Clinical, Histopathologic, and Genetic Features of Patients With Myofibrillary and Distal Myopathies: Experience From the Italian Network.

BACKGROUND AND OBJECTIVES: The diagnostic process for myofibrillar myopathies (MFM) and distal myopathies (DM) is particularly complex because of the large number of causative genes, the existence of still molecularly undefined disease entities, and the overlapping features between the 2 categories. This study aimed to characterize a large cohort of patients affected by MFM and DM and identify the most important diagnostic and prognostic aspects of these diseases. METHODS: Patients with either a myopathological diagnosis of MFM or a clinical diagnosis of DM were included in this retrospective multicentric national study. Demographic, genetic, clinical, and histopathologic data of anonymized patients were collected from the neuromuscular centers of the Italian Association of Myology network. RESULTS: Data regarding 132 patients with MFM (mean age 57.0 ± 15.8 years, 49% female) and 298 patients with DM (mean age 50.7 ± 15.9 years, 40% female) were gathered from 20 neuromuscular centers. 69 patients fulfilled the criteria for both groups (distal myopathies with myofibrillar pathology, DM-MP). Molecular confirmation was achieved in 63% of the patients. Fifty-two percent of the patients with MFM carried pathogenic variants in either DES (n = 30), MYOT (n = 20), or DNAJB6 (n = 18), which were also the most frequent disease-causing genes in DM-MP, while GNE (n = 44) and MYH7 (n = 23) were the genes most commonly carrying pathogenic variants in DM. The mean age at onset varied from <25 years in patients with causative variants in MYH7 and DYSF to 59 years in patients with myotilinopathies. Cardiac involvement was reported in 29% of patients with MFM and 16% of patients with DM, with DES and MYH7 variants significantly associated with the development of cardiomyopathy. Respiratory impairment was more prevalent in patients with TTN and DES variants and rare in other disorders such as GNE myopathy and dysferlinopathies, which were instead associated, together with DNAJB6-related and PLIN4-related myopathies, with the risk of losing ambulation during the disease course. DISCUSSION: The Italian cohort of patients with MFM and DM recapitulates the phenotypic heterogeneity and the partial overlap between the 2 groups. However, in relative contrast to the encountered phenotypic variability, only 5 genes accounted for most of the molecular diagnoses. Specific genetic entities are associated with significantly increased risk of developing cardiorespiratory complications or loss of ambulation, which has relevant prognostic implications.

Neonatal X-linked myotubular myopathy with a de novo mutation: A case report and literature review. / æ–°å‘çªå˜åŸºå› çš„æ–°ç”Ÿå„¿X-è¿žé”è‚Œå°ç®¡è‚Œç— 1例并文献回顾.

X-linked myotubular myopathy (XLMTM) is a rare congenital myopathy. In February 2021, a male neonate was admitted to the West China Second University Hospital, Sichuan University, with clinical manifestations of hypotonia, accompanied by distinctive facial features, and requiring continuous ventilatory support. He was born prematurely at 36+2 weeks gestation and developed respiratory distress postnatally, followed by difficulty in weaning from mechanical ventilation. Additional clinical features included hypotonia of the limbs, swallowing dysfunction, and specific facial characteristics (elongated limbs, narrow face, high-arched palate, wrist drop, empty scrotum, elongated fingers/toes). Genetic testing confirmed the diagnosis of XLMTM. Whole-exome sequencing analysis of the family revealed no mutations in the father, paternal grandfather, or paternal grandmother, while the mother had a heterozygous mutation. The pathogenic mutation was identified as MTM1 gene (OMIM: 300415), chromosome position chrX-150649714, with a nucleotide change of c.868-2A>C. The patient exhibited typical facial features. Genetic testing is crucial for accurate diagnosis of XLMTM in infants presenting with abnormal muscle tone and distinctive facial features.

Exome sequencing in undiagnosed congenital myopathy reveals new genes and refines genes-phenotypes correlations.

BACKGROUND: Congenital myopathies are severe genetic diseases with a strong impact on patient autonomy and often on survival. A large number of patients do not have a genetic diagnosis, precluding genetic counseling and appropriate clinical management. Our objective was to find novel pathogenic variants and genes associated with congenital myopathies and to decrease diagnostic odysseys and dead-end. METHODS: To identify pathogenic variants and genes implicated in congenital myopathies, we established and conducted the MYOCAPTURE project from 2009 to 2018 to perform exome sequencing in a large cohort of 310 families partially excluded for the main known genes. RESULTS: Pathogenic variants were identified in 156 families (50%), among which 123 families (40%) had a conclusive diagnosis. Only 44 (36%) of the resolved cases were linked to a known myopathy gene with the corresponding phenotype, while 55 (44%) were linked to pathogenic variants in a known myopathy gene with atypical signs, highlighting that most genetic diagnosis could not be anticipated based on clinical-histological assessments in this cohort. An important phenotypic and genetic heterogeneity was observed for the different genes and for the different congenital myopathy subtypes, respectively. In addition, we identified 14 new myopathy genes not previously associated with muscle diseases (20% of all diagnosed cases) that we previously reported in the literature, revealing novel pathomechanisms and potential therapeutic targets. CONCLUSIONS: Overall, this approach illustrates the importance of massive parallel gene sequencing as a comprehensive tool for establishing a molecular diagnosis for families with congenital myopathies. It also emphasizes the contribution of clinical data, histological findings on muscle biopsies, and the availability of DNA samples from additional family members to the diagnostic success rate. This study facilitated and accelerated the genetic diagnosis of congenital myopathies, improved health care for several patients, and opened novel perspectives for either repurposing of existing molecules or the development of novel treatments.

[Clinical characteristics and genetic analysis of two children with X-linked Centronuclear myopathy due to variants of MTM1 gene].

OBJECTIVE: To explore the clinical and genetic characteristics of two newborns with Central nuclear myopathy (CNM). METHODS: Two newborns with CNM diagnosed clinically at Wuhan Children's Hospital Affiliated to Tongji Medical College, Huazhong University of Science and Technology in April 2019 and November 2021 were selected as the study subjects, and their clinical data was collected. Both newborns and their parents were subjected chromosomal karyotyping analysis and whole exome sequencing (WES). Candidate variants were verified by Sanger sequencing. Pathogenicity of the candidate variants was evaluated based on the guidelines from the American College of Medical Genetics and Genomics (ACMG). RESULTS: Patient 1 was a male neonate and Patient 2 was a 20-day-old male infant. Both newborns had featured difficulty in breathing and swallowing. WES revealed that both had harbored hemizygous variants of the MTM1 gene, which were verified by Sanger sequencing. Patient 1 had harbored a c.1261A>G variant. Based on the ACMG guidelines, it was rated as pathogenic (PVS1+PM2_Supporting+PP3). Patient 2 harbored a c.342delT variant, which was also rated as pathogenic (PVS1+PM2_Supporting+PP3). CONCLUSION: The c.1261A>G and c.342delT variants of the MTM1 gene probably underlay the pathogenesis of CNM in the two patients.

Uncovering the BIN1-SH3 interactome underpinning centronuclear myopathy.

Truncation of the protein-protein interaction SH3 domain of the membrane remodeling Bridging Integrator 1 (BIN1, Amphiphysin 2) protein leads to centronuclear myopathy. Here, we assessed the impact of a set of naturally observed, previously uncharacterized BIN1 SH3 domain variants using conventional in vitro and cell-based assays monitoring the BIN1 interaction with dynamin 2 (DNM2) and identified potentially harmful ones that can be also tentatively connected to neuromuscular disorders. However, SH3 domains are typically promiscuous and it is expected that other, so far unknown partners of BIN1 exist besides DNM2, that also participate in the development of centronuclear myopathy. In order to shed light on these other relevant interaction partners and to get a holistic picture of the pathomechanism behind BIN1 SH3 domain variants, we used affinity interactomics. We identified hundreds of new BIN1 interaction partners proteome-wide, among which many appear to participate in cell division, suggesting a critical role of BIN1 in the regulation of mitosis. Finally, we show that the identified BIN1 mutations indeed cause proteome-wide affinity perturbation, signifying the importance of employing unbiased affinity interactomic approaches.

A CCG expansion in ABCD3 causes oculopharyngodistal myopathy in individuals of European ancestry.

Oculopharyngodistal myopathy (OPDM) is an inherited myopathy manifesting with ptosis, dysphagia and distal weakness. Pathologically it is characterised by rimmed vacuoles and intranuclear inclusions on muscle biopsy. In recent years CGG • CCG repeat expansion in four different genes were identified in OPDM individuals in Asian populations. None of these have been found in affected individuals of non-Asian ancestry. In this study we describe the identification of CCG expansions in ABCD3, ranging from 118 to 694 repeats, in 35 affected individuals across eight unrelated OPDM families of European ancestry. ABCD3 transcript appears upregulated in fibroblasts and skeletal muscle from OPDM individuals, suggesting a potential role of over-expression of CCG repeat containing ABCD3 transcript in progressive skeletal muscle degeneration. The study provides further evidence of the role of non-coding repeat expansions in unsolved neuromuscular diseases and strengthens the association between the CGG • CCG repeat motif and a specific pattern of muscle weakness.

Cognitive, adaptive and perseverative aspects characterization of children with XLMTM: An explorative study.

BACKGROUND: X-Linked Myotubular Myopathy (XLMTM) is a severe congenital myopathy, potentially fatal within the first years. Patients present several complications and their cognitive development has never been explored deeply so far. An in-depth knowledge on the disease natural history, including the neurocognitive and adaptive profile, is essential in light of the promising new therapeutic perspectives. METHODS: We included all XLMTM patients seen in our clinical Unit between January 2021 and December 2023, irrespective to their disease's severity. Demographic and clinical data, including motor, respiratory and swallowing functions were collected. Patients were assessed with gold-standard international scales, according to their age and communication skills. RESULTS: We assessed nine patients in total, four with a severe phenotype, four with an intermediate phenotype and one with mild phenotype. The cognitive profile was within the lower limits or lower than the norm, with a global adaptive deficit for the majority of patients. A perseverative behavioural trait was also observed in some patients. CONCLUSION: This study shows that XLMTM patients in the cohort had a neurodevelopmental profile within the lower limits of the norm, irrespective to the disease's severity, while the adaptive difficulties seems to be related to patients' global clinical impairment. Our observation would deserve a confirmation on a wider range of patients and we consider it essential for better defining the XLMTM phenotype, also considering the incoming promising therapeutic approaches.

Interplay between myotubularins and Ca2+ homeostasis.

The myotubularin family, encompassing myotubularin 1 (MTM1) and 14 myotubularin-related proteins (MTMRs), represents a conserved group of phosphatases featuring a protein tyrosine phosphatase domain. Nine members are characterized by an active phosphatase domain C(X)5R, dephosphorylating the D3 position of PtdIns(3)P and PtdIns(3,5)P2. Mutations in myotubularin genes result in human myopathies, and several neuropathies including X-linked myotubular myopathy and Charcot-Marie-Tooth type 4B. MTM1, MTMR6 and MTMR14 also contribute to Ca2+ signaling and Ca2+ homeostasis that play a key role in many MTM-dependent myopathies and neuropathies. Here we explore the evolving roles of MTM1/MTMRs, unveiling their influence on critical aspects of Ca2+ signaling pathways.

A healthcare claims analysis to identify and characterize patients with suspected X-Linked Myotubular Myopathy (XLMTM) in the Brazilian Healthcare System.

BACKGROUND: X-linked myotubular myopathy (XLMTM) is a rare, life-threatening congenital disease, which is not well-defined. To our knowledge, no studies characterizing the XLMTM disease burden have been conducted in Brazil. We identified and described patients with suspected XLMTM using administrative claims data from the Brazilian public healthcare system. METHODS: Data from 2015 to 2019 were extracted from the DATASUS database. As no XLMTM-specific ICD-10 code was available, a stepwise algorithm was applied to identify patients with suspected XLMTM by selecting male patients with a congenital myopathies code (G71.2), aged < 18 years at index date (first claim of G71.2), with an associated diagnostic procedure (muscle biopsy/genetic test) and without spinal muscular atrophy or Duchenne muscular dystrophy. We attempted to identify patients with suspected severe XLMTM based on use of both respiratory and feeding support, which are nearly universal in the care of XLMTM patients. Analyses were performed for the overall cohort and stratified by age at index date < 5 years old and ≥ 5 years old. RESULTS: Of 173 patients with suspected XLMTM identified, 39% were < 5 years old at index date. Nearly all (N = 166) patients (96%) were diagnosed by muscle biopsy (91% of patients < 5 years old and 99% of patients ≥ 5 years old), six (3.5%) were diagnosed by clinical evaluation (8% of patients < 5 years old and 1% of patients ≥ 5 years old), and one was diagnosed by a genetic test. Most patients lived in Brasilia (n = 55), São Paulo (n = 33) and Minas Gerais (n = 27). More than 85% of patients < 5 years old and approximately 75% of patients ≥ 5 years old had physiotherapy at the index date. In both age groups, nearly 50% of patients required hospitalization at some point and 25% required mobility support. Respiratory and feeding support were required for 3% and 12% of patients, respectively, suggesting that between 5 and 21 patients may have had severe XLMTM. CONCLUSION: In this real-world study, genetic testing for XLMTM appears to be underutilized in Brazil and may contribute to underdiagnosis of the disease. Access to diagnosis and care is limited outside of specific regions with specialized clinics and hospitals. Substantial use of healthcare resources included hospitalization, physiotherapy, mobility support, and, to a lesser extent, feeding support and respiratory support.

Dynamics of membrane tubulation coupled with fission by a two-component module.

Membrane tubulation coupled with fission (MTCF) is a widespread phenomenon but mechanisms for their coordination remain unclear, partly because of the lack of assays to monitor dynamics of membrane tubulation and subsequent fission. Using polymer cushioned bilayer islands, we analyze the membrane tubulator Bridging Integrator 1 (BIN1) mixed with the fission catalyst dynamin2 (Dyn2). Our results reveal this mixture to constitute a minimal two-component module that demonstrates MTCF. MTCF is an emergent property and arises because BIN1 facilitates recruitment but inhibits membrane binding of Dyn2 in a dose-dependent manner. MTCF is therefore apparent only at high Dyn2 to BIN1 ratios. Because of their mutual involvement in T-tubules biogenesis, mutations in BIN1 and Dyn2 are associated with centronuclear myopathies and our analysis links the pathology with aberrant MTCF. Together, our results establish cushioned bilayer islands as a facile template for the analysis of membrane tubulation and inform of mechanisms that coordinate MTCF.

ORAI1 inhibition as an efficient preclinical therapy for tubular aggregate myopathy and Stormorken syndrome.

Tubular aggregate myopathy (TAM) and Stormorken syndrome (STRMK) are clinically overlapping disorders characterized by childhood-onset muscle weakness and a variable occurrence of multisystemic signs, including short stature, thrombocytopenia, and hyposplenism. TAM/STRMK is caused by gain-of-function mutations in the Ca2+ sensor STIM1 or the Ca2+ channel ORAI1, both of which regulate Ca2+ homeostasis through the ubiquitous store-operated Ca2+ entry (SOCE) mechanism. Functional experiments in cells have demonstrated that the TAM/STRMK mutations induce SOCE overactivation, resulting in excessive influx of extracellular Ca2+. There is currently no treatment for TAM/STRMK, but SOCE is amenable to manipulation. Here, we crossed Stim1R304W/+ mice harboring the most common TAM/STRMK mutation with Orai1R93W/+ mice carrying an ORAI1 mutation partially obstructing Ca2+ influx. Compared with Stim1R304W/+ littermates, Stim1R304W/+Orai1R93W/+ offspring showed a normalization of bone architecture, spleen histology, and muscle morphology; an increase of thrombocytes; and improved muscle contraction and relaxation kinetics. Accordingly, comparative RNA-Seq detected more than 1,200 dysregulated genes in Stim1R304W/+ muscle and revealed a major restoration of gene expression in Stim1R304W/+Orai1R93W/+ mice. Altogether, we provide physiological, morphological, functional, and molecular data highlighting the therapeutic potential of ORAI1 inhibition to rescue the multisystemic TAM/STRMK signs, and we identified myostatin as a promising biomarker for TAM/STRMK in humans and mice.

Novel TUBA4A variant causes congenital myopathy with focal myofibrillar disorganisation.

BACKGROUND: Congenital myopathies are a clinical, histopathological and genetic heterogeneous group of inherited muscle disorders that are defined on peculiar architectural abnormalities in the muscle fibres. Although there have been at least 33 different genetic causes of the disease, a significant percentage of congenital myopathies remain genetically unresolved. The present study aimed to report a novel TUBA4A variant in two unrelated Chinese patients with sporadic congenital myopathy. METHODS: A comprehensive strategy combining laser capture microdissection, proteomics and whole-exome sequencing was performed to identify the candidate genes. In addition, the available clinical data, myopathological changes, the findings of electrophysiological examinations and thigh muscle MRIs were also reviewed. A cellular model was established to assess the pathogenicity of the TUBA4A variant. RESULTS: We identified a recurrent novel heterozygous de novo c.679C>T (p.L227F) variant in the TUBA4A (NM_006000), encoding tubulin alpha-4A, in two unrelated patients with clinicopathologically diagnosed sporadic congenital myopathy. The prominent myopathological changes in both patients were muscle fibres with focal myofibrillar disorganisation and rimmed vacuoles. Immunofluorescence showed ubiquitin-positive TUBA4A protein aggregates in the muscle fibres with rimmed vacuoles. Overexpression of the L227F mutant TUBA4A resulted in cytoplasmic aggregates which colocalised with ubiquitin in cellular model. CONCLUSION: Our findings expanded the phenotypic and genetic manifestations of TUBA4A as well as tubulinopathies, and added a new type of congenital myopathy to be taken into consideration in the differential diagnosis.

Myofibrillar myopathies due to a novel mutation in exon 8 of the LDB3 gene.

Myofibrillar myopathies (MFMs) are a group of genetically heterogeneous diseases affecting the skeletal and cardiac muscles. Myofibrillar myopathies are characterized by focal lysis of myogenic fibers and integration of degraded myogenic fiber products into inclusion bodies, which are typically rich in desmin and many other proteins. Herein, we report a case of a 54-year-old woman who experienced bilateral thigh weakness for over three years. She was diagnosed with MFMs based on muscle biopsy findings and the presence of a novel mutation in exon 8 of the LDB3 gene. Myofibrillar myopathies caused by a mutation in the LDB3 gene are extremely uncommon and often lack distinct clinical characteristics and typically exhibit a slow disease progression. When considering a diagnosis of MFMs, particularly in complex instances of autosomal dominant myopathies where muscle biopsies do not clearly indicate MFMs, it becomes crucial for clinicians to utilize genetic test as a diagnostic tool.

Generation of human induced pluripotent stem cell lines from five patients with Myofibrillar myopathy carrying different heterozygous mutations in the DES gene.

Myofibrillar myopathy (MFM) is a rare genetic disorder characterized by muscular dystrophy that is often associated with cardiac disease. This disease is caused by mutations in several genes, among them DES (encoding desmin) is the most frequently affected. Peripheral blood mononuclear cells from 5 different MFM patients with different DES mutations were reprogrammed into induced pluripotent stem cells (IPSC) using non-integrative vectors. For each patient, one IPSC clone was selected and demonstrated pluripotency hallmarks without genomic abnormalities. SNP profiles were identical to the cells of origin and all the clones have the capacity to differentiate into all three germ layers.

The experience of clinical study and trial participation in rare diseases: A scoping review of centronuclear myopathy and other neuromuscular disorders.

The design of a clinical trial for a rare disease can be challenging. An optimal study design is required to effectively study the clinical outcomes for possible therapies for these types of disorders. Understanding the study participants' experiences as well as barriers and facilitators of participation are important to optimize future research and to inform clinical trial management. Centronuclear myopathies (CNMs) including X-linked myotubular myopathy (XLMTM) are a group of rare congenital myopathies for which there is no cure currently. Since 2014, a number of natural history studies and clinical trials have been conducted in CNMs. Two trials have been prematurely terminated because of severe adverse events. Since no research has been conducted regarding trial experience in CNM, we performed a scoping literature research on clinical trial experience of patients with neuromuscular disorders in general. The most common barriers to trial participation of patients comprise concerns about potential harmful effects, opportunity loss and the expected burden on daily life. The most common facilitators were an expected benefit on the disease course, altruism and collateral benefit. While several results are in line with trial experiences of other types of patients, for example oncological patients, distinctions can be made for patients with CNM and other neuromuscular disorders. However, the limited availability of relevant literature suggests that future (qualitative) research should focus on trial experiences in CNM patients.

Tubular aggregate myopathy causing progressive fatiguable weakness.

Tubular aggregate myopathies comprise a rare group of disorders with characteristic pathological findings and heterogeneous phenotypes, including myasthenic syndrome. We describe a patient with tubular aggregate myopathy who presented with fatiguable weakness improving with pyridostigmine, respiratory involvement and possible cardiac manifestations. We highlight the utility of muscle biopsy in atypical myasthenic syndrome.

Shaping transverse-tubules: central mechanisms that play a role in the cytosol zoning for muscle contraction.

A transverse-tubule (T-tubule) is an invagination of the plasma membrane penetrating deep into muscle cells. An extensive membrane network of T-tubules is crucial for rapid and synchronized signal transmission from the cell surface to the entire sarcoplasmic reticulum for Ca2+ release, leading to muscle contraction. T-tubules are also indispensable for the formation and positioning of other muscle organelles. Their structure and physiological roles are relatively well established; however, the mechanisms shaping T-tubules require further elucidation. Centronuclear myopathy (CNM), an inherited muscular disorder, accompanies structural defects in T-tubules. Membrane traffic-related genes, including MTM1 (Myotubularin 1), DNM2 (Dynamin 2), and BIN1 (Bridging Integrator-1), were identified as causative genes of CNM. In addition, causative genes for other muscle diseases are also reported to be involved in the formation and maintenance of T-tubules. This review summarizes current knowledge on the mechanisms of how T-tubule formation and maintenance is regulated.

The myotubular and centronuclear myopathy patient registry: a multifunctional tool for translational research.

The Myotubular and Centronuclear Myopathy Registry is an international research database containing key longitudinal data on a diverse and growing cohort of individuals affected by this group of rare and ultra-rare neuromuscular conditions. It can inform and support all areas of translational research including epidemiological and natural history studies, clinical trial feasibility planning, recruitment for clinical trials or other research studies, stand-alone clinical studies, standards of care development, and provision of real-world evidence data. For ten years, it has also served as a valuable communications tool and provided a link between the scientific and patient communities. With the anticipated advent of disease-modifying therapies for these conditions, the registry is a key resource for the generation of post-authorisation data for regulatory decision-making, real world evidence, and patient-reported outcome measures. In this paper we present some key data from the current 444 registered individuals with the following genotype split: MTM1 n=270, DNM2 n=42, BIN1 n=4, TTN n=4, RYR1 n=12, other n=4, unknown n=108. The data presented are consistent with the current literature and the common understanding of a strong genotype/phenotype correlations in CNM, most notably the data supports the current knowledge that XLMTM is typically the most severe form of CNM. Additionally, we outline the ways in which the registry supports research, and, more generally, the importance of continuous investment and development to maintain the relevance of registries for all stakeholders. Further information on the registry and contact details are available on the registry website at www.mtmcnmregistry.org.

Effects of gene replacement therapy with resamirigene bilparvovec (AT132) on skeletal muscle pathology in X-linked myotubular myopathy: results from a substudy of the ASPIRO open-label clinical trial.

BACKGROUND: X-linked myotubular myopathy (XLMTM) is a rare, life-threatening congenital muscle disease caused by mutations in the MTM1 gene that result in profound muscle weakness, significant respiratory insufficiency, and high infant mortality. There is no approved disease-modifying therapy for XLMTM. Resamirigene bilparvovec (AT132; rAAV8-Des-hMTM1) is an investigational adeno-associated virus (AAV8)-mediated gene replacement therapy designed to deliver MTM1 to skeletal muscle cells and achieve long-term correction of XLMTM-related muscle pathology. The clinical trial ASPIRO (NCT03199469) investigating resamirigene bilparvovec in XLMTM is currently paused while the risk:benefit balance associated with this gene therapy is further investigated. METHODS: Muscle biopsies were taken before treatment and 24 and 48 weeks after treatment from ten boys with XLMTM in a clinical trial of resamirigene bilparvovec (ASPIRO; NCT03199469). Comprehensive histopathological analysis was performed. FINDINGS: Baseline biopsies uniformly showed findings characteristic of XLMTM, including small myofibres, increased internal or central nucleation, and central aggregates of organelles. Biopsies taken at 24 weeks post-treatment showed marked improvement of organelle localisation, without apparent increases in myofibre size in most participants. Biopsies taken at 48 weeks, however, did show statistically significant increases in myofibre size in all nine biopsies evaluated at this timepoint. Histopathological endpoints that did not demonstrate statistically significant changes with treatment included the degree of internal/central nucleation, numbers of triad structures, fibre type distributions, and numbers of satellite cells. Limited (predominantly mild) treatment-associated inflammatory changes were seen in biopsy specimens from five participants. INTERPRETATION: Muscle biopsies from individuals with XLMTM treated with resamirigene bilparvovec display statistically significant improvement in organelle localisation and myofibre size during a period of substantial improvements in muscle strength and respiratory function. This study identifies valuable histological endpoints for tracking treatment-related gains with resamirigene bilparvovec, as well as endpoints that did not show strong correlation with clinical improvement in this human study. FUNDING: Astellas Gene Therapies (formerly Audentes Therapeutics, Inc.).