Heterozygous germline mutations in SDHA, SDHB, SDHC or SDHD, which are the 4 subunits in the mitochondrial protein succinate dehydrogenase, lead to hereditary paraganglioma-pheochromocytoma syndromes.  Is the mechanism for how this occurs understood?  My naïve first thought is that a mitochondrial defect would be particularly BAD for fast-growing cells such as tumor cells.