Mitochondrial topoisomerases, nucleoid architecture and mtDNA repair in human disease

DNA topoisomerases are essential for maintaining DNA topology, gene expression and the accurate transmission of genetic information. Mitochondria possess circular DNA (mtDNA), which, unlike nuclear chromosomes, lacks protective histones and exists in nucleoprotein complexes called nucleoids, which are vital for mtDNA stability. Although the mitochondrial genome encodes essential genes involved in ATP production via oxidative phosphorylation, it does not encode crucial mtDNA maintenance genes and depends entirely on nuclear-encoded proteins for mtDNA maintenance. These include nuclear-encoded topoisomerases (i.e. Top1mt, Top2α, Top2β and Top3α), which alleviate topological stress during mtDNA transcription and replication, and mitochondrial transcription factor A (TFAM), are crucial for ensuring proper nucleoid structure and mtDNA packaging. Furthermore, tyrosyl-DNA phosphodiesterase 1 and 2 (TDP1 and TDP2) participate in the repair of mtDNA damage associated with trapped topoisomerase-mtDNA complexes, which can compromise mtDNA integrity and contribute to neurodegeneration, cancer and premature aging. Drugs that stabilize these protein-DNA adducts (PDAs) to induce mtDNA damage and mitochondrial dysfunction are promising new strategies for cancer therapy. This Review explores the essential roles of mitochondrial topoisomerases, overviews mechanisms involved in mtDNA repair and discusses how mitochondrial fission and mitophagy are employed as a survival strategy for clearing damaged mtDNA.