Frataxin deficiency and mitochondrial dysfunction.

Friedreich ataxia (FA) is an inherited recessive disorder characterized by progressive neurological disability and heart abnormalities. The Friedreich ataxia gene (FRDA) encodes a small mitochondrial protein, frataxin, which is produced in insufficient amounts in the disease as a consequence of a GAA triplet repeat expansion in the first intron of the gene. Frataxin deficiency leads to excessive free radical production, dysfunction of Fe-S center containing enzymes (in particular respiratory complexes I, II and III, and aconitase), and progressive iron accumulation in mitochondria. Frataxin may be a mitochondrial iron-binding protein that prevents this metal from participating in Fenton chemistry to generate toxic hydroxyl radicals. We investigated whether frataxin deficiency may in addition interfere with signaling pathways. First, we showed that exposure of FA fibroblasts to iron fails to produce the normally observed increase in expression of the stress defense protein manganese superoxide dismutase. This impaired induction involves a nuclear factor-kappaB-independent pathway that does not require free radical signaling intermediates. We also examined the role of frataxin in neuronal differentiation by using stably transfected clones of P19 embryonic carcinoma cells with antisense or sense frataxin constructs. We found that during retinoic acid-induced neurogenesis frataxin deficiency enhances apoptosis and reduces the number of terminally differentiated neuronal-like cells. The addition of the antioxidant N-acetyl-cysteine only rescues cells non-committed to the neuronal lineage, indicating that frataxin deficiency impairs differentiation mechanisms and survival responses through different mechanisms. Both studies suggest that some abnormalities in frataxin-deficient cells are related to free radical independent signaling pathways.