Homozygous COQ7 mutation: a new cause of potentially treatable distal hereditary motor neuropathy.

Distal hereditary motor neuropathy represents a group of motor inherited neuropathies leading to distal weakness. We report a family of two brothers and a sister affected by distal hereditary motor neuropathy in whom a homozygous variant c.3G>T (p.1Met?) was identified in the COQ7 gene. This gene encodes a protein required for coenzyme Q10 biosynthesis, a component of the respiratory chain in mitochondria. Mutations of COQ7 were previously associated with severe multi-organ disorders characterized by early childhood onset and developmental delay. Using patient blood samples and fibroblasts derived from a skin biopsy, we investigated the pathogenicity of the variant of unknown significance c.3G>T (p.1Met?) in the COQ7 gene and the effect of coenzyme Q10 supplementation in vitro. We showed that this variation leads to a severe decrease in COQ7 protein levels in the patient's fibroblasts, resulting in a decrease in coenzyme Q10 production and in the accumulation of 6-demethoxycoenzyme Q10, the COQ7 substrate. Interestingly, such accumulation was also found in the patient's plasma. Normal coenzyme Q10 and 6-demethoxycoenzyme Q10 levels were restored in vitro by using the coenzyme Q10 precursor 2,4-dihydroxybenzoic acid, thus bypassing the COQ7 requirement. Coenzyme Q10 biosynthesis deficiency is known to impair the mitochondrial respiratory chain. Seahorse experiments showed that the patient's cells mainly rely on glycolysis to maintain sufficient ATP production. Consistently, the replacement of glucose by galactose in the culture medium of these cells reduced their proliferation rate. Interestingly, normal proliferation was restored by coenzyme Q10 supplementation of the culture medium, suggesting a therapeutic avenue for these patients. Altogether, we have identified the first example of recessive distal hereditary motor neuropathy caused by a homozygous variation in the COQ7 gene, which should thus be included in the gene panels used to diagnose peripheral inherited neuropathies. Furthermore, 6-demethoxycoenzyme Q10 accumulation in the blood can be used to confirm the pathogenic nature of the mutation. Finally, supplementation with coenzyme Q10 or derivatives should be considered to prevent the progression of COQ7-related peripheral inherited neuropathy in diagnosed patients.

Repurposing pentamidine using hyaluronic acid-based nanocarriers for skeletal muscle treatment in myotonic dystrophy.

In a context of drug repurposing, pentamidine (PTM), an FDA-approved antiparasitic drug, has been proposed to reverse the splicing defects associated in myotonic dystrophy type 1 (DM1). However, clinical use of PTM is hinder by substantial toxicity, leading to find alternative delivery strategies. In this work we proposed hyaluronic acid-based nanoparticles as a novel encapsulation strategy to efficiently deliver PTM to skeletal muscles cells. In vitro studies on C2C12 myoblasts and myotubes showed an efficient nanoparticles' internalization with minimal toxicity. More interestingly, our findings evidenced for the first time the endosomal escape of hyaluronic acid-based nanocarriers. Ex vivo studies showed an efficient nanoparticles' internalization within skeletal muscle fibers. Finally, the therapeutic efficacy of PTM-loaded nanosystems to reduce the number of nuclear foci has been demonstrated in a novel DM1 in vitro model. So far, current data demonstrated the potency of hyaluronic acid-based nanosystems as efficient nanocarrier for delivering PTM into skeletal muscle and mitigate DM1 pathology.

Expanding the phenotypic variability of MORC2 gene mutations: From Charcot-Marie-Tooth disease to late-onset pure motor neuropathy.

MORC2 gene encodes a ubiquitously expressed nuclear protein involved in chromatin remodeling, DNA repair, and transcriptional regulation. Heterozygous mutations in MORC2 gene have been associated with a spectrum of disorders affecting the peripheral nervous system such as Charcot-Marie-Tooth (CMT2Z), spinal muscular atrophy-like with or without cerebellar involvement, and a developmental syndrome associated with impaired growth, craniofacial dysmorphism and axonal neuropathy (DIGFAN syndrome). Such variability in clinical manifestations associated with the increasing number of variants of unknown significance detected by next-generation sequencing constitutes a serious diagnostic challenge. Here we report the characterization of an in vitro model to evaluate the pathogenicity of variants of unknown significance based on MORC2 overexpression in a neuroblastoma cell line SH-EP or cortical neurons. Likewise, we show that MORC2 mutants affect survival and trigger apoptosis over time in SH-EP cell line. Furthermore, overexpression in primary cortical neurons increases apoptotic cell death and decreases neurite outgrowth. Altogether, these approaches establish the pathogenicity of two new variants p.Gly444Arg and p.His446Gln in three patients from two families. These new mutations in MORC2 gene are associated with autosomal dominant CMT and with adult late onset proximal motor neuropathy, further increasing the spectrum of clinical manifestations associated with MORC2 mutations.

Severe congenital myasthenic syndromes caused by agrin mutations affecting secretion by motoneurons.

Congenital myasthenic syndromes (CMS) are predominantly characterized by muscle weakness and fatigability and can be caused by a variety of mutations in genes required for neuromuscular junction formation and maintenance. Among them, AGRN encodes agrin, an essential synaptic protein secreted by motoneurons. We have identified severe CMS patients with uncharacterized p.R1671Q, p.R1698P and p.L1664P mutations in the LG2 domain of agrin. Overexpression in primary motoneurons cultures in vitro and in chick spinal motoneurons in vivo revealed that the mutations modified agrin trafficking, leading to its accumulation in the soma and/or in the axon. Expression of mutant agrins in cultured cells demonstrated accumulation of agrin in the endoplasmic reticulum associated with induction of unfolded protein response (UPR) and impaired secretion in the culture medium. Interestingly, evaluation of the specific activity of individual agrins on AChR cluster formation indicated that when secreted, mutant agrins retained a normal capacity to trigger the formation of AChR clusters. To confirm agrin accumulation and secretion defect, iPS cells were derived from a patient and differentiated into motoneurons. Patient iPS-derived motoneurons accumulated mutant agrin in the soma and increased XBP1 mRNA splicing, suggesting UPR activation. Moreover, co-cultures of patient iPS-derived motoneurons with myotubes confirmed the deficit in agrin secretion and revealed a reduction in motoneuron survival. Altogether, we report the first mutations in AGRN gene that specifically affect agrin secretion by motoneurons. Interestingly, the three patients carrying these mutations were initially suspected of spinal muscular atrophy (SMA). Therefore, in the presence of patients with a clinical presentation of SMA but without mutation in the SMN1 gene, it can be worth to look for mutations in AGRN.

Charcot-Marie-Tooth disease type 2CC due to NEFH variants causes a progressive, non-length-dependent, motor-predominant phenotype.

OBJECTIVE: Neurofilaments are the major scaffolding proteins for the neuronal cytoskeleton, and variants in NEFH have recently been described to cause axonal Charcot-Marie-Tooth disease type 2CC (CMT2CC). METHODS: In this large observational study, we present phenotype-genotype correlations on 30 affected and 3 asymptomatic mutation carriers from eight families. RESULTS: The majority of patients presented in adulthood with motor-predominant and lower limb-predominant symptoms and the average age of onset was 31.0±15.1 years. A prominent feature was the development of proximal weakness early in the course of the disease. The disease progressed rapidly, unlike other Charcot-Marie-Tooth disease (CMT) subtypes, and half of the patients (53%) needed to use a wheelchair on average 24.1 years after symptom onset. Furthermore, 40% of patients had evidence of early ankle plantarflexion weakness, a feature which is observed in only a handful of CMT subtypes. Neurophysiological studies and MRI of the lower limbs confirmed the presence of a non-length-dependent neuropathy in the majority of patients.All families harboured heterozygous frameshift variants in the last exon of NEFH, resulting in a reading frameshift to an alternate open reading frame and the translation of approximately 42 additional amino acids from the 3' untranslated region (3'-UTR). CONCLUSIONS: This phenotype-genotype study highlights the unusual phenotype of CMT2CC, which is more akin to spinal muscular atrophy rather than classic CMT. Furthermore, the study will enable more informative discussions on the natural history of the disease and will aid in NEFH variant interpretation in the context of the disease's unique molecular genetics.

Novel combinatorial screening identifies neurotrophic factors for selective classes of motor neurons.

Numerous neurotrophic factors promote the survival of developing motor neurons but their combinatorial actions remain poorly understood; to address this, we here screened 66 combinations of 12 neurotrophic factors on pure, highly viable, and standardized embryonic mouse motor neurons isolated by a unique FACS technique. We demonstrate potent, strictly additive, survival effects of hepatocyte growth factor (HGF), ciliary neurotrophic factor (CNTF), and Artemin through specific activation of their receptor complexes in distinct subsets of lumbar motor neurons: HGF supports hindlimb motor neurons through c-Met; CNTF supports subsets of axial motor neurons through CNTFRα; and Artemin acts as the first survival factor for parasympathetic preganglionic motor neurons through GFRα3/Syndecan-3 activation. These data show that neurotrophic factors can selectively promote the survival of distinct classes of embryonic motor neurons. Similar studies on postnatal motor neurons may provide a conceptual framework for the combined therapeutic use of neurotrophic factors in degenerative motor neuron diseases such as amyotrophic lateral sclerosis, spinal muscular atrophy, and spinobulbar muscular atrophy.

Mutant superoxide dismutase 1 (SOD1), a cause of amyotrophic lateral sclerosis, disrupts the recruitment of SMN, the spinal muscular atrophy protein to nuclear Cajal bodies.

Spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS) are among the most common motor neuron diseases to afflict the human population. A deficiency of the survival of motor neuron (SMN) protein causes SMA and is also reported to be an exacerbating factor in the development of ALS. However, pathways linking the two diseases have yet to be defined and it is not clear precisely how the pathology of ALS is aggravated by reduced SMN or whether mutant proteins underlying familial forms of ALS interfere with SMN-related biochemical pathways to exacerbate the neurodegenerative process. In this study, we show that mutant superoxide dismutase-1 (SOD1), a cause of familial ALS, profoundly alters the sub-cellular localization of the SMN protein, preventing the formation of nuclear 'gems' by disrupting the recruitment of the protein to Cajal bodies. Overexpressing the SMN protein in mutant SOD1 mice, a model of familial ALS, alleviates this phenomenon, most likely in a cell-autonomous manner, and significantly mitigates the loss of motor neurons in the spinal cord and in culture dishes. In the mice, the onset of the neuromuscular phenotype is delayed and motor function enhanced, suggestive of a therapeutic benefit for ALS patients treated with agents that augment the SMN protein. Nevertheless, this finding is tempered by an inability to prolong survival, a limitation most likely imposed by the inexorable denervation that characterizes ALS and eventually disrupts the neuromuscular synapses even in the presence of increased SMN.

Genetic characterization of non-5q proximal spinal muscular atrophy in a French cohort: the place of whole exome sequencing.

Proximal spinal muscular atrophy (SMA) is defined by a degeneration of the anterior horn cells resulting in muscle weakness predominantly in the proximal lower limbs. While most patients carry a biallelic deletion in the SMN1 gene (localized in chromosome 5q), little is known regarding patients without SMN1-mutation, and a genetic diagnosis is not always possible. Here, we report a cohort of 24 French patients with non-5q proximal SMA from five neuromuscular centers who all, except two, had next-generation sequencing (NGS) gene panel, followed by whole exome sequencing (WES) if gene panel showed a negative result. The two remaining patients benefited directly from WES or whole genome sequencing (WGS). A total of ten patients with causative variants were identified, nine of whom were index cases (9/23 families = 39%). Eight variants were identified by gene panel: five variants in DYNC1H1, and three in BICD2. Compound heterozygous causative variants in ASAH1 were identified directly by WES, and one variant in DYNC1H1 was identified directly by WGS. No causative variant was found using WES in patients with a previous panel with negative results (14 cases). We thus recommend using primarily NGS panels in patients with non-5q-SMA and using WES, especially when several members of the same family are affected and/or when trio analyses are possible, or WGS as second-line testing if available.

Nucleolar reorganization after cellular stress is orchestrated by SMN shuttling between nuclear compartments.

Spinal muscular atrophy is an autosomal recessive neuromuscular disease caused by mutations in the multifunctional protein Survival of Motor Neuron, or SMN. Within the nucleus, SMN localizes to Cajal bodies, which are associated with nucleoli, nuclear organelles dedicated to the first steps of ribosome biogenesis. The highly organized structure of the nucleolus can be dynamically altered by genotoxic agents. RNAP1, Fibrillarin, and nucleolar DNA are exported to the periphery of the nucleolus after genotoxic stress and, once DNA repair is fully completed, the organization of the nucleolus is restored. We find that SMN is required for the restoration of the nucleolar structure after genotoxic stress. During DNA repair, SMN shuttles from the Cajal bodies to the nucleolus. This shuttling is important for nucleolar homeostasis and relies on the presence of Coilin and the activity of PRMT1.

Microrchidia CW-Type Zinc Finger 2, a Chromatin Modifier in a Spectrum of Peripheral Neuropathies.

Microrchidia CW-type zinc finger 2 (MORC2) gene encodes a protein expressed in all tissues and enriched in the brain. MORC2 protein is composed of a catalytic ATPase domain, three coil-coiled domains allowing dimerization or protein complex interaction, a zinc-finger CW domain allowing DNA interaction, and a CHROMO-like (CHRromatin Organization Modifier) domain. Recently, de novo or dominantly inherited heterozygous mutations have been associated with a spectrum of disorders affecting the peripheral nervous system such as the Charcot-Marie-Tooth disease, spinal muscular atrophy-like phenotype disorder, or a neurodevelopmental syndrome associated with developmental delay, impaired growth, dysmorphic facies, and axonal neuropathy (DIGFAN). In this review, we detail the various mutations of MORC2 and their consequences on clinical manifestations. Possible genotype-phenotype correlations as well as intra and inter-family variability are discussed. MORC2 molecular functions such as transcriptional modulation, DNA damage repair, and lipid metabolism are then reviewed. We further discuss the impact of MORC2 mutations on the epigenetic landscape in the neuromuscular system and hypothesize probable pathophysiological mechanisms underlying the phenotypic variability observed.

Novel Intronic Mutation in VMA21 Causing Severe Phenotype of X-Linked Myopathy with Excessive Autophagy-Case Report.

X-linked Myopathy with Excessive Autophagy (XMEA) is a rare autophagic vacuolar myopathy caused by mutations in the Vacuolar ATPase assembly factor VMA21 gene; onset usually occurs during childhood and rarely occurs during adulthood. We described a 22-year-old patient with XMEA, whose onset was declared at 11 through gait disorder. He had severe four-limb proximal weakness and amyotrophy, and his proximal muscle MRC score was between 2 and 3/5 in four limbs; creatine kinase levels were elevated (1385 IU/L), and electroneuromyography and muscle MRI were suggestive of myopathy. Muscle biopsy showed abnormalities typical of autophagic vacuolar myopathy. We detected a hemizygous, unreported, intronic, single-nucleotide substitution c.164-20T>A (NM_001017980.4) in intron 2 of the VMA21 gene. Fibroblasts derived from this patient displayed a reduced level of VMA21 transcripts (at 40% of normal) and protein, suggesting a pathogenicity related to an alteration of the splicing efficiency associated with an intron retention. This patient with XMEA displayed a severe phenotype (rapid weakness of upper and lower limbs) due to a new intronic variant of VMA21, related to an alteration in the splicing efficiency associated with intron retention, suggesting that phenotype severity is closely related to the residual expression of the VMA21 protein.

Inflammation in ALS and SMA: sorting out the good from the evil.

Indices of neuroinflammation are found in a variety of diseases of the CNS including amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA). Over the years, neuroinflammation, in degenerative disorders of the CNS, has evolved from being regarded as an innocent bystander accomplishing its housekeeping function secondary to neurodegeneration to being considered as a bona fide contributor to the disease process and, in some situations, as a putative initiator of the disease. Herein, we will review neuroinflammation in both ALS and SMA not only from the angle of neuropathology but also from the angle of its potential role in the pathogenesis and treatment of these two dreadful paralytic disorders.

Systematic elucidation of neuron-astrocyte interaction in models of amyotrophic lateral sclerosis using multi-modal integrated bioinformatics workflow.

Cell-to-cell communications are critical determinants of pathophysiological phenotypes, but methodologies for their systematic elucidation are lacking. Herein, we propose an approach for the Systematic Elucidation and Assessment of Regulatory Cell-to-cell Interaction Networks (SEARCHIN) to identify ligand-mediated interactions between distinct cellular compartments. To test this approach, we selected a model of amyotrophic lateral sclerosis (ALS), in which astrocytes expressing mutant superoxide dismutase-1 (mutSOD1) kill wild-type motor neurons (MNs) by an unknown mechanism. Our integrative analysis that combines proteomics and regulatory network analysis infers the interaction between astrocyte-released amyloid precursor protein (APP) and death receptor-6 (DR6) on MNs as the top predicted ligand-receptor pair. The inferred deleterious role of APP and DR6 is confirmed in vitro in models of ALS. Moreover, the DR6 knockdown in MNs of transgenic mutSOD1 mice attenuates the ALS-like phenotype. Our results support the usefulness of integrative, systems biology approach to gain insights into complex neurobiological disease processes as in ALS and posit that the proposed methodology is not restricted to this biological context and could be used in a variety of other non-cell-autonomous communication mechanisms.

Modeling Charcot-Marie-Tooth Disease In Vitro by Transfecting Mouse Primary Motoneurons.

Neurodegeneration of spinal motoneurons (MNs) is implicated in a large spectrum of neurological disorders including amyotrophic lateral sclerosis, Charcot-Marie-Tooth disease, and spinal muscular atrophy, which are all associated with muscular atrophy. Primary cultures of spinal MNs have been used widely to demonstrate the involvement of specific genes in such diseases and characterize the cellular consequences of their mutations. This protocol models a primary MN culture derived from the seminal work of Henderson and colleagues more than twenty years ago. First, we detail a method of dissecting the anterior horns of the spinal cord from a mouse embryo and isolating the MNs from neighboring cells using a density gradient. Then, we present a new way of efficiently transfecting MNs with expression plasmids using magnetofection. Finally, we illustrate how to fix and immunostain primary MNs. Using neurofilament mutations that cause Charcot-Marie-Tooth disease type 2, this protocol demonstrates a qualitative approach to expressing proteins of interest and studying their involvement in MN growth, maintenance, and survival.

Sensory neuropathy in progressive motor neuronopathy (pmn) mice is associated with defects in microtubule polymerization and axonal transport.

Motor neuron diseases such as amyotrophic lateral sclerosis (ALS) are now recognized as multi-system disorders also involving various non-motor neuronal cell types. The precise extent and mechanistic basis of non-motor neuron damage in human ALS and ALS animal models remain however unclear. To address this, we here studied progressive motor neuronopathy (pmn) mice carrying a missense loss-of-function mutation in tubulin binding cofactor E (TBCE). These mice manifest a particularly aggressive form of motor axon dying back and display a microtubule loss, similar to that induced by human ALS-linked TUBA4A mutations. Using whole nerve confocal imaging of pmn × thy1.2-YFP16 fluorescent reporter mice and electron microscopy, we demonstrate axonal discontinuities, bead-like spheroids and ovoids in pmn suralis nerves indicating prominent sensory neuropathy. The axonal alterations qualitatively resemble those in phrenic motor nerves but do not culminate in the loss of myelinated fibers. We further show that the pmn mutation decreases the level of TBCE, impedes microtubule polymerization in dorsal root ganglion (DRG) neurons and causes progressive loss of microtubules in large and small caliber suralis axons. Live imaging of axonal transport using GFP-tagged tetanus toxin C-fragment (GFP-TTC) demonstrates defects in microtubule-based transport in pmn DRG neurons, providing a potential explanation for the axonal alterations in sensory nerves. This study unravels sensory neuropathy as a pathological feature of mouse pmn, and discusses the potential contribution of cytoskeletal defects to sensory neuropathy in human motor neuron disease.

Cryptic amyloidogenic elements in mutant NEFH causing Charcot-Marie-Tooth 2 trigger aggresome formation and neuronal death.

Neurofilament heavy chain (NEFH) gene was recently identified to cause autosomal dominant axonal Charcot-Marie-Tooth disease (CMT2cc). However, the clinical spectrum of this condition and the physio-pathological pathway remain to be delineated. We report 12 patients from two French families with axonal dominantly inherited form of CMT caused by two new mutations in the NEFH gene. A remarkable feature was the early involvement of proximal muscles of the lower limbs associated with pyramidal signs in some patients. Nerve conduction velocity studies indicated a predominantly motor axonal neuropathy. Unique deletions of two nucleotides causing frameshifts near the end of the NEFH coding sequence were identified: in family 1, c.3008_3009del (p.Lys1003Argfs*59), and in family 2 c.3043_3044del (p.Lys1015Glyfs*47). Both frameshifts lead to 40 additional amino acids translation encoding a cryptic amyloidogenic element. Consistently, we show that these mutations cause protein aggregation which are recognised by the autophagic pathway in motoneurons and triggered caspase 3 activation leading to apoptosis in neuroblastoma cells. Using electroporation of chick embryo spinal cord, we confirm that NEFH mutants form aggregates in vivo and trigger apoptosis of spinal cord neurons. Thus, our results provide a physiological explanation for the overlap between CMT and amyotrophic lateral sclerosis (ALS) clinical features in affected patients.

The Regulatory Machinery of Neurodegeneration in In Vitro Models of Amyotrophic Lateral Sclerosis.

Neurodegenerative phenotypes reflect complex, time-dependent molecular processes whose elucidation may reveal neuronal class-specific therapeutic targets. The current focus in neurodegeneration has been on individual genes and pathways. In contrast, we assembled a genome-wide regulatory model (henceforth, "interactome"), whose unbiased interrogation revealed 23 candidate causal master regulators of neurodegeneration in an in vitro model of amyotrophic lateral sclerosis (ALS), characterized by a loss of spinal motor neurons (MNs). Of these, eight were confirmed as specific MN death drivers in our model of familial ALS, including NF-κB, which has long been considered a pro-survival factor. Through an extensive array of molecular, pharmacological, and biochemical approaches, we have confirmed that neuronal NF-κB drives the degeneration of MNs in both familial and sporadic models of ALS, thus providing proof of principle that regulatory network analysis is a valuable tool for studying cell-specific mechanisms of neurodegeneration.

PlexinA1 is a new Slit receptor and mediates axon guidance function of Slit C-terminal fragments.

Robo-Slit and Plexin-Semaphorin signaling participate in various developmental and pathogenic processes. During commissural axon guidance in the spinal cord, chemorepulsion by Semaphorin3B and Slits controls midline crossing. Slit processing generates an N-terminal fragment (SlitN) that binds to Robo1 and Robo2 receptors and mediates Slit repulsive activity, as well as a C-terminal fragment (SlitC) with an unknown receptor and bioactivity. We identified PlexinA1 as a Slit receptor and found that it binds the C-terminal Slit fragment specifically and transduces a SlitC signal independently of the Robos and the Neuropilins. PlexinA1-SlitC complexes are detected in spinal cord extracts, and ex vivo, SlitC binding to PlexinA1 elicits a repulsive commissural response. Analysis of various ligand and receptor knockout mice shows that PlexinA1-Slit and Robo-Slit signaling have complementary roles during commissural axon guidance. Thus, PlexinA1 mediates both Semaphorin and Slit signaling, and Slit processing generates two active fragments, each exerting distinct effects through specific receptors.

Use of pHluorin to assess the dynamics of axon guidance receptors in cell culture and in the chick embryo.

During development, axon guidance receptors play a crucial role in regulating axons sensitivity to both attractive and repulsive cues. Indeed, activation of the guidance receptors is the first step of the signaling mechanisms allowing axon tips, the growth cones, to respond to the ligands. As such, the modulation of their availability at the cell surface is one of the mechanisms that participate in setting the growth cone sensitivity. We describe here a method to precisely visualize the spatio-temporal cell surface dynamics of an axon guidance receptor both in vitro and in vivo in the developing chick spinal cord. We took advantage of the pH-dependent fluorescence property of a green fluorescent protein (GFP) variant to specifically detect the fraction of the axon guidance receptor that is addressed to the plasma membrane. We first describe the in vitro validation of such pH-dependent constructs and we further detail their use in vivo, in the chick spinal chord, to assess the spatio-temporal dynamics of the axon guidance receptor of interest.