Interruptions in the FXN GAA repeat tract delay age at onset of Friedreich's Ataxia

Background and Objective: Friedreich's ataxia (FRDA) is the most common inherited ataxia. It is primarily caused by the homozygous expansion of a GAA trinucleotide repeat in intron 1 of the FXN gene. GAA repeat expansion causes gene silencing and consequent deficiency of the frataxin protein leading to mitochondrial dysfunction, oxidative stress, and cell death. The GAA repeat tract may be impure and have sequence variations called interruptions. However, large interruptions, determined by abnormal MboII digestion, are very rare. We used triplet repeat primed PCR (TP PCR) assays to identify small interruptions at the 5' and 3' ends of the GAA repeat tract through alterations in the electropherogram trace signal. We subsequently examined how these interruptions modulate the disease phenotype. Methods: 101 peripheral blood genomic DNA samples from patients with FRDA were analysed by Reverse TP PCR (RTP) and Forward TP PCR (FTP) to examine the 5' and 3' ends of the FXN GAA repeat tract, respectively. The cohort was subsequently stratified based on the presence and location of interruptions. The age at disease onset was then modelled by a group-specific exponential decay. Results: 71% of our cohort had an interruption at either end of the repeat tract, with 3' interruptions being most frequent. Interruption at the 3' end of the repeat tract is associated with shorter GAA1 repeat sizes and later ages at disease onset. Our modelling revealed that a 3' interruption delays disease onset by approximately 9 years relative to those lacking 5' and 3' interruptions. Discussion and Conclusion: TP PCR can quickly and easily screen for small interruptions towards the ends of the GAA repeat tract, which are more common than large interruptions. These interruptions play a key role in modulating the disease phenotype. Interruption-specific stratification and modelling can therefore facilitate more tailored and precise patient prognoses.