Graphical Abstract Highlights d Tumorigenesis depends on functional OXPHOS d OXPHOS-derived ATP is not required for tumor formation d DHODH-driven pyrimidine biosynthesis requires CoQ redoxcycling d CoQ redox-cycling via OXPHOS drives tumorigenesis through pyrimidine biosynthesis

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 CIIlow, 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 CIIlow 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 CIIlow is a core complex inside mitochondria that provides homeostatic control of cellular metabolism depending on the availability of energy.

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 subjects 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 eight germline variants in the GTP-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 CII and faulty assembly of the complex, 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.

Pheochromocytomas and paragangliomas (PPGLs) are rare neuroendocrine tumors derived from neural crest cells that are frequently linked to mutations including those in Krebs cycle enzymes, particularly succinate dehydrogenase (SDH). Succinyl-CoA ligase (SUCL) catalyzes reversible conversion of succinyl-CoA to succinate providing the substrate for SDH. While mitochondrial diseases were documented for the mutations in SUCL subunits G1 and A2, the association of GDP/GTP-specific subunit SUCLG2 mutations with specific pathologies including cancer have not been reported. In our study, 352 patients with apparently sporadic PPGLs underwent genetic testing using a panel of 54 genes developed at the National Institutes of Health. Additionally, human pheochromocytoma (hPheo1) cells were used for gene manipulation to produce SUCLG2 knock-out (KO). Tumor tissues and hPheo1 SUCLG2 KO cells were used for further analysis focusing on mechanism of germline variants effect on mitochondrial functions. We detected eight germline SUCLG2 mutations in 15 patients which represents 4.3% of the cohort. Germline variants together with LOH led to decreased levels of SDH subunit B resulting in aberrant respiration and accumulation of succinate, well recognized oncometabolite. Manipulation of SUCLG2 in hPheo1 cells confirmed decrease in SDHB leading to faulty assembly of mitochondrial complex II and alteration of its respiration and activity. In summary, our study identified an association between SUCLG2 and PPGL. Larger scale sequencing and uncovering additional cases bearing SUCLG2 variants will further clarify the relationship between SUCLG2 and SDHx, particularly SDHB, as well as their role in disease etiology.