Expanding the spectrum of LAMB2: Pierson syndrome associated with neuromuscular junction disorder in two patients.

LAMB2 gene disorders present with different phenotypes. Pierson syndrome (PS) is a common phenotype associated with LAMB2 variants. Neuromuscular phenotype has been reported including hypotonia and developmental delay. However, neuromuscular junction abnormalities represented as congenital myasthenic syndrome (CMS) was reported in one adult patient only. Here, in this paper, we present two pediatric cases with a severe presentation of PS and have CMS so expanding the knowledge of LAMB2 related phenotypes. The first patient had hypotonia and global developmental delay. Targeted genetic testing panel demonstrated homozygous pathogenic variant in the LAMB2 gene (c.5182C>T, pGln1728*) which was reported by Maselli et al. 2009. Repetitive nerve stimulation (RNS) showed a decremental response at low frequency of 3 Hz. On the other hand, the second patient had profound weakness since birth. Tri-Whole exome sequencing showed homozygous pathogenic variant in the LAMB2 gene c.2890C>T, pArg964*. A trial of salbutamol did not improve the symptoms. Both patients passed away from sequala of PS. The spectrum of phenotypic changes associated with LAMB2 mutations is still expanding, and further investigation into the various clinical and morphologic presentations associated with these mutations is important to better identify and manage affected individuals.

How Inflammation Pathways Contribute to Cell Death in Neuro-Muscular Disorders.

Neuro-muscular disorders include a variety of diseases induced by genetic mutations resulting in muscle weakness and waste, swallowing and breathing difficulties. However, muscle alterations and nerve depletions involve specific molecular and cellular mechanisms which lead to the loss of motor-nerve or skeletal-muscle function, often due to an excessive cell death. Morphological and molecular studies demonstrated that a high number of these disorders seem characterized by an upregulated apoptosis which significantly contributes to the pathology. Cell death involvement is the consequence of some cellular processes that occur during diseases, including mitochondrial dysfunction, protein aggregation, free radical generation, excitotoxicity and inflammation. The latter represents an important mediator of disease progression, which, in the central nervous system, is known as neuroinflammation, characterized by reactive microglia and astroglia, as well the infiltration of peripheral monocytes and lymphocytes. Some of the mechanisms underlying inflammation have been linked to reactive oxygen species accumulation, which trigger mitochondrial genomic and respiratory chain instability, autophagy impairment and finally neuron or muscle cell death. This review discusses the main inflammatory pathways contributing to cell death in neuro-muscular disorders by highlighting the main mechanisms, the knowledge of which appears essential in developing therapeutic strategies to prevent the consequent neuron loss and muscle wasting.

Biallelic loss of OTUD7A causes severe muscular hypotonia, intellectual disability, and seizures.

The heterozygous deletion of 15q13.3 is a recurrently observed microdeletion syndrome associated with a relatively mild phenotype including learning disability and language impairment. In contrast, the homozygous deletion of 15q13.3 is extremely rare and is associated with a much severer phenotype that includes epileptic encephalopathy, profound intellectual disability, and hypotonia. Which of the genes within the deleted interval is responsible for the more severe features when biallelically deleted is currently unknown. Here, we report a patient with profound hypotonia, severe intellectual disability, and seizures who had biallelic loss-of-function variants in OTUD7A: a 15q13.3 deletion including the OTUD7A locus, and a frameshift OTUD7A variant c.1125del, p.(Glu375Aspfs*11). Unexpectedly, both aberrations occurred de novo. Our experiment using Caenorhabditis elegans showed that worms carrying a corresponding homozygous variant in the homolog OTUB-2 exhibited weakened muscle contraction suggestive of aberrant neuromuscular transmission. We concluded that the biallelic complete loss of OTUD7A in humans represents a presumably new autosomal recessive disorder characterized by profound hypotonia, severe intellectual disability, and seizures.

Skeletal Muscle Channelopathies.

Skeletal muscle channelopathies are rare genetic neuromuscular conditions that include the nondystrophic myotonias and periodic paralyses. They cause disabling muscle symptoms and can limit educational potential, work opportunities, socialization, and quality of life. Effective therapy is available, making it essential to recognize and treat this group of disorders. Here, the authors highlight important aspects regarding diagnosis and management using illustrative case reports.

Loss of mitochondrial protein CHCHD10 in skeletal muscle causes neuromuscular junction impairment.

The neuromuscular junction (NMJ) is a synapse between motoneurons and skeletal muscles to control motor behavior. Acetylcholine receptors (AChRs) are restricted at the synaptic region for proper neurotransmission. Mutations in the mitochondrial CHCHD10 protein have been identified in multiple neuromuscular disorders; however, the physiological roles of CHCHD10 at NMJs remain elusive. Here, we report that CHCHD10 is highly expressed at the postsynapse of NMJs in skeletal muscles. Muscle conditional knockout CHCHD10 mice showed motor defects, abnormal neuromuscular transmission and NMJ structure. Mechanistically, we found that mitochondrial CHCHD10 is required for ATP production, which facilitates AChR expression and promotes agrin-induced AChR clustering. Importantly, ATP could effectively rescue the reduction of AChR clusters in the CHCHD10-ablated muscles. Our study elucidates a novel physiological role of CHCHD10 at the peripheral synapse. It suggests that mitochondria dysfunction contributes to neuromuscular pathogenesis.

Presynaptic congenital myasthenic syndrome with a homozygous sequence variant in LAMA5 combines myopia, facial tics, and failure of neuromuscular transmission.

Defects in genes encoding the isoforms of the laminin alpha subunit have been linked to various phenotypic manifestations, including brain malformations, muscular dystrophy, ocular defects, cardiomyopathy, and skin abnormalities. We report here a severe defect of neuromuscular transmission in a consanguineous patient with a homozygous variant in the laminin alpha-5 subunit gene (LAMA5). The variant c.8046C>T (p.Arg2659Trp) is rare and has a predicted deleterious effect. The affected individual, who also carries a rare homozygous sequence variant in LAMA1, had muscle weakness, myopia, and facial tics. Magnetic resonance imaging of brain showed mild volume loss and periventricular T2 prolongation. Repetitive nerve stimulation revealed 50% decrement of compound muscle action potential amplitudes and 250% facilitation immediately after exercise, Endplate studies identified a profound reduction of the endplate potential quantal content and endplates with normal postsynaptic folding that were denuded or partially occupied by small nerve terminals. Expression studies revealed that p.Arg2659Trp caused decreased binding of laminin alpha-5 to SV2A and impaired laminin-521 cell-adhesion and cell projection support in primary neuronal cultures. In summary, this report describing severe neuromuscular transmission failure in a patient with a LAMA5 mutation expands the list of phenotypes associated with defects in genes encoding alpha-laminins.

Ethical issues in the evaluation of adults with suspected genetic neuromuscular disorders.

Genetic testing is rapidly becoming an increasingly significant part of the diagnostic armamentarium of neuromuscular clinicians. Although technically easy to order, the results of such testing, whether positive or negative, have potentially enormous consequences for the individual tested and for family members. As a result, ethical considerations must be in the forefront of the physician's agenda when obtaining genetic testing. Informed consent is an important starting point for discussions between physicians and patients, but the counseling embedded in the informed consent process must be an ongoing part of subsequent interactions, including return of results and follow-up. Patient autonomy, including the right to know and right not-to-know results, must be respected. Considerations of capacity, physician beneficence and nonmaleficence, and privacy all play roles in the process. Muscle Nerve 54: 997-1006, 2016.

Structural and Functional Abnormalities of the Neuromuscular Junction in the Trembler-J Homozygote Mouse Model of Congenital Hypomyelinating Neuropathy.

Mutations in peripheral myelin protein 22 (PMP22) result in the most common form of Charcot-Marie-Tooth (CMT) disease, CMT1A. This hereditary peripheral neuropathy is characterized by dysmyelination of peripheral nerves, reduced nerve conduction velocity, and muscle weakness. APMP22 point mutation in L16P (leucine 16 to proline) underlies a form of human CMT1A as well as the Trembler-J mouse model of CMT1A. Homozygote Trembler-J mice (Tr(J)) die early postnatally, fail to make peripheral myelin, and, therefore, are more similar to patients with congenital hypomyelinating neuropathy than those with CMT1A. Because recent studies of inherited neuropathies in humans and mice have demonstrated that dysfunction and degeneration of neuromuscular synapses or junctions (NMJs) often precede impairments in axonal conduction, we examined the structure and function of NMJs in Tr(J)mice. Although synapses appeared to be normally innervated even in end-stage Tr(J)mice, the growth and maturation of the NMJs were altered. In addition, the amplitudes of nerve-evoked muscle endplate potentials were reduced and there was transmission failure during sustained nerve stimulation. These results suggest that the severe congenital hypomyelinating neuropathy that characterizes Tr(J)mice results in structural and functional deficits of the developing NMJ.

Plasticity of motor network and function in the absence of corticospinal projection.

Despite the obvious clinical interest, our understanding of how developmental mechanisms are redeployed during degeneration and regeneration after brain and spinal cord injuries remains quite rudimentary. In animal models of spinal cord injury, although spontaneous regeneration of descending axons is limited, compensation by intact corticospinal axons, descending tracts from the brainstem, and local intrinsic spinal networks all contribute to the recovery of motor function. Here, we investigated spontaneous motor compensation and plasticity that occur in the absence of corticospinal tract, using Celsr3|Emx1 mice in which the corticospinal tract is completely and specifically absent as a consequence of Celsr3 inactivation in the cortex. Mutant mice had no paresis, but displayed hyperactivity in open-field, and a reduction in skilled movements in food pellet manipulation tests. The number of spinal motoneurons was reduced and their terminal arbors at neuromuscular junctions were atrophic, which was reflected in electromyography deficits. Rubrospinal projections, calretinin-positive propriospinal projections, afferent innervation of motoneurons by calretinin-positive segmental interneurons, and terminal ramifications of monoaminergic projections were significantly increased. Contrary to control animals, mutants also developed a severe and persistent disability of forelimb use following the section of the rubrospinal tract at the C4 spinal level. These observations demonstrate for the first time that the congenital absence of the corticospinal tract induces spontaneous plasticity, both at the level of the motor spinal cord and in descending monoaminergic and rubrospinal projections. Such compensatory mechanisms could be recruited in case of brain or spinal cord lesion or degeneration.

LMO7-null mice exhibit phenotypes consistent with emery-dreifuss muscular dystrophy.

INTRODUCTION: Mutations in the inner nuclear envelope protein emerin cause Emery-Dreifuss muscular dystrophy (EDMD), which is characterized by progressive skeletal muscle wasting, cardiac conduction defects, and tendon contractures. We previously showed that emerin binds directly to the transcription regulator Lmo7 and attenuates its activity to regulate the proper temporal expression of important myogenic differentiation genes. METHODS: The skeletal muscle and cardiac phenotypes were analyzed in a newly generated Lmo7-null mouse using histological analysis, echocardiography, and various neuromuscular tests to determine if Lmo7 was important for skeletal muscle and cardiac function. RESULTS: Lmo7-null mice had growth retardation, decreased fiber size, and impaired skeletal muscle and cardiac function. Lmo7-null mice also had lower levels of phosphorylated retinoblastoma (Rb), extracellular signal-regulated kinase, and c-Jun N-terminal kinase, which is consistent with altered Rb and mitogen-activated protein kinase signaling. CONCLUSIONS: These findings demonstrate that loss of Lmo7 in mice causes myopathic phenotypes similar to those seen in other EDMD mouse models.

The MuSK activator agrin has a separate role essential for postnatal maintenance of neuromuscular synapses.

The motoneural control of skeletal muscle contraction requires the neuromuscular junction (NMJ), a midmuscle synapse between the motor nerve and myotube. The formation and maintenance of NMJs are orchestrated by the muscle-specific receptor tyrosine kinase (MuSK). Motor neuron-derived agrin activates MuSK via binding to MuSK's coreceptor Lrp4, and genetic defects in agrin underlie a congenital myasthenic syndrome (an NMJ disorder). However, MuSK-dependent postsynaptic differentiation of NMJs occurs in the absence of a motor neuron, indicating a need for nerve/agrin-independent MuSK activation. We previously identified the muscle protein Dok-7 as an essential activator of MuSK. Although NMJ formation requires agrin under physiological conditions, it is dispensable for NMJ formation experimentally in the absence of the neurotransmitter acetylcholine, which inhibits postsynaptic specialization. Thus, it was hypothesized that MuSK needs agrin together with Lrp4 and Dok-7 to achieve sufficient activation to surmount inhibition by acetylcholine. Here, we show that forced expression of Dok-7 in muscle enhanced MuSK activation in mice lacking agrin or Lrp4 and restored midmuscle NMJ formation in agrin-deficient mice, but not in Lrp4-deficient mice, probably due to the loss of Lrp4-dependent presynaptic differentiation. However, these NMJs in agrin-deficient mice rapidly disappeared after birth, and postsynaptic specializations emerged ectopically throughout myotubes whereas exogenous Dok-7-mediated MuSK activation was maintained. These findings demonstrate that the MuSK activator agrin plays another role essential for the postnatal maintenance, but not for embryonic formation, of NMJs and also for the postnatal, but not prenatal, midmuscle localization of postsynaptic specializations, providing physiological and pathophysiological insight into NMJ homeostasis.

Congenital myopathies with secondary neuromuscular transmission defects; a case report and review of the literature.

Congenital myopathies are a clinically and genetically heterogeneous group of disorders characterized by early onset hypotonia, weakness and characteristic, but not pathognomonic, structural abnormalities in muscle fibres. The clinical features overlap with muscular dystrophies, myofibrillar myopathies, neurogenic conditions and congenital myasthenic syndromes. We describe a case of cap myopathy with myasthenic features due to a mutation in the TPM2 gene that responded to anticholinesterase therapy. We also review other published cases of congenital myopathies with neuromuscular transmission abnormalities. This report expands the spectrum of congenital myopathies with secondary neuromuscular transmission defects. The recognition of these cases is important since these conditions can benefit from treatment with drugs enhancing neuromuscular transmission.

Morphological analysis of neuromuscular junction development and degeneration in rodent lumbrical muscles.

BACKGROUND: The neuromuscular junction (NMJ) is a specialised synapse formed between a lower motor neuron and a skeletal muscle fibre, and is an early pathological target in numerous nervous system disorders, including amyotrophic lateral sclerosis (ALS), Charcot-Marie-Tooth disease (CMT), and spinal muscular atrophy (SMA). Being able to accurately visualise and quantitatively characterise the NMJ in rodent models of neurological conditions, particularly during the early stages of disease, is thus of clear importance. NEW METHOD: We present a method for dissection of rodent deep lumbrical muscles located in the hind-paw, and describe how to perform immunofluorescent morphological analysis of their NMJs. RESULTS: These techniques allow the temporal assessment of a number of developmental and pathological NMJ phenotypes in lumbrical muscles. COMPARISON WITH EXISTING METHODS: Small muscles, such as the distal hind-limb lumbrical muscles, possess a major advantage over larger muscles, such as gastrocnemius, in that they can be whole-mounted and the entire innervation pattern visualised. This reduces preparation time and ambiguity when evaluating important neuromuscular phenotypes. CONCLUSIONS: Together, these methods will allow the reader to perform a detailed and accurate analysis of the neuromuscular system in rodent models of disease in order to identify pertinent features of neuropathology.

Survival motor neuron affects plastin 3 protein levels leading to motor defects.

The actin-binding protein plastin 3 (PLS3) has been identified as a modifier of the human motoneuron disease spinal muscular atrophy (SMA). SMA is caused by decreased levels of the survival motor neuron protein (SMN) and in its most severe form causes death in infants and young children. To understand the mechanism of PLS3 in SMA, we have analyzed pls3 RNA and protein in zebrafish smn mutants. We show that Pls3 protein levels are severely decreased in smn(-/-) mutants without a reduction in pls3 mRNA levels. Moreover, we show that both pls3 mRNA and protein stability are unaffected when Smn is reduced. This indicates that SMN affects PLS3 protein production. We had previously shown that, in smn mutants, the presynaptic protein SV2 is decreased at neuromuscular junctions. Transgenically driving human PLS3 in motoneurons rescues the decrease in SV2 expression. To determine whether PLS3 could also rescue function, we performed behavioral analysis on smn mutants and found that they had a significant decrease in spontaneous swimming and turning. Driving PLS3 transgenically in motoneurons rescued both of these defects. These data show that PLS3 protein levels are dependent on SMN and that PLS3 is able to rescue the neuromuscular defects and corresponding movement phenotypes caused by low levels of Smn suggesting that decreased PLS3 contributes to SMA motor phenotypes.

Prenatal diagnosis and genetic analysis of fetal akinesia deformation sequence and multiple pterygium syndrome associated with neuromuscular junction disorders: a review.

Fetal akinesia deformation sequence is a clinically and genetically heterogeneous disorder characterized by a variable combination of arthrogryposis, fetal akinesia, intrauterine growth restriction, developmental abnormalities such as cystic hygroma, pulmonary hypoplasia, cleft palate, cryptorchidism, cardiac defects and intestinal malrotation, and occasional pterygia of the limbs. Multiple pterygium syndrome is a clinically and genetically heterogeneous disorder characterized by pterygia of the neck, elbows and/or knees, arthrogryposis, and other phenotypic features such as short stature, genital abnormalities, craniofacial abnormalities, clubfoot, kyphoscoliosis, and cardiac abnormalities. Fetal akinesia deformation sequence may phenotypically overlap with the lethal type of multiple pterygium syndrome. This article provides a comprehensive review of prenatal diagnosis and genetic analysis of fetal akinesia deformation sequence and multiple pterygium syndrome associated with neuromuscular junction disorders. Prenatal diagnosis of fetal akinesia along with cystic hygroma, increased nuchal translucency, nuchal edema, hydrops fetalis, arthrogryposis, pterygia, and other structural abnormalities should include a differential diagnosis of neuromuscular junction disorders. Genetic analysis of mutations in the neuromuscular junction genes such as CHRNA1, CHRND, CHRNG, CNTN1, DOK7, RAPSN, and SYNE1 may unveil the pathogenetic cause of fetal akinesia deformation sequence and multiple pterygium syndrome, and the information acquired is helpful for genetic counseling and clinical management.

Protein-anchoring strategy for delivering acetylcholinesterase to the neuromuscular junction.

Acetylcholinesterase (AChE) at the neuromuscular junction (NMJ) is anchored to the synaptic basal lamina via a triple helical collagen Q (ColQ). Congenital defects of ColQ cause endplate AChE deficiency and myasthenic syndrome. A single intravenous administration of adeno-associated virus serotype 8 (AAV8)-COLQ to Colq(-/-) mice recovered motor functions, synaptic transmission, as well as the morphology of the NMJ. ColQ-tailed AChE was specifically anchored to NMJ and its amount was restored to 89% of the wild type. We next characterized the molecular basis of this efficient recovery. We first confirmed that ColQ-tailed AChE can be specifically targeted to NMJ by an in vitro overlay assay in Colq(-/-) mice muscle sections. We then injected AAV1-COLQ-IRES-EGFP into the left tibialis anterior and detected AChE in noninjected limbs. Furthermore, the in vivo injection of recombinant ColQ-tailed AChE protein complex into the gluteus maximus muscle of Colq(-/-) mice led to accumulation of AChE in noninjected forelimbs. We demonstrated for the first time in vivo that the ColQ protein contains a tissue-targeting signal that is sufficient for anchoring itself to the NMJ. We propose that the protein-anchoring strategy is potentially applicable to a broad spectrum of diseases affecting extracellular matrix molecules.

Motor neuron, nerve, and neuromuscular junction disease.

PURPOSE OF REVIEW: The aim is to review the most relevant findings published during the last year concerning clinical, genetic, pathogenic, and therapeutic advances in motor neuron disease, neuropathies, and neuromuscular junction disorders. RECENT FINDINGS: Studies on animal and cell models have improved the understanding of how mutated survival motor neuron protein in spinal muscular atrophy governs the pathogenetic processes. New phenotypes of SOD1 mutations have been described. Moreover, animal models enhanced the insight into the pathogenetic background of sporadic and familial amyotrophic lateral sclerosis. Novel treatment options for motor neuron disease have been described in humans and animal models. Considerable progress has been achieved also in elucidating the genetic background of many forms of inherited neuropathies and high clinical and genetic heterogeneity has been demonstrated. Mutations in MuSK and GFTP1 have been shown to cause new types of congenital myasthenic syndromes. A third type of autoantibodies (Lrp4) has been detected to cause myasthenia gravis. SUMMARY: Advances in the clinical and genetic characterization of motor neuron diseases, neuropathies, and neuromuscular transmission defects have important implications on the fundamental understanding, diagnosis, and management of these disorders. Identification of crucial steps of the pathogenetic process may provide the basis for the development of novel therapeutic strategies.

Endplate structure and parameters of neuromuscular transmission in sporadic centronuclear myopathy associated with myasthenia.

Centronuclear myopathy is a pathologically diagnosed congenital myopathy. The disease genes encode proteins with membrane modulating properties (MTM1, DNM2, and BIN1) or alter excitation-contraction coupling (RYR1). Some patients also have myasthenic symptoms but electrodiagnostic and endplate studies in these are limited. A sporadic patient had fatigable weakness and a decremental EMG response. Analysis of centronuclear myopathy disease- and candidate-genes identified no mutations. Quantitative endplate electron microscopy studies revealed simplified postsynaptic regions, endplate remodeling with normal nerve terminal size, normal synaptic vesicle density, and mild acetylcholine receptor deficiency. The amplitude of the miniature endplate potential was decreased to 60% of normal. Quantal release by nerve impulse was reduced to 40% of normal due to a decreased number of releasable quanta. The safety margin of neuromuscular transmission is compromised by decreased quantal release by nerve impulse and by a reduced postsynaptic response to the released quanta.

Therapy update in nerve, neuromuscular junction and myopathic disorders.

PURPOSE OF REVIEW: The objective of this review is to summarize recent advances in the treatment of various neuromuscular disorders including neuropathies, neuromuscular junction disorders, and myopathies. RECENT FINDINGS: Immunotherapy with sophisticated agents for myasthenia gravis and inflammatory myopathies, neuroprotection with vitamin E for chemotherapy-induced neuropathy, and promising gene transfer and exon-skipping therapies for muscular dystrophy are among the most exciting recent developments in the treatment of neuromuscular disorders. SUMMARY: In spite of significant advances, therapy in many neuromuscular diseases remains far from satisfactory. Better understanding of the underlying molecular and pathophysiologic processes for both hereditary and acquired disorders should lead to more refined and successful therapeutic approaches, reducing physical and other types of disability while posing fewer side effects.

Neuromuscular defects and breathing disorders in a new mouse model of spinal muscular atrophy.

Spinal muscular atrophy (SMA) is caused by insufficient levels of the survival motor neuron (SMN) protein leading to muscle paralysis and respiratory failure. In mouse, introducing the human SMN2 gene partially rescues Smn(-)(/)(-) embryonic lethality. However current models were either too severe or nearly unaffected precluding convenient drug testing for SMA. We report here new SMN2;Smn(-/-) lines carrying one to four copies of the human SMN2 gene. Mice carrying three SMN2 copies exhibited an intermediate phenotype with delayed appearance of motor defects and developmental breathing disorders reminiscent of those found in severe SMA patients. Although normal at birth, at 7 days of age respiratory rate was decreased and apnea frequency was increased in SMA mice in parallel with the appearance of neuromuscular junction defects in the diaphragm. With median survival of 15 days and postnatal onset of neurodegeneration, these mice could be an important tool for evaluating new therapeutics.