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International audience; The autosomal recessive white matter disorder LBSL (leukoencephalopathy with brain stem and spinal cord involvement and lactate elevation) is caused by mutations in DARS2, coding for mtAspRS (mitochondrial aspartyl-tRNA synthetase). Generally, patients are compound heterozygous for mutations in DARS2. Many different mutations have been identified in patients, including several missense mutations. In the present study, we have examined the effects of missense mutations found in LBSL patients on the expression, enzyme activity, localization and dimerization of mtAspRS, which is important for understanding the cellular defect underlying the pathogenesis of the disease. Nine different missense mutations were analysed and were shown to have various effects on mtAspRS properties. Several mutations have a direct effect on the catalytic activity of the enzyme; others have an effect on protein expression or dimerization. Most mutations have a clear impact on at least one of the properties of mtAspRS studied, probably resulting in a small contribution of the missense variants to the mitochondrial aspartylation activity in the cell.

LBSL (leukoencephalopathy with brain stem and spinal cord involvement and lactate elevation) is an autosomal recessive white matter disorder with slowly progressive cerebellar ataxia, spasticity and dorsal column dysfunction. Magnetic resonance imaging shows characteristic abnormalities in the cerebral white matter and specific brain stem and spinal cord tracts. LBSL is caused by mutations in the gene DARS2, which encodes mtAspRS (mitochondrial aspartyl-tRNA synthetase). The selective involvement of specific white matter tracts in LBSL is striking since this protein is ubiquitously expressed. Almost all LBSL patients have one mutation in intron 2 of DARS2, affecting the splicing of the third exon. Using a splicing reporter construct, we find cell-type-specific differences in the sensitivity to these mutations: the mutations have a larger effect on exon 3 exclusion in neural cell lines, especially neuronal cell lines, than in nonneural cell lines. Furthermore, correct inclusion of exon 3 in the normal mtAspRS mRNA occurs less efficiently in neural cells than in other cell types, and this effect is again most pronounced in neuronal cells. The combined result of these two effects may explain the selective vulnerability of specific white matter tracts in LBSL patients. © The Authors Journal compilation © 2012 Biochemical Society.

The segregation and myelination of axons in the developing PNS, results from a complex series of cellular and molecular interactions between Schwann cells and axons. Previously we identified theLgi4gene (leucine-rich glioma-inactivated4) as an important regulator of myelination in the PNS, and its dysfunction results in arthrogryposis as observed in claw paw mice. Lgi4 is a secreted protein and a member of a small family of proteins that are predominantly expressed in the nervous system. Their mechanism of action is unknown but may involve binding to members of the Adam (A disintegrin and metalloprotease) family of transmembrane proteins, in particular Adam22. We found that Lgi4 and Adam22 are both expressed in Schwann cells as well as in sensory neurons and that Lgi4 binds directly to Adam22 without a requirement for additional membrane associated factors. To determine whether Lgi4-Adam22 function involves a paracrine and/or an autocrine mechanism of action we performed heterotypic Schwann cell sensory neuron cultures and cell type-specific ablation of Lgi4 and Adam22 in mice. We show that Schwann cells are the principal cellular source of Lgi4 in the developing nerve and that Adam22 is required on axons. Our results thus reveal a novel paracrine signaling axis in peripheral nerve myelination in which Schwann cell secreted Lgi4 functions through binding of axonal Adam22 to drive the differentiation of Schwann cells.

Sir, We thank Drs Bocca Vieira de Rezende Pinto and Sgobbi de Souza for their interest in our recent overview paper on a cohort of patients with leukoencephalopathy with brainstem and spinal cord involvement and lactate elevation (LBSL), caused by DARS2 mutations (Van Berge et al. , 2014). Their letter provides us with the opportunity to make specific points clearer. The authors give a summary of the information published to date and pay special attention to exceptional cases, such as described by Miyake et al. (2011) and Synofzik et al. (2011). Additionally, they comment that magnetic resonance spectroscopy does not invariably reveal elevated lactate in patients with LBSL. They therefore suggest replacing the name LBSL by ‘ DARS2 -related conditions’ or ‘ DARS2 -related spectrum disorders’. We agree that the use of an acronym as the name for a disease may suggest that all patients fulfil all letters of the acronym at all stages of the disease. What is …

s ingediend voor het Amsterdam Kindersymposium 2013 47 Leukoencephalopathy with Brain Stem and Spinal Cord Involvement and Lactate Elevation Study of the phenotype and genotype and their correlation Eline M. Hamilton (1), Laura van Berge (1), Marjan E. Steenweg (1), Gert C. Scheper (1), Truus E. Abbink (1), Tarja Linnankivi (2), Marjo S. van der Knaap (1) (1) Department of Child Neurology, VU University medical center, Amsterdam (2) Department of Pediatric Neurology, Helsinki University Central Hospital, Helsinki, Finland INTRODUCTION Leukoencephalopathy with Brain Stem and Spinal Cord Involvement and Lactate Elevation (LBSL) is a rare autosomal recessive disorder with a distinctive MRI and spectroscopy pattern, clinically characterized by slowly progressive ataxia, spasticity and dorsal column dysfunction. The disease is caused by mutations in the DARS2 gene. There is a wide clinical variation. The aim of this study was to make an inventory of the clinical severity, disease course and genotype in LBSL patients and to study the possible infl uence of the genotype on the phenotype. METHODS A cross sectional observational study in 73 patients (64 families) with two proven DARS2 mutations was conducted. Clinical information was collected through questionnaires for physicians and families. RESULTS The clinical course in LBSL patients varied from neonatal onset, rapidly fatal disease up to an adult onset, mild disorder. The most common phenotype was characterized by childhood onset, slowly progressive neurological deterioration and normal intelligence. Apart from exceptional, severe cases, full wheelchair dependency was rare and predominantly occurred in adulthood. All patients were compound heterozygous and 44 diff erent DARS2 mutations were observed; 18 were novel. Almost all patients had at least one splice-site mutation. With this heterogeneity of genotypes, no genotype-phenotype correlation could be demonstrated. CONCLUSION This study describes the natural course and genotype in the largest cohort of LBSL patients to date. LBSL is a very heterogeneous disorder caused by numerous diff erent mutations. Larger groups of patients are required to study the genotype-phenotype correlation.