Analyzing the type and frequency of patient-specific mutations that give rise to Duchenne muscular dystrophy (DMD) is an invaluable tool for diagnostics, basic scientific research, trial planning, and improved clinical care. Locus-specific databases allow for the collection, organization, storage, and analysis of genetic variants of disease. Here, we describe the development and analysis of the TREAT-NMD DMD Global database (http://umd.be/TREAT_DMD/). We analyzed genetic data for 7,149 DMD mutations held within the database. A total of 5,682 large mutations were observed (80% of total mutations), of which 4,894 (86%) were deletions (1 exon or larger) and 784 (14%) were duplications (1 exon or larger). There were 1,445 small mutations (smaller than 1 exon, 20% of all mutations), of which 358 (25%) were small deletions and 132 (9%) small insertions and 199 (14%) affected the splice sites. Point mutations totalled 756 (52% of small mutations) with 726 (50%) nonsense mutations and 30 (2%) missense mutations. Finally, 22 (0.3%) mid-intronic mutations were observed. In addition, mutations were identified within the database that would potentially benefit from novel genetic therapies for DMD including stop codon read-through therapies (10% of total mutations) and exon skipping therapy (80% of deletions and 55% of total mutations).

Mutations in mitofusin 2 (MFN2) have been reported in Charcot-Marie-Tooth type 2 (CMT2) families. To study the distribution of mutations in MFN2 we screened 323 families and isolated patients with distinct CMT phenotypes. In 29 probands, we identified 22 distinct MFN2 mutations, and 14 of these mutations have not been reported before. All mutations were located in the cytoplasmic domains of the MFN2 protein. Patients presented with a classical but rather severe CMT phenotype, since 28% of them were wheelchair-dependent. Some had additional features as optic atrophy. Most patients had an early onset and severe disease status, whereas a smaller group experienced a later onset and milder disease course. Electrophysiological data showed in the majority of patients normal to slightly reduced nerve conduction velocities with often severely reduced amplitudes of the compound motor and sensory nerve action potentials. Examination of sural nerve specimens showed loss of large myelinated fibres and degenerative mitochondrial changes. In patients with a documented family history of CMT2 the frequency of MFN2 mutations was 33% indicating that MFN2 mutations are a major cause in this population.

Charcot-Marie-Tooth (CMT) neuropathies are common disorders of the peripheral nervous system caused by demyelination or axonal degeneration, or a combination of both features. We previously assigned the locus for autosomal dominant intermediate CMT neuropathy type C (DI-CMTC) to chromosome 1p34-p35. Here we identify two heterozygous missense mutations (G41R and E196K) and one de novo deletion (153-156delVKQV) in tyrosyl-tRNA synthetase (YARS) in three unrelated families affected with DI-CMTC. Biochemical experiments and genetic complementation in yeast show partial loss of aminoacylation activity of the mutant proteins, and mutations in YARS, or in its yeast ortholog TYS1, reduce yeast growth. YARS localizes to axonal termini in differentiating primary motor neuron and neuroblastoma cultures. This specific distribution is significantly reduced in cells expressing mutant YARS proteins. YARS is the second aminoacyl-tRNA synthetase found to be involved in CMT, thereby linking protein-synthesizing complexes with neurodegeneration.

MFN2 is a mitochondrial membrane protein that was recently reported to cause axonal CMT type 2A. It is intriguing that MFN2 shows functional overlap with optic atrophy 1 (OPA1), the protein underlying the most common form of autosomal dominant optic atrophy, and mitochondrial encoded oxidative phosphorylation components as seen in Leber's hereditary optic atrophy. We conclude that autosomal dominant HMSN VI is caused by mutations in MFN2, emphasizing the important role of mitochondrial function for both optic atrophies and peripheral neuropathies.

Summary Selective neuronal loss is a hallmark of neurodegenerative diseases, which counter-intuitively are often caused by mutations in widely-expressed genes1. Charcot-Marie-Tooth (CMT) diseases are the most common hereditary peripheral neuropathies, for which there are no effective therapies2,3. A subtype of the diseases—CMT2D—is caused by dominant mutations in GARS, encoding the ubiquitously expressed enzyme glycyl-tRNA synthetase (GlyRS). Despite the broad requirement of GlyRS for protein biosynthesis in all cells, mutations in this gene cause a selective degeneration of peripheral axons leading to deficits in distal motor function4. How mutations in GlyRS (GlyRSCMT2D) are linked to motor neuron vulnerability has remained elusive. Here we report that GlyRSCMT2D acquires a neomorphic binding activity that directly antagonizes an essential signaling pathway for motor neuron survival. We find that CMT2D mutations alter the conformation of GlyRS, enabling GlyRSCMT2D to bind the Neuropilin 1 (Nrp1) receptor. This aberrant interaction competitively interferes with the binding of the cognate ligand vascular endothelial growth factor (VEGF) to Nrp1. Genetic reduction of Nrp1 in mice worsens CMT2D symptoms, whereas enhanced expression of VEGF improves motor function. These findings link the selective pathology of CMT2D to the neomorphic binding activity of GlyRSCMT2D that antagonizes the VEGF/Nrp1 interaction, and indicate the VEGF/Nrp1 signaling axis is an actionable target for treating CMT2D.

Neuromuscular junctions (NMJs) are synapses that transmit impulses from motor neurons to skeletal muscle fibers leading to muscle contraction. Study of hereditary disorders of neuromuscular transmission, termed congenital myasthenic syndromes (CMS), has helped elucidate fundamental processes influencing development and function of the nerve-muscle synapse. Using genetic linkage, we find 18 different biallelic mutations in the gene encoding glutamine-fructose-6-phosphate transaminase 1 (GFPT1) in 13 unrelated families with an autosomal recessive CMS. Consistent with these data, downregulation of the GFPT1 ortholog gfpt1 in zebrafish embryos altered muscle fiber morphology and impaired neuromuscular junction development. GFPT1 is the key enzyme of the hexosamine pathway yielding the amino sugar UDP-N-acetylglucosamine, an essential substrate for protein glycosylation. Our findings provide further impetus to study the glycobiology of NMJ and synapses in general.

Distal myopathies represent a heterogeneous group of inherited skeletal muscle disorders. One type of adult-onset, progressive autosomal-dominant distal myopathy, frequently associated with dysphagia and dysphonia (vocal cord and pharyngeal weakness with distal myopathy [VCPDM]), has been mapped to chromosome 5q31 in a North American pedigree. Here, we report the identification of a second large VCPDM family of Bulgarian descent and fine mapping of the critical interval. Sequencing of positional candidate genes revealed precisely the same nonconservative S85C missense mutation affecting an interspecies conserved residue in the MATR3 gene in both families. MATR3 is expressed in skeletal muscle and encodes matrin 3, a component of the nuclear matrix, which is a proteinaceous network that extends throughout the nucleus. Different disease related haplotype signatures in the two families provided evidence that two independent mutational events at the same position in MATR3 cause VCPDM. Our data establish proof of principle that the nuclear matrix is crucial for normal skeletal muscle structure and function and put VCPDM on the growing list of monogenic disorders associated with the nuclear proteome.

The 8-10 million European Roma/Gypsies are a founder population of common origins that has subsequently split into multiple socially divergent and geographically dispersed Gypsy groups. Unlike other founder populations, whose genealogy has been extensively documented, the demographic history of the Gypsies is not fully understood and, given the lack of written records, has to be inferred from current genetic data. In this study, we have used five disease loci harboring private Gypsy mutations to examine some missing historical parameters and current structure. We analyzed the frequency distribution of the five mutations in 832-1,363 unrelated controls, representing 14 Gypsy populations, and the diversification of chromosomal haplotypes in 501 members of affected families. Sharing of mutations and high carrier rates supported a strong founder effect, and the identity of the congenital myasthenia 1267delG mutation in Gypsy and Indian/Pakistani chromosomes provided the best evidence yet of the Indian origins of the Gypsies. However, dramatic differences in mutation frequencies and haplotype divergence and very limited haplotype sharing pointed to strong internal differentiation and characterized the Gypsies as a founder population comprising multiple subisolates. Using disease haplotype coalescence times at the different loci, we estimated that the entire Gypsy population was founded approximately 32-40 generations ago, with secondary and tertiary founder events occurring approximately 16-25 generations ago. The existence of multiple subisolates, with endogamy maintained to the present day, suggests a general approach to complex disorders in which initial gene mapping could be performed in large families from a single Gypsy group, whereas fine mapping would rely on the informed sampling of the divergent subisolates and searching for the shared genomic region that displays the strongest linkage disequilibrium with the disease.