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Clinical manifestation and a new ISCU mutation in iron-sulphur cluster deficiency myopathy.

Journal article
Authors Gittan Kollberg
Mar Tulinius
Atle Melberg
Niklas Darin
Oluf Andersen
Daniel Holmgren
Anders Oldfors
Elisabeth Holme
Published in Brain : a journal of neurology
Volume 132
Issue Pt 8
Pages 2170-9
ISSN 1460-2156
Publication year 2009
Published at Institute of Neuroscience and Physiology, Department of Clinical Neuroscience and Rehabilitation
Institute of Biomedicine, Department of Clinical Chemistry and Transfusion Medicine
Institute of Biomedicine, Department of Pathology
Institute of Clinical Sciences
Pages 2170-9
Language en
Links dx.doi.org/10.1093/brain/awp152
Subject categories Medical and Health Sciences

Abstract

Myopathy with deficiency of succinate dehydrogenase and aconitase is a recessively inherited disorder characterized by childhood-onset early fatigue, dyspnoea and palpitations on trivial exercise. The disease is non-progressive, but life-threatening episodes of widespread weakness, severe metabolic acidosis and rhabdomyolysis may occur. The disease has so far only been identified in northern Sweden. The clinical, histochemical and biochemical phenotype is very homogenous and the patients are homozygous for a deep intronic IVS5 + 382G>C splicing affecting mutation in ISCU, which encodes the differently spliced cytosolic and mitochondrial iron-sulphur cluster assembly protein IscU. Iron-sulphur cluster containing proteins are essential for iron homeostasis and respiratory chain function, with IscU being among the most conserved proteins in evolution. We identified a shared homozygous segment of only 405,000 base pair with the deep intronic mutation in eight patients with a phenotype consistent with the original description of the disease. Two other patients, two brothers, had an identical biochemical and histochemical phenotype which is probably pathognomonic for muscle iron-sulphur cluster deficiency, but they presented with a disease where the clinical phenotype was characterized by early onset of a slowly progressive severe muscle weakness, severe exercise intolerance and cardiomyopathy. The brothers were compound heterozygous for the deep intronic mutation and had a c.149 G>A missense mutation in exon 3 changing a completely conserved glycine residue to a glutamate. The missense mutation was inherited from their mother who was of Finnish descent. The intronic mutation affects mRNA splicing and results in inclusion of pseudoexons in most transcripts in muscle. The pseudoexon inclusion results in a change in the reading frame and appearance of a premature stop codon. In western blot analysis of protein extracts from fibroblasts, there was no pronounced reduction of IscU in any of the patients, but the analysis revealed that the species corresponding to mitochondrial IscU migrates slower than a species present only in whole cells. In protein extracted from isolated skeletal muscle mitochondria the western blot analysis revealed a severe deficiency of IscU in the homozygous patients and appearance of a faint new fraction that could represent a truncated protein. There was only a slight reduction of mitochondrial IscU in the compound heterozygotes, despite their severe phenotype, indicating that the IscU expressed in these patients is non-functional.

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