Friedreich's Ataxia (FA) is an autosomal recessive neurodegenerative and cardiac disease due to a trinucleotide (GAA) repeat expansion in intron 1 of the FXN (FXN) gene. This mutation does not modify the amino acids of the FXN protein but does reduce its expression. Thus FA is representative of a typical haplo-insufficiency disease.
The level of FXN is related to the number of the trinucleotide repeats, it is 30% in patients with 200 GAA repeats and only 5% in patients with 900 GAA repeats. However, carriers of this disease produce about only 50% of the normal level of FXN but do not develop symptoms.