Friedreich ataxia is not only a GAA repeats expansion disorder: implications for molecular testing and counselling

Friedreich ataxia (FRDA) is the most common hereditary ataxia. It is an autosomal recessive disorder caused by mutations of the FXN gene, mainly the biallelic expansion of the (GAA)n repeats in its first intron. Heterozygous expansion/point mutations or deletions are rare; no patients with two point mutations or a point mutation/deletion have been described, suggesting that loss of the FXN gene product, frataxin, is lethal. This is why routine FRDA molecular diagnostics is focused on (GAA)n expansion analysis. Additional tests are considered only in cases of heterozygous expansion carriers and an atypical clinical picture. Analyses of the parent's carrier status, together with diagnostic tests, are performed in rare cases, and, because of that, we may underestimate the frequency of deletions. Even though FXN deletions are characterised as 'exquisitely rare,' we were able to identify one case (2.4 %) of a (GAA)n expansion/exonic deletion in a group of 41 probands. This was a patient with very early onset of disease with rapid progression of gait instability and hypertrophic cardiomyopathy. We compared the patient's clinical data to expansion/deletion carriers available in the literature and suggest that, in clinical practice, the FXN deletion test should be taken into account in patients with early-onset, rapid progressive ataxia and severe scoliosis.