Introduction of disease-related mitochondrial DNA deletions into HeLa cells lacking mitochondrial DNA results in mitochondrial dysfunction.

TLDR

Large‑scale deletions of mitochondrial DNA are frequently found in patients with chronic progressive external ophthalmoplegia, a mitochondrial encephalomyopathy. The study aimed to determine whether nuclear genes contribute to the mitochondrial dysfunction seen in CPEO. This was achieved by transferring patient mtDNA into mtDNA‑free HeLa cells to generate cybrid clones. Cybrids containing higher proportions of delta‑mtDNA grew more slowly, and when delta‑mtDNA exceeded 60 % translation of mitochondrial proteins and cytochrome c oxidase activity were progressively impaired, indicating that large‑scale mtDNA deletions alone can cause the mitochondrial dysfunction characteristic of CPEO.

Abstract

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 less than 60% of the total mtDNA, whereas accumulation of delta-mtDNA to greater than 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.