Glutaredoxin 5 deficiency causes sideroblastic anemia by specifically impairing heme biosynthesis and depleting cytosolic iron in human erythroblasts

Abstract

Glutaredoxin 5 (GLRX5) deficiency has previously been identified as a cause of anemia in a zebrafish model and of sideroblastic anemia in a human patient. Here we report that GLRX5 is essential for iron-sulfur cluster biosynthesis and the maintenance of normal mitochondrial and cytosolic iron homeostasis in human cells. GLRX5, a mitochondrial protein that is highly expressed in erythroid cells, can homodimerize and assemble [2Fe-2S] in vitro. In GLRX5-deficient cells, [Fe-S] cluster biosynthesis was impaired, the iron-responsive element-binding (IRE-binding) activity of iron regulatory protein 1 (IRP1) was activated, and increased IRP2 levels, indicative of relative cytosolic iron depletion, were observed together with mitochondrial iron overload. Rescue of patient fibroblasts with the WT GLRX5 gene by transfection or viral transduction reversed a slow growth phenotype, reversed the mitochondrial iron overload, and increased aconitase activity. Decreased aminolevulinate δ, synthase 2 (ALAS2) levels attributable to IRP-mediated translational repression were observed in erythroid cells in which GLRX5 expression had been downregulated using siRNA along with marked reduction in ferrochelatase levels and increased ferroportin expression. Erythroblasts express both IRP-repressible ALAS2 and non-IRP-repressible ferroportin 1b. The unique combination of IRP targets likely accounts for the tissue-specific phenotype of human GLRX5 deficiency.

Histidine 103 in Fra2 Is an Iron-Sulfur Cluster Ligand in the [2Fe-2S] Fra2-Grx3 Complex and Is Required for in Vivo Iron Signaling in Yeast

“…These results suggest that the cytosolic CGFS-type Grxs play an additional role beyond iron sensing and regulation. Chloroplast and mitochondrial monothiol Grxs (Grx5 in yeast) are proposed to be involved in assembly, storage, and/or delivery of [2Fe-2S] clusters (10,(35)(36)(37). Although this function has not been established for cytosolic Grxs, yeast Grx3 and Grx4 can partially rescue the Fe-S cluster biogenesis and growth defects of yeast grx5⌬ mutants when artificially targeted to the mitochondria, suggesting a conservation of function among proteins in the CGFStype Grx family (38).…”

The Role of Mitochondria in Cellular Iron-Sulfur Protein Biogenesis: Mechanisms, Connected Processes, and Diseases

“…Nevertheless, deletion of the yeast GRX5 gene leads to a severe Fe/S protein assembly defect both inside and outside mitochondria and is associated with a pronounced sensitivity to oxidative stress, possibly as a result of the iron accumulation in mitochondria (see below). In both humans and zebrafish, GLRX5 is essential for life (Wingert et al 2005;Camaschella et al 2007;Ye et al 2010). In humans, a mutation leading to decreased amounts of GLRX5 causes a severe iron-storage disease with a characteristic cellular Fe/S protein and heme synthesis defect as well as with an iron accumulation in mitochondria as indicated by the occurrence of ringed sideroblasts (Cazzola and Invernizzi 2011).…”

BOLA1 Is an Aerobic Protein That Prevents Mitochondrial Morphology Changes Induced by Glutathione Depletion

“…Alternatively, to directly affecting the thiol oxidation state of target proteins, BOLA1 could have an indirect effect, for example, by being involved in the assembly of Fe-S clusters, like its mitochondrial paralog BOLA3 (6). Interactions between Grx5 and the Fe-S assembly protein ISA1 have been measured in S. cerevisiae (36) and in Schizosaccharomyces pombe (19), implicating Grx5 in Fe-S assembly, and GLRX5 has been implicated in Fe-S cluster assembly in human erythroblasts (41). The nonassembly of Fe-S clusters in mitochondria will lead to free iron, which via Fenton chemistry can lead to the formation of hydroxyl radicals, and therewith to increased oxidation of biomolecules.…”