Age-dependent loss of mitochondrial DNA integrity in mammalian muscle

MtDNA integrity is important for mitochondrial function. We analysed mtDNA over the course of mouse lifespan and found that its integrity, as measured by electrophoretic mobility under denaturing conditions, decreased significantly in old animals whereas its ribonucleotide (rNMP) content increased with age. To investigate whether the rNMPs incorporated into mtDNA were responsible for this loss of integrity in old animals, we used mice deficient in SAMHD1, a dNTPase that decreases rNTP/dNTP ratios. Knockout of SAMHD1 almost completely eliminated rNMPs from mtDNA, but did not restore mtDNA integrity in older animals, indicating that rNMP incorporation does not explain the loss of mtDNA integrity. Neither did mtDNA from older animals contain more gaps, deletions or double-strand breaks than that of younger animals. By contrast, mtDNA integrity was restored by treatment with ligase. Thus, we conclude that the loss of mtDNA integrity in old mice is caused by accumulation of nicks.