Distal muscular dystrophies

Computational insights into dynamics and conformational stability of N-acetylmannosamine kinase mutations

The activity of UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE) is essential for the biosynthesis of sialic acid, which is involved in cellular processes in health and diseases. GNE contains an N-terminal epimerase domain and a C-terminal kinase domain (N-acetylmannosamine kinase, MNK). Mutations of the GNE protein led to hypoactivity of the enzyme and cause sialurea or autosomal recessive inclusion body myopathy/Nonaka myopathy. Here, we used all-atom molecular dynamics (MD) simulations to comprehend the folding, dynamics and conformational stability of MNK variants, including the wild type (WT) and three mutants (H677R, V696M and H677R/V696M). The deleterious and destabilizing nature of MNK mutants were predicted using different prediction tools. Results predicted that mutations modulate the stability, flexibility and function of MNK. The effect of mutations on the conformational stability and dynamics of MNK was next studied through the free-energy landscape (FEL), hydrogen-bonds and secondary structure changes. The FEL results show that the mutations interfere with various conformational transitions in both WT and mutants, exposing the structural underpinnings of protein destabilization and unfolding brought on by mutation. We discover that, when compared to the other two mutations, V696M and H677R/V696M, H677R has the most harmful effects. These findings have a strong correlation with published experimental studies that demonstrate how these mutations disrupt MNK activity. Hence, this computational study describes the structural details to unravel the mutant effects at the atomistic resolution and has implications for understanding the GNE's physiological and pathological role.Communicated by Ramaswamy H. Sarma.

Porcine Circovirus type 2 infected myocardial tissue transcriptome signature

The goal of this study was to compare the global gene expression profile in cardiac tissues of pig infected with porcine circovirus 2 (PCV2) to that of healthy cells. Since PCV2 infection causes severe cardiovascular lesions, the myocardial tissue model was chosen for this study. In High-throughput transcriptome analysis, DESeq2 and CLC genomics workbench analyses revealed a total of 196 significantly differentially expressed genes (DEGs) (p-value < 0.05). Furthermore, 194 transcripts were upregulated, while only two were downregulated (HSPA6 and DNAJA1), with fold changes ranging from 16.293 to -10.002. Among the KEGG canonical pathways targeted by the DEGs in the functional analysis, adrenergic signalling in cardiomyocytes, Cardiac Muscle Contraction, Hypertrophic Cardiomyopathy (HCM), and Dilated Cardiomyopathy (DCM) tends to be enriched. The differentially expressed highly connected (DEHC) biomarker genes in pathogenicity of PCV2 infection, such as LDB3, MYOZ2, CASQ2, TNNT2, MLC2V, MYBPC3, ACTC1, TCAP, TNNI3, TRDN, CSRP3, MYL3, RYR2, LMOD2, MYH7, etc., were identified using protein-protein interaction (PPI) network analysis. The study might provide detailed information on the dysregulated genes and biological pathways in infected myocardial tissues that may be essential for PCV2-related heart pathology.

Diagnosis of Cardiac Abnormalities in Muscular Dystrophies

Muscular disorders are mainly characterized by progressive skeletal muscle weakness. There are several aspects that can be monitored, which are used to differentiate between the types of muscular disorders, ranging from the targeted muscle up to the mutated gene. An aspect that holds critical importance when managing muscular dystrophies is that most of them exhibit cardiac abnormalities. Therefore, cardiac imaging is an essential part of muscular disorder monitoring and management. In the first section of the review, several cardiac abnormalities are introduced; afterward, different muscular dystrophies' pathogenesis is presented. Not all muscular dystrophies necessarily present cardiac involvement; however, the ones that do are linked with the cardiac abnormalities described in the first section. Moreover, studies from the last 3 years on muscular disorders are presented alongside imaging techniques used to determine cardiac abnormalities.

Role of RNA Binding Proteins with prion-like domains in muscle and neuromuscular diseases

A number of neuromuscular and muscular diseases, including amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA) and several myopathies, are associated to mutations in related RNA-binding proteins (RBPs), including TDP-43, FUS, MATR3 or hnRNPA1/B2. These proteins harbor similar modular primary sequence with RNA binding motifs and low complexity domains, that enables them to phase separate and create liquid microdomains. These RBPs have been shown to critically regulate multiple events of RNA lifecycle, including transcriptional events, splicing and RNA trafficking and sequestration. Here, we review the roles of these disease-related RBPs in muscle and motor neurons, and how their dysfunction in these cell types might contribute to disease.

Muscular dystrophies

Muscular dystrophies are primary diseases of muscle due to mutations in more than 40 genes, which result in dystrophic changes on muscle biopsy. Now that most of the genes responsible for these conditions have been identified, it is possible to accurately diagnose them and implement subtype-specific anticipatory care, as complications such as cardiac and respiratory muscle involvement vary greatly. This development and advances in the field of supportive medicine have changed the standard of care, with an overall improvement in the clinical course, survival, and quality of life of affected individuals. The improved understanding of the pathogenesis of these diseases is being used for the development of novel therapies. In the most common form, Duchenne muscular dystrophy, a few personalised therapies have recently achieved conditional approval and many more are at advanced stages of clinical development. In this Seminar, we concentrate on clinical manifestations, molecular pathogenesis, diagnostic strategy, and therapeutic developments for this group of conditions.

GNE-Myopathy in a Greek Romani Family with Unusual Calf Phenotype and Protein Aggregation Pathology

BACKGROUND

GNE-myopathy is increasingly diagnosed in different ethnicities worldwide. No clear genotype-phenotype correlation has been established to date.

CASE REPORTS

We describe two affected members of the same family from Balkan population carrying an already known homozygous pathogenic mutation in the kinase domain of the UDP-N-acetylglucosamine 2 epimerase/N-acetylmannosamime kinase (GNE) gene. The patients presented with severe distal weakness of lower legs combined with rimmed vacuoles in muscle biopsy. However, in contrast to the typical pattern of muscle involvement, one of them showed severe involvement of posterior calf muscles with spared anterior compartment of the lower leg muscles.

CONCLUSIONS

These patients provide evidence for a larger variability and further extend the phenotypic spectrum of GNE-myopathy to include preferential calf involvement.

Impairment of respiratory function in late-onset distal myopathy due to MATR3 Mutation

INTRODUCTION

Recently, mutations in the MATR3 gene were found to cause late-onset distal myopathy. The frequency and impact of respiratory involvement are not clear.

METHODS

Respiratory parameters [maximum vital capacity (VCmax); forced expiratory volume (FEV1 ); peak expiratory flow (PEF), postural drop of VCmax from sitting to supine, maximum inspiratory muscle pressure (PImax), mouth occlusion pressure after 100 ms (P 0.1), peak cough flow, and blood-gas analysis] were monitored prospectively at baseline, and then 6 months and 12 months later in 8 patients with genetically confirmed MATR3 myopathy.

RESULTS

All patients showed involvement of respiratory function. Six of 8 reported exertional dyspnea. At the end of follow-up, 5 of 8 had decreased VC, 7 of 8 had reduced PImax, and 5 of 7 had decreased partial pressure of oxygen (PO2 ). Within 12 months, respiratory parameters deteriorated non-significantly. No patient required non-invasive ventilation.

CONCLUSIONS

There is a high risk of abnormal respiratory function with progressive worsening in MATR3 myopathy.

Two recurrent mutations are associated with GNE myopathy in the North of Britain

OBJECTIVE

GNE myopathy is a rare recessive myopathy associated with inclusion bodies on muscle biopsy. The clinical phenotype is associated with distal muscle weakness with quadriceps sparing. Most of the current information on GNE myopathy has been obtained through studies of Jewish and Japanese patient cohorts carrying founder mutations in the GNE gene. However, little is known about GNE myopathy in Europe where the prevalence is thought to be very low.

METHODS

Patients were referred through the National Specialist Commissioning Team service for limb-girdle muscular dystrophies at Newcastle (UK). All patients harbouring mutations in the GNE gene were recruited for our study. Detailed clinical and genetic data as well as muscle MRIs and muscle biopsies were reviewed.

RESULTS

We identified 26 patients harbouring mutations in the GNE gene. Two previously reported mutations (c.1985C>T, p.Ala662Val and c.1225G>T, p.Asp409Tyr) were prevalent in the Scottish, Northern Irish and Northern English populations; with 90% of these patients carrying at least one of the two mutations. Clinically, we confirmed the homogenous pattern of selective quadriceps sparing but noted additional features like asymmetry of weakness at disease onset.

CONCLUSIONS

GNE myopathy is an important diagnosis to consider in patients presenting with distal leg muscle weakness. We report, for the first time, two common mutations in the north of Britain and highlight the broader spectrum of clinical phenotypes. We also propose that the prevalence of GNE myopathy may be underestimated due to the frequent absence of rimmed vacuoles in the muscle biopsy.

Muscular dystrophies

Muscular dystrophies are a heterogeneous group of inherited disorders that share similar clinical features and dystrophic changes on muscle biopsy. An improved understanding of their molecular bases has led to more accurate definitions of the clinical features associated with known subtypes. Knowledge of disease-specific complications, implementation of anticipatory care, and medical advances have changed the standard of care, with an overall improvement in the clinical course, survival, and quality of life of affected people. A better understanding of the mechanisms underlying the molecular pathogenesis of several disorders and the availability of preclinical models are leading to several new experimental approaches, some of which are already in clinical trials. In this Seminar, we provide a comprehensive review that integrates clinical manifestations, molecular pathogenesis, diagnostic strategy, and therapeutic developments.

Filamin C-related myopathies: pathology and mechanisms

The term filaminopathy was introduced after a truncating mutation in the dimerization domain of filamin C (FLNc) was shown to be responsible for a devastating muscle disease. Subsequently, the same mutation was found in patients from diverse ethnical origins, indicating that this specific alteration is a mutational hot spot. Patients initially present with proximal muscle weakness, while distal and respiratory muscles become affected with disease progression. Muscle biopsies of these patients show typical signs of myofibrillar myopathy, including disintegration of myofibrils and aggregation of several proteins into distinct intracellular deposits. Highly similar phenotypes were observed in patients with other mutations in Ig-like domains of FLNc that result in expression of a noxious protein. Biochemical and biophysical studies showed that the mutated domains acquire an abnormal structure causing decreased stability and eventually becoming a seed for abnormal aggregation with other proteins. The disease usually presents only after the fourth decade of life possibly as a result of ageing-related impairments in the machinery that is responsible for disposal of damaged proteins. This is confirmed by mutations in components of this machinery that cause a highly similar phenotype. Transfection studies of cultured muscle cells reflect the events observed in patient muscles and, therefore, may provide a helpful model for testing future dedicated therapeutic strategies. More recently, FLNC mutations were also found in families with a distal myopathy phenotype, caused either by mutations in the actin-binding domain of FLNc that result in increased actin-binding and non-specific myopathic abnormalities without myofibrillar myopathy pathology, or a nonsense mutation in the rod domain that leads to RNA instability, haploinsufficiency with decreased expression levels of FLNc in the muscle fibers and myofibrillar abnormalities, but not to the formation of desmin-positive protein aggregates required for the diagnosis of myofibrillar myopathy.