Maintenance of human rearranged mitochondrial DNAs in long-term cultured transmitochondrial cell lines

Large-scale rearrangements of mitochondrial DNA (mtDNA; i.e., partial dupli cations [dup-mtDNAs] and deletions [Delta-mtDNAs]) coexist in tissues in a subset of patients with sporadic mitochondrial disorders. In order to study the dynamic relationship among rearranged and wild-type mtDNA (wt-mtDNA) s pecies, we created transmitochondrial cell lines harboring various proporti ons of wt-, Delta-, and dup-mtDNAs from two patients. After prolonged cultu re in nonselective media, cells that contained initially 100% dup-mtDNAs be came heteroplasmic, containing both wild-type and rearranged mtDNAs, likely generated via intramolecular recombination events. However, in cells that contained initially a mixture of both wt- and Delta-mtDNAs, we did not obse rve any dup-mtDNAs or other new forms of rearranged mtDNAs, perhaps because the two species were physically separated and were therefore unable to rec ombine. The ratio of wt-mtDNA to Delta-mtDNAs remained stable in all cells examined, suggesting that there was no replicative advantage for the smalle r deleted molecules. Finally, in cells containing a mixture of monomeric an d dimeric forms of a specific Delta-mtDNA, we found that the mtDNA populati on shifted towards homoplasmic dimers, suggesting that there may be circums tances under which the cells favor molecules with multiple replication orig ins, independent of the size of the molecule.