Background:
Succinate dehydrogenase (SDH)-loss paraganglioma (PGL), a rare neuroendocrine tumor derived from autonomic paraganglia, is a model for tumorigenesis driven by metabolic derangement. SDH is an enzyme of the tricarboxylic acid (TCA) cycle that oxidizes succinate to fumarate. Accumulation of succinate in the setting of SDH loss is believed to drive PGL tumorigenesis via activation of pseudohypoxic signaling and induction of hypermethylation of histones and DNA. SDH loss may obligate PGL tumor cells to rely upon otherwise non-essential metabolic pathways for survival. Previous evidence has suggested that SDH-deficient cells rely on lactate dehydrogenase A (LDHA) for regeneration of NAD+, raising the possibility that LDH inhibition might be selectively toxic to SDH-loss cells. Likewise, SDH loss may induce dependence on pyruvate carboxylase (PCx) for uptake of extracellular pyruvate and increased aspartate synthesis. In the current work, we investigate the effects on cell viability of SDH loss in the context of inhibition of putative PGL therapeutic targets, LDHA and PCx.
Methods:
We developed an immortalized mouse embryonic fibroblast (iMEF) cell line with doxycycline-triggered conditional disruption of SDHC alleles. Using homozygous SDH loss model and a heterozygous control, we characterized the time-course of SDHC gene loss and protein loss following doxycycline treatment. We then performed lentiviral short hairpin RNA (shRNA) knockdown of LDHA and PCx in SDHC-loss and control iMEFs and assessed apoptosis induction via a fluorescence image-based caspase activity assay. Similarly, we assessed differences in cell viability between SDH-loss and control iMEF lines following exposure to known pharmacologic inhibitors of LDH.
Results:
We present quantitative apoptosis data showing that LDHA and PCx loss are synthetically lethal with SDH loss. These results are compared with new data testing differential vulnerability of SDH-loss iMEFs to small molecule inhibitors. SDH-loss PGL may be selectively vulnerable to growth inhibition by non-cytotoxic metabolic inhibitors.