Complex II (CII) activity controls phenomena that require crosstalk between metabolism and signaling, including neurodegeneration, cancer metabolism, immune activation, and ischemia-reperfusion injury. CII activity can be regulated at the level of assembly, a process that leverages metastable assembly intermediates. The nature of these intermediates and how CII subunits transfer between metastable complexes remains unclear. In this work, we identify metastable species containing the SDHA subunit and its assembly factors, and we assign a preferred temporal sequence of appearance of these species during CII assembly. Structures of two species show that the assembly factors undergo disordered-to-ordered transitions without the appearance of significant secondary structure. The findings identify that intrinsically disordered regions are critical in regulating CII assembly, an observation that has implications for the control of assembly in other biomolecular complexes.

• MeSH
• Catalytic Domain * MeSH
• Protein Structure, Secondary MeSH
• Publication type
• Journal Article MeSH

BACKGROUND: Pheochromocytoma and paraganglioma (PPGL) are neuroendocrine tumors with frequent mutations in genes linked to the tricarboxylic acid cycle. However, no pathogenic variant has been found to date in succinyl-CoA ligase (SUCL), an enzyme that provides substrate for succinate dehydrogenase (SDH; mitochondrial complex II [CII]), a known tumor suppressor in PPGL. METHODS: A cohort of 352 patients with apparently sporadic PPGL underwent genetic testing using a panel of 54 genes developed at the National Institutes of Health, including the SUCLG2 subunit of SUCL. Gene deletion, succinate levels, and protein levels were assessed in tumors where possible. To confirm the possible mechanism, we used a progenitor cell line, hPheo1, derived from a human pheochromocytoma, and ablated and re-expressed SUCLG2. RESULTS: We describe 8 germline variants in the guanosine triphosphate-binding domain of SUCLG2 in 15 patients (15 of 352, 4.3%) with apparently sporadic PPGL. Analysis of SUCLG2-mutated tumors and SUCLG2-deficient hPheo1 cells revealed absence of SUCLG2 protein, decrease in the level of the SDHB subunit of SDH, and faulty assembly of the complex II, resulting in aberrant respiration and elevated succinate accumulation. CONCLUSIONS: Our study suggests SUCLG2 as a novel candidate gene in the genetic landscape of PPGL. Large-scale sequencing may uncover additional cases harboring SUCLG2 variants and provide more detailed information about their prevalence and penetrance.

• MeSH
• Pheochromocytoma * genetics   pathology   MeSH
• Humans MeSH
• Adrenal Gland Neoplasms * genetics   pathology   MeSH
• Paraganglioma * genetics   pathology   MeSH
• Succinate Dehydrogenase genetics   metabolism   MeSH
• Germ-Line Mutation MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Research Support, N.I.H., Intramural MeSH
• Names of Substances
• Succinate Dehydrogenase MeSH

Increasing evidence points to the respiratory Complex II (CII) as a source and modulator of reactive oxygen species (ROS). Both functional loss of CII as well as its pharmacological inhibition can lead to ROS generation in cells, with a relevant impact on the development of pathophysiological conditions, i.e. cancer and neurodegenerative diseases. While the basic framework of CII involvement in ROS production has been defined, the fine details still await clarification. It is important to resolve these aspects to fully understand the role of CII in pathology and to explore its therapeutic potential in cancer and other diseases.

• Keywords
• OXPHOS, Respiratory complex II, cancer, mitochondria, reactive oxygen species, succinate, succinate dehydrogenase, tricarboxylic acid cycle,
• MeSH
• Molecular Targeted Therapy * MeSH
• Humans MeSH
• Mitochondrial Diseases drug therapy   metabolism   pathology   MeSH
• Mitochondria metabolism   pathology   MeSH
• Reactive Oxygen Species metabolism   MeSH
• Electron Transport Complex II metabolism   MeSH
• Electron Transport MeSH
• Animals MeSH
• Check Tag
• Humans MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Review MeSH
• Names of Substances
• Reactive Oxygen Species MeSH
• Electron Transport Complex II MeSH

Cancer cells without mitochondrial DNA (mtDNA) do not form tumors unless they reconstitute oxidative phosphorylation (OXPHOS) by mitochondria acquired from host stroma. To understand why functional respiration is crucial for tumorigenesis, we used time-resolved analysis of tumor formation by mtDNA-depleted cells and genetic manipulations of OXPHOS. We show that pyrimidine biosynthesis dependent on respiration-linked dihydroorotate dehydrogenase (DHODH) is required to overcome cell-cycle arrest, while mitochondrial ATP generation is dispensable for tumorigenesis. Latent DHODH in mtDNA-deficient cells is fully activated with restoration of complex III/IV activity and coenzyme Q redox-cycling after mitochondrial transfer, or by introduction of an alternative oxidase. Further, deletion of DHODH interferes with tumor formation in cells with fully functional OXPHOS, while disruption of mitochondrial ATP synthase has little effect. Our results show that DHODH-driven pyrimidine biosynthesis is an essential pathway linking respiration to tumorigenesis, pointing to inhibitors of DHODH as potential anti-cancer agents.

• Keywords
• OXPHOS, cancer, coenzyme Q, dihydroorotate dehydrogenase, mitochondria, pyrimidine biosynthesis, respiration,
• MeSH
• Cell Respiration MeSH
• Dihydroorotate Dehydrogenase MeSH
• Humans MeSH
• DNA, Mitochondrial metabolism   MeSH
• Mitochondria metabolism   MeSH
• Mice, Inbred BALB C MeSH
• Mice, Inbred C57BL MeSH
• Mice MeSH
• Cell Line, Tumor MeSH
• Neoplasms metabolism   MeSH
• Oxidative Phosphorylation MeSH
• Oxidoreductases Acting on CH-CH Group Donors physiology   MeSH
• Pyrimidines metabolism   MeSH
• Ubiquinone metabolism   MeSH
• Animals MeSH
• Check Tag
• Humans MeSH
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Research Support, N.I.H., Intramural MeSH
• Names of Substances
• Dihydroorotate Dehydrogenase MeSH
• DNA, Mitochondrial MeSH
• Oxidoreductases Acting on CH-CH Group Donors MeSH
• pyrimidine MeSH Browser
• Pyrimidines MeSH
• Ubiquinone MeSH