Molecular Pathogenesis
The pathophysiology of exercise in affected individuals was described in the classic papers of Linderholm and colleagues [1969]. It consists of low muscle oxidative capacity in which the muscle mitochondrial defect limits the extraction of available O2 from blood and is associated with a hyperkinetic circulatory response in which O2 delivery greatly exceeds O2 utilization. Similarly, exercise ventilation is greatly exaggerated relative to metabolic rate, accounting for the prominence of exertional dyspnea [Heinicke et al 2011]. Impaired oxygen utilization by working muscle combined with exaggerated oxygen delivery by the circulation are now recognized to be a feature of all severe muscle mitochondrial defects [Taivassalo et al 2003]. Fibroblast growth factor 21 (FGF21) expression and protein levels in muscle and plasma are greatly increased in affected individuals in keeping with the observation that plasma FGF21 may be increased in muscle-manifesting mitochondrial disease [Suomalainen et al 2011].
Selective skeletal muscle involvement in affected individuals with the common splice site variant relates to several factors. First, ISCU messenger RNA and protein are low in skeletal muscle compared to other tissues [Sanaker et al 2010, Crooks et al 2012]. In affected individuals, levels of misspliced ISCU RNA are much higher and levels of ISCU protein lower in skeletal muscle than in other tissues [Sanaker et al 2010, Nordin et al 2011, Crooks et al 2012]. Accordingly, succinate dehydrogenase, aconitase, and related iron-sulfur proteins that are severely deficient in skeletal muscle are relatively preserved in other tissues, and iron deposition, which is present in skeletal muscle mitochondria, is not detected in other tissues [Nordin et al 2012]. The incorrect splicing of ISCU in muscle may be driven by specific splicing factors [Nordin et al 2012]. Differential expression of these factors in a tissue-specific manner could underlie the severity of the phenotype in the skeletal muscle of individuals with ISCU. Additionally ISCU protein is further decreased by oxidative stress that is likely promoted by physical activity, mitochondrial dysfunction, and mitochondrial iron deposition [Crooks et al 2012]. Remarkably, two months after an episode of rhabdomyolysis, an individual homozygous for the common Swedish pathogenic variant had higher muscle levels of normally spliced ISCU RNA and ISCU protein and normal levels of histochemically assessed succinate dehydrogenase in regenerating muscle [Kollberg et al 2011]. This observation suggests that increased levels of misspliced ISCU and decreased levels of ISCU protein that accompany muscle maturation and physical activity-related oxidative stress may play important roles in the development of the classic biochemical phenotype [Crooks et al 2012].
Gene structure.
ISCU (isoform ISCU2) comprises five exons. Two ISCU splice variants have been identified to date: ISCU1 and ISCU2 type [Tong & Rouault 2000, Tong et al 2003]. The two variants share the same transcription initiation site but differ in the presence (ISCU1) or absence (ISCU2) of exon 1B.
ISCU1 (NM_014301.3, NP_055116.1) encodes a deduced 142-amino acid protein with 13 unique N-terminal residues, and ISCU2 (NM_213595.2, NP_998760.1) encodes a deduced 167-amino acid protein with 38 unique N-terminal residues, including a mitochondrial targeting signal.
Pathogenic variants. The common pathogenic variant c.418+382G>C is a homozygous splice variant in intron 4 of ISCU (), originating from a founder haplotype in northern Sweden [Mochel et al 2008, Olsson et al 2008]. This variant leads to the inclusion of an additional exon 4A that is predicted to result in a premature stop codon [Mochel et al 2008]. Two brothers were heterozygous for the common splice variant and a missense c.149G>A variant in exon 3 converting a highly conserved glycine to glutamate [Kollberg et al 2009].
Table 2.
ISCU Pathogenic Variants Discussed in This GeneReview
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| DNA Nucleotide Change | Predicted Protein Change | Reference Sequence |
|---|
| c.149G>A | p.Gly50Glu | NM_213595.3
NP_998760.1 |
| c.418+382G>C 1 | -- |
Variants listed in the table have been provided by the authors. GeneReviews staff have not independently verified the classification of variants.
GeneReviews follows the standard naming conventions of the Human Genome Variation Society (varnomen.hgvs.org). See Quick Reference for an explanation of nomenclature.
- 1.
Normal gene product. The iron-sulfur cluster assembly enzyme ISCU, or iron-sulfur cluster scaffold protein, is a highly conserved protein [Liu et al 2005]. Iron-sulfur clusters are prosthetic groups composed of iron and sulfur and usually ligated to proteins via the sulfhydryl side chains of cysteine. Iron-sulfur clusters often function as electron acceptors or donors; they are important for function of the mitochondrial respiratory chain, which contains 12 iron-sulfur clusters in respiratory complexes I-III. In humans, the citric acid cycle enzymes succinate dehydrogenase and aconitase are iron-sulfur proteins. In addition to their importance in electron transfer, iron-sulfur clusters can ligate substrate in enzymes such as aconitase, which converts citrate to isocitrate; iron-sulfur proteins can also have important structural and sensing roles.
In mammalian iron sulfur-cluster assembly, a cysteine desulfurase known as ISCS, encoded by NFS1, provides sulfur, and assembly of nascent iron-sulfur clusters takes place on ISCU, which functions as a scaffold on which the cluster is assembled [Rouault & Tong 2008]. ISCU has also been reported to interact with the Friedreich ataxia gene product frataxin in iron-sulfur cluster biosynthesis; this interaction is thought to facilitate delivery of iron from frataxin to nascent iron-sulfur clusters on ISCU [Shan et al 2007, Maio & Rouault 2015].
Abnormal gene product. The pathogenic splice variant detected in persons from northern Sweden results in aberrant splicing, with the increased retention of an additional exon (exon 4A) and the introduction of a premature stop codon in the penultimate exon; this ultimately alters the C terminus of the protein and decreases levels of ISCU protein [Mochel et al 2008]. Impaired iron-sulfur synthesis results in deficiency of multiple Fe-S-containing mitochondrial enzymes including succinate dehydrogenase (complex II), aconitase, and respiratory chain complexes I and III. The iron-sulfur protein, ferrochelatase, which catalyzes the terminal step in heme biosynthesis, is also deficient [Crooks et al 2010]. This may impair cytochrome synthesis and account for a variable reduction of cytochrome c oxidase (which does not contain Fe-S subunits) in some affected individuals [Kollberg et al 2009].
The missense pathogenic variant in exon 3 changes a glycine residue to a glutamate at amino acid position 50 [Kollberg et al 2009]. This amino acid residue is totally conserved among species from bacteria to mammals.
