A molecular overview of the primary dystroglycanopathies

The missense mutation C667F in murine β-dystroglycan causes embryonic lethality, myopathy and blood-brain barrier destabilization

Dystroglycan (DG) is an extracellular matrix receptor consisting of an α- and a β-DG subunit encoded by the DAG1 gene. The homozygous mutation (c.2006G>T, p.Cys669Phe) in β-DG causes muscle-eye-brain disease with multicystic leukodystrophy in humans. In a mouse model of this primary dystroglycanopathy, approximately two-thirds of homozygous embryos fail to develop to term. Mutant mice that are born undergo a normal postnatal development but show a late-onset myopathy with partially penetrant histopathological changes and an impaired performance on an activity wheel. Their brains and eyes are structurally normal, but the localization of mutant β-DG is altered in the glial perivascular end-feet, resulting in a perturbed protein composition of the blood-brain and blood-retina barrier. In addition, α- and β-DG protein levels are significantly reduced in muscle and brain of mutant mice. Owing to the partially penetrant developmental phenotype of the C669F β-DG mice, they represent a novel and highly valuable mouse model with which to study the molecular effects of β-DG functional alterations both during embryogenesis and in mature muscle, brain and eye, and to gain insight into the pathogenesis of primary dystroglycanopathies.

Analysis of the GFP-labelled β-dystroglycan interactome in HEK-293 transfected cells reveals novel intracellular networks

Dystroglycan (DG) is a cell adhesion complex that is widely expressed in tissues. It is composed by two subunits, α-DG, a highly glycosylated protein that interacts with several extracellular matrix proteins, and transmembrane β-DG whose, cytodomain binds to the actin cytoskeleton. Glycosylation of α-DG is crucial for functioning as a receptor for its multiple extracellular binding partners. Perturbation of α-DG glycosylation is the central event in the pathogenesis of severe pathologies such as muscular dystrophy and cancer. β-DG acts as a scaffold for several cytoskeletal and nuclear proteins and very little is known about the fine regulation of some of these intracellular interactions and how they are perturbed in diseases. To start filling this gap by identifying uncharacterized intracellular networks preferentially associated with β-DG, HEK-293 cells were transiently transfected with a plasmid carrying the β-DG subunit with GFP fused at its C-terminus. With this strategy, we aimed at forcing β-DG to occupy multiple intracellular locations instead of sitting tightly at its canonical plasma membrane milieu, where it is commonly found in association with α-DG. Immunoprecipitation by anti-GFP antibodies followed by shotgun proteomic analysis led to the identification of an interactome formed by 313 exclusive protein matches for β-DG binding. A series of already known β-DG interactors have been found, including ezrin and emerin, whilst significant new matches, which include potential novel β-DG interactors and their related networks, were identified in diverse subcellular compartments, such as cytoskeleton, endoplasmic reticulum/Golgi, mitochondria, nuclear membrane and the nucleus itself. Of particular interest amongst the novel identified matches, Lamina-Associated Polypeptide-1B (LAP1B), an inner nuclear membrane protein, whose mutations are known to cause nuclear envelopathies characterized by muscular dystrophy, was found to interact with β-DG in HEK-293 cells. This evidence was confirmed by immunoprecipitation, Western blotting and immunofluorescence experiments. We also found by immunofluorescence experiments that LAP1B looses its nuclear envelope localization in C2C12 DG-knock-out cells, suggesting that LAP1B requires β-DG for a proper nuclear localization. These results expand the role of β-DG as a nuclear scaffolding protein and provide novel evidence of a possible link between dystroglycanopathies and nuclear envelopathies displaying with muscular dystrophy.

Removal of pomt1 in zebrafish leads to loss of α-dystroglycan glycosylation and dystroglycanopathy phenotypes

Biallelic mutations in Protein O-mannosyltransferase 1 (POMT1) are among the most common causes of a severe group of congenital muscular dystrophies (CMDs) known as dystroglycanopathies. POMT1 is a glycosyltransferase responsible for the attachment of a functional glycan mediating interactions between the transmembrane glycoprotein dystroglycan and its binding partners in the extracellular matrix (ECM). Disruptions in these cell-ECM interactions lead to multiple developmental defects causing brain and eye malformations in addition to CMD. Removing Pomt1 in the mouse leads to early embryonic death due to the essential role of dystroglycan during placental formation in rodents. Here, we characterized and validated a model of pomt1 loss of function in the zebrafish showing that developmental defects found in individuals affected by dystroglycanopathies can be recapitulated in the fish. We also discovered that pomt1 mRNA provided by the mother in the oocyte supports dystroglycan glycosylation during the first few weeks of development. Muscle disease, retinal synapse formation deficits, and axon guidance defects can only be uncovered during the first week post fertilization by generating knock-out embryos from knock-out mothers. Conversely, maternal pomt1 from heterozygous mothers was sufficient to sustain muscle, eye, and brain development only leading to loss of photoreceptor synapses at 30 days post fertilization. Our findings show that it is important to define the contribution of maternal mRNA while developing zebrafish models of dystroglycanopathies and that offspring generated from heterozygous and knock-out mothers can be used to differentiate the role of dystroglycan glycosylation in tissue formation and maintenance.

Pharmacotherapeutic Approaches to Treatment of Muscular Dystrophies

Muscular dystrophies are a heterogeneous group of genetic muscle-wasting disorders that are subdivided based on the region of the body impacted by muscle weakness as well as the functional activity of the underlying genetic mutations. A common feature of the pathophysiology of muscular dystrophies is chronic inflammation associated with the replacement of muscle mass with fibrotic scarring. With the progression of these disorders, many patients suffer cardiomyopathies with fibrosis of the cardiac tissue. Anti-inflammatory glucocorticoids represent the standard of care for Duchenne muscular dystrophy, the most common muscular dystrophy worldwide; however, long-term exposure to glucocorticoids results in highly adverse side effects, limiting their use. Thus, it is important to develop new pharmacotherapeutic approaches to limit inflammation and fibrosis to reduce muscle damage and promote repair. Here, we examine the pathophysiology, genetic background, and emerging therapeutic strategies for muscular dystrophies.

Fetal Presentation of Walker-Warburg Syndrome with Compound Heterozygous POMT2 Missense Mutations

Background: Walker-Warburg syndrome (WWS) (OMIM #236670) is an autosomal recessive disorder characterized by congenital muscular dystrophy, hydrocephalus, cobblestone lissencephaly, and retinal dysplasia. The main genes involved are: POMT1, POMT2, POMGNT1, FKTN, LARGE1, and FKRP. Case report: We present a fetus with WWS showing at ultrasound severe triventricular hydrocephalus. Pregnancy was legally terminated at 21 weeks +2 days of gestation. In vivo and postmortem magnetic resonance revealed corpus callosum agenesis and cerebellar hypoplasia. Cobblestone lissencephaly was observed at post-mortem. Next generation sequencing (NGS) of 193 genes, performed on fetal DNA extracted from amniocytes, detected two heterozygous mutations in the POMT2 gene. The c.1238G > C p.(Arg413Pro) mutation was paternally inherited and is known to be pathogenic. The c.553G > A p.(Gly185Arg) mutation was maternally inherited and has not been previously described. Conclusion: Compound heterozygous mutations in the POMT2 gene caused a severe cerebral fetal phenotype diagnosed prenatally at midgestation allowing therapeutic pregnancy termination.

A combinatorial code of neurexin-3 alternative splicing controls inhibitory synapses via a trans-synaptic dystroglycan signaling loop

Disrupted synaptic inhibition is implicated in neuropsychiatric disorders, yet the molecular mechanisms that shape and sustain inhibitory synapses are poorly understood. Here, we show through rescue experiments performed using Neurexin-3 conditional knockout mice that alternative splicing at SS2 and SS4 regulates the release probability, but not the number, of inhibitory synapses in the olfactory bulb and prefrontal cortex independent of sex. Neurexin-3 splice variants that mediate Neurexin-3 binding to dystroglycan enable inhibitory synapse function, whereas splice variants that don't allow dystroglycan binding do not. Furthermore, a minimal Neurexin-3 protein that binds to dystroglycan fully sustains inhibitory synaptic function, indicating that trans-synaptic dystroglycan binding is necessary and sufficient for Neurexin-3 function in inhibitory synaptic transmission. Thus, Neurexin-3 enables a normal release probability at inhibitory synapses via a trans-synaptic feedback signaling loop consisting of presynaptic Neurexin-3 and postsynaptic dystroglycan.

The many roles of dystroglycan in nervous system development and function: Dystroglycan and neural circuit development: Dystroglycan and neural circuit development

The glycoprotein dystroglycan was first identified in muscle, where it functions as part of the dystrophin glycoprotein complex to connect the extracellular matrix to the actin cytoskeleton. Mutations in genes involved in the glycosylation of dystroglycan cause a form of congenital muscular dystrophy termed dystroglycanopathy. In addition to its well-defined role in regulating muscle integrity, dystroglycan is essential for proper central and peripheral nervous system development. Patients with dystroglycanopathy can present with a wide range of neurological perturbations, but unraveling the complex role of Dag1 in the nervous system has proven to be a challenge. Over the past two decades, animal models of dystroglycanopathy have been an invaluable resource that has allowed researchers to elucidate dystroglycan's many roles in neural circuit development. In this review, we summarize the pathways involved in dystroglycan's glycosylation and its known interacting proteins, and discuss how it regulates neuronal migration, axon guidance, synapse formation, and its role in non-neuronal cells.

The role of the dystrophin glycoprotein complex in muscle cell mechanotransduction

Dystrophin is the central protein of the dystrophin-glycoprotein complex (DGC) in skeletal and heart muscle cells. Dystrophin connects the actin cytoskeleton to the extracellular matrix (ECM). Severing the link between the ECM and the intracellular cytoskeleton has a devastating impact on the homeostasis of skeletal muscle cells, leading to a range of muscular dystrophies. In addition, the loss of a functional DGC leads to progressive dilated cardiomyopathy and premature death. Dystrophin functions as a molecular spring and the DGC plays a critical role in maintaining the integrity of the sarcolemma. Additionally, evidence is accumulating, linking the DGC to mechanosignalling, albeit this role is still less understood. This review article aims at providing an up-to-date perspective on the DGC and its role in mechanotransduction. We first discuss the intricate relationship between muscle cell mechanics and function, before examining the recent research for a role of the dystrophin glycoprotein complex in mechanotransduction and maintaining the biomechanical integrity of muscle cells. Finally, we review the current literature to map out how DGC signalling intersects with mechanical signalling pathways to highlight potential future points of intervention, especially with a focus on cardiomyopathies.

A review of the function of the Dystrophic Glycoprotein Complex (DGC) in mechanosignaling provides an overview of the various components of DGC and potential mechanopathogenic mechanisms, particularly as they relate to muscular dystrophy.

Involvement of muscle satellite cell dysfunction in neuromuscular disorders: Expanding the portfolio of satellite cell-opathies

Neuromuscular disorders are a heterogeneous group of acquired or hereditary conditions that affect striated muscle function. The resulting decrease in muscle strength and motility irreversibly impacts quality of life. In addition to directly affecting skeletal muscle, pathogenesis can also arise from dysfunctional crosstalk between nerves and muscles, and may include cardiac impairment. Muscular weakness is often progressive and paralleled by continuous decline in the ability of skeletal muscle to functionally adapt and regenerate. Normally, the skeletal muscle resident stem cells, named satellite cells, ensure tissue homeostasis by providing myoblasts for growth, maintenance, repair and regeneration. We recently defined 'Satellite Cell-opathies' as those inherited neuromuscular conditions presenting satellite cell dysfunction in muscular dystrophies and myopathies (doi:10.1016/j.yexcr.2021.112906). Here, we expand the portfolio of Satellite Cell-opathies by evaluating the potential impairment of satellite cell function across all 16 categories of neuromuscular disorders, including those with mainly neurogenic and cardiac involvement. We explore the expression dynamics of myopathogenes, genes whose mutation leads to skeletal muscle pathogenesis, using transcriptomic analysis. This revealed that 45% of myopathogenes are differentially expressed during early satellite cell activation (0 - 5 hours). Of these 271 myopathogenes, 83 respond to Pax7, a master regulator of satellite cells. Our analysis suggests possible perturbation of satellite cell function in many neuromuscular disorders across all categories, including those where skeletal muscle pathology is not predominant. This characterisation further aids understanding of pathomechanisms and informs on development of prognostic and diagnostic tools, and ultimately, new therapeutics.

Defining and identifying satellite cell-opathies within muscular dystrophies and myopathies

Muscular dystrophies and congenital myopathies arise from specific genetic mutations causing skeletal muscle weakness that reduces quality of life. Muscle health relies on resident muscle stem cells called satellite cells, which enable life-course muscle growth, maintenance, repair and regeneration. Such tuned plasticity gradually diminishes in muscle diseases, suggesting compromised satellite cell function. A central issue however, is whether the pathogenic mutation perturbs satellite cell function directly and/or indirectly via an increasingly hostile microenvironment as disease progresses. Here, we explore the effects on satellite cell function of pathogenic mutations in genes (myopathogenes) that associate with muscle disorders, to evaluate clinical and muscle pathological hallmarks that define dysfunctional satellite cells. We deploy transcriptomic analysis and comparison between muscular dystrophies and myopathies to determine the contribution of satellite cell dysfunction using literature, expression dynamics of myopathogenes and their response to the satellite cell regulator PAX7. Our multimodal approach extends current pathological classifications to define Satellite Cell-opathies: muscle disorders in which satellite cell dysfunction contributes to pathology. Primary Satellite Cell-opathies are conditions where mutations in a myopathogene directly affect satellite cell function, such as in Progressive Congenital Myopathy with Scoliosis (MYOSCO) and Carey-Fineman-Ziter Syndrome (CFZS). Primary satellite cell-opathies are generally characterised as being congenital with general hypotonia, and specific involvement of respiratory, trunk and facial muscles, although serum CK levels are usually within the normal range. Secondary Satellite Cell-opathies have mutations in myopathogenes that affect both satellite cells and muscle fibres. Such classification aids diagnosis and predicting probable disease course, as well as informing on treatment and therapeutic development.

Determination of Agrin and Related Proteins Levels as a Function of Age in Human Hearts

Background

Mature cardiomyocytes are unable to proliferate, preventing the injured adult heart from repairing itself. Studies in rodents have suggested that the extracellular matrix protein agrin promotes cardiomyocyte proliferation in the developing heart and that agrin expression is downregulated shortly after birth, resulting in the cessation of proliferation. Agrin based therapies have proven successful at inducing repair in animal models of cardiac injury, however whether similar pathways exist in the human heart is unknown.

Methods

Right ventricular (RV) biopsies were collected from 40 patients undergoing surgery for congenital heart disease and the expression of agrin and associated proteins was investigated.

Results

Agrin transcripts were found in all samples and their levels were significantly negatively correlated to age (p = 0.026), as were laminin transcripts (p = 0.023), whereas no such correlation was found for the other proteins analyzed. No significant correlations for any of the proteins were found when grouping patients by their gender or pathology. Immunohistochemistry and western blots to detect and localize agrin and the other proteins under analysis in RV tissue, confirmed their presence in patients of all ages.

Conclusions

We show that agrin is progressively downregulated with age in human RV tissue but not as dramatically as has been demonstrated in mice; highlighting both similarities and differences to findings in rodents. Our results lay the groundwork for future studies exploring the potential of agrin-based therapies in the repair of damaged human hearts.

Urine-Derived Stem Cells Express 571 Neuromuscular Disorders Causing Genes, Making Them a Potential in vitro Model for Rare Genetic Diseases

Background: Neuromuscular disorders (NMDs) are a heterogeneous group of genetic diseases, caused by mutations in genes involved in spinal cord, peripheral nerve, neuromuscular junction, and muscle functions. To advance the knowledge of the pathological mechanisms underlying NMDs and to eventually identify new potential drugs paving the way for personalized medicine, limitations regarding the availability of neuromuscular disease-related biological samples, rarely accessible from patients, are a major challenge. Aim: We characterized urinary stem cells (USCs) by in-depth transcriptome and protein profiling to evaluate whether this easily accessible source of patient-derived cells is suitable to study neuromuscular genetic diseases, focusing especially on those currently involved in clinical trials. Methods: The global transcriptomics of either native or MyoD transformed USCs obtained from control individuals was performed by RNA-seq. The expression of 610 genes belonging to 16 groups of disorders (http://www.musclegenetable.fr/) whose mutations cause neuromuscular diseases, was investigated on the RNA-seq output. In addition, protein expression of 11 genes related to NMDs including COL6A, EMD, LMNA, SMN, UBA1, DYNC1H1, SOD1, C9orf72, DYSF, DAG1, and HTT was analyzed in native USCs by immunofluorescence and/or Western blot (WB). Results: RNA-seq profile of control USCs shows that 571 out of 610 genes known to be involved in NMDs, are expressed in USCs. Interestingly, the expression levels of the majority of NMD genes remain unmodified following USCs MyoD transformation. Most genes involved in the pathogenesis of all 16 groups of NMDs are well represented except for channelopathies and malignant hyperthermia related genes. All tested proteins showed high expression values, suggesting consistency between transcription and protein representation in USCs. Conclusion: Our data suggest that USCs are human cells, obtainable by non-invasive means, which might be used as a patient-specific cell model to study neuromuscular disease-causing genes and that they can be likely adopted for a variety of in vitro functional studies such as mutation characterization, pathway identification, and drug screening.

High degree of conservation of the enzymes synthesizing the laminin-binding glycoepitope of α-dystroglycan

The dystroglycan (DG) complex plays a pivotal role for the stabilization of muscles in Metazoa. It is formed by two subunits, extracellular α-DG and transmembrane β-DG, originating from a unique precursor via a complex post-translational maturation process. The α-DG subunit is extensively glycosylated in sequential steps by several specific enzymes and employs such glycan scaffold to tightly bind basement membrane molecules. Mutations of several of these enzymes cause an alteration of the carbohydrate structure of α-DG, resulting in severe neuromuscular disorders collectively named dystroglycanopathies. Given the fundamental role played by DG in muscle stability, it is biochemically and clinically relevant to investigate these post-translational modifying enzymes from an evolutionary perspective. A first phylogenetic history of the thirteen enzymes involved in the fabrication of the so-called 'M3 core' laminin-binding epitope has been traced by an overall sequence comparison approach, and interesting details on the primordial enzyme set have emerged, as well as substantial conservation in Metazoa. The optimization along with the evolution of a well-conserved enzymatic set responsible for the glycosylation of α-DG indicate the importance of the glycosylation shell in modulating the connection between sarcolemma and surrounding basement membranes to increase skeletal muscle stability, and eventually support movement and locomotion.

Essential roles of the dystrophin-glycoprotein complex in different cardiac pathologies

The dystrophin-glycoprotein complex (DGC), situated at the sarcolemma dynamically remodels during cardiac disease. This review examines DGC remodeling as a common denominator in diseases affecting heart function and health. Dystrophin and the DGC serve as broad cytoskeletal integrators that are critical for maintaining stability of muscle membranes. The presence of pathogenic variants in genes encoding proteins of the DGC can cause absence of the protein and/or alterations in other complex members leading to muscular dystrophies. Targeted studies have allowed the individual functions of affected proteins to be defined. The DGC has demonstrated its dynamic function, remodeling under a number of conditions that stress the heart. Beyond genetic causes, pathogenic processes also impinge on the DGC, causing alterations in the abundance of dystrophin and associated proteins during cardiac insult such as ischemia-reperfusion injury, mechanical unloading, and myocarditis. When considering new therapeutic strategies, it is important to assess DGC remodeling as a common factor in various heart diseases. The DGC connects the internal F-actin-based cytoskeleton to laminin-211 of the extracellular space, playing an important role in the transmission of mechanical force to the extracellular matrix. The essential functions of dystrophin and the DGC have been long recognized. DGC based therapeutic approaches have been primarily focused on muscular dystrophies, however it may be a beneficial target in a number of disorders that affect the heart. This review provides an account of what we now know, and discusses how this knowledge can benefit persistent health conditions in the clinic.

Exocyst-mediated membrane trafficking of the lissencephaly-associated ECM receptor dystroglycan is required for proper brain compartmentalization

To assemble a brain, differentiating neurons must make proper connections and establish specialized brain compartments. Abnormal levels of cell adhesion molecules disrupt these processes. Dystroglycan (Dg) is a major non-integrin cell adhesion receptor, deregulation of which is associated with dramatic neuroanatomical defects such as lissencephaly type II or cobblestone brain. The previously established Drosophila model for cobblestone lissencephaly was used to understand how Dg is regulated in the brain. During development, Dg has a spatiotemporally dynamic expression pattern, fine-tuning of which is crucial for accurate brain assembly. In addition, mass spectrometry analyses identified numerous components associated with Dg in neurons, including several proteins of the exocyst complex. Data show that exocyst-based membrane trafficking of Dg allows its distinct expression pattern, essential for proper brain morphogenesis. Further studies of the Dg neuronal interactome will allow identification of new factors involved in the development of dystroglycanopathies and advance disease diagnostics in humans.

Characteristic Cochlear Hypoplasia in Patients with Walker-Warburg Syndrome: A Radiologic Study of the Inner Ear in α-Dystroglycan-Related Muscular Disorders

BACKGROUND AND PURPOSE

Walker-Warburg syndrome, muscle-eye-brain disease, and Fukuyama congenital muscular dystrophy are α-dystroglycan-related muscular disorders associated with brain malformations and eye abnormalities in which no structural inner ear abnormality has been described radiologically. We collected patients from 6 tertiary pediatric hospitals and reported the radiologic features and frequency of inner ear dysplasias.

MATERIALS AND METHODS

Patients previously diagnosed clinicoradiologically with Walker-Warburg syndrome, muscle-eye-brain disease, or Fukuyama congenital muscular dystrophy were included. We recorded the pathogenic variant, when available. Brain MR imaging and/or CT findings were reviewed in consensus, and inner ear anomalies were classified according to previous description in the literature. We then correlated the clinicoradiologic phenotype with the inner ear phenotype.

RESULTS

Thirteen patients fulfilled the criteria for the Walker-Warburg syndrome phenotype, 8 for muscle-eye-brain disease, and 3 for Fukuyama congenital muscular dystrophy. A dysplastic cochlea was demonstrated in 17/24. The most frequent finding was a pronounced cochlear hypoplasia type 4 with a very small anteriorly offset turn beyond the normal-appearing basal turn (12/13 patients with Walker-Warburg syndrome and 1/11 with muscle-eye-brain disease or Fukuyama congenital muscular dystophy). Two of 8 patients with muscle-eye-brain disease, 1/3 with Fukuyama congenital muscular dystrophy, and 1/13 with Walker-Warburg syndrome showed a less severe cochlear hypoplasia type 4. The remaining patients without Walker-Warburg syndrome were healthy. The vestibule and lateral semicircular canals of all patients were normal. Cranial nerve VIII was present in all patients with diagnostic MR imaging.

CONCLUSIONS

Most patients with the severe α-dystroglycanopathy Walker-Warburg syndrome phenotype have a highly characteristic cochlear hypoplasia type 4. Patients with the milder variants, muscle-eye-brain disease and Fukuyama congenital muscular dystrophy, more frequently have a normal cochlea or milder forms of hypoplasia.

Congenital Disorders of Glycosylation: What Clinicians Need to Know?

Congenital disorders of glycosylation (CDG) are a group of clinically heterogeneous disorders characterized by defects in the synthesis of glycans and their attachment to proteins and lipids. This manuscript aims to provide a classification of the clinical presentation, diagnostic methods, and treatment of CDG based on the literature review and our own experience (referral center in Poland). A diagnostic algorithm for CDG was also proposed. Isoelectric focusing (IEF) of serum transferrin (Tf) is still the method of choice for diagnosing N-glycosylation disorders associated with sialic acid deficiency. Nowadays, high-performance liquid chromatography, capillary zone electrophoresis, and mass spectrometry techniques are used, although they are not routinely available. Since next-generation sequencing became more widely available, an improvement in diagnostics has been observed, with more patients and novel CDG subtypes being reported. Early and accurate diagnosis of CDG is crucial for timely implementation of appropriate therapies and improving clinical outcomes. However, causative treatment is available only for few CDG types.

Genetic cause of heterogeneous inherited myopathies in a cohort of Greek patients

Inherited muscle disorders are caused by pathogenic changes in numerous genes. Herein, we aimed to investigate the etiology of muscle disease in 24 consecutive Greek patients with myopathy suspected to be genetic in origin, based on clinical presentation and laboratory and electrophysiological findings and absence of known acquired causes of myopathy. Of these, 16 patients (8 females, median 24 years-old, range 7 to 67 years-old) were diagnosed by Whole Exome Sequencing as suffering from a specific type of inherited muscle disorder. Specifically, we have identified causative variants in 6 limb-girdle muscular dystrophy genes (6 patients; ANO5, CAPN3, DYSF, ISPD, LAMA2, SGCA), 3 metabolic myopathy genes (4 patients; CPT2, ETFDH, GAA), 1 congenital myotonia gene (1 patient; CLCN1), 1 mitochondrial myopathy gene (1 patient; MT-TE) and 3 other myopathy-associated genes (4 patients; CAV3, LMNA, MYOT). In 6 additional family members affected by myopathy, we reached genetic diagnosis following identification of a causative variant in an index patient. In our patients, genetic diagnosis ended a lengthy diagnostic process and, in the case of Multiple acyl-CoA dehydrogenase deficiency and Pompe's disease, it enabled specific treatment to be initiated. These results further expand the genotypic and phenotypic spectrum of inherited myopathies.

Dystrophic muscle distribution in late-stage muscular dystrophy

There is scant information about the comprehensive distribution of dystrophic muscles in muscular dystrophy. Despite different clinical presentations of muscular dystrophy, a recent multi-center study concluded that phenotypic distribution of dystrophic muscles is independent of clinical phenotype and suggested that there is a common pattern of involved muscles. To evaluate this possibility, the present case report used cadaveric dissection to determine the whole-body distribution of fat-infiltrated, dystrophic muscles from a 72-year-old white male cadaver with adult-onset, late-stage muscular dystrophy. Severely dystrophic muscles occupied the pectoral, gluteal and pelvic regions, as well as the arm, thigh and posterior leg. In contrast, muscles of the head, neck, hands and feet largely appeared unaffected. Histopathology and a CT-scan supported these observations. This pattern of dystrophic muscles generally conformed with that described in the multi-center study, and provides prognostic insight for patients and the physicians treating them.

Agrin-Mediated Cardiac Regeneration: Some Open Questions

After cardiac injury, the mammalian adult heart has a very limited capacity to regenerate, due to the inability of fully differentiated cardiomyocytes (CMs) to efficiently proliferate. This has been directly linked to the extracellular matrix (ECM) surrounding and connecting cardiomyocytes, as its increasing rigidity during heart maturation has a crucial impact over the proliferative capacity of CMs. Very recent studies using mouse models have demonstrated how the ECM protein agrin might promote heart regeneration through CMs de-differentiation and proliferation. In maturing CMs, this proteoglycan would act as an inducer of a specific molecular pathway involving ECM receptor(s) within the transmembrane dystrophin-glycoprotein complex (DGC) as well as intracellular Yap, an effector of the Hippo pathway involved in the replication/regeneration program of CMs. According to the mechanism proposed, during mice heart development agrin gets progressively downregulated and ultimately replaced by other ECM proteins eventually leading to loss of proliferation/ regenerative capacity in mature CMs. Although the role played by the agrin-DGC-YAP axis during human heart development remains still largely to be defined, this scenario opens up fascinating and promising therapeutic avenues. Herein, we discuss the currently available relevant information on this system, with a view to explore how the fundamental understanding of the regenerative potential of this cellular program can be translated into therapeutic treatment of injured human hearts.

Translational Regulation of Pmt1 and Pmt2 by Bfr1 Affects Unfolded Protein O-Mannosylation

O-mannosylation is implicated in protein quality control in Saccharomyces cerevisiae due to the attachment of mannose to serine and threonine residues of un- or misfolded proteins in the endoplasmic reticulum (ER). This process also designated as unfolded protein O-mannosylation (UPOM) that ends futile folding cycles and saves cellular resources is mainly mediated by protein O-mannosyltransferases Pmt1 and Pmt2. Here we describe a genetic screen for factors that influence O-mannosylation in yeast, using slow-folding green fluorescent protein (GFP) as a reporter. Our screening identifies the RNA binding protein brefeldin A resistance factor 1 (Bfr1) that has not been linked to O-mannosylation and ER protein quality control before. We find that Bfr1 affects O-mannosylation through changes in Pmt1 and Pmt2 protein abundance but has no effect on PMT1 and PMT2 transcript levels, mRNA localization to the ER membrane or protein stability. Ribosome profiling reveals that Bfr1 is a crucial factor for Pmt1 and Pmt2 translation thereby affecting unfolded protein O-mannosylation. Our results uncover a new level of regulation of protein quality control in the secretory pathway.

Metabolomics Analysis of Skeletal Muscles from FKRP-Deficient Mice Indicates Improvement After Gene Replacement Therapy

Muscular dystrophy-dystroglycanopathies comprise a heterogeneous and complex group of disorders caused by loss-of-function mutations in a multitude of genes that disrupt the glycobiology of α-dystroglycan, thereby affecting its ability to function as a receptor for extracellular matrix proteins. Of the various genes involved, FKRP codes for a protein that plays a critical role in the maturation of a novel glycan found only on α-dystroglycan. Yet despite knowing the genetic cause of FKRP-related dystroglycanopathies, the molecular pathogenesis of disease and metabolic response to therapeutic intervention has not been fully elucidated. To address these challenges, we utilized mass spectrometry-based metabolomics to generate comprehensive metabolite profiles of skeletal muscle across diseased, treated, and normal states. Notably, FKRP-deficient mice elicit diverse metabolic abnormalities in biomarkers of extracellular matrix remodeling and/or aging, pentoses/pentitols, glycolytic intermediates, and lipid metabolism. More importantly, the restoration of FKRP protein activity following AAV-mediated gene therapy induced a substantial correction of these metabolic impairments. While interconnections of the affected molecular mechanisms remain unclear, our datasets support the notion that global metabolic profiling can be valuable for determining the involvement of previously unsuspected regulatory or pathological pathways as well as identifying potential targets for drug discovery and diagnostics.

A molecular overview of the primary dystroglycanopathies

Dystroglycan is a major non-integrin adhesion complex that connects the cytoskeleton to the surrounding basement membranes, thus providing stability to skeletal muscle. In Vertebrates, hypoglycosylation of α-dystroglycan has been strongly linked to muscular dystrophy phenotypes, some of which also show variable degrees of cognitive impairments, collectively termed dystroglycanopathies. Only a small number of mutations in the dystroglycan gene, leading to the so called primary dystroglycanopathies, has been described so far, as opposed to the ever-growing number of identified secondary or tertiary dystroglycanopathies (caused by genetic abnormalities in glycosyltransferases or in enzymes involved in the synthesis of the carbohydrate building blocks). The few mutations found within the autonomous N-terminal domain of α-dystroglycan seem to destabilise it to different degrees, without influencing the overall folding and targeting of the dystroglycan complex. On the contrary other mutations, some located at the α/β interface of the dystroglycan complex, seem to be able to interfere with its maturation, thus compromising its stability and eventually leading to the intracellular engulfment and/or partial or even total degradation of the dystroglycan uncleaved precursor.