Mitochondrial citrate carrier deficiency causes neuromuscular transmission impairment

The mitochondrial citrate carrier (CIC), encoded by the SLC25A1 gene, catalyzes the export of citrate from mitochondria to the cytosol where it is broken into into acetyl-CoA and oxaloacetate. We identified for the first time two pathogenic SLC25A1 variants in a patient that suffered from a severe neurodevelopmental syndrome [1]. Recessive mutations in SLC25A1 have been since identified in more than a dozen patients and CIC deficiency has been classified as an inborn disorder of metabolism (OMIM: 615182) whose biochemical hallmark is combined D-2- and L-2-hydroxyglutaric aciduria [2]. More recently we reported a novel homozygous mutation in the SLC25A1 gene in an affected sib pair presenting with myasthenia and impaired neuromuscular junction (NMJ) transmission [3]. Upon functional reconstitution of recombinant proteins into liposomes, we showed that the newly identified mutation caused a milder activity impairment than the previously reported mutations suggesting a fundamental role of CIC in neuromuscular transmission whose defect was previously masked by the harsher phenotypes. Using the CRISPR/CAS9 approach we obtained stable lines of Caenorhabditis elegans knocked-out in the SLC25A1 ortholog that showed resistance to levamisole, a nicotinic acetylcholine receptor agonist, that causes continued stimulation of the worm muscles, leading to paralysis. This phenotype was, at least in part, rescued by the expression of wild-type human SLC25A1 under the control of a neuron-specific promoter, pointing towards an underlying presynaptic defect. Altogether these data demonstrate a conserved role of CIC in neuromuscular transmission and validate the worm C. elegans as a suitable animal model for further investigation of the molecular and cellular underpinnings of the NMJ transmission defect associated to CIC deficiency.