Frataxin is localized to the mitochondrion. The function of frataxin is not entirely clear, but it seems to be involved in assembly of iron-sulfur clusters. It has been proposed to act as either an iron chaperone or an iron storage protein.[3]
Frataxin mRNA is predominantly expressed in tissues with a high metabolicrate (including liver, kidney, brown fat and heart). Mouse and yeast frataxin homologues contain a potential N-terminal mitochondrial targeting sequence, and human frataxin has been observed to co-localise with a mitochondrial protein. Furthermore, disruption of the yeast gene has been shown to result in mitochondrial dysfunction. Friedreich's ataxia is thus believed to be a mitochondrial disease caused by a mutation in the nuclear genome (specifically, expansion of an intronic GAA triplet repeat in the FXN gene, which encodes the protein frataxin.).[1][4][5]
Clinical significance
Reduced expression of frataxin is the cause of Friedreich's ataxia (FRDA), a lethal neurodegenerative disease. The reduction in frataxin gene expression may be attributable from either the silencing of transcription of the frataxin gene because of epigenetic modifications in the chromosomal entity[6] or from the inability of splicing the expanded GAA repeats in the first intron of the pre-mRNA as seen in Bacteria[7] and Human cells[8] or both. The expansion of intronic trinucleotide repeat GAA results in Friedreich's ataxia.[9] This expanded repeat causes R-loop formation, and using a repeat-targeted oligonucleotide to disrupt the R-loop can reactivate frataxin expression.[10]
Animal studies
An overexpression of frataxin in Drosophila has shown an increase in antioxidant capability, resistance to oxidative stress insults and longevity.[11]
Fibroblasts from a mouse model of Friedreich’s ataxia and Friedreich’s ataxia patient fibroblasts show increased levels of DNA double-strand breaks.[12] A lentivirus gene delivery system was used to deliver the frataxin gene to the Friedreich’s ataxia mouse model and human patient cells, and this resulted in long-term restored expression of frataxin mRNA and frataxin protein. This restored expression of the frataxin gene was accompanied by a substantial reduction in the number of DNA double-strand breaks.[12] The impaired frataxin in Friedreich’s ataxia cells appears to cause reduced capacity for repair of DNA damage and this may contribute to neurodegeneration.[12]
Interactions
Frataxin has been shown to biologically interact with the enzyme PMPCB.[13]
References
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↑Adinolfi S, Iannuzzi C, Prischi F, Pastore C, Iametti S, Martin SR, Bonomi F, Pastore A (Apr 2009). "Bacterial frataxin CyaY is the gatekeeper of iron-sulfur cluster formation catalyzed by IscS". Nature Structural & Molecular Biology. 16 (4): 390–6. doi:10.1038/nsmb.1579. PMID19305405.
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↑Koutnikova H, Campuzano V, Foury F, Dollé P, Cazzalini O, Koenig M (Aug 1997). "Studies of human, mouse and yeast homologues indicate a mitochondrial function for frataxin". Nature Genetics. 16 (4): 345–51. doi:10.1038/ng0897-345. PMID9241270.
↑Pan X, Ding Y, Shi L (Nov 2009). "The roles of SbcCD and RNaseE in the transcription of GAA x TTC repeats in Escherichia coli". DNA Repair. 8 (11): 1321–7. doi:10.1016/j.dnarep.2009.08.001. PMID19733517.
↑Runko AP, Griswold AJ, Min KT (Mar 2008). "Overexpression of frataxin in the mitochondria increases resistance to oxidative stress and extends lifespan in Drosophila". FEBS Letters. 582 (5): 715–9. doi:10.1016/j.febslet.2008.01.046. PMID18258192.
↑Koutnikova H, Campuzano V, Koenig M (Sep 1998). "Maturation of wild-type and mutated frataxin by the mitochondrial processing peptidase". Human Molecular Genetics. 7 (9): 1485–9. doi:10.1093/hmg/7.9.1485. PMID9700204.
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