Institut NeuroMyoGène

Calpainopathies, including limb-girdle muscular dystrophy recessive type 1 (LGMD R1) and the rare dominant type 4 (LGMD D4), are genetic neuromuscular disorders caused by pathogenic variants in the CAPN3 gene, which encodes calpain-3, a muscle-specific cysteine protease. This protein plays a crucial role in muscle remodelling, calcium homeostasis, and myogenesis regulation. LGMD R1, the most common form of LGMD, is characterized by progressive, symmetrical muscle weakness primarily affecting the shoulder and pelvic girdles, with an onset ranging from childhood to adulthood. It affects about 1 in 100,000 individuals. The clinical spectrum is wide, with three main phenotypes: pelvifemoral myopathy, scapulohumeral myopathy, and isolated hyperCKemia. LGMD D4 is characterized generally by a milder phenotype with variable but frequent axial muscle involvement. Diagnostic algorithm include serum CK levels dosage, muscle imaging, and sometimes a muscle biopsy, with definitive diagnosis confirmed by CAPN3 gene testing. Calpainopathies have no cure and care is focused on physiotherapy, management of muscle contractures, moderate physical activity, and orthosis. The newly established French National Diagnosis and Care Protocol (NDCP) aim to standardize the diagnosis and care of calpainopathies. The protocol emphasizes early diagnosis, personalized patient care, genetic assessment and prevention of disease progression. This initiative aims to reduce diagnostic delays and improve patient outcomes through a harmonized multidisciplinary approach, informed by the latest clinical and research expertise.

Exertional Heat Stroke (EHS) is a life-threatening disease defined by severe hyperthermia, with a body temperature above 40°C, associated with a sudden neurological dysfunction, observed in healthy individuals exposed to intensive physical exercise. EHS shares many clinical and biological similarities with Malignant Hyperthermia (MH), a pharmacogenetic disorder associated with mutations in the RYR1, CACNA1S and STAC3 genes involved in calcium release required for skeletal muscle contraction. Several pieces of evidence support the existence of inherited genetic mutations that predispose some patients to EHS, but to date, the genetic bases of EHS remain poorly documented. Whole Exome Sequencing on a cohort of soldiers presenting with an EHS episode led to the identification of variants in the CACNA2D4 gene, encoding the α2δ4 isoform of regulatory subunits of voltage-gated calcium channels. In vitro expression of these variants suggested they can affect the function of the protein, and modulate the properties of the Cav1.1 channel. A mouse model with a knock-in for the S299R variant showed EHS-like crisis with signs of rhabdomyolysis and an elevation in core body temperature when submitted to an intense exercise protocol. We show that a CACNA2D4 transcript is expressed in mouse skeletal muscle and that the presence of the S299R variant in α2δ4 induces a modification in the calcium flux triggered by muscle cells depolarization. Altogether, our data point to the involvement of the CACNA2D4 gene in skeletal muscle function and suggest that its genetic variants could be responsible for EHS susceptibility.

Even brief periods of physical inactivity can induce metabolic disruptions; however, the underlying cellular and molecular mechanisms initiating these alterations remain unclear. We investigated whole-body and skeletal muscle-specific metabolic responses to short-term inactivity induced by dry immersion (DI), a model of rapid physical deconditioning. Eighteen healthy men (age = 33.6 [SD 5.5] years, body mass index (BMI) = 23.3 [1.8] kg/m2) underwent five days of DI in a longitudinal within-subject design, with each participant serving as his own control. DI-induced inactivity reduced V̇o2max (-7.4%, P = 0.003), fat mass [dual energy X-ray absorptiometry (DXA), -2.6%, P = 0.002], fat-free mass (DXA, -2.6%, P < 0.001), and quadriceps cross-sectional area (MRI, -2.8%, P < 0.001). Fat content increased in the liver (MRI, +21%, P < 0.001), but not in the muscles (MRI, +0.1%, P = 0.218). Urinary nitrogen excretion rose (+28%, P < 0.001), indicating increased whole-body protein catabolism. Fasting insulin (+46%, P = 0.009) and triglycerides (+14%, P = 0.013), as well as postprandial incremental glucose (+49%, P = 0.002) and insulin (+90%, P < 0.001) concentrations following a carbohydrate-rich meal were increased. Fasting and postprandial total lipid and carbohydrate oxidation measured by indirect calorimetry and adjusted for body composition remained unchanged (P > 0.05 for all). In differentiated myotubes isolated from vastus lateralis biopsies, insulin-stimulated Akt Thr308 phosphorylation (P = 0.03), in vitro glycogen synthesis assessed from U-14C glucose (P < 0.01), and the ability to suppress in vitro palmitate oxidation (1-14Cpalmitate) following incremental glucose concentrations were impaired (P = 0.02). The ability to increase palmitate oxidation when palmitate availability rises was not significantly altered. These results suggest that early intrinsic skeletal muscle cell changes may contribute to the onset of whole-body metabolic disorders induced by physical inactivity.NEW & NOTEWORTHY Five days of dry immersion led to reduced cardiovascular fitness, muscle atrophy, hepatic fat accumulation, and lower glucose tolerance. These alterations occur despite no detectable changes in whole body fat and carbohydrate oxidation following a carbohydrate-rich meal. In cultured primary myotubes, insulin action and metabolic flexibility (fuel switching) are impaired. Early alterations in intrinsic muscle cell metabolism likely reflect rapid epigenetic imprinting of satellite cells and may contribute to systemic metabolic disturbances induced by physical inactivity.

Background: Patients with neurodevelopmental and neuromuscular disorders often show overlapping clinical phenotypes. Pathogenic variants in KMT5B, a histone lysine methyltransferase, have been linked to neurodevelopmental disorders, yet their effects on human skeletal muscle remain unexplored. We report on a patient with KMT5B-linked disease who presented to a neuromuscular specialty clinic with significant involvement of skeletal muscle, where a multi-omics approach established the genetic diagnosis and revealed neuromuscular findings relevant for diagnosis, care and rehabilitation. Methods: Whole-exome sequencing was performed from blood and data was analyzed using the RD-Connect Genome Phenome Analysis Platform. Histological analysis and proteomic profiling were performed on muscle tissue. Results: Whole-exome sequencing revealed a pathogenic heterozygous variant (c.554_557del, p.Tyr185Cysfs*27) in KMT5B. Histological examination revealed fiber-type grouping, angular fibers, increased fast-twitch fiber proportion, and lipid droplet accumulation, indicative of muscle denervation. Proteomic profiling identified 77 dysregulated proteins, including upregulation of sarcomeric proteins, mitochondrial and glycolytic enzymes, acute-phase and complement factors, and extracellular matrix components, reflecting structural remodeling, metabolic adaptation, and inflammatory activation. These findings align with the role types observed in Kmt5b mouse models, supporting a role of KMT5B in neuromuscular function. Conclusions: We present the first combined histological and proteomic analysis of quadriceps muscle from a patient carrying a pathogenic KMT5B variant with a neuromuscular phenotype. The convergence of histological and proteomic alterations suggests that KMT5B haploinsufficiency may be associated with fiber-type shifts, denervation, and metabolic stress in human skeletal muscle. Understanding these processes provides mechanistic insight into motor deficits and informs targeted therapeutic strategies, including physiotherapeutic interventions, and early compensatory measures.

Fast skeletal muscle fibers from zebrafish share a number of functional properties with mammalian twitch muscle fibers, making this vertebrate a precious model to investigate the pathophysiology of neuromuscular disorders. We previously reported that action potentials (APs) from zebrafish fast fibers exhibit low amplitude and require unusually strong negative resting membrane voltage levels to be elicited. In this study, using voltage-clamp and current-clamp techniques, we explored the properties of voltage-gated Na+ channels (NaV) responsible for initiation and propagation of AP in isolated adult zebrafish fast skeletal muscle fibers and compared them to mouse fast-twitch muscle fibers using the same experimental conditions. We found that kinetics of activation and inactivation of NaV were faster in zebrafish fibers and, overall, that the voltage-dependence of inactivation was shifted by 25 mV toward negative voltages as compared to mouse fibers, yielding a mean half-inactivation potential of -90 mV. In agreement with these findings, recording of APs at various resting membrane potentials indicated that APs vanished for resting membrane potentials less negative than -80 mV in zebrafish, whereas APs could still be elicited from resting membrane potentials as low as -60 mV in mice. In addition, Ca2+ transients induced by field stimulation were insensitive to Na+ current blockade in zebrafish but not in mouse fibers. Fluorescence labeling of nicotinic acetylcholine receptors showed that zebrafish fast fibers were multi-innervated with a mean distance between extra-synaptic sarcolemma and motor endplates of 14 µm, expected to lead to negligible attenuation of depolarization propagated from endplates. Finally, knock out of the two genes encoding pore-forming NaV subunits in zebrafish muscles did not induce any change in locomotion and escape behavior of the animals. Taken together, these data question the role of NaV and the occurrence of APs in zebrafish fast muscle.

In recent decades, medical and scientific advances have led to the development of new therapeutic approaches for Duchenne muscular dystrophy (DMD), including gene therapy (GT), which is currently being evaluated. Recruiting enough children in clinical trials remains a challenge, depending on parental decisions. Numerous studies have already been carried out to understand these decision-making factors. To date, no study in Europe has been conducted among the various stakeholders lived experience in a DMD GT trial. Our qualitative study explored participants' perceptions using a social representation method and compared them. We recruited 42 participants, divided into 2 groups comprising 21 parents and 21 professionals participating in GNT-014, a DMD natural history study. Each participant was interviewed on four questions about clinical trials, GT, and the facilitators and barriers of the clinical trial pathway. A prototypical and categorical analysis was carried out using "Pointe-au-Sel" software to analyze the data quantitatively. This method highlights which perceptions are shared within the same group and brings out the most important and most frequently evoked terms. We exported the data as a superimposed scatterplot of the representations of both groups for each question. We obtained a total of 453 evocations for the parents' group and a total of 611 evocations for the professionals' group. For clinical trial and GT, hope and scientific progress are common to the core of both groups but are not at the same level of representation. Parents evoked human contact as the main facilitator and what their child may undergo and become for barriers. For professionals, the facilitators and barriers are centered on the terms that can influence the proper conduct of the trial. These comparative results imply that the vision of the different stakeholders is not totally shared in trial participation. On the contrary, the term GT may also have an influence on professionals, including caregivers.

Background and ObjectivesAnti-IgLON5 disease manifests by various neurologic symptoms, the severity of which can be evaluated using the anti-IgLON5 composite score (ICS). This study assessed the correlation of neurofilament light chain (NfL) and glial fibrillary acidic protein (GFAP) with the ICS and investigated these biomarkers as predictors of long-term clinical severity and mortality in anti-IgLON5 disease.MethodsPatients diagnosed with anti-IgLON5 disease at a national reference center (2016–2024) with available serum and CSF samples were included. NfL and GFAP concentrations were measured in these samples using Simoa assay Neurology 2-Plex B Kit. The severity of symptoms was classified according to the ICS, which was retrospectively evaluated at diagnosis, at last clinical evaluation, and at any other timepoint when samples were collected.ResultsThirty patients (60% male, median age 72 years) were included. Serum NfL concentration was significantly correlated with the total ICS (rho = 0.38, p = 0.025) and its partial bulbar score (rho = 0.39, p = 0.020); serum GFAP concentration was significantly correlated only with the bulbar ICS (rho = 0.34, p = 0.044). CSF NfL and GFAP concentrations were not significantly correlated with the total ICS nor any of its partial scores. Anti-IgLON5 antibody CSF titers, but not serum titers, showed a significant inverse correlation with the total ICS (rho = −0.44, p = 0.04). In 26 patients sampled <4 months after diagnosis, neither NfL nor GFAP predicted the total or partial ICS at last clinical evaluation (median 18 months after diagnosis), but serum NfL increased the risk of 1-year mortality independently of age (hazard ratio for each 10 pg/mL increase: 2.05, 95% CI [1.21–3.45], p = 0.007). In patients with bulbar involvement (n = 22), serum NfL concentration was lower than in 10 controls with bulbar amyotrophic lateral sclerosis (median interquartile range [IQR] 26 pg/mL [21–51] vs 158 pg/mL [100–340], p < 0.001) and higher than in 10 controls with bulbar myasthenia gravis (median [IQR] 15 pg/mL [8–26], p = 0.040).DiscussionIn anti-IgLON5 disease, serum NfL and GFAP are elevated and correlate with the clinical severity, especially of bulbar symptoms. In clinical practice, serum NfL could be useful for disease monitoring and to predict the risk of death.