9377 Exploring Strategies To Reverse Cellular Characteristics Of Succinate Dehydrogenase B-loss Immortalized Chromaffin Cells

Abstract

Abstract Disclosure: L. Rahimi: None. F. Al-Khazal: None. N. Becker: None. L. Maher, III: None. Carriers of loss-of-function alleles in genes encoding the four succinate dehydrogenase (SDH) subunits exhibit a heightened risk for pheochromocytoma/paraganglioma (PPGL), a rare neuroendocrine tumor of chromaffin cells. Tumorigenesis begins with the somatic loss of the remaining functional gene copy, resulting in a cell devoid of functional SDH. Compared to wild-type immortalized chromaffin cells (WT imCCs), SDHB-KO immortalized chromaffin cells (SDHB-KO imCCs) are larger, grow more slowly, display deformed mitochondria, show strikingly heightened succinate accumulation, and altered metabolism. Among several mechanisms proposed to link SDH deficiency with PPGL tumorigenesis, the accumulation of succinate as an oncometabolite is a leading hypothesis. We describe two attempts to suppress succinate accumulation in SDHB-KO imCCs. First, riboflavin has previously been shown to enhance residual SDHA flavinylation and enzymatic function, possibly reducing succinate levels by supporting conversion of accumulated succinate to fumarate in the absence of SDHB. We tested this by treating WT and SDHB-KO imCCs with 5 µM riboflavin for 2 weeks. Metabolomic analysis did not show decreased succinate, suggesting that enzyme activity of any remaining SDHA subunit was not rescued by increased flavinylation. Second, we envisioned a gene therapy approach to replace defective SDHB with a rescuing transgene. SDHB-KO imCCs received DNA encoding SDHB or SDHC cDNAs via transposon technology, confirmed by PCR and analyzed by western blot analyses. These studies showed that whereas WT imCCs successfully expressed processed transgene products in mitochondria, SDHB-KO imCCs could not process or transport rescuing SDH subunits into mitochondria. This shows that transposed SDHB-KO imCCs cannot properly express a rescuing SDHB subunit and do not revert to a wild-type phenotype. It is likely that the severely damaged mitochondria of SDHB-KO imCCs are incompetent to process and import rescuing SDHB subunits, so unprocessed protein accumulates without effect. This defect may lie in the mitochondrial targeting peptide recognition system because the targeting peptide of rescuing SDHB protein is not removed in SDHB-KO imCCs as judged by western blotting. In summary, we show the failure of two conceptually simple tactics to suppress succinate accumulation in SDHB-KO imCCs with highly defective mitochondria: rescue of SDHA by supporting flavinylation, or rescue of SDHB with a replacement gene copy. Further research may allow screening for small molecules that suppress succinate accumulation by other mechanisms. It remains to be determined if blocking succinate accumulation would be therapeutic in SDH-loss PPGL tumors. Presentation: 6/1/2024