A SINGLE-CELL COMPLEMENTATION CLASS IS COMMON TO SEVERAL CASES OF CYTOCHROME-C OXIDASE-DEFECTIVE LEIGHS SYNDROME

A generalized defect of complex IV (cytochrome c oxidase, COX) is freq uently found In subacute necrotizing encephalomyelopathy (Leigh's synd rome), the most common mitochondrial disorder in infancy, We previousl y demonstrated the nuclear origin of the COX defect in one case, by fu sing nuclear DNA-less cytoplasts derived from normal fibroblasts with mitochondrial DNA (mtDNA)-less transformant fibroblasts derived from a patient with COX-defective [COX((-))] Leigh's syndrome, The resulting cybrid line showed a specific and severe COX((-)) phenotype, Converse ly, in the present study, we demonstrate that a COX((+)) phenotype cou ld be restored in hybrids obtained by fusing COX((-)) transformant fib roblasts of seven additional Leigh's syndrome patients with mtDNA-less , COX((-)) tumor-derived rho degrees cells, Both these results are exp lained by the presence of a mutation in a nuclear gene. In a second se t of experiments, in order to demonstrate whether COX((-)) Leigh's syn drome is due to a defect in the same gene, or in different genes, we t ested several hybrids derived by fusing our original COX((-)) cell lin e with each of the remaining seven cell lines, COX activity was evalua ted in situ by histochemical techniques and in cell extracts by a spec trophotometric assay, No COX complementers were found among the result ing hybrid lines, This result demonstrates that all our cases were gen etically homogeneous, and suggests that a major nuclear disease locus is associated with several, perhaps most, of the cases of infantile CO X((-)) Leigh's syndrome, This information should make it easier to ide ntify the gene responsible.