A pathogenic 15-base pair deletion in mitochondrial DNA-encoded cytochromec oxidase subunit III results in the absence of functional cytochrome c oxidase

A 15-base pair, in-frame, deletion (9480del15) in the mitochondrial DNA (mt DNA)-encoded cytochrome c oxidase subunit III (COX III) gene was identified previously in a patient with recurrent episodes of myoglobinuria and an is olated COX deficiency. Transmitochondrial cell lines harboring 0, 97, and 1 00% of the 9480del15 deletion were created by fusing human cells lacking mt DNA (rho(o) cells) with platelet and lymphocyte fractions isolated from the patient. The COX III gene mutation resulted in a severe respiratory chain defect in all mutant cell lines. Cells homoplasmic for the mutation had no detectable COX activity or respiratory ATP synthesis, and required uridine and pyruvate supplementation for growth, a phenotype similar to rho(o) cell s. The cells with 97% mutated mtDNA exhibited severe reductions in both COX activity (6% of wild-type levels) and rates of ATP synthesis (9% of wild-t ype). The COX III polypeptide in the mutant cells, although translated at r ates similar to wild-type, had reduced stability. There was no evidence for assembly of COX I, COX II, or COX III subunits in a multisubunit complex i n cells homoplasmic for the mutation, thus indicating that there was no sta ble assembly of COX I with COX II in the absence of wild-type COX III. In c ontrast, the COX I and COX II subunits were assembled in cells with 97% mut ated mtDNA.