Mammalian cytochrome c oxidase (COX) catalyses the transfer of reducing equivalents from cytochrome c to molecular oxygen and pumps protons across the inner mitochondrial membrane1. 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 described7, no mutations in the nDNA-encoded subunits have been uncovered in any mendelian-inherited COX deficiency disorder8,9,10,11,12,13. 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 syndrome14,15.
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We thank F. Guo, P. Kranz-Eberle, F. Pallotti, P. Magalhães, G. Manfredi, R. Pons and S. Tadesse for technical assistance; E. Holme, M. Huttermann, B. Kadenbach and M. Tulinius for patient samples; and A. Tzagoloff for communicating unpublished data. This work was supported by grants from the National Institutes of Health (NS28828, NS32527, NS11766, HL59657 and HD32062), the Muscular Dystrophy Association and a Neil Hamilton Fairley NHMRC Postdoctoral Fellowship (C.M.S.).
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