Succinate dehydrogenase and hereditary paraganglioma syndromes: linking Krebs cycle dysfunction to cancer
Kolling Institute University of Sydney and Department of Endocrinology, Royal North Shore Hospital.
Succinate dehydrogenase (SDH) is located on the inner mitochondrial membrane and functions in the mitochondrial respiratory chain and the Krebs cycle. In the respiratory chain, SDH transports electrons to the ubiquinone pool, then to cytochrome c of complex III. In the Krebs cycle, SDH catalyses conversion of succinate to fumarate. Two predictable consequences therefore of SDH inactivation are succinate accumulation, and increased production of reactive oxygen species. Both outcomes have been suggested to contribute to cellular accumulation of hypoxia-inducible factors (HIFs) and tumours associated with SDH deficiency display notable upregulation of hypoxia-responsive genes. Mitochondrial dysfunction due to mutations in genes encoding the subunits of SDH (SDHA-D) leads to adrenal phaeochromocytomas (PCs), sympathetic and parasympathetic paragangliomas (PGLs), renal cell carcinomas (RCCs), gastrointestinal stromal tumours (GISTs), and pituitary tumours. SDHB mutations in particular are associated with metastatic PC/PGLs. We have developed several orthogonal models to test genotype-phenotype correlations of SDHB variants, including immunohistochemistry and metabolomic assays of tumour samples, structural modeling, and in vitro localization and enzymatic assays. Loss of SDHB staining in tumours is a reliable marker for mutations in any of the SDH subunit genes. This is corroborated by measurement of succinate in tumour samples by LC/MS-MS: high succinate measurement relative to fumarate in the tumors represents a direct link to functional aspects associated with SDH-deficiency. Elevated succinate:fumarate ratios are a consistent biomolecular phenotype of SDH-deficient tumors including PC/PGLs, GISTs and RCCs. A homology model for human SDH was developed from a crystallographic structure. Structural modelling showed that many mutations within SDHB are predicted to disrupt the electron path. In vitro assessment by immunoprecipation from transfected cells demonstrated that most SDHB mutations result in impaired mitochondrial localisation and/or SDH enzymatic activity. In conclusion, studying SDH mutations represent fertile ground for understanding the association between Krebs cycle dysfunction and cancer.