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Open Access | 
10.1172/JCI182474
Mutations in the cytoplasmic iron-sulfur assembly protein CIAO1 cause neuromuscular deficits
Puneet Opal
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Published June 17, 2024 - More info
J Clin Invest. 2024;134(12):e182474. https://doi.org/10.1172/JCI182474.
© 2024 Opal et al. This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.
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Abstract
Cytoplasmic and nuclear iron-sulfur (Fe-S) enzymes that are essential for genome maintenance and replication depend on the cytoplasmic Fe-S assembly (CIA) machinery for cluster acquisition. The core of the CIA machinery consists of a complex of CIAO1, MMS19 and FAM96B. The physiological consequences of loss of function in the components of the CIA pathway have thus far remained uncharacterized. Our study revealed that patients with biallelic loss of function in CIAO1 developed proximal and axial muscle weakness, fluctuating creatine kinase elevation, and respiratory insufficiency. In addition, they presented with CNS symptoms including learning difficulties and neurobehavioral comorbidities, along with iron deposition in deep brain nuclei, mild normocytic to macrocytic anemia, and gastrointestinal symptoms. Mutational analysis revealed reduced stability of the variants compared with WT CIAO1. Functional assays demonstrated failure of the variants identified in patients to recruit Fe-S recipient proteins, resulting in compromised activities of DNA helicases, polymerases, and repair enzymes that rely on the CIA complex to acquire their Fe-S cofactors. Lentivirus-mediated restoration of CIAO1 expression reversed all patient-derived cellular abnormalities. Our study identifies CIAO1 as a human disease gene and provides insights into the broader implications of the cytosolic Fe-S assembly pathway in human health and disease.
Authors
Nunziata Maio, Rotem Orbach, Irina T. Zaharieva, Ana Töpf, Sandra Donkervoort, Pinki Munot, Juliane Mueller, Tracey Willis, Sumit Verma, Stojan Peric, Deepa Krishnakumar, Sniya Sudhakar, A. Reghan Foley, Sarah Silverstein, Ganka Douglas, Lynn Pais, Stephanie DiTroia, Christopher Grunseich, Ying Hu, Caroline Sewry, Anna Sarkozy, Volker Straub, Francesco Muntoni, Tracey A. Rouault, Carsten G. Bönnemann
Iron-sulfur clusters emerged over three billion years ago in microbes, allowing the eventual evolution of biological processes such as respiration, photosynthesis, and nitrogen fixation (1). In eukaryotes, iron-sulfur proteins contribute to cellular activities from DNA replication and repair to heme synthesis and electron transfer in mitochondria. Despite these diverse functions, the biochemical pathways that synthesize and assemble iron-sulfur clusters onto client proteins are highly conserved.
Studies in yeast and mammalian cells have determined that mitochondrial iron-sulfur proteins are assembled from a core iron-sulfur complex (ISC) involving multiple components. The cysteine desulfurase NFS1 (stabilized by the accessory proteins ISD11/LYRM4 and acyl carrier protein ACP) converts cysteine to alanine to generate a sulfur group, which is then transferred to a cysteine of a scaffold protein, ISCU, with iron provided by the iron donor frataxin (1). Mutations in most of these proteins, along with accessory cluster carriers and chaperones, have been associated with human diseases that manifest as mitochondrial dysfunction (2). Some of the diseases feature anemia as a major component, reflecting the role of these clusters in erythrocytes.
Iron-sulfur enzymes localized outside mitochondria require an additional cytosolic iron-sulfur assembly (CIA) complex (3, 4). This complex comprises CIAO1, MMS19, and FAM96B, which add iron-sulfur clusters to client proteins in the nucleus or cytosol. Their contribution to human health is far less studied, in part because no genetic diseases have been described — until now.
In this issue of the JCI, Maio et al. describe four unrelated individuals with biallelic mutations in CIAO1 (5). These patients experienced childhood or juvenile onset of limb, facial, and bulbar weakness, accompanied by respiratory weakness and elevated muscle enzymes. The authors describe variable learning and behavioral problems, along with anemia and constipation. Three patients were compound heterozygous for missense variants in CIAO1, and one patient has a missense variant on one allele and a deletion on the other. Computational analyses predicted the missense variants to be pathogenic, while the deletion was expected to generate an out-of-frame transcript undergoing nonsense-mediated decay.
The researchers investigated the biogenesis and activity of client enzymes dependent on Fe-S clusters from the CIA complex. They demonstrated a reduction in the levels of several iron-sulfur proteins and in the biosynthesis of two paradigmatic enzymes, POLD1 and DPYD, which are involved in DNA replication and the degradation of pyrimidine bases, respectively. Importantly, lentivirus-mediated transduction with wild-type CIAO1 rescued the phenotypic abnormalities in patient-derived cells (5).
One notable aspect of CIAO1 deficiency is its overlap with mitochondrial myopathies, including mitochondrial ultrastructural and functional deficits. This finding suggests that extramitochondrial iron-sulfur proteins influence mitochondrial function (5). Further investigation into additional mutations in CIAO1 and other CIA complex members will likely shed light on this puzzling issue and the role of extramitochondrial iron-sulfur proteins more generally in health and disease.
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