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Abstract

Résumé Chez l'homme, les dysfonctions des enzymes du cycle de Krebs liées à des mutations causent des encéphalopathies sévères du jeune enfant, ou différents types de tumeurs et de cancers touchant différents organes. Ainsi des mutations des gènes de la succinate-déshydrogénase ont été identifiées dans des formes familiales de phéochromocytomes et paragangliomes. Les mutations du gène de la fumarase s'observent dans des léiomyomes utérins en association avec des cancers rénaux, mais des cas d'encéphalopathies de l'enfant ont aussi été observés. Les mutations des gènes de l'isocitrate-déshydrogénase peuvent représenter jusqu'à 20% des leucémies myéloïdes aiguës et jusqu'à 80% dans certains gliomes. On ne sait actuellement expliciter la diversité des conséquences possibles de ces mutations : encéphalopathies versus cancers, nature des tissus affectés, cinétique variable d'apparition des symptômes. Il semble en revanche établi que les anomalies dans les équilibres métaboliques qui résultent de ces mutations soient à la base des processus tumoraux en modulant l'activité d'hydroxylases susceptibles d'affecter certains facteurs de transcriptions, mais aussi la méthylation de l'ADN et les histones. Dans ce contexte, il semble étonnant que soient largement utilisés certains fongicides dont l'effet est de bloquer une étape centrale du cycle de Krebs, la succinate-déshydrogénase. Le principe de précaution voudrait sans aucun doute de réévaluer urgemment la nécessité de l'usage de telles substances pour l'homme et l'environnement.

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... Les inhibiteurs de la succinate déshydrogénase (SDHI) sont des fongicides à large spectre utilisés depuis les années 60 contre divers types de moisissures. Les cultures protégées peuvent être des cultures de céréales, de fruits et de légumes, des semences, mais aussi des pelouses publiques ou des terrains de golf 103,104 . Leur rôle principal est d'inhiber la succinate déshydrogénase chez les champignons phytopathogènes. ...
... En effet, la succinate déshydrogénase étant un complexe universel, la spécificité des SDHI n'est pas garantie. Une perte d'activité de la succinate déshydrogénase provoque une accumulation de succinate dans l'organisme, or ce dernier est un oncométabolite 104 . ...
Thesis
Les travaux de recherche présentés ici ont pour objectif la synthèse de fongicides innovants de la manière la plus éco-compatible qu’il soit. Pour cela, plusieurs axes de réflexion ont été étudiés. Tout d’abord, la synthèse d’un intermédiaire de fongicides bien connu, l’o-crésol1, a été réalisée à partir d’un substrat naturel, le salicylaldéhyde2. Les conditions expérimentales ont été éco-conçues afin de correspondre au mieux aux exigences de la chimie verte. Des solvants et des réactifs classés verts ont été utilisés, comme l’éthanol, l’eau et l’acide acétique3 par exemple, ainsi que des techniques alternatives comme l’activation par micro-ondes (Schéma 1). Dans un second temps, des synthèses éco-compatibles de nouveaux fongicides ont été réalisées. Le salicylaldéhyde, substrat naturel de référence, a été mis à réagir avec différents alcools afin de réaliser des réactions d’éthérifications réductrices. Les molécules obtenues sont composées d’une partie phénolique portant un groupement alcoxyle en position 2. Ce groupement alcoxyle se compose d’une longueur de chaîne variable ayant une influence sur les propriétés fongicides des produits synthétisés (Schéma 1). Troisièmement, des réactions d’homo- et d’hétéro-couplages pinacoliques ont été réalisées entre divers aldéhydes, aromatiques et aliphatiques, afin d’obtenir des diols vicinaux, appelés pinacols, symétriques ou non (Schéma 1). Dernièrement, les propriétés fongicides et anti-oxydantes de toutes les molécules obtenues de manière stable, référencées ou non, ainsi que les substrats de départ, ont été testés. Les tests microbiologiques ont été réalisés sur une gamme de diverses souches fongiques.
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Although the alteration of cellular metabolism in cancer was reported by Warburg in the early 1930s, a regain of interest in cancer metabolism has more recently followed the discovery of germline or somatic mutations in genes coding for metabolic enzymes (succinate dehydrogenase, fumarate hydratase and isocitrate dehydrogenase) that are associated with tumor susceptibility. Mutations in these genes are found in numerous tumor types including paragangliomas, kidney cancers, leiomyomas, glioblastomas and acute myeloid leukemia. They lead to the accumulation of so-called oncometabolites that behave as competitors of 2-oxoglutarate-dependent dioxygenases, involved in a broad spectrum of pathways such as hypoxic response and epigenetic reprogramming. Here, we review the diverse pathways affected by oncometabolites, their potential role in cancer formation, maintenance, metastasis and sensitivity to chemotherapies, as well as emerging new therapeutic strategies.
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Seventy years from the formalization of the Krebs cycle as the central metabolic turntable sustaining the cell respiratory process, key functions of several of its intermediates, especially succinate, and fumarate, have been recently uncovered. The presumably immutable organization of the cycle has been challenged by a number of observations, and the variable subcellular location of a number of its constitutive protein components is now well recognized, although yet unexplained. Nonetheless, the most striking observations have been made in the recent period while investigating human diseases, especially a set of specific cancers, revealing the crucial role of Krebs cycle intermediates as factors affecting genes methylation and thus cell remodeling. We review here the recent advances and persisting incognita about the role of Krebs cycle acids in diverse aspects of cellular life and human pathology.
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Mutations in the enzyme cytosolic isocitrate dehydrogenase 1 (IDH1) are a common feature of a major subset of primary human brain cancers. These mutations occur at a single amino acid residue of the IDH1 active site, resulting in loss of the enzyme's ability to catalyse conversion of isocitrate to {alpha}-ketoglutarate. However, only a single copy of the gene is mutated in tumours, raising the possibility that the mutations do not result in a simple loss of function. Here we show that cancer-associated IDH1 mutations result in a new ability of the enzyme to catalyse the NADPH-dependent reduction of {alpha}-ketoglutarate to R(-)-2-hydroxyglutarate (2HG). Structural studies demonstrate that when arginine 132 is mutated to histidine, residues in the active site are shifted to produce structural changes consistent with reduced oxidative decarboxylation of isocitrate and acquisition of the ability to convert {alpha}-ketoglutarate to 2HG. Excess accumulation of 2HG has been shown to lead to an elevated risk of malignant brain tumours in patients with inborn errors of 2HG metabolism. Similarly, in human malignant gliomas harbouring IDH1 mutations, we find markedly elevated levels of 2HG. These data demonstrate that the IDH1 mutations result in production of the onco-metabolite 2HG, and indicate that the excess 2HG which accumulates in vivo contributes to the formation and malignant progression of gliomas.
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Paragangliomas are neuroendocrine tumors frequently associated with mutations in RET, NF1, VHL, and succinate dehydrogenase (SDHx) genes. Methylome analysis of a large paraganglioma cohort identified three stable clusters, associated with distinct clinical features and mutational status. SDHx-related tumors displayed a hypermethylator phenotype, associated with downregulation of key genes involved in neuroendocrine differentiation. Succinate accumulation in SDH-deficient mouse chromaffin cells led to DNA hypermethylation by inhibition of 2-OG-dependent histone and DNA demethylases and established a migratory phenotype reversed by decitabine treatment. Epigenetic silencing was particularly severe in SDHB-mutated tumors, potentially explaining their malignancy. Finally, inactivating FH mutations were identified in the only hypermethylated tumor without SDHx mutations. These findings emphasize the interplay between the Krebs cycle, epigenomic changes, and cancer.
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Two siblings are described who present with fumaric aciduria, a hitherto unreported organic aciduria. The results of our analytical investigations using gas chromatography/mass spectrometry, and the clinical presentation of the patients, are consistent with the notion that the fumaric aciduria is caused by an inherited defect which leads to a net secretion of fumaric acid by the renal tubules.
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During the last decade much progress has been made in the analysis of the anaerobic fermentation of carbohydrate, but very little is so far known about the intermediate stages of the oxidative breakdown of carbohydrate. A number of reactions are known in which derivatives of carbohydrate take part and which are probably steps in the breakdown of carbohydrate; we know furthermore, from the work of Szent-Györgyi20) that succinic acid, fumaric acid and oxaloacetic acid play some role in the oxidation of carbohydrate, but the details of this role are still obscure.In the present paper experiments are reported which throw new light on the problem of the intermediate stages of oxidation of carbohydrate; in conjunction with the work of Szent-Györgyi20), Stare and Baumann19) and Martius and Knoop13,14) the new experiments allow us to outline the principal steps of the oxidation of sugar in animal tissues.
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We report an inborn error of the tricarboxylic acid cycle, fumarase deficiency, in two siblings born to first cousin parents. They presented with progressive encephalopathy, dystonia, leucopenia, and neutropenia. Elevation of lactate in the cerebrospinal fluid and high fumarate excretion in the urine led us to investigate the activities of the respiratory chain and of the Krebs cycle, and to finally identify fumarase deficiency in these two children. The deficiency was profound and present in all tissues investigated, affecting the cytosolic and the mitochondrial fumarase isoenzymes to the same degree. Analysis of fumarase cDNA demonstrated that both patients were homozygous for a missense mutation, a G-955-->C transversion, predicting a Glu-319-->Gln substitution. This substitution occurred in a highly conserved region of the fumarase cDNA. Both parents exhibited half the expected fumarase activity in their lymphocytes and were found to be heterozygous for this substitution. The present study is to our knowledge the first molecular characterization of tricarboxylic acid deficiency, a rare inherited inborn error of metabolism in childhood.
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Hereditary paragangliomas are usually benign tumors of the autonomic nervous system that are composed of cells derived from the primitive neural crest. Even though three genes (SDHD, SDHC, and SDHB), which encode three protein subunits of cytochrome b of complex II in the mitochondrial respiratory chain, have been identified, the molecular mechanisms leading to tumorigenesis are unknown. We studied a family in which the father and his eldest son had bilateral neck paragangliomas, whereas the second son had a left carotid-body paraganglioma and an ectopic mediastinal pheochromocytoma. A nonsense mutation (R22X) in the SDHD gene was found in these three affected subjects. Loss of heterozygosity was observed for the maternal chromosome 11q21-q25 within the tumor but not in peripheral leukocytes. Assessment of the activity of respiratory-chain enzymes showed a complete and selective loss of complex II enzymatic activity in the inherited pheochromocytoma, that was not detected in six sporadic pheochromocytomas. In situ hybridization and immunohistochemistry experiments showed a high level of expression of markers of the angiogenic pathway. Real-time quantitative reverse transcriptase (RT)-PCR measurements confirmed that vascular endothelial growth factor and endothelial PAS domain protein 1 mRNA levels were significantly higher (three- and sixfold, respectively) than those observed in three sporadic benign pheochromocytomas. Thus, inactivation of the SDHD gene in hereditary paraganglioma is associated with a complete loss of mitochondrial complex II activity and with a high expression of angiogenic factors.
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Several mitochondrial proteins are tumor suppressors. These include succinate dehydrogenase (SDH) and fumarate hydratase, both enzymes of the tricarboxylic acid (TCA) cycle. However, to date, the mechanisms by which defects in the TCA cycle contribute to tumor formation have not been elucidated. Here we describe a mitochondrion-to-cytosol signaling pathway that links mitochondrial dysfunction to oncogenic events: succinate, which accumulates as a result of SDH inhibition, inhibits HIF-alpha prolyl hydroxylases in the cytosol, leading to stabilization and activation of HIF-1alpha. These results suggest a mechanistic link between SDH mutations and HIF-1alpha induction, providing an explanation for the highly vascular tumors that develop in the absence of VHL mutations.
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