Myopathy with lactic acidosis is linked to chromosome 12q23.3-24.11 and caused by an intron mutation in the ISCU gene resulting in a splicing defect

Abstract

We describe the mapping and identification of the gene for hereditary myopathy with lactic acidosis (HML). HML is characterized by low physical performance, resulting in physical exertion that causes early exhaustion, dyspnoea and palpitations. Using an autosomal recessive mode of inheritance, we mapped the trait to chromosome 12q23.3-24.11, with a maximum lod score of 5.26. The 1.6-Mb disease-critical region contained one obvious candidate gene-ISCU-specifying a protein involved in iron-sulphur cluster assembly. IscU is produced in two isoforms; one cytosolic and one mitochondrial, coded for by different splice variants of the ISCU gene. Mutational analysis of all exon and intron sequences as well as 1000 bp of the promoter of the ISCU gene revealed one intron mutation that was specific for the disease haplotype. The mutation is located in a region with homology to the interferon-stimulated response element (ISRE), but we could not see any effect of the mutation on expression levels in vitro or in vivo. We did, however, observe a drastic difference in the splicing pattern between patients and controls. In controls the mRNA was, as expected, mainly in the mitochondrial form, while in the patients a larger mRNA transcript was predominant. Sequencing of the product revealed that the mutation activates cryptic splice sites in intron 5 resulting in aberrant mRNA containing 100 bp of the intron. To conclude, our data strongly suggest that an intron mutation in the ISCU gene, leading to incorrectly spliced mRNA, is the cause of myopathy with lactic acidosis in this family.

miR-210 Targets Iron-Sulfur Cluster Scaffold Homologue in Human Trophoblast Cell Lines

“…This disorder is characterized by muscle weakness, exercise-induced myoglobinuria, and muscle iron overload. 41,42 ISCU is a key protein involved in Fe-S cluster biogenesis, and isoforms originating from alternative splicing have been found in the mitochondria and the cytosol, indicating that Fe-S cluster biogenesis is a compartmentalized process in mammalian cells. 48,50 Impairment of cytosolic Fe-S cluster assembly after selective depletion of cytosolic ISCU in HeLa cells indicated that its function cannot be compensated by mitochondrial ISCU.…”

Human Nbp35 Is Essential for both Cytosolic Iron-Sulfur Protein Assembly and Iron Homeostasis

“…In humans, impaired functions of XPD/ Ercc2 and FancJ/Bach1 lead to a certain type of xeroderma pigmentosum (35) and to Fanconi's anemia (67), respectively. Since more and more diseases are identified to be related to an impaired assembly of Fe/S proteins, including Friedreich's ataxia (52) and X-linked sideroblastic anemia and cerebellar ataxia (2), as well as some forms of myopathies (44,49), microcytic anemia (11), and erythropoietic protoporphyria (59), the elucidation of the mechanisms leading to the maturation of Fe/S proteins within various eukaryotic compartments is of utmost medical importance. The identification of huNbp35 as a critical component of the cytosolic CIA machinery for Fe/S protein maturation may help to achieve deeper insights into these mechanisms.…”

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

“…Although these [Fe-S] synthetic components work at different points in the same pathway, different phenotypes arise from inherited mutations in these genes, including Friedreich ataxia (FRDA) (20,21), ISCU myopathy (22,23), and the recently characterized sideroblastic anemia (SA), associated with a mutation in glutaredoxin 5 (GLRX5) (24). The patient with GLRX5 deficiency and SA has an A294G mutation in the last nucleotide of exon-1, which impedes intron-1 splicing and dramatically reduces GLRX5 mRNA levels.…”