Damaged mitochondrial DNA replication system and the development of diabetic retinopathy

In the pathogenesis of diabetic retinopathy, retinal mitochondria are damaged, superoxide levels are elevated, and mitochondrial DNA (mtDNA) biogenesis is impaired. mtDNA has a noncoding region, displacement loop (D-loop), which has essential transcription and replication elements, and this region is highly vulnerable to oxidative damage. The aim of this study is to investigate the effect of diabetes on the D-loop damage and the mtDNA replication machinery.

Using retina from wild-type (WT) and mitochondrial superoxide dismutase transgenic (Tg) mice, we have investigated the effect of diabetes on retinal D-loop damage and on the replication system. The results were confirmed in the isolated retinal endothelial cells in which the DNA polymerase gamma 1 (POLG1) function was genetically manipulated. Diabetes damaged retinal mtDNA, and the damage was more at the D-loop region compared with the cytochrome B region. Gene transcripts and mitochondrial accumulation of POLG1, POLG2, and mtDNA helicase, the enzymes that form replisome to bind/unwind and extend mtDNA, were also decreased in WT-diabetic mice compared with WT-normal mice. Tg-diabetic mice were protected from diabetes-induced damage to the D-loop region. Overexpression of POLG1 prevented high glucose-induced D-loop damage. This was accompanied by a decrease in mitochondrial superoxide levels.

Integrity of the retinal D-loop region and the mtDNA replication play important roles in the mtDNA damage experienced by the retina in diabetes, and these are under the control of superoxide. Thus, the regulation of mtDNA replication/repair machinery has the potential to prevent mitochondrial dysfunction and the development of diabetic retinopathy.