FXN
Available structures
PDBOrtholog search: PDBe RCSB
Identifiers
AliasesFXN, CyaY, FA, FARR, FRDA, X25, frataxin
External IDsOMIM: 606829 MGI: 1096879 HomoloGene: 47908 GeneCards: FXN
Orthologs
SpeciesHumanMouse
Entrez

2395

14297

Ensembl

ENSG00000165060

ENSMUSG00000059363

UniProt

Q16595

O35943

RefSeq (mRNA)

NM_181425
NM_000144
NM_001161706

NM_008044

RefSeq (protein)

NP_000135
NP_852090

NP_032070

Location (UCSC)Chr 9: 69.04 – 69.08 MbChr 19: 24.24 – 24.26 Mb
PubMed search[3][4]
Wikidata
View/Edit HumanView/Edit Mouse

Frataxin is a protein that in humans is encoded by the FXN gene.[5][6]

It is located in the mitochondrion and Frataxin mRNA is mostly expressed in tissues with a high metabolic rate. The function of frataxin is not clear but it is involved in assembly of iron-sulfur clusters. It has been proposed to act as either an iron chaperone or an iron storage protein. Reduced expression of frataxin is the cause of Friedreich's ataxia.

Structure

X-ray crystallography has shown that human frataxin consists of a β-sheet that supports a pair of parallel α-helices, forming a compact αβ sandwich.[7] Frataxin homologues in other species are similar, sharing the same core structure. However, the frataxin tail sequences, extending from the end of one helix, diverge in sequence and differ in length. Human frataxin has a longer tail sequence than frataxin found in bacteria or yeast. It is hypothesized that the purpose of the tail is to stabilize the protein.[7]

Like most mitochondrial proteins, frataxin is synthesized in cytoplasmic ribosomes as large precursor molecules with mitochondrial targeting sequences. Upon entry into mitochondria, the molecules are broken down by a proteolytic reaction to yield mature frataxin.[8]

Function

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.[9]

Frataxin mRNA is predominantly expressed in tissues with a high metabolic rate (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.).[5][10][11]

Clinical significance

Reduced expression of frataxin is the cause of Friedreich's ataxia (FRDA), a 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[12] or from the inability of splicing the expanded GAA repeats in the first intron of the pre-mRNA as seen in bacteria[13] and Human cells[14] or both. The expansion of intronic trinucleotide repeat GAA results in Friedreich's ataxia.[15] This expanded repeat causes R-loop formation, and using a repeat-targeted oligonucleotide to disrupt the R-loop can reactivate frataxin expression.[16]

96% of FRDA patients have a GAA trinucleotide repeat expansion in intron 1 of both alleles of their FXN gene.[17] Overall, this leads to a decrease in frataxin mRNA synthesis and a decrease (but not absence) in frataxin protein in people with FRDA. (A subset of FRDA patients have GAA expansion in one chromosome and a point mutation in the FXN exon in the other chromosome.) In the typical case, the length of the allele with the shorter GAA expansion inversely correlates with frataxin levels. FRDA patients’ peripheral tissues typically have less than 10% of the frataxin levels exhibited by unaffected people.[17] Lower levels of frataxin result in earlier disease onset and faster progression.

FRDA is characterized by ataxia, sensory loss, and cardiomyopathy. The reason frataxin deficiency causes these symptoms is not entirely clear. On a cellular level, it is linked to iron accumulation in the mitochondria and increased oxidant sensitivity. For reasons that are not well understood, this primarily affects the tissue of the dorsal root ganglia, cerebellum, and heart muscle.[8]

Animal studies

In mice, complete inactivation of the FXN homolog (Frda) is lethal in the early embryonic stage.[18] Although nearly all organisms express a frataxin homologue, the GAA repeat in intron 1 only exists in humans and other primates, so the mutation that causes FDRA can't occur naturally in other animals. Scientists have developed several options to model this disease in mice. One approach is to silence frataxin expression in just one specific tissue type of interest: the heart (mice modified this way are called MCK), all neurons (NSE), or just the spinal cord and cerebellum (PRP).[19] Another approach involves inserting a GAA expansion into the first intron of the mouse FXN gene, which should inhibit frataxin production, just like in humans. Mice that are homozygous for this modified gene are called KIKI (knock-in knock-in), and the compound heterozygotes formed by crossing KIKI mice with frataxin knockout mice are called KIKO (knock-in knock-out). However, even KIKO mice still express 25-36% of the normal frataxin level, and show very mild symptoms. The final approach involves creating transgenic mice with a GAA-expanded version of the human frataxin gene. These mice are called YG22R (one GAA sequence of 190 repeats) and YG22R (two GAA sequences of 90 and 190 repeats). These mice show symptoms similar to human patients.[19]

An overexpression of frataxin in Drosophila has shown an increase in antioxidant capability, resistance to oxidative stress insults and longevity,[20] supporting the theory that the role of frataxin is to protect the mitochondria from oxidative stress and the ensuing cellular damage.

Fibroblasts from a mouse model of FRDA and FRDA patient fibroblasts show increased levels of DNA double-strand breaks.[21] A lentivirus gene delivery system was used to deliver the frataxin gene to the FRDA 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.[21] The impaired frataxin in FRDA cells appears to cause reduced capacity for repair of DNA damage and this may contribute to neurodegeneration.[21]

Interactions

Frataxin has been shown to biologically interact with the enzyme PMPCB.[22]

References

  1. 1 2 3 GRCh38: Ensembl release 89: ENSG00000165060 - Ensembl, May 2017
  2. 1 2 3 GRCm38: Ensembl release 89: ENSMUSG00000059363 - Ensembl, May 2017
  3. ↑ "Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  4. ↑ "Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  5. 1 2 Campuzano V, Montermini L, Moltò MD, Pianese L, Cossée M, Cavalcanti F, Monros E, Rodius F, Duclos F, Monticelli A, Zara F, Cañizares J, Koutnikova H, Bidichandani SI, Gellera C, Brice A, Trouillas P, De Michele G, Filla A, De Frutos R, Palau F, Patel PI, Di Donato S, Mandel JL, Cocozza S, Koenig M, Pandolfo M (Mar 1996). "Friedreich's ataxia: autosomal recessive disease caused by an intronic GAA triplet repeat expansion". Science. 271 (5254): 1423–7. Bibcode:1996Sci...271.1423C. doi:10.1126/science.271.5254.1423. PMID 8596916. S2CID 20303793.
  6. ↑ Carvajal JJ, Pook MA, dos Santos M, Doudney K, Hillermann R, Minogue S, Williamson R, Hsuan JJ, Chamberlain S (Oct 1996). "The Friedreich's ataxia gene encodes a novel phosphatidylinositol-4- phosphate 5-kinase". Nature Genetics. 14 (2): 157–62. doi:10.1038/ng1096-157. PMID 8841185. S2CID 6324358.
  7. 1 2 Dhe-Paganon S, Shigeta R, Chi YI, Ristow M, Shoelson SE (Oct 2000). "Crystal structure of human frataxin". The Journal of Biological Chemistry. 275 (40): 30753–6. doi:10.1074/jbc.C000407200. PMID 10900192.
  8. 1 2 Stemmler TL, Lesuisse E, Pain, Dancis (August 2010). "Frataxin and Mitochondrial FeS Cluster Biogenesis". Journal of Biological Chemistry. 285 (35): 26737–26743. doi:10.1074/jbc.R110.118679. PMC 2930671. PMID 20522547.
  9. ↑ 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. PMID 19305405. S2CID 205522816.
  10. ↑ Dürr A, Cossee M, Agid Y, Campuzano V, Mignard C, Penet C, Mandel JL, Brice A, Koenig M (Oct 1996). "Clinical and genetic abnormalities in patients with Friedreich's ataxia". The New England Journal of Medicine. 335 (16): 1169–75. doi:10.1056/NEJM199610173351601. PMID 8815938.
  11. ↑ 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. PMID 9241270. S2CID 5883249.
  12. ↑ Kim E, Napierala M, Dent SY (Oct 2011). "Hyperexpansion of GAA repeats affects post-initiation steps of FXN transcription in Friedreich's ataxia". Nucleic Acids Research. 39 (19): 8366–77. doi:10.1093/nar/gkr542. PMC 3201871. PMID 21745819.
  13. ↑ 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. PMID 19733517.
  14. ↑ Baralle M, Pastor T, Bussani E, Pagani F (Jul 2008). "Influence of Friedreich ataxia GAA noncoding repeat expansions on pre-mRNA processing". American Journal of Human Genetics. 83 (1): 77–88. doi:10.1016/j.ajhg.2008.06.018. PMC 2443835. PMID 18597733.
  15. ↑ "Entrez Gene: FXN frataxin".
  16. ↑ Li L, Matsui M, Corey DR (2016-01-01). "Activating frataxin expression by repeat-targeted nucleic acids". Nature Communications. 7: 10606. Bibcode:2016NatCo...710606L. doi:10.1038/ncomms10606. PMC 4742999. PMID 26842135.
  17. 1 2 Clark E, Johnson J, Dong YN, Mercado-Ayon, Warren N, Zhai M, McMillan E, Salovin A, Lin H, Lynch DR (November 2018). "Role of frataxin protein deficiency and metabolic dysfunction in Friedreich ataxia, an autosomal recessive mitochondrial disease". Neuronal Signaling. 2 (4): NS20180060. doi:10.1042/NS20180060. PMC 7373238. PMID 32714592.
  18. ↑ Cossée M, Puccio H, Gansmuller A, Koutnikova H, Dierich A, LeMeur M, Fischbeck K, Dollé P, Kœnig M (May 2000). "Inactivation of the Friedreich ataxia mouse gene leads to early embryonic lethality without iron accumulation". Human Molecular Genetics. 9 (8): 1219–1226. doi:10.1093/hmg/9.8.1219. PMID 10767347. Archived from the original on 2 June 2018. Retrieved 5 April 2019.
  19. 1 2 Perdomini M, Hick A, Puccio H (17 July 2013). "Animal and cellular models of Friedreich ataxia". Journal of Neurochemistry. 126: 65–79. doi:10.1111/jnc.12219. PMID 23859342. S2CID 1427817.
  20. ↑ Runko AP, Griswold AJ, Min KT (March 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. PMID 18258192. S2CID 207603250.
  21. 1 2 3 Khonsari H, Schneider M, Al-Mahdawi S, Chianea YG, Themis M, Parris C, Pook MA, Themis M (December 2016). "Lentivirus-meditated frataxin gene delivery reverses genome instability in Friedreich ataxia patient and mouse model fibroblasts". Gene Ther. 23 (12): 846–856. doi:10.1038/gt.2016.61. PMC 5143368. PMID 27518705.
  22. ↑ 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. PMID 9700204.

Further reading

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