Triplex H-DNA structure: the long and winding road from the discovery to its role in human disease.

H-DNA is an intramolecular DNA triplex formed by homopurine/homopyrimidine mirror repeats. Since its discovery, the field has advanced from characterizing the structure in vitro to discovering its existence and role in vivo. H-DNA interacts with cellular machinery in unique ways, stalling DNA and RNA polymerases and causing genome instability. The foundational S1 nuclease and chemical probing technologies originally used to show H-DNA formation have been updated and combined with genome-wide sequencing methods for large-scale mapping of secondary structures. There is evidence for triplex H-DNA's role in polycystic kidney disease (PKD), cancer, and numerous repeat expansion diseases (REDs). In PKD, an H-DNA forming repeat region within the PKD1 gene stalls DNA replication and induces fragility. H-DNA-forming repeats in various genes have a role in cancer; the most well-studied examples involve H-DNA-mediated fragility causing translocations in multiple lymphomas. Lastly, H-DNA-forming repeats have been implicated in four REDs: Friedreich's ataxia, GAA-FGF14-related ataxia, X-linked Dystonia Parkinsonism, and cerebellar ataxia, neuropathy and vestibular areflexia syndrome. In this review, we summarize H-DNA's discovery and characterization, evidence for its existence and function in vivo, and the field's current knowledge on its role in physiology and pathology.