Cytochrome c Oxidase (COX)-deficiency is associated with nuclear DNA (nDNA) damage and replicative stress

Mitochondrial cytochrome c oxidase (COX) contributes to ATP production via oxidative phosphorylation (OXPHOS). The mammalian COX is a dimeric multi-protein complex composed of 14 subunits. The three mitochondrial-encoded subunits comprise the catalytic core, while the remaining subunits, encoded by the nDNA perform regulatory functions. Isolated COX-deficiency is a mitochondrial disease, which is characterized by clinical heterogeneity that affecting numerous organs including brain, heart, muscle, etc. Mutations in more than 30 genes, in both mitochondrial and nDNA affecting either structural subunits of the enzyme or proteins involved in its biogenesis are associated with human disease. Among these, is the K101N mutation in the common isoform of COX4 (COX4-1), we recently reported. The relatively mild presentation could be explained by upregulation of the COX4-2 isoform, while the Fanconi anemia-like features, suggested genomic instability. This was confirmed in the patient’s fibroblasts by detecting increased frequency of double-stranded DNA breaks (DSB) in the nDNA, by both γH2AX-staining and neutral comet assay. DSB was also present in potassium cyanide (KCN)-treated control fibroblasts and in COX4-1 shRNA cell line grown in high-glucose medium where ATP was normal and without evidence of oxidative stress. In contrast, DSB was not detected when cell growth was impaired (glucose-free or serum-free medium). Interestingly, nicotinamide riboside (NR) ameliorated DSB in the COX mutated fibroblasts while polymerase-(PARP) inhibitor had a marked negative effetc. Our findings raise the possibility that the pathomechanism of COX-deficiency is explained not only by the decreased OXPHOS activity but also by genomic instability, while independent of oxidative stress.