Fatal infantile cardioencephalomyopathy with COX deficiency and mutations in SCO2, a COX assembly gene

Mammalian cytochrome c oxidase (COX) catalyses the transfer of reducing equivalents from cytochrome c to molecular oxygen and pumps protons across the inner mitochondrial membrane. Mitochondrial DNA (mtDNA) encodes three COX subunits (I-III) and nuclear DNA (nDNA) encodes ten. In addition, ancillary proteins are required for the correct assembly and function of COX (refs 2, 3, 4, 5, 6). Although pathogenic mutations in mtDNA-encoded COX subunits have been described, no mutations in the nDNA-encoded subunits have been uncovered in any mendelian-inherited COX deficiency disorder. In yeast, two related COX assembly genes, SCO1 and SCO2 (for synthesis of cytochrome c oxidase), enable subunits I and II to be incorporated into the holoprotein. Here we have identified mutations in the human homologue, SCO2, in three unrelated infants with a newly recognized fatal cardioencephalomyopathy and COX deficiency. Immunohistochemical studies implied that the enzymatic deficiency, which was most severe in cardiac and skeletal muscle, was due to the loss of mtDNA-encoded COX subunits. The clinical phenotype caused by mutations in human SCO2 differs from that caused by mutations in SURF1, the only other known COX assembly gene associated with a human disease, Leigh syndrome.

Oligomycin induces a decrease in the cellular content of a pathogenic mutation in the human mitochondrial ATPase 6 gene

A T --> G mutation at position 8993 in human mitochondrial DNA is associated with the syndrome neuropathy, ataxia, and retinitis pigmentosa and with a maternally inherited form of Leigh's syndrome. The mutation substitutes an arginine for a leucine at amino acid position 156 in ATPase 6, a component of the F0 portion of the mitochondrial ATP synthase complex. Fibroblasts harboring high levels of the T8993G mutation have decreased ATP synthesis activity, but do not display any growth defect under standard culture conditions. Combining the notions that cells with respiratory chain defects grow poorly in medium containing galactose as the major carbon source, and that resistance to oligomycin, a mitochondrial inhibitor, is associated with mutations in the ATPase 6 gene in the same transmembrane domain where the T8993G amino acid substitution is located, we created selective culture conditions using galactose and oligomycin that elicited a pathological phenotype in T8993G cells and that allowed for the rapid selection of wild-type over T8993G mutant cells. We then generated cytoplasmic hybrid clones containing heteroplasmic levels of the T8993G mutation, and showed that selection in galactose-oligomycin caused a significant increase in the fraction of wild-type molecules (from 16 to 28%) in these cells.

Isolation of a cDNA specifying subunit VIIb of human cytochrome c oxidase

Human cytochrome c oxidase (COX) is a complex of 13 subunits: three are encoded by mitochondrial DNA and ten by nuclear DNA. We have now isolated a full-length cDNA specifying subunit VIIb, the last remaining uncharacterized nuclear-encoded subunit cDNA of human COX. The cDNA encodes a deduced 80-aa polypeptide, including a 24-amino acid (aa) N-terminal leader sequence and a 56-aa mature polypeptide with 82% identity to mature bovine COX VIIb. Southern blot hybridization of human muscle genomic DNA showed multiple hybridizing bands, implying the presence of a large coxVIIb gene family, including a potential processed pseudogene.