Additive effect of nuclear and mitochondrial mutations in a patient with mitochondrial encephalomyopathy.

We describe the case of a woman in whom combination of a mitochondrial (MT-CYB) and a nuclear (SDHB) mutation was associated with clinical and metabolic features suggestive of a mitochondrial disorder. The mutations impaired overall energy metabolism in the patient's muscle and fibroblasts and increased cellular susceptibility to oxidative stress. To clarify the contribution of each mutation to the phenotype, mutant yeast strains were generated. A significant defect in strains carrying the Sdh2 mutation, either alone or in combination with the cytb variant, was observed. Our data suggest that the SDHB mutation was causative of the mitochondrial disorder in our patient with a possible cumulative contribution of the MT-CYB variant. To our knowledge, this is the first association of bi-genomic variants in the mtDNA and in a nuclear gene encoding a subunit of complex II.