Genotype-phenotype correlations in a group of 100 patients with typical Friedreich ataxia (FRDA), and in three smaller clinically atypical groups (Arcadian FRDA, late-onset FRDA (LOFA), and FRDA with retained reflexes (FARR)), were studied at the Centre de Recherche Louis-Charles Simard, Service de Genetique Medicale, Service de Neurologie, Hopital Sainte-Justine, Departments of Genetics and Medicine, McGill University, Montreal General Hospital, and other centers. Almost all FRDA patients carry a GAA triplet repeat expansion on chromosome 9, despite phenotypic variation. Larger expansions lead to earlier onset and more severe disease. Arcadian FRDA and FARR were unrelated to expansion sizes. [1]

COMMENT. Testing for the GAA expansion is essential for the molecular diagnosis of the various forms of FRDA, and particularly atypical cases of late-onset, those with retained reflexes, and lack of the characteristic hypertrophic cardiomyopathy.