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  • Genetics of sporadic amyotrophic lateral sclerosis.

    24 October 2018

    Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized clinically by rapidly progressive paralysis leading ultimately to death from respiratory failure. There is substantial evidence suggesting that ALS is a heritable disease, and a number of genes have been identified as being causative in familial ALS. In contrast, the genetics of the much commoner sporadic form of the disease is poorly understood and no single gene has been definitively shown to increase the risk of developing ALS. In this review, we discuss the genetic evidence for each candidate gene that has been putatively associated with increased risk of sporadic ALS. We also review whole genome association studies of ALS and discuss the potential of this methodology for identifying genes relevant to motor neuron degeneration.

  • A two-stage genome-wide association study of sporadic amyotrophic lateral sclerosis.

    24 October 2018

    The cause of sporadic amyotrophic lateral sclerosis (ALS) is largely unknown, but genetic factors are thought to play a significant role in determining susceptibility to motor neuron degeneration. To identify genetic variants altering risk of ALS, we undertook a two-stage genome-wide association study (GWAS): we followed our initial GWAS of 545 066 SNPs in 553 individuals with ALS and 2338 controls by testing the 7600 most associated SNPs from the first stage in three independent cohorts consisting of 2160 cases and 3008 controls. None of the SNPs selected for replication exceeded the Bonferroni threshold for significance. The two most significantly associated SNPs, rs2708909 and rs2708851 [odds ratio (OR) = 1.17 and 1.18, and P-values = 6.98 x 10(-7) and 1.16 x 10(-6)], were located on chromosome 7p13.3 within a 175 kb linkage disequilibrium block containing the SUNC1, HUS1 and C7orf57 genes. These associations did not achieve genome-wide significance in the original cohort and failed to replicate in an additional independent cohort of 989 US cases and 327 controls (OR = 1.18 and 1.19, P-values = 0.08 and 0.06, respectively). Thus, we chose to cautiously interpret our data as hypothesis-generating requiring additional confirmation, especially as all previously reported loci for ALS have failed to replicate successfully. Indeed, the three loci (FGGY, ITPR2 and DPP6) identified in previous GWAS of sporadic ALS were not significantly associated with disease in our study. Our findings suggest that ALS is more genetically and clinically heterogeneous than previously recognized. Genotype data from our study have been made available online to facilitate such future endeavors.

  • A mutation in the small heat-shock protein HSPB1 leading to distal hereditary motor neuronopathy disrupts neurofilament assembly and the axonal transport of specific cellular cargoes.

    24 October 2018

    Distal hereditary motor neuronopathies (dHMNs) are a clinically and genetically heterogeneous group of disorders in which motor neurons selectively undergo age-dependant degeneration. Mutations in the small heat-shock protein HSPB1 (HSP27) are responsible for one form of dHMN. In this study, we have analysed the effect of expressing a form of mutant HSPB1 in primary neuronal cells in culture. Mutant (P182L) but not wild-type HSPB1 led to the formation of insoluble intracellular aggregates and to the sequestration in the cytoplasm of selective cellular components, including neurofilament middle chain subunit (NF-M) and p150 dynactin. These findings suggest a possible pathogenic mechanism for HSPB1 whereby the mutation may lead to preferential motor neuron loss by disrupting selective components essential for axonal structure and transport.

  • Severe childhood SMA and axonal CMT due to anticodon binding domain mutations in the GARS gene.

    24 October 2018

    We screened 100 patients with inherited and sporadic lower motor neuron degeneration and identified three novel missense mutations in the glycyl-tRNA synthetase (GARS) gene. One mutation was in the anticodon binding domain and associated with onset in early childhood and predominant involvement of the lower limbs, thus extending the phenotype associated with GARS mutations.

  • Compound heterozygosity for loss-of-function lysyl-tRNA synthetase mutations in a patient with peripheral neuropathy.

    24 October 2018

    Charcot-Marie-Tooth (CMT) disease comprises a genetically and clinically heterogeneous group of peripheral nerve disorders characterized by impaired distal motor and sensory function. Mutations in three genes encoding aminoacyl-tRNA synthetases (ARSs) have been implicated in CMT disease primarily associated with an axonal pathology. ARSs are ubiquitously expressed, essential enzymes responsible for charging tRNA molecules with their cognate amino acids. To further explore the role of ARSs in CMT disease, we performed a large-scale mutation screen of the 37 human ARS genes in a cohort of 355 patients with a phenotype consistent with CMT. Here we describe three variants (p.Leu133His, p.Tyr173SerfsX7, and p.Ile302Met) in the lysyl-tRNA synthetase (KARS) gene in two patients from this cohort. Functional analyses revealed that two of these mutations (p.Leu133His and p.Tyr173SerfsX7) severely affect enzyme activity. Interestingly, both functional variants were found in a single patient with CMT disease and additional neurological and non-neurological sequelae. Based on these data, KARS becomes the fourth ARS gene associated with CMT disease, indicating that this family of enzymes is specifically critical for axon function.

  • Activation of mutant protein kinase Cgamma leads to aberrant sequestration and impairment of its cellular function.

    24 October 2018

    Mutations in protein kinase Cgamma (PKCgamma) cause the neurodegenerative disease spinocerebellar ataxia type 14 (SCA14). In this study, expression of an extensive panel of known SCA14-associated PKCgamma mutations as fusion proteins in cell culture led to the consistent formation of cytoplasmic aggregates in response to purinoceptor stimulation. Aggregates co-stained with antibodies to phosphorylated PKCgamma and the early endosome marker EEA1 but failed to redistribute to the cell membrane under conditions of oxidative stress. These studies suggest that Purkinje cell damage in SCA14 may result from a reduction of PKCgamma activity due its aberrant sequestration in the early endosome compartment.

  • Selective vulnerability of motor neurons and dissociation of pre- and post-synaptic pathology at the neuromuscular junction in mouse models of spinal muscular atrophy.

    24 October 2018

    Proximal spinal muscular atrophy (SMA) is a common autosomal recessive childhood form of motor neuron disease. Previous studies have highlighted nerve- and muscle-specific events in SMA, including atrophy of muscle fibres and post-synaptic motor endplates, loss of lower motor neuron cell bodies and denervation of neuromuscular junctions caused by loss of pre-synaptic inputs. Here we have undertaken a detailed morphological investigation of neuromuscular synaptic pathology in the Smn-/-;SMN2 and Smn-/-;SMN2;Delta7 mouse models of SMA. We show that neuromuscular junctions in the transversus abdominis (TVA), levator auris longus (LAL) and lumbrical muscles were disrupted in both mouse models. Pre-synaptic inputs were lost and abnormal accumulations of neurofilament were present, even in early/mid-symptomatic animals in the most severely affected muscle groups. Neuromuscular pathology was more extensive in the postural TVA muscle compared with the fast-twitch LAL and lumbrical muscles. Pre-synaptic pathology in Smn-/-;SMN2;Delta7 mice was reduced compared with Smn-/-;SMN2 mice at late-symptomatic time-points, although post-synaptic pathology was equally severe. We demonstrate that shrinkage of motor endplates does not correlate with loss of motor nerve terminals, signifying that one can occur in the absence of the other. We also demonstrate selective vulnerability of a subpopulation of motor neurons in the caudal muscle band of the LAL. Paralysis with botulinum toxin resulted in less terminal sprouting and ectopic synapse formation in the caudal band compared with the rostral band, suggesting that motor units conforming to a Fast Synapsing (FaSyn) phenotype are likely to be more vulnerable than those with a Delayed Synapsing (DeSyn) phenotype.

  • Dismutase-competent SOD1 mutant accumulation in myelinating Schwann cells is not detrimental to normal or transgenic ALS model mice.

    24 October 2018

    Mutant superoxide dismutase 1 (SOD1) action within non-neuronal cells is implicated in damage to spinal motor neurons in a genetic form of amyotrophic lateral sclerosis (ALS). Central nervous system glial cells such as astrocytes and microglia drive progression in transgenic mutant SOD1 mice, however, the role of myelinating glia remains unclear. Specifically, peripheral myelinating glial cells are likely candidates for mediating degeneration of distal synapses and axons of motor neurons in ALS. Here, we examine the potential contribution of peripheral axon ensheathing Schwann cells to ALS by constructing transgenic mice expressing dismutase active mutant SOD1(G93A) driven by the myelin protein zero (P0) promoter. In this model, mutant SOD1 accumulation in Schwann cells was comparable to levels in mice ubiquitously expressing a SOD1(G93A) transgene that become paralysed. Growth, locomotion and survival of these P0-SOD1(G93A) mice were indistinguishable from normal animals. There was no evidence for spinal motor neuron loss, distal axonal degeneration and p75 neurotrophin receptor (p75(NTR)) upregulation in the periphery of P0-SOD1(G93A) mice, unlike transgenic SOD1(G93A) mice with presymptomatic p75(NTR) induction and death-signalling. Furthermore, Schwann cells were resistant to mutant SOD1 aggregation in vivo and in transfected primary cultures. Increasing mutant SOD1 synthesis in Schwann cells by cross-breeding transgenic P0-SOD1(G93A) and SOD1(G93A) mice did not affect disease onset or survival. We conclude that dismutase-competent mutant SOD1 accumulation within Schwann cells is not pathological to spinal motor neurons or deleterious to disease course in transgenic ALS model mice, in contrast to astrocytes and microglia.