Next-generation enzyme replacement therapies (ERTs) for late-onset Pompe disease (LOPD), including avalglucosidase alfa and cipaglucosidase alfa with miglustat, have been developed to improve muscle targeting and enzyme stability. Real-world evidence on therapy switching between ERT preparations remains limited. A prospective, observational, multicenter cohort study was conducted across German neuromuscular centers. Adults with genetically confirmed LOPD who transitioned to avalglucosidase alfa (Aval) or cipaglucosidase alfa with miglustat (Cipa/Mig) between August/2022 and September/2024 were included. Clinical data were extracted from medical records using a standardized case report form. Following EPOC recommendations, data captured comprised six-minute walk test (6MWT), ten-meter walk test (10MWT), Rasch-built Pompe-specific Activity scale (R-PAct), upright and supine forced vital capacity (FVC), maximal inspiratory/expiratory pressure (MIP/MEP), and maximal voluntary ventilation (MVV). Data were analyzed descriptively and using linear mixed-effects models. Paired comparisons between baseline and 12 months were performed as a secondary analysis in patients with available data. Thirty-nine patients (43 switches; alglucosidase alfa (AlGlu) → Aval n = 32, AlGlu → Cipa/Mig n = 7) were included; four double switches were analyzed under AlGlu → Aval. Main reasons for switching were patient request (42

BACKGROUND:Diabetes mellitus is a common but incompletely characterised manifestation of mitochondrial diseases (MD). Data on risk factors, clinical course, and treatment recommendations are lacking. METHODS:In this multinational cohort study, we analysed longitudinal data of patients with a genetically confirmed MD from the GENOMIT registry included at German, Austrian, and Italian sites between 07/2009-01/2025. Our objectives were to (1) expand the genetic spectrum of mitochondrial diabetes mellitus (mDM), (2) identify risk factors, (3) delineate the clinical course, and (4) characterise real-world use of antidiabetic therapies. FINDINGS:Of 2399 patients, 1225 (51%) were female, and 281 (12%; 172 female) had mDM. Diabetes occurred across 31 genotypes and exhibited marked genotype dependence, with the highest prevalence in m.3243A>G carriers (177/360 [49%]). Only the m.3243A>G variant was associated with a significantly increased risk of mDM (HR = 10.3; 95% CI 5.2-20.4, p < 0.0001), whereas single mtDNA deletions, multiple mtDNA deletions, and primary LHON variants, as well as sex, BMI, ethnicity, smoking, hypertension and dyslipidaemia did not show a significant association. Median diabetes onset in patients with the m.3243A>G variant was at 47.7 years (SD 45.2-52.0). Among patients with mDM, 140/281 (50%) used insulin, and 111/281 (40%) received non-insulin antidiabetic drugs, most commonly metformin, which was discontinued in 8/50 users. Literature review revealed neurological events temporally linked to metformin application in m.3243A>G carriers, though long-term use without adverse events was likewise reported. INTERPRETATION:mDM is frequent in patients with MD, and the individual risk is strongly genotype dependent. While caution is warranted, our data do not justify universal avoidance of metformin; prospective, genotype-informed studies are needed to guide management. FUNDING:German Ministry of Research, Technology and Space; Italian Ministry of Health; European Union.

Background and Objectives:Mitochondrial DNA (mtDNA) disorders exhibit striking clinical variability that is poorly explained by known factors such as variant heteroplasmy, age, or sex. Nuclear genetic modifiers likely play a significant role in this heterogeneity. We aimed to characterize the nature of nuclear genetic involvement for 2 common syndromic presentations of the common pathogenic mtDNA variant, m.3243A>G: mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) and maternally inherited diabetes and deafness (MIDD). Methods:We assembled a multicenter cohort of clinically ascertained carriers of m.3243A>G (total n = 488), identifying 198 individuals across 76 pedigrees suitable for genetic linkage analysis. We investigated 4 clinical features characteristic of MELAS and MIDD: diabetes, hearing impairment, stroke-like episodes, and encephalopathy. Haseman-Elston regression-based genetic linkage analysis was performed to identify regions of the nuclear genome cosegregating with these features. The effects of m.3243A>G heteroplasmy, age, and sex were accounted for using logistic regression; empirical significance thresholds were determined through feature-specific gene-dropping simulations. Association analyses were performed in 247 individuals using single-variant (SAIGE) and gene-based approaches (SAIGE-GENE+ and MAGMA) to refine candidate loci within a significant linkage region. Results:We identified significant genetic linkage to encephalopathy (chromosome 7q22; LOD = 3.72), and regions suggestive of genetic linkage on chromosomes 1, 5, 6, 11, and 13, for encephalopathy and stroke-like episodes. No linkage was identified for diabetes or hearing impairment. Association analysis within the chromosome 7 region identified variant rs62500792 (intergenic between SDHAF3 and TAC1) with the lowest p value (3.7 × 10-5), yet no variants reached the proportional significance threshold (5.3 × 10-6). Gene-based analyses highlighted PLOD3 (p = 3.9 × 10-3) and IMMP2L (p = 6.4 × 10-3) as candidates, as each showed the strongest gene-level signals within the linkage region across complementary burden-testing methods, although neither reached corrected significance thresholds. Discussion:The nuclear genetic architecture modifying m.3243A>G differs across clinical features. Severe neurologic features (encephalopathy and stroke-like episodes) may be influenced by a small number of nuclear genes with relatively large effect sizes, whereas the nuclear contribution to diabetes and hearing impairment appears more polygenic. This study highlights the value of large, well-characterized patient cohorts in identifying modifier loci and advancing knowledge of the mechanisms underlying phenotypic variability in mtDNA disease.

BACKGROUND:Losmapimod is an orally administered small molecule and selective p38α/β mitogen-activated protein kinase (MAPK) inhibitor able to reduce aberrant expression of DUX4 in vitro and thereby potentially slowing disease progression in patients with facioscapulohumeral muscular dystrophy (FSHD). OBJECTIVE:This global, randomized, placebo-controlled, double-blind phase 3 study in patients with FSHD1 and FSHD2 examined the efficacy and safety of losmapimod over a 48-week treatment period compared to placebo (NCT05397470, EUDRACT 2022-000389-16). METHODS:The primary endpoint was change in quantification of reachable workspace (RWS) expressed as relative surface area (RSA). Other endpoints included measures of muscle composition (fat content and lean muscle) using magnetic resonance imaging (MRI), muscle strength using quantitative dynamometry, and quality of life measures. RESULTS:130 participants received losmapimod and 130 participants received placebo, with 252 participants completing the 48-week treatment period. There were no statistically significant differences between groups in change in RSA and all secondary efficacy endpoints from baseline to Week 48. Losmapimod treatment was well-tolerated, and most adverse events were mild. CONCLUSIONS:Losmapimod was generally well tolerated with a favorable safety profile at a dose of 15 mg twice daily. Although none of the efficacy endpoints were met, study design and data from the study may inform future studies of FSHD therapies.

Background Mevalonate kinase deficiency (MKD) is a rare genetic disorder, resulting in the lack of the mevalonate kinase enzyme (MVK), which is involved in the biosynthesis of cholesterol, non-sterol isoprenoids, and coenzyme Q10 (CoQ10).The more severe phenotype of MKD is known as mevalonic aciduria (MA), typically presenting as a multisystemic inflammatory syndrome with possible neurological manifestations, such as developmental delay, cerebellar ataxia, and retinopathy. Myopathy or isolated hyperCKemia have been rarely reported in association with MA. However, a few studies evidenced mitochondrial dysfunction in MVK deficient cells. Aim To point out the connection between MKD, myopathy, and mitochondrial dysfunction, describing two cases of MA. Methods We report on two unrelated patients with myopathy and ataxia, providing clinical, histological, biochemical, and genetic data of MKD. Results Both patients were referred to the Neurology Department in the first year of life, due to muscle weakness, gait disturbances, and increased levels of CK value. Muscle biopsy was performed, showing some mitochondrial alterations and mild lipid storage. Interestingly, biochemical studies on muscle homogenate revealed a reduction of mitochondrial respiratory chain activities and CoQ10 levels. Genetic analysis confirmed the MKD diagnosis, evidencing a homozygous MVK gene mutation in the first case, and compound heterozygous mutations in the second one. Conclusion This report describes two MKD cases with clinical and morphological evidence of muscle involvement in the spectrum of MA related to mitochondrial dysfunction.

Background and Purpose Mitochondrial Encephalomyopathy, Lactic acidosis and Stroke-like episodes (MELAS) is a rare multisystem mitochondrial disorder with clinical heterogeneity. Diagnostic criteria and management strategies for MELAS and mitochondrial stroke-like episodes (SLE) remain inconsistent. This work provides international consensus recommendations on the definition, diagnosis, and management of MELAS and SLE in pediatric and adult populations.Methods An international Delphi consensus process was conducted within the European Reference Network for Neuromuscular Diseases (ERN EURO-NMD), in collaboration with the US Mitochondrial Medicine Society, the ERN for Hereditary Metabolic Disorders (MetabERN), and patient representatives. Following a systematic literature review, 54 statements addressing diagnostic definitions and management of MELAS were evaluated. Statements not reaching consensus were revised and re-evaluated during a face-to-face meeting.Results Consensus supported defining MELAS as a clinical syndrome characterized by one or more SLE in the context of mitochondrial dysfunction caused by a pathogenic mitochondrial DNA variant, particularly m.3243A>G in MT-TL1. The use of terms such as "MELAS-like" or "MELAS spectrum" was discouraged. The panel agreed that the efficacy of L-arginine, L-taurine, L-citrulline, coenzyme Q10, vitamins, and other supplements remains unproven and requires validation in clinical trials. Antiseizure medications should be initiated promptly when seizures are suspected during SLE, and intravenous corticosteroids may be beneficial acutely. Multidisciplinary management of neurological, neuropsychiatric, and systemic complications was endorsed.Conclusions This international consensus provides updated definitions and practical guidance for the diagnosis and management of MELAS and SLE, aiming to harmonize clinical practice and inform future evidence-based research.

Abstract Myofibrillar myopathy 6 is a rare, autosomal-dominant neuromuscular disorder caused by an amino acid exchange Pro209Leu in the co-chaperone BAG3, which disrupts muscle protein turnover and causes severe muscle weakness and shortened lifespan. We generated transgenic mice overexpressing the human mutant BAG3P209L-GFP, which rapidly develop skeletal muscle weakness unlike controls expressing BAG3WT-GFP. Here we show that mutant mice exhibit sarcomere breakdown, inflammation, protein aggregates, centralized nuclei and mitochondrial defects in their skeletal muscles, thereby reducing contraction force by ~90%. Omics profiling uncovered impaired protein synthesis, blocked autophagy, impaired mitophagy and loss of sarcomere proteins. Pathway modulation in vitro and in vivo showed autophagy dysfunction as the primary driver for the pathology, while BAG3 knockdown gene therapy markedly restored muscle function in vivo. In summary, this model recapitulates core disease features, revealing how BAG3 aggregates and loss of BAG3 function impair autophagy to drive muscle degeneration.

Mitochondrial disorders are multiorgan disorders resulting in significant morbidity and mortality. We aimed to characterize death-associated factors in an international cohort of deceased individuals with mitochondrial disorders. This cross-sectional multicenter observational study used data provided by 26 mitochondrial disease centers from 8 countries from January 2022 to March 2023. Individuals with genetically confirmed mitochondrial disorders were included, along with patients with clinically or genetically diagnosed Leigh syndrome. Collected data included demographic and genetic diagnosis variables, clinical phenotype, involvement of organs and systems, conditions leading to death, and supportive care. We defined pediatric and adult groups based on age at death before or after 18 years, respectively. We used Kruskal-Wallis with post hoc Dunn test with Bonferroni correction and Fisher exact test for comparisons, Spearman rank test for correlations, and multiple linear regression for multivariable analysis. Data from 330 deceased individuals with mitochondrial disorders (191 [57.9%] pediatric) were analyzed. The shortest survival times were observed in hepatocerebral syndrome (median 0.3, interquartile range [IQR] 0.2-0.6 years) and mitochondrial cardiomyopathy (median 0.3, IQR 0.2-5.2 years) and the longest in chronic progressive external ophthalmoplegia plus (median 26.5, IQR 22.8-40.2 years) and sensory ataxic neuropathy, dysarthria, and ophthalmoparesis (median 21.0, IQR 13.8-28.5 years). Respiratory failure and pulmonary infections were the most common conditions associated with death (52/330, 15.7% and 46/330, 13.9%, respectively). Noninvasive ventilation was required more often in children (57/191, 29.8%) than adults (12/139, 8.6%, p < 0.001), as was nasogastric or gastric tube (131/191, 68.6% in children and 39/139, 28.1% in adults, p < 0.001). On multivariate analysis, individuals with movement disorders and nuclear gene involvement had increased odds of any respiratory support use (OR 2.42 (95% CI 1.17-5.22) and OR 2.39 (95% CI 1.16-5.07), respectively). This international collaboration highlights the importance of respiratory care and infection management and provides a reference for prognostication across different mitochondrial disorders.

Background: Titin, the largest human protein, is essential for sarcomere structure and function. The TTN gene, spanning 364 exons, undergoes extensive alternative splicing thus producing multiple isoforms. The M-band region, encoded by exons 359-364, plays a critical role in sarcomere integrity and mechanical stability. Exon 363 is of interest due to its involvement in titinopathies. Pathogenic truncating variants in this exon have been linked to recessive myopathies, including and mainly young-onset recessive distal titinopathy. Methods: A multicenter study was conducted on six patients from five unrelated families with confirmed recessive titinopathy and truncating variants in exon 363. Clinical evaluations were performed. Genetic testing and segregation analysis confirmed the phase of the variants. Results: A novel truncating variant c.107578C>T, p.(Gln35860Ter) was identified in four unrelated patients of Eastern European ancestry, all carrying a second pathogenic variant in a canonical TTN exon. These patients exhibited juvenile/young-adult onset recessive distal titinopathy with progressive lower limb weakness, frequently asymmetric muscle involvement, and no cardiac or respiratory complications. A Belgian family presented with a congenital myopathy caused by a novel frameshift deletion c.107430delA, p.(Ser35811AlafsTer32) in exon 363, in compound heterozygosity with a truncating variant in exon 208. These patients showed a more severe phenotype. Conclusions: This study expands the spectrum of TTN-related myopathies, emphasizing exon 363's pathogenic significance. Truncating exon 363 variants contribute to young onset recessive distal and sometimes early onset titinopathy with contractures, and the phenotype severity is influenced by the second variant's location and exon usage.

BACKGROUND AND OBJECTIVES:Mitochondrial disorders are multiorgan disorders resulting in significant morbidity and mortality. We aimed to characterize death-associated factors in an international cohort of deceased individuals with mitochondrial disorders. METHODS:This cross-sectional multicenter observational study used data provided by 26 mitochondrial disease centers from 8 countries from January 2022 to March 2023. Individuals with genetically confirmed mitochondrial disorders were included, along with patients with clinically or genetically diagnosed Leigh syndrome. Collected data included demographic and genetic diagnosis variables, clinical phenotype, involvement of organs and systems, conditions leading to death, and supportive care. We defined pediatric and adult groups based on age at death before or after 18 years, respectively. We used Kruskal-Wallis with post hoc Dunn test with Bonferroni correction and Fisher exact test for comparisons, Spearman rank test for correlations, and multiple linear regression for multivariable analysis. RESULTS:Data from 330 deceased individuals with mitochondrial disorders (191 [57.9%] pediatric) were analyzed. The shortest survival times were observed in hepatocerebral syndrome (median 0.3, interquartile range [IQR] 0.2-0.6 years) and mitochondrial cardiomyopathy (median 0.3, IQR 0.2-5.2 years) and the longest in chronic progressive external ophthalmoplegia plus (median 26.5, IQR 22.8-40.2 years) and sensory ataxic neuropathy, dysarthria, and ophthalmoparesis (median 21.0, IQR 13.8-28.5 years). Respiratory failure and pulmonary infections were the most common conditions associated with death (52/330, 15.7% and 46/330, 13.9%, respectively). Noninvasive ventilation was required more often in children (57/191, 29.8%) than adults (12/139, 8.6%, p < 0.001), as was nasogastric or gastric tube (131/191, 68.6% in children and 39/139, 28.1% in adults, p < 0.001). On multivariate analysis, individuals with movement disorders and nuclear gene involvement had increased odds of any respiratory support use (OR 2.42 (95% CI 1.17-5.22) and OR 2.39 (95% CI 1.16-5.07), respectively). DISCUSSION:This international collaboration highlights the importance of respiratory care and infection management and provides a reference for prognostication across different mitochondrial disorders.

Dermatomyositis (DM) is an autoimmune disease characterised by muscle weakness, myalgia, and extramuscular manifestations and is classified as an idiopathic inflammatory myopathy (IIM). Imaging modalities are important for diagnosis and disease monitoring. Vascular adhesion protein-1 (VAP-1), an inflammation-specific adhesion molecule, may represent a promising target for disease assessment. Recently, the radiotracer [68Ga]Ga-DOTA-Siglec-9, which binds to VAP-1, was introduced for visualising vascular inflammation in giant cell arteritis. This case report aimed to generate preliminary insights into the pathophysiological involvement of VAP-1 and the possible diagnostic utility of [68Ga]Ga-DOTA-Siglec-9 positron emission tomography/computed tomography (PET/CT) in a patient with DM. A 59-year-old female patient with newly diagnosed, untreated DM was recruited. Demographic data, disease history, and laboratory parameters, including soluble VAP-1 (sVAP-1) levels, were collected. The patient underwent a [68Ga]Ga-DOTA-Siglec-9 PET/CT scan, followed by muscle biopsy of the vastus lateralis muscle. The patient’s sVAP-1 level was 416.1 pg/mL, compared to 726.1 ± 126.7 pg/mL in controls (n = 8). The [68Ga]Ga-DOTA-Siglec-9 PET/CT scan demonstrated increased tracer uptake in the muscles of the shoulder girdle, back, forearm, and pharynx, consistent with clinical symptoms. Immunohistochemistry demonstrated endothelial VAP-1 expression in perimysial vessels along with DM-specific muscle alterations. In conclusion, we report the first application of [68Ga]Ga-DOTA-Siglec-9 PET/CT in a patient with IIM. While the findings support the feasibility of this imaging modality and provide preliminary evidence for the involvement of both soluble and endothelial-bound VAP-1 in immune cell trafficking and inflammation, any conclusions regarding its diagnostic or monitoring utility must remain cautious. Further studies with larger cohorts and longitudinal imaging are needed to evaluate [68Ga]Ga-DOTA-Siglec-9 PET/CT as a potential molecular biomarker for inflammation in IIM.

BACKGROUND:Neuromuscular diseases (NMDs) comprise a range of genetic and acquired rare disorders that affect motor neurons, peripheral nerves, neuromuscular junctions and skeletal muscles, leading to significant impairments such as muscle weakness and fatigue resulting in functional limitations. This study aims to investigate the prevalence and severity of disease-related symptoms in adult patients with NMDs registered in the European Reference Network (ERN) EURO-NMD. A cross-sectional electronic survey was conducted with 1,253 participants who reported the severity of 28 symptoms, which were scored using multi-criteria decision analysis (MCDA). RESULTS:The results identified muscle fatigue, weakness and impaired physical function/activity as the most severe and prevalent symptoms in all NMD groups, followed by coordination and/or balance problems, muscle stiffness, mental fatigue, and pain. Notably, the analysis highlighted differences in symptom severity between disease subtypes and underlined the need for standardised patient-reported outcome measures (PROMs) to address the broad heterogeneity of NMDs. CONCLUSIONS:The findings stress the critical importance of capturing patient perspectives to guide clinical care, research priorities and therapeutic development. This work argues for the development of uniform PROMs to better assess disease impact, natural history and treatment efficacy, contributing to improved patient-centred care across diverse NMD populations.

BACKGROUND:Variants in CASQ1, encoding a calcium-binding protein in the fast-twitch fibers of skeletal muscle, cause sarcoplasmic reticulum aberrations such as large vacuoles with CASQ1 inclusions or, less commonly, tubular aggregates. To date, seven pathogenic variants have been described, all dominant missense variants. The typical symptoms of the disease include muscle weakness, cramps, myalgia, and fatigue. METHODS:We used genome and exome sequencing to identify the disease-causing variants in two families with dominant myopathy. The candidate variants were further characterized by cell-transfection studies and western blotting. RESULTS:In Family 1, three patients presented with exercise intolerance, cramps, and myalgia. Additionally, the proband had muscle weakness and her muscle biopsy showed nemaline bodies. In electron microscopy, there were morphological changes in the triads and the SR-feet in all patients. A variant in CASQ1, p.(Glu89Lys), was found in all patients, whereas the proband had also two compound heterozygous variants in NEB. In Family 2, three patients presented with progressive muscle weakness. The proband's muscle biopsy showed marked atrophy. The frameshift variant p.(Gly383Alafs*39) in CASQ1 was found in all three patients. In silico analysis indicated that the variant results in protein extension, which was confirmed by western blotting of patient muscle. Cell-transfection studies showed that the variant protein forms aggregates. CONCLUSION:This study expands the spectrum of pathogenic CASQ1 variants. The morphological changes in the SR-feet indicate a novel pathogenetic mechanism.

The 280th ENMC International Workshop, held in Hoofddorp, The Netherlands, November 22-24, 2024, focused on primary mitochondrial myopathies (PMM). The workshop aimed to update diagnostic criteria, outcome measures, and explore new digital health technologies (DHTs) in the context of clinical trial design and conduct for PMM. Key points discussed included: (i) PMM definition and phenotypes; PMM are genetically determined mitochondrial disorders with prominent skeletal muscle involvement with two major phenotypes: mitochondrial myopathy (MiMy) either with or without chronic progressive external ophthalmoplegia (PEO); (ii) diagnostic criteria, with emphasis on the importance of genetic testing and muscle biopsy for accurate diagnosis; (iii) outcome measures: consensus on clinical scales, functional tests, performance measures, and patient-reported outcome measures (PROMs) for both adults and children; (iv) digital health technologies, with exploration of wearable and non-wearable technologies for gait analysis, physical activity monitoring, and other assessments; (v) potential and limitations of biomarkers for PMM diagnosis and monitoring. The workshop concluded with a strong consensus on the updated definition of PMM, its phenotypes, and the recommended outcome measures for clinical studies. Further research is needed to validate digital health technologies and biomarkers for PMM.

Background and purpose Primary mitochondrial diseases (PMDs) are rare diseases for which diagnosis is challenging, and management and training programs are not well defined in Europe. To capture and assess care needs, five different European Reference Networks have conducted an exploratory survey. Methods The survey covering multiple topics relating to PMDs was sent to all ERNs healthcare providers (HCPs) in Europe. Results We have collected answers from 220 members based in 24/27 European member states and seven non-European member states. Even though most of the responders are aware of neurogenetic diseases, difficulties arise in the ability to deliver comprehensive genetic testing. While single gene analysis is widely available in Europe, whole exome and genome sequencing are not easily accessible, with considerable variation between countries and average waiting time for results frequently above 6 months. Only 12.7% of responders were happy with the ICD-10 codes for classifying patients with PMDs discharged from the hospital, and more than 70% of them consider that PMDs deserve specific ICD codes to improve clinical management, including tailored healthcare, and for reimbursement reasons. Finally, 90% of responders declared that there is a need for further education and training in these diseases. Conclusions This survey provides information on the current difficulties in the care of PMDs in Europe. We believe that the results of this survey are important to help rare disease stakeholders in European countries identify key care and research priorities.

BACKGROUND:Magnetic resonance (MRI) imaging of the skeletal muscles (muscle MRI for short) is increasingly being used in clinical routine for diagnosis and longitudinal assessment of muscle disorders. However, cross-centre standards for measurement protocol and radiological assessment are still lacking. OBJECTIVES:The aim of this expert recommendation is to present standards for the application and interpretation of muscle MRI in hereditary and inflammatory muscle disorders. METHODS:This work was developed in collaboration between neurologists, neuroradiologists, radiologists, neuropaediatricians, neuroscientists and MR physicists from different university hospitals in Germany. The recommendations are based on expert knowledge and a focused literature search. RESULTS:The indications for muscle MRI are explained, including the detection and monitoring of structural tissue changes and oedema in the muscle, as well as the identification of a suitable biopsy site. Recommendations for the examination procedure and selection of appropriate MRI sequences are given. Finally, steps for a structured radiological assessment are presented. CONCLUSIONS:The present work provides concrete recommendations for the indication, implementation and interpretation of muscle MRI in muscle disorders. Furthermore, it provides a possible basis for the standardisation of the measurement protocols at all clinical centres in Germany.