System-level analysis of genes mutated in muscular dystrophies reveals a functional pattern associated with muscle weakness distribution.

Muscular dystrophies (MDs) are inherited genetic diseases causing weakness and degeneration of muscles. The distribution of muscle weakness differs between MDs, involving distal muscles or proximal muscles. While the mutations in most of the MD-associated genes lead to either distal or proximal onset, there are also genes whose mutations can cause both types of onsets. We hypothesized that the genes associated with different MD onsets code proteins with distinct cellular functions. To investigate this, we collected the MD-associated genes and assigned them to three onset groups: genes mutated only in distal onset dystrophies, genes mutated only in proximal onset dystrophies, and genes mutated in both types of onsets. We then systematically evaluated the cellular functions of these gene sets with computational strategies based on functional enrichment analysis and biological network analysis. Our analyses demonstrate that genes mutated in either distal or proximal onset MDs code proteins linked with two distinct sets of cellular processes. Interestingly, these two sets of cellular processes are relevant for the genes that are associated with both onsets. Moreover, the genes associated with both onsets display high centrality and connectivity in the network of muscular dystrophy genes. Our findings support the hypothesis that the proteins associated with distal or proximal onsets have distinct functional characteristics, whereas the proteins associated with both onsets are multifunctional.

Association of IGF-1 and IGF-2 genotypes with respiratory muscle strength in individuals with COPD: A cross-sectional study.

Introduction: Chronic obstructive pulmonary disease is a systemic disease characterized not only by respiratory symptoms but also by physical deconditioning and muscle weakness. One prominent manifestation of this disease is the decline in respiratory muscle strength. Previous studies have linked the genotypes of insulin-like growth factor 1 and 2 (IGF-1 and IGF-2) to muscle weakness in other populations without this disease. However, there is a notable knowledge gap regarding the biological mechanisms underlying respiratory muscle weakness, particularly the role of IGF-1 and IGF-2 genotypes in this pulmonary disease. Therefore, this study aimed to investigate, for the first time, the association between IGF-1 and IGF-2 genotypes with respiratory muscle strength in individuals with chronic obstructive pulmonary disease. In addition, we analyzed the relationship between oxidative stress, chronic inflammation, and vitamin D with respiratory muscle strength. Methods: A cross sectional study with 61 individuals with chronic obstructive pulmonary disease. Polymerase chain reaction of gene polymorphisms IGF-1 (rs35767) and IGF-2 (rs3213221) was analyzed. Other variables, related to oxidative stress, inflammation and Vitamin D were dosed from peripheral blood. Maximal inspiratory and expiratory pressure were measured. Results: The genetic polymorphisms were associated with respiratory muscle strength ( 3.0 and 3.5; = 0.57). Specific genotypes of IGF-1 and IGF-2 presented lower maximal inspiratory and expiratory pressure (<0.05 for all). Oxidative stress, inflammatory biomarkers, and vitamin D were not associated with respiratory muscle strength. Conclusion: The polymorphisms of IGF-1 and IGF-2 displayed stronger correlations with respiratory muscle strength compared to blood biomarkers in patients with chronic obstructive pulmonary disease. Specific genotypes of IGF-1 and IGF-2 were associated with reduced respiratory muscle strength in this population.

Introducción: La enfermedad pulmonar obstructiva crónica es una enfermedad sistémica caracterizada no solo por síntomas respiratorios, sino también por el deterioro físico y la debilidad muscular. Una manifestación destacada de esta enfermedad es el declive en la fuerza de los músculos respiratorios. Estudios previos han vinculado los genotipos de factor de crecimiento insulínico 1 y 2 (IGF-1 e IGF-2) con la debilidad muscular en poblaciones sin esta enfermedad. Sin embargo, existe un vacío de conocimiento con respecto a los mecanismos biológicos subyacentes a la debilidad de los músculos respiratorios, en particular el papel de los genotipos IGF-1 e IGF-2 en esta enfermedad pulmonar. Por lo tanto, este estudio tuvo como objetivo investigar, por primera vez, la asociación de los genotipos IGF-1 e IGF-2 con la fuerza de los músculos respiratorios en individuos con enfermedad pulmonar obstructiva crónica. Además, analizamos la relación entre el estrés oxidativo, la inflamación crónica y la vitamina D con la fuerza de los músculos respiratorios. Métodos: Un estudio transversal con 61 individuos con enfermedad pulmonar obstructiva crónica. Se analizó la reacción en cadena de la polimerasa de los polimorfismos genéticos IGF-1 (rs35767) e IGF-2 (rs3213221). Otras variables relacionadas con el estrés oxidativo, la inflamación y la vitamina D se dosificaron a partir de muestras de sangre periférica. Se midieron las presiones inspiratorias y espiratorias máximas. Resultados: Los polimorfismos genéticos están asociados con la fuerza de los músculos respiratorios (F: 3.0 y 3.5; R2= 0.57). Genotipos específicos de IGF-1 e IGF-2 presentaron bajos valores en las presiones inspiratorias y espiratorias (p<0.05 en todos los casos). El estrés oxidativo, los biomarcadores inflamatorios y la vitamina D no se asociaron con la fuerza de los músculos respiratorios. Conclusión: Los polimorfismos de IGF-1 e IGF-2 mostraron correlaciones más sólidas con la fuerza de los músculos respiratorios en pacientes con enfermedad pulmonar obstructiva crónica en comparación con los biomarcadores sanguíneos. Genotipos específicos de IGF-1 e IGF-2 se asociaron con una disminución de la fuerza de los músculos respiratorios en esta población.

The D84G mutation in STIM1 causes nuclear envelope dysfunction and myopathy in mice.

Stromal interaction molecule 1 (STIM1) is a Ca2+ sensor located in the sarcoplasmic reticulum (SR) of skeletal muscle, where it is best known for its role in store-operated Ca2+ entry (SOCE). Genetic syndromes resulting from STIM1 mutations are recognized as a cause of muscle weakness and atrophy. Here, we focused on a gain-of-function mutation that occurs in humans and mice (STIM1+/D84G mice), in which muscles exhibited constitutive SOCE. Unexpectedly, this constitutive SOCE did not affect global Ca2+ transients, SR Ca2+ content, or excitation-contraction coupling (ECC) and was therefore unlikely to underlie the reduced muscle mass and weakness observed in these mice. Instead, we demonstrate that the presence of D84G STIM1 in the nuclear envelope of STIM1+/D84G muscle disrupted nuclear-cytosolic coupling, causing severe derangement in nuclear architecture, DNA damage, and altered lamina A-associated gene expression. Functionally, we found that D84G STIM1 reduced the transfer of Ca2+ from the cytosol to the nucleus in myoblasts, resulting in a reduction of [Ca2+]N. Taken together, we propose a novel role for STIM1 in the nuclear envelope that links Ca2+ signaling to nuclear stability in skeletal muscle.

A novel likely pathogenic homozygous RBCK1 variant in dilated cardiomyopathy with muscle weakness.

AIMS: Polyglucosan body myopathy 1 (PGBM1) is a type of glycogen storage disease where polyglucosan accumulation leads to cardiomyopathy and skeletal muscle myopathy. Variants of RBCK1 is related with PGBM1. We present a newly discovered pathogenic RBCK1 variant resulting in dilated cardiomyopathy (DCM) and a comprehensive literature review. METHODS AND RESULTS: Whole-exome sequencing (WES) was utilized to detect genetic variations in a 7-year-old girl considered the proband. Sanger sequencing was performed to validate the variant in the patient and all the available family members, whether affected or unaffected. The variant's pathogenicity was assessed by conducting a cosegregation analysis within the family with in silico predictive software. WES showed that the proband's RBCK1 gene contained a missense likely pathogenic homozygous nucleotide variant, c.598_599insT: p.His200LeufsTer14 (NM_001323956.1), in exon 8. The computational analysis supported the variant's pathogenicity. The variant was identified in a heterozygous form among all the healthy members of the family. Variants with changes in N-terminal part of the protein were more likely to manifest immunodeficiency and auto-inflammation than those with C-terminal protein modifications according to prior variations of RBCK1 reported in the literature. CONCLUSIONS: Our study offers novel findings indicating an RBCK1 variant in individuals of Iranian ancestry presenting with DCM leading to heart transplantation and myopathy without immunodeficiency or auto-inflammation.

PABPN1 loss-of-function causes APA-shift in oculopharyngeal muscular dystrophy.

Alternative polyadenylation (APA) at the 3' UTR of transcripts contributes to the cell transcriptome. APA is suppressed by the nuclear RNA-binding protein PABPN1. Aging-associated reduced PABPN1 levels in skeletal muscles lead to muscle wasting. Muscle weakness in oculopharyngeal muscular dystrophy (OPMD) is caused by short alanine expansion in PABPN1 exon1. The expanded PABPN1 forms nuclear aggregates, an OPMD hallmark. Whether the expanded PABPN1 affects APA and how it contributes to muscle pathology is unresolved. To investigate these questions, we developed a procedure including RNA library preparation and a simple pipeline calculating the APA-shift ratio as a readout for PABPN1 activity. Comparing APA-shift results to previously published PAS utilization and APA-shift results, we validated this procedure. The procedure was then applied on the OPMD cell model and on RNA from OPMD muscles. APA-shift was genome-wide in the mouse OPMD model, primarily affecting muscle transcripts. In OPMD individuals, APA-shift was enriched with muscle transcripts. In an OPMD cell model APA-shift was not significant. APA-shift correlated with reduced expression levels of a subset of PABPN1 isoforms, whereas the expression of the expanded PABPN1 did not correlate with APA-shift. PABPN1 activity is not affected by the expression of expanded PABPN1, but rather by reduced PABPN1 expression levels. In muscles, PABPN1 activity initially affects muscle transcripts. We suggest that muscle weakness in OPMD is caused by PABPN1 loss-of-function leading to APA-shift that primarily affects in muscle transcripts.

New mutation within a common haplotype is associated with calf muscle weakness in Holsteins.

A recessive haplotype resulting in elevated calf mortality but with apparent incomplete penetrance was previously linked to the end of chromosome 16 (78.7-80.7 Mbp). Genotype analysis of 5.6 million Holsteins indicated that the haplotype was common and traced back to 1952, with a key ancestor born in 1984 (HOUSA1964484, Southwind) identified from chip genotypes as homozygous for the suspect haplotype. Sequence data from Southwind (an affected calf) and the sire of the affected calf were scanned for candidate mutations. A missense mutation with a deleterious projected impact at 79,613,592 bp was homozygous in the affected calf and heterozygous in the calf's sire and Southwind. Sequence data available from the Cooperative Dairy DNA Repository for 299 other Holsteins indicated a 97% concordance with the haplotype and an 89% call rate. The exon amino acid sequence appears to be broadly conserved in the CACNA1S gene, and mutations in humans and mice can cause phenotypes of temporary or permanent paralysis analogous to those in calves with the haplotype causing muscle weakness (HMW). Improved methods for using pedigree to track new mutations within existing haplotypes were developed and applied to the haplotypes for both muscle weakness and Holstein cholesterol deficiency (HCD). For HCD, concordance of the gene test with its haplotype status was greatly improved. For both defects, haplotype status was matched to heifer livability records for 558,000 calves. For HMW, only 46 heifers with livability records were homozygous and traced only to Southwind on both sides. Of those, 52% died before 18 mo at an average age of 1.7 ± 1.6 mo, but that death rate may be underestimated if only healthier calves were genotyped. The death rate was 2.4% for noncarriers. Different reporting methods or dominance effects may be needed to include HMW and other partially lethal effects in selection and mating. Direct tests are needed for new mutations within existing common haplotypes because tracking can be difficult even with accurate pedigrees when the original haplotype has a high frequency.

Skeletal muscle-specific inducible AMPKα1/α2 knockout mice develop muscle weakness, glycogen depletion, and fibrosis that persists during disuse atrophy.

The 5' adenosine monophosphate-activated protein kinase (AMPK) is an important skeletal muscle regulator implicated as a possible therapeutic target to ameliorate the local undesired deconditioning of disuse atrophy. However, the muscle-specific role of AMPK in regulating muscle function, fibrosis, and transcriptional reprogramming during physical disuse is unknown. The purpose of this study was to determine how the absence of both catalytic subunits of AMPK in skeletal muscle influences muscle force production, collagen deposition, and the transcriptional landscape. We generated skeletal muscle-specific tamoxifen-inducible AMPKα1/α2 knockout (AMPKα-/-) mice that underwent 14 days of hindlimb unloading (HU) or remained ambulatory for 14 days (AMB). We found that AMPKα-/- during ambulatory conditions altered body weight and myofiber size, decreased muscle function, depleted glycogen stores and TBC1 domain family member 1 (TBC1D1) phosphorylation, increased collagen deposition, and altered transcriptional pathways. Primarily, pathways related to cellular senescence and mitochondrial biogenesis and function were influenced by the absence of AMPKα. The effects of AMPKα-/- persisted, but were not worsened, following hindlimb unloading. Together, we report that AMPKα is necessary to maintain skeletal muscle quality.NEW & NOTEWORTHY We determined that skeletal muscle-specific AMPKα knockout (KO) mice display functional, fibrotic, and transcriptional alterations before and during muscle disuse atrophy. We also observed that AMPKα KO drives muscle fibrosis and pathways related to cellular senescence that continues during the hindlimb unloading period.

A case report on multiple acyl-CoA dehydrogenase deficiency with severe myopathy and osteoporosis.

A 35-year-old male patient presented fluctuating bilateral lower extremity weakness for 3 years. Physical examination showed grade 4 proximal muscle weakness in both lower extremities and grade 5 distal muscle weakness. Laboratory data revealed elevated creatine kinase, triglycerides, and cholesterol. Muscle pathology showed deposition of lipid droplet under the sarcolemma. Bone densitometry indicated severe osteoporosis. Next-generation sequencing revealed a pathogenic mutation in the ETFDH gene. The patient was diagnosed with late-onset multiple acyl-CoA dehydrogenase deficiency. After riboflavin treatment, symptoms of the patient were relieved, physical endurance was restored, and bone mineral density was improved.

Overexpression of manganese superoxide dismutase mitigates ACL injury-induced muscle atrophy, weakness and oxidative damage.

Oxidative stress has been implicated in the etiology of skeletal muscle weakness following joint injury. We investigated longitudinal patient muscle samples following knee injury (anterior cruciate ligament tear). Following injury, transcriptomic analysis revealed downregulation of mitochondrial metabolism-related gene networks, which were supported by reduced mitochondrial respiratory flux rates. Additionally, enrichment of reactive oxygen species (ROS)-related pathways were upregulated in muscle following knee injury, and further investigation unveiled marked oxidative damage in a progressive manner following injury and surgical reconstruction. We then investigated whether antioxidant protection is effective in preventing muscle atrophy and weakness after knee injury in mice that overexpress Mn-superoxide dismutase (MnSOD+/-). MnSOD+/- mice showed attenuated oxidative damage, atrophy, and muscle weakness compared to wild type littermate controls following ACL transection surgery. Taken together, our results indicate that ROS-related damage is a causative mechanism of muscle dysfunction after knee injury, and that mitochondrial antioxidant protection may hold promise as a therapeutic target to prevent weakness and development of disability.

Causal influence of muscle weakness on cardiometabolic diseases and osteoporosis.

The causal roles of muscle weakness in cardiometabolic diseases and osteoporosis remain elusive. This two-sample Mendelian randomization (MR) study aims to explore the causal roles of muscle weakness in the risk of cardiometabolic diseases and osteoporosis. 15 single nucleotide polymorphisms (SNPs, P < 5 × 10-8) associated with muscle weakness were used as instrumental variables. Genetic predisposition to muscle weakness led to increased risk of coronary artery disease (inverse variance weighted [IVW] analysis, beta-estimate: 0.095, 95% confidence interval [CI]: 0.023 to 0.166, standard error [SE]:0.036, P-value = 0.009) and reduced risk of heart failure (weight median analysis, beta-estimate: - 0.137, 95% CI - 0.264 to - 0.009, SE:0.065, P-value = 0.036). In addition, muscle weakness may reduce the estimated bone mineral density (eBMD, weight median analysis, beta-estimate: - 0.059, 95% CI - 0.110 to - 0.008, SE:0.026, P-value = 0.023). We found no MR associations between muscle weakness and atrial fibrillation, type 2 diabetes or fracture. This study provides robust evidence that muscle weakness is causally associated with the incidence of coronary artery disease and heart failure, which may provide new insight to prevent and treat these two cardiometabolic diseases.

Epilepsy and Coenzyme Q10 deficiency with COQ4 variants.

Coenzyme Q10 (CoQ10) is one of the essential substances for mitochondrial energy synthesis and extra-mitochondrial vital function. Primary CoQ10 deficiency is a rare disease resulting from interruption of CoQ10 biosynthetic pathway and biallelic COQ4 variants are one of the genetic etiologies recognized in this hereditary disorder. The clinical heterogenicity is broad with wide onset age from prenatal period to adulthood. The typical manifestations include early pharmacoresistant seizure, severe cognition and/or developmental delay, dystonia, ataxia, and spasticity. Patients may also have multisystemic involvements such as cardiomyopathy, lactic acidosis or gastro-esophageal regurgitation disease. Oral CoQ10 supplement is the major therapeutic medication currently. Among those patients, c.370G > A variant is the most common pathogenic variant detected, especially in Asian population. This phenomenon also suggests that this specific allele may be the founder variants in Asia. In this article, we report two siblings with infantile onset seizures, developmental delay, cardiomyopathy, and diffuse brain atrophy. Genetic analysis of both two cases revealed homozygous COQ4 c.370G > A (p.Gly124Ser) variants. We also review the clinical manifestations of primary CoQ10 deficiency patients and possible treatment categories, which are still under survey. As oral CoQ10 supplement may improve or stabilize disease severity, early precise diagnosis of primary CoQ10 deficiency and early treatment are the most important issues. This review article helps to further understand clinical spectrum and treatment categories of primary CoQ10 deficiency with COQ4 variant.

A Novel Variant in TPM3 Causing Muscle Weakness and Concomitant Hypercontractile Phenotype.

A novel variant of unknown significance c.8A > G (p.Glu3Gly) in TPM3 was detected in two unrelated families. TPM3 encodes the transcript variant Tpm3.12 (NM_152263.4), the tropomyosin isoform specifically expressed in slow skeletal muscle fibers. The patients presented with slowly progressive muscle weakness associated with Achilles tendon contractures of early childhood onset. Histopathology revealed features consistent with a nemaline rod myopathy. Biochemical in vitro assays performed with reconstituted thin filaments revealed defects in the assembly of the thin filament and regulation of actin-myosin interactions. The substitution p.Glu3Gly increased polymerization of Tpm3.12, but did not significantly change its affinity to actin alone. Affinity of Tpm3.12 to actin in the presence of troponin ± Ca2+ was decreased by the mutation, which was due to reduced interactions with troponin. Altered molecular interactions affected Ca2+-dependent regulation of the thin filament interactions with myosin, resulting in increased Ca2+ sensitivity and decreased relaxation of the actin-activated myosin ATPase activity. The hypercontractile molecular phenotype probably explains the distal joint contractions observed in the patients, but additional research is needed to explain the relatively mild severity of the contractures. The slowly progressive muscle weakness is most likely caused by the lack of relaxation and prolonged contractions which cause muscle wasting. This work provides evidence for the pathogenicity of the TPM3 c.8A > G variant, which allows for its classification as (likely) pathogenic.

Pharmacological Inhibition of Myostatin in a Mouse Model of Typical Nemaline Myopathy Increases Muscle Size and Force.

Nemaline myopathy is one of the most common non-dystrophic congenital myopathies. Individuals affected by this condition experience muscle weakness and muscle smallness, often requiring supportive measures like wheelchairs or respiratory support. A significant proportion of patients, approximately one-third, exhibit compound heterozygous nebulin mutations, which usually give rise to the typical form of the disease. Currently, there are no approved treatments available for nemaline myopathy. Our research explored the modulation of myostatin, a negative regulator of muscle mass, in combating the muscle smallness associated with the disease. To investigate the effect of myostatin inhibition, we employed a mouse model with compound heterozygous nebulin mutations that mimic the typical form of the disease. The mice were treated with mRK35, a myostatin antibody, through weekly intraperitoneal injections of 10 mg/kg mRK35, commencing at two weeks of age and continuing until the mice reached four months of age. The treatment resulted in an increase in body weight and an approximate 20% muscle weight gain across most skeletal muscles, without affecting the heart. The minimum Feret diameter of type IIA and IIB fibers exhibited an increase in compound heterozygous mice, while only type IIB fibers demonstrated an increase in wild-type mice. In vitro mechanical experiments conducted on intact extensor digitorum longus muscle revealed that mRK35 augmented the physiological cross-sectional area of muscle fibers and enhanced absolute tetanic force in both wild-type and compound heterozygous mice. Furthermore, mRK35 administration improved grip strength in treated mice. Collectively, these findings indicate that inhibiting myostatin can mitigate the muscle deficits in nebulin-based typical nemaline myopathy, potentially serving as a much-needed therapeutic option.

CACNA1S Variant Associated With a Myalgic Myopathy Phenotype.

BACKGROUND AND OBJECTIVES: This study aimed to characterize the phenotype of a novel myalgic myopathy encountered in a Finnish family. METHODS: Four symptomatic and 3 asymptomatic individuals from 2 generations underwent clinical, neurophysiologic, imaging, and muscle biopsy examinations. Targeted sequencing of all known myopathy genes was performed. RESULTS: A very rare CACNA1S gene variant c.2893G>C (p.E965Q) was identified in the family. The symptomatic patients presented with exercise-induced myalgia, cramping, muscle stiffness, and fatigue and eventually developed muscle weakness. Examinations revealed mild ptosis and unusual muscle hypertrophy in the upper limbs. In the most advanced disease stage, muscle weakness and muscle atrophy of the limbs were evident. In some patients, muscle biopsy showed mild myopathic findings and creatine kinase levels were slightly elevated. DISCUSSION: Myalgia is a very common symptom affecting quality of life. Widespread myalgia may be confused with other myalgic syndromes such as fibromyalgia. In this study, we show that variants in CACNA1S gene may be one cause of severe exercise-induced myalgia.

A novel nonsense variant in the ATL3 gene is associated with disturbed pain sensitivity, numbness of distal limbs and muscle weakness.

Introduction Hereditary sensory neuropathy (HSN) describes as a heterogeneous group of peripheral neuropathies. HSN type 1 (HSN1) is one subtype characterized by distal sensory impairment that occurs in the form of numbness, tingling, or pain. To date, only two variants in the atlastin GTPase 3 (ATL3) gene have been identified that result in hereditary sensory neuropathy type 1F (HSN1F) with autosomal dominantinheritance. Methods We sudied and examined who present with sensory disturbances and muscle weakness in their lower limb. Patients underwent Whole Exome Sequencing and Sanger sequencing was performed in families for validation of detected variant. Results Here, we identified two Iranian families carrying the novel heterozygous stop variant NM_015459.5: c.16C>T, p.Arg6Ter in ATL3 that led to disturbed pain and touch sensitivity. This variant in the ATL3 gene was detected in both families (NM_015459.5: c.16C>T, p.Arg6Ter) by whole-exome sequencing and confirmed by Sanger sequencing. Conclusion In this study, the subjects manifested weakness of distal limb muscles and numbness of the lower extremities. In addition, some unusual features, including hearing problems and inability to sit and walk presented in one of the patients. Eventually, we provide a case-based review of the clinical features associated with HSN1F. Hitherto, only 11 patients with HSN1F have been reported. We compared our findings to previously reported cases, suggesting that the clinical features are generally variable in the HSN1F patients.

PCYT2-regulated lipid biosynthesis is critical to muscle health and ageing.

Muscle degeneration is the most prevalent cause for frailty and dependency in inherited diseases and ageing. Elucidation of pathophysiological mechanisms, as well as effective treatments for muscle diseases, represents an important goal in improving human health. Here, we show that the lipid synthesis enzyme phosphatidylethanolamine cytidyltransferase (PCYT2/ECT) is critical to muscle health. Human deficiency in PCYT2 causes a severe disease with failure to thrive and progressive weakness. pcyt2-mutant zebrafish and muscle-specific Pcyt2-knockout mice recapitulate the participant phenotypes, with failure to thrive, progressive muscle weakness and accelerated ageing. Mechanistically, muscle Pcyt2 deficiency affects cellular bioenergetics and membrane lipid bilayer structure and stability. PCYT2 activity declines in ageing muscles of mice and humans, and adeno-associated virus-based delivery of PCYT2 ameliorates muscle weakness in Pcyt2-knockout and old mice, offering a therapy for individuals with a rare disease and muscle ageing. Thus, PCYT2 plays a fundamental and conserved role in vertebrate muscle health, linking PCYT2 and PCYT2-synthesized lipids to severe muscle dystrophy and ageing.

X-linked myotubular myopathy: a clinical report and a review of the mild phenotype. / Miopatía miotubular ligada al cromosoma X: informe clínico y revisión del fenotipo leve.

INTRODUCTION: X-linked myotubular myopathy is a rare centronuclear myopathy that affects approximately 1 in 50,000 male newborns caused by pathogenic variants in the myotubularin 1 gene (MTM1). The clinical severity varies, however the need for ventilatory support occurs almost invariably. CASE REPORT: We report the case of a 4-year-old boy presenting mild muscle hypotonia at 12 months-old, expressive language disorder, global developmental delay, and a sensory processing disorder. Clinical exome sequencing identified the hemizygous variant c.722G>A p.(Arg241His) in exon 9 of the myotubularin 1 gene (NM_000252.2). The mother is a heterozygous carrier of the same variant. A diagnosis of a mild form of maternal inherited X-linked myotubular myopathy was established. The child presented significant improvement with speech, occupational, and physical therapies, with no respiratory intercurrences or ventilator dependency. CONCLUSION: The presentation of a mild form of this myotubular myopathy, being less commonly reported, added challenge to the diagnosis. The combination of mild hypotonia, feeding difficulties and expressive language disorder should raise suspicion of a neuromuscular disease. There is a lack of verified motor or developmental scores specific to this myopathy to further determine prognosis and need of other therapies. While currently the severity myotubular myopathy is classified according to ventilator dependency, this may be insufficient and unapplicable to milder cases. There is an evident need for a grading system for mild and moderate cases assessing muscle weakness and fatigue, daily life limitations, motor developmental delay, early phenotypical scores, or recurrent respiratory infections.

TITLE: Miopatía miotubular ligada al cromosoma X: informe clínico y revisión del fenotipo leve.Introducción. La miopatía miotubular ligada al X es una miopatía centronuclear rara que afecta aproximadamente a 1 de cada 50.000 recién nacidos varones causada por variantes patógenas en el gen de la miotubularina 1 (MTM1). La gravedad clínica varía; sin embargo, la necesidad de soporte ventilatorio ocurre casi invariablemente. Caso clínico. Presentamos el caso de un niño de 4 años que presentaba hipotonía muscular leve a los 12 meses, trastorno del lenguaje expresivo, retraso global del desarrollo y trastorno del procesamiento sensorial. La secuenciación clínica del exoma identificó la variante hemicigótica c.722G>A p.(Arg241His) en el exón 9 del gen de la miotubularina 1 (NM_000252.2). La madre es portadora heterocigota de la misma variante. Se estableció el diagnóstico de una forma leve de miopatía miotubular ligada al cromosoma X de herencia materna. El niño presentó una mejoría significativa con terapias del habla, ocupacional y física, sin intercurrencias respiratorias ni dependencia de ventilador. Conclusión. La presentación de una forma leve de esta miopatía miotubular, al notificarse más raramente, añadió desafío al diagnóstico. La combinación de hipotonía leve, dificultades de alimentación y trastorno del lenguaje expresivo debe hacer sospechar una enfermedad neuromuscular. Se carece de puntuaciones motoras o de desarrollo verificadas específicas de esta miopatía para determinar el pronóstico y la necesidad de otras terapias. Aunque actualmente la gravedad de la miopatía miotubular se clasifica según la dependencia del ventilador, esto puede ser insuficiente e inaplicable a los casos más leves. Es evidente la necesidad de un sistema de clasificación para los casos leves y moderados que evalúe la debilidad muscular y la fatiga, las limitaciones de la vida diaria, el retraso del desarrollo motor, las puntuaciones fenotípicas tempranas o las infecciones respiratorias recurrentes.

A new homozygous missense variant in LMOD3 gene causing mild nemaline myopathy with prominent facial weakness.

Nemaline myopathy (NEM) type 10, caused by biallelic mutations in LMOD3, is a severe congenital myopathy clinically characterized by generalized hypotonia and muscle weakness, respiratory insufficiency, joint contractures, and bulbar weakness. Here, we describe a family with two adult patients presenting mild nemaline myopathy due to a novel homozygous missense variant in LMOD3. Both patients presented mild delayed motor milestones, frequent falls during infancy, prominent facial weakness and mild muscle weakness in the four limbs. Muscle biopsy showed mild myopathic changes and small nemaline bodies in a few fibers. A neuromuscular gene panel revealed a homozygous missense variant in LMOD3 that co-segregated with the disease in the family (NM_198271.4: c.1030C>T; p.Arg344Trp). The patients described here provide evidence of the phenotype-genotype correlation, suggesting that non-truncating variants in LMOD3 lead to milder phenotypes of NEM type 10.

ACTA1 H40Y mutant iPSC-derived skeletal myocytes display mitochondrial defects in an in vitro model of nemaline myopathy.

Nemaline myopathies (NM) are a group of congenital myopathies that lead to muscle weakness and dysfunction. While 13 genes have been identified to cause NM, over 50% of these genetic defects are due to mutations in nebulin (NEB) and skeletal muscle actin (ACTA1), which are genes required for normal assembly and function of the thin filament. NM can be distinguished on muscle biopsies due to the presence of nemaline rods, which are thought to be aggregates of the dysfunctional protein. Mutations in ACTA1 have been associated with more severe clinical disease and muscle weakness. However, the cellular pathogenesis linking ACTA1 gene mutations to muscle weakness are unclear To evaluate cellular disease phenotypes, iPSC-derived skeletal myocytes (iSkM) harboring an ACTA1 H40Y point mutation were used to model NM in skeletal muscle. These were generated by Crispr-Cas9, and include one non-affected healthy control (C) and 2 NM iPSC clone lines, therefore representing isogenic controls. Fully differentiated iSkM were characterized to confirm myogenic status and subject to assays to evaluate nemaline rod formation, mitochondrial membrane potential, mitochondrial permeability transition pore (mPTP) formation, superoxide production, ATP/ADP/phosphate levels and lactate dehydrogenase release. C- and NM-iSkM demonstrated myogenic commitment as evidenced by mRNA expression of Pax3, Pax7, MyoD, Myf5 and Myogenin; and protein expression of Pax4, Pax7, MyoD and MF20. No nemaline rods were observed with immunofluorescent staining of NM-iSkM for ACTA1 or ACTN2, and these mRNA transcript and protein levels were comparable to C-iSkM. Mitochondrial function was altered in NM, as evidenced by decreased cellular ATP levels and altered mitochondrial membrane potential. Oxidative stress induction revealed the mitochondrial phenotype, as evidenced by collapsed mitochondrial membrane potential, early formation of the mPTP and increased superoxide production. Early mPTP formation was rescued with the addition of ATP to media. Together, these findings suggest that mitochondrial dysfunction and oxidative stress are disease phenotypes in the in vitro model of ACTA1 nemaline myopathy, and that modulation of ATP levels was sufficient to protect NM-iSkM mitochondria from stress-induced injury. Importantly, the nemaline rod phenotype was absent in our in vitro model of NM. We conclude that this in vitro model has the potential to recapitulate human NM disease phenotypes, and warrants further study.

Ryanodine receptor 1 related myasthenia like myopathy responsive to pyridostigmine.

Disease causing variants in the Ryanodine receptor 1 (RYR1) gene are a common cause for congenital myopathy and for malignant hyperthermia susceptibility. We report a 17 year old boy with congenital muscle weakness progressing to a myasthenia like myopathy with muscle weakness, fatigability, ptosis, and ophthalmoplegia. Muscle biopsy showed predominance and atrophy of type 1 fibers. Whole-exome trio sequencing revealed three variants in the RYR1-gene in the patient: c.6721C > T,p.(Arg2241*) and c.2122G > A,p.(Asp708Asn) in cis position, and the c.325C > T,p.(Arg109Trp) variant in trans. Treatment with pyridostigmine improved symptoms. This case supports that a myasthenia like phenotype is part of the phenotypic spectrum of RYR1 related disorders, and that treatment with pyridostigmine can be beneficial for patients with this phenotype.