Diaphragmatic spinal muscular atrophy with respiratory distress is heterogeneous, and one form Is linked to chromosome 11q13-q21

The Clinical Heterogeneity of Spinal Muscular Atrophy with Respiratory Distress Type 1 (SMARD1)-A Report of Three Cases, Including Twins

Spinal muscular atrophy with respiratory distress type 1 (SMARD1; OMIM #604320, ORPHA:98920) is a rare autosomal recessive congenital motor neuron disease. It is caused by variants in the IGHMBP2 gene. Clinically, it presents with respiratory failure due to diaphragmatic paralysis, progressive muscle weakness starting in the distal parts of the limbs, dysphagia, and damage to sensory and autonomic nerves. Unlike spinal muscular atrophy (SMA), SMARD1 has a distinct genetic etiology and is not detected in the population newborn screening programs. Most children with SMARD1 do not survive beyond the first year of life due to progressive respiratory failure. Artificial ventilation can prolong survival, but no specific treatment is available. Therapy focuses on mechanical ventilation and improving the patient's quality of life. Research into gene therapy is ongoing. We report three female patients with SMARD1, including twins from a triplet pregnancy. In twin sisters (patient no. 1 and patient no. 2), two heterozygous variants in the IGHMBP2 gene were identified: c.595G>C/p.Ala199Pro and c.1615_1623del/p.Ser539_Tyr541del. In patient no. 3, a variant c.1478C>T/p.Thr493Ile and a variant c.439C>T/p.Arg147* in the IGHMBP2 gene were detected. Our findings underscore the variability of clinical presentations, even among patients sharing the same pathogenic variants in the IGHMBP2 gene, and emphasize the importance of early genetic diagnosis in patients presenting with respiratory failure, with or without associated diaphragmatic muscle paralysis.

Childhood spinal muscular atrophy

Spinal muscular atrophy (SMA) is caused by biallelic mutations in the SMN1 (survival motor neuron 1) gene on chromosome 5q13.2, which leads to a progressive degeneration of alpha motor neurons in the spinal cord and in motor nerve nuclei in the caudal brainstem. It is characterized by progressive proximally accentuated muscle weakness with loss of already acquired motor skills, areflexia and, depending on the phenotype, varying degrees of weakness of the respiratory and bulbar muscles. Over the past decade, disease-modifying therapies have become available based on splicing modulation of the SMN2 with SMN1 gene replacement, which if initiated significantly modifies the natural course of the disease. Newborn screening for SMA has been implemented in an increasing number of centers; however, available evidence for these new treatments is often limited to a small spectrum of patients concerning age and disease stage.

Therapy development for spinal muscular atrophy: perspectives for muscular dystrophies and neurodegenerative disorders

BACKGROUND

Major efforts have been made in the last decade to develop and improve therapies for proximal spinal muscular atrophy (SMA). The introduction of Nusinersen/Spinraza™ as an antisense oligonucleotide therapy, Onasemnogene abeparvovec/Zolgensma™ as an AAV9-based gene therapy and Risdiplam/Evrysdi™ as a small molecule modifier of pre-mRNA splicing have set new standards for interference with neurodegeneration.

MAIN BODY

Therapies for SMA are designed to interfere with the cellular basis of the disease by modifying pre-mRNA splicing and enhancing expression of the Survival Motor Neuron (SMN) protein, which is only expressed at low levels in this disorder. The corresponding strategies also can be applied to other disease mechanisms caused by loss of function or toxic gain of function mutations. The development of therapies for SMA was based on the use of cell culture systems and mouse models, as well as innovative clinical trials that included readouts that had originally been introduced and optimized in preclinical studies. This is summarized in the first part of this review. The second part discusses current developments and perspectives for amyotrophic lateral sclerosis, muscular dystrophies, Parkinson's and Alzheimer's disease, as well as the obstacles that need to be overcome to introduce RNA-based therapies and gene therapies for these disorders.

CONCLUSION

RNA-based therapies offer chances for therapy development of complex neurodegenerative disorders such as amyotrophic lateral sclerosis, muscular dystrophies, Parkinson's and Alzheimer's disease. The experiences made with these new drugs for SMA, and also the experiences in AAV gene therapies could help to broaden the spectrum of current approaches to interfere with pathophysiological mechanisms in neurodegeneration.

Defining the optimal dose and therapeutic window in SMA with respiratory distress type I model mice, FVB/NJ-Ighmpb2 nmd-2J

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is an autosomal recessive disorder that develops in infancy and arises from mutation of the immunoglobulin helicase μ-binding protein 2 (IGHMBP2) gene. Whereas IGHMBP2 is ubiquitously expressed, loss or reduction of function leads to alpha motor neuron loss and skeletal muscle atrophy. We previously developed a gene therapy strategy for SMARD1 using a single-stranded AAV9-IGHMBP2 vector and compared two different delivery methods in a validated SMARD1 mouse model. An important question in the field relates to the temporal requirements for this or any potential treatment. To examine the therapeutic window, we utilized our recently developed SMARD1 model, FVB/NJ-Ighmpb2 nmd-2J , to deliver AAV9-IGHMBP2 at four different time points starting at post-natal day 2 (P2) through P8. At each time point, significant improvements were observed in survival, weight gain, and motor function. Similarly, treatment improved important hallmarks of disease, including motor unit pathology. Whereas improvements were more pronounced in the early-treatment groups, even the later-treatment groups displayed significant phenotypic improvements. This work suggests that an effective gene therapy strategy could provide benefits to pre-symptomatic and early-symptomatic individuals, thereby expanding the potential therapeutic window for SMARD1.

A 20-year Clinical and Genetic Neuromuscular Cohort Analysis in Lebanon: An International Effort

BACKGROUND

Clinical and molecular data on the occurrence and frequency of inherited neuromuscular disorders (NMD) in the Lebanese population is scarce.

OBJECTIVE

This study aims to provide a retrospective overview of hereditary NMDs based on our clinical consultations in Lebanon.

METHODS

Clinical and molecular data of patients referred to a multi-disciplinary consultation for neuromuscular disorders over a 20-year period (1999-2019) was reviewed.

RESULTS

A total of 506 patients were diagnosed with 62 different disorders encompassing 10 classes of NMDs. 103 variants in 49 genes were identified. In this cohort, 81.4%of patients were diagnosed with motor neuron diseases and muscular dystrophies, with almost half of these described with spinal muscular atrophy (SMA) (40.3%of patients). We estimate a high SMA incidence of 1 in 7,500 births in Lebanon. Duchenne and Becker muscular dystrophy were the second most frequently diagnosed NMDs (17%of patients). The latter disorders were associated with the highest number of variants (39) identified in this study. A highly heterogeneous presentation of Limb Girdle Muscular Dystrophy and Charcot-Marie-Tooth disease was notably identified. The least common disorders (5.5%of patients) involved congenital, metabolic, and mitochondrial myopathies, congenital myasthenic syndromes, and myotonic dystrophies. A review of the literature for selected NMDs in Lebanon is provided.

CONCLUSIONS

Our study indicates a high prevalence and underreporting of heterogeneous forms of NMDs in Lebanon- a major challenge with many novel NMD treatments in the pipeline. This report calls for a regional NMD patient registry.

Models for IGHMBP2-associated diseases: an overview and a roadmap for the future

Models are practical tools with which to establish the basic aspects of a diseases. They allow systematic research into the significance of mutations, of cellular and molecular pathomechanisms, of therapeutic options and of functions of diseases associated proteins. Thus, disease models are an integral part of the study of enigmatic proteins such as immunoglobulin mu-binding protein 2 (IGHMBP2). IGHMBP2 has been well defined as a helicase, however there is little known about its role in cellular processes. Notably, it is unclear why changes in such an abundant protein lead to specific neuronal disorders including spinal muscular atrophy with respiratory distress type 1 (SMARD1) and Charcot-Marie-Tooth type 2S (CMT2S). SMARD1 is caused by a loss of motor neurons in the spinal cord that results in muscle atrophy and is accompanied by rapid respiratory failure. In contrast, CMT2S manifests as a severe neuropathy, but typically without critical breathing problems. Here, we present the clinical manifestation of IGHMBP2 mutations, function of protein and models that may be used for the study of IGHMBP2-associated disorders. We highlight the strengths and weaknesses of specific models and discuss the orthologs of IGHMBP2 that are found in different systems with regard to their similarity to human IGHMBP2.

Current understanding of and emerging treatment options for spinal muscular atrophy with respiratory distress type 1 (SMARD1)

Spinal muscular atrophy (SMA) with respiratory distress type 1 (SMARD1) is an autosomal recessive motor neuron disease that is characterized by distal and proximal muscle weakness and diaphragmatic palsy that leads to respiratory distress. Without intervention, infants with the severe form of the disease die before 2 years of age. SMARD1 is caused by mutations in the IGHMBP2 gene that determine a deficiency in the encoded IGHMBP2 protein, which plays a critical role in motor neuron survival because of its functions in mRNA processing and maturation. Although it is rare, SMARD1 is the second most common motor neuron disease of infancy, and currently, treatment is primarily supportive. No effective therapy is available for this devastating disease, although multidisciplinary care has been an essential element of the improved quality of life and life span extension in these patients in recent years. The objectives of this review are to discuss the current understanding of SMARD1 through a summary of the presently known information regarding its clinical presentation and pathogenesis and to discuss emerging therapeutic approaches. Advances in clinical care management have significantly extended the lives of individuals affected by SMARD1 and research into the molecular mechanisms that lead to the disease has identified potential strategies for intervention that target the underlying causes of SMARD1. Gene therapy via gene replacement or gene correction provides the potential for transformative therapies to halt or possibly prevent neurodegenerative disease in SMARD1 patients. The recent approval of the first gene therapy approach for SMA associated with mutations in the SMN1 gene may be a turning point for the application of this strategy for SMARD1 and other genetic neurological diseases.

Overlapping spectrums: The clinicogenetic commonalities between Charcot-Marie-Tooth and other neurodegenerative diseases

Charcot-Marie-Tooth (CMT) disease is a progressive and heterogeneous inherited peripheral neuropathy. A myriad of genetic factors have been identified that contribute to the degeneration of motor and sensory axons in a length-dependent manner. Emerging biological themes underlying disease include defects in axonal trafficking, dysfunction in RNA metabolism and protein homeostasis, as well deficits in the cellular stress response. Moreover, genetic contributions to CMT can have overlap with other neuropathies, motor neuron diseases (MNDs) and neurodegenerative disorders. Recent progress in understanding the molecular biology of CMT and overlapping syndromes aids in the search for necessary therapeutic targets.

Spinal muscular atrophy with respiratory distress type 1: Clinical phenotypes, molecular pathogenesis and therapeutic insights

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a rare autosomal recessive neuromuscular disorder caused by mutations in the IGHMBP2 gene, which encodes immunoglobulin μ‐binding protein 2, leading to progressive spinal motor neuron degeneration. We review the data available in the literature about SMARD1. The vast majority of patients show an onset of typical symptoms in the first year of life. The main clinical features are distal muscular atrophy and diaphragmatic palsy, for which permanent supportive ventilation is required. No effective treatment is available yet, but novel therapeutic approaches, such as gene therapy, have shown encouraging results in preclinical settings and thus represent possible methods for treating SMARD1. Significant advancements in the understanding of both the SMARD1 clinical spectrum and its molecular mechanisms have allowed the rapid translation of preclinical therapeutic strategies to human patients to improve the poor prognosis of this devastating disease.

Development of a novel severe mouse model of spinal muscular atrophy with respiratory distress type 1: FVB-nmd

Spinal Muscular Atrophy with Respiratory Distress type 1 (SMARD1) is an autosomal recessive disease that develops early during infancy. The gene responsible for disease development is immunoglobulin helicase μ-binding protein 2 (IGHMBP2). IGHMBP2 is a ubiquitously expressed gene but its mutation results in the loss of alpha-motor neurons and subsequent muscle atrophy initially of distal muscles. The current SMARD1 mouse model arose from a spontaneous mutation originally referred to as neuromuscular degeneration (nmd). The nmd mice have the C57BL/6 genetic background and contain an A to G mutation in intron 4 of the endogenous Ighmbp2 gene. This mutation causes aberrant splicing, resulting in only 20-25% of full-length functional protein. Several congenital conditions including hydrocephalus are common in the C57BL/6 background, consistent with our previous observations when developing a gene therapy approach for SMARD1. Additionally, a modifier allele exists on chromosome 13 in nmd mice that can partially suppress the phenotype, resulting in some animals that have extended life spans (up to 200 days). To eliminate the intrinsic complications of the C57BL/6 background and the variation in survival due to the genetic modifier effect, we created a new SMARD1 mouse model that contains the same intron 4 mutation in Ighmbp2 as nmd mice but is now on a FVB congenic background. FVB-nmd are consistently more severe than the original nmd mice with respect to survival, weigh and motor function. The relatively short life span (18-21 days) of FVB-nmd mice allows us to monitor therapeutic efficacy for a variety of novel therapeutics in a timely manner without the complication of the small percentage of longer-lived animals that were observed in our colony of nmd mice.

Muscle fiber-type selective propensity to pathology in the nmd mouse model of SMARD1

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is an autosomal recessive disease that causes distal limb muscle atrophy, due to motor neuron degeneration. Similar to other motor neuron diseases, SMARD1 shows differential vulnerability to denervation in various muscle groups, which is recapitulated in the nmd mouse, a model of SMARD1. In multiple neurodegenerative disease models, transcriptomic analysis has identified differentially expressed genes between vulnerable motor neuron populations, but the mechanism leading to susceptibility is largely unknown. To investigate if denervation vulnerability is linked to intrinsic muscle properties, we analyzed muscle fiber-type composition in muscles from motor units that show different degrees of denervation in nmd mice: gastrocnemius, tibialis anterior (TA), and extensor digitorum longus (EDL). Our results revealed that denervation vulnerability correlated with atrophy and loss of MyHC-IIb and MyHC-IIx muscle fiber types. Interestingly, increased vulnerability also correlated with an increased abundance of MyHC-I and MyHC-IIa muscle fibers. These results indicated that MyHC-IIx muscle fibers are the most vulnerable to denervation, followed by MyHC-IIb muscle fibers. Moreover, our data indicate that type MyHC-IIa and MyHC-IIb muscle fibers show resistance to denervation and compensate for the loss of MyHC-IIx and MyHC-IIb muscle fibers in the most vulnerable muscles. Taken together these results provide a basis for the selective vulnerability to denervation of specific muscles in nmd mice and identifies new targets for potential therapeutic intervention.

Diagnostic Odyssey and Application of Targeted Exome Sequencing in the Investigation of Recurrent Infant Deaths in a Syrian Consanguineous Family: a Case of Spinal Muscular Atrophy with Respiratory Distress Type 1

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a rare autosomal recessive disorder caused by a defect in the immunoglobulin mu binding protein 2 (IGHMBP2) gene, leading to motor neuron degeneration. We identified an infant with SMARD1 by targeted exome sequencing from a consanguineous Syrian family having a history of recurrent infant deaths. The patient initially presented intrauterine growth retardation, poor sucking, failure to thrive, and respiratory failure at the age of two months, and an inborn error of metabolism was suspected at first. Over a period of one month, the infant showed rapid progression of distal muscular weakness with hand and foot contractures, which were suggestive of neuromuscular disease. Using targeted exome sequencing, the mutation in IGHMBP2 was confirmed, although the first report was normal. Targeted exome sequencing enabled identification of the genetic cause of recurrent mysterious deaths in the consanguineous family. Additionally, it is suggested that a detailed phenotypic description and communication between bioinformaticians and clinicians is important to reduce false negative results in exome sequencing.

Selective vulnerability in neuronal populations in nmd/SMARD1 mice

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is an autosomal recessive motor neuron disease causing distal limb muscle atrophy that progresses proximally and is accompanied by diaphragmatic paralysis. Neuromuscular junction (NMJ) alterations have been reported in muscles of SMARD1 model mice, known as nmd mice, with varying degrees of severity, suggesting that different muscles are specifically and selectively resistant or susceptible to denervation. To evaluate the extent of NMJ pathology in a broad range of muscles, a panel of axial and appendicular muscles were isolated and immunostained from nmd mice. These analyses revealed that selective distal appendage muscles were highly vulnerable to denervation. Susceptibility to pathology was not limited to NMJ alterations, but included defects in myelination within those neurons innervating susceptible muscles. Interestingly, end plate fragmentation was present within all muscles independent of the extent of NMJ alterations, suggesting that end plate fragmentation is an early hallmark of SMARD1 pathogenesis. Expressing the full-length IGHMBP2 cDNA using an adeno-associated virus (AAV9) significantly decreased all aspects of muscle and nerve disease pathology. These results shed new light onto the pathogenesis of SMARD1 by identifying specific motor units that are resistant and susceptible to neurodegeneration in an important model of SMARD1.

IGHMBP2 mutation associated with organ-specific autonomic dysfunction

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Novel IGHMBP2 variant found in a patient with early onset severe peripheral neuropathy.

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IGHMBP2 mutations may cause enteral autonomic dysfunction.

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Autonomic dysfunction in IGHMBP2-related disorders may be severe requiring parenteral nutrition.

Biallelic mutations in the IGHMBP2 have been associated with two distinct phenotypes: spinal muscular atrophy with respiratory distress type 1 (SMARD1) and CMT2S. We describe a patient who developed progressive muscle weakness and wasting in her upper and lower limbs from infancy. She developed respiratory involvement at age 9, eventually requiring 24-h non-invasive ventilation, and severe autonomic dysfunction restricted to the gastrointestinal tract. Neurophysiological studies at age 27 years revealed absent sensory and motor responses and severe chronic denervation changes in proximal muscles of the upper limbs. Targeted multigene panel sequencing detected a novel homozygous missense variant in the IGHMBP2 gene (c.1325A > G; p.Tyr442Cys). This variant was validated by Sanger sequencing and co-segregation analysis confirmed that both parents were asymptomatic heterozygous carriers. This case report confirms that IGHMBP2 related disorders can result in a severe peripheral neuropathy with gastrointestinal autonomic dysfunction requiring parenteral nutrition.

A Direct Comparison of IV and ICV Delivery Methods for Gene Replacement Therapy in a Mouse Model of SMARD1

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is an infantile autosomal recessive disease caused by the loss of the ubiquitously expressed IGHMBP2 gene. SMARD1 causes degeneration of alpha-motor neurons, resulting in distal muscle weakness, diaphragm paralysis, and respiratory malfunction. We have reported that delivery of a low dose of AAV9-IGHMBP2 to the CNS results in a significant rescue of the SMARD1 mouse model (nmd). To examine how a delivery route can impact efficacy, a direct comparison of intravenous (IV) and intracerebroventricular (ICV) delivery of AAV9-IGHMBP2 was performed. Using a low-dose, both IV and ICV delivery routes led to a significant extension in survival and increased body weight. Conversely, only ICV-treated animals demonstrated improvements in the hindlimb muscle, neuromuscular junction, and motor function. The hindlimb phenotype of IV-treated mice resembled the untreated nmd mice. We investigated whether the increased survival of IV-treated nmd mice was the result of a positive impact on the cardiac function. Our results revealed that cardiac function and pathology were similarly improved in IV- and ICV-treated mice. We concluded that while IV delivery of a low dose does not improve the hindlimb phenotype and motor function, partial restoration of cardiac performance is sufficient to significantly extend survival.

Rescue of a Mouse Model of Spinal Muscular Atrophy With Respiratory Distress Type 1 by AAV9-IGHMBP2 Is Dose Dependent

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is an autosomal recessive disease occurring during childhood. The gene responsible for disease development is a ubiquitously expressed protein, IGHMBP2. Mutations in IGHMBP2 result in the loss of α-motor neurons leading to muscle atrophy in the distal limbs accompanied by respiratory complications. Although genetically and clinically distinct, proximal SMA is also caused by the loss of a ubiquitously expressed gene (SMN). Significant preclinical success has been achieved in proximal SMA using viral-based gene replacement strategies. We leveraged the technologies employed in SMA to demonstrate gene replacement efficacy in an SMARD1 animal model. Intracerebroventricular (ICV) injection of single-stranded AAV9 expressing the full-length cDNA of IGHMBP2 in a low dose led to a significant level of rescue in treated SMARD1 animals. Consistent with drastically increased survival, weight gain, and strength, the rescued animals demonstrated a significant improvement in muscle, NMJ, motor neurons, and axonal pathology. In addition, increased levels of IGHMBP2 in lumbar motor neurons verified the efficacy of the virus to transduce the target tissues. Our results indicate that AAV9-based gene replacement is a viable strategy for SMARD1, although dosing effects and potential negative impacts of high dose and ICV injection should be thoroughly investigated.

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) Report of a Spanish case with extended clinicopathological follow-up

Background: Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a clinically and genetically distinct and uncommon variant of SMA that results from irreversible degeneration of α-motor neurons in the anterior horns of the spinal cord and in ganglion cells on the spinal root ganglia. Aims: To describe the clinical, electrophysiological, neuropathological, and genetic findings, at different stages from birth to death, of a Spanish child diagnosed with SMARD1. Patient and methods: We report the case of a 3-month-old girl with severe respiratory insufficiency and, later, intense hypotonia. Paraclinical tests included biochemistry, chest X-ray, and electrophysiological studies, among others. Muscle and nerve biopsies were performed at 5 and 10 months and studied under light and electron microscopy. Post-mortem examination and genetic investigations were performed. Results: Pre- and post-mortem histopathological findings demonstrated the disease progression over time. Muscle biopsy at 5 months of age was normal, however a marked neurogenic atrophy was present in post-mortem samples. Peripheral motor and sensory nerves were severely involved likely due to a primary axonal disorder. Automatic sequencing of IGHMBP2 revealed a compound heterozygous mutation. Conclusions: The diagnosis of SMARD1 should be considered in children with early respiratory insufficiency or in cases of atypical SMA. Direct sequencing of the IGHMBP2 gene should be performed.

Clinical and molecular features and therapeutic perspectives of spinal muscular atrophy with respiratory distress type 1

Spinal muscular atrophy with respiratory distress (SMARD1) is an autosomal recessive neuromuscular disease caused by mutations in the IGHMBP2 gene, encoding the immunoglobulin μ-binding protein 2, leading to motor neuron degeneration. It is a rare and fatal disease with an early onset in infancy in the majority of the cases. The main clinical features are muscular atrophy and diaphragmatic palsy, which requires prompt and permanent supportive ventilation. The human disease is recapitulated in the neuromuscular degeneration (nmd) mouse. No effective treatment is available yet, but novel therapeutical approaches tested on the nmd mouse, such as the use of neurotrophic factors and stem cell therapy, have shown positive effects. Gene therapy demonstrated effectiveness in SMA, being now at the stage of clinical trial in patients and therefore representing a possible treatment for SMARD1 as well. The significant advancement in understanding of both SMARD1 clinical spectrum and molecular mechanisms makes ground for a rapid translation of pre-clinical therapeutic strategies in humans.

One novel and one recurrent mutation in IGHMBP2 gene, causing severe spinal muscular atrophy respiratory distress 1 with onset soon after birth

A family with 2 siblings with severe spinal muscular atrophy with respiratory distress 1 (SMARD1) was genetically proved to be caused by mutations in IGHMBP2 gene. Both patients developed progressive muscular weakness and respiratory distress and died before 6 months of age. One novel deletion, c.780delG;p.(Gln260Hisfs*24), inherited from the father and a nonsense mutation, c.1488C>A;p.(Cys496*), inherited from the mother were detected. An attempt was made to correlate the genetic-clinical data available in the literature. The clinical case presented in this study might be considered as the most severe form of spinal muscular atrophy respiratory distress 1 reported so far, presumably because of the total absence of IGHMBP2 enzyme activity.

Severe phenotypes of SMARD1 associated with novel mutations of the IGHMBP2 gene and nuclear degeneration of muscle and Schwann cells

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a very rare autosomal recessive form of spinal muscular atrophy manifested in low birth weight, diaphragmatic palsy and distal muscular atrophy. Caused by a mutation in the IGHMBP2 gene, the disease is addressed here by reference to five Polish patients in which SMARD1 has been confirmed genetically. All presented a severe form of the disease and had evident symptoms during the second month of life; with four displaying weak cries, feeding difficulties and hypotonia from birth. Two were afflicted by severe dysfunction of the autonomic nervous system. Ultrastructural analysis of a muscle biopsy revealed progressive degeneration within the nuclei of the muscle cells and Schwann cells. Neuromuscular junctions were also defective. It proved possible to identify in our patients 6 novel IGHMBP2 mutations: three missense (c.595G>C, c.1682T>C and c.1794C>A), two nonsense (c.94C>T and c.1336C>T) and one in-frame deletion (c.1615_1623del). One nonsense mutation (c.429C>T) that had been described previously was also identified. Observation of our patients makes it clear that clinical picture is still the most important factor suggesting diagnosis of SMARD1, though further investigations concerning some of the symptoms are required. As the IGHMBP2 gene is characterized by significant heterogeneity, genetic counseling of affected families is rendered more complex. IGHMBP2 protein deficiency can lead to the degeneration of nuclei, in both muscle and Schwann cells.

Fast motor axon loss in SMARD1 does not correspond to morphological and functional alterations of the NMJ

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a childhood motoneuron disease caused by mutations in the gene encoding for IGHMBP2, an ATPase/Helicase. Paralysis of the diaphragm is an early and prominent clinical sign resulting both from denervation and myopathy. In skeletal muscles, muscle atrophy mainly results from loss of motoneuron cell bodies and axonal degeneration. Although it is well known that loss of motoneurons at the lumbar spinal cord is an early event in the pathogenesis of the disease, it is not clear whether the corresponding proximal axons and NMJs are also early affected. In order to address this question, we have investigated the time course of the disease progression at the level of the motoneuron cell body, proximal axon (ventral root), distal axon (sciatic nerve), NMJ, and muscle fiber in Nmd(2J) mice, a mouse model for SMARD1. Our results show an early and apparently parallel loss of motoneurons, proximal axons, and NMJs. In affected muscles, however, denervated fibers coexist with NMJs with normal morphology and unaltered neurotransmission. Furthermore, unaffected axons are able to sprout and reinnervate muscle fibers, suggesting selective vulnerability of neurons to Ighmbp2 deficiency. The preservation of the NMJ morphology and neurotransmission in the Nmd(2J) mouse until motor axon loss takes place, differs from that observed in SMA mouse models in which NMJ impairment is an early and more general phenomenon in affected muscles.

Recessively transmitted predominantly motor neuropathies

Recessively transmitted predominantly motor neuropathies are rare and show a severe phenotype. They are frequently observed in populations with a high rate of consanguineous marriages. At least 15 genes and six loci have been found to be associated with autosomal recessive CMT (AR-CMT) and X-linked CMT (AR-CMTX) and also distal hereditary motor neuronopathy (AR-dHMN). These disorders are genetically heterogeneous but the clinical phenotype is relatively homogeneous. Distal muscle weakness and atrophy predominating in the lower extremities, diminished or absent deep tendon reflexes, distal sensory loss, and pes cavus are the main clinical features of this disorder with occasional cranial nerve involvement. Although genetic diagnosis of some of subtypes of AR-CMT are now available, rapid advances in the molecular genetics and cell biology show a great complexity. Animal models for the most common subtypes of human AR-CMT disease provide clues for understanding the pathogenesis of CMT and also help to reveal possible treatment strategies of inherited neuropathies. This chapter highlights the clinical features and the recent genetic and biological findings in these disorders based on the current classification.

The natural course of infantile spinal muscular atrophy with respiratory distress type 1 (SMARD1)

BACKGROUND

Only scarce information is available on the long-term outcome and the natural course of children with infantile spinal muscular atrophy with respiratory distress type 1 (SMARD1) due to mutations in the IGHMBP2 gene.

OBJECTIVE

To describe the natural disease course, to systematically quantify the residual capacities of children with SMARD1 who survive on permanent mechanical respiration, and to identify markers predicting the disease outcome at the time of manifestation.

METHODS

We conducted a longitudinal study of 11 infantile SMARD1 patients over a mean observational period of 7.8 (SD 3.2) years. Disease-specific features were continuously assessed by using a semiquantitative scoring system. Additionally, we analyzed the residual enzymatic activity of 6 IGHMBP2 mutants in our patients.

RESULTS

After an initial rapid decline of the clinical score until the age of 2 years, residual capabilities reached a plateau or even improved. The overall clinical outcome was markedly heterogeneous, but clinical scores at the age of 3 months showed a positive linear correlation with the clinical outcome at 1 year and at 4 years of age. If expressed in an in vitro recombinant system, mutations of patients with more favorable outcomes retained residual enzymatic activity.

CONCLUSIONS

Despite their severe disabilities and symptoms, most SMARD1 patients are well integrated into their home environment and two thirds of them are able to attend kindergarten or school. This information will help to counsel parents at the time of disease manifestation.

Myopathy and neurogenic muscular atrophy in unexpected cardiopulmonary arrest

BACKGROUND

  Neuromuscular disorders can be the cause of sudden death of infants because of their weakness and gastroesophageal reflux (GER).

METHODS

  Muscle biopsy and genetic studies were performed by usual method.

RESULTS

  In this report four cases of infants with neuromuscular disorders (two cases of congenital myopathy and two cases of spinal muscular atrophy) who had unexpected cardiopulmonary arrest on arrival (CPAOA) are presented. Two of the cases did not show any symptoms, such as muscle weakness prior to CPAOA. The diagnosis was based on the results of the muscle biopsy and genetic examination.

CONCLUSION

  These results suggest that sudden infant death caused by neuromuscular disorders should be considered.

Infantile spinal muscular atrophy with respiratory distress type 1: a case report

The condition, currently known as spinal muscular atrophy with respiratory distress type 1, is an unusual variant of spinal muscular atrophy type 1 that is characterized by early respiratory failure due to diaphragmatic paralysis. The defective gene, the immunoglobulin mu-binding protein 2 (IGHMBP2 gene), of this autosomal recessive disorder is located on chromosome 11q13 and encodes immunoglobulin mu-binding protein 2. The natural history and phenotypic spectrum of the disease are still not clear. The authors present the first genetically proven case of spinal muscular atrophy with respiratory distress type 1 to be reported from Saudi Arabia. The parents are first cousins and the causative gene sequencing revealed mutation in exon 7 reported for the first time in a homozygous form. The clinical scenario of the case is discussed. The findings in the muscle magnetic resonance imaging (MRI) are presented.

Interfamilial phenotypic heterogeneity in SMARD1

Spinal muscular atrophy with respiratory distress (SMARD1: mu-binding protein 2 gene mutation) is characterised by low birth weight, progressive distal limb weakness, diaphragmatic paralysis and subsequent respiratory failure manifesting before 13 months of age. Our case report illustrates marked phenotype variability in two siblings with an identical genetic mutation of SMARD1, one of whom died of fulminant respiratory failure aged 6 months, whereas the other shows limb weakness but, only mild sleep hypoventilation aged 12 years. This suggests other compensatory mechanisms may play a role in modifying SMARD1; broadening our perception of phenotype. Therefore, SMARD1 phenotype should be considered in cases of atypical spinal muscular atrophy even in the absence of overt diaphragmatic weakness.

[Distal spinal-muscular atrophy 1 (DSMA1 or SMARD1)]

In this article, we review the clinical, neuropathological and genetic aspects of distal spinal-muscular atrophy 1 (DSMA1; MIM#604320), formerly designated as autosomal recessive spinal muscular atrophy with respiratory distress type 1 (SMARD1) and also known as distal hereditary-motor neuropathy type 6 (dHMN6 or HMN6).

A novel locus for dHMN with pyramidal features maps to chromosome 4q34.3-q35.2

The distal hereditary motor neuropathy (dHMN) is a rare genetically and clinically heterogeneous disorder characterized by weakness and wasting of distal limb muscles in absence of overt sensory abnormalities. Recently, pyramidal signs have been also described in some patients with dominant or recessive dHMN, and two different loci have been identified in families affected by dHMN complicated with pyramidal dysfunction. We investigated an Italian family affected by an autosomal dominant dHMN complicated by pyramidal signs in order to map a new gene locus. The disease maps to a novel locus in a 26-cM region flanked by D4S1552 and D4S2930 on chromosome 4q34.3-35.2. Three candidate genes (SNX25, CASP3 and TUBB4Q) located in the critical region were screened for the presence of mutations by heteroduplex analysis. No mutations have been detected in the analyzed genes. In conclusion, the new private genetic locus we reported further confirms the wide heterogeneity of dHMN.

Spinal muscular atrophy with respiratory distress type 1 (SMARD1)

Autosomal recessive spinal muscular atrophy with respiratory distress type 1 (SMARD1), recently referred to as distal spinal muscular atrophy 1 (DSMA1; MIM#604320) and also known as distal hereditary motor neuropathy type 6 (dHMN6 or HMN6), results from mutations in the IGHMBP2 gene on chromosome 11q13.3 encoding the immunoglobulin micro-binding protein 2. In contrast to the infantile spinal muscular atrophy type 1 (SMA1; Werdnig-Hoffmann disease) with weakness predominantly of proximal muscles and bell-shaped thorax deformities due to intercostal muscle atrophy, infants with distal spinal muscular atrophy 1 usually present with distal muscle weakness, foot deformities, and sudden respiratory failure due to diaphragmatic paralysis that often requires urgent intubation. In this article, the authors review the clinical, neuropathological, and genetic aspects of distal spinal muscular atrophy 1 and discuss differential diagnoses.

Charcot-Marie-Tooth disease: a clinico-genetic confrontation

Charcot-Marie-Tooth disease (CMT) is the most common neuromuscular disorder. It represents a group of clinically and genetically heterogeneous inherited neuropathies. Here, we review the results of molecular genetic investigations and the clinical and neurophysiological features of the different CMT subtypes. The products of genes associated with CMT phenotypes are important for the neuronal structure maintenance, axonal transport, nerve signal transduction and functions related to the cellular integrity. Identifying the molecular basis of CMT and studying the relevant genes and their functions is important to understand the pathophysiological mechanisms of these neurodegenerative disorders, and the processes involved in the normal development and function of the peripheral nervous system. The results of molecular genetic investigations have impact on the appropriate diagnosis, genetic counselling and possible new therapeutic options for CMT patients.

REVIEW ARTILCE: Cell therapy and stem cells in animal models of motor neuron disorders

Amyotrophic lateral sclerosis (ALS), spinal bulbar muscular atrophy (or Kennedy's disease), spinal muscular atrophy and spinal muscular atrophy with respiratory distress 1 are neurodegenerative disorders mainly affecting motor neurons and which currently lack effective therapies. Recent studies in animal models as well as primary and embryonic stem cell models of ALS, utilizing over-expression of mutated forms of Cu/Zn superoxide dismutase 1, have shown that motor neuron degeneration in these models is in part a non cell-autonomous event and that by providing genetically non-compromised supporting cells such as microglia or growth factor-excreting cells, onset can be delayed and survival increased. Using models of acute motor neuron injury it has been shown that embryonic stem cell-derived motor neurons implanted into the spinal cord can innervate muscle targets and improve functional recovery. Thus, a rationale exists for the development of cell therapies in motor neuron diseases aimed at either protecting and/or replacing lost motor neurons, interneurons as well as non-neuronal cells. This review evaluates approaches used in animal models of motor neuron disorders and their therapeutic relevance.

Clinical and mutational profile in spinal muscular atrophy with respiratory distress (SMARD): defining novel phenotypes through hierarchical cluster analysis

Autosomal recessive spinal muscular atrophy with respiratory distress (SMARD) is a heterogeneous disorder. Mutations in the immunoglobulin micro-binding protein gene (IGHMBP2) lead to SMARD1, but clinical criteria that delineate SMARD1 from other SMARD syndromes are not well established. Here we present a retrospective clinical and genetic study to determine the criteria that would predict the presence or absence of IGHMBP2 mutations. From 141 patients with respiratory distress and a spinal muscular atrophy phenotype we recorded the clinical features through a questionnaire and sequenced the entire coding region of IGHMBP2. In 47 (33%) patients we identified IGHMBP2 mutations, 14 of which were not described before. Clinical features and combinations thereof associated with the presence of IGHMBP2 mutations were discovered through hierarchical cluster analysis. This method detects common traits not evident at first sight by grouping items according to their similarity. The combination of "manifestation of respiratory failure between 6 weeks and 6 months" AND ("presence of diaphragmatic eventration" OR "preterm birth") predicted the presence of IGHMBP2 mutations with 98% sensitivity and 92% specificity. Non-SMARD1 patients fell into two different symptom clusters, mainly separated by the age at respiratory failure and the presence of multiple congenital contractures. The 14 novel IGHMBP2 mutations comprised missense, frameshift, splice-site, and nonsense mutations. All missense mutations altered conserved residues within or adjacent to the putative DNA helicase domain. The c.1235+3A>G splice-site mutation did not entirely suppress correct splicing and we found a residual wild-type IGHMBP2 mRNA steady-state level of 24.4+/-6.9%, which was, however, not sufficient to avert SMARD1 in this patient.

Mutation of gene in spinal muscular atrophy respiratory distress type I

Spinal muscular atrophy with respiratory distress type I (SMARD1, MIM #604 320) is an uncommon variant of infantile spinal muscular atrophy type I. Distinguishing features include diaphragmatic palsy, early-onset distal limb wasting, and contracture. This report describes a Chinese male with typical features of spinal muscular atrophy with respiratory distress type I. Direct sequencing of the causative gene, the immunoglobulin mu-binding protein 2 (IGHMBP2) gene, revealed the presence of a novel frameshift mutation caused by deletion of G in exon 13 and a single base pair substitution of G to A in exon 12 resulting in substitution of isoleucine for valine.

Unraveling the genetics of distal hereditary motor neuronopathies

The hereditary motor neuronopathies (HMN [MIM 158590]) are a heterogeneous group of disorders characterized by an exclusive involvement of the motor part of the peripheral nervous system. They are usually subdivided in proximal HMN, i.e., the classical spinal muscular atrophy syndromes and distal hereditary motor neuronopathies (distal HMN) that clinically resemble Charcot-Marie-Tooth syndromes. In this review, we concentrate on distal HMN. The distal HMN are clinically and genetically heterogeneous and were initially subdivided in seven subtypes according to mode of inheritance, age at onset, and clinical evolution. Recent studies have shown that these subtypes are still heterogeneous at the molecular genetic level and novel clinical and genetic entities have been delineated. Since the introduction of positional cloning, 13 chromosomal loci and seven disease-associated genes have been identified for autosomal-dominant, autosomal-recessive, and X-linked recessive distal HMN. Most of the genes involved encode protein with housekeeping functions, such as RNA processing, translation synthesis, stress response, apoptosis, and others code for proteins involved in retrograde survival. Motor neurons of the anterior horn of the spinal cord seems to be vulnerable to defects in these housekeeping proteins, likely because their large axons have higher metabolic requirements for maintenance, transport over long distances and precise connectivity. Understanding the molecular pathomechanisms for mutations in these genes that are ubiquitous expressed will help unravel the neuronal mechanisms that underlie motor neuropathies leading to denervation of distal limb muscles, and might generate new insights for future therapeutic strategies.

The ultrastructure of peripheral nerve, motor end-plate and skeletal muscle in patients suffering from spinal muscular atrophy with respiratory distress type 1 (SMARD1)

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is genetically and clinically distinct from classic spinal muscular atrophy (SMA1). It results from mutations in the gene encoding immunoglobulin mu-binding protein 2 (IGHMBP2) on chromosome 11q13. Patients develop distally pronounced muscular weakness and early involvement of the diaphragm, resulting in respiratory failure. Sensory and autonomic nerves are also affected at later stages of the disease. We investigated peripheral nerves, skeletal muscles and neuromuscular junctions (NMJ) ultrastructurally in five unrelated patients and three siblings with genetically confirmed SMARD1. In mixed motor and sensory nerves we detected Wallerian degeneration and axonal atrophy similar to the ultrastructural findings described in SMA1. Isolated axonal atrophy was evident in purely sensory nerves. All investigated NMJ of patients with SMARD1 were dysmorphic and lacked a terminal axon. Moreover, we also observed characteristics of neuropathies, such as abnormalities in myelination, that have not been described in spinal muscular atrophies so far. Based on these findings we conclude that impairment of IGHMBP2 function leads to axonal degeneration, abnormal myelin formation, and motor end-plate degeneration.

Treatment with trkC agonist antibodies delays disease progression in neuromuscular degeneration (nmd) mice

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a fatal autosomal recessive disorder seen in infants. It is characterized by lower motor neuron degeneration, progressive muscle paralysis and respiratory failure, for which no effective treatment exists. The phenotype of neuromuscular degeneration (nmd) mice closely resembles the human SMARD1. The identification of the mutated mouse gene in nmd mice, Ighmbp2, led to the discovery of mutations of the homologous gene in humans with SMARD1. We have studied the nmd mouse model with in vivo electrophysiological techniques and evaluated the efficacy of Mab2256, a monoclonal antibody with agonist effect on the tyrosine kinase receptor C, trkC, on disease progression in nmd mice. Treatment with Mab2256 resulted in a significant but transient improvement of muscle strength in nmd mice, as well as normalization of the neuromuscular depression during high-frequency nerve stimulation. These results suggest the potential of using monoclonal agonist antibodies for neurotrophin receptors in lower motor neuron diseases such as SMARD1.

A new mutation of IGHMBP2 gene in spinal muscular atrophy with respiratory distress type 1

This report presents a new mutation in the first Japanese female infant with spinal muscular atrophy with respiratory distress type 1. She manifested the characteristic clinical features, including early-onset respiratory failure due to diaphragmatic paralysis and severe distal muscle weakness. Muscle biopsy in the femoral muscle indicated massive neurogenic changes. Sural nerve biopsy disclosed a moderate reduction of myelinated fibers, predominantly reduced large fibers. She had a novel homozygous missense mutation 2685 C -->A, leading to a T879K substitution in the immunoglobulin mu-binding protein 2 gene. Both parents were heterozygous for this mutation.

Genomic rearrangements at the IGHMBP2 gene locus in two patients with SMARD1

Autosomal recessive spinal muscular atrophy with respiratory distress type 1 (SMARD1) is caused by mutations in the immunoglobulin mu-binding protein 2 (IGHMBP2) gene. Patients affected by the infantile form of SMARD1 present with early onset respiratory distress. So far, patients with neither juvenile onset nor with larger deletions/rearrangements in IGHMBP2 have been reported. In this study, we investigated one patient with infantile (4 months) and another with juvenile (4.3 years) onset of respiratory distress. Direct sequencing of all exons and flanking intron sequences in both patients revealed a mutation on only one allele. In both patients, we identified genomic rearrangements of the other allele of IGHMBP2 by means of Southern blotting. Putative breakpoints were confirmed by polymerase chain reaction on genomic and cDNA. The patient with juvenile onset had an Alu/Alu mediated rearrangement, which resulted in the loss of aproximately 18.5 kb genomic DNA. At the mRNA level, this caused an in-frame deletion of exons 3-7. The patient with infantile onset had a complex rearrangement with two deletions and an inversion between intron 10 and 14. This rearrangement led to a frameshift at the mRNA level. Our results show that SMARD1 can be caused by genomic rearrangements at the IGHMBP2 gene locus. This may be missed by mere sequence analysis. Additionally, we demonstrate that juvenile onset SMARD1 may also be caused by mutations of IGHMBP2. The complex nature of the genomic rearrangement in the patient with infantile SMARD1 is discussed and a deletion mechanism is proposed.

Transgenic rescue of neurogenic atrophy in the nmd mouse reveals a role for Ighmbp2 in dilated cardiomyopathy

Immunoglobulin mu binding protein 2 (IGHMBP2) is a DNA/RNA helicase with a putative role in transcriptional regulation and splicing. A recessive mutation of the Ighmbp2 gene in neuromuscular degeneration (nmd) mice causes progressive neurogenic atrophy of limb muscles. Affected mice show significant loss of motor neurons with large caliber axons and a moderate reduction of neurons with small caliber axons in the ventral nerve roots of the spinal cord. To investigate the role of Ighmbp2 in the pathogenesis of neuromuscular degeneration, we generated two independent lines of transgenic mice expressing the full-length Ighmbp2 cDNA specifically in neurons. Histopathological evaluation of L4 ventral nerve roots revealed that transgenic expression of the Ighmbp2 cDNA prevented primary motor neuron degeneration, while restoring the normal axonal morphology and density in nmd mice. A similar neuronal improvement is found in mutant mice carrying the CAST/EiJ-derived modifier of nmd (Mnm(C)). Intriguingly, both the transgenic and modified nmd mice went on to develop a previously unobserved cardiac and skeletal myopathy. Necropsy of nmd mice in end-stage heart failure revealed a primary dilated cardiomyopathy with secondary respiratory failure that was confirmed by in vivo ECG and echocardiographic measures. Our results suggest that reduced levels of IGHMBP2 in nmd mice compromise the integrity and function not only of motor neurons but also of skeletal and cardiac myocytes. These findings highlight the important role of IGHMBP2 in the maintenance and survival of these terminally differentiated cell types.

Refined genetic mapping of autosomal recessive chronic distal spinal muscular atrophy to chromosome 11q13.3 and evidence of linkage disequilibrium in European families

Chronic distal spinal muscular atrophy (Chronic DSMA, MIM (*)607088) is a rare autosomal recessive disorder characterized by a progressive motor weakness and muscular atrophy, predominating in the distal parts of the limbs. A form of Chronic DSMA gene has been previously mapped to chromosome 11q13 in the 10.3 cM interval defined by loci D11S1889 and D11S1321. By linkage analysis in 12 European Chronic DSMA families, we showed that a disease gene maps to chromosome 11q13.3 (Z(max)=6.66 at theta=0.00 at the DSM4 locus) and suggested that this condition is genetically homogeneous. Recombination events allowed us to reduce the genetic interval to a 2.6 cM region, telomeric to the IGHMBP2 gene, excluding this gene as the disease causing gene in Chronic DSMA. Moreover, partial linkage disequilibrium was found between three rare alleles at loci D11S1369, DSM4 and D11S4184 and the mutant chromosome in European patients. Analysis of the markers at these loci strongly suggests that most Chronic DSMA chromosomes are derived from a single ancestor. Refinement of the Chronic DSMA locus will hopefully allow to test candidate genes and lead to identification of the disease-causing mutations.

Infantile spinal muscular atrophy with respiratory distress type 1 (SMARD1)

Autosomal recessive spinal muscular atrophy with respiratory distress type 1 (SMARD1) is the second anterior horn cell disease in infants in which the genetic defect has been defined. SMARD1 results from mutations in the gene encoding the immunoglobulin micro-binding protein 2 (IGHMBP2) on chromosome 11q13. Our aim was to review the clinical features of 29 infants affected with SMARD1 and report on 26 novel IGHMBP2 mutations. Intrauterine growth retardation, weak cry, and foot deformities were the earliest symptoms of SMARD1. Most patients presented at the age of 1 to 6 months with respiratory distress due to diaphragmatic paralysis and progressive muscle weakness with predominantly distal lower limb muscle involvement. Sensory and autonomic nerves are also affected. Because of the poor prognosis, there is a demand for prenatal diagnosis, and clear diagnostic criteria for infantile SMARD1 are needed. The diagnosis of SMARD1 should be considered in infants with non-5q spinal muscular atrophy, neuropathy, and muscle weakness and/or respiratory distress of unclear cause. Furthermore, consanguineous parents of a child with sudden infant death syndrome should be examined for IGHMBP2 mutations.

Hereditary motor neuropathies and motor neuron diseases: which is which

When Charcot first defined amyotrophic lateral sclerosis (ALS) he used the clinical and neuropathological pattern of vulnerability as a guideline. Similarly other motor neuron diseases such as the spinal muscular atrophies (SMA) and the motor neuropathies (MN) were grouped following clinical criteria. However, ever since the etiology of these diseases has started to be disclosed by genetics, we have learnt that the limits of the syndromes are not as well defined as our forefathers thought. A mutation leading to ALS can also be associated with the clinical picture of spinal muscular atrophy; even more unexpected is the overlap of the so-called motor neuropathies with the clinical syndrome of slowly progressive ALS or that primary lateral sclerosis (PLS) can be caused by the same gene as that responsible for some cases of ALS. In this review we summarise recent work showing that there is a considerable overlap between CMT, MN, SMA, ALS and PLS. Insights into these phenotypes should lead to study of the variants of motor neuron disease and possibly to a reclassification. This comprehensive review should help to improve understanding of the pathogenesis of motor neuron degeneration and finally may aid the research for urgently needed new treatment strategies, perhaps with validity for the entire group of motor neuron diseases.