Increased mitophagy and mitochondrial DNA (mtDNA) turnover in defects of mtDNA maintenance associated with severe neurodegenerative disease: towards new therapies Eszter Dombi1, Natnita Mattawanon1, Tiffany Lodge1, Janet Carver1, Joanna Poulton1 1Nuffield Department of Obstetrics and Gynaecology, University of Oxford Mutations in the mitochondrial DNA polymerase (pol γ) have been associated with mitochondrial diseases that evolve over time and have an overlapping range of symptoms. The holoenzyme of DNA pol γ consists of a catalytic subunit (encoded by POLG) and a dimeric form of its accessory subunit (encoded by POLG2). Mutations in POLG may lead not only to mtDNA depletion, but also inaccuracies such as deletions and point mutations of any nucleotides in the mitochondrial DNA. We previously demonstrated excessive mitochondrial fragmentation and increased mitophagy in fibroblasts from patients with mitochondrial fission/fusion defects. Here we used high-throughput imaging to investigate fibroblasts harbouring mutations in POLG. We demonstrated increased mtDNA turnover in mouse embryonic fibroblasts (MEFs) harbouring POLG exonuclease mutations and an increased number of Rho zero cells. POLG fibroblasts derived from a patient with compound heterozygous mutations located in the exonuclease and linker domain were severely depleted of mtDNA and also had increased mitochondrial turnover. In these patient derived cells we also found low levels of OXPHOS complexes (CI, CII, and CIV ), complex V was not changed. In order to alleviate the mtDNA depletion and deregulated mitophagy we supplemented POLG mutant fibroblasts with deoxyguanosine monophosphate (dGMP) and deoxyadenosine monophosphate (dAMP) for 7 days. Mitochondrial DNA depletion is hallmark of cells with POLG mutation. We hypothesized that a combination of increased mitochondrial turnover and depletion of the nucleotide pool due to the erroneous activity of pol γ are responsible for the depletion of mitochondrial DNA. This results in a vicious circle as mDNA depletions and deletions further increase mitophagy which in turn deplete nucleotide pools even more. Supplementation of cells with dAMP and dGMP increased mtDNA content and reduced mitophagic flux. We conclude that nucleotide deficiency may exacerbate mtDNA depletion due to POLG and that novel therapies that supplement the dNTP pool may be helpful in these patients.
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