An out-of-frame cytochrome b gene deletion from a patient with parkinsonism is associated with impaired complex III assembly and an increase in free radical production

We have isolated transmitochondrial cybrids containing a mitochondrial DNA cytochrome b 4-base pair deletion previously identified in a patient with p arkinsonism. This presentation is in contrast to that of most patients with cytochrome b mutations, who present with exercise intolerance. Clones cont aining different levels of the cytochrome b 4-base pair deletion showed tha t high levels of the mutation were associated with a respiratory deficiency and a specific complex III defect. Newly synthesized full-length cytochrom e b was undetectable by metabolic labeling of mutant cells, and these cells were unable to grow in media that restricts proliferation of cells with de fective oxidative phosphorylation. Steady state levels of some subunits pre viously found to be in close association with cytochrome 6 by crystallograp hy and biochemical analysis (ie, Rieske [2Fe-2S] protein and subunit VI) we re drastically reduced in clones containing high levels of the mutation, wh ereas the reduction in the core-1 subunit was milder. The absence of cytoch rome 6 and complex III activity was also associated with increased hydrogen peroxide production. These findings, together with the variable tissue dis tribution of pathogenic mitochondrial DNA molecules, provide clues to the h eterogeneous phenotypes associated with mitochondrial DNA mutations and est ablish a link between different forms of parkinsonism and oxidative phospho rylation defects.