Defective assembly of the respiratory chain.

A functional respiratory chain is dependent on protein components encoded by both mtDNA and nuclear DNA. Isolated cytochrome c oxidase (COX) deficiency is often caused by mutations in nuclear genes regulating the assembly of the 13 protein subunits of this complex. The accompanying paper by Zeman and co-workers reports that mutations in SCO2 are common in infantile COX deficiency and are associated with a very poor prognosis.

Conclusions: Molecular diagnosis is often feasible in patients with COX deficiency and particular attention should be paid to mutations in COX assembly genes.