Human Molecular Genetics, 2001, Vol. 10, No. 26 3025-3035
© 2001 Oxford University Press
Cytochrome c oxidase deficiency due to mutations in SCO2, encoding a mitochondrial copper-binding protein, is rescued by copper in human myoblasts
1Metabolic Disease Centre Munich-Schwabing and 2Institute of Clinical Chemistry, Molecular Diagnostics and Mitochondrial Genetics, Koelner Platz 1, 80804 Munich, Germany, 3Institute of Genetics, TU Dresden, Germany, 4Friedrich-Baur-Institute, Department of Neurology and Gene Center, Ludwig-Maximilians-University, Munich, Germany, 5JFK Institute, Glostrup, Denmark, 6Institute of Pathology, LMU Munich, Germany, 7Childrens Hospital, TU, Munich, Germany, 8Institute of Analytical Chemistry, TU Dresden, Germany and 9Montreal Neurological Institute and Department of Human Genetics, McGill University, Montreal, Quebec, Canada
Mutations in SCO2, a cytochrome c oxidase (COX) assembly gene, have been reported in nine infants with early onset fatal cardioencephalomyopathy and a severe COX deficiency in striated muscle. Studies on a yeast homolog have suggested that human Sco2 acts as a copper chaperone, transporting copper to the CuA site on the Cox II subunit, but the mechanism of action remains unclear. To investigate the molecular basis of pathogenesis of Sco2 defects in humans we performed genetic and biochemical studies on tissues, myoblasts and fibroblasts from affected patients, as well as on a recombinant human C-terminal Sco2 segment (22 kDa), bearing the putative CxxxC metal-binding motif. Recombinant Sco2 was shown to bind copper with a 1:1 stoichiometry and to form homomeric complexes in vitro, independent of the metal-binding motif. Immunohistochemistry using antibodies directed against different COX subunits showed a marked tissue-specific decrease in the Cox II/III subunits that form part of the catalytic core, consistent with the differential tissue involvement, but a more uniform distribution of Cox Vab, a nuclear-encoded subunit. Sco2 was severely reduced in patient fibroblasts and myoblasts by immunoblot analysis. Patient fibroblasts showed increased 64Cu uptake but normal retention values and, consistent with this, the copper concentration was four times higher in Sco2-deficient myoblasts than in controls. COX activity in patient myoblasts was completely rescued by transduction with a retroviral vector expressing the human SCO2 coding sequence, and more interestingly by addition of copper–histidine (300 µM) to the culture medium. Whether the latter is accomplished by the very low residual levels of Sco2 in the patient cells, direct addition of copper to the CuA site, or by another copper-binding protein remains unknown. Whatever the mechanism, this result suggests a possible therapy for the early treatment of this fatal infantile disease.
+ To whom correspondence should be addressed at: Metabolic Disease Center Munich-Schwabing, Koelner Platz 1, 80804 Munich, Germany. Tel: +49 89 3068 2670; Fax: +49 89 3068 3911; Email: michaela.jaksch@lrz.uni-muenchen.de
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