Uniparental IsoDisomy: a case study on a new mechanism of Friedreich ataxia

Friedreich's Ataxia (FRDA) is the most common hereditary ataxia and is mainly caused by biallelic GAA repeat expansion in the FXN gene. Rare patients carrying FXN point mutations or intragenic deletions are reported. We describe the first FRDA patient with a chromosome 9 segmental Uniparental isoDisomy (UPiD) unmasking a homozygous FXN expansion initially undetected by TP-PCR. The child presented with a progressive proprioceptive ataxia associated with peripheral sensory neuronopathy and severe scoliosis. Whole genome sequencing (WGS) identified a maternal segmental Uniparental Isodisomy (UPiD) encompassing FXN. Short tandem repeats analysis on WGS showed a biallelic FXN expansion. The identification of a deletion in the primer-annealing region of the TP-PCR explained the initial TP-PCR failure. This is the first documented case of FRDA caused by segmental UPiD. This case highlights the complexity of the molecular diagnosis of FRDA, and emphasises the importance of integrating results from various technical diagnostic approaches.