Interruptions as disease modifiers and repeat regulators

Background Friedreich ataxia (FRDA) is the most common recessive ataxia and is predominantly caused by abnormal expansion of GAA repeats in the first intron of the frataxin gene (FXN). Repeat expansion leads to transcriptional silencing and a subsequent deficiency in frataxin protein. There is an inverse correlation between GAA repeat length of the shorter allele and age of onset, whereby the greater the expansion the earlier the age at onset and more severe disease phenotype. However, GAA repeat length only accounts for 36% of the variability in the age at onset (Reetz et al., 2015). These GAA repeats are unstable and tend expand or contract. Interruption of the GAA repeat may have a stabilising effect on the repeat and lead to a less severe phenotype, as we have observed in other triplet repeat disorders. We have previously shown that large interruptions are very rare in FRDA patients and that the 3' end of the GAA repeat tract is commonly interrupted (Al-Mahdawi et al., 2018). Materials and methods We analysed 133 peripheral blood genomic DNA samples from FRDA patients by triplet repeat primed PCR (TP-PCR). PCR products underwent fragment analysis on a 3730xl DNA analyser. Samples that were deemed interrupted had an alteration in the standard TP-PCR trace. Results We show that in our cohort the GAA repeat tract is interrupted 10% more frequently at the 5' end compared to the 3'end. 56% of the patients are either interrupted at 5' or 3' end of GAA repeats and only 16% of patients have pure GAA repeats. When repeat size and age of onset were plotted in linear regression fit model, nearly 80% of the samples which didn't fit the model were potentially interrupted. The model was significant with a p-value less than 0.0001 and R2-value of 0.3533. Conclusions We confirm that sequence interruptions are present more frequently at the 5' end than the 3' end of the GAA tract. These interruptions should be considered disease modifiers and play a major role in defining the age of onset and disease severity.