Prominent mitochondrial DNA recombination intermediates in human heart muscle

Recombination intermediates containing four-way (Holliday) junctions are ge nerated during DNA repair and replication in many systems, including yeast mitochondrial DNA (mtDNA). In contrast, convincing evidence for recombinati on in mammalian mtDNA is lacking. We have used two-dimensional agarose-gel electrophoresis to analyse non-linear forms of mtDNA in human heart muscle. Replication intermediates from both the coupled and strand-asynchronous mt DNA replication pathways were detected. An additional class of non-linear m olecules, with the electrophoretic properties of four-way junctions, was al so prominent. These molecules were insensitive to topoisomerase I or RNase H, but were diminished by branch migration or RuvC treatment. Junctional mo lecules were detected in all regions of the mitochondrial genome, were foun d in myocardial DNA from young and old adults, but were present at lower le vels in skeletal muscle and placenta. We suggest that they could represent intermediates of mtDNA repair, given their prevalence in the oxyradical-ric h environment of heart muscle mitochondria.