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.

BACKGROUND: Since 2014, several clinical studies focusing on centronuclear myopathies have been conducted, including a prospective natural history study, a gene transfer clinical trial and a clinical trial using an antisense oligonucleotide. Dedicated patient organizations have played an important role in this process. The experience of members of these organizations, either as a study participant, parent or as a patient organization member communicating with the sponsors are potentially very informative for future trial design.
METHODS: We investigated the burden of and the lessons learned from the first natural history studies and clinical trials from a patient perspective using a qualitative approach. We arranged 4 focus groups with a total of 37 participants from 3 large international patient organizations: ZNM-ZusammenStark!, the Myotubular Trust, and the MTM-CNM Family Connection. 4 themes, based on a systematic literature search were discussed: Expectations and preparation, Clinical study participation, Communication and Recommendations for future clinical trials. The focus group recordings were transcribed, anonymized, and uploaded to Atlas-ti version 8.1 software. The data were analyzed using a thematic content analysis.
RESULTS: Overall, participants were realistic in their expectations, hoping for small improvements of function and quality of life. The realization that trial participation does not equate to a treatment was challenging. Participating in a clinical study had a huge impact on many aspects of daily life, both for patients and their immediate families. First-hand insights into the burden of the design and its possible effect on performance were provided, resulting in numerous compelling recommendations for future clinical studies. Furthermore, participants stressed the importance of clear communication, which was considered to be especially vital in cases of severe adverse events. Finally, while patients were understanding of the importance of adhering to the regulations of good clinical practice, they indicated that they would strongly appreciate a greater understanding and/or acknowledgment of the patient perspective and a reflection of this perspective in future clinical trial design.
CONCLUSIONS: The acknowledgment and inclusion of patients\' perspectives and efficient and effective communication is expected to improve patient recruitment and retention in future clinical studies, as well as more accurate assessment of the patient performance related to suitable planning of the study visits.

X-linked myotubular myopathy (XLMTM) is a rare congenital myopathy resulting from dysfunction of the protein myotubularin encoded by the MTM1 gene. XLMTM has a high neonatal and infantile mortality rate due to a severe myopathic phenotype and respiratory failure. However, in a minority of XLMTM cases, patients present with milder phenotypes and achieve ambulation and adulthood. Notable facial dysmorphia is also present.
We investigated the genotype-phenotype correlations in newly diagnosed XLMTM patients in a patients\' cohort (previously published data plus three novel variants, n = 414). Based on the facial gestalt difference between XLMTM patients and unaffected controls, we investigated the use of the Face2Gene application.
Significant associations between severe phenotype and truncating variants (p < 0.001), frameshift variants (p < 0.001), nonsense variants (p = 0.006), and in/del variants (p = 0.036) were present. Missense variants were significantly associated with the mild and moderate phenotype (p < 0.001). The Face2Gene application showed a significant difference between XLMTM patients and unaffected controls (p = 0.001).
Using genotype-phenotype correlations could predict the disease course in most XLMTM patients, but still with limitations. The Face2Gene application seems to be a practical, non-invasive diagnostic approach in XLMTM using the correct algorithm.

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.
RESULTS: 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.
CONCLUSIONS: 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.
BACKGROUND: Astellas Gene Therapies (formerly Audentes Therapeutics, Inc.).

UNASSIGNED: Ryanodine receptor 1 (RYR1)-related myopathies are a group of congenital muscle diseases caused by RYR1 mutations. These mutations may cause centronuclear myopathy, a congenital neuromuscular disorder characterized by clinical muscle weakness and pathological presence of centrally placed nuclei on muscle biopsy. Mutations in RYR2 cause ventricular arrhythmias that can be treated with flecainide; however, reports of ventricular arrhythmias in RYR1-related myopathies are rare. Herein we report a case of centronuclear myopathy with RYR1 mutations who exhibited frequent premature ventricular contractions (PVCs) and non-sustained ventricular tachycardia (NSVT), which was successfully treated with verapamil and flecainide.
UNASSIGNED: At 7 months, the patient presented neurological manifestations of hypotonia and delayed motor development. A skeletal muscle biopsy performed at age 4 years led to the diagnosis of centronuclear myopathy. At age 15 years, frequent PVCs and NSVT were identified on the electrocardiogram and 24 h Holter monitoring. Treatment with verapamil was initiated; however, it was not beneficial. Therefore, flecainide was added to the treatment, decreasing the frequency of PVCs and NSVT. Non-sustained ventricular tachycardia disappeared at the age of 21, and PVCs almost disappeared at the age of 22. Genetic testing revealed c.13216delG (p.E4406Rfs*35), c.14874G>C (p.K4958N), and c.9892G>A (p.A3298T) in RYR1, and the compound heterozygosity of variants was confirmed by analysis of the parents.
UNASSIGNED: This is the first report of ventricular arrhythmia associated with RYR1-related myopathy that was successfully treated with verapamil and flecainide. The combination of verapamil and flecainide may be a useful treatment option for ventricular arrhythmias in patients with RYR1-related myopathies.

X-linked myotubular myopathy (XLMTM), a centronuclear congenital myopathy secondary to pathogenic variants in the MTM1 gene encoding myotubularin, is typically recognized for its classic and severe phenotype which includes neonatal hypotonia, severe muscle weakness, long-term ventilator dependence, markedly delayed gross motor milestones with inability to independently ambulate, and a high neonatal and childhood mortality. However, milder congenital forms of the condition and other phenotypes are recognized. We describe a 6-year-old boy with a mild XLMTM phenotype with independent gait and no respiratory insufficiency even in the neonatal period. The child has a hemizygous novel splice site variant in the MTM1 gene (c.232-25A > T) whose pathogenicity was confirmed by cDNA studies (exon 5 skipping) and muscle biopsy findings. We also compared the phenotype of our patient with the few reported cases that presented a mild XLMTM phenotype and no respiratory distress at birth, and discussed the potential mechanisms underlying this phenotype such as the presence of residual expression of the normal myotubularin transcript.

Dynamin 2 (DNM2) is a ubiquitously expressed GTPase regulating membrane trafficking and cytoskeleton dynamics. Heterozygous dominant mutations in DNM2 cause centronuclear myopathy (CNM), associated with muscle weakness and atrophy and histopathological hallmarks as fiber hypotrophy and organelles mis-position. Different severities range from the severe neonatal onset form to the moderate form with childhood onset and to the mild adult onset form. No therapy is approved for CNM. Here we aimed to validate and rescue a mouse model for the moderate form of DNM2-CNM harboring the common DNM2 R369W missense mutation. Dnm2R369W/+ mice presented with increased DNM2 protein level in muscle and moderate CNM-like phenotypes with force deficit, muscle and fiber hypotrophy, impaired mTOR signaling, and progressive mitochondria and nuclei mis-position with age. Molecular analyses revealed a fiber type switch toward oxidative metabolism correlating with decreased force and alteration of mitophagy markers paralleling mitochondria structural defects. Normalization of DNM2 levels through intramuscular injection of AAV-shDnm2 targeting Dnm2 mRNA significantly improved histopathology and muscle and myofiber hypotrophy. These results showed that the Dnm2R369W/+ mouse is a faithful model for the moderate form of DNM2-CNM and revealed that DNM2 normalization after a short 4-week treatment is sufficient to improve the CNM phenotypes.

Endocytic dynamins self-assemble into helical scaffolds and utilize energy from GTP hydrolysis to constrict and sever tubular membranous necks of budded endocytic intermediates. They bind the membrane using a pleckstrin-homology domain (PHD). The PHD is characterized by four unstructured loops, two of which partially insert into the membrane. Recent studies reveal that loop insertion lowers the bending rigidity of the membrane and that mutations in these two loops produce separable and opposite effects on the efficiency of dynamin-catalyzed membrane fission. Here, we review the current understanding of dynamin-catalyzed membrane fission and attempt to reconcile contrasting notions that have emerged from biochemical and cellular studies evaluating the role of the PHD in this process. We propose that two membrane-inserting loops act as \"gears\" that define the catalytic efficiency of the dynamin helical scaffold in membrane fission.

Centronuclear myopathy (CNM) is a heterogeneous group of muscle disorders primarily characterized by muscle weakness and variable degrees of respiratory dysfunction caused by mutations in MTM1, DNM2, RYR1, TTN and BIN1. X-linked myotubular myopathy has been the focus of recent natural history studies and clinical trials. Data on respiratory function for other genotypes is limited. To better understand the respiratory properties of the CNM spectrum, we performed a retrospective study in a non-selective Dutch CNM cohort. Respiratory dysfunction was defined as an FVC below 70% of predicted and/or a daytime pCO2 higher than 6 kPa. We collected results of other pulmonary function values (FEV1/FVC ratio) and treatment data from the home mechanical ventilation centres. Sixty-one CNM patients were included. Symptoms of respiratory weakness were reported by 15/47 (32%) patients. Thirty-three individuals (54%) with different genotypes except autosomal dominant (AD)-BIN1-related CNM showed respiratory dysfunction. Spirometry showed decreased FVC, FEV1 & PEF values in all but two patients. Sixteen patients were using HMV (26%), thirteen of them only during night-time. In conclusion, this study provides insight into the prevalence of respiratory symptoms in four genetic forms of CNM in the Netherlands and offers the basis for future natural history studies.

Dynamin-2 is a large GTPase, a member of the dynamin superfamily that regulates membrane remodelling and cytoskeleton dynamics. Mutations in the dynamin-2 gene (DNM2) cause autosomal dominant centronuclear myopathy (CNM), a congenital neuromuscular disorder characterised by progressive weakness and atrophy of the skeletal muscles. Cognitive defects have been reported in some DNM2-linked CNM patients suggesting that these mutations can also affect the central nervous system (CNS). Here we studied how a dynamin-2 CNM-causing mutation influences the CNS function.
Heterozygous mice harbouring the p.R465W mutation in the dynamin-2 gene (HTZ), the most common causing autosomal dominant CNM, were used as disease model. We evaluated dendritic arborisation and spine density in hippocampal cultured neurons, analysed excitatory synaptic transmission by electrophysiological field recordings in hippocampal slices, and evaluated cognitive function by performing behavioural tests.
HTZ hippocampal neurons exhibited reduced dendritic arborisation and lower spine density than WT neurons, which was reversed by transfecting an interference RNA against the dynamin-2 mutant allele. Additionally, HTZ mice showed defective hippocampal excitatory synaptic transmission and reduced recognition memory compared to the WT condition.
Our findings suggest that the dynamin-2 p.R465W mutation perturbs the synaptic and cognitive function in a CNM mouse model and support the idea that this GTPase plays a key role in regulating neuronal morphology and excitatory synaptic transmission in the hippocampus.