title: Decreased expression of genes involved in sulfur amino acid metabolism in frataxin-deficient cells.
abstract: Inherited deficiency of the mitochondrial protein frataxin causes neural and cardiac cell degeneration, and Friedreich's ataxia. Five hypotheses for frataxin's mitochondrial function have been generated, largely from work in non-human cells: iron transporter, iron-sulfur cluster assembler, iron-storage protein, antioxidant and stimulator of oxidative phosphorylation. We analyzed gene expression in three human cell types using microarrays, and identified just 48 transcripts whose expression was significantly frataxin-dependent in at least two cell types. Significant decreases in seven transcripts occurred in the sulfur amino acid (SAA) biosynthetic pathway and the iron-sulfur cluster (ISC) biosynthetic pathway to which it is connected. By contrast, we did not observe a single frataxin-dependent transcript that fits with the other four current hypotheses. Quantitative reverse-transcriptase PCR analysis of ISC-S and rhodanese transcripts confirmed that the expression of these genes involved in ISC metabolism was lower in mutants. Amino acid analysis confirmed the defect in SAA metabolism: homocystine, cysteine, cystathionine and serine were significantly decreased in frataxin-deficient cell extracts and mitochondria. An ISC defect was further confirmed by observing decreases in succinate dehydrogenase and aconitase activities, whose activities require ISCs. The ISC-U scaffold protein was specifically decreased in frataxin-deficient cells, suggesting a role for frataxin in its expression or maintenance, and sodium sulfide partially rescued the oxidant-sensitivity of the FRDA cells. Also, multiple transcripts involved in the Fas/TNF/INF apoptosis pathway were up-regulated in frataxin-deficient cells, consistent with a multi-step mechanism of Friedreich's ataxia pathophysiology, and suggesting alternative possibilities for therapeutic intervention.
authors.affiliation: Department of Molecular Biosciences, 1311 Haring Hall, University of California, Davis, CA 95616, USA.
authors.forename: Guolin
authors.initials: G
authors.lastname: Tan
authors.forename: Eleonora
authors.initials: E
authors.lastname: Napoli
authors.forename: Franco
authors.initials: F
authors.lastname: Taroni
authors.forename: Gino
authors.initials: G
authors.lastname: Cortopassi
mesh_terms: D000603:Amino Acids, Sulfur / Q000235:genetics / Q000378:metabolism*; D017209:Apoptosis; D005347:Fibroblasts; D005621:Friedreich Ataxia / Q000235:genetics / Q000378:metabolism; D006801:Humans; D033862:Iron-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D009474:Neurons / Q000378:metabolism; D018911:Trinucleotide Repeats; D000096923:Frataxin
chemical_list: D000603:Amino Acids, Sulfur; D033862:Iron-Binding Proteins; D000096923:Frataxin