Nuclear DNA origin of cytochrome c oxidase deficiency in Leigh's syndrome: genetic evidence based on patient's-derived rho{degrees} transformants

Defects of the respiratory chain carrying out oxidative phosphorylation(OXPHOS) are the biochemical hallmark of human mitochondrialdisorders. Faulty OXPHOS can be due to mutations in either nuclearor mitochondrial genes, that are involved in the synthesis ofindividual respiratory subunits or in their post-translationalcontrol. The most common mitochondrial disorder of infancy andchildhood is Leigh's syndrome, a severe encephalopathy, oftenassociated with a defect of cytochrome c oxidase (COX). In orderto demonstrate which genome is primarily involved in COX-deficient(COX(^–))-Leigh's syndrome, we generated two lines of transmitochondrialcybrids. The first was obtained by fusing nuclear DNA-less cytoplastsderived from normal fibroblasts, with mitochondrial DNA-less(rho°) transformant fibroblasts derived from a patient withCOX(^–)-Leigh's syndrome. The second cybrid line was obtainedby fusing rho° cells derived from 143B.TK^– human osteosarcomacells, with cytoplasts derived from the same patient. The firstcybrid line showed a specific and severe COX(-) phenotype, whilein the second all the respiratory chain complexes, includingCOX, were normal. These results indicate that the COX defectin our patient is due to a mutation of a nuclear gene. The useof cybrids obtained from ‘customized’, patient-derivedrho° cells can have wide applications in the identificationof respiratory chain defects originated by nuclear DNA—encodedmutations, and in the study of nuclear DNA-mitochondrial DNAinteractions.