Mutations in the Mitochondrial Citrate Carrier SLC25A1 are Associated with Impaired Neuromuscular Transmission

Abstract

Background and Objective Congenital myasthenic syndromes are rare inherited disorders characterized by fatigable weakness caused by malfunction of the neuromuscular junction. We performed whole exome sequencing to unravel the genetic aetiology in an English sib pair with clinical features suggestive of congenital myasthenia. Methods We used homozygosity mapping and whole exome sequencing to identify the candidate gene variants. Mutant protein expression and function were assessed in vitro and a knockdown zebrafish model was generated to assess neuromuscular junction development. Results We identified a novel homozygous missense mutation in the SLC25A1 gene, encoding the mitochondrial citrate carrier. Mutant SLC25A1 showed abnormal carrier function. SLC25A1 has recently been linked to a severe, often lethal clinical phenotype. Our patients had a milder phenotype presenting primarily as a neuromuscular (NMJ) junction defect. Of note, a previously reported patient with different compound heterozygous missense mutations of SLC25A1 has since been shown to suffer from a neuromuscular transmission defect. Using knockdown of SLC25A1 expression in zebrafish, we were able to mirror the human disease in terms of variable brain, eye and cardiac involvement. Importantly, we show clear abnormalities in the neuromuscular junction, regardless of the severity of the phenotype. Conclusions Based on the axonal outgrowth defects seen in SLC25A1 knockdown zebrafish, we hypothesize that the neuromuscular junction impairment may be related to pre-synaptic nerve terminal abnormalities. Our findings highlight the complex machinery required to ensure efficient neuromuscular function, beyond the proteomes exclusive to the neuromuscular synapse.

Quantitative metabolic flux analysis reveals an unconventional pathway of fatty acid synthesis in cancer cells deficient for the mitochondrial citrate transport protein

“…The first is a neurodevelopmental syndrome associated with combined L-/D-2-hydroxyglutaric acidaemia (Nota et al, 2013) and lactic acidosis (Prasun et al, 2015) alongside variable dysmorphic features (Smith et al, 2016). The second is an isolated early-onset congenital myasthenia (muscle weakness) in the absence of accumulated organic acids (Chaouch et al, 2014). In the latter phenotype, it appears that a moderate degree of residual CTP activity tempers the clinical severity.…”

A novel homozygous SLC25A1 mutation with impaired mitochondrial complex V: Possible phenotypic expansion

“…In the cytoplasm, acetyl‐coenzyme‐A serves as the main substrate in fatty acid, triacylglycerol, and cholesterol biosynthesis, and also regulates glycolysis and fatty acid metabolism (Sun et al, ). It should be noted that there is significant variability in the phenotype and the severity of the disease stemming from SLC25A1 mutations (Chaouch et al, ) with a spectrum of presentations, from neonatal onset and abrupt deterioration as in the patients described here, to other previously described mutations that are more compatible with life (Chaouch et al, ; Nota et al, ; Soden et al, ). Patient II3 underwent muscle biopsy because he presented with neonatal‐onset encephalopathy with severe muscular weakness, intractable seizures as well as elevated lactate in plasma and CSF, and was thus highly suspected of having a mitochondrial disorder.…”

“…SLC25A1 mutations underlying the disorder were found both in those patients as well as in 11 other individuals affected with DL‐2HGA, (Muntau et al, ; Edvardson et al, ; Nota et al, ). To date, at least 20 affected individuals have been reported, with at least 16 different SLC25A1 mutations, summarized in Table (Chaouch et al, ; Edvardson et al, ; Nota et al, ; Muntau et al, ; Prasun et al, ; Smith et al, ; Soden et al, ). DL‐2HGA, as opposed to the disorders of increased urinary excretion of each of the two enantiomers, D‐2‐ and L‐2‐hydroxyglutaric acid, alone, is attributed only to SLC25A1 mutations.…”

“…DL‐2HGA, as opposed to the disorders of increased urinary excretion of each of the two enantiomers, D‐2‐ and L‐2‐hydroxyglutaric acid, alone, is attributed only to SLC25A1 mutations. SLC25A1 encodes the mitochondrial citrate carrier (CIC), which mediates efflux of the mitochondrial tricarboxylic acid (TCA) cycle intermediates citrate and isocitrate in exchange for cytosolic malate, and is thus believed to be a key player in fatty acid and sterol biosynthesis in the cytoplasm, as well as gluconeogenesis in the mitochondria (Chaouch et al, ; Kaplan, Mayor, & Wood, ; Muhlhausen et al, ). Here we present a series of five individuals with a phenotype closely resembling that of known DL‐2HGA patients, with a homozygous novel SLC25A1 mutation, demonstrating a mitochondrial complex V defect in the only affected individual that was tested for mitochondrial function.…”

Missense mutations in SLC25A1 are associated with congenital myasthenic syndrome type 23

“…1,2 In 2014, Chaouch and colleagues reported on a consanguineous family with two affected individuals. 3 They presented with mild intellectual disability and symptoms of a CMS (CMS type 23 [CMS23]; OMIM: 618197). Homozygosity for SLC25A1 variant (NM_005984.5:c.740G>A:p.Arg247Gln) was implicated as causative of the disease.…”