Congenital myopathies: pathophysiological mechanisms and promising therapies.

Congenital myopathies (CMs) are a kind of non-progressive or slow-progressive muscle diseases caused by genetic mutations, which are currently defined and categorized mainly according to their clinicopathological features. CMs exhibit pleiotropy and genetic heterogeneity. Currently, supportive treatment and pharmacological remission are the mainstay of treatment, with no cure available. Some adeno-associated viruses show promising prospects in the treatment of MTM1 and BIN1-associated myopathies; however, such gene-level therapeutic interventions target only specific mutation types and are not generalizable. Thus, it is particularly crucial to identify the specific causative genes. Here, we outline the pathogenic mechanisms based on the classification of causative genes: excitation-contraction coupling and triadic assembly (RYR1, MTM1, DNM2, BIN1), actin-myosin interaction and production of myofibril forces (NEB, ACTA1, TNNT1, TPM2, TPM3), as well as other biological processes. Furthermore, we provide a comprehensive overview of recent therapeutic advancements and potential treatment modalities of CMs. Despite ongoing research endeavors, targeted strategies and collaboration are imperative to address diagnostic uncertainties and explore potential treatments.

Phenotype-Genotype Correlation of a Cohort of Patients with Congenital Myopathy: A Single Centre Experience from India.

Background: Congenital myopathies (CMs) are a diverse group of inherited muscle disorders with broad genotypic and phenotypic heterogeneity. While the literature on CM is available from European countries, comprehensive data from the Indian subcontinent is lacking. Objectives: This study aims to describe the clinical and histopathological characteristics of a cohort of genetically confirmed CMs from India and attempts to do phenotype-genotype correlation. Methods: A retrospective chart review of genetically confirmed CMs was evaluated between January 2016 and December 2020 at the neuromuscular clinic. The clinical, genetic, and follow-up data were recorded in a pre-structured proforma as per the medical records, and the data was analyzed. Results: A total of 31(M: F = 14 : 17) unrelated patients were included. The median age at onset and duration of illness are 2.0(IQR:1-8) years and 6.0(IQR:3-10) years respectively. Clinical features observed were proximodistal weakness (54.8%), facial weakness (64.5%), and myopathic facies (54.8%), followed by ptosis (33.3%), and ophthalmoplegia (19.4%). Muscle histopathology was available in 38.7% of patients, and centronuclear myopathy was the most common histopathology finding. The pathogenic genetic variants were identified in RYR1 (29.0%), DNM2 (19.4%), SELENON (12.9%), KBTBD13 (9.7%), NEB (6.5%), and MYPN (6.5%) genes. Novel mutations were observed in 30.3% of the cohort. Follow-up details were available in 77.4% of children, and the median duration of follow-up and age at last follow-up was 4.5 (Range 0.5-11) years and 13 (Range 3-35) years, respectively. The majority were ambulant with minimal assistance at the last follow-up. Mortality was noted in 8.3% due to respiratory failure in Centronuclear myopathy 1 and congenital myopathy 3 with rigid spines (SELENON). Conclusion: This study highlights the various phenotypes and patterns of genetic mutations in a cohort of pediatric patients with congenital myopathy from India. Centronuclear myopathy was the most common histological classification and the mutations in RYR1 followed by DNM2 gene were the common pathogenic variants identified. The majority were independent in their activities of daily living during the last follow-up, highlighting the fact that the disease has slow progression irrespective of the genotype.

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.

Common Pathogenic Mechanisms in Centronuclear and Myotubular Myopathies and Latest Treatment Advances.

Centronuclear myopathies (CNM) are rare congenital disorders characterized by muscle weakness and structural defects including fiber hypotrophy and organelle mispositioning. The main CNM forms are caused by mutations in: the MTM1 gene encoding the phosphoinositide phosphatase myotubularin (myotubular myopathy), the DNM2 gene encoding the mechanoenzyme dynamin 2, the BIN1 gene encoding the membrane curvature sensing amphiphysin 2, and the RYR1 gene encoding the skeletal muscle calcium release channel/ryanodine receptor. MTM1, BIN1, and DNM2 proteins are involved in membrane remodeling and trafficking, while RyR1 directly regulates excitation-contraction coupling (ECC). Several CNM animal models have been generated or identified, which confirm shared pathological anomalies in T-tubule remodeling, ECC, organelle mispositioning, protein homeostasis, neuromuscular junction, and muscle regeneration. Dynamin 2 plays a crucial role in CNM physiopathology and has been validated as a common therapeutic target for three CNM forms. Indeed, the promising results in preclinical models set up the basis for ongoing clinical trials. Another two clinical trials to treat myotubular myopathy by MTM1 gene therapy or tamoxifen repurposing are also ongoing. Here, we review the contribution of the different CNM models to understanding physiopathology and therapy development with a focus on the commonly dysregulated pathways and current therapeutic targets.

Estimation of the Quality-of-Life Impact of X-Linked Myotubular Myopathy.

X-linked myotubular myopathy (XLMTM) is a rare, severe, neuromuscular disorder for which novel treatments are under investigation. This study estimated quality-of-life weights (or utilities) for children with XLMTM. The state that was rated the worst described a child unable to sit and requiring invasive ventilation for≥16 hours a day (utility = -0.07 or -0.27 depending on method used). The state describing a child who can stand and walk and does not require invasive ventilation was the most highly rated state and had a utility of 0.91 or 0.77 (depending on method used).Nine health state vignettes were developed for XLMTM defined in terms of respiratory and motor function based on clinical trial data from parents completing the Assessment of Caregiver Experience with Neuromuscular Disease (ACEND) Domain 1 scale assessing mobility, transfers, sitting, playing, eating, grooming and dressing. These data were supplemented with qualitative data from parent interviews on the daily impact of XLMTM, especially in terms of psychological wellbeing, pain and discomfort, and communication. Seven clinical experts reviewed the draft vignettes for accuracy. Vignettes were rated by members of the UK general public using a time trade-off (TTO) interview and an EQ-5D-5L assessment. This study demonstrated a substantial impact of XLMTM on utility weights.

c.3G>A mutation in the CRYAB gene that causes fatal infantile hypertonic myofibrillar myopathy in the Chinese population.

Infantile hypertonic myofibrillar myopathy is characterized by the rapid development of rigid muscles and respiratory insufficiency soon after birth, with very high mortality. It is extremely rare, and only a few cases having been reported until now. Here we report four Chinese infants with fatal neuromuscular disorders characterized by abdominal and trunk skeletal muscle stiffness and rapid respiratory insufficiency progression. Electromyograms showed increased insertion activities and profuse fibrillation potentials with complex repetitive discharges. Immunohistochemistry staining of muscle biopsies showed accumulations of desmin in the myocytes. Powdery Z-bands with dense granules across sarcomeres were observed in muscle fibers using electron microscopy. All patients carry a homozygous c.3G>A mutation in the CRYAB gene, which resulted in the loss of the initiating methionine and the absence of protein. This study's findings help further understand the disease and highlight a founder mutation in the Chinese population.

Myopathy associated LDB3 mutation causes Z-disc disassembly and protein aggregation through PKCα and TSC2-mTOR downregulation.

Mechanical stress induced by contractions constantly threatens the integrity of muscle Z-disc, a crucial force-bearing structure in striated muscle. The PDZ-LIM proteins have been proposed to function as adaptors in transducing mechanical signals to preserve the Z-disc structure, however the underlying mechanisms remain poorly understood. Here, we show that LDB3, a well-characterized striated muscle PDZ-LIM protein, modulates mechanical stress signaling through interactions with the mechanosensing domain in filamin C, its chaperone HSPA8, and PKCα in the Z-disc of skeletal muscle. Studies of Ldb3Ala165Val/+ mice indicate that the myopathy-associated LDB3 p.Ala165Val mutation triggers early aggregation of filamin C and its chaperones at muscle Z-disc before aggregation of the mutant protein. The mutation causes protein aggregation and eventually Z-disc myofibrillar disruption by impairing PKCα and TSC2-mTOR, two important signaling pathways regulating protein stability and disposal of damaged cytoskeletal components at a major mechanosensor hub in the Z-disc of skeletal muscle.

A novel recessive mutation affecting DNAJB6a causes myofibrillar myopathy.

Mutations in the DNAJB6 gene have been identified as rare causes of myofibrillar myopathies. However, the underlying pathophysiologica mechanisms remain elusive. DNAJB6 has two known isoforms, including the nuclear isoform DNAJB6a and the cytoplasmic isoform DNAJB6b, which was thought to be the pathogenic isoform. Here, we report a novel recessive mutation c.695_699del (p. Val 232 Gly fs*7) in the DNAJB6 gene, associated with an apparently recessively inherited late onset distal myofibrillar myopathy in a Chinese family. Notably, the novel mutation localizes to exon 9 and uniquely encodes DNAJB6a. We further identified that this mutation decreases the mRNA and protein levels of DNAJB6a and results in an age-dependent recessive toxic effect on skeletal muscle in knock-in mice. Moreover, the mutant DNAJB6a showed a dose-dependent anti-aggregation effect on polyglutamine-containing proteins in vitro. Taking together, these findings reveal the pathogenic role of DNAJB6a insufficiency in myofibrillar myopathies and expand upon the molecular spectrum of DNAJB6 mutations.

Use of the Children's Hospital of Philadelphia Infant Test of Neuromuscular Disorders (CHOP INTEND) in X-Linked Myotubular Myopathy: Content Validity and Psychometric Performance.

X-linked myotubular myopathy (XLMTM) is a life-threatening, congenital myopathy characterized by extreme hypotonia, weakness, delayed motor milestones, and respiratory failure, often resulting in pediatric mortality. This study evaluated the content validity and psychometric performance of the Children's Hospital of Philadelphia Infant Test of Neuromuscular Disorders as a measure of neuromuscular functioning in children with X-linked myotubular myopathy. This study was conducted in two phases. Phase I assessed the content validity of the measure for use in an XLMTM pediatric population through: literature review, clinical expert interviews, caregiver interviews, and a modified-Delphi panel among clinicians. Phase II assessed psychometric performance based on the INCEPTUS observational clinical study and the ASPIRO interventional gene therapy study, including tests of reliability (internal consistency, test-retest, and interrater), validity (construct and criterion), and responsiveness based on observational and interventional clinical trial data analyses. Data established construct validity and reliability of the Children's Hospital of Philadelphia Infant Test of Neuromuscular Disorders among XLMTM patients before administration of resamirigene bilparvovec, and sensitivity to study drug administration as evidenced by the significant post-administration response in ASPIRO. Findings support the Children's Hospital of Philadelphia Infant Test of Neuromuscular Disorders as an appropriate neuromuscular functioning assessment in a pediatric X-linked myotubular myopathy patient population.

The Phenotype and Genotype of Congenital Myopathies Based on a Large Pediatric Cohort.

BACKGROUND: Congenital myopathies (CMs) are a clinically and genetically heterogeneous group of hereditary muscular disorders. The distribution of genetic and histologic subtypes has been addressed in only a few cohorts, and the relationship between phenotypes and genotypes is only partially understood. METHODS: This is a retrospective cross-sectional data collection study conducted at a single center. The clinical, histopathological, and molecular characterization of 104 patients with CM is reported. RESULTS: The most common histopathological subtype was core myopathy (42%). Patients with severe endomysial fibrosis were more commonly unable to walk than patients with only a mild-grade endomysial fibrosis (56% vs 16%). Inability to walk was also more prevalent in patients with severe fatty replacement (44% vs 19%). The genetic etiology was more frequently identified among those patients with "specific" histologic findings (74% vs 62%). A definite molecular diagnosis was reached in 65 of 104 patients (62%), with RYR1 (24/104) and TTN (8/104) being the most frequent causative genes. Neonatal onset occurred in 56%. Independent ambulation was achieved by 74%. Patients who walked late were more likely to become wheelchair-dependent. Respiratory support was needed in one of three patients. Gastrostomy placement was required in 15%. Cardiac involvement was observed in 3%, scoliosis in 43%, and intellectual disability in 6%. CONCLUSIONS: This study provides an updated picture of the clinical, histopathological, and molecular landscape of CMs. Independently of the causative gene, fibrosis and fatty replacement in muscle biopsy specimens are associated with clinical severity. Mutations in TTN are responsible for a higher proportion of cases than previously thought.

Diagnosing X-linked Myotubular Myopathy - A German 20-year Follow Up Experience.

X-linked myotubular myopathy (XLMTM) is a life-threatening rare neuromuscular disease, which is caused by pathogenic variants in the MTM1 gene. It has a large phenotypic heterogeneity, ranging from patients, who are able to walk independently to immobile patients who are only able to bring hand to mouth and depend on a respirator 24 hours a day every day. This suggests that ventilator requirements may not illustrate the full clinical picture of patients with XLMTM. At present, there is no curative therapy available, despite first promising results from ongoing gene therapy studies.In this study, we evaluated in detail the data from 13 German XLMTM patients, which was collected over a period of up to 20 years in our university hospital. We compared it to the international prospective longitudinal natural history study (NHS) data from 45 patients (containing 11 German patients). To highlight the broad phenotypic spectrum of the disease, we additionally focused on the clinical presentation of three cases at a glance.Comparing our data with the above mentioned natural history study, it appears the patients of the present German cohort seem to be more often severely affected, with higher frequency of non-ambulatory patients and patients on ventilation (and for longer time) and a higher proportion of patients needing a percutaneous endoscopic gastrostomy. Another key finding is a potential gap in time between first clinical presentation and final diagnosis, showing a need for patients to be treated in a specialized center for neuromuscular diseases.

rAAV-related therapy fully rescues myonuclear and myofilament function in X-linked myotubular myopathy.

X-linked myotubular myopathy (XLMTM) is a life-threatening skeletal muscle disease caused by mutations in the MTM1 gene. XLMTM fibres display a population of nuclei mispositioned in the centre. In the present study, we aimed to explore whether positioning and overall distribution of nuclei affects cellular organization and contractile function, thereby contributing to muscle weakness in this disease. We also assessed whether gene therapy alters nuclear arrangement and function. We used tissue from human patients and animal models, including XLMTM dogs that had received increasing doses of recombinant AAV8 vector restoring MTM1 expression (rAAV8-cMTM1). We then used single isolated muscle fibres to analyze nuclear organization and contractile function. In addition to the expected mislocalization of nuclei in the centre of muscle fibres, a novel form of nuclear mispositioning was observed: irregular spacing between those located at the fibre periphery, and an overall increased number of nuclei, leading to dramatically smaller and inconsistent myonuclear domains. Nuclear mislocalization was associated with decreases in global nuclear synthetic activity, contractile protein content and intrinsic myofilament force production. A contractile deficit originating at the myofilaments, rather than mechanical interference by centrally positioned nuclei, was supported by experiments in regenerated mouse muscle. Systemic administration of rAAV8-cMTM1 at doses higher than 2.5 × 1013 vg kg-1 allowed a full rescue of all these cellular defects in XLMTM dogs. Altogether, these findings identify previously unrecognized pathological mechanisms in human and animal XLMTM, associated with myonuclear defects and contractile filament function. These defects can be reversed by gene therapy restoring MTM1 expression in dogs with XLMTM.

A family with adult-onset myofibrillar myopathy with BAG3 mutation (P470S) presenting with axonal polyneuropathy.

We report a family with adult-onset myofibrillar myopathy with BAG3 mutation who presented peroneal weakness and axonal polyneuropathy, mimicking axonal Charcot-Marie-Tooth disease. The male proband noticed difficulty in tiptoeing at age 34. At age 42, the examination showed muscle weakness and atrophy in distal lower extremities with diminished patellar and Achilles tendon reflexes. Thermal and vibration sensations were also impaired in both feet. The serum CK level was 659 U/L. On muscle imaging, predominant semitendinosus muscle atrophy coexisted with atrophies in the quadriceps, gastrocnemius and lumbar paraspinal muscles. The muscle biopsy showed myofibrillar myopathy with fiber type grouping. His 68-year-old mother also had suffered from distal leg weakness and sensory impairment since her forties. A heterozygous mutation in BAG3 (P470S) was identified in both patients. Clinical features of myofibrillar myopathy with axonal polyneuropathy were consistent with BAG3-related myopathy. Our patients showed remarkably mild presentations without cardiomyopathy, unlike the majorities of previously reported cases.

Regulation of myonuclear positioning and muscle function by the skeletal muscle-specific CIP protein.

The appropriate arrangement of myonuclei within skeletal muscle myofibers is of critical importance for normal muscle function, and improper myonuclear localization has been linked to a variety of skeletal muscle diseases, such as centronuclear myopathy and muscular dystrophies. However, the molecules that govern myonuclear positioning remain elusive. Here, we report that skeletal muscle-specific CIP (sk-CIP) is a regulator of nuclear positioning. Genetic deletion of sk-CIP in mice results in misalignment of myonuclei along the myofibers and at specialized structures such as neuromuscular junctions (NMJs) and myotendinous junctions (MTJs) in vivo, impairing myonuclear positioning after muscle regeneration, leading to severe muscle dystrophy in mdx mice, a mouse model of Duchenne muscular dystrophy. sk-CIP is localized to the centrosome in myoblasts and relocates to the outer nuclear envelope in myotubes upon differentiation. Mechanistically, we found that sk-CIP interacts with the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex and the centriole Microtubule Organizing Center (MTOC) proteins to coordinately modulate myonuclear positioning and alignment. These findings indicate that sk-CIP may function as a muscle-specific anchoring protein to regulate nuclear position in multinucleated muscle cells.

Contractile properties are impaired in congenital myopathies.

The ratio between muscle strength and muscle cross-sectional area is called the specific force. Fatty replacement of muscles is seen in many myopathies, affecting the specific force, without necessarily affecting the ability of the remaining muscle fibers to contract. This ability is called the contractility and is the ratio between muscle strength and the lean muscle cross-sectional area, i.e. the contractile cross-sectional area. We hypothesized that contractility is disrupted in patients with congenital myopathy, because of defects in contractile proteins of the sarcomere. Peak torque across ankle and knee joints was measured by isokinetic dynamometry in 16 patients with congenital myopathy and 13 healthy controls. Five patients only participated partially in the dynamometer measurements due to severe muscle weakness. Dixon MRI technique was used to quantify muscle fat fractions and calculate cross-sectional area. Patients with congenital myopathy had lower cross-sectional area in all muscle groups (P<0.01), higher fat fraction (P<0.01) and less strength (P<0.005) in all studied muscle groups. Their fat content was more than doubled and peak torque lower than half that in healthy controls. Muscle contractility was reduced (P<0.01) in three of four patient muscle groups. In conclusion, muscle contractility was reduced in patients with congenital myopathy, across different diagnoses, and was independent of the level of muscle fat fraction, suggesting that intrinsic defects of the myocyte are responsible for reduced contractility.

A mutation in MTM1 causes X-Linked myotubular myopathy in Boykin spaniels.

The purpose of this study was to report the findings of clinical and genetic evaluation of a 3-month old male Boykin spaniel (the proband) that presented with progressive weakness. The puppy underwent a physical and neurological examination, serum biochemistry and complete blood cell count, electrophysiological testing, muscle biopsy and whole genome sequencing. Clinical evaluation revealed generalized neuromuscular weakness with tetraparesis and difficulty holding the head up and a dropped jaw. There was diffuse spontaneous activity on electromyography, most severe in the cervical musculature. Nerve conduction studies were normal, the findings were interpreted as consistent with a myopathy. Skeletal muscle was grossly abnormal on biopsy and there were necklace fibers and abnormal triad structure localization on histopathology, consistent with myotubular myopathy. Whole genome sequencing revealed a premature stop codon in exon 13 of MTM1 (ChrX: 118,903,496 C > T, c.1467C>T, p.Arg512X). The puppy was humanely euthanized at 5 months of age. The puppy's dam was heterozygous for the variant, and 3 male puppies from a subsequent litter all of which died by 2 weeks of age were hemizygous for the variant. This naturally occurring mutation in Boykin spaniels causes a severe form of X-linked myotubular myopathy, comparable to the human counterpart.

Update on Congenital Myopathies in Adulthood.

Congenital myopathies (CMs) constitute a group of heterogenous rare inherited muscle diseases with different incidences. They are traditionally grouped based on characteristic histopathological findings revealed on muscle biopsy. In recent decades, the ever-increasing application of modern genetic technologies has not just improved our understanding of their pathophysiology, but also expanded their phenotypic spectrum and contributed to a more genetically based approach for their classification. Later onset forms of CMs are increasingly recognised. They are often considered milder with slower progression, variable clinical presentations and different modes of inheritance. We reviewed the key features and genetic basis of late onset CMs with a special emphasis on those forms that may first manifest in adulthood.

A novel heterozygous mutation in the C-terminal region of HSPB8 leads to limb-girdle rimmed vacuolar myopathy.

Mutations in heat shock protein B8 were initially identified in inherited neuropathies and were more recently found to cause a predominantly distal myopathy with myofibrillar pathology and rimmed vacuoles. Rare patients also had proximal weakness. Only very few pathogenic variants have been identified in HSPB8. Disruption of the chaperone activity of heat shock protein B8 impairs chaperone-assisted selective autophagy and results in protein aggregation. We report a 23-year-old patient who presented with a 4-year history of predominantly proximal lower limb weakness due to a novel variant in HSPB8. The creatine kinase level was mildly elevated. Electrodiagnostic studies demonstrated a proximal-predominant myopathy without evidence of neuropathy, and muscle histopathology revealed rimmed vacuoles and myofibrillar protein aggregates. Whole exome sequencing identified a de novo frameshift variant in the C-terminal region of HSPB8 (c.577_580dupGTCA, p.Thr194Serfs*23). This case demonstrates that HSPB8-related disorders can present with early onset limb-girdle myopathy without associated neuropathy.

Cylindrical spirals in two families: Clinical and genetic investigations.

Cylindrical spirals are a rare ultrastructural finding on muscle biopsy, with fewer than 20 reported cases since its first description in 1979. These structures are sometimes observed with tubular aggregates and are thought to comprise longitudinal sarcoplasmic reticulum. While mutations in genes encoding key components of Ca2+ handling (ORAI1 and STIM1) underlie tubular aggregate myopathy, no causative genes have been associated with cylindrical spirals. Here we describe two families with cylindrical spirals on muscle biopsy with a suspected genetic cause. In one family we identified a known truncating variant in EBF3, previously associated with a neurodevelopmental disorder. The affected individuals in this family present with clinical features overlapping with those described for EBF3 disease. An isolated proband in the second family harbours bi-allelic truncating variants in TTN and her clinical course and other features on biopsy are highly concordant for titinopathy. From experimental studies, EBF3 is known to be involved in Ca2+ regulation in muscle, thus EBF3 dysregulation may represent a novel mechanism of impaired Ca2+ handling leading to cylindrical spirals. Additional cases of EBF3 disease or titinopathy with cylindrical spirals need to be identified to support the involvement of these genes in the pathogenesis of cylindrical spirals.