Introduction: Instability expressed as the propensity to expand or contract is a major feature of long, pathogenic tandem repeat sequences. Expansion of these sequences is a cause of more than 50 human diseases, including Friedreich’s ataxia (FRDA). Large, biallelic expansion of intronic GAA repeats results in decreased transcription of the frataxin (FXN) gene leading to deficiency of frataxin protein and progressive multi-organ deterioration. While length of the shorter of the two expanded GAA tracts (GAA1) corelates well with disease onset and progression, the high degree of variability observed between FRDA patients cannot be solely explained by the size of GAAs established at diagnosis. We postulate that tissue-specific somatic instability affects clinical presentation.
Methods: Using CRISPR-Cas9, we generated several FRDA induced pluripotent stem cell (iPSC) lines to determine the role of active transcription, proximity of DNA replication origins, level of frataxin as well as length, chromosomal context and repeat orientation on the propensity of GAA sequences to expand. GAA repeat expansion assays were utilized to monitor size changes over an extended period of time.
Results: Transcription, tract length and orientation of the repeats relative to transcription direction are the major factors affecting GAA repeat expansion.
Conclusions: Considering potential long-term treatment of FRDA patients, the effect of therapy, especially those affecting processes occurring at the endogenous FXN gene, on GAA expansion should be incorporated in clinical trial design and patient monitoring.