Mitochondrial aspartyl-tRNA synthetase deficiency causes leukoencephalopathy with brain stem and spinal cord involvement and lactate elevation.

Leukoencephalopathy with brain stem and spinal cord involvement and lactate elevation (LBSL) has recently been defined based on a highly characteristic constellation of abnormalities observed by magnetic resonance imaging and spectroscopy. LBSL is an autosomal recessive disease, most often manifesting in early childhood. Affected individuals develop slowly progressive cerebellar ataxia, spasticity and dorsal column dysfunction, sometimes with a mild cognitive deficit or decline. We performed linkage mapping with microsatellite markers in LBSL families and found a candidate region on chromosome 1, which we narrowed by means of shared haplotypes. Sequencing of genes in this candidate region uncovered mutations in DARS2, which encodes mitochondrial aspartyl-tRNA synthetase, in affected individuals from all 30 families. Enzyme activities of mutant proteins were decreased. We were surprised to find that activities of mitochondrial complexes from fibroblasts and lymphoblasts derived from affected individuals were normal, as determined by different assays.

Megalencephalic leukoencephalopathy with subcortical cysts: an update and extended mutation analysis of MLC1.

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is an autosomal recessive cerebral white matter disorder in children. This disease is histopathologically characterized by myelin splitting and intramyelinic vacuole formation. MLC is caused by mutations in the gene MLC1, which encodes a novel protein, MLC1. Since the first report, 50 mutations in this gene have been found. Mutations occur throughout the entire coding region and include all different types: 11 splice-site mutations; one nonsense mutation; 24 missense mutations; and 14 deletions and insertions. Until now, six polymorphisms within the coding sequence of MLC1 had been reported. In about 20% of the patients with a typical clinical and MRI picture, no mutations in the MLC1 gene are found. Several of the families, in which no mutations are found, also do not show linkage with the MLC1 locus, which suggests a second gene involved in MLC. The absence of mutations may also be the consequence of performing standard mutation analysis that can miss heterozygous deletions, mutations in the promoter, 3' and 5' untranslated regions (UTRs), and intron mutations, which may influence the amino acid composition of the end product. In this work we describe 13 novel mutations, including those found with extended mutation analysis on MLC patients. This study shows that extended mutation analysis is a valuable tool to identify at least some of the missing mutations. Therefore, we suggest extended mutation analysis for the MLC1 gene, if no mutations are found during standard analysis.

Vanishing white matter disease.

Vanishing white matter disease (VWM) is one of the most prevalent inherited childhood leucoencephalopathies. The classical phenotype is characterised by early childhood onset of chronic neurological deterioration, dominated by cerebellar ataxia. VWM is unusual because of its clinically evident sensitivity to febrile infections, minor head trauma, and acute fright, which may cause rapid neurological deterioration and unexplained coma. Most patients die a few years after onset. The phenotypic variation is extremely wide, including antenatal onset and early demise and adult-onset, slowly progressive disease. MRI findings are diagnostic in almost all patients and are indicative of vanishing of the cerebral white matter. The basic defect of this striking disease resides in either one of the five subunits of eukaryotic translation initiation factor eIF2B. eIF2B is essential in all cells of the body for protein synthesis and its regulation under different stress conditions. Although the defect is in housekeeping genes, oligodendrocytes and astrocytes are predominantly affected, whereas other cell types are surprisingly spared. Recently, undue activation of the unfolded-protein response has emerged as important in the pathophysiology of VWM, but the selective vulnerability of glia for defects in eIF2B is poorly understood.

MLC1: a novel protein in distal astroglial processes.

Megaloencephalic leukoencephalopathy with subcortical cysts (MLC) is a progressive cerebral white matter disease in children caused by mutations in the MLC1 gene. This disease is histopathologically characterized by myelin splitting and intramyelinic vacuole formation. MLC1 encodes a novel protein, MLC1, which is mainly expressed in the brain and leukocytes. The function is unknown, although a transport function has been suggested. In this article, we provide experimental data addressing the membrane topology and cellular localization of MLC1. We show that MLC1 contains an even number of transmembrane domains, supporting the possible transport function of MLC1. We demonstrate that MLC1 is specifically expressed in distal astroglial processes in perivascular, subependymal, and subpial regions. This localization suggests a role for MLC1 in a transport process across the blood-brain and brain-cerebrospinal fluid barriers. Astrocyte functions have long been debated. It is becoming increasingly clear that these cells are of fundamental importance in maintaining the structural and functional integrity of neural tissue. Elucidation of the function of MLC1 will contribute to a better understanding of not only the pathophysiology of the disease, but also the role of astrocytes in normal neural tissue.

Leukoencephalopathy with vanishing white matter: from magnetic resonance imaging pattern to five genes.

Leukoencephalopathy with vanishing white matter can be diagnosed on the basis of distinct clinical and magnetic resonance imaging (MRI) findings. It is a recessively inherited disorder, most often presenting in young children. The clinical symptoms include a slowly progressive cerebellar ataxia, spasticity, variable optic atrophy, and relatively preserved mental capacities. In addition, there are episodes of rapid and major deterioration following infections with fever and minor head trauma. These episodes can end in unexplained coma. MRI findings suggest that over time there is a progressive vanishing of the abnormal white matter, which is replaced by cerebrospinal fluid. We performed a genome-wide search and localized a gene for vanishing white matter on chromosome 3q27. We demonstrated that mutations in the gene EIF2B5 cause the disease. This gene encodes one of the five subunits of the translation factor eIF2B. Patients without mutations in the EIF2B5 gene were found to be mutated in one of the other genes that encode eIF2B subunits: EIF2B1 to EIF2B4.

MLC1 is associated with the dystrophin-glycoprotein complex at astrocytic endfeet.

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a progressive cerebral white matter disease with onset in childhood, caused by mutations in the MLC1 gene. MLC1 is a protein with unknown function that is mainly expressed in the brain in astrocytic endfeet at the blood-brain and cerebrospinal fluid-brain barriers. It shares its localization at astrocytic endfeet with the dystrophin-associated glycoprotein complex (DGC). The objective of the present study was to investigate the possible association of MLC1 with the DGC. To test this hypothesis, (co)-localization of DGC-proteins and MLC1 was analyzed by immunohistochemical stainings in gliotic brain tissue from a patient with multiple sclerosis, in glioblastoma tissue and in brain tissue from an MLC patient. In control tissue, a direct protein interaction was tested by immunoprecipitation. Results revealed that MLC1 is co-localized with DGC-proteins in gliotic brain tissue. We demonstrated that both MLC1 and aquaporin-4, a member of the DGC, were redistributed in glioblastoma cells. In MLC brain tissue, we showed absence of MLC1 and altered expression of several DGC-proteins. We demonstrated a direct protein interaction between MLC1 and Kir4.1. From these results we conclude that MLC1 is associated with the DGC at astrocytic endfeet.

Vanishing white matter disease: a review with focus on its genetics.

Leukoencephalopathy with vanishing white matter (VWM) is an autosomal recessive brain disorder, most often with a childhood onset. Magnetic resonance imaging and spectroscopy indicate that, with time, increasing amounts of cerebral white matter vanish and are replaced by fluid. Autopsy confirms white matter rarefaction and cystic degeneration. The process of localization and identification of the first two genes related to VWM, EIF2B5 and EIF2B2, was facilitated by two founder effects in the Dutch population. EIF2B5 and EIF2B2 encode the epsilon and beta subunits of translation initiation factor eIF2B. Soon it was shown that mutations in all five eIF2B subunit genes can cause VWM. EIF2B is essential for the initiation of translation of RNA into protein and is involved in regulation of the process, especially under stress conditions, which may explain the sensitivity to stress conditions observed in VWM patients. The pathophysiology of the disease is still poorly understood.

Regulation of protein synthesis in lymphoblasts from vanishing white matter patients.

Leukoencephalopathy with vanishing white matter (VWM) is an inherited childhood white matter disorder, caused by mutations in the genes encoding eukaryotic initiation factor 2B (eIF2B). The present study showed that, while the eIF2B activity was reduced in VWM lymphoblasts, the expression levels of the eIF2B subunits were similar to control lymphoblast lines. The mutations in eIF2B did not affect the interaction with eIF2. Strikingly, no apparent differences for the regulation of protein synthesis, measured by [35S]-methionine incorporation, were found between control and VWM lymphoblasts. Western blotting showed that, in some VWM cells, exposure to heat shock caused a decrease in the expression of specific eIF2B subunits. Most importantly, the increase in phosphorylation of eIF2alpha in response to heat shock was lower in VWM lymphoblasts than in control cells. These findings could form part of the explanation for the episodes of rapid and severe deterioration in VWM patients that are precipitated by febrile infections.

Mutations in each of the five subunits of translation initiation factor eIF2B can cause leukoencephalopathy with vanishing white matter.

Leukoencephalopathy with vanishing white matter is a recently defined autosomal recessive disorder. The course is chronic progressive with additional episodes of rapid deterioration, provoked by fever and minor head trauma. A previous study showed that mutations in the genes encoding the epsilon- or the beta-subunit of the eukaryotic translation initiation factor eIF2B, a complex consisting of five subunits, cause the disease in most patients. Seven unsolved patients remained. The unsolved patients were investigated by mutation analysis of the genes encoding the alpha-, gamma-, and delta-subunit of eIF2B and the gene encoding the alpha-subunit of eIF2, because phosphorylation of this latter subunit regulates eIF2B activity. Mutations were found in the genes encoding the alpha- (1 patient), gamma- (2 patients), and delta-subunits (2 patients) of eIF2B, but no mutations were found in the gene encoding the alpha-subunit of eIF2. In 2, both less typical patients, no mutations were found. Mutations in all five genes eIF2B subunit genes can cause VWM. eIF2B is essential for the initiation of translation of RNA into protein and is involved in regulation of the process, especially under circumstances of stress, such as fever. A defect in eIF2B may explain the sensitivity to stress factors in vanishing white matter patients.

eIF2B-related disorders: antenatal onset and involvement of multiple organs.

Leukoencephalopathy with vanishing white matter, also called "childhood ataxia with central nervous system hypomyelination," is the first human disease related to mutations in any of the five genes encoding subunits of eukaryotic initiation factor eIF2B or any translation factor at all. eIF2B is essential in all cells of the body for protein synthesis and the regulation of this protein synthesis under different stress conditions. It is surprising that mutations in the eIF2B genes have been reported to lead to abnormalities of the white matter of the brain only, although it has been shown recently that ovarian failure may accompany the leukoencephalopathy. Another surprising observation is that the onset of the disease varies from early childhood to adulthood, with the exception of Cree leukoencephalopathy, a disease related to a particular mutation in one of the eIF2B genes, which invariably has its onset within the first year of life. We analyzed the eIF2B genes of nine patients with an antenatal- or early-infantile-onset encephalopathy and an early demise and found mutations in eight of the patients. In addition to signs of a serious encephalopathy, we found oligohydramnios, intrauterine growth retardation, cataracts, pancreatitis, hepatosplenomegaly, hypoplasia of the kidneys, and ovarian dysgenesis. Until now, no evidence had been found for a genotype-phenotype correlation, but the consistently severe phenotype in affected siblings among our patients and in Cree encephalopathy patients suggests an influence of the genotype on the phenotype.