In plants as well as in a number of micro-organisms, and in several animal phyla, but not in mammals, the
alternative oxidase (AOX) possibly by-passes the
cytochrome segment of the respiratory chain. The AOX is located at the inner surface of the inner mitochondrial membrane, being activated by over-reduction of the
quinone pool and accumulation of
keto-acids such as
pyruvate. Since these conditions are frequently encountered in patients with
mitochondrial diseases, we hypothesized that the expression of the ascidian Ciona intestinalis AOX might alleviate the consequences of a blockade of the
cytochrome segment of the respiratory chain in human cells. We previously expressed the C. intestinalis AOX in human embryonic kidney (HEK 293-T-derived) cells conferring
cyanide-resistance to cell respiration without any detectable detrimental effect (Hakkaart et al. 2006). We have now expressed the AOX in human cultured fibroblasts either with a functional respiratory chain (foreskin immortalized fibroblasts, BJ1-htert) or presenting a
cytochrome c oxidase deficiency resulting from an impaired
heme aa3 biogenesis. We used immortalized COX15-deficient skin fibroblasts from a patient who died from an early fatal
cardiomyopathy. We found that the expression of the AOX in these cells was well tolerated and corrected for the various consequences of the
cytochrome c oxidase deficiency in COX15-mutant cells, i.e. decreased cell respiration,
glucose and
pyruvate dependency. We thus validated our hypothesis that AOX could compensate for
cytochrome c oxidase deficiency in human cells.