Rearrangements of human mitochondrial DNA (mtDNA): New insights into the regulation of mtDNA copy number and gene expression

Mitochondria from patients with Kearns-Sayre syndrome harboring large-scale rearrangements of human mitochondrial DNA (mtDNA; both partial deletions a nd a partial duplication) were introduced into human cells lacking endogeno us mtDNA. Cytoplasmic hybrids containing 100% wild-type mtDNA, 100% mtDNA w ith partial duplications, and 100% mtDNA with partial deletions were isolat ed and characterized. The cell Lines with 100% deleted mtDNAs exhibited a c omplete impairment of respiratory chain function and oxidative phosphorylat ion. Ln contrast, there were no detectable respiratory chain or protein syn thesis defects in the cell lines with 100% duplicated mtDNAs. Unexpectedly, the mass of mtDNA was identical in all cell lines, despite the fact that d ifferent Lines contained mtDNAs of vastly different sizes and with differen t numbers of replication origins, suggesting that mtDNA copy number may be regulated by tightly controlled mitochondrial dNTP pools. In addition, quan titation of mtDNA-encoded RNAs and polypeptides in these lines provided evi dence that mtDNA gene copy number affects gene expression, which, in turn, is regulated at both the post-transcriptional and translational levels.