The accumulation of H(2)S over time causes progressive COX deficiency in animal tissues and human cells, which is associated with reduced amount of COX holoenzyme, and of several COX subunits, including mitochondrially encoded cytochrome c oxidase 1 (MTCO1), MTCO2, COX4, and COX5A.
Our report of two patients with isolated COX deficiency and new mutations in COX subunit genes may help to draw more attention to this type of mtDNA defects and provide new aspects for counselling affected families.