Friedreich's ataxia-associated GAA repeats induce replication-fork reversal and unusual molecular junctions.

Expansion of GAA/TTC repeats is the causative event in Friedreich's ataxia. GAA repeats have been shown to hinder replication in model systems, but the mechanisms of replication interference and expansion in human cells remained elusive. To study in vivo replication structures at GAA repeats, we designed a new plasmid-based system that permits the analysis of human replication intermediates by two-dimensional gel electrophoresis and EM. We found that replication forks transiently pause and reverse at long GAA/TTC tracts in both orientations. Furthermore, we identified replication-associated intramolecular junctions, located between GAA/TTC repeats and other homopurine-homopyrimidine tracts, that were associated with breakage of the plasmid fork not traversing the repeats. Finally, we detected postreplicative, sister-chromatid hemicatenanes on control plasmids, which were replaced by persistent homology-driven junctions at GAA/TTC repeats. These data prove that GAA/TTC tracts interfere with replication in humans and implicate postreplicative mechanisms in trinucleotide repeat expansion.