Mitochondrial DNA deletion detection from human tissues

Accumulation of deletions in the mitochondrial genome is a pathological hallmark of both aging and a broad range of neurometabolic disorders. Prior mtDNA deletion studies have been limited by reliance upon either long range PCR or NGS strategies that target specific regions of the mitochondrial genome. We developed an ultrasensitive high throughput sequencing process (LostArc) to identify and quantify all deletions in circular DNA molecules while minimizing interference from linear DNA and PCR artefacts. We applied this process to skeletal muscle biopsies from 19 unaffected individuals (aged 17 to 93 years) and from 22 patients with heritable mitochondrial disease (17 to 80 years) bearing pathogenic variants in POLG, the nuclear gene encoding the catalytic subunit of the mitochondrial replicative DNA polymerase gamma (Pol gamma). To our astonishment, LostArc revealed 35 million mtDNA deletions with ~470,000 unique deletion spans. This unprecedented volume of data enabled detailed and highly informative bioinformatics analyses. We show that ablation of mtDNA is sufficient to explain skeletal muscle phenotypes associated with aging and POLG-derived disease, with a clear threshold separating symptomatic and asymptomatic individuals. Unsupervised hierarchical clustering and principle component analysis reveal distinct age- and disease-correlated deletion patterns that implicate DNA replication by Pol gamma in the formation of mtDNA deletions. Deletion patterns also point to active mtDNA replication but little or no purifying selection against deleted mtDNA by mitophagy in postmitotic muscle fibers. Monte Carlo simulations of varied hypotheses for deletion formation, based in part on competing models of mtDNA replication, suggest the observed patterns are most consistent with deletions initiated by replication fork stalling during strand displacement mtDNA synthesis.