Selected missense mutations impair frataxin processing in Friedreich ataxia

Objective: Frataxin (FXN) is a highly conserved mitochondrial protein. Reduced FXN levels cause Friedreich ataxia, a recessive neurodegenerative disease. Typical patients carry GAA repeat expansions on both alleles, while a subgroup of patients carry a missense mutation on one allele and a GAA repeat expansion on the other. Here, we report that selected disease-related FXN missense mutations impair FXN localization, interaction with mitochondria processing peptidase, and processing. Methods: Immunocytochemical studies and subcellular fractionation were performed to study FXN import into the mitochondria and examine the mechanism by which mutations impair FXN processing. Coimmunoprecipitation was performed to study the interaction between FXN and mitochondrial processing peptidase. A proteasome inhibitor was used to model traditional therapeutic strategies. In addition, clinical profiles of subjects with and without point mutations were compared in a large natural history study. Results: FXN^{I 154F} and FXN^{G 130V} missense mutations decrease FXN^81–210 levels compared with FXN^WT, FXN^{R 165C}, and FXN^{W 155R}, but do not block its association with mitochondria. FXN^{I 154F} and FXN^{G 130V} also impair FXN maturation and enhance the binding between FXN^42–210 and mitochondria processing peptidase. Furthermore, blocking proteosomal degradation does not increase FXN^81–210 levels. Additionally, impaired FXN processing also occurs in fibroblasts from patients with FXN^{G 130V}. Finally, clinical data from patients with FXN^{G 130V} and FXN^{I 154F} mutations demonstrates a lower severity compared with other individuals with Friedreich ataxia. Interpretation: These data suggest that the effects on processing associated with FXN^{G 130V} and FXN^{I 154F} mutations lead to higher levels of partially processed FXN, which may contribute to the milder clinical phenotypes in these patients.