De novo LMNA mutations cause a new form of congenital muscular dystrophy

Congenital LMNA-Related Muscular Dystrophy in Paediatrics: Cardiac Management in Monozygotic Twins

Pathogenic variants in LMNA have been associated with a wide spectrum of muscular conditions: the laminopathies. LMNA-related congenital muscular dystrophy is a laminopathy characterised by the early onset of symptoms and often leads to a fatal outcome at young ages. Children face a heightened risk of malignant arrhythmias. No established paediatric protocols for managing this condition are available. We review published cases and provide insights into disease progression in two twin sisters with LMNA-related muscular dystrophy. Our objective is to propose a cardiac surveillance and management plan tailored specifically for paediatric patients. We present a family of five members, including two twin sisters with LMNA-related muscular dystrophy. A comprehensive neuromuscular and cardiac work-up was performed in all family members. Genetic analysis using massive sequencing technology was performed in both twins. Clinical assessment showed that only the twins showed diagnoses of LMNA-related muscular dystrophy. Follow-up showed an early onset of symptoms and life-threatening arrhythmias, with differing disease progressions despite both twins passing away. Genetic analysis identified a de novo rare missense deleterious variant in the LMNA gene. Other additional rare variants were identified in genes associated with myasthenic syndrome. Early-onset neuromuscular symptoms could be related to a prognosis of worse life-threatening arrhythmias in LMNA related muscular dystrophy. Being a carrier of other rare variants may be a modifying factor in the progression of the phenotype, although further studies are needed. There is a pressing need for specific cardiac recommendations tailored to the paediatric population to mitigate the risk of malignant arrhythmias.

Proteomic characterization of human LMNA-related congenital muscular dystrophy muscle cells

LMNA-related congenital muscular dystrophy (L-CMD) is caused by mutations in the LMNA gene, encoding lamin A/C. To further understand the molecular mechanisms of L-CMD, proteomic profiling using DIA mass spectrometry was conducted on immortalized myoblasts and myotubes from controls and L-CMD donors each harbouring a different LMNA mutation (R249W, del.32 K and L380S). Compared to controls, 124 and 228 differentially abundant proteins were detected in L-CMD myoblasts and myotubes, respectively, and were associated with enriched canonical pathways including synaptogenesis and necroptosis in myoblasts, and Huntington's disease and insulin secretion in myotubes. Abnormal nuclear morphology and reduced lamin A/C and emerin abundance was evident in all L-CMD cell lines compared to controls, while nucleoplasmic aggregation of lamin A/C was restricted to del.32 K cells, and mislocalization of emerin was restricted to R249W cells. Abnormal nuclear morphology indicates loss of nuclear lamina integrity as a common feature of L-CMD, likely rendering muscle cells vulnerable to mechanically induced stress, while differences between L-CMD cell lines in emerin and lamin A localization suggests that some molecular alterations in L-CMD are mutation specific. Nonetheless, identifying common proteomic alterations and molecular pathways across all three L-CMD lines has highlighted potential targets for the development of non-mutation specific therapies.

The structure and function of lamin A/C: Special focus on cardiomyopathy and therapeutic interventions

Lamins are inner nuclear membrane proteins that belong to the intermediate filament family. Lamin A/C lie adjacent to the heterochromatin structure in polymer form, providing skeletal to the nucleus. Based on the localization, lamin A/C provides nuclear stability and cytoskeleton to the nucleus and modulates chromatin organization and gene expression. Besides being the structural protein making the inner nuclear membrane in polymer form, lamin A/C functions as a signalling molecule involved in gene expression as an enhancer inside the nucleus. Lamin A/C regulates various cellular pathways like autophagy and energy balance in the cytoplasm. Its expression is highly variable in differentiated tissues, higher in hard tissues like bone and muscle cells, and lower in soft tissues like the liver and brain. In muscle cells, including the heart, lamin A/C must be expressed in a balanced state. Lamin A/C mutation is linked with various diseases, such as muscular dystrophy, lipodystrophy, and cardiomyopathies. It has been observed that a good number of mutations in the LMNA gene impact cardiac activity and its function. Although several works have been published, there are still several unexplored areas left regarding the lamin A/C function and structure in the cardiovascular system and its pathological state. In this review, we focus on the structural organization, expression pattern, and function of lamin A/C, its interacting partners, and the pathophysiology associated with mutations in the lamin A/C gene, with special emphasis on cardiovascular diseases. With the recent finding on lamin A/C, we have summarized the possible therapeutic interventions to treat cardiovascular symptoms and reverse the molecular changes.

Somatic Mosaicism in Amyotrophic Lateral Sclerosis and Frontotemporal Dementia Reveals Widespread Degeneration from Focal Mutations

Although mutations in dozens of genes have been implicated in familial forms of amyotrophic lateral sclerosis (fALS) and frontotemporal degeneration (fFTD), most cases of these conditions are sporadic (sALS and sFTD), with no family history, and their etiology remains obscure. We tested the hypothesis that somatic mosaic mutations, present in some but not all cells, might contribute in these cases, by performing ultra-deep, targeted sequencing of 88 genes associated with neurodegenerative diseases in postmortem brain and spinal cord samples from 404 individuals with sALS or sFTD and 144 controls. Known pathogenic germline mutations were found in 20.6% of ALS, and 26.5% of FTD cases. Predicted pathogenic somatic mutations in ALS/FTD genes were observed in 2.7% of sALS and sFTD cases that did not carry known pathogenic or novel germline mutations. Somatic mutations showed low variant allele fraction (typically <2%) and were often restricted to the region of initial discovery, preventing detection through genetic screening in peripheral tissues. Damaging somatic mutations were preferentially enriched in primary motor cortex of sALS and prefrontal cortex of sFTD, mirroring regions most severely affected in each disease. Somatic mutation analysis of bulk RNA-seq data from brain and spinal cord from an additional 143 sALS cases and 23 controls confirmed an overall enrichment of somatic mutations in sALS. Two adult sALS cases were identified bearing pathogenic somatic mutations in DYNC1H1 and LMNA, two genes associated with pediatric motor neuron degeneration. Our study suggests that somatic mutations in fALS/fFTD genes, and in genes associated with more severe diseases in the germline state, contribute to sALS and sFTD, and that mosaic mutations in a small fraction of cells in focal regions of the nervous system can ultimately result in widespread degeneration.

Multi-omic analysis of mandibuloacral dysplasia type A patient iPSC-derived MSC senescence reveals miR-311 as a novel biomarker for MSC senescence

Mandibuloacral dysplasia type A (MADA) is a rare genetic progeroid syndrome associated with lamin A/C (LMNA) mutations. Pathogenic mutations of LMNA result in nuclear structural abnormalities, mesenchymal tissue damage and progeria phenotypes. However, it remains elusive how LMNA mutations cause mesenchymal-derived cell senescence and disease development. Here, we established an in vitro senescence model using induced pluripotent stem cell-derived mesenchymal stem cells (iMSCs) from MADA patients with homozygous LMNA p.R527C mutation. When expanded to passage 13 in vitro, R527C iMSCs exhibited marked senescence and attenuation of stemness potential, accompanied by immunophenotypic changes. Transcriptome and proteome analysis revealed that cell cycle, DNA replication, cell adhesion and inflammation might play important roles in senescence. In-depth evaluation of changes in extracellular vesicle (EV) derived iMSCs during senescence revealed that R527C iMSC-EVs could promote surrounding cell senescence by carrying pro-senescence microRNAs (miRNAs), including a novel miRNA called miR-311, which can serve as a new indicator for detecting chronic and acute mesenchymal stem cell (MSC) senescence and play a role in promoting senescence. Overall, this study advanced our understanding of the impact of LMNA mutations on MSC senescence and provided novel insights into MADA therapy as well as the link between chronic inflammation and aging development.

Somatic and germinal mosaicism in a Han Chinese family with laminopathies

"Laminopathies" refers to a wide spectrum of myopathies caused by mutations in the LMNA gene. These myopathies include limb girdle muscular dystrophy type 1B (LGMD1B) and dilated cardiomyopathy 1 A (DCM1A), which are both autosomal dominant neurogenetic diseases. There have been few studies on mosaicism in laminopathies. Herein, a Han Chinese family with laminopathies was enrolled in our study. Genetic analysis revealed that the proband carried a novel splice site mutation, c. 1158-3 C > T, in the LMNA gene due to her mother having de novo somatic and gonadal mosaicism. Reverse-transcription polymerase chain reaction (RT-PCR) analysis revealed reduced levels of LMNA mRNA in the proband, which were probably due to nonsense-mediated mRNA decay (NMD). Western blotting revealed reduced lamin A/C protein levels in the skeletal muscle tissue of the proband. In this family, the clinical phenotypes of the proband's mother were normal, and the c. 1158-3 C > T splicing mutation was identified in the blood sample of the proband's mother. Thus, the mutation could be easily considered to be nonpathogenic. Our study emphasizes the importance of mosaicism in the identification of pathogenic variants and genetic counseling.

Caenorhabditis elegans models for striated muscle disorders caused by missense variants of human LMNA

Striated muscle laminopathies caused by missense mutations in the nuclear lamin gene LMNA are characterized by cardiac dysfunction and often skeletal muscle defects. Attempts to predict which LMNA variants are pathogenic and to understand their physiological effects lag behind variant discovery. We created Caenorhabditis elegans models for striated muscle laminopathies by introducing pathogenic human LMNA variants and variants of unknown significance at conserved residues within the lmn-1 gene. Severe missense variants reduced fertility and/or motility in C. elegans. Nuclear morphology defects were evident in the hypodermal nuclei of many lamin variant strains, indicating a loss of nuclear envelope integrity. Phenotypic severity varied within the two classes of missense mutations involved in striated muscle disease, but overall, variants associated with both skeletal and cardiac muscle defects in humans lead to more severe phenotypes in our model than variants predicted to disrupt cardiac function alone. We also identified a separation of function allele, lmn-1(R204W), that exhibited normal viability and swimming behavior but had a severe nuclear migration defect. Thus, we established C. elegans avatars for striated muscle laminopathies and identified LMNA variants that offer insight into lamin mechanisms during normal development.

Nuclear proteostasis imbalance in laminopathy-associated premature aging diseases

Laminopathies are a group of rare genetic disorders with heterogeneous clinical phenotypes such as premature aging, cardiomyopathy, lipodystrophy, muscular dystrophy, microcephaly, epilepsy, and so on. The cellular phenomena associated with laminopathy invariably show disruption of nucleoskeleton of lamina due to deregulated expression, localization, function, and interaction of mutant lamin proteins. Impaired spatial and temporal tethering of lamin proteins to the lamina or nucleoplasmic aggregation of lamins are the primary molecular events that can trigger nuclear proteotoxicity by modulating differential protein-protein interactions, sequestering quality control proteins, and initiating a cascade of abnormal post-translational modifications. Clearly, laminopathic cells exhibit moderate to high nuclear proteotoxicity, raising the question of whether an imbalance in nuclear proteostasis is involved in laminopathic diseases, particularly in diseases of early aging such as HGPS and laminopathy-associated premature aging. Here, we review nuclear proteostasis and its deregulation in the context of lamin proteins and laminopathies.

Characterization of cardiac involvement in children with LMNA-related muscular dystrophy

Introduction: LMNA-related muscular dystrophy is a rare entity that produce "laminopathies" such as Emery-Dreifuss muscular dystrophy (EDMD), limb-girdle muscular dystrophy type 1B (LGMD1B), and LMNA-related congenital muscular dystrophy (L-CMD). Heart failure, malignant arrhythmias, and sudden death may occur. No consensus exists on cardiovascular management in pediatric laminopathies. The aim was to perform an exhaustive cardiologic follow-up in pediatric patients diagnosed with LMNA-related muscular dystrophy. Methods: Baseline cardiac work-up consisted of clinical assessment, transthoracic Doppler echocardiography, 12-lead electrocardiogram, electrophysiological study, and implantation of a long-term implantable cardiac loop recorder (ILR). Results: We enrolled twenty-eight pediatric patients diagnosed with EDMD (13 patients), L-CMD (11 patients), LGMD1B (2 patients), and LMNA-related mild weakness (2 patients). Follow-up showed dilated cardiomyopathy (DCM) in six patients and malignant arrhythmias in five (four concomitant with DCM) detected by the ILR that required implantable cardioverter defibrillator (ICD) implantation. Malignant arrhythmias were detected in 20% of our cohort and early-onset EDMD showed worse cardiac prognosis. Discussion: Patients diagnosed with early-onset EDMD are at higher risk of DCM, while potentially life-threatening arrhythmias without DCM appear earlier in L-CMD patients. Early onset neurologic symptoms could be related with worse cardiac prognosis. Specific clinical guidelines for children are needed to prevent sudden death.

LMNA-related muscular dystrophy: Identification of variants in alternative genes and personalized clinical translation

Background: Laminopathies are caused by rare alterations in LMNA, leading to a wide clinical spectrum. Though muscular dystrophy begins at early ages, disease progression is different in each patient. We investigated variability in laminopathy phenotypes by performing a targeted genetic analysis of patients diagnosed with LMNA-related muscular dystrophy to identify rare variants in alternative genes, thereby explaining phenotypic differences. Methods: We analyzed 105 genes associated with muscular diseases by targeted sequencing in 26 pediatric patients of different countries, diagnosed with any LMNA-related muscular dystrophy. Family members were also clinically assessed and genetically analyzed. Results: All patients carried a pathogenic rare variant in LMNA. Clinical diagnoses included Emery-Dreifuss muscular dystrophy (EDMD, 13 patients), LMNA-related congenital muscular dystrophy (L-CMD, 11 patients), and limb-girdle muscular dystrophy 1B (LGMD1B, 2 patients). In 9 patients, 10 additional rare genetic variants were identified in 8 genes other than LMNA. Genotype-phenotype correlation showed additional deleterious rare variants in five of the nine patients (3 L-CMD and 2 EDMD) with severe phenotypes. Conclusion: Analysis f known genes related to muscular diseases in close correlation with personalized clinical assessments may help identify additional rare variants of LMNA potentially associated with early onset or most severe disease progression.

Childhood muscular dystrophies

Infancy- and childhood-onset muscular dystrophies are associated with a characteristic distribution and progression of motor dysfunction. The underlying causes of progressive childhood muscular dystrophies are heterogeneous involving diverse genetic pathways and genes that encode proteins of the plasma membrane, extracellular matrix, sarcomere, and nuclear membrane components. The prototypical clinicopathological features in an affected child may be adequate to fully distinguish it from other likely diagnoses based on four common features: (1) weakness and wasting of pelvic-femoral and scapular muscles with involvement of heart muscle; (2) elevation of serum muscle enzymes in particular serum creatine kinase; (3) necrosis and regeneration of myofibers; and (4) molecular neurogenetic assessment particularly utilizing next-generation sequencing of the genome of the likeliest candidates genes in an index case or family proband. A number of different animal models of therapeutic strategies have been developed for gene transfer therapy, but so far these techniques have not yet entered clinical practice. Treatment remains for the most part symptomatic with the goal of ameliorating locomotor and cardiorespiratory manifestations of the disease.

Surgical Treatment for Severe Cervical Hyperlordosis and Thoracolumar Kyphoscoliosis with Emery-Dreifuss Muscular Dystrophy: A Case Report and Literature Review

BACKGROUND

Emery-Dreifuss muscular dystrophy (EDMD) is an uncommon, gradually progressive X-linked myopathy, and it could result in rigid spinal deformity. Only a few case reports have described surgical treatment of cervical hyperlordosis and thoracolumbar kyphoscoliosis secondary to EDMD. We report a rare case of EDMD to present the surgical strategies of severe cervical hyperlordosis and thoracolumbar kyphoscoliosis.

CASE PRESENTATION

The patient was a 22-year-old man with EDMD who had severe cervical hyperlordosis and thoracolumbar kyphoscoliosis. A posterior spinal fusion from T9-S2 was performed to correct the thoracolumbar kyphoscoliosis at the age of 21 years. Six months later, with an anterior C7-T1 closing wedge bone-disc-bone osteotomy and a posterior-anterior-posterior cervicothoracic fusion from C4-T4, the cervical deformity was corrected, thus achieving a horizontal gaze. During 1.5-year follow-up, no loss of correction was observed.

CONCLUSION

Cervical posterior-anterior-posterior closing-wedge osteotomy combined with long fusion at thoracolumbar spine can be a reliable surgical technique to correct severe spine deformity in EDMD. This two-stage revision surgical strategy can help restore a horizontal gaze on the basis of a balanced trunk. Cervical deformity in such patients should be corrected in the first stage considering its role as a "driver" of the global spine deformity.

The most severe form of LMNA-associated congenital muscular dystrophy

Alterations in the LMNA gene cause a wide spectrum of diseases collectively called laminopathies. LMNA-associated congenital muscular dystrophy is a form of laminopathy, which usually causes infantile onset of muscle weakness, predominantly in the cervical-axial muscles, and motor developmental retardation. Cardiac symptoms during the first decade of life are rare. We report a case of LMNA-associated congenital muscular dystrophy in which the patient did not achieve head control and experienced facial muscle weakness. Cardiac dysrhythmias were observed at 5 years with development of dilated cardiomyopathy and ischemic strokes at 7 years. Despite intensive medical intervention, he died suddenly at 9 years. This report broadens the spectrum of phenotypes of this disorder with the most severe symptoms during the first decade of life. Our case underscores the need for early genetic testing for LMNA in patients with congenital muscular dystrophy to screen for cardiac manifestations and intervene as necessary.

The LMNA p.R541C mutation causes dilated cardiomyopathy in human and mice

Dilated cardiomyopathy (DCM) is a major cause of heart failure. LMNA variants contribute to 6-10% DCM cases, but the underlying mechanisms remain incompletely understood. Here, we reported two patients carrying the LMNA c.1621C > T/ p.R541C variant and generated a knock-in mouse model (Lmna^RC) to study the role of this variant in DCM pathogenesis. We found Lmna^RC/RC mice exhibited ventricular dilation and reduced systolic functions at 6 months after birth. The Lmna^RC/RC cardiomyocytes increased in size but no nuclear morphology defects were detected. Transcriptomic and microscopic analyses revealed suppressed gene expression and perturbed ultrastructure in Lmna^RC/RC mitochondria. These defects were associated with increased heterochromatin structures and epigenetic markers including H3K9me2/3. Together, these data implied that the LMNA c.1621C > T/ p.R541C variant enhanced heterochromatic gene suppression and disrupted mitochondria functions as a cause of DCM.

LMNA Determines Nuclear Morphology During Syncytialization of Human Trophoblast Stem Cells

Upon implantation, the trophectoderm differentiates into the multi-nucleated primitive syncytiotrophoblast (pSTB) through a process called primary syncytialization to facilitate maternal-fetal interactions and to establish a pregnancy. However, ethical issues and limited access to human embryos around the time of embryo implantation hinder the investigation of the detailed molecular mechanisms underpinning this event in humans. Here we established human trophoblast stem cells (hTSCs) from human blastocysts. We characterized nuclear enlargement in STB differentiated from hTSCs, which recapitulate morphological nuclear features of pSTB in human embryos. Specifically, we revealed that CRISPR/Cas9-mediated LMNA disruption perturbated nuclear volume during hTSCs syncytialization. Overall, our results not only provide an interesting insight into mechanisms underlying nuclear enlargement during primary syncytialization but highlight the hTSCs as an indispensable model in understanding human trophoblast differentiation during implantation.

Novel SEPN1 Mutations in Exon 1 Are Common in Rigid Spine With Muscular Dystrophy Type 1 in Chinese Patients

Congenital muscular dystrophy with early rigid spine, also known as the rigid spine with muscular dystrophy type 1 (RSMD1), is caused by SEPN1 mutation. We investigated the clinical manifestations, pathological features, and genetic characteristics of 8 Chinese RSMD1 patients in order to improve diagnosis and management of the disease. Eight patients presented with delayed motor development, muscle weakness, hypotonia, and a myopathic face with high palatine arches. All patients could walk independently, though with poor running and jumping, and most had a rigid spine, lordosis, or scoliosis. The symptoms of respiratory involvement were present early, and upper respiratory tract infections and pneumonia often occurred. Five patients had severe pneumonia, pulmonary hypertension, and respiratory failure. Lung function tests showed variable restrictive ventilation dysfunction. Polysomnography suggested hypoxia and hypoventilation. The serum creatine kinase (CK) level was normal or mildly increased. Muscle biopsy indicated chronic myopathic changes and minicores. Muscle magnetic resonance imaging (MRI) showed diffuse fatty infiltration of the gluteus maximus and thigh muscle. SEPN1 gene analysis revealed 16 compound heterozygous variants, 81.3% of which are unreported, including 7 exon 1 variants. Our study expands the spectrum of clinical and genetic findings in RSMD1 to improve diagnosis, management, and standards of care. SEPN1 mutations in exon 1 are common and easily missed, and exon 1 should be carefully analyzed when RSMD1 is suspected, which will provide valuable genetic counseling for the family and useful information for future natural history studies and clinical trials.

Molecular Pathology of Laminopathies

The nuclear envelope is composed of the nuclear membranes, nuclear lamina, and nuclear pore complexes. Laminopathies are diseases caused by mutations in genes encoding protein components of the lamina and these other nuclear envelope substructures. Mutations in the single gene encoding lamin A and C, which are expressed in most differentiated somatic cells, cause diseases affecting striated muscle, adipose tissue, peripheral nerve, and multiple systems with features of accelerated aging. Mutations in genes encoding other nuclear envelope proteins also cause an array of diseases that selectively affect different tissues or organs. In some instances, the molecular and cellular consequences of laminopathy-causing mutations are known. However, even when these are understood, mechanisms explaining specific tissue or organ pathology remain enigmatic. Current mechanistic hypotheses focus on how alterations in the nuclear envelope may affect gene expression, including via the regulation of signaling pathways, or cellular mechanics, including responses to mechanical stress.

Is the Next Generation Sequencing the Essential Tool for the Early Diagnostic Approach in Congenital Muscular Dystrophy? New Mutation in the Gen LMNA Associated with Serious Phenotype

Background

Laminopathies are a group of diseases caused by mutations in the LMNA gene. Congenital dystrophy of the LMN is a rare disease, with less than 100 cases described in the literature.

Objectives and Materials and Methods

We present the clinical case of a patient with congenital muscular dystrophy associated with an undescribed mutation in the LMNA gene.

Results

The patient presented progressive motor delay from 10 months with a physical examination consisting of global hypotonia, bilateral winged scapula, areflexia, hip and knee flexion posture, and positive Gowers. The patient developed progressive weakness with neck tone loss, functional impairment, and loss of gait at 5 years.

Conclusions

To date, more than 20 mutations associated with congenital LMNA muscular dystrophy have been identified, most due to a single amino acid change (aa), few due to the gain or loss of several aa as in our patient.

[A case of laminopathy with the mutation of LMNA gene identified by the exome analysis of disease-related genes]

Laminopathy, caused by mutations in the LMNA gene, include a variety of diseases, such as Emery-Dreifuss muscular dystrophy. A Japanese woman developed progressive muscle weakness, muscle atrophy and joint contractures of upper and lower limbs after the age of two years old. She had restrictive respiratory dysfunction, and developed both supraventricular and ventricular arrhythmias after the fourth decade of life. At 55 years old, she had tracheostomy, required mechanical ventilation and was implanted with the implantable cardioverter defibrillator. The serum level of creatine kinase was within normal range. Electromyography showed polyphasic or large motor unit potentials and reduced interference pattern, while relatively normal recruitment. The exome analysis of disease-related genes revealed a heterozygous pathogenic variant c.1072G>A (p.E358K) in the LMNA gene, which contributed to the diagnosis of laminopathy.

Preclinical Advances of Therapies for Laminopathies

Laminopathies are a group of rare disorders due to mutation in LMNA gene. Depending on the mutation, they may affect striated muscles, adipose tissues, nerves or are multisystemic with various accelerated ageing syndromes. Although the diverse pathomechanisms responsible for laminopathies are not fully understood, several therapeutic approaches have been evaluated in patient cells or animal models, ranging from gene therapies to cell and drug therapies. This review is focused on these therapies with a strong focus on striated muscle laminopathies and premature ageing syndromes.

In Silico and In Vivo Analysis of Amino Acid Substitutions That Cause Laminopathies

Mutations in the LMNA gene cause diseases called laminopathies. LMNA encodes lamins A and C, intermediate filaments with multiple roles at the nuclear envelope. LMNA mutations are frequently single base changes that cause diverse disease phenotypes affecting muscles, nerves, and fat. Disease-associated amino acid substitutions were mapped in silico onto three-dimensional structures of lamin A/C, revealing no apparent genotype–phenotype connections. In silico analyses revealed that seven of nine predicted partner protein binding pockets in the Ig-like fold domain correspond to sites of disease-associated amino acid substitutions. Different amino acid substitutions at the same position within lamin A/C cause distinct diseases, raising the question of whether the nature of the amino acid replacement or genetic background differences contribute to disease phenotypes. Substitutions at R249 in the rod domain cause muscular dystrophies with varying severity. To address this variability, we modeled R249Q and R249W in Drosophila Lamin C, an orthologue of LMNA. Larval body wall muscles expressing mutant Lamin C caused abnormal nuclear morphology and premature death. When expressed in indirect flight muscles, R249W caused a greater number of adults with wing posturing defects than R249Q, consistent with observations that R249W and R249Q cause distinct muscular dystrophies, with R249W more severe. In this case, the nature of the amino acid replacement appears to dictate muscle disease severity. Together, our findings illustrate the utility of Drosophila for predicting muscle disease severity and pathogenicity of variants of unknown significance.

The non-muscle ADF/cofilin-1 controls sarcomeric actin filament integrity and force production in striated muscle laminopathies

Cofilins are important for the regulation of the actin cytoskeleton, sarcomere organization, and force production. The role of cofilin-1, the non-muscle-specific isoform, in muscle function remains unclear. Mutations in LMNA encoding A-type lamins, intermediate filament proteins of the nuclear envelope, cause autosomal Emery-Dreifuss muscular dystrophy (EDMD). Here, we report increased cofilin-1 expression in LMNA mutant muscle cells caused by the inability of proteasome degradation, suggesting a protective role by ERK1/2. It is known that phosphorylated ERK1/2 directly binds to and catalyzes phosphorylation of the actin-depolymerizing factor cofilin-1 on Thr25. In vivo ectopic expression of cofilin-1, as well as its phosphorylated form on Thr25, impairs sarcomere structure and force generation. These findings present a mechanism that provides insight into the molecular pathogenesis of muscular dystrophies caused by LMNA mutations.

Floppy infant syndrome as a first manifestation of LMNA-related congenital muscular dystrophy

LMNA-related congenital muscular dystrophy (L-CMD) is the most severe phenotypic form of skeletal muscle laminopathies. This paper reports clinical presentation of the disease in 15 Polish patients from 13 families with genetically confirmed skeletal muscle laminopathy. In all these patients floppy infant syndrome was the first manifestation of the disease. The genetic diagnosis was established by next generation sequencing (targeted panel or exome; 11 patients) or classic Sanger sequencing (4 patients). In addition to known pathogenic LMNA variants: c.116A > G (p.Asn39Ser), c.745C > T (p.Arg249Trp), c.746G > A (p.Arg249Gln), c.1072G > A (p.Glu358Lys), c.1147G > A (p.Glu383Lys), c.1163G > C (p.Arg388Pro), c.1357C > T (p.Arg453Trp), c.1583C > G (p.Thr528Arg), we have identified three novel ones: c.121C > G (p.Arg41Gly), c.1127A > G (p.Tyr376Cys) and c.1160T > C (p.Leu387Pro). Eleven patients had de novo mutations, 4 - familial. In one family we observed intrafamilial variability of clinical course: severe L-CMD in the male proband, intermediate form in his sister and asymptomatic in their mother. One asymptomatic father had somatic mosaicism. L-CMD should be suspected in children with hypotonia in infancy and delayed motor development, who have poor head control, severe hyperlordosis and unstable and awkward gait. Serum creatine kinase may be high (~1000IU/l). Progression of muscle weakness is fast, leading to early immobilization. In some patients with L-CMD joint contractures can develop with time. MRI shows that the most frequently affected muscles are the serratus anterior, lumbar paraspinal, gluteus, vastus, adductor magnus, hamstrings, medial head of gastrocnemius and soleus. Ultra-rare laminopathies can be a relatively common cause of generalized hypotonia in children. Introduction of wide genome sequencing methods was a breakthrough in diagnostics of diseases with great clinical and genetic variability and allowed approach "from genotype do phenotype". However target sequencing of LMNA gene could be considered in selected patients with clinical picture suggestive for laminopathy.

Skeletal and Cardiac Muscle Disorders Caused by Mutations in Genes Encoding Intermediate Filament Proteins

Intermediate filaments are major components of the cytoskeleton. Desmin and synemin, cytoplasmic intermediate filament proteins and A-type lamins, nuclear intermediate filament proteins, play key roles in skeletal and cardiac muscle. Desmin, encoded by the DES gene (OMIM *125660) and A-type lamins by the LMNA gene (OMIM *150330), have been involved in striated muscle disorders. Diseases include desmin-related myopathy and cardiomyopathy (desminopathy), which can be manifested with dilated, restrictive, hypertrophic, arrhythmogenic, or even left ventricular non-compaction cardiomyopathy, Emery–Dreifuss Muscular Dystrophy (EDMD2 and EDMD3, due to LMNA mutations), LMNA-related congenital Muscular Dystrophy (L-CMD) and LMNA-linked dilated cardiomyopathy with conduction system defects (CMD1A). Recently, mutations in synemin (SYNM gene, OMIM *606087) have been linked to cardiomyopathy. This review will summarize clinical and molecular aspects of desmin-, lamin- and synemin-related striated muscle disorders with focus on LMNA and DES-associated clinical entities and will suggest pathogenetic hypotheses based on the interplay of desmin and lamin A/C. In healthy muscle, such interplay is responsible for the involvement of this network in mechanosignaling, nuclear positioning and mitochondrial homeostasis, while in disease it is disturbed, leading to myocyte death and activation of inflammation and the associated secretome alterations.

Bi-allelic truncating mutations in VWA1 cause neuromyopathy

The von Willebrand Factor A domain containing 1 protein, encoded by VWA1, is an extracellular matrix protein expressed in muscle and peripheral nerve. It interacts with collagen VI and perlecan, two proteins that are affected in hereditary neuromuscular disorders. Lack of VWA1 is known to compromise peripheral nerves in a Vwa1 knock-out mouse model. Exome sequencing led us to identify bi-allelic loss of function variants in VWA1 as the molecular cause underlying a so far genetically undefined neuromuscular disorder. We detected six different truncating variants in 15 affected individuals from six families of German, Arabic, and Roma descent. Disease manifested in childhood or adulthood with proximal and distal muscle weakness predominantly of the lower limbs. Myopathological and neurophysiological findings were indicative of combined neurogenic and myopathic pathology. Early childhood foot deformity was frequent, but no sensory signs were observed. Our findings establish VWA1 as a new disease gene confidently implicated in this autosomal recessive neuromyopathic condition presenting with child-/adult-onset muscle weakness as a key clinical feature.

International retrospective natural history study of LMNA-related congenital muscular dystrophy

Muscular dystrophies due to heterozygous pathogenic variants in LMNA gene cover a broad spectrum of clinical presentations and severity with an age of onset ranging from the neonatal period to adulthood. The natural history of these conditions is not well defined, particularly in patients with congenital or early onset who arguably present with the highest disease burden. Thus the definition of natural history endpoints along with clinically revelant outcome measures is essential to establishing both clinical care planning and clinical trial readiness for this patient group. We designed a large international cross-sectional retrospective natural history study of patients with genetically proven muscle laminopathy who presented with symptoms before two years of age intending to identify and characterize an optimal clinical trial cohort with pertinent motor, cardiac and respiratory endpoints. Quantitative statistics were used to evaluate associations between LMNA variants and distinct clinical events. The study included 151 patients (median age at symptom onset 0.9 years, range: 0.0–2.0). Age of onset and age of death were significantly lower in patients who never acquired independent ambulation compared to patients who achieved independent ambulation. Most of the patients acquired independent ambulation (n = 101, 66.9%), and subsequently lost this ability (n = 86; 85%). The age of ambulation acquisition (median: 1.2 years, range: 0.8–4.0) and age of ambulation loss (median: 7 years, range: 1.2–38.0) were significantly associated with the age of the first respiratory interventions and the first cardiac symptoms. Respiratory and gastrointestinal interventions occurred during first decade while cardiac interventions occurred later. Genotype–phenotype analysis showed that the most common mutation, p.Arg249Trp (20%), was significantly associated with a more severe disease course. This retrospective natural history study of early onset LMNA-related muscular dystrophy confirms the progressive nature of the disorder, initially involving motor symptoms prior to onset of other symptoms (respiratory, orthopaedic, cardiac and gastrointestinal). The study also identifies subgroups of patients with a range of long-term outcomes. Ambulatory status was an important mean of stratification along with the presence or absence of the p.Arg249Trp mutation. These categorizations will be important for future clinical trial cohorts. Finally, this study furthers our understanding of the progression of early onset LMNA-related muscular dystrophy and provides important insights into the anticipatory care needs of LMNA-related respiratory and cardiac manifestations.

Ryanodine receptor remodeling in cardiomyopathy and muscular dystrophy caused by lamin A/C gene mutation

Mutations in the lamin A/C gene (LMNA), which encodes A-type lamins, cause several diseases called laminopathies, the most common of which is dilated cardiomyopathy with muscular dystrophy. The role of Ca2+ regulation in these diseases remain poorly understood. We now show biochemical remodeling of the ryanodine receptor (RyR)/intracellular Ca2+ release channel in heart samples from human subjects with LMNA mutations, including protein kinase A-catalyzed phosphorylation, oxidation and depletion of the stabilizing subunit calstabin. In the LmnaH222P/H222P murine model of Emery-Dreifuss muscular dystrophy caused by LMNA mutation, we demonstrate an age-dependent biochemical remodeling of RyR2 in the heart and RyR1 in skeletal muscle. This RyR remodeling is associated with heart and skeletal muscle dysfunction. Defective heart and muscle function are ameliorated by treatment with a novel Rycal small molecule drug (S107) that fixes 'leaky' RyRs. SMAD3 phosphorylation is increased in hearts and diaphragms of LmnaH222P/H222P mice, which enhances NADPH oxidase binding to RyR channels, contributing to their oxidation. There is also increased generalized protein oxidation, increased calcium/calmodulin-dependent protein kinase II-catalyzed phosphorylation of RyRs and increased protein kinase A activity in these tissues. Our data show that RyR remodeling plays a role in cardiomyopathy and skeletal muscle dysfunction caused by LMNA mutation and identify these Ca2+ channels as a potential therapeutic target.

A splicing LMNA mutation causing laminopathies accompanied by aortic valve malformation

BACKGROUND

Laminopathies caused by LMNA gene mutations are characterized by different clinical manifestations. Among them, cardiac involvement is one of the most severe phenotypes.

CASE PRESENTATION

A 30-year-old man visited the hospital because of palpitations, shortness of breath, and fatigue. He also had muscular dystrophy, joint contractures, scoliosis, and mild dysphagia. A novel de novo heterozygous LMNA splice variant (c.810+1G>T) with dilated cardiomyopathy, Emery-Dreifuss muscular dystrophy, and progressive cardiac conduction defect was identified by genetic analysis. The patient also presented with congenital aortic valve malformation, which has never been reported in laminopathies.

CONCLUSIONS

The LMNA mutation (c.810+1G>T) was identified for the first time, enriching the mutation spectrum of the LMNA gene. The correlation between an LMNA mutation and congenital aortic valve malformation deserves further study.

Nuclear Mechanotransduction in Skeletal Muscle

Skeletal muscle is composed of multinucleated, mature muscle cells (myofibers) responsible for contraction, and a resident pool of mononucleated muscle cell precursors (MCPs), that are maintained in a quiescent state in homeostatic conditions. Skeletal muscle is remarkable in its ability to adapt to mechanical constraints, a property referred as muscle plasticity and mediated by both MCPs and myofibers. An emerging body of literature supports the notion that muscle plasticity is critically dependent upon nuclear mechanotransduction, which is transduction of exterior physical forces into the nucleus to generate a biological response. Mechanical loading induces nuclear deformation, changes in the nuclear lamina organization, chromatin condensation state, and cell signaling, which ultimately impacts myogenic cell fate decisions. This review summarizes contemporary insights into the mechanisms underlying nuclear force transmission in MCPs and myofibers. We discuss how the cytoskeleton and nuclear reorganizations during myogenic differentiation may affect force transmission and nuclear mechanotransduction. We also discuss how to apply these findings in the context of muscular disorders. Finally, we highlight current gaps in knowledge and opportunities for further research in the field.

The nuclear envelope protein Net39 is essential for muscle nuclear integrity and chromatin organization

Lamins and transmembrane proteins within the nuclear envelope regulate nuclear structure and chromatin organization. Nuclear envelope transmembrane protein 39 (Net39) is a muscle nuclear envelope protein whose functions in vivo have not been explored. We show that mice lacking Net39 succumb to severe myopathy and juvenile lethality, with concomitant disruption in nuclear integrity, chromatin accessibility, gene expression, and metabolism. These abnormalities resemble those of Emery-Dreifuss muscular dystrophy (EDMD), caused by mutations in A-type lamins (LMNA) and other genes, like Emerin (EMD). We observe that Net39 is downregulated in EDMD patients, implicating Net39 in the pathogenesis of this disorder. Our findings highlight the role of Net39 at the nuclear envelope in maintaining muscle chromatin organization, gene expression and function, and its potential contribution to the molecular etiology of EDMD.

Laminopathies' Treatments Systematic Review: A Contribution Towards a 'Treatabolome'

BACKGROUND

Variants in the LMNA gene, encoding lamins A/C, are responsible for a growing number of diseases, all of which complying with the definition of rare diseases. LMNA-related disorders have a varied phenotypic expression with more than 15 syndromes described, belonging to five phenotypic groups: Muscular Dystrophies, Neuropathies, Cardiomyopathies, Lipodystrophies and Progeroid Syndromes. Overlapping phenotypes are also reported. Linking gene and variants with phenotypic expression, disease mechanisms, and corresponding treatments is particularly challenging in laminopathies. Treatment recommendations are limited, and very few are variant-based.

OBJECTIVE

The Treatabolome initiative aims to provide a shareable dataset of existing variant-specific treatment for rare diseases within the Solve-RD EU project. As part of this project, we gathered evidence of specific treatments for laminopathies via a systematic literature review adopting the FAIR (Findable, Accessible, Interoperable, and Reusable) guidelines for scientific data production.

METHODS

Treatments for LMNA-related conditions were systematically collected from MEDLINE and Embase bibliographic databases and clinical trial registries (Cochrane Central Registry of Controlled Trials, clinicaltrial.gov and EudraCT). Two investigators extracted and analyzed the literature data independently. The included papers were assessed using the Oxford Centre for Evidence-Based Medicine 2011 Levels of Evidence.

RESULTS

From the 4783 selected articles by a systematic approach, we identified 78 papers for our final analysis that corresponded to the profile of data defined in the inclusion and exclusion criteria. These papers include 2 guidelines/consensus papers, 4 meta-analyses, 14 single-arm trials, 15 case series, 13 cohort studies, 21 case reports, 8 expert reviews and 1 expert opinion. The treatments were summarized electronically according to significant phenome-genome associations. The specificity of treatments according to the different laminopathic phenotypical presentations is variable.

CONCLUSIONS

We have extracted Treatabolome-worthy treatment recommendations for patients with different forms of laminopathies based on significant phenome-genome parings. This dataset will be available on the Treatabolome website and, through interoperability, on genetic diagnosis and treatment support tools like the RD-Connect's Genome Phenome Analysis Platform.

Lamin-Related Congenital Muscular Dystrophy Alters Mechanical Signaling and Skeletal Muscle Growth

Laminopathies are a clinically heterogeneous group of disorders caused by mutations in the LMNA gene, which encodes the nuclear envelope proteins lamins A and C. The most frequent diseases associated with LMNA mutations are characterized by skeletal and cardiac involvement, and include autosomal dominant Emery–Dreifuss muscular dystrophy (EDMD), limb-girdle muscular dystrophy type 1B, and LMNA-related congenital muscular dystrophy (LMNA-CMD). Although the exact pathophysiological mechanisms responsible for LMNA-CMD are not yet understood, severe contracture and muscle atrophy suggest that mutations may impair skeletal muscle growth. Using human muscle stem cells (MuSCs) carrying LMNA-CMD mutations, we observe impaired myogenic fusion with disorganized cadherin/β catenin adhesion complexes. We show that skeletal muscle from Lmna-CMD mice is unable to hypertrophy in response to functional overload, due to defective fusion of activated MuSCs, defective protein synthesis and defective remodeling of the neuromuscular junction. Moreover, stretched myotubes and overloaded muscle fibers with LMNA-CMD mutations display aberrant mechanical regulation of the yes-associated protein (YAP). We also observe defects in MuSC activation and YAP signaling in muscle biopsies from LMNA-CMD patients. These phenotypes are not recapitulated in closely related but less severe EDMD models. In conclusion, combining studies in vitro, in vivo, and patient samples, we find that LMNA-CMD mutations interfere with mechanosignaling pathways in skeletal muscle, implicating A-type lamins in the regulation of skeletal muscle growth.

Whole-exome analyses of congenital muscular dystrophy and congenital myopathy patients from India reveal a wide spectrum of known and novel mutations

BACKGROUND AND PURPOSE

Congenital muscular dystrophies (CMDs) and congenital myopathies (CMs) are a group of genetically and clinically heterogeneous degenerative primary muscle disorders with onset at birth or during infancy. Due to vast heterogeneity, clinical examination and protein-based analyses often fail to identify the genetic causes of these diseases. The aim of this study was to genetically diagnose a cohort of 36 difficult-to-diagnose CMD and CM cases of Indian origin using next-generation sequencing methods.

METHODS

Whole-exome sequencing (WES) was performed to identify pathogenic mutations in previously reported CMD and CM-related genes using variant calling and stringent variant filtration process. Subsequently, in silico homology modelling and molecular dynamics simulations (MDS) studies were undertaken for a number of novel and missense variants.

RESULTS

A total of 33 and 21 rare and deleterious mutations were identified in 28 genes previously reported in CMD and CM based on OMIM, ClinVar and Orphanet, respectively. We could accurately diagnose 54% patients (n = 12/22) in the CMD group and 35% patients (n = 5/14) in the CM group. Furthermore, MDS studies for mutations located in LMNA, LAMA2 and RYR1 suggest that the wild-type proteins are more stable than their mutant counterparts, implying a potential mechanism of pathogenesis.

CONCLUSION

The WES findings led us to identify reported as well as novel variants for the first time in Indian patients with CMD and CM. This allowed us to achieve an accurate genetic diagnosis, which was difficult using conventional diagnostic tools. Transferring these WES findings to clinical practice will help guide clinical care of the affected patients and inform genetic counselling.

Clinical and Genomic Evaluation of 207 Genetic Myopathies in the Indian Subcontinent

Objective: Inherited myopathies comprise more than 200 different individually rare disease-subtypes, but when combined together they have a high prevalence of 1 in 6,000 individuals across the world. Our goal was to determine for the first time the clinical- and gene-variant spectrum of genetic myopathies in a substantial cohort study of the Indian subcontinent.

Methods: In this cohort study, we performed the first large clinical exome sequencing (ES) study with phenotype correlation on 207 clinically well-characterized inherited myopathy-suspected patients from the Indian subcontinent with diverse ethnicities.

Results: Clinical-correlation driven definitive molecular diagnosis was established in 49% (101 cases; 95% CI, 42–56%) of patients with the major contributing pathogenicity in either of three genes, GNE (28%; GNE-myopathy), DYSF (25%; Dysferlinopathy), and CAPN3 (19%; Calpainopathy). We identified 65 variant alleles comprising 37 unique variants in these three major genes. Seventy-eight percent of the DYSF patients were homozygous for the detected pathogenic variant, suggesting the need for carrier-testing for autosomal-recessive disorders like Dysferlinopathy that are common in India. We describe the observed clinical spectrum of myopathies including uncommon and rare subtypes in India: Sarcoglycanopathies (SGCA/B/D/G), Collagenopathy (COL6A1/2/3), Anoctaminopathy (ANO5), telethoninopathy (TCAP), Pompe-disease (GAA), Myoadenylate-deaminase-deficiency-myopathy (AMPD1), myotilinopathy (MYOT), laminopathy (LMNA), HSP40-proteinopathy (DNAJB6), Emery-Dreifuss-muscular-dystrophy (EMD), Filaminopathy (FLNC), TRIM32-proteinopathy (TRIM32), POMT1-proteinopathy (POMT1), and Merosin-deficiency-congenital-muscular-dystrophy-type-1 (LAMA2). Thirteen patients harbored pathogenic variants in >1 gene and had unusual clinical features suggesting a possible role of synergistic-heterozygosity/digenic-contribution to disease presentation and progression.

Conclusions: Application of clinically correlated ES to myopathy diagnosis has improved our understanding of the clinical and genetic spectrum of different subtypes and their overlaps in Indian patients. This, in turn, will enhance the global gene-variant-disease databases by including data from developing countries/continents for more efficient clinically driven molecular diagnostics.

Protein Kinase C Alpha Cellular Distribution, Activity, and Proximity with Lamin A/C in Striated Muscle Laminopathies

Striated muscle laminopathies are cardiac and skeletal muscle conditions caused by mutations in the lamin A/C gene (LMNA). LMNA codes for the A-type lamins, which are nuclear intermediate filaments that maintain the nuclear structure and nuclear processes such as gene expression. Protein kinase C alpha (PKC-α) interacts with lamin A/C and with several lamin A/C partners involved in striated muscle laminopathies. To determine PKC-α’s involvement in muscular laminopathies, PKC-α’s localization, activation, and interactions with the A-type lamins were examined in various cell types expressing pathogenic lamin A/C mutations. The results showed aberrant nuclear PKC-α cellular distribution in mutant cells compared to WT. PKC-α activation (phos-PKC-α) was decreased or unchanged in the studied cells expressing LMNA mutations, and the activation of its downstream targets, ERK 1/2, paralleled PKC-α activation alteration. Furthermore, the phos-PKC-α-lamin A/C proximity was altered. Overall, the data showed that PKC-α localization, activation, and proximity with lamin A/C were affected by certain pathogenic LMNA mutations, suggesting PKC-α involvement in striated muscle laminopathies.

Need for NAD+: Focus on Striated Muscle Laminopathies

Laminopathies are a heterogeneous group of rare diseases caused by genetic mutations in the LMNA gene, encoding A-type lamins. A-type lamins are nuclear envelope proteins which associate with B-type lamins to form the nuclear lamina, a meshwork underlying the inner nuclear envelope of differentiated cells. The laminopathies include lipodystrophies, progeroid phenotypes and striated muscle diseases. Research on striated muscle laminopathies in the recent years has provided novel perspectives on the role of the nuclear lamina and has shed light on the pathological consequences of altered nuclear lamina. The role of altered nicotinamide adenine dinucleotide (NAD⁺) in the physiopathology of striated muscle laminopathies has been recently highlighted. Here, we have summarized these findings and reviewed the current knowledge about NAD⁺ alteration in striated muscle laminopathies, providing potential therapeutic approaches.

The clinical, histologic, and genotypic spectrum of SEPN1-related myopathy: A case series

OBJECTIVE

To clarify the prevalence, long-term natural history, and severity determinants of SEPN1-related myopathy (SEPN1-RM), we analyzed a large international case series.

METHODS

Retrospective clinical, histologic, and genetic analysis of 132 pediatric and adult patients (2-58 years) followed up for several decades.

RESULTS

The clinical phenotype was marked by severe axial muscle weakness, spinal rigidity, and scoliosis (86.1%, from 8.9 ± 4 years), with relatively preserved limb strength and previously unreported ophthalmoparesis in severe cases. All patients developed respiratory failure (from 10.1±6 years), 81.7% requiring ventilation while ambulant. Histopathologically, 79 muscle biopsies showed large variability, partly determined by site of biopsy and age. Multi-minicores were the most common lesion (59.5%), often associated with mild dystrophic features and occasionally with eosinophilic inclusions. Identification of 65 SEPN1 mutations, including 32 novel ones and the first pathogenic copy number variation, unveiled exon 1 as the main mutational hotspot and revealed the first genotype-phenotype correlations, bi-allelic null mutations being significantly associated with disease severity (p = 0.017). SEPN1-RM was more severe and progressive than previously thought, leading to loss of ambulation in 10% of cases, systematic functional decline from the end of the third decade, and reduced lifespan even in mild cases. The main prognosis determinants were scoliosis/respiratory management, SEPN1 mutations, and body mass abnormalities, which correlated with disease severity. We propose a set of severity criteria, provide quantitative data for outcome identification, and establish a need for age stratification.

CONCLUSION

Our results inform clinical practice, improving diagnosis and management, and represent a major breakthrough for clinical trial readiness in this not so rare disease.

Clinical spectrum and genetic variations of LMNA-related muscular dystrophies in a large cohort of Chinese patients

Background

LMNA-related muscular dystrophy is caused by mutations in LMNA gene. We aimed to identify genetic variations and clinical features in a large cohort of Chinese patients with LMNA mutations in an attempt to establish genotype-phenotype correlation.

Methods

The clinical presentations of patients with LMNA-related muscular dystrophy were recorded using retrospective and prospective cohort study. LMNA mutation analysis was performed by Sanger sequencing or next-generation sequencing. Mosaicism was detected by personal genome machine amplicon deep sequencing for mosaicism.

Results

Eighty-four patients were identified to harbour LMNA mutations. Forty-one of those were diagnosed with LMNA-related congenital muscular dystrophy (L-CMD), 32 with Emery-Dreifuss muscular dystrophy (EDMD) and 11 with limb-girdle muscular dystrophy type 1B (LGMD1B). We identified 21 novel and 29 known LMNA mutations. Two frequent mutations were identified: c.745C>T and c.1357C>T. A correlation between the location of mutation and the clinical phenotype was observed: mutations affecting the head and coil 2A domains mainly occurred in L-CMD, while the coil 2B and Ig-like domains mainly related to EDMD and LGMD1B. We found somatic mosaicism in one parent of four probands. Muscle biopsies revealed 11 of 20 biopsied L-CMD exhibited inflammatory changes, and muscle cell ultrastructure showed abnormal nuclear morphology.

Conclusions

Our detailed clinical and genetic analysis of 84 patients with LMNA-related muscular dystrophy expands clinical spectrum and broadens genetic variations caused by LMNA mutations. We identified 21 novel and 29 known LMNA mutations and found two frequent mutations. A correlation between the location of mutation and the clinical severity was observed. Preliminary data suggested that low-dose corticosteroid treatment may be effective.

An Omics View of Emery-Dreifuss Muscular Dystrophy

Recent progress in Omics technologies has started to empower personalized healthcare development at a thorough biomolecular level. Omics have subsidized medical breakthroughs that have started to enter clinical proceedings. The use of this scientific know-how has surfaced as a way to provide a more far-reaching view of the biological mechanisms behind diseases. This review will focus on the discoveries made using Omics and the utility of these approaches for Emery–Dreifuss muscular dystrophy.

Mutated lamin A modulates stiffness in muscle cells

The cytoskeleton is a complex network interlinking filaments that extend throughout the cytoplasm from the nucleus to the plasma membrane. Three major types of filaments are found in the cytoskeleton: actin filaments, microtubules, and intermediate filaments. They play a key role in the ability of cells to both resist mechanical stress and generate force. However, the precise involvement of intermediate filament proteins in these processes remains unclear. Here, we focused on nuclear A-type lamins, which are connected to the cytoskeleton via the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex. Using micro-constriction rheology, we investigated the impact of A-type lamins (p.H222P) mutation on the mechanical properties of muscle cells. We demonstrate that the expression of point mutation of lamin A in muscle cells increases cellular stiffness compared with cells expressing wild type lamin A and that the chemical agent selumetinib, an inhibitor of the ERK1/2 signaling, reversed the mechanical alterations in mutated cells. These results highlight the interplay between A-type lamins and mechano-signaling, which are supported by cell biology measurements.

Linking skeletal muscle aging with osteoporosis by lamin A/C deficiency

The nuclear lamina protein lamin A/C is a key component of the nuclear envelope. Mutations in the lamin A/C gene (LMNA) are identified in patients with various types of laminopathy-containing diseases, which have features of accelerated aging and osteoporosis. However, the underlying mechanisms for laminopathy-associated osteoporosis remain largely unclear. Here, we provide evidence that loss of lamin A/C in skeletal muscles, but not osteoblast (OB)-lineage cells, results in not only muscle aging-like deficit but also trabecular bone loss, a feature of osteoporosis. The latter is due in large part to elevated bone resorption. Further cellular studies show an increase of osteoclast (OC) differentiation in cocultures of bone marrow macrophages/monocytes (BMMs) and OBs after treatment with the conditioned medium (CM) from lamin A/C-deficient muscle cells. Antibody array screening analysis of the CM proteins identifies interleukin (IL)-6, whose expression is markedly increased in lamin A/C-deficient muscles. Inhibition of IL-6 by its blocking antibody in BMM-OB cocultures diminishes the increase of osteoclastogenesis. Knockout (KO) of IL-6 in muscle lamin A/C-KO mice diminishes the deficits in trabecular bone mass but not muscle. Further mechanistic studies reveal an elevation of cellular senescence marked by senescence-associated beta-galactosidase (SA-β-gal), p16Ink4a, and p53 in lamin A/C-deficient muscles and C2C12 muscle cells, and the p16Ink4a may induce senescence-associated secretory phenotype (SASP) and IL-6 expression. Taken together, these results suggest a critical role for skeletal muscle lamin A/C to prevent cellular senescence, IL-6 expression, hyperosteoclastogenesis, and trabecular bone loss, uncovering a pathological mechanism underlying the link between muscle aging/senescence and osteoporosis.

Lamin A/C Mechanotransduction in Laminopathies

Mechanotransduction translates forces into biological responses and regulates cell functionalities. It is implicated in several diseases, including laminopathies which are pathologies associated with mutations in lamins and lamin-associated proteins. These pathologies affect muscle, adipose, bone, nerve, and skin cells and range from muscular dystrophies to accelerated aging. Although the exact mechanisms governing laminopathies and gene expression are still not clear, a strong correlation has been found between cell functionality and nuclear behavior. New theories base on the direct effect of external force on the genome, which is indeed sensitive to the force transduced by the nuclear lamina. Nuclear lamina performs two essential functions in mechanotransduction pathway modulating the nuclear stiffness and governing the chromatin remodeling. Indeed, A-type lamin mutation and deregulation has been found to affect the nuclear response, altering several downstream cellular processes such as mitosis, chromatin organization, DNA replication-transcription, and nuclear structural integrity. In this review, we summarize the recent findings on the molecular composition and architecture of the nuclear lamina, its role in healthy cells and disease regulation. We focus on A-type lamins since this protein family is the most involved in mechanotransduction and laminopathies.

Lamin A/C Cardiomyopathy with E203K Pathogenic Mutation

Lamin A/C (LMNA) cardiomyopathy is an adult-onset, autosomal dominant, rapidly progressive cardiomyopathy which belongs to a spectrum of familial idiopathic cardiomyopathies. It is the most common type of familial dilated cardiomyopathy that is associated with conduction defects. A 76-year-old African American female with second-degree atrioventricular (AV) block presented for evaluation of persistent fatigue. Her family history was significant for sudden deaths of her son and brother at the age of 6 and 48 years, respectively, and AV block in her sister with a pacemaker implant at the age of 64 years. Physical examination was within normal limits. Electrocardiogram showed a Mobitz type II, second-degree AV block. Mild dilated cardiomyopathy was present on echocardiogram. Stress echocardiography had to be stopped due to premature ventricular contractions. Cardiac catheterization, coronary angiography, and cardiac MRI revealed no significant etiology for rhythm disturbance. Holter monitoring revealed intermittent bradycardia with a heart rate falling as low as 28 beats per minute, which led to the decision of dual-chamber pacemaker implantation. RhythmNext genetic testing (Ambry Genetics, Aliso Viejo, CA) was done due to the significant family history of sudden death; it revealed a heterozygous E203K pathologic mutation in the LMNA gene. Sudden death is the most common mode of death in LMNA cardiomyopathy; hence, the implantation of intracardiac cardioverter-defibrillator for primary prophylaxis was discussed with the patient. Clinicians should suspect LMNA cardiomyopathy in patients with rhythm disorders and family history of sudden death, which can help to identify individuals at risk and prevent sudden death by appropriate interventions.

Congenital Muscular Dystrophy and Congenital Myopathy

PURPOSE OF REVIEW

Congenital muscular dystrophies and congenital myopathies are a heterogeneous group of disorders resulting in hypotonia, muscle weakness, and dystrophic or myopathic features on muscle biopsy. This article summarizes the clinical and genetic aspects of these disorders.

RECENT FINDINGS

Historically, diagnoses of congenital muscular dystrophy and congenital myopathy have been made by clinical features and histopathology; however, recent advances in genetics have changed diagnostic practice by relying more heavily on genetic findings. This article reviews the clinical and genetic features of the most common congenital muscular dystrophies including laminin subunit alpha 2 (LAMA2)-related (merosin deficient), collagen VI-related, and α-dystroglycan-related congenital muscular dystrophies and reviews the most common congenital myopathies including nemaline rod, core, and centronuclear myopathies. With the increasing accessibility of genetic testing, the number of genes found to be associated with these disorders has increased dramatically. A wide spectrum of severity and onset (from birth to adulthood) exist across all subtypes. Progression and other features are variable depending on the subtype and severity of the specific genetic mutation.

SUMMARY

Congenital muscular dystrophy and congenital myopathy are increasingly recognized disorders. A growing appreciation for the breadth of phenotypic variability and overlap between established subtypes has challenged long-standing phenotypic and histopathologic classifications of these disorders but has driven a greater understanding of pathogenesis and opened the door to the development of novel treatments.

Lamin A/C Assembly Defects in LMNA-Congenital Muscular Dystrophy Is Responsible for the Increased Severity of the Disease Compared with Emery-Dreifuss Muscular Dystrophy

LMNA encodes for Lamin A/C, type V intermediate filaments that polymerize under the inner nuclear membrane to form the nuclear lamina. A small fraction of Lamin A/C, less polymerized, is also found in the nucleoplasm. Lamin A/C functions include roles in nuclear resistance to mechanical stress and gene regulation. LMNA mutations are responsible for a wide variety of pathologies, including Emery–Dreifuss (EDMD) and LMNA-related congenital muscular dystrophies (L-CMD) without clear genotype–phenotype correlations. Both diseases presented with striated muscle disorders although L-CMD symptoms appear much earlier and are more severe. Seeking for pathomechanical differences to explain the severity of L-CMD mutations, we performed an in silico analysis of the UMD-LMNA database and found that L-CMD mutations mainly affect residues involved in Lamin dimer and tetramer stability. In line with this, we found increased nucleoplasmic Lamin A/C in L-CMD patient fibroblasts and mouse myoblasts compared to the control and EDMD. L-CMD myoblasts show differentiation defects linked to their inability to upregulate muscle specific nuclear envelope (NE) proteins expression. NE proteins were mislocalized, leading to misshapen nuclei. We conclude that these defects are due to both the absence of Lamin A/C from the nuclear lamina and its maintenance in the nucleoplasm of myotubes.

Two novel cases further expand the phenotype of TOR1AIP1-associated nuclear envelopathies

Biallelic variants in TOR1AIP1, encoding the integral nuclear membrane protein LAP1 (lamina-associated polypeptide 1) with two functional isoforms LAP1B and LAP1C, have initially been linked to muscular dystrophies with variable cardiac and neurological impairment. Furthermore, a recurrent homozygous nonsense alteration, resulting in loss of both LAP1 isoforms, was identified in seven likely related individuals affected by multisystem anomalies with progeroid-like appearance and lethality within the 1st decade of life. Here, we have identified compound heterozygosity in TOR1AIP1 affecting both LAP1 isoforms in two unrelated individuals affected by congenital bilateral hearing loss, ventricular septal defect, bilateral cataracts, mild to moderate developmental delay, microcephaly, mandibular hypoplasia, short stature, progressive muscular atrophy, joint contractures and severe chronic heart failure, with much longer survival. Cellular characterization of primary fibroblasts of one affected individual revealed absence of both LAP1B and LAP1C, constitutively low lamin A/C levels, aberrant nuclear morphology including nuclear cytoplasmic channels, and premature senescence, comparable to findings in other progeroid forms of nuclear envelopathies. We additionally observed an abnormal activation of the extracellular signal-regulated kinase 1/2 (ERK 1/2). Ectopic expression of wild-type TOR1AIP1 mitigated these cellular phenotypes, providing further evidence for the causal role of identified genetic variants. Altogether, we thus further expand the TOR1AIP1-associated phenotype by identifying individuals with biallelic loss-of-function variants who survived beyond the 1st decade of life and reveal novel molecular consequences underlying the TOR1AIP1-associated disorders.

Emery-Dreifuss muscular dystrophy

Emery-Dreifuss muscular dystrophy (EDMD) is a rare muscular dystrophy, but is particularly important to diagnose due to frequent life-threatening cardiac complications. EDMD classically presents with muscle weakness, early contractures, cardiac conduction abnormalities and cardiomyopathy, although the presence and severity of these manifestations vary by subtype and individual. Associated genes include EMD, LMNA, SYNE1, SYNE2, FHL1, TMEM43, SUN1, SUN2, and TTN, encoding emerin, lamin A/C, nesprin-1, nesprin-2, FHL1, LUMA, SUN1, SUN2, and titin, respectively. The Online Mendelian Inheritance in Man database recognizes subtypes 1 through 7, which captures most but not all of the associated genes. Genetic diagnosis is essential whenever available, but traditional diagnostic tools can help steer the evaluation toward EDMD and assist with interpretation of equivocal genetic test results. Management is primarily supportive, but it is important to monitor patients closely, especially for potential cardiac complications. There is a high potential for progress in the treatment of EDMD in the coming years.

Importance of early diagnosis in LMNA-related muscular dystrophy for cardiac surveillance

INTRODUCTION

The identification of LMNA-related muscular dystrophy is important because it poses life-threatening cardiac complications. However, diagnosis of LMNA-related muscular dystrophy based on clinical features is challenging.

METHODS

We reviewed the clinical phenotypes of 14 children with LMNA variants, focusing on the cardiac function and genotypes.

RESULTS

Most patients presented with motor developmental delay or gait abnormalities. Eight (57%) patients had prominent neck extensor weakness or contractures. All patients showed ankle contractures at an early stage. Regular cardiac surveillance allowed for the detection of dysrhythmias in 57% of patients at a mean age of 14 years (range, 5-26). All patients had missense variants; however, there were no clear phenotype-genotype correlations.

DISCUSSION

Early diagnosis of LMNA-related muscular dystrophy provides an opportunity for cardiac surveillance, potentially leading to the prevention of cardiac mortality in children.