Temporal requirement of dystroglycan glycosylation during brain development and rescue of severe cortical dysplasia via gene delivery in the fetal stage

Molecular mechanisms and therapeutic strategies for neuromuscular diseases

Neuromuscular diseases encompass a heterogeneous array of disorders characterized by varying onset ages, clinical presentations, severity, and progression. While these conditions can stem from acquired or inherited causes, this review specifically focuses on disorders arising from genetic abnormalities, excluding metabolic conditions. The pathogenic defect may primarily affect the anterior horn cells, the axonal or myelin component of peripheral nerves, the neuromuscular junction, or skeletal and/or cardiac muscles. While inherited neuromuscular disorders have been historically deemed not treatable, the advent of gene-based and molecular therapies is reshaping the treatment landscape for this group of condition. With the caveat that many products still fail to translate the positive results obtained in pre-clinical models to humans, both the technological development (e.g., implementation of tissue-specific vectors) as well as advances on the knowledge of pathogenetic mechanisms form a collective foundation for potentially curative approaches to these debilitating conditions. This review delineates the current panorama of therapies targeting the most prevalent forms of inherited neuromuscular diseases, emphasizing approved treatments and those already undergoing human testing, offering insights into the state-of-the-art interventions.

Protein O-mannosylation: one sugar, several pathways, many functions

Recent research has unveiled numerous important functions of protein glycosylation in development, homeostasis, and diseases. A type of glycosylation taking the center stage is protein O-mannosylation, a posttranslational modification conserved in a wide range of organisms, from yeast to humans. In animals, protein O-mannosylation plays a crucial role in the nervous system, whereas protein O-mannosylation defects cause severe neurological abnormalities and congenital muscular dystrophies. However, the molecular and cellular mechanisms underlying protein O-mannosylation functions and biosynthesis remain not well understood. This review outlines recent studies on protein O-mannosylation while focusing on the functions in the nervous system, summarizes the current knowledge about protein O-mannosylation biosynthesis, and discusses the pathologies associated with protein O-mannosylation defects. The evolutionary perspective revealed by studies in the Drosophila model system are also highlighted. Finally, the review touches upon important knowledge gaps in the field and discusses critical questions for future research on the molecular and cellular mechanisms associated with protein O-mannosylation functions.

Germline C1GALT1C1 mutation causes a multisystem chaperonopathy

Mutations in genes encoding molecular chaperones can lead to chaperonopathies, but none have so far been identified causing congenital disorders of glycosylation. Here we identified two maternal half-brothers with a novel chaperonopathy, causing impaired protein O-glycosylation. The patients have a decreased activity of T-synthase (C1GALT1), an enzyme that exclusively synthesizes the T-antigen, a ubiquitous O-glycan core structure and precursor for all extended O-glycans. The T-synthase function is dependent on its specific molecular chaperone Cosmc, which is encoded by X-chromosomal C1GALT1C1. Both patients carry the hemizygous variant c.59C>A (p.Ala20Asp; A20D-Cosmc) in C1GALT1C1. They exhibit developmental delay, immunodeficiency, short stature, thrombocytopenia, and acute kidney injury (AKI) resembling atypical hemolytic uremic syndrome. Their heterozygous mother and maternal grandmother show an attenuated phenotype with skewed X-inactivation in blood. AKI in the male patients proved fully responsive to treatment with the complement inhibitor Eculizumab. This germline variant occurs within the transmembrane domain of Cosmc, resulting in dramatically reduced expression of the Cosmc protein. Although A20D-Cosmc is functional, its decreased expression, though in a cell or tissue-specific manner, causes a large reduction of T-synthase protein and activity, which accordingly leads to expression of varied amounts of pathological Tn-antigen (GalNAcα1-O-Ser/Thr/Tyr) on multiple glycoproteins. Transient transfection of patient lymphoblastoid cells with wild-type C1GALT1C1 partially rescued the T-synthase and glycosylation defect. Interestingly, all four affected individuals have high levels of galactose-deficient IgA1 in sera. These results demonstrate that the A20D-Cosmc mutation defines a novel O-glycan chaperonopathy and causes the altered O-glycosylation status in these patients.

Compound variants of FKTN, POMGNT1, and LAMB1 gene identified by prenatal whole-exome sequencing in three fetuses with congenital hydrocephalus

Congenital hydrocephalus (CH) is a severe birth defect, and genetics components is an important etiology. Whole-exome sequencing (WES) has been proven to be a feasible approach for prenatal diagnosis of CH. In this study, we carried out WES on three fetuses with cerebral ventriculomegaly. After bioinformation analysis and data filtering, three compound variants, c.919C>T(p.Arg307Ter)/c.1100del(p.Phe369fs) in FKTN, c.1449_1450insACAACG/c.1490G>C(p.Arg497Pro) in POMGNT1, and c.2690+1G>A/c.1447C>T(p.Arg483Cys) in LAMB1 were detected in the three fetuses. All the six variants were classified as likely pathogenic or pathogenic in accordance with the American College of Medical Genetics and Genomics-Association for Molecular Pathology guidelines. This study provides support for the potential of WES for the accurate prenatal diagnosis of fetal hydrocephalus and further demonstrated the genetic heterogeneity in patients with CH. The novel variants (c.1449_1450insACAACG and c.1490G>C in POMGNT1, c.2690+1G>A in LAMB1) expanded the gene mutational spectrum of CH and contributes to genetics counseling and pregnancy management.

CDP-ribitol prodrug treatment ameliorates ISPD-deficient muscular dystrophy mouse model

Ribitol-phosphate modification is crucial for the functional maturation of α-dystroglycan. Its dysfunction is associated with muscular dystrophy, cardiomyopathy, and central nervous system abnormalities; however, no effective treatments are currently available for diseases caused by ribitol-phosphate defects. In this study, we demonstrate that prodrug treatments can ameliorate muscular dystrophy caused by defects in isoprenoid synthase domain containing (ISPD), which encodes an enzyme that synthesizes CDP-ribitol, a donor substrate for ribitol-phosphate modification. We generated skeletal muscle-selective Ispd conditional knockout mice, leading to a pathogenic reduction in CDP-ribitol levels, abnormal glycosylation of α-dystroglycan, and severe muscular dystrophy. Adeno-associated virus-mediated gene replacement experiments suggested that the recovery of CDP-ribitol levels rescues the ISPD-deficient pathology. As a prodrug treatment strategy, we developed a series of membrane-permeable CDP-ribitol derivatives, among which tetraacetylated CDP-ribitol ameliorated the dystrophic pathology. In addition, the prodrug successfully rescued abnormal α-dystroglycan glycosylation in patient fibroblasts. Consequently, our findings provide proof-of-concept for supplementation therapy with CDP-ribitol and could accelerate the development of therapeutic agents for muscular dystrophy and other diseases caused by glycosylation defects.

Targeting cell surface glycans with lectin-coated fluorescent nanodiamonds

Glycosylation is arguably the most important functional post-translational modification in brain cells and abnormal cell surface glycan expression has been associated with neurological diseases and brain cancers. In this study we developed a novel method for uptake of fluorescent nanodiamonds (FND), carbon-based nanoparticles with low toxicity and easily modifiable surfaces, into brain cell subtypes by targeting their glycan receptors with carbohydrate-binding lectins. Lectins facilitated uptake of 120 nm FND with nitrogen-vacancy centers in three types of brain cells - U87-MG astrocytes, PC12 neurons and BV-2 microglia cells. The nanodiamond/lectin complexes used in this study target glycans that have been described to be altered in brain diseases including sialic acid glycans via wheat (Triticum aestivum) germ agglutinin (WGA), high mannose glycans via tomato (Lycopersicon esculentum) lectin (TL) and core fucosylated glycans via Aleuria aurantia lectin (AAL). The lectin conjugated nanodiamonds were taken up differently by the various brain cell types with fucose binding AAL/FNDs taken up preferentially by glioblastoma phenotype astrocyte cells (U87-MG), sialic acid binding WGA/FNDs by neuronal phenotype cells (PC12) and high mannose binding TL/FNDs by microglial cells (BV-2). With increasing recognition of glycans having a role in many diseases, the lectin bioconjugated nanodiamonds developed here are well suited for further investigation into theranostic applications.

Dystroglycanopathy: From Elucidation of Molecular and Pathological Mechanisms to Development of Treatment Methods

Dystroglycanopathy is a collective term referring to muscular dystrophies with abnormal glycosylation of dystroglycan. At least 18 causative genes of dystroglycanopathy have been identified, and its clinical symptoms are diverse, ranging from severe congenital to adult-onset limb-girdle types. Moreover, some cases are associated with symptoms involving the central nervous system. In the 2010s, the structure of sugar chains involved in the onset of dystroglycanopathy and the functions of its causative gene products began to be identified as if they were filling the missing pieces of a jigsaw puzzle. In parallel with these discoveries, various dystroglycanopathy model mice had been created, which led to the elucidation of its pathological mechanisms. Then, treatment strategies based on the molecular basis of glycosylation began to be proposed after the latter half of the 2010s. This review briefly explains the sugar chain structure of dystroglycan and the functions of the causative gene products of dystroglycanopathy, followed by introducing the pathological mechanisms involved as revealed from analyses of dystroglycanopathy model mice. Finally, potential therapeutic approaches based on the pathological mechanisms involved are discussed.

Restoration of the defect in radial glial fiber migration and cortical plate organization in a brain organoid model of Fukuyama muscular dystrophy

Fukuyama congenital muscular dystrophy (FCMD) is a severe, intractable genetic disease that affects the skeletal muscle, eyes, and brain and is attributed to a defect in alpha dystroglycan (αDG) O-mannosyl glycosylation. We previously established disease models of FCMD; however, they did not fully recapitulate the phenotypes observed in human patients. In this study, we generated induced pluripotent stem cells (iPSCs) from a human FCMD patient and differentiated these cells into three-dimensional brain organoids and skeletal muscle. The brain organoids successfully mimicked patient phenotypes not reliably reproduced by existing models, including decreased αDG glycosylation and abnormal radial glial (RG) fiber migration. The basic polycyclic compound Mannan-007 (Mn007) restored αDG glycosylation in the brain and muscle models tested and partially rescued the abnormal RG fiber migration observed in cortical organoids. Therefore, our study underscores the importance of αDG O-mannosyl glycans for normal RG fiber architecture and proper neuronal migration in corticogenesis.

Congenital muscular dystrophies: What is new?

Congenital muscular dystrophies (CMDs) are a group of inherited conditions defined by muscle weakness occurring before the acquisition of ambulation, delayed motor milestones, and characterised by muscle dystrophic pathology. A large number of genes - at least 35- are responsible for CMD phenotypes, and it is therefore not surprising that CMDs comprise a wide spectrum of phenotypes, with variable involvement of cardiac/respiratory muscles, central nervous system, and ocular structures. The identification of several new genes over the past few years has further expanded both the clinical and the molecular spectrum underlying CMDs. Comprehensive gene panels allow to arrive at a final diagnosis in around 60% of cases, suggesting that both new genes, and unusual mutations of the currently known genes are likely to account for the remaining cases. The aim of this review is to present the most recent advances in this field. We will outline recent natural history studies that provide additional information on disease progression, discuss recently discovered genes and the current status of the most promising therapeutic options.

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.

Identification of a novel missense c.386G > A variant in a boy with the POMGNT1-related muscular dystrophy-dystroglycanopathy

Muscular dystrophy-dystroglycanopathies are autosomal recessive neurologic disorders, caused by homozygous or compound heterozygous mutations in the POMGNT1 gene-encoding protein O-mannose beta-1,2-N-acetylglucosaminyl transferase. This type of muscular dystrophy is characterized by early-onset muscle weakness, gait ataxia, microcephaly, and developmental delay.We performed whole-exome sequencing to detect the disease-causing variants in a 4 year-old boy. Afterwards, Sanger sequencing was performed to confirm the detected variant in the patient and his family. We evaluated a 4 year-old Iranian boy presented with delayed speech and language development, gait ataxia, global developmental delay, motor delay, neurodevelopmental delay, postnatal microcephaly and strabismus. His parents were first cousins, and the mother had a history of spontaneous abortion. In this study, we report a novel missense c.386G > A; p.(Arg129Gln) variant in the POMGNT1 gene which was confirmed by Sanger sequencing in the patient and segregated with the disease in the family.

Human embryoid bodies as a 3D tissue model of the extracellular matrix and α-dystroglycanopathies

The basal lamina is a specialized sheet of dense extracellular matrix (ECM) linked to the plasma membrane of specific cell types in their tissue context, which serves as a structural scaffold for organ genesis and maintenance. Disruption of the basal lamina and its functions is central to many disease processes, including cancer metastasis, kidney disease, eye disease, muscular dystrophies and specific types of brain malformation. The latter three pathologies occur in the α-dystroglycanopathies, which are caused by dysfunction of the ECM receptor α-dystroglycan. However, opportunities to study the basal lamina in various human disease tissues are restricted owing to its limited accessibility. Here, we report the generation of embryoid bodies from human induced pluripotent stem cells that model the basal lamina. Embryoid bodies cultured via this protocol mimic pre-gastrulation embryonic development, consisting of an epithelial core surrounded by a basal lamina and a peripheral layer of ECM-secreting endoderm. In α-dystroglycanopathy patient embryoid bodies, electron and fluorescence microscopy reveal ultrastructural basal lamina defects and reduced ECM accumulation. By starting from patient-derived cells, these results establish a method for the in vitro synthesis of patient-specific basal lamina and recapitulate disease-relevant ECM defects seen in the α-dystroglycanopathies. Finally, we apply this system to evaluate an experimental ribitol supplement therapy on genetically diverse α-dystroglycanopathy patient samples.This article has an associated First Person interview with the first author of the paper.

Elimination of fukutin reveals cellular and molecular pathomechanisms in muscular dystrophy-associated heart failure

Heart failure is the major cause of death for muscular dystrophy patients, however, the molecular pathomechanism remains unknown. Here, we show the detailed molecular pathogenesis of muscular dystrophy-associated cardiomyopathy in mice lacking the fukutin gene (Fktn), the causative gene for Fukuyama muscular dystrophy. Although cardiac Fktn elimination markedly reduced α-dystroglycan glycosylation and dystrophin-glycoprotein complex proteins in sarcolemma at all developmental stages, cardiac dysfunction was observed only in later adulthood, suggesting that membrane fragility is not the sole etiology of cardiac dysfunction. During young adulthood, Fktn-deficient mice were vulnerable to pathological hypertrophic stress with downregulation of Akt and the MEF2-histone deacetylase axis. Acute Fktn elimination caused severe cardiac dysfunction and accelerated mortality with myocyte contractile dysfunction and disordered Golgi-microtubule networks, which were ameliorated with colchicine treatment. These data reveal fukutin is crucial for maintaining myocyte physiology to prevent heart failure, and thus, the results may lead to strategies for therapeutic intervention.

Mutations in Ftkn cause Fukuyama muscular dystrophy, and heart failure is the main cause of death in thes patients. Here the authors show that acute elimination of Fktn in adult mice causes early mortality, and this is associated with myocyte dysfunction, with disorganised Golg-microtubule networks, and that the pathology can be ameliorated with colchicine treatment.

Prenatal whole exome sequencing detects a new homozygous fukutin (FKTN) mutation in a fetus with an ultrasound suspicion of familial Dandy-Walker malformation

BACKGROUND

Posterior fossa malformations are among the most diagnosed central nervous system (CNS) anomalies detected by ultrasound (US) in prenatal age. We identified the pathogenic gene mutation in a male fetus of 17 weeks of gestation with US suspicion of familial Dandy-Walker spectrum malformation, using Next Generation Sequencing approach in prenatal diagnosis.

METHODS

Whole exome sequencing (WES) approach has been performed on fetal genomic DNA. After reads preprocessing, mapping, variant calling, and annotation, a filtering strategy based on allelic frequency, recessive inheritance, and phenotypic ontologies has been applied. A fetal magnetic resonance imaging (MRI) at 18 weeks of gestation has been performed. An in silico analysis of a potential causative missense variant in the fukutin protein has been carried out through a structural modeling approach.

RESULTS

We identified a new homozygous missense mutation in fukutin gene (FKTN, NM_006731.2: c.898G>A; NP_006722.2: p.Gly300Arg). Fetal MRI supported molecular findings. Structural modeling analyses indicated a potential pathogenetic mechanism of the variant, through a reduced activation of the sugar moieties, which in turn impairs transfer to dystroglycan and thus its glycosylation. These findings pointed to a redefinition of the US suspicion of recurrence of Dandy-Walker malformation (DWM) to a muscular dystrophy-dystroglycanopathy type A4.

CONCLUSIONS

The present case confirmed WES as a reliable tool for the prenatal identification of the molecular bases of early-detected CNS malformations.

Myo-Glyco disease Biology: Genetic Myopathies Caused by Abnormal Glycan Synthesis and Degradation

Glycosylation is a major form of post-translational modification and plays various important roles in organisms by modifying proteins or lipids, which generates functional variability and can increase their stability. Because of the physiological importance of glycosylation, defects in genes encoding proteins involved in glycosylation or glycan degradation are sometimes associated with human diseases. A number of genetic neuromuscular diseases are caused by abnormal glycan modification or degeneration. Heterogeneous and complex modification machinery, and difficulties in structural and functional analysis of glycans have impeded the understanding of how glycosylation contributes to pathology. However, recent rapid advances in glycan and genetic analyses, as well as accumulating genetic and clinical information have greatly contributed to identifying glycan structures and modification enzymes, which has led to breakthroughs in the understanding of the molecular pathogenesis of various diseases and the possible development of therapeutic strategies. For example, studies on the relationship between glycosylation and muscular dystrophy in the last two decades have significantly impacted the fields of glycobiology and neuromyology. In this review, the basis of glycan structure and biosynthesis will be briefly explained, and then molecular pathogenesis and therapeutic concepts related to neuromuscular diseases will be introduced from the point of view of the life cycle of a glycan molecule.

The roles of dystroglycan in the nervous system: insights from animal models of muscular dystrophy

Dystroglycan is a cell membrane protein that binds to the extracellular matrix in a variety of mammalian tissues. The α-subunit of dystroglycan (αDG) is heavily glycosylated, including a special O-mannosyl glycoepitope, relying upon this unique glycosylation to bind its matrix ligands. A distinct group of muscular dystrophies results from specific hypoglycosylation of αDG, and they are frequently associated with central nervous system involvement, ranging from profound brain malformation to intellectual disability without evident morphological defects. There is an expanding literature addressing the function of αDG in the nervous system, with recent reports demonstrating important roles in brain development and in the maintenance of neuronal synapses. Much of these data are derived from an increasingly rich array of experimental animal models. This Review aims to synthesize the information from such diverse models, formulating an up-to-date understanding about the various functions of αDG in neurons and glia of the central and peripheral nervous systems. Where possible, we integrate these data with our knowledge of the human disorders to promote translation from basic mechanistic findings to clinical therapies that take the neural phenotypes into account.

Summary: Dystroglycan is a ubiquitous matrix receptor linked to brain and muscle disease. Unraveling the functions of this protein will inform basic and translational research on neural development and muscular dystrophies.