Mutation analysis of genes within the dynactin complex in a cohort of hereditary peripheral neuropathies

The cytoplasmic dynein–dynactin genes are attractive candidates for neurodegenerative disorders given their functional role in retrograde transport along neurons. The cytoplasmic dynein heavy chain (DYNC1H1) gene has been implicated in various neurodegenerative disorders, and dynactin 1 (DCTN1) genes have been implicated in a wide spectrum of disorders including motor neuron disease, Parkinson's disease, spinobulbar muscular atrophy and hereditary spastic paraplegia. However, the involvement of other dynactin genes with inherited peripheral neuropathies (IPN) namely, hereditary sensory neuropathy, hereditary motor neuropathy and Charcot–Marie–Tooth disease is under reported. We screened eight genes; DCTN1‐6 and ACTR1A and ACTR1B in 136 IPN patients using whole‐exome sequencing and high‐resolution melt (HRM) analysis. Eight non‐synonymous variants (including one novel variant) and three synonymous variants were identified. Four variants have been reported previously in other studies, however segregation analysis within family members excluded them from causing IPN in these families. No variants of disease significance were identified in this study suggesting the dynactin genes are unlikely to be a common cause of IPNs. However, with the ease of querying gene variants from exome data, these genes remain worthwhile candidates to assess unsolved IPN families for variants that may affect the function of the proteins.     

Recent advances in the genetics of distal hereditary motor neuropathy give insight to a disease mechanism involving copper homeostasis that may extend to other motor neuron disorders

Distal hereditary motor neuropathy (dHMN) is a sub-group of Charcot-Marie-Tooth disease (CMT), the most common peripheral neuropathy, that affects only motor neurons. The recent observation of ATP7A mutations in dHMN provides insight for a common disease mechanism that may involve copper homeostasis. Functionally, diverse proteins were previously shown to underlie dHMN and a convergent link is destined to unfold for some of these. We propose connections between copper and known dHMN genes that overlap also with the causative genes of other motor neuron disorders (MNDs).     

Non-primate EIAV-based lentiviral vectors as gene delivery system for motor neuron diseases

Motor neuron diseases such as amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA) are neurodegenerative diseases, which cause progressive paralysis and premature death in affected adults and children. The treatment rational for these diseases is to halt or delay the degeneration of motor neurons but to date there are no effective drugs. This may however change with recent advances in gene therapy using lentiviral vectors. These vectors can transfer genes to motor neurons with high efficiency and give long term expression. One of these vector systems, based on the equine infectious anaemia virus (EIAV), can insert genes into the cells of the central nervous system after remote delivery including delivery into the muscle by exploiting retrograde transport pathways. This opens up the exciting possibility of rescuing the denervation of key muscle groups in patients by simple injections of these neurotropic lentiviral vectors into the muscle. This review will describe the general features of lentiviral vectors derived from the EIAV. It will then describe some key examples of gene transfer and genetic correction in animal models of motor neuron disease. The prospects for the clinical evaluation of lentiviral vectors for the treatment of human motor neuron disease will be outlined.     

Small heat shock protein 27 mutation in a Japanese patient with distal hereditary motor neuropathy

Heat shock protein 27 (HSP27) belongs to a family of small heat shock proteins that play significant roles in the cellular stress response and are also involved in the control of protein–protein interactions as chaperons. Mutation in HSP27 has been identified as the cause of axonal Charcot–Marie–Tooth disease (CMT) and distal hereditary motor neuropathy (HMN). Heat shock protein 22 (HSP22) is a molecular counterpart of HSP27, and its mutation is another cause of distal HMN. We screened the mutation of HSP27 and HSP22 in 68 Japanese patients with axonal CMT or unclassified CMT and six Japanese patients with distal HMN. We detected a heterozygous P182S mutation of HSP27 in a patient with distal HMN, but we found no mutations in HSP22. Mutation in HSP27 may impair the formation of the stable neurofilament network that is indispensable for the maintenance of peripheral nerves.     

New ALS‐Related Genes Expand the Spectrum Paradigm of Amyotrophic Lateral Sclerosis

Amyotrophic Lateral Sclerosis (ALS) is characterized by the degeneration of upper and lower motor neurons. Clinical heterogeneity is a well‐recognized feature of the disease as age of onset, site of onset and the duration of the disease can vary greatly among patients. A number of genes have been identified and associated to familial and sporadic forms of ALS but the majority of cases remains still unexplained. Recent breakthrough discoveries have demonstrated that clinical manifestations associated with ALS‐related genes are not circumscribed to motor neurons involvement. In this view, ALS appears to be linked to different conditions over a continuum or spectrum in which overlapping phenotypes may be identified. In this review, we aim to examine the increasing number of spectra, including ALS/Frontotemporal Dementia and ALS/Myopathies spectra. Considering all these neurodegenerative disorders as different phenotypes of the same spectrum can help to identify common pathological pathways and consequently new therapeutic targets in these incurable diseases.     

Autosomal dominant late-onset spinal motor neuronopathy is linked to a new locus on chromosome 22q11.2-q13.2

Spinal muscular atrophies (SMAs) are hereditary disorders characterized by degeneration of lower motor neurons. Different SMA types are clinically and genetically heterogeneous and many of them show significant phenotypic overlap. We recently described the clinical phenotype of a new disease in two Finnish families with a unique autosomal dominant late-onset lower motor neuronopathy. The studied families did not show linkage to any known locus of hereditary motor neuron disease and thus seemed to represent a new disease entity. For this study, we recruited two more family members and performed a more thorough genome-wide scan. We obtained significant linkage on chromosome 22q, maximum LOD score being 3.43 at marker D22S315. The linked area is defined by flanking markers D22S686 and D22S276, comprising 18.9 Mb. The region harbours 402 genes, none of which is previously known to be associated with SMAs. This study confirms that the disease in these two families is a genetically distinct entity and also provides evidence for a founder mutation segregating in both pedigrees.     

Prader–Willi and Other Syndromes Associated with Obesity and Mental Retardation

Constitutional obesity and mental retardation cooccur in several multiple congenital anomaly syndromes, including Prader–Willi syndrome, Bardet–Biedl syndrome, Cohen syndrome, Albright hereditary osteodystrophy, and Borjeson–Forssman–Lehmann syndrome as well as some rarer disorders. Although hypothalamic–pituitary axis abnormalities are thought to be a possible causative mechanism in some of these disorders, current knowledge is insufficient to explain the pathophysiologic mechanism of obesity in most multiple congenital anomaly/mental retardation syndromes. The chromosomal location of many of these syndromes is known, and studies are ongoing to identify the causative genes. Further delineation of the functions of the underlying genes will likely be instructive regarding mechanisms of appetite, satiety, and obesity in the general population. This review details current knowledge of the clinical and molecular genetic findings of multiple congenital anomaly/mental retardation syndromes associated with intrinsic obesity in an effort to delineate causative mechanisms and genetic abnormalities contributing to obesity.     

Germline mutations in the von Hippel–Lindau gene in Italian patients

von Hippel–Lindau syndrome (VHL) is a dominantly inherited familial cancer syndrome predisposing to a variety of malignant and benign tumours, most frequently retinal, cerebellar, and spinal hemangioblastoma, renal cell carcinoma, pheochromocytoma, and pancreatic tumours.The current study investigated the occurrence of VHL mutations in Italian patients with classic VHL disease or with atypical VHL-like clinical features referred to the Service of Medical Genetics for VHL molecular diagnosis. In addition, an RQ-PCR protocol was validated in order to introduce it in the routine VHL laboratory diagnosis.     

X‐linked spinal muscular atrophy in mice caused by autonomous loss of ATP7A in the motor neuron

ATP7A is a copper‐transporting P‐type ATPase that is essential for cellular copper homeostasis. Loss‐of‐function mutations in the ATP7A gene result in Menkes disease, a fatal neurodegenerative disorder resulting in seizures, hypotonia and failure to thrive, due to systemic copper deficiency. Most recently, rare missense mutations in ATP7A that do not impact systemic copper homeostasis have been shown to cause X‐linked spinal muscular atrophy type 3 (SMAX3), a distal hereditary motor neuropathy. An understanding of the mechanistic and pathophysiological basis of SMAX3 is currently lacking, in part because the disease‐causing mutations have been shown to confer both loss‐ and gain‐of‐function properties to ATP7A, and because there is currently no animal model of the disease. In this study, the Atp7a gene was specifically deleted in the motor neurons of mice, resulting in a degenerative phenotype consistent with the clinical features in affected patients with SMAX3, including the progressive deterioration of gait, age‐dependent muscle atrophy, denervation of neuromuscular junctions and a loss of motor neuron cell bodies. Taken together, these data reveal autonomous requirements for ATP7A that reveal essential roles for copper in the maintenance and function of the motor neuron, and suggest that SMAX3 is caused by a loss of ATP7A function that specifically impacts the spinal motor neuron. Copyright © 2015 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.     

Central motor conduction in hereditary motor and sensory neuropathy and hereditary spastic paraplegia.

Conduction of the central motor pathways (CMCT) by magnetic stimulation of the motor cortex (TMS) was performed in 17 patients with hereditary motor sensory neuropathy (HMSN) and 2 siblings with hereditary spastic paraplegia (HSP). CMCT was prolonged in two patients with HMSN I with associated pyramidal features and in two subjects with HMSN II without clinical pyramidal signs. CMCT may be abnormal in HMSN due to central motor pathways involvement or altered spinal excitability with increased synaptic delay. CMCT was normal in the upper limbs in patients with HSP but increased in the legs. Diagnostic yield of TMS increased in less disabled cases with HSP when selective conduction at the spinal level (C7-S1) was calculated. Abnormal spinal conduction in HSP is consistent with degeneration of the crossed corticospinal tracts at the thoracic level found in neuropathologic observations.     

Distal hereditary motor neuropathy type II (distal HMN II): mapping of a locus to chromosome 12q24

The distal hereditary motor neuropathy (distal HMN) or the spinal form of Charcot-Marie-Tooth (CMT) disease is an exclusively motor disorder of the peripheral nervous system. The disorder clinically resembles the hereditary motor and sensory neuropathies (HMSN) type I and type II or CMT type 1 and type 2. Distal HMN might also be related to the spinal muscular atrophies (SMA) since, in both disorders, the lower motor neurons are affected. Electrophysiological and neuropathological examinations of peripheral nerves show the absence of sensory involvement. We performed a genome search in an extended Belgian family with autosomal dominant distal HMN type II. Significant linkage was obtained with markers located at chromosome 12q24, and the gene for distal HMN II was assigned to the 13 cM interval between D12S86 and D12S340.      

Modulation of neuronal activity in dorsal column nuclei by upper cervical spinal cord stimulation in rats

Clinical human and animal studies show that upper cervical spinal cord stimulation (cSCS) has beneficial effects in treatment of some cerebral disorders, including those due to deficient cerebral circulation. However, the underlying mechanisms and neural pathways activated by cSCS using clinical parameters remain unclear. We have shown that a cSCS-induced increase in cerebral blood flow is mediated via rostral spinal dorsal column fibers implying that the dorsal column nuclei (DCN) are involved. The aim of this study was to examine how cSCS modulated neuronal activity of DCN. A spring-loaded unipolar ball electrode was placed on the left dorsal column at cervical (C2) spinal cord in pentobarbital anesthetized, ventilated and paralyzed male rats. Stimulation with frequencies of 1, 10, 20, 50 Hz (0.2 ms, 10 s) and an intensity of 90% of motor threshold was applied. Extracellular potentials of single neurons in DCN were recorded and examined for effects of cSCS. In total, 109 neurons in DCN were isolated and tested for effects of cSCS. Out of these, 56 neurons were recorded from the cuneate nucleus and 53 from the gracile nucleus. Mechanical somatic stimuli altered activity of 87/109 (83.2%) examined neurons. Of the neurons receiving somatic input, 62 were classified as low-threshold and 25 as wide dynamic range. The cSCS at 1 Hz changed the activity of 96/109 (88.1%) of the neurons. Neuronal responses to cSCS exhibited multiple patterns of excitation and/or inhibition: excitation (E, n=21), inhibition (I, n=19), E–I (n=37), I–E (n=8) and E–I–E (n=11). Furthermore, cSCS with high-frequency (50 Hz) altered the activity of 92.7% (51/55) of tested neurons, including 30 E, 24 I, and 2 I–E responses to cSCS. These data suggested that cSCS significantly modulates neuronal activity in DCN. These nuclei might serve as a neural relay for cSCS-induced effects on cerebral dysfunction and diseases.     

TDP‐43 Redistribution is an Early Event in Sporadic Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder consisting of progressive loss of motor neurons. TDP‐43 has been identified as a component of ubiquitin‐immunoreactive inclusions of motor neurons in ALS. We focused on the diffuse cytoplasmic TDP‐43 immunoreactivity in ALS neurons, and quantitatively assessed it in comparison with skein/round TDP‐43 and ubiquitin immunostaining in motor neurons of 30 sporadic ALS cases. The percentage of spinal motor neurons with cytoplasmic TDP‐43 immunoreactivity was higher than that of ubiquitin‐immunoreactive ones. The percentage of TDP‐43‐positive motor neurons was independent of neuron counts in anterior horns, while the percentage of ubiquitinated neurons was inversely correlated. Aiming to define the cytosolic localization of TDP‐43, the immunoblot analysis of spinal cord and frontal cortex showed that full‐length TDP‐43, the 45 kDa form and ubiquitinated TDP‐43 are found in the soluble inclusion‐free fraction. The present data suggest that delocalization, accumulation and ubiquitination of TDP‐43 in the cytoplasm of motor neurons are early dysfunctions in the cascade of the events leading to motor neuron degeneration in ALS, preceding the formation of insoluble inclusion bodies. Being cytoplasmic accumulation an ongoing event during the course of the illness, a therapeutic approach to this incurable disease can be envisaged.     

Exome sequencing reveals HINT1 mutations as a cause of distal hereditary motor neuropathy

Distal hereditary motor neuropathies (dHMNs) are a heterogenous group of genetic disorders with length-dependent degeneration of motor axons. Obtaining a genetic diagnosis in patients with dHMN remains challenging. We performed exome sequencing in a diagnostic setting in 12 patients with a clinical diagnosis of dHMN. Potential disease-causing variants in genes associated with dHMN and other forms of inherited neuropathies/motor neuron diseases were validated using Sequenom. The coverage in the genes studied was >95% with an average coverage of >50 times. In none of the patients a mutations was found in genes previously reported to be associated with dHMN. However, in 2/12 patients a recessive mutation in histidine triad nucleotide binding protein 1 (HINT1, recently discovered as a cause of axonal neuropathy with neuromyotonia) was identified. Our results demonstrate the diagnostic value of exome sequencing for patients with inherited neuropathies. The phenotypic spectrum of recessive mutations in HINT1 includes dHMN. HINT1 should be added to the list of genes to check for in dHMN.     

The Protein-Protein Interaction Network of Hereditary Parkinsonism Genes Is a Hierarchical Scale-Free Network

PurposeHereditary parkinsonism genes consist of causative genes of familial Parkinson’s disease (PD) with a locus symbol prefix (PARK genes) and hereditary atypical parkinsonian disorders that present atypical features and limited responsiveness to levodopa (non-PARK genes). Although studies have shown that hereditary parkinsonism genes are related to idiopathic PD at the phenotypic, gene expression, and genomic levels, no study has systematically investigated connectivity among the proteins encoded by these genes at the protein-protein interaction (PPI) level.Materials and MethodsTopological measurements and physical interaction enrichment were performed to assess PPI networks constructed using some or all the proteins encoded by hereditary parkinsonism genes (n=96), which were curated using the Online Mendelian Inheritance in Man database and literature.ResultsNon-PARK and PARK genes were involved in common functional modules related to autophagy, mitochondrial or lysosomal organization, catecholamine metabolic process, chemical synapse transmission, response to oxidative stress, neuronal apoptosis, regulation of cellular protein catabolic process, and vesicle-mediated transport in synapse. The hereditary parkinsonism proteins formed a single large network comprising 51 nodes, 83 edges, and three PPI pairs. The probability of degree distribution followed a power-law scaling behavior, with a degree exponent of 1.24 and a correlation coefficient of 0.92. LRRK2 was identified as a hub gene with the highest degree of betweenness centrality; its physical interaction enrichment score was 1.28, which was highly significant.ConclusionBoth PARK and non-PARK genes show high connectivity at the PPI and biological functional levels.     

Activation of bulbar reticular neurons by visceral afferents

Conclusion Synaptic activation of reticular neurons of the bulbar gigantocellular nucleus has been investigated with stimulation of the left greater splanchnic nerve, Synaptic activation of these neurons has been shown to be produced by ascending pathways formed by somatic and visceral afferents, running in the ventro-lateral part of the lateral, and the ventral, columns of the spinal cord. Synaptic processes arising in reticular neurons are uniform and differ only temporally. The pattern of development of synaptic processes in reticular neurons suggests that transmission of visceral impulses into the bulbar reticular formation takes place along two pathways. One of these, formed by the spinoreticular fibers of the lateral columns, activates neurons after a short latency (8–10 msec). Besides this there are also polysynaptic, nonspecific fibers in the ventrolateral and ventral columns, organized on the principle of long sensory tracts and including an intermediate link in the segmental interneurons. Synaptic processes in reticular neurons evoked by the polysynaptic pathways develop after 10–20 msec and originate from excitation of somatic and visceral afferents possessing lower excitability and a lower conduction velocity.Division of afferent fibers of the splanchnic nerve running in the dorsal columns and dorsolateral part of the lateral columns of the spinal cord has no significant influence on synaptic processes in reticular neurons.     

AIF translocates to the nucleus in the spinal motor neurons in a mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by selective loss of motor neurons in the brain stem and the spinal cords. One of the causes for the familial ALS has been attributed to the mutations in copper-zinc superoxide dismutase (SOD1). Although the toxic function of the mutant enzyme has not been fully understood, the final cell death pathway has been suggested as caspase-dependent. In the present study, we present evidence that the activation of apoptosis inducing factor (AIF) may play a role to induce motor neuron death during ALS pathogenesis. In the spinal cord of SOD1 G93A transgenic mice, expression of AIF was detected in the motor neurons and astrocytes. The level of AIF expression increased as the disease progressed. In the symptomatic SOD1 G93A transgenic mice, AIF released from the mitochondria and translocated into the nucleus in the motor neurons as evidenced by confocal microscopy and biochemical analysis. These results suggest that AIF may play a role to induce motor neuron death in a mouse model of ALS.     

Mediterranean fever gene variants modify clinical phenotypes of idiopathic multi‐centric Castleman disease

Four cases of idiopathic multi‐centric Castleman disease (iMCD) reportedly have variants in hereditary autoinflammatory disease‐related genes; however, the frequency and role of these variants in iMCD is still unknown. We therefore investigated such gene variants among patients with iMCD and aimed to reveal the relationship between iMCD and autoinflammatory disease‐related genes. We reviewed 14 Japanese iMCD patients who were recruited between January 2015 and September 2019. All patients met both the Japanese tentative diagnostic criteria for Castleman disease and the international consensus diagnostic criteria for iMCD. We performed genetic analyses for 31 autoinflammatory disease‐related genes by targeted next‐generation sequencing. The MEFV gene variants were observed in 10 of 14 patients with iMCD. Although iMCD had a high percentage of exons 2 or 3 variants of MEFV, comparison of data from healthy Japanese subjects indicated that there was no significant difference in the percentage between healthy Japanese subjects and patients with iMCD. Variants of uncertain significance (VUS) in the TNFRSF1A and CECR1 genes were observed in two of the patients, respectively. We divided patients into two groups—those with MEFV variants (excluding E148Q variants) and those without MEFV variants—and compared the clinical characteristics between these two groups. Patients with MEFV variants, excluding E148Q variants, exhibited a significantly higher likelihood of fever and significantly lower levels of hemoglobin than those lacking MEFV variants. Our results indicated that patients with iMCD tended to have a high frequency of MEFV gene variants and the presence of such variants can affect iMCD clinical phenotypes.     

A nonstop variant in REEP1 causes peripheral neuropathy by unmasking a 3′UTR‐encoded,aggregation‐inducing motif

Single‐nucleotide variants that abolish the stop codon (“nonstop” alterations) are a unique type of substitution in genomic DNA. Whether they confer instability of the mutant mRNA or result in expression of a C‐terminally extended protein depends on the absence or presence of a downstream in‐frame stop codon, respectively. Of the predicted protein extensions, only few have been functionally characterized. In a family with autosomal dominant Charcot‐Marie‐Tooth disease type 2, that is, an axonopathy affecting sensory neurons as well as lower motor neurons, we identified a heterozygous nonstop variant in REEP1. Mutations in this gene have classically been associated with the upper motor neuron disorder hereditary spastic paraplegia (HSP). We show that the C‐terminal extension resulting from the nonstop variant triggers self‐aggregation of REEP1 and of several reporters. Our findings support the recently proposed concept of 3′UTR‐encoded “cryptic amyloidogenic elements.” Together with a previous report on an aggregation‐prone REEP1 deletion variant in distal hereditary motor neuropathy, they also suggest that toxic gain of REEP1 function, rather than loss‐of‐function as relevant for HSP, specifically affects lower motor neurons. A search for similar correlations between genotype, phenotype, and effect of mutant protein may help to explain the wide clinical spectra also in other genetically determined disorders.