INTRODUCTION OF DISEASE-RELATED MITOCHONDRIAL-DNA DELETIONS INTO HELA-CELLS LACKING MITOCHONDRIAL-DNA RESULTS IN MITOCHONDRIAL DYSFUNCTION

Mutant mitochondrial DNA with large-scale deletions (DELTA-mtDNA) has been frequently observed in patients with chronic progressive external ophthalmoplegia (CPEO), a subgroup of the mitochondrial encephalomyopathies. To exclude involvement of the nuclear genome in expression of the mitochondrial dysfunction characteristic of CPEO, we introduced the mtDNA of a CPEO patient into clonal mtDNA-less HeLa cells and isolated cybrid clones. Quantitation of DELTA-MtDNA in the cybrids revealed that DELTA-mtDNA was selectively propagated with higher levels of DELTA-mtDNA correlating with slower cellular growth rate. In these cybrid clones, translational complementation of the missing tRNAs occurred only when DELTA-mtDNA was < 60% of the total mtDNA, whereas accumulation of DELTA-mtDNA to > 60% resulted in progressive inhibition of overall mitochondrial translation as well as reduction of cytochrome c oxidase activity throughout the organelle population. Because these cybrids shared the same nuclear background as HeLa cells, these results suggest that large-scale deletion mutations of mtDNA alone are sufficient for the mitochondrial dysfunction characteristic of CPEO.