Abstract
Oxidative phosphorylation (OXPHOS) dysfunction has been implicated in several human diseases. It is caused by the mutations that affect the function of electron transport chain enzyme complexes and/or ATP production. We have studied the potential consequences of the mtDNA mutations and mtDNA damage on OXPHOS function in a human colorectal cancer cell line, V425 and in a mouse model of mitochondrial myopathy. We found that the nonsense and nearly homoplasmic mutations in the catalytic subunits of the OXPHOS enzymes did not significantly alter the OXPHOS function in V425 cancer cells. However, the deleterious effects of the mutations on OXPHOS function were evident when the mitochondria from the V425 cells were transferred to an osteosarcoma nuclear background. We found that the V425 cancer cells had adapted to maintain an efficient OXPHOS function by upregulating the steady-state levels of several mitochondrial respiratory chain proteins and the transcriptional coactivator genes of the PGC-1 family that in turn regulate the expression of nuclear genes involved in mitochondrial biogenesis. We also studied the potential consequences of the mtDNA damage in the skeletal muscle of PstI-transgenic mice. We found that the mtDNA double-strand breaks induced by the expression of a restriction endonuclease in the skeletal muscle mitochondria were associated with the loss of functional mtDNA molecules leading to mtDNA depletion and the formation of large-scale mtDNA deletions. The molecular features of the deletions were similar to those found in humans with multiple mtDNA deletions suggesting that the double-strand breaks in mammalian mtDNA mediate the formation of large-scale deletions. We also devised a strategy to modulate mtDNA heteroplasmy. Since most human mitochondrial diseases are heteroplasmic and are manifested when the percentage of mutant mtDNA exceeds a critical threshold, a slight shift in the heteroplasmy could be useful in disease prevention. We showed that a mitochondrially-targeted restriction endonuclease has the ability to selectively degrade mtDNA haplotypes that harbor the restriction enzyme sites. The therapeutic potential of this approach applies to a subgroup of mitochondrial diseases where the pathogenic mtDNA mutation creates a novel restriction enzyme site which is absent from the wild-type mtDNA haplotype.