSIONS

The advent of rapid genetic screening using a gene-panel makes it feasible to screen large cohorts of patients and provides a valuable tool to contribute to the prediction of bilateral and malignant disease and to screen family members.

Expression of Contactin 4 Is Associated With Malignant Behavior in Pheochromocytomas and Paragangliomas

CONTEXT

Pheochromocytomas and paragangliomas (PPGLs) are rare neuroendocrine, usually benign, tumors. Currently, the only reliable criterion of malignancy is the presence of metastases.

OBJECTIVE

The aim of this study was to identify genes associated with malignancy in PPGLs.

DESIGN

Transcriptomic profiling was performed on 40 benign and 11 malignant PPGLs. Genes showing a significantly different expression between benign and malignant PPGLs with a ratio ≥4 were confirmed and tested in an independent series by quantitative real-time polymerase chain reaction (qRT-PCR). Immunohistochemistry was performed for the validated genes on 109 benign and 32 malignant PPGLs. Functional assays were performed with hPheo1 cells.

SETTING

This study was conducted at the Department of Pathology of the Erasmus MC University Medical Center Rotterdam Human Molecular Genetics laboratory of the de Duve Institute, University of Louvain.

PATIENTS

PPGL samples from 179 patients, diagnosed between 1972 and 2015, were included.

MAIN OUTCOME MEASURES

Associations between gene expression and malignancy were tested using supervised clustering approaches.

RESULTS

Ten differentially expressed genes were selected based on messenger RNA (mRNA) expression array data. Contactin 4 (CNTN4) was overexpressed in malignant vs benign tumors [4.62-fold; false discovery rate (FDR), 0.001]. Overexpression at the mRNA level was confirmed using qRT-PCR (2.90-fold, P = 0.02; validation set: 4.26-fold, P = 0.005). Consistent findings were obtained in The Cancer Genome Atlas cohort (2.7-fold; FDR, 0.02). CNTN4 protein was more frequently expressed in malignant than in benign PPGLs by immunohistochemistry (58% vs 17%; P = 0.002). Survival after 7 days of culture under starvation conditions was significantly enhanced in hPheo1 cells transfected with CNTN4 complementary DNA.

CONCLUSION

CNTN4 expression is consistently associated with malignant behavior in PPGLs.

DIAGNOSIS of ENDOCRINE DISEASE: SDHx mutations: beyond pheochromocytomas and paragangliomas

Mutations in one of the five genes encoding the succinate dehydrogenase (SDHx) or mitochondrial complex II cause the corresponding family syndromes characterized by the occurrence of pheochromocytomas (PHEO) and paragangliomas (PGL). Recently, other solid growths, such as gastrointestinal stromal tumors (GISTs), renal cell carcinomas (RCCs) and pituitary adenomas (PAs) have been associated with these syndromes. In the absence of prospective studies assessing their frequency, at present, their occurrence seems too infrequent to suggest systematic screening for SDHx mutation carriers. However, SDHB immunohistochemistry (IHC) on tumor tissues or SDHx genetic testing on blood or tumor samples should be performed in patients affected by GISTs, RCCs or PAs with clinicopathologic phenotypes suggesting an etiologic role of SDHx genes.

Characterization and metabolic synthetic lethal testing in a new model of SDH-loss familial pheochromocytoma and paraganglioma

Succinate dehydrogenase (SDH)-loss pheochromocytoma and paraganglioma (PPGL) are tumors driven by metabolic derangement. SDH loss leads to accumulation of intracellular succinate, which competitively inhibits dioxygenase enzymes, causing activation of pseudohypoxic signaling and hypermethylation of histones and DNA. The mechanisms by which these alterations lead to tumorigenesis are unclear, however. In an effort to fundamentally understand how SDH loss reprograms cell biology, we developed an immortalized mouse embryonic fibroblast cell line with conditional disruption of Sdhc and characterize the kinetics of Sdhc gene rearrangement, SDHC protein loss, succinate accumulation, and the resultant hypoproliferative phenotype. We further perform global transcriptomic, epigenomic, and proteomic characterization of changes resulting from SDHC loss, identifying specific perturbations at each biological level. We compare the observed patterns of epigenomic derangement to another previously-described immortalized mouse chromaffin cell model of SDHB loss, and compare both models to human SDH-loss tumors. Finally, we perform analysis of SDHC synthetic lethality with lactate dehydrogenase A (LDHA) and pyruvate carboxylase (PCX), which are important for regeneration of NAD+ and aspartate biosynthesis, respectively. Our data show that SDH-loss cells are selectively vulnerable to LDH genetic knock-down or chemical inhibition, suggesting that LDH inhibition may be an effective therapeutic strategy for SDH-loss PPGL.

New Perspectives on Pheochromocytoma and Paraganglioma: Toward a Molecular Classification

A molecular biology-based taxonomy has been proposed for pheochromocytoma and paraganglioma (PPGL). Data from the Cancer Genome Atlas revealed clinically relevant prognostic and predictive biomarkers and stratified PPGLs into three main clusters. Each subgroup has a distinct molecular-biochemical-imaging signature. Concurrently, new methods for biochemical analysis, functional imaging, and medical therapies have also become available. The research community now strives to match the cluster biomarkers with the best intervention. The concept of precision medicine has been long awaited and holds great promise for improved care. Here, we review the current and future PPGL classifications, with a focus on hereditary syndromes. We discuss the current strengths and shortcomings of precision medicine and suggest a condensed manual for diagnosis and treatment of both adult and pediatric patients with PPGL. Finally, we consider the future direction of this field, with a particular focus on how advanced molecular characterization of PPGL can improve a patient's outcome, including cures and, ultimately, disease prevention.

Sterol Oxidation Mediates Stress-Responsive Vms1 Translocation to Mitochondria

Vms1 translocates to damaged mitochondria in response to stress, whereupon its binding partner, Cdc48, contributes to mitochondrial protein homeostasis. Mitochondrial targeting of Vms1 is mediated by its conserved mitochondrial targeting domain (MTD), which, in unstressed conditions, is inhibited by intramolecular binding to the Vms1 leucine-rich sequence (LRS). Here, we report a 2.7 Å crystal structure of Vms1 that reveals that the LRS lies in a hydrophobic groove in the autoinhibited MTD. We also demonstrate that the oxidized sterol, ergosterol peroxide, is necessary and sufficient for Vms1 localization to mitochondria, through binding the MTD in an interaction that is competitive with binding to the LRS. These data support a model in which stressed mitochondria generate an oxidized sterol receptor that recruits Vms1 to support mitochondrial protein homeostasis.

Clinical evaluation and treatment of phaeochromocytoma

Phaeochromocytoma and extra adrenal paraganglioma are rare neuroendocrine tumours and have the potential to secrete adrenaline, noradrenaline and dopamine causing a myriad of clinical symptoms. Prompt diagnosis is essential for clinicians and requires a multidisciplinary specialist approach for the clinical and laboratory investigation, diagnosis, treatment and follow-up of patients. This paper is an integrated review of the clinical and laboratory evaluation and treatment of patients suspected to have phaeochromocytoma or paraganglioma, highlighting recent developments and best practices from recent published clinical guidelines.

New Insights into the Nuclear Imaging Phenotypes of Cluster 1 Pheochromocytoma and Paraganglioma

Pheochromocytomas and paragangliomas (PPGLs) belong to the family of neural crest cell-derived neoplasms. In up to 70% of cases they are associated with germline and somatic mutations in 15 well-characterized PPGL driver or fusion genes. PPGLs can be grouped into three main clusters, where cluster 1 includes PPGLs characterized by a pseudohypoxic signature. Although cluster 1 tumors share several common features, they exhibit unique behaviors. We present here unique insights into the imaging phenotypes of cluster 1 PPGLs based on glucose uptake, catecholamine metabolism, and somatostatin receptor expression. Recent data suggest that succinate is a major player in the imaging phenotype of succinate dehydrogenase-deficient PPGLs. This review emphasizes the emerging stromal cell-succinate interaction and highlights new perspectives in PPGL theranostics.

Biochemical, Molecular, and Clinical Characterization of Succinate Dehydrogenase Subunit A Variants of Unknown Significance

Purpose: Patients who inherit a pathogenic loss-of-function genetic variant involving one of the four succinate dehydrogenase (SDH) subunit genes have up to an 86% chance of developing one or more cancers by the age of 50. If tumors are identified and removed early in these high-risk patients, they have a higher potential for cure. Unfortunately, many alterations identified in these genes are variants of unknown significance (VUS), confounding the identification of high-risk patients. If we could identify misclassified SDH VUS as benign or pathogenic SDH mutations, we could better select patients for cancer screening procedures and remove tumors at earlier stages.Experimental Design: In this study, we combine data from clinical observations, a functional yeast model, and a computational model to determine the pathogenicity of 22 SDHA VUS. We gathered SDHA VUS from two primary sources: The OHSU Knight Diagnostics Laboratory and the literature. We used a yeast model to identify the functional effect of a VUS on mitochondrial function with a variety of biochemical assays. The computational model was used to visualize variants' effect on protein structure.Results: We were able to draw conclusions on functional effects of variants using our three-prong approach to understanding VUS. We determined that 16 (73%) of the alterations are actually pathogenic, causing loss of SDH function, and six (27%) have no effect upon SDH function.Conclusions: We thus report the reclassification of the majority of the VUS tested as pathogenic, and highlight the need for more thorough functional assessment of inherited SDH variants. Clin Cancer Res; 23(21); 6733-43. ©2017 AACR.

Gene of the month: SDH

Succinate dehydrogenase (SDH) is a heterotetrameric nuclear encoded mitochondrial protein complex which plays a role in the citric acid cycle and the electron transfer chain. Germline mutations in SDHA are associated with Leigh syndrome. Mutations in SDHB, SDHC and SDHD are found in an increasing number of neoplasms, most notably paragangliomas and wild-type gastrointestinal stromal tumours. SDH deficiency in these tumours has important prognostic implications, and also provides a novel target for molecular therapy. In this article, we outline the structure and function of SDH and provide a summary of its role in various diseases.

Oncometabolites: A New Paradigm for Oncology, Metabolism, and the Clinical Laboratory

BACKGROUND

Pediatric clinical laboratories commonly measure tricarboxylic acid cycle intermediates for screening, diagnosis, and monitoring of specific inborn errors of metabolism, such as organic acidurias. In the past decade, the same tricarboxylic acid cycle metabolites have been implicated and studied in cancer. The accumulation of these metabolites in certain cancers not only serves as a biomarker but also directly contributes to cellular transformation, therefore earning them the designation of oncometabolites.

CONTENT

D-2-hydroxyglutarate, L-2-hydroxyglutarate, succinate, and fumarate are the currently recognized oncometabolites. They are structurally similar and share metabolic proximity in the tricarboxylic acid cycle. As a result, they promote tumorigenesis in cancer cells through similar mechanisms. This review summarizes the currently understood common and distinct biological features of these compounds. In addition, we will review the current laboratory methodologies that can be used to quantify these metabolites and their downstream targets.

SUMMARY

Oncometabolites play an important role in cancer biology. The metabolic pathways that lead to the production of oncometabolites and the downstream signaling pathways that are activated by oncometabolites represent potential therapeutic targets. Clinical laboratories have a critical role to play in the management of oncometabolite-associated cancers through development and validation of sensitive and specific assays that measure oncometabolites and their downstream effectors. These assays can be used as screening tools and for follow-up to measure response to treatment, as well as to detect minimal residual disease and recurrence.

Loss of maternal chromosome 11 is a signature event in SDHAF2, SDHD, and VHL-related paragangliomas, but less significant in SDHB-related paragangliomas

Germline mutations in the succinate dehydrogenase (SDHA, SDHB, SDHC, SDHD, SDHAF2) or Von Hippel-Lindau (VHL) genes cause hereditary paraganglioma/pheochromocytoma. While SDHB (1p36) and VHL (3p25) are associated with autosomal dominant disease, SDHD (11q23) and SDHAF2 (11q13) show a remarkable parent-of-origin effect whereby tumor formation is almost completely dependent on paternal transmission of the mutant allele. Loss of the entire maternal copy of chromosome 11 occurs frequently in SDHD-linked tumors, and has been suggested to be the basis for this typical inheritance pattern.Using fluorescent in situ hybridization, microsatellite marker and SNP array analysis, we demonstrate that loss of the entire copy of chromosome 11 is also frequent in SDHAF2-related PGLs, occurring in 89% of tumors. Analysis of two imprinted differentially methylated regions (DMR) in 11p15, H19-DMR and KvDMR, showed that this loss always affected the maternal copy of chromosome 11. Likewise, loss of maternal chromosome 11p15 was demonstrated in 85% of SDHD and 75% of VHL-related PGLs/PCCs. By contrast, both copies of chromosome 11 were found to be retained in 62% of SDHB-mutated PGLs/PCCs, while only 31% showed loss of maternal chromosome 11p15. Genome-wide copy number analysis revealed frequent loss of 1p in SDHB mutant tumors and show greater genomic instability compared to SDHD and SDHAF2.These results show that loss of the entire copy of maternal chromosome 11 is a highly specific and statistically significant event in SDHAF2, SDHD and VHL-related PGLs/PCCs, but is less significant in SDHB-mutated tumors, suggesting that these tumors have a distinct genetic etiology.

ARHI is a novel epigenetic silenced tumor suppressor in sporadic pheochromocytoma

Pheochromocytoma (PCC) is related to germline mutations in 12 susceptibility genes. Although comparative genomic hybridization array has revealed some putative tumor suppressor genes on the short arm of chromosome 1 that are likely to be involved in PCC tumorigenesis, the molecules involved, except for those encoded by known susceptibility genes, have not been found in the generation of sporadic tumors. In the present work, we first identified that the unmethylated allele of Aplasia Ras homolog member I (ARHI) was deleted in most PCC tumors which retained a hypermethylated copy, while its mRNA level was significantly correlated with the unmethylated copy. De-methylation experiments confirmed that expression of ARHI was also regulated by the methylation level of the remaining allele. Furthermore, ARHI overexpression inhibited cell proliferation, with cell cycle arrest and induction of apoptosis, in ARHI-negative primary human PCC cells, whereas knockdown of ARHI demonstrated the opposite effect in ARHI-positive primary human PCC cells. Finally, we demonstrated that ARHI has the ability to suppress pAKT and pErK1/2, to promote the expression of p21^Waf1/Cip1 and p27^Kip1, and also to increase p27^Kip1 protein stability. In summary, ARHI was silenced or downregulated in PCC tissues harboring only one hypermethylated allele. ARHI contributes to tumor suppression through inhibition of PI3K/AKT and MAKP/ERK pathways, to upregulate cell cycle inhibitors such as p27^Kip1. We therefore reasoned that ARHI is a novel epigenetic silenced tumor suppressor gene on chromosome 1p that is involved in sporadic PCC tumorigenesis.

Interplay between Metabolism and Epigenetics: A Nuclear Adaptation to Environmental Changes

The physiological identity of every cell is maintained by highly specific transcriptional networks that establish a coherent molecular program that is in tune with nutritional conditions. The regulation of cell-specific transcriptional networks is accomplished by an epigenetic program via chromatin-modifying enzymes, whose activity is directly dependent on metabolites such as acetyl-coenzyme A, S-adenosylmethionine, and NAD+, among others. Therefore, these nuclear activities are directly influenced by the nutritional status of the cell. In addition to nutritional availability, this highly collaborative program between epigenetic dynamics and metabolism is further interconnected with other environmental cues provided by the day-night cycles imposed by circadian rhythms. Herein, we review molecular pathways and their metabolites associated with epigenetic adaptations modulated by histone- and DNA-modifying enzymes and their responsiveness to the environment in the context of health and disease.

The emerging role and targetability of the TCA cycle in cancer metabolism

The tricarboxylic acid (TCA) cycle is a central route for oxidative phosphorylation in cells, and fulfills their bioenergetic, biosynthetic, and redox balance requirements. Despite early dogma that cancer cells bypass the TCA cycle and primarily utilize aerobic glycolysis, emerging evidence demonstrates that certain cancer cells, especially those with deregulated oncogene and tumor suppressor expression, rely heavily on the TCA cycle for energy production and macromolecule synthesis. As the field progresses, the importance of aberrant TCA cycle function in tumorigenesis and the potentials of applying small molecule inhibitors to perturb the enhanced cycle function for cancer treatment start to evolve. In this review, we summarize current knowledge about the fuels feeding the cycle, effects of oncogenes and tumor suppressors on fuel and cycle usage, common genetic alterations and deregulation of cycle enzymes, and potential therapeutic opportunities for targeting the TCA cycle in cancer cells. With the application of advanced technology and in vivo model organism studies, it is our hope that studies of this previously overlooked biochemical hub will provide fresh insights into cancer metabolism and tumorigenesis, subsequently revealing vulnerabilities for therapeutic interventions in various cancer types.

Analysis of SDHAF3 in familial and sporadic pheochromocytoma and paraganglioma

BACKGROUND

Germline mutations in genes encoding subunits of succinate dehydrogenase (SDH) are associated with the development of pheochromocytoma (PC) and/or paraganglioma (PGL). As assembly factors have been identified as playing a role in maturation of individual SDH subunits and assembly of the functioning SDH complex, we hypothesized that SDHAF3 variants may be associated with PC/PGL and functionality of SDH.

METHODS

DNA was extracted from the blood of 37 individuals (from 23 families) with germline SDH mutations and 18 PC/PGL (15 sporadic, 3 familial) and screened for mutations using a custom gene panel, containing SDHAF3 (SDH assembly factor 3) as well as eight known PC/PGL susceptibility genes. Molecular and functional consequences of an identified sequence variant of SDHAF3 were assessed in yeast and mammalian cells (HEK293).

RESULTS

Using massively parallel sequencing, we identified a variant in SDHAF3, c.157 T > C (p.Phe53Leu), associated with increased prevalence in familial and sporadic PC/PGL (6.6%) when compared to normal populations (1.2% [1000 Genomes], p = 0.003; 2.1% [Exome Aggregation Consortium], p = 0.0063). In silico prediction tools suggest this variant is probably damaging to protein function, hence we assessed molecular and functional consequences of the resulting amino acid change (p.Phe53Leu) in yeast and human cells. We showed that introduction of SDHAF3 p.Phe53Leu into Sdh7 (ortholog of SDHAF3 in humans) null yeast resulted in impaired function, as observed by its failure to restore SDH activity when expressed in Sdh7 null yeast relative to WT SDHAF3. As SDHAF3 is involved in maturation of SDHB, we tested the functional impact of SDHAF3 c.157 T > C and various clinically relevant SDHB mutations on this interaction. Our in vitro studies in human cells show that SDHAF3 interacts with SDHB (residues 46 and 242), with impaired interaction observed in the presence of the SDHAF3 c.157 T > C variant.

CONCLUSIONS

Our studies reveal novel insights into the biogenesis of SDH, uncovering a vital interaction between SDHAF3 and SDHB. We have shown that SDHAF3 interacts directly with SDHB (residue 242 being key to this interaction), and that a variant in SDHAF3 (c.157 T > C [p.Phe53Leu]) may be more prevalent in individuals with PC/PGL, and is hypomorphic via impaired interaction with SDHB.

Targeted Exome Sequencing of Krebs Cycle Genes Reveals Candidate Cancer-Predisposing Mutations in Pheochromocytomas and Paragangliomas

Purpose: Mutations in Krebs cycle genes are frequently found in patients with pheochromocytomas/paragangliomas. Disruption of SDH, FH or MDH2 enzymatic activities lead to accumulation of specific metabolites, which give rise to epigenetic changes in the genome that cause a characteristic hypermethylated phenotype. Tumors showing this phenotype, but no alterations in the known predisposing genes, could harbor mutations in other Krebs cycle genes.Experimental Design: We used downregulation and methylation of RBP1, as a marker of a hypermethylation phenotype, to select eleven pheochromocytomas and paragangliomas for targeted exome sequencing of a panel of Krebs cycle-related genes. Methylation profiling, metabolite assessment and additional analyses were also performed in selected cases.Results: One of the 11 tumors was found to carry a known cancer-predisposing somatic mutation in IDH1 A variant in GOT2, c.357A>T, found in a patient with multiple tumors, was associated with higher tumor mRNA and protein expression levels, increased GOT2 enzymatic activity in lymphoblastic cells, and altered metabolite ratios both in tumors and in GOT2 knockdown HeLa cells transfected with the variant. Array methylation-based analysis uncovered a somatic epigenetic mutation in SDHC in a patient with multiple pheochromocytomas and a gastrointestinal stromal tumor. Finally, a truncating germline IDH3B mutation was found in a patient with a single paraganglioma showing an altered α-ketoglutarate/isocitrate ratio.Conclusions: This study further attests to the relevance of the Krebs cycle in the development of PCC and PGL, and points to a potential role of other metabolic enzymes involved in metabolite exchange between mitochondria and cytosol. Clin Cancer Res; 23(20); 6315-24. ©2017 AACR.

Structural and biochemical analyses reveal insights into covalent flavinylation of the Escherichia coli Complex II homolog quinol:fumarate reductase

The Escherichia coli Complex II homolog quinol:fumarate reductase (QFR, FrdABCD) catalyzes the interconversion of fumarate and succinate at a covalently attached FAD within the FrdA subunit. The SdhE assembly factor enhances covalent flavinylation of Complex II homologs, but the mechanisms underlying the covalent attachment of FAD remain to be fully elucidated. Here, we explored the mechanisms of covalent flavinylation of the E. coli QFR FrdA subunit. Using a ΔsdhE E. coli strain, we show that the requirement for the assembly factor depends on the cellular redox environment. We next identified residues important for the covalent attachment and selected the FrdA^E245 residue, which contributes to proton shuttling during fumarate reduction, for detailed biophysical and structural characterization. We found that QFR complexes containing FrdA^E245Q have a structure similar to that of the WT flavoprotein, but lack detectable substrate binding and turnover. In the context of the isolated FrdA subunit, the anticipated assembly intermediate during covalent flavinylation, FrdA^E245 variants had stability similar to that of WT FrdA, contained noncovalent FAD, and displayed a reduced capacity to interact with SdhE. However, small-angle X-ray scattering (SAXS) analysis of WT FrdA cross-linked to SdhE suggested that the FrdA^E245 residue is unlikely to contribute directly to the FrdA-SdhE protein-protein interface. We also found that no auxiliary factor is absolutely required for flavinylation, indicating that the covalent flavinylation is autocatalytic. We propose that multiple factors, including the SdhE assembly factor and bound dicarboxylates, stimulate covalent flavinylation by preorganizing the active site to stabilize the quinone-methide intermediate.

PheoSeq: A Targeted Next-Generation Sequencing Assay for Pheochromocytoma and Paraganglioma Diagnostics

Genetic diagnosis is recommended for all pheochromocytoma and paraganglioma (PPGL) cases, as driver mutations are identified in approximately 80% of the cases. As the list of related genes expands, genetic diagnosis becomes more time-consuming, and targeted next-generation sequencing (NGS) has emerged as a cost-effective tool. This study aimed to optimize targeted NGS in PPGL genetic diagnostics. A workflow based on two customized targeted NGS assays was validated to study the 18 main PPGL genes in germline and frozen tumor DNA, with one of them specifically directed toward formalin-fixed paraffin-embedded tissue. The series involved 453 unrelated PPGL patients, of whom 30 had known mutations and were used as controls. Partial screening using Sanger had been performed in 275 patients. NGS results were complemented with the study of gross deletions. NGS assay showed a sensitivity ≥99.4%, regardless of DNA source. We identified 45 variants of unknown significance and 89 pathogenic mutations, the latter being germline in 29 (7.2%) and somatic in 58 (31.7%) of the 183 tumors studied. In 37 patients previously studied by Sanger sequencing, the causal mutation could be identified. We demonstrated that both assays are an efficient and accurate alternative to conventional sequencing. Their application facilitates the study of minor PPGL genes, and enables genetic diagnoses in patients with incongruent or missing clinical data, who would otherwise be missed.

Restoring of Glucose Metabolism of Engineered Yarrowia lipolytica for Succinic Acid Production via a Simple and Efficient Adaptive Evolution Strategy

Succinate dehydrogenase inactivation in Yarrowia lipolytica has been demonstrated for robust succinic acid production, whereas the inefficient glucose metabolism has hindered its practical application. In this study, a simple and efficient adaptive evolution strategy via cell immobilization was conducted in shake flasks, with an aim to restore the glucose metabolism of Y. lipolytica mutant PGC01003. After 21 days with 14 generations evolution, glucose consumption rate increased to 0.30 g/L/h in YPD medium consisting of 150 g/L initial glucose concentration, while poor yeast growth was observed in the same medium using the initial strain without adaptive evolution. Succinic acid productivity of the evolved strain also increased by 2.3-fold, with stable cell growth in YPD medium with high initial glucose concentration. Batch fermentations resulted in final succinic acid concentrations of 65.7 g/L and 87.9 g/L succinic acid using YPD medium and food waste hydrolysate, respectively. The experimental results in this study show that a simple and efficient strategy could facilitate the glucose uptake rate in succinic acid fermentation using glucose-rich substrates.

Metabolic reprogramming and epithelial-to-mesenchymal transition in cancer

Several lines of evidence indicate that during transformation epithelial cancer cells can acquire mesenchymal features via a process called epithelial-to-mesenchymal transition (EMT). This process endows cancer cells with increased invasive and migratory capacity, enabling tumour dissemination and metastasis. EMT is associated with a complex metabolic reprogramming, orchestrated by EMT transcription factors, which support the energy requirements of increased motility and growth in harsh environmental conditions. The discovery that mutations in metabolic genes such as FH, SDH and IDH activate EMT provided further evidence that EMT and metabolism are intertwined. In this review, we discuss the role of EMT in cancer and the underpinning metabolic reprogramming. We also put forward the hypothesis that, by altering chromatin structure and function, metabolic pathways engaged by EMT are necessary for its full activation.

Mitochondrial Deficiencies in the Predisposition to Paraganglioma

Paragangliomas and pheochromocytomas are rare neuroendocrine tumours with a very strong genetic component. It is estimated that around 40% of all cases are caused by a germline mutation in one of the 13 predisposing genes identified so far. Half of these inherited cases are intriguingly caused by mutations in genes encoding tricarboxylic acid enzymes, namely SDHA, SDHB, SDHC, SDHD, and SDHAF2 genes, encoding succinate dehydrogenase and its assembly protein, FH encoding fumarate hydratase, and MDH2 encoding malate dehydrogenase. These mutations may also predispose to other type of cancers, such as renal cancer, leiomyomas, or gastro-intestinal stromal tumours. SDH, which is also the complex II of the oxidative respiratory chain, was the first mitochondrial enzyme to be identified having tumour suppressor functions, demonstrating that 80 years after his initial proposal, Otto Warburg may have actually been right when he hypothesized that low mitochondrial respiration was the origin of cancer. This review reports the current view on how such metabolic deficiencies may lead to cancer predisposition and shows that the recent data may lead to the development of innovative therapeutic strategies and establish precision medicine approaches for the management of patients affected by these rare diseases.

SDHA related tumorigenesis: a new case series and literature review for variant interpretation and pathogenicity

PURPOSE

To evaluate the role of germline SDHA mutation analysis by (1) comprehensive literature review, (2) description of novel germline SDHA mutations and (3) in silico structural prediction analysis of missense substitutions in SDHA.

PATIENTS AND METHODS

A systematic literature review and a retrospective review of the molecular and clinical features of patients identified with putative germline variants in UK molecular genetic laboratories was performed. To evaluate the molecular consequences of SDHA missense variants, a novel model of the SDHA/B/C/D complex was generated and the structural effects of missense substitutions identified in the literature, our UK novel cohort and a further 32 "control missense variants" were predicted by the mCSM computational platform. These structural predictions were correlated with the results of tumor studies and other bioinformatic predictions.

RESULTS

Literature review revealed reports of 17 different germline SDHA variants in 47 affected individuals from 45 kindreds. A further 10 different variants in 15 previously unreported cases (seven novel variants in eight patients) were added from our UK series. In silico structural prediction studies of 11 candidate missense germline mutations suggested that most (63.7%) would destabilize the SDHA protomer, and that most (78.1%) rare SDHA missense variants present in a control data set (ESP6500) were also associated with impaired protein stability.

CONCLUSION

The clinical spectrum of SDHA-associated neoplasia differs from that of germline mutations in other SDH-subunits. The interpretation of the significance of novel SDHA missense substitutions is challenging. We recommend that multiple investigations (e.g. tumor studies, metabolomic profiling) should be performed to aid classification of rare missense variants before genetic testing results are used to influence clinical management.

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.

Salicylic Acid-Dependent Plant Stress Signaling via Mitochondrial Succinate Dehydrogenase

Mitochondria are known for their role in ATP production and generation of reactive oxygen species, but little is known about the mechanism of their early involvement in plant stress signaling. The role of mitochondrial succinate dehydrogenase (SDH) in salicylic acid (SA) signaling was analyzed using two mutants: disrupted in stress response1 (dsr1), which is a point mutation in SDH1 identified in a loss of SA signaling screen, and a knockdown mutant (sdhaf2) for SDH assembly factor 2 that is required for FAD insertion into SDH1. Both mutants showed strongly decreased SA-inducible stress promoter responses and low SDH maximum capacity compared to wild type, while dsr1 also showed low succinate affinity, low catalytic efficiency, and increased resistance to SDH competitive inhibitors. The SA-induced promoter responses could be partially rescued in sdhaf2, but not in dsr1, by supplementing the plant growth media with succinate. Kinetic characterization showed that low concentrations of either SA or ubiquinone binding site inhibitors increased SDH activity and induced mitochondrial H₂O₂ production. Both dsr1 and sdhaf2 showed lower rates of SA-dependent H₂O₂ production in vitro in line with their low SA-dependent stress signaling responses in vivo. This provides quantitative and kinetic evidence that SA acts at or near the ubiquinone binding site of SDH to stimulate activity and contributes to plant stress signaling by increased rates of mitochondrial H₂O₂ production, leading to part of the SA-dependent transcriptional response in plant cells.

PRECISION MEDICINE: AN UPDATE ON GENOTYPE/BIOCHEMICAL PHENOTYPE RELATIONSHIPS IN PHEOCHROMOCYTOMA/PARAGANGLIOMA PATIENTS

OBJECTIVE

Pheochromocytomas and paragangliomas (PPGLs) are rare neuroendocrine tumors known to produce and secrete high levels of circulating catecholamines and their metabolites. The biochemical characteristics of these tumors can be used to divide them into three major phenotypes. The adrenergic, noradrenergic and dopaminergic phenotypes are defined by predominant elevations in epinephrine and metanephrine, norepinephrine and normetanephrine, and dopamine and 3-methoxytyramine, respectively. There are over 15 well-identified tumor-susceptibility genes responsible for approximately 40% of the cases. The objective of this review article is to outline specific genotype/biochemical phenotype relationships.

METHODS

Literature review.

RESULTS

None.

CONCLUSION

Biochemical phenotype of PPGL is determined by the underlying genetic mutation and the associated molecular pathway. Identification of genotype/biochemical relationships is valuable in prioritizing testing for specific genes, making treatment decisions and monitoring disease progression.

ABBREVIATIONS

3-MT = 3-methoxytyramine; EPAS1 = endothelial pas domain protein 1; FH = fumarate hydratase; HIF2A = hypoxia inducible factor type 2A; MEN2 = multiple endocrine neoplasia type 2; NF1 = neurofibromatosis type 1; PNMT = phenylethanolamine N-methyltransferase; PPGL = pheochromocytoma and paraganglioma; RET = rearranged during transfection; SDH = succinate dehydrogenase; SDHAF2 = succinate dehydrogenase complex assembly factor 2; TCA = tricarboxylic acid; TH = tyrosine hydroxylase; TMEM127 = transmembrane protein 127; VHL = von Hippel-Lindau.

Targeting the Metabolic Reprogramming That Controls Epithelial-to-Mesenchymal Transition in Aggressive Tumors

The epithelial-to-mesenchymal transition (EMT) process allows the trans-differentiation of a cell with epithelial features into a cell with mesenchymal characteristics. This process has been reported to be a key priming event for tumor development and therefore EMT activation is now considered an established trait of malignancy. The transcriptional and epigenetic reprogramming that governs EMT has been extensively characterized and reviewed in the last decade. However, increasing evidence demonstrates a correlation between metabolic reprogramming and EMT execution. The aim of the current review is to gather the recent findings that illustrate this correlation to help deciphering whether metabolic changes are causative or just a bystander effect of EMT activation. The review is divided accordingly to the catabolic and anabolic pathways that characterize carbohydrate, aminoacid, and lipid metabolism. Moreover, at the end of each part, we have discussed a series of potential metabolic targets involved in EMT promotion and execution for which drugs are either available or that could be further investigated for therapeutic intervention.

Central role of lactate and proton in cancer cell resistance to glucose deprivation and its clinical translation

Targeting common weaknesses of cancer is an important strategy for cancer therapy. Glucose is a nutrient that maintains essential cellular metabolism, supporting cancer cell survival, growth and proliferation. Depriving glucose rapidly kills cancer cells. Most cancer cells possess a feature called Warburg effect, which refers to that cancer cells even with ample oxygen exhibit an exceptionally high glycolysis rate and convert most incoming glucose to lactate. Although it is recognized that Warburg effect confers growth advantage to cancer cells when glucose supply is sufficient, this feature could be considered as a fatal weakness of cancer cells when glucose supply is a problem. As glucose supply in many solid tumors is poor, and as most cancer cells have exceptionally high glycolytic capacity, maximizing cancer cell glycolysis rate would possibly exhaust intratumoral glucose, leading cancer cell to death. Lactate and proton are two common factors in solid tumors, they jointly protect cancer cells against glucose deprivation, and they are also powerful regulators dictating glucose metabolic phenotypes of cancer cells. Disrupting the joint action of lactate and proton, for example, by means of bicarbonate infusion into tumor, could maximize cancer cell glycolytic rate to rapidly use up glucose, expose their vulnerability to glucose deprivation and ultimately kill cancer cells. A pilot clinical study demonstrated that this approach achieved a remarkable improvement in local control of large and huge hepatocellular carcinoma.

Emerging role of dopamine in neovascularization of pheochromocytoma and paraganglioma

Dopamine is a catecholamine that acts both as a neurotransmitter and as a hormone, exerting its functions via dopamine (DA) receptors that are present in a broad variety of organs and cells throughout the body. In the circulation, DA is primarily stored in and transported by blood platelets. Recently, the important contribution of DA in the regulation of angiogenesis has been recognized. In vitro and in vivo studies have shown that DA inhibits angiogenesis through activation of the DA receptor type 2. Overproduction of catecholamines is the biochemical hallmark of pheochromocytoma (PCC) and paraganglioma (PGL). The increased production of DA has been shown to be an independent predictor of malignancy in these tumors. The precise relationship underlying the association between DA production and PCC and PGL behavior needs further clarification. Herein, we review the biochemical and physiologic aspects of DA with a focus on its relations with VEGF and hypoxia inducible factor related angiogenesis pathways, with special emphasis on DA producing PCC and PGL.-Osinga, T. E., Links, T. P., Dullaart, R. P. F., Pacak, K., van der Horst-Schrivers, A. N. A., Kerstens, M. N., Kema, I. P. Emerging role of dopamine in neovascularization of pheochromocytoma and paraganglioma.

Mitochondrial Complex II: At the Crossroads

Mitochondrial complex II (CII), also called succinate dehydrogenase (SDH), is a central purveyor of the reprogramming of metabolic and respiratory adaptation in response to various intrinsic and extrinsic stimuli and abnormalities. In this review we discuss recent findings regarding SDH biogenesis, which requires four known assembly factors, and modulation of its enzymatic activity by acetylation, succinylation, phosphorylation, and proteolysis. We further focus on the emerging role of both genetic and epigenetic aberrations leading to SDH dysfunction associated with various clinical manifestations. This review also covers the recent discovery of the role of SDH in inflammation-linked pathologies. Conceivably, SDH is a potential target for several hard-to-treat conditions, including cancer, that remains to be fully exploited.

SDH6 and SDH7 Contribute to Anchoring Succinate Dehydrogenase to the Inner Mitochondrial Membrane in Arabidopsis thaliana

The succinate dehydrogenase complex (complex II) is a highly conserved protein complex composed of the SDH1 to SDH4 subunits in bacteria and in the mitochondria of animals and fungi. The reason for the occurrence of up to four additional subunits in complex II of plants, termed SDH5 to SDH8, so far is a mystery. Here, we present a biochemical approach to investigate the internal subunit arrangement of Arabidopsis (Arabidopsis thaliana) complex II. Using low-concentration detergent treatments, the holo complex is dissected into subcomplexes that are analyzed by a three-dimensional gel electrophoresis system. Protein identifications by mass spectrometry revealed that the largest subcomplex (IIa) represents the succinate dehydrogenase domain composed of SDH1 and SDH2. Another subcomplex (IIb) is composed of the SDH3, SDH4, SDH6, and SDH7 subunits. All four proteins include transmembrane helices and together form the membrane anchor of complex II. Sequence analysis revealed that SDH3 and SDH4 lack helices conserved in other organisms. Using homology modeling and phylogenetic analyses, we present evidence that SDH6 and SDH7 substitute missing sequence stretches of SDH3 and SDH4 in plants. Together with SDH5, which is liberated upon dissection of complex II into subcomplexes, SDH6 and SDH7 also add some hydrophilic mass to plant complex II, which possibly inserts further functions into this smallest protein complex of the oxidative phosphorylation system (which is not so small in plants).

Interplay between epigenetics and metabolism in oncogenesis: mechanisms and therapeutic approaches

Epigenetic and metabolic alterations in cancer cells are highly intertwined. Oncogene-driven metabolic rewiring modifies the epigenetic landscape via modulating the activities of DNA and histone modification enzymes at the metabolite level. Conversely, epigenetic mechanisms regulate the expression of metabolic genes, thereby altering the metabolome. Epigenetic-metabolomic interplay has a critical role in tumourigenesis by coordinately sustaining cell proliferation, metastasis and pluripotency. Understanding the link between epigenetics and metabolism could unravel novel molecular targets, whose intervention may lead to improvements in cancer treatment. In this review, we summarized the recent discoveries linking epigenetics and metabolism and their underlying roles in tumorigenesis; and highlighted the promising molecular targets, with an update on the development of small molecule or biologic inhibitors against these abnormalities in cancer.