Pitfalls in molecular diagnosis of Friedreich ataxia

We report on an initial pitfall in the molecular characterization of a 15 year-old girl with Freidreich ataxia (FRDA) who carried a rare deletion in intron 1 of the FXN gene. Due to this deletion TP-PCR failed to amplify the GAA expansion. This exceptional configuration induced misinterpretation of the molecular defect in this patient, who was first reported as having no FXN expansion. NGS analysis of a panel of 212 genes involved in nuclear mitochondrial disorders further revealed an intragenic deletion encompassing exons 4-5 of the FXN gene. Modified TP-PCR analysis confirmed the presence of a classical (GAA)n expansion located in trans. This case points out the possible pitfalls in molecular diagnosis of FRDA in affected patients and their relatives: detection of the FXN expansion may be impaired by several non-pathological or pathological variants around the FXN (GAA)n repeat. We propose a new molecular strategy to accurately detect expansion by TP-PCR in FRDA patients.