Outcomes for patients in the RESTORE registry with spinal muscular atrophy and four or more SMN2 gene copies treated with onasemnogene abeparvovec.

OBJECTIVE: We describe outcomes following onasemnogene abeparvovec monotherapy for patients with ≥four survival motor neuron 2 (SMN2) gene copies in RESTORE, a noninterventional spinal muscular atrophy patient registry. METHODS: We evaluated baseline characteristics, motor milestone achievement, post-treatment motor function, use of ventilatory/nutritional support, and adverse events as of December 22, 2022. RESULTS: At data cutoff, 19 patients in RESTORE had ≥four SMN2 copies and were treated with onasemnogene abeparvovec monotherapy (n=12 [63.2%] four copies; n=7 [36.8%] >four copies). All patients were identified by newborn screening and were reported as asymptomatic at diagnosis. Median age at onasemnogene abeparvovec administration was 3.0 months. Median time from treatment to last recorded visit was 15.4 months, with a range of post-treatment follow-up of 0.03-39.4 months. All 12 children who were assessed for motor development achieved new milestones, including standing alone (n=2) and walking alone (n=5). Five children reported one or more treatment-emergent adverse events (one Grade 3 or greater). No deaths or use of ventilatory/nutritional support were reported. CONCLUSIONS: Real-world findings from the RESTORE registry indicate that patients with ≥four SMN2 gene copies treated with onasemnogene abeparvovec monotherapy demonstrated improvements in motor function. Adverse events experienced by these patients were consistent with previously reported findings.

Real-world multidisciplinary outcomes of onasemnogene abeparvovec monotherapy in patients with spinal muscular atrophy type 1: experience of the French cohort in the first three years of treatment.

BACKGROUND: Spinal muscular atrophy type 1 (SMA1) is the most severe and early form of SMA, a genetic disease with motor neuron degeneration. Onasemnogene abeparvovec gene transfer therapy (GT) has changed the natural history of SMA1, but real-world data are scarce. METHODS: A French national expert committee identified 95 newly diagnosed treatment-naive SMA1 patients between June 2019 and June 2022. We prospectively report on children treated with GT as the first and only therapy who had more than one-year of follow-up. RESULTS: Forty-six SMA1 patients received GT. Twelve patients received other treatments. Patients with respiratory insufficiency were oriented toward palliative care after discussion with families. Twenty-nine of the treated patients with more than 12 months of follow-up were included in the follow-up analysis. Among them, 17 had 24 months of follow-up. The mean age at treatment was 7.5 (2.1-12.5) months. Twenty-two patients had two SMN2 copies, and seven had three copies. One infant died in the month following GT due to severe thrombotic microangiopathy, and another died due to respiratory distress. Among the 17 patients with 24 months of follow-up, 90% required spinal bracing (15/17), three patients required nocturnal noninvasive ventilation, and two needed gastrostomy. Concerning motor milestones at the 24-month follow-up, all patients held their head, 15/17 sat for 30 s unassisted, and 12/17 stood with aid. Motor scores (CHOPINTEND and HINE-2) and thoracic circumference significantly improved in all patients. CONCLUSIONS: Our study shows favorable motor outcomes and preserved respiratory and feeding functions in treatment-naive SMA1 infants treated by GT as the first and only therapy before respiratory and bulbar dysfunctions occurred. Nevertheless, almost all patients developed spinal deformities.

Functional benefit of CRISPR-Cas9-induced allele deletion for RYR1 dominant mutation.

More than 700 pathogenic or probably pathogenic variations have been identified in the RYR1 gene causing various myopathies collectively known as "RYR1-related myopathies." There is no treatment for these myopathies, and gene therapy stands out as one of the most promising approaches. In the context of a dominant form of central core disease due to a RYR1 mutation, we aimed at showing the functional benefit of inactivating specifically the mutated RYR1 allele by guiding CRISPR-Cas9 cleavages onto frequent single-nucleotide polymorphisms (SNPs) segregating on the same chromosome. Whole-genome sequencing was used to pinpoint SNPs localized on the mutant RYR1 allele and identified specific CRISPR-Cas9 guide RNAs. Lentiviruses encoding these guide RNAs and the SpCas9 nuclease were used to transduce immortalized patient myoblasts, inducing the specific deletion of the mutant RYR1 allele. The efficiency of the deletion was assessed at DNA and RNA levels, and at the functional level after monitoring calcium release induced by the stimulation of the RyR1-channel. This study provides in cellulo proof of concept regarding the benefits of mutant RYR1 allele deletion, in the case of a dominant RYR1 mutation, from both a molecular and functional perspective, and could apply potentially to 20% of all patients with a RYR1 mutation.

Complex SMN Hybrids Detected in a Cohort of 31 Patients With Spinal Muscular Atrophy.

Background and Objectives: Spinal muscular atrophy (SMA) is a recessive neuromuscular disorder caused by the loss or presence of point pathogenic variants in the SMN1 gene. The main positive modifier of the SMA phenotype is the number of copies of the SMN2 gene, a paralog of SMN1, which only produces around 10%-15% of functional SMN protein. The SMN2 copy number is inversely correlated with phenotype severity; however, discrepancies between the SMA type and the SMN2 copy number have been reported. The presence of SMN2-SMN1 hybrids has been proposed as a possible modifier of SMA disease. Methods: We studied 31 patients with SMA, followed at a single center and molecularly diagnosed by Multiplex Ligand-Dependent Probe Amplification (MLPA), with a specific next-generation sequencing protocol to investigate their SMN2 genes in depth. Hybrid characterization also included bioinformatics haplotype phasing and specific PCRs to resolve each SMN2-SMN1 hybrid structure. Results: We detected SMN2-SMN1 hybrid genes in 45.2% of the patients (14/31), the highest rate reported to date. This represents a total of 25 hybrid alleles, with 9 different structures, of which only 4 are detectable by MLPA. Of particular interest were 2 patients who presented 4 SMN2-SMN1 hybrid copies each and no pure SMN2 copies, an event reported here for the first time. No clear trend between the presence of hybrids and a milder phenotype was observed, although 5 of the patients with hybrid copies showed a better-than-expected phenotype. The higher hybrid detection rate in our cohort may be due to both the methodology applied, which allows an in-depth characterization of the SMN genes and the ethnicity of the patients, mainly of African origin. Discussion: Although hybrid genes have been proposed to be beneficial for patients with SMA, our work revealed great complexity and variability between hybrid structures; therefore, each hybrid structure should be studied independently to determine its contribution to the SMA phenotype. Large-scale studies are needed to gain a better understanding of the function and implications of SMN2-SMN1 hybrid copies, improving genotype-phenotype correlations and prediction of the evolution of patients with SMA.

2024 update: European consensus statement on gene therapy for spinal muscular atrophy.

Spinal muscular atrophy (SMA) is one of the most common genetic diseases and was, until recently, a leading genetic cause of infant mortality. Three disease-modifying treatments have dramatically changed the disease trajectories and outcome for severely affected infants (SMA type 1), especially when initiated in the presymptomatic phase. One of these treatments is the adeno-associated viral vector 9 (AAV9) based gene therapy onasemnogene abeparvovec (Zolgensma®), which is delivered systemically and has been approved by the European Medicine Agency for SMA patients with up to three copies of the SMN2 gene or with the clinical presentation of SMA type 1. While this broad indication provides flexibility in patient selection, it also raises concerns about the risk-benefit ratio for patients with limited or no evidence supporting treatment. In 2020, we convened a European neuromuscular expert working group to support the rational use of onasemnogene abeparvovec, employing a modified Delphi methodology. After three years, we have assembled a similar yet larger group of European experts who assessed the emerging evidence of onasemnogene abeparvovec's role in treating older and heavier SMA patients, integrating insights from recent clinical trials and real-world evidence. This effort resulted in 12 consensus statements, with strong consensus achieved on 9 and consensus on the remaining 3, reflecting the evolving role of onasemnogene abeparvovec in treating SMA.

The recurrent deep intronic pseudoexon-inducing variant COL6A1 c.930+189C>T results in a consistently severe phenotype of COL6-related dystrophy: Towards clinical trial readiness for splice-modulating therapy.

Collagen VI-related dystrophies (COL6-RDs) manifest with a spectrum of clinical phenotypes, ranging from Ullrich congenital muscular dystrophy (UCMD), presenting with prominent congenital symptoms and characterised by progressive muscle weakness, joint contractures and respiratory insufficiency, to Bethlem muscular dystrophy, with milder symptoms typically recognised later and at times resembling a limb girdle muscular dystrophy, and intermediate phenotypes falling between UCMD and Bethlem muscular dystrophy. Despite clinical and immunohistochemical features highly suggestive of COL6-RD, some patients had remained without an identified causative variant in COL6A1, COL6A2 or COL6A3. With combined muscle RNA-sequencing and whole-genome sequencing we uncovered a recurrent, de novo deep intronic variant in intron 11 of COL6A1 (c.930+189C>T) that leads to a dominantly acting in-frame pseudoexon insertion. We subsequently identified and have characterised an international cohort of forty-four patients with this COL6A1 intron 11 causative variant, one of the most common recurrent causative variants in the collagen VI genes. Patients manifest a consistently severe phenotype characterised by a paucity of early symptoms followed by an accelerated progression to a severe form of UCMD, except for one patient with somatic mosaicism for this COL6A1 intron 11 variant who manifests a milder phenotype consistent with Bethlem muscular dystrophy. Characterisation of this individual provides a robust validation for the development of our pseudoexon skipping therapy. We have previously shown that splice-modulating antisense oligomers applied in vitro effectively decreased the abundance of the mutant pseudoexon-containing COL6A1 transcripts to levels comparable to the in vivo scenario of the somatic mosaicism shown here, indicating that this therapeutic approach carries significant translational promise for ameliorating the severe form of UCMD caused by this common recurrent COL6A1 causative variant to a Bethlem muscular dystrophy phenotype.

SEPN1-related myopathy depends on the oxidoreductase ERO1A and is druggable with the chemical chaperone TUDCA.

Selenoprotein N (SEPN1) is a protein of the endoplasmic reticulum (ER) whose inherited defects originate SEPN1-related myopathy (SEPN1-RM). Here, we identify an interaction between SEPN1 and the ER-stress-induced oxidoreductase ERO1A. SEPN1 and ERO1A, both enriched in mitochondria-associated membranes (MAMs), are involved in the redox regulation of proteins. ERO1A depletion in SEPN1 knockout cells restores ER redox, re-equilibrates short-range MAMs, and rescues mitochondrial bioenergetics. ERO1A knockout in a mouse background of SEPN1 loss blunts ER stress and improves multiple MAM functions, including Ca2+ levels and bioenergetics, thus reversing diaphragmatic weakness. The treatment of SEPN1 knockout mice with the ER stress inhibitor tauroursodeoxycholic acid (TUDCA) mirrors the results of ERO1A loss. Importantly, muscle biopsies from patients with SEPN1-RM exhibit ERO1A overexpression, and TUDCA-treated SEPN1-RM patient-derived primary myoblasts show improvement in bioenergetics. These findings point to ERO1A as a biomarker and a viable target for intervention and to TUDCA as a pharmacological treatment for SEPN1-RM.

270th ENMC International Workshop: Consensus for SMN2 genetic analysis in SMA patients 10-12 March, 2023, Hoofddorp, the Netherlands.

The 270th ENMC workshop aimed to develop a common procedure to optimize the reliability of SMN2 gene copy number determination and to reinforce collaborative networks between molecular scientists and clinicians. The workshop involved neuromuscular and clinical experts and representatives of patient advocacy groups and industry. SMN2 copy number is currently one of the main determinants for therapeutic decision in SMA patients: participants discussed the issues that laboratories may encounter in this molecular test and the cruciality of the accurate determination, due the implications as prognostic factor in symptomatic patients and in individuals identified through newborn screening programmes. At the end of the workshop, the attendees defined a set of recommendations divided into four topics: SMA molecular prognosis assessment, newborn screening for SMA, SMN2 copies and treatments, and modifiers and biomarkers. Moreover, the group draw up a series of recommendations for the companies manufacturing laboratory kits, that will help to minimize the risk of errors, regardless of the laboratories' expertise.

Real-World Outcomes in Patients with Spinal Muscular Atrophy Treated with Onasemnogene Abeparvovec Monotherapy: Findings from the RESTORE Registry.

Background: Long-term, real-world effectiveness and safety data of disease-modifying treatments for spinal muscular atrophy (SMA) are important for assessing outcomes and providing information for a larger number and broader range of SMA patients than included in clinical trials. Objective: We sought to describe patients with SMA treated with onasemnogene abeparvovec monotherapy in the real-world setting. Methods: RESTORE is a prospective, multicenter, multinational, observational registry that captures data from a variety of sources. Results: Recruitment started in September 2018. As of May 23, 2022, data were available for 168 patients treated with onasemnogene abeparvovec monotherapy. Median (IQR) age at initial SMA diagnosis was 1 (0-6) month and at onasemnogene abeparvovec infusion was 3 (1-10) months. Eighty patients (47.6%) had two and 70 (41.7%) had three copies of SMN2, and 98 (58.3%) were identified by newborn screening. Infants identified by newborn screening had a lower age at final assessment (mean age 11.5 months) and greater mean final (SD) CHOP INTEND score (57.0 [10.0] points) compared with clinically diagnosed patients (23.1 months; 52.1 [8.0] points). All patients maintained/achieved motor milestones. 48.5% (n = 81/167) experienced at least one treatment-emergent adverse event (AE), and 31/167 patients (18.6%) experienced at least one serious AE, of which 8/31 were considered treatment-related. Conclusion: These real-world outcomes support findings from the interventional trial program and demonstrate effectiveness of onasemnogene abeparvovec over a large patient population, which was consistent with initial clinical data and published 5-year follow-up data. Observed AEs were consistent with the established safety profile of onasemnogene abeparvovec.

Criterion validity of the spatial exploration test of upper limb mobility to evaluate the active horizontal workspace of children with spinal muscular atrophy.

PURPOSE: To determine the criterion validity of the SET-ULM (Spatial Exploration Test of Upper Limb Mobility), a functional workspace test. MATERIALS AND METHODS: A prospective study from July 2017 to November 2018 in 30 children with SMA type 1 or 2. All children underwent assessment with the SET-ULM and the Motor Function Measure (MFM). RESULTS: We included 30 children. Median (Q1; Q3) MFM D1 (standing ability, ambulation and transfers), D2 (axial and proximal motor function), D3 (distal motor function) scores, Total MFM and Total SET-ULM active score were respectively 2.6% (2.6-3.8); 45.8% (19.9-65.3); 57.7% (36.9-80.9); 35.4% (16.7-43.2) and 70.2% (49.7-97.9). Total SET-ULM active score was strongly correlated with the MFM D2 dimension score (rho 0.82; p < 001), with the D3 dimension (rho 0.86; p < 0.001) and with the Total MFM score (rho 0.89; p < 0.005). Total SET-ULM active score differed between SMA types (p < 0.01). CONCLUSION: The SET-ULM has good criterion validity for the evaluation of available horizontal active upper limb workspace in children with SMA1 and SMA2. Future studies should evaluate reliability and sensitivity to change during a longitudinal follow-up study, as well as in a longitudinal trial of therapeutic effectiveness. CLINICAL TRIALS: NCT03223051IMPLICATIONS FOR REHABILITATIONThe Spatial Exploration Test for Upper Limb Mobility is a useful adjunct to the Motor Function Measure.It provides a precise evaluation of horizontal reaching ability.The Spatial Exploration Test for Upper Limb Mobility will be of great clinical utility for the evaluation of the effects of treatments for spinal muscular atrophy.

Life-Saving Treatments for Spinal Muscular Atrophy: Global Access and Availability.

Background and Objectives: Spinal muscular atrophy (SMA) is a neurodegenerative disorder manifesting with progressive muscle weakness and atrophy. SMA type 1 used to be fatal within the first 2 years of life, but is now treatable with therapies targeting splicing modification and gene replacement. Nusinersen, risdiplam, and onasemnogene abeparvovec-xioi improve survival, motor strength, endurance, and ability to thrive, allowing many patients to potentially attain a normal life; all have been recently approved by major regulatory agencies. Although these therapies have revolutionized the world of SMA, they are associated with a high economic burden, and access to these therapies is limited in some countries. The primary objective of this study was to compare the availability and implementation of treatment of SMA from different regions of the world. Methods: In this qualitative study, we surveyed health care providers from 21 countries regarding their experiences caring for patients with SMA. The main outcome measures were provider survey responses on newborn screening, drug availability/access, barriers to treatment, and related questions. Results: Twenty-four providers from 21 countries with decades of experience (mean 26 years) in treating patients with SMA responded to the survey. Nusinersen was the most available therapy for SMA. Our survey showed that while genetic testing is usually available, newborn screening is still unavailable in many countries. The provider-reported treatment cost also varied between countries, and economic burden was a major barrier in treating patients with SMA. Discussion: Overall, this survey highlights the global inequality in managing patients with SMA. The spread of newborn screening is essential in ensuring improved access to care for patients with SMA. With the advancement of neurotherapeutics, more genetic diseases will soon be treatable, and addressing the global inequality in clinical care will require novel approaches to mitigate such inequality in the future.

Effect of nusinersen after 3 years of treatment in 57 young children with SMA in terms of SMN2 copy number or type.

BACKGROUND: Spinal muscular atrophy (SMA) is a rare genetic neuromuscular disorder due to an autosomal recessive mutation in the survival motor neuron 1 gene (SMN1), causing degeneration of the anterior horn cells of the spinal cord and resulting in muscle atrophy. This study aimed to report on the 36-month follow-up of children with SMA treated with nusinersen before the age of 3 years. Changes in motor function, nutritional and ventilatory support, and orthopedic outcomes were evaluated at baseline and 36 months after intrathecal administration of nusinersen and correlated with SMA type and SMN2 copy number. RESULTS: We found that 93% of the patients gained new motor skills during the 3 years-standing without help for 12 of 37 and walking with help for 11 of 37 patients harboring three SMN2 copies. No patients with two copies of SMN2 can stand alone or walk. Patients bearing three copies of SMN2 are more likely to be spared from respiratory, nutritional, and orthopedic complications than patients with two SMN2 copies. CONCLUSION: Children with SMA treated with nusinersen continue to make motor acquisitions at 3 years after initiation of treatment. Children with two SMN2 copies had worse motor, respiratory, and orthopedic outcomes after 3 years of treatment than children with three copies.

Combination disease-modifying treatment in spinal muscular atrophy: A proposed classification.

We sought to devise a rational, systematic approach for defining/grouping survival motor neuron-targeted disease-modifying treatment (DMT) scenarios. The proposed classification is primarily based on a two-part differentiation: initial DMT, and persistence/discontinuation of subsequent DMT(s). Treatment categories were identified: monotherapy add-on, transient add-on, combination with onasemnogene abeparvovec, bridging to onasemnogene abeparvovec, and switching to onasemnogene abeparvovec. We validated this approach by applying the classification to the 443 patients currently in the RESTORE registry and explored the demographics of these different groups of patients. This work forms the basis to explore the safety and efficacy profile of the different combinations of DMT in SMA.

Characterization of cardiac involvement in children with LMNA-related muscular dystrophy.

Introduction: LMNA-related muscular dystrophy is a rare entity that produce "laminopathies" such as Emery-Dreifuss muscular dystrophy (EDMD), limb-girdle muscular dystrophy type 1B (LGMD1B), and LMNA-related congenital muscular dystrophy (L-CMD). Heart failure, malignant arrhythmias, and sudden death may occur. No consensus exists on cardiovascular management in pediatric laminopathies. The aim was to perform an exhaustive cardiologic follow-up in pediatric patients diagnosed with LMNA-related muscular dystrophy. Methods: Baseline cardiac work-up consisted of clinical assessment, transthoracic Doppler echocardiography, 12-lead electrocardiogram, electrophysiological study, and implantation of a long-term implantable cardiac loop recorder (ILR). Results: We enrolled twenty-eight pediatric patients diagnosed with EDMD (13 patients), L-CMD (11 patients), LGMD1B (2 patients), and LMNA-related mild weakness (2 patients). Follow-up showed dilated cardiomyopathy (DCM) in six patients and malignant arrhythmias in five (four concomitant with DCM) detected by the ILR that required implantable cardioverter defibrillator (ICD) implantation. Malignant arrhythmias were detected in 20% of our cohort and early-onset EDMD showed worse cardiac prognosis. Discussion: Patients diagnosed with early-onset EDMD are at higher risk of DCM, while potentially life-threatening arrhythmias without DCM appear earlier in L-CMD patients. Early onset neurologic symptoms could be related with worse cardiac prognosis. Specific clinical guidelines for children are needed to prevent sudden death.

Use of MFM-20 to monitor SMA types 1 and 2 patients treated with nusinersen.

OBJECTIVE: To evaluate sensitivity to change and discriminant validity of the 20-item Motor Function Measure (MFM-20) in 2-7-year-old patients with spinal muscular atrophy types 1 (SMA1) or 2 (SMA2) treated with nusinersen. METHODS: Children aged 2 to 7 years old with SMA1 or SMA2 treated with nusinersen were assessed at least three times using the MFM-20 over an average follow-up time of 17 months. Evolution of 4-month-standardized MFM-20 scores was calculated for each MFM-20 domain (D1 standing and transfers, D2 axial and proximal, D3 distal) and for the total score (TS). RESULTS: Included in the study were 22 SMA1 subjects and 19 SMA2 subjects. Baseline MFM scores were significantly lower in patients with SMA1 than SMA2 (TS 29.5% vs. 48.3%, D1 4.5% vs. 10.6%, D2 43.6% vs. 72.6%, D3 51.2% vs. 75.0%). When considering the mean change during nusinersen treatment, standardized over a 4-month period, TS was improved for both SMA1 (+ 4.1%, SRM 1.5) and SMA2 (+ 2.8%, SRM 0.89) patients. For SMA1 patients, considerable changes were observed in D2 (+ 6.2%, SRM 0.89) and D3 (+ 6.0%, SRM 0.72), whereas the change in D1 was small (+ 0.5%, SRM 0.44). In SMA2 2 subjects, D3 was improved to a larger extent (+ 4.2%, SRM 0.53) than D1 (+ 1.8% SRM 0.63) or D2 (+ 3.2%, SRM 0.69). CONCLUSION: Our results validate use of MFM-20 to monitor function of young SMA1 and SMA2 subjects treated with nusinersen. Significant motor function improvements following treatment were observed in both SMA1 and SMA2 patients.

Perioperative complications after posterior spinal fusion versus minimally invasive fusionless surgery in neuromuscular scoliosis: a comparative study.

INTRODUCTION: Early-onset scoliosis is a common deformity in neuromuscular disease. When conservative treatment becomes ineffective, several surgical options can be proposed. The most common technique is posterior spinal fusion (PSF) consisting of performing a multiple segmental instrumentation with pedicular screws on the full spine associated with decortication and bone graft. Minimally invasive fusionless surgery (MIFS) is an alternative to correct and fix definitively the spine without graft. The objective of this study was to compare early surgical inpatient period between PSF and MIFS in neuromuscular scoliosis. MATERIAL AND METHODS: 140 NMS operated by PSF or MIFS between 2012 and 2017 was retrospectively reviewed. The following data were compared between groups: general characteristics (age, sex, etiology), preoperative preparation (halo traction, noninvasive ventilation or tracheostomy), Cobb angle and pelvic obliquity correction, use of drugs (vasopressor and/or inotropes, expansion fluids, transfusion and volumes), postoperative complications, and need of noninvasive ventilation. RESULTS: 75 patients were managed by PSF with a mean age of 14.3 ± 2.3y and 65 by MIFS with a mean age of 11.8 ± 3y. Average pelvic obliquity and major curve correction were similar postoperatively. Intraoperative blood transfusion was significantly more common in PSF group (OR, 14; 95% CI [6.3-33.0]). Vasopressors were used non-significantly more often in the PSF group and expansion fluids similar in the two groups. PSF group had more overall complications (OR, 4.6; 95% CI [2.3-9.8]), more infections (OR, 3.6; 95% CI [1.5-9.3]) and more hemodynamic complications (OR, 4.1; 95% CI [1.4-15.1]). Average intubation duration was 5 days in the PSF and 4 days in MIFS (p = 0.05). CONCLUSION: In this series of neuromuscular patients, the complication rate was reduced in MIFS comparatively to PSF, with lower blood transfusion and less infections.

Severe ACTA1-related nemaline myopathy: intranuclear rods, cytoplasmic bodies, and enlarged perinuclear space as characteristic pathological features on muscle biopsies.

Nemaline myopathy (NM) is a muscle disorder with broad clinical and genetic heterogeneity. The clinical presentation of affected individuals ranges from severe perinatal muscle weakness to milder childhood-onset forms, and the disease course and prognosis depends on the gene and mutation type. To date, 14 causative genes have been identified, and ACTA1 accounts for more than half of the severe NM cases. ACTA1 encodes α-actin, one of the principal components of the contractile units in skeletal muscle. We established a homogenous cohort of ten unreported families with severe NM, and we provide clinical, genetic, histological, and ultrastructural data. The patients manifested antenatal or neonatal muscle weakness requiring permanent respiratory assistance, and most deceased within the first months of life. DNA sequencing identified known or novel ACTA1 mutations in all. Morphological analyses of the muscle biopsy specimens showed characteristic features of NM histopathology including cytoplasmic and intranuclear rods, cytoplasmic bodies, and major myofibrillar disorganization. We also detected structural anomalies of the perinuclear space, emphasizing a physiological contribution of skeletal muscle α-actin to nuclear shape. In-depth investigations of the nuclei confirmed an abnormal localization of lamin A/C, Nesprin-1, and Nesprin-2, forming the main constituents of the nuclear lamina and the LINC complex and ensuring nuclear envelope integrity. To validate the relevance of our findings, we examined muscle samples from three previously reported ACTA1 cases, and we identified the same set of structural aberrations. Moreover, we measured an increased expression of cardiac α-actin in the muscle samples from the patients with longer lifespan, indicating a potential compensatory effect. Overall, this study expands the genetic and morphological spectrum of severe ACTA1-related nemaline myopathy, improves molecular diagnosis, highlights the enlargement of the perinuclear space as an ultrastructural hallmark, and indicates a potential genotype/phenotype correlation.