Congenital muscular dystrophies with defective glycosylation of dystroglycan: a population study

Inhibitory CCK+ basket synapse defects in mouse models of dystroglycanopathy

Dystroglycan (Dag1) is a transmembrane glycoprotein that links the extracellular matrix to the actin cytoskeleton. Mutations in Dag1 or the genes required for its glycosylation result in dystroglycanopathy, a type of congenital muscular dystrophy characterized by a wide range of phenotypes including muscle weakness, brain defects, and cognitive impairment. We investigated interneuron (IN) development, synaptic function, and associated seizure susceptibility in multiple mouse models that reflect the wide phenotypic range of dystroglycanopathy neuropathology. Mice that model severe dystroglycanopathy due to forebrain deletion of Dag1 or Pomt2, which is required for Dystroglycan glycosylation, show significant impairment of CCK+/CB1R+ IN development. CCK+/CB1R+ IN axons failed to properly target the somatodendritic compartment of pyramidal neurons in the hippocampus, resulting in synaptic defects and increased seizure susceptibility. Mice lacking the intracellular domain of Dystroglycan have milder defects in CCK+/CB1R+ IN axon targeting, but exhibit dramatic changes in inhibitory synaptic function, indicating a critical postsynaptic role of this domain. In contrast, CCK+/CB1R+ IN synaptic function and seizure susceptibility was normal in mice that model mild dystroglycanopathy due to partially reduced Dystroglycan glycosylation. Collectively, these data show that inhibitory synaptic defects and elevated seizure susceptibility are hallmarks of severe dystroglycanopathy, and show that Dystroglycan plays an important role in organizing functional inhibitory synapse assembly.

Misfolding of fukutin-related protein (FKRP) variants in congenital and limb girdle muscular dystrophies

Fukutin-related protein (FKRP, MIM ID 606596) variants cause a range of muscular dystrophies associated with hypo-glycosylation of the matrix receptor, α-dystroglycan. These disorders are almost exclusively caused by homozygous or compound heterozygous missense variants in the FKRP gene that encodes a ribitol phosphotransferase. To understand how seemingly diverse FKRP missense mutations may contribute to disease, we examined the synthesis, intracellular dynamics, and structural consequences of a panel of missense mutations that encompass the disease spectrum. Under non-reducing electrophoresis conditions, wild type FKRP appears to be monomeric whereas disease-causing FKRP mutants migrate as high molecular weight, disulfide-bonded aggregates. These results were recapitulated using cysteine-scanning mutagenesis suggesting that abnormal disulfide bonding may perturb FKRP folding. Using fluorescence recovery after photobleaching, we found that the intracellular mobility of most FKRP mutants in ATP-depleted cells is dramatically reduced but can, in most cases, be rescued with reducing agents. Mass spectrometry showed that wild type and mutant FKRP differentially associate with several endoplasmic reticulum (ER)-resident chaperones. Finally, structural modelling revealed that disease-associated FKRP missense variants affected the local environment of the protein in small but significant ways. These data demonstrate that protein misfolding contributes to the molecular pathophysiology of FKRP-deficient muscular dystrophies and suggest that molecules that rescue this folding defect could be used to treat these disorders.

Exome Sequencing and Optical Genome Mapping in Molecularly Unsolved Cases of Duchenne Muscular Dystrophy: Identification of a Causative X-Chromosomal Inversion Disrupting the DMD Gene

Duchenne muscular dystrophy (DMD) is a severe progressive muscle disease that mainly affects boys due to X-linked recessive inheritance. In most affected individuals, MLPA or sequencing-based techniques detect deletions, duplications, or point mutations in the dystrophin-encoding DMD gene. However, in a small subset of patients clinically diagnosed with DMD, the molecular cause is not identified with these routine methods. Evaluation of the 60 DMD patients in our center revealed three cases without a known genetic cause. DNA samples of these patients were analyzed using whole-exome sequencing (WES) and, if unconclusive, optical genome mapping (OGM). WES led to a diagnosis in two cases: one patient was found to carry a splice mutation in the DMD gene that had not been identified during previous Sanger sequencing. In the second patient, we detected two variants in the fukutin gene (FKTN) that were presumed to be disease-causing. In the third patient, WES was unremarkable, but OGM identified an inversion disrupting the DMD gene (~1.28 Mb) that was subsequently confirmed with long-read sequencing. These results highlight the importance of reanalyzing unsolved cases using WES and demonstrate that OGM is a useful method for identifying large structural variants in cases with unremarkable exome sequencing.

An adhesion signaling axis involving Dystroglycan, β1-Integrin, and Cas adaptor proteins regulates the establishment of the cortical glial scaffold

The mature mammalian cortex is composed of 6 architecturally and functionally distinct layers. Two key steps in the assembly of this layered structure are the initial establishment of the glial scaffold and the subsequent migration of postmitotic neurons to their final position. These processes involve the precise and timely regulation of adhesion and detachment of neural cells from their substrates. Although much is known about the roles of adhesive substrates during neuronal migration and the formation of the glial scaffold, less is understood about how these signals are interpreted and integrated within these neural cells. Here, we provide in vivo evidence that Cas proteins, a family of cytoplasmic adaptors, serve a functional and redundant role during cortical lamination. Cas triple conditional knock-out (Cas TcKO) mice display severe cortical phenotypes that feature cobblestone malformations. Molecular epistasis and genetic experiments suggest that Cas proteins act downstream of transmembrane Dystroglycan and β1-Integrin in a radial glial cell-autonomous manner. Overall, these data establish a new and essential role for Cas adaptor proteins during the formation of cortical circuits and reveal a signaling axis controlling cortical scaffold formation.

A pathogenic CTBP1 variant featuring HADDTS with dystrophic myopathology

HADDTS (Hypotonia, Ataxia, Developmental-Delay and Tooth-enamel defects) is a newly emerging syndrome caused by CTBP1 mutations. Only five reports (13 cases) are available; three contained muscle-biopsy results but none presented illustrated histomyopathology. We report a patient in whom whole-exome sequencing revealed a heterozygous de novo CTBP1 missense mutation (c.1024 C>T; p.(Arg342Trp)). Progressive muscular weakness and myopathic electromyography suggested a myopathological substrate; muscle-biopsy revealed dystrophic features with endomysial-fibrosis, fiber-size variability, necrotic/degenerative vacuolar myopathy, sarcoplasmic/myofibrillar- and striation-alterations, and enzyme histochemical and structural mitochondrial alterations/defects including vacuolar mitochondriopathy. Our report expands the number of cases in this extremely rare condition and provides illustrated myopathology, muscle-MRI, and electron-microscopy. These are crucial for elucidating the nature and extent of the underlying myopathological-correlates and to characterize the myopatholgical phenotype spectrum in this genetic neurodevelopmental condition.

Long-term clinical and MRI follow-up in two POMT2-related limb girdle muscular dystrophy (LGMDR14) patients

INTRODUCTION

POMT2-related limb girdle muscular dystrophy (LGMDR14) is a rare muscular dystrophy caused by mutations in the POMT2 gene. Thus far only 26 LGMDR14 subjects have been reported and no longitudinal natural history data are available.

CASE REPORT

We describe two LGMDR14 patients followed for 20 years since infancy. Both patients presented a childhood-onset, slowly progressive pelvic girdle muscular weakness leading to loss of ambulation in the second decade in one patient, and cognitive impairment without detectable brain structural abnormalities. Glutei, paraspinal, and adductor muscles were the primarily involved muscles at MRI.

DISCUSSION

This report provides natural history data on LGMDR14 subjects, with a focus on longitudinal muscle MRI. We also reviewed the LGMDR14 literature data, providing information on the LGMDR14 disease progression. Considering the high prevalence of cognitive impairment in LGMDR14 patients, a reliable application of functional outcome measures can be challenging, therefore a muscle MRI follow-up to assess disease evolution is recommended.

Clinical spectrum of orbital and ocular abnormalities on fetal MRI

BACKGROUND

Fetal magnetic resonance imaging (MRI) may reveal sonographically occult ocular abnormalities. When discovered, acquired causes and genetic associations must be sought.

OBJECTIVE

We aim to evaluate a fetal cohort with orbit and/or globe malformations to determine whether there are imaging patterns that suggest the underlying cause.

MATERIALS AND METHODS

We searched all fetal MRI reports performed at an academic children's hospital over 9 consecutive years for orbit and/or globe abnormalities. Each positive exam and all follow-up MRIs were evaluated for interocular distance, globe size, shape and signal, and brain malformations. Genetic and clinical diagnoses were recorded from the medical record.

RESULTS

Seventy-six of 3,085 fetuses (2.5%) were diagnosed with ocular and/or globe abnormalities; 50% had postnatal follow-up MR exams, all confirming the fetal MRI findings. Ninety-two percent (70/76) had concurrent brain malformations. Sixty-seven percent (51/76) were diagnosed with an underlying disorder and 39% of these were genetically proven. The most common diagnoses with ocular globe abnormalities included CHARGE (coloboma of the eye, heart anomaly, choanal atresia, retardation and genital and ear anomalies) syndrome, trisomy 13 syndrome, dystroglycanopathy, holoprosencephaly and diencephalic-mesencephalic junction dysplasia. Genetic diagnoses were more likely with ocular globe abnormalities than isolated orbital abnormalities (P=0.04). Sixty-seven percent of fetuses with ocular calcifications, hemorrhage and/or lens abnormalities had potential maternal risk factors (P=0.03).

CONCLUSION

Malformed ocular globes are associated with brain malformations and genetic abnormalities. Ocular calcifications, hemorrhage and/or lens abnormalities may be associated with maternal risk factors. Genetic work-up should be considered when an ocular globe size or shape abnormality is detected.

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.

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.

Structural basis for matriglycan synthesis by the LARGE1 dual glycosyltransferase

LARGE1 is a bifunctional glycosyltransferase responsible for generating a long linear polysaccharide termed matriglycan that links the cytoskeleton and the extracellular matrix and is required for proper muscle function. This matriglycan polymer is made with an alternating pattern of xylose and glucuronic acid monomers. Mutations in the LARGE1 gene have been shown to cause life-threatening dystroglycanopathies through the inhibition of matriglycan synthesis. Despite its major role in muscle maintenance, the structure of the LARGE1 enzyme and how it assembles in the Golgi are unknown. Here we present the structure of LARGE1, obtained by a combination of X-ray crystallography and single-particle cryo-EM. We found that LARGE1 homo-dimerizes in a configuration that is dictated by its coiled-coil stem domain. The structure shows that this enzyme has two canonical GT-A folds within each of its catalytic domains. In the context of its dimeric structure, the two types of catalytic domains are brought into close proximity from opposing monomers to allow efficient shuttling of the substrates between the two domains. Together, with putative retention of matriglycan by electrostatic interactions, this dimeric organization offers a possible mechanism for the ability of LARGE1 to synthesize long matriglycan chains. The structural information further reveals the mechanisms in which disease-causing mutations disrupt the activity of LARGE1. Collectively, these data shed light on how matriglycan is synthesized alongside the functional significance of glycosyltransferase oligomerization.

Deciphering the molecular landscape of microcephaly in 87 Indian families by exome sequencing

Microcephaly is a frequent feature of neurodevelopmental disorders (NDDs). Our study presents the heterogeneous spectrum of genetic disorders in patients with microcephaly either in isolated form or in association with other neurological and extra-neural abnormalities. We present data of 91 patients from 87 unrelated families referred to our clinic during 2016-2020 and provide a comprehensive clinical and genetic landscape in the studied cohort. Molecular diagnosis using exome sequencing was made in 45 families giving a yield of 51.7%. In 9 additional families probable causative variants were detected. We identified disease causing variations in 49 genes that are involved in different functional pathways Among these, 36 had an autosomal recessive pattern, 8 had an autosomal dominant pattern (all inherited de novo), and 5 had an X-linked pattern. In 41 probands where sequence variations in autosomal recessive genes were identified 31 were homozygotes (including 16 from non-consanguineous families). The study added 28 novel pathogenic/likely pathogenic variations. The study also calls attention to phenotypic variability and expansion in spectrum as well as uncovers genes where microcephaly is not reported previously or is a rare finding. We here report phenotypes associated with the genes for ultra-rare NDDs with microcephaly namely ATRIP, MINPP1, PNPLA8, AIMP2, ANKLE2, NCAPD2 and TRIT1.

Clinical and Molecular Spectrum of Muscular Dystrophies (MDs) with Intellectual Disability (ID): a Comprehensive Overview

Muscular dystrophies encompass a wide and heterogeneous subset of hereditary myopathies that manifest by the structural or functional abnormalities in the skeletal muscle. Some pathogenic mutations induce a dysfunction or loss of proteins that are critical for the stability of muscle cells, leading to progressive muscle degradation and weakening. Several studies have well-established cognitive deficits in muscular dystrophies which are mainly due to the disruption of brain-specific expression of affected muscle proteins. We provide a comprehensive overview of the types of muscular dystrophies that are accompanied by intellectual disability by detailed consulting of the main libraries. The current paper focuses on the clinical and molecular evidence about Duchenne, congenital, limb-girdle, and facioscapulohumeral muscular dystrophies as well as myotonic dystrophies. Because these syndromes impose a heavy burden of psychological and financial problems on patients, their families, and the health care community, a thorough examination is necessary to perform timely psychological and medical interventions and thus improve the quality of life.

Neuronal Dystroglycan regulates postnatal development of CCK/cannabinoid receptor-1 interneurons

Background

The development of functional neural circuits requires the precise formation of synaptic connections between diverse neuronal populations. The molecular pathways that allow GABAergic interneuron subtypes in the mammalian brain to initially recognize their postsynaptic partners remain largely unknown. The transmembrane glycoprotein Dystroglycan is localized to inhibitory synapses in pyramidal neurons, where it is required for the proper function of CCK+ interneurons. However, the precise temporal requirement for Dystroglycan during inhibitory synapse development has not been examined.

Methods

In this study, we use NEX^Cre or Camk2a^CreERT2 to conditionally delete Dystroglycan from newly-born or adult pyramidal neurons, respectively. We then analyze forebrain development from postnatal day 3 through adulthood, with a particular focus on CCK+ interneurons.

Results

In the absence of postsynaptic Dystroglycan in developing pyramidal neurons, presynaptic CCK+ interneurons fail to elaborate their axons and largely disappear from the cortex, hippocampus, amygdala, and olfactory bulb during the first two postnatal weeks. Other interneuron subtypes are unaffected, indicating that CCK+ interneurons are unique in their requirement for postsynaptic Dystroglycan. Dystroglycan does not appear to be required in adult pyramidal neurons to maintain CCK+ interneurons. Bax deletion did not rescue CCK+ interneurons in Dystroglycan mutants during development, suggesting that they are not eliminated by canonical apoptosis. Rather, we observed increased innervation of the striatum, suggesting that the few remaining CCK+ interneurons re-directed their axons to neighboring areas where Dystroglycan expression remained intact.

Conclusion

Together these findings show that Dystroglycan functions as part of a synaptic partner recognition complex that is required early for CCK+ interneuron development in the forebrain.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13064-021-00153-1.

Muscular dystrophy-dystroglycanopathy in a family of Labrador retrievers with a LARGE1 mutation

Alpha-dystroglycan (αDG) is a highly glycosylated cell surface protein with a significant role in cell-to-extracellular matrix interactions in muscle. αDG interaction with extracellular ligands relies on the activity of the LARGE1 glycosyltransferase that synthesizes and extends the heteropolysaccharide matriglycan. Abnormalities in αDG glycosylation and formation of matriglycan are the pathogenic mechanisms for the dystroglycanopathies, a group of congenital muscular dystrophies. Muscle biopsies were evaluated from related 6-week-old Labrador retriever puppies with poor suckling, small stature compared to normal litter mates, bow-legged stance and markedly elevated creatine kinase activities. A dystrophic phenotype with marked degeneration and regeneration, multifocal mononuclear cell infiltration and endomysial fibrosis was identified on muscle cryosections. Single nucleotide polymorphism (SNP) array genotyping data on the family members identified three regions of homozygosity in 4 cases relative to 8 controls. Analysis of whole genome sequence data from one of the cases identified a stop codon mutation in the LARGE1 gene that truncates 40% of the protein. Immunofluorescent staining and western blotting demonstrated the absence of matriglycan in skeletal muscle and heart from affected dogs. Compared to control, LARGE enzyme activity was not detected. This is the first report of a dystroglycanopathy in dogs.

Analysis of genotype-phenotype correlation in Walker-Warburg syndrome with a novel CRPPA mutation in different clinical manifestations

PURPOSE

Walker-Warburg syndrome (WWS) is a rare autosomal recessive disorder characterized by congenital muscular dystrophy and severe brain and eye malformations. This study aims to analyze genotype-phenotype correlations in WWS with a novel cytidine diphosphate-l-ribitol pyrophosphorylase A (CRPPA) mutation in different clinical manifestations.

CASE DESCRIPTION

We report a girl with a presentation of multiple brain and ocular anomalies. Her ophthalmological evaluation showed a shallow anterior chamber, cortical cataract, iris hypoplasia, persistent hyperplastic primary vitreous in the right eye, punctate cataract, iris hypoplasia, primary congenital glaucoma, and a widespread loss of fundus pigmentation in the left eye. She was hypotonic, and her deep tendon reflexes were absent. Laboratory investigations showed high serum levels of serum creatine kinase. Brain magnetic resonance imaging demonstrated hydrocephalus, agenesis of the corpus callosum, retrocerebellar cyst, cerebellar dysplasia and hypoplasia, cobblestone lissencephaly, and hypoplastic brainstem. Whole exome sequencing revealed a novel homozygous nonsense mutation in the first exon of the CRPPA gene (NM_001101426.4, c.217G>T, p.Glu73Ter).

CONCLUSIONS

The study findings expand the phenotypic variability of the ocular manifestations in the CRPPA gene-related WWS. Iris hypoplasia can be a part of clinical manifestations of the CRPPA gene-related WWS. The uncovering of the genes associated with ocular features can provide preventative methods, early diagnosis, and improved therapeutic strategies.

A local regulatory network in the testis mediated by laminin and collagen fragments that supports spermatogenesis

It is almost five decades since the discovery of the hypothalamic-pituitary-testicular axis. This refers to the hormonal axis that connects the hypothalamus, pituitary gland and testes, which in turn, regulates the production of spermatozoa through spermatogenesis in the seminiferous tubules, and testosterone through steroidogenesis by Leydig cells in the interstitium, of the testes. Emerging evidence has demonstrated the presence of a regulatory network across the seminiferous epithelium utilizing bioactive molecules produced locally at specific domains of the epithelium. Studies have shown that biologically active fragments are produced from structural laminin and collagen chains in the basement membrane. Additionally, bioactive peptides are also produced locally in non-basement membrane laminin chains at the Sertoli-spermatid interface known as apical ectoplasmic specialization (apical ES, a testis-specific actin-based anchoring junction type). These bioactive peptides are derived from structural laminins and/or collagens at the corresponding sites through proteolytic cleavage by matrix metalloproteinases (MMPs). They in turn serve as autocrine and/or paracrine factors to modulate and coordinate cellular events across the epithelium by linking the apical and basal compartments, the apical and basal ES, the blood-testis barrier (BTB), and the basement membrane of the tunica propria. The cellular events supported by these bioactive peptides/fragments include the release of spermatozoa at spermiation, remodeling of the immunological barrier to facilitate the transport of preleptotene spermatocytes across the BTB, and the transport of haploid spermatids across the epithelium to support spermiogenesis. In this review, we critically evaluate these findings. Our goal is to identify research areas that deserve attentions in future years. The proposed research also provides the much needed understanding on the biology of spermatogenesis supported by a local network of regulatory biomolecules.

Phenotypic Spectrum of α-Dystroglycanopathies Associated With the c.919T>a Variant in the FKRP Gene in Humans and Mice

Mutations in the fukutin-related protein gene, FKRP, are the most frequent single cause of α-dystroglycanopathy. Rare FKRP mutations are clinically not well characterized. Here, we review the phenotype associated with the rare c.919T>A mutation in FKRP in humans and mice. We describe clinical and paraclinical findings in 6 patients, 2 homozygous, and 4-compound heterozygous for c.919T>A, and compare findings with a mouse model we generated, which is homozygous for the same mutation. In patients, the mutation at the homozygous state is associated with a severe congenital muscular dystrophy phenotype invariably characterized by severe multisystem disease and early death. Compound heterozygous patients have a severe limb-girdle muscular dystrophy phenotype, loss of ambulation before age 20 and respiratory insufficiency. In contrast, mice homozygous for the same mutation show no symptoms or signs of muscle disease. Evidence therefore defines the FKRP c.919T>A as a very severe mutation in humans. The huge discrepancy between phenotypes in humans and mice suggests that differences in protein folding/processing exist between human and mouse Fkrp. This emphasizes the need for more detailed structural analyses of FKRP and shows the challenges of developing appropriate animal models of dystroglycanopathies that mimic the disease course in humans.

Genetic variations and clinical spectrum of dystroglycanopathy in a large cohort of Chinese patients

Dystroglycanopathy is a group of muscular dystrophies with deficient glycosylation of alpha-dystroglycan (α-DG). We recruited patients from 36 tertiary academic hospitals in China. In total, 143 patients with genetically diagnosed dystroglycanopathy were enrolled. Of these, limb girdle muscular dystrophy was the most common initial diagnosis (83 patients) and Walker-Warburg syndrome was the least common (1 patient). In 143 patients, mutations in FKRP gene were the most prevalent (62 patients), followed by POMT2, POMT1 (16), POMGNT1, ISPD (14), FKTN, GMPPB, B3GALNT2, DPM3, and DAG1. Several frequent mutations were identified in FKRP, POMT1, POMGNT1, ISPD, and FKTN genes. Many of these were founder mutations. Patients with FKRP mutations tended to have milder phenotypes, while those with mutations in POMGNT1 genes had more severe phenotypes. Mental retardation was a clinical feature associated with mutations of POMT1 gene. Detailed clinical data of 83 patients followed up in Peking University First Hospital were further analyzed. Our clinical and genetic analysis of a large cohort of Chinese patients with dystroglycanopathy expanded the genotype variation and clinical spectrum of congenital muscular dystrophies.

Prenatal presentation of Walker–Warburg syndrome with a POMT2 mutation: an extended fetal phenotype

Background

Walker–Warburg syndrome (WWS) is a rare, lethal, genetically, and clinically heterogeneous congenital muscular dystrophy resulting from defective glycosylation of α-dystroglycan (α-DG) and is associated with both cranial and ocular malformations. Prenatal detection of posterior fossa anomalies in association with hydrocephalus are nonspecific, however, an additional finding of eye anomalies are typical for WWS. The purpose of this report is to elucidate the pattern of associated malformations in a fetus with WWS born to 3rd degree consanguineously married couple. Additionally, the fetal ultrasonography revealed congenital heart disease, clenched hands, and talipes equinovarus; these findings have not been previously reported and represent an expansion of prenatal spectrum associated with WWS.

Case presentation

We report on a specific sonographic pattern of congenital anomalies including hydrocephalus, agenesis of corpus callosum, and Dandy–Walker malformation. Ocular abnormalities include microphthalmia, cataract, and an echoic structure suggestive of persistent primary vitreous. Other features include congenital heart disease, unilateral multicystic kidney, and previously unreported findings of bilateral clenched hands and talipes equinovarus. The molecular analysis detected a homozygous splicing mutation, c.924-2A>C, in the POMT2 gene; this variant segregated with the phenotype.

Conclusion

WWS syndrome has characteristic prenatal ultrasound findings which can improve the prenatal identification of this condition and help in guiding the molecular diagnosis and counseling. The detection of bilateral clenched hands and talipes equinovarus is a novel finding that further expands the phenotypic spectrum of WWS.

Congenital hearing impairment associated with peripheral cochlear nerve dysmyelination in glycosylation-deficient muscular dystrophy

Hearing loss (HL) is one of the most common sensory impairments and etiologically and genetically heterogeneous disorders in humans. Muscular dystrophies (MDs) are neuromuscular disorders characterized by progressive degeneration of skeletal muscle accompanied by non-muscular symptoms. Aberrant glycosylation of α-dystroglycan causes at least eighteen subtypes of MD, now categorized as MD-dystroglycanopathy (MD-DG), with a wide spectrum of non-muscular symptoms. Despite a growing number of MD-DG subtypes and increasing evidence regarding their molecular pathogeneses, no comprehensive study has investigated sensorineural HL (SNHL) in MD-DG. Here, we found that two mouse models of MD-DG, Large^myd/myd and POMGnT1-KO mice, exhibited congenital, non-progressive, and mild-to-moderate SNHL in auditory brainstem response (ABR) accompanied by extended latency of wave I. Profoundly abnormal myelination was found at the peripheral segment of the cochlear nerve, which is rich in the glycosylated α-dystroglycan–laminin complex and demarcated by “the glial dome.” In addition, patients with Fukuyama congenital MD, a type of MD-DG, also had latent SNHL with extended latency of wave I in ABR. Collectively, these findings indicate that hearing impairment associated with impaired Schwann cell-mediated myelination at the peripheral segment of the cochlear nerve is a notable symptom of MD-DG.

Hearing loss (HL) is one of the most common sensory impairments and heterogeneous disorders in humans. Up to 60% of HL cases are caused by genetic factors, and approximately 30% of genetic HL cases are syndromic. Although 400–700 genetic syndromes are associated with sensorineural HL (SNHL), caused due to problems in the nerve pathways from the cochlea to the brain, only about 45 genes are known to be associated with syndromic HL. Muscular dystrophies (MDs) are neuromuscular disorders characterized by progressive degeneration of skeletal muscle accompanied by non-muscular symptoms. MD-dystroglycanopathy (MD-DG), caused by aberrant glycosylation of α-dystroglycan, is an MD subtype with a wide spectrum of non-muscular symptoms. Despite a growing number of MD-DG subtypes (at least 18), no comprehensive study has investigated SNHL in MD-DG. Here, we found that hearing impairment was associated with abnormal myelination of the peripheral segment of the cochlear nerve caused by impaired dystrophin–dystroglycan complex in two mouse models (type 3 and 6) of MD-DG and in patients (type 4) with MD-DG. This is the first comprehensive study investigating SNHL in MD-DG. Our findings may provide new insights into understanding the pathogenic characteristics and mechanisms underlying inherited syndromic hearing impairment.

Identification and Modeling of a GT-A Fold in the α-Dystroglycan Glycosylating Enzyme LARGE1

The acetylglucosaminyltransferase-like protein LARGE1 is an enzyme that is responsible for the final steps of the post-translational modifications of dystroglycan (DG), a membrane receptor that links the cytoskeleton with the extracellular matrix in the skeletal muscle and in a variety of other tissues. LARGE1 acts by adding the repeating disaccharide unit [-3Xyl-α1,3GlcAβ1-] to the extracellular portion of the DG complex (α-DG); defects in the LARGE1 gene result in an aberrant glycosylation of α-DG and consequent impairment of its binding to laminin, eventually affecting the connection between the cell and the extracellular environment. In the skeletal muscle, this leads to degeneration of the muscular tissue and muscular dystrophy. So far, a few missense mutations have been identified within the LARGE1 protein and linked to congenital muscular dystrophy, and because no structural information is available on this enzyme, our understanding of the molecular mechanisms underlying these pathologies is still very limited. Here, we generated a 3D model structure of the two catalytic domains of LARGE1, combining different molecular modeling approaches. Furthermore, by using molecular dynamics simulations, we analyzed the effect on the structure and stability of the first catalytic domain of the pathological missense mutation S331F that gives rise to a severe form of muscle–eye–brain disease.

Duchenne muscular dystrophy-like phenotype in an LGMD2I patient with novel FKRP gene variants

A 32-year-old man initially received a diagnosis of Duchenne muscular dystrophy (DMD). Genetic analysis revealed two novel heterozygous FKRP variants: c.169G>A (p.Glu57Lys) and c.692G>A (p.Trp231*). These results indicated that the patient had limb-girdle muscular dystrophy type 2I (LGMD2I) caused by recessive FKRP variants. Patients with LGMD2I and DMD have many overlapping phenotypes. LGMD2I should be considered in patients who have a DMD phenotype but not a DMD pathogenic variant.

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.

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.

Structure of the eukaryotic protein O-mannosyltransferase Pmt1-Pmt2 complex

In eukaryotes, a nascent peptide entering the endoplasmic reticulum (ER) is scanned by two Sec61-translocon-associated large membrane machines for protein N-glycosylation and protein O-mannosylation, respectively. While the structure of the eight-protein oligosaccharyltransferase complex has been determined recently, the structures of mannosyltransferases of the PMT family, which are an integral part of ER protein homeostasis, are still unknown. Here we report cryo-EM structures of the S. cerevisiae Pmt1–Pmt2 complex bound to a donor and an acceptor peptide at 3.2-Å resolution, showing that each subunit contains 11 transmembrane helices and a lumenal β-trefoil fold termed the MIR domain. The structures reveal the substrate recognition model and confirm an inverting mannosyl-transferring reaction mechanism by the enzyme complex. Furthermore, we found that the transmembrane domains of Pmt1 and Pmt2 share a structural fold with the catalytic subunits of oligosaccharyltransferases, confirming a previously proposed evolutionary relationship between protein O-mannosylation and protein N-glycosylation.

Compound heterozygous POMGNT1 mutations leading to muscular dystrophy-dystroglycanopathy type A3: a case report

Background

Dystroglycanopathies, which are caused by reduced glycosylation of alpha-dystroglycan, are a heterogeneous group of neurodegenerative disorders characterized by variable brain and skeletal muscle involvement. Muscle-eye-brain disease (or muscular dystrophy-dystroglycanopathy type 3 A) is an autosomal recessive disorder characterized by congenital muscular dystrophy, ocular abnormalities, and lissencephaly.

Case presentation

We report clinical and genetic characteristics of a 6-year-old boy affected by muscular dystrophy-dystroglycanopathy. He has severe a delay in psychomotor and speech development, muscle hypotony, congenital myopia, partial atrophy of the optic nerve disc, increased level of creatine kinase, primary-muscle lesion, polymicrogyria, ventriculomegaly, hypoplasia of the corpus callosum, cysts of the cerebellum. Exome sequencing revealed compound heterozygous mutations in POMGNT1 gene (transcript NM_001243766.1): c.1539 + 1G > A and c.385C > T.

Conclusions

The present case report shows diagnostic algorithm step by step and helps better understand the clinical and genetic features of congenital muscular dystrophy.

Neuronal migration in the CNS during development and disease: insights from in vivo and in vitro models

Neuronal migration is a fundamental process that governs embryonic brain development. As such, mutations that affect essential neuronal migration processes lead to severe brain malformations, which can cause complex and heterogeneous developmental and neuronal migration disorders. Our fragmented knowledge about the aetiology of these disorders raises numerous issues. However, many of these can now be addressed through studies of in vivo and in vitro models that attempt to recapitulate human-specific mechanisms of cortical development. In this Review, we discuss the advantages and limitations of these model systems and suggest that a complementary approach, using combinations of in vivo and in vitro models, will broaden our knowledge of the molecular and cellular mechanisms that underlie defective neuronal positioning in the human cerebral cortex.

Keeping an eye on congenital disorders of O-glycosylation: A systematic literature review

Congenital disorders of glycosylation (CDG) are a rapidly growing family comprising >100 genetic diseases. Some 25 CDG are pure O-glycosylation defects. Even among this CDG subgroup, phenotypic diversity is broad, ranging from mild to severe poly-organ/system dysfunction. Ophthalmic manifestations are present in 60% of these CDG. The ophthalmic manifestations in N-glycosylation-deficient patients have been described elsewhere. The present review documents the spectrum and incidence of eye disorders in patients with pure O-glycosylation defects with the aim of assisting diagnosis and management and promoting research.

Neonatal hypotonia and neuromuscular conditions

The differential diagnosis of neonatal hypotonia is a complex task, as in newborns hypotonia can be the presenting sign of different underlying causes, including peripheral and central nervous system involvement and genetic and metabolic diseases. This chapter describes how a combined approach, based on the combination of clinical signs and new genetic techniques, can help not only to establish when the hypotonia is related to peripheral involvement but also to achieve an accurate and early diagnosis of the specific neuromuscular diseases with neonatal onset. The early identification of such disorders is important, as this allows early intervention with disease-specific standards of care and, more importantly, because of the possibility to treat some of them, such as spinal muscular atrophy, with therapeutic approaches that have recently become available.

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.

Prevalence, pathological mechanisms, and genetic basis of limb-girdle muscular dystrophies: A review

Limb-girdle muscular dystrophies (LGMDs) are a highly heterogeneous group of neuromuscular disorders that are associated with weakness and wasting of muscles in legs and arms. Signs and symptoms may begin at any age and usually worsen by time. LGMDs are autosomal disorders with different types and their prevalence is not the same in different areas. New technologies such as next-generation sequencing can accelerate their diagnosis. Several important pathological mechanisms that are involved in the pathology of the LGMD include abnormalities in dystrophin-glycoprotein complex, the sarcomere, glycosylation of dystroglycan, vesicle and molecular trafficking, signal transduction pathways, and nuclear functions. Here, we provide a comprehensive review that integrates LGMD clinical manifestations, prevalence, and some pathological mechanisms involved in LGMDs.

Molecular genetics of the POMT1-related muscular dystrophy-dystroglycanopathies

Protein O-mannosyltransferase 1 (POMT1) is a critical enzyme participating in the first step of protein O-mannosylation. Mutations in the coding gene, POMT1, have been described to be related to a series of autosomal recessive disorders associated with defective alpha-dystroglycan glycosylation, later termed muscular dystrophy-dystroglycanopathies (MDDGs). MDDGs are characterized by a broad phenotypic spectrum of congenital muscular dystrophy or later-onset limb-girdle muscular dystrophy, accompanied by variable degrees of intellectual disability, brain defects, and ocular abnormalities. To date, at least 76 disease-associated mutations in the POMT1 gene, including missense, nonsense, splicing, deletion, insertion/duplication, and insertion-deletion mutations, have been reported in the literature. In this review, we highlight the present knowledge of the identified disease-associated POMT1 gene mutations and genetic animal models related to the POMT1 gene. This review may help further normative classification of phenotypes, assist in definite clinical and genetic diagnoses, and genetic counseling, and may comprehensively improve our understanding of the basis of complex phenotypes and possible pathogenic mechanisms involved.

Detection of variants in dystroglycanopathy-associated genes through the application of targeted whole-exome sequencing analysis to a large cohort of patients with unexplained limb-girdle muscle weakness

Background

Dystroglycanopathies are a clinically and genetically heterogeneous group of disorders that are typically characterised by limb-girdle muscle weakness. Mutations in 18 different genes have been associated with dystroglycanopathies, the encoded proteins of which typically modulate the binding of α-dystroglycan to extracellular matrix ligands by altering its glycosylation. This results in a disruption of the structural integrity of the myocyte, ultimately leading to muscle degeneration.

Methods

Deep phenotypic information was gathered using the PhenoTips online software for 1001 patients with unexplained limb-girdle muscle weakness from 43 different centres across 21 European and Middle Eastern countries. Whole-exome sequencing with at least 250 ng DNA was completed using an Illumina exome capture and a 38 Mb baited target. Genes known to be associated with dystroglycanopathies were analysed for disease-causing variants.

Results

Suspected pathogenic variants were detected in DPM3, ISPD, POMT1 and FKTN in one patient each, in POMK in two patients, in GMPPB in three patients, in FKRP in eight patients and in POMT2 in ten patients. This indicated a frequency of 2.7% for the disease group within the cohort of 1001 patients with unexplained limb-girdle muscle weakness. The phenotypes of the 27 patients were highly variable, yet with a fundamental presentation of proximal muscle weakness and elevated serum creatine kinase.

Conclusions

Overall, we have identified 27 patients with suspected pathogenic variants in dystroglycanopathy-associated genes. We present evidence for the genetic and phenotypic diversity of the dystroglycanopathies as a disease group, while also highlighting the advantage of incorporating next-generation sequencing into the diagnostic pathway of rare diseases.

Electronic supplementary material

The online version of this article (10.1186/s13395-018-0170-1) contains supplementary material, which is available to authorized users.

Calculating the statistical significance of rare variants causal for Mendelian and complex disorders

BACKGROUND

With the expanding use of next-gen sequencing (NGS) to diagnose the thousands of rare Mendelian genetic diseases, it is critical to be able to interpret individual DNA variation. To calculate the significance of finding a rare protein-altering variant in a given gene, one must know the frequency of seeing a variant in the general population that is at least as damaging as the variant in question.

METHODS

We developed a general method to better interpret the likelihood that a rare variant is disease causing if observed in a given gene or genic region mapping to a described protein domain, using genome-wide information from a large control sample. Based on data from 2504 individuals in the 1000 Genomes Project dataset, we calculated the number of individuals who have a rare variant in a given gene for numerous filtering threshold scenarios, which may be used for calculating the significance of an observed rare variant being causal for disease. Additionally, we calculated mutational burden data on the number of individuals with rare variants in genic regions mapping to protein domains.

RESULTS

We describe methods to use the mutational burden data for calculating the significance of observing rare variants in a given proportion of sequenced individuals. We present SORVA, an implementation of these methods as a web tool, and we demonstrate application to 20 relevant but diverse next-gen sequencing studies. Specifically, we calculate the statistical significance of findings involving multi-family studies with rare Mendelian disease and a large-scale study of a complex disorder, autism spectrum disorder. If we use the frequency counts to rank genes based on intolerance for variation, the ranking correlates well with pLI scores derived from the Exome Aggregation Consortium (ExAC) dataset (ρ = 0.515), with the benefit that the scores are directly interpretable.

CONCLUSIONS

We have presented a strategy that is useful for vetting candidate genes from NGS studies and allows researchers to calculate the significance of seeing a variant in a given gene or protein domain. This approach is an important step towards developing a quantitative, statistics-based approach for presenting clinical findings.

Retrospective genotype-phenotype analysis in a 305 patient cohort referred for testing of a targeted epilepsy panel

PURPOSE

Epilepsy is a diverse neurological condition with extreme genetic and phenotypic heterogeneity. The introduction of next-generation sequencing into the clinical laboratory has made it possible to investigate hundreds of associated genes simultaneously for a patient, even in the absence of a clearly defined syndrome. This has resulted in the detection of rare and novel mutations at a rate well beyond our ability to characterize their effects. This retrospective study reviews genotype data in the context of available phenotypic information on 305 patients spanning the epileptic spectrum to identify established and novel patterns of correlation.

METHODS

Our epilepsy panel comprising 377 genes was used to sequence 305 patients referred for genetic testing. Qualifying variants were annotated with phenotypic data obtained from either the test requisition form or supporting clinical documentation. Observed phenotypes were compared with established phenotypes in OMIM, published literature and the ILAEs 2010 report on genetic testing to assess congruity with known gene aberrations.

RESULTS

We identified a number of novel and recognized genetic variants consistent with established epileptic phenotypes. Forty-one pathogenic or predicted deleterious variants were detected in 39 patients with accompanying clinical documentation. Twenty-five of these variants across 15 genes were novel. Furthermore, evaluation of phenotype data for 194 patients with variants of unknown significance in genes with autosomal dominant and X-linked disease inheritance elucidated potentially disease-causing variants that were not currently characterized in the literature.

CONCLUSIONS

Assessment of key genotype-phenotype correlations from our cohort provide insight into variant classification, as well as the importance of including ILAE recommended genes as part of minimum panel content for comprehensive epilepsy tests. Many of the reported VUSs are likely genuine pathogenic variants driving the observed phenotypes, but not enough evidence is available for assertive classifications. Similar studies will provide more utility via mounting independent genotype-phenotype data from unrelated patients. The possible outcome would be a better molecular diagnostic product, with fewer indeterminate reports containing only VUSs.

NINDS Common Data Elements for Congenital Muscular Dystrophy Clinical Research: A National Institute for Neurological Disorders and Stroke Project

BACKGROUND

A Congenital Muscular Dystrophy (CMD) Working Group (WG) consisting of international experts reviewed common data elements (CDEs) previously developed for other neuromuscular diseases (NMDs) and made recommendations for all types of studies on CMD.

OBJECTIVES

To develop a comprehensive set of CDEs, data definitions, case report forms and guidelines for use in CMD clinical research to facilitate interoperability of data collection, as part of the CDE project at the National Institute of Neurological Disorders and Stroke (NINDS).

METHODS

One working group composed of ten experts reviewed existing NINDS CDEs and outcome measures, evaluated the need for new elements, and provided recommendations for CMD clinical research. The recommendations were compiled, internally reviewed by the CMD working group, and posted online for external public comment. The CMD working group and the NIH CDE team reviewed the final version before release.

RESULTS

The NINDS CMD CDEs and supporting documents are publicly available on the NINDS CDE website (https://www.commondataelements.ninds.nih.gov/CMD.aspx#tab=Data_Standards). Content areas include demographics, social status, health history, physical examination, diagnostic tests, and guidelines for a variety of specific outcomes and endpoints. The CMD CDE WG selected these documents from existing versions that were generated by other disease area working groups. Some documents were tailored to maximize their suitability for the CMD field.

CONCLUSIONS

Widespread use of CDEs can facilitate CMD clinical research and trial design, data sharing and retrospective analyses. The CDEs that are most relevant to CMD research are like those generated for other NMDs, and CDE documents tailored to CMD are now available to the public. The existence of a single source for these documents facilitates their use in research studies and offers a clear mechanism for the discussion and update of the information as knowledge is gained.

Novel copy number variation of POMGNT1 associated with muscle-eye-brain disease detected by next-generation sequencing

The protein O-mannose beta-1,2-N-acetylglucosaminyltransferase 1 (POMGNT1) gene is one of 18 genes involved in the pathogenesis of α-dystroglycanopathies(α-DGPs) such as muscle-eye-brain disease (MEB). Our study aimed to retrospectively analyze and characterize the clinical and genetic features of three MEB patients with POMGNT1 mutations. One female and two male patients from three unrelated families were diagnosed with MEB, manifesting hypotonia at birth, mental retardation, structural brain defects, and ocular malformations. The novel missense mutations c.296 T > C and c.794 G > C were revealed in patient 2 and patient 3 respectively by next-generation sequencing (NGS). Further NGS data analysis revealed that all three patients had the same novel copy number variations (CNV) g.6668-8257del, which was homozygous in patient 1 and heterozygous in patients 2 and 3. By long-range polymerase chain reaction (PCR) and Sanger sequencing, it was shown that the two breakpoints of the CNV localized to two AluY elements and displayed 42-bp of microhomology. The CNV was confirmed as a founder mutation by haplotype analysis. Our study indicated that NGS is a clinically useful method of detecting α-DGPs genes -related CNV, and the CNV is likely to be caused by Alu-Alu recombination or from a single ancestor bearing the deletion chromosome.

22q12.3 microduplication overlapping the LARGE gene as a male-only affected loci responsible for increasing the risk of autism spectrum disorder

The present study describes a three-generation Chinese family with one male who was diagnosed with an autism spectrum disorder (ASD) disease. The male proband presented with features of an autism spectrum disorder. Magnetic resonance imaging demonstrated an abnormal high-intensity zone in the frontal white matter. Whole-genome single nucleotide polymorphism-microarray demonstrated an interstitial 575-kb duplication of chromosome 22p12.3 that involved the LARGE gene among the six family members, which included three healthy female carriers, the affected boy and two male fetuses. Fluorescence in situ hybridization analysis, using special probes, and LARGE gene sequencing were performed, which exhibited a submicroscopic 22q13 duplication that involved the LARGE gene. Combined with a review of the literature, the present findings support the hypothesis that the 22q12.3 microduplication overlapping the LARGE gene may be a male-only affected loci, which is responsible for increasing the ASD risk.

Congenital muscular dystrophies in the UK population: Clinical and molecular spectrum of a large cohort diagnosed over a 12-year period

Congenital muscular dystrophies (CMDs) are clinically and genetically heterogeneous conditions; some fatal in the first few years of life and with central nervous system involvement, whereas others present a milder course. We provide a comprehensive report of the relative frequency and clinical and genetic spectrum of CMD in the UK. Genetic analysis of CMD genes in the UK is centralised in London and Newcastle. Between 2001 and 2013, a genetically confirmed diagnosis of CMD was obtained for 249 unrelated individuals referred to these services. The most common CMD subtype was laminin-α2 related CMD (also known as MDC1A, 37.4%), followed by dystroglycanopathies (26.5%), Ullrich-CMD (15.7%), SEPN1 (11.65%) and LMNA (8.8%) gene related CMDs. The most common dystroglycanopathy phenotype was muscle-eye-brain-like disease. Fifteen patients carried mutations in the recently discovered ISPD, GMPPB and B3GALNT2 genes. Pathogenic allelic mutations in one of the CMD genes were also found in 169 unrelated patients with milder phenotypes, such as limb girdle muscular dystrophy and Bethlem myopathy. In all, we identified 362 mutations, 160 of which were novel. Our results provide one of the most comprehensive reports on genetics and clinical features of CMD subtypes and should help diagnosis and counselling of families with this group of conditions.

Large is required for normal astrocyte migration and retinal vasculature development

BACKGROUND

Persistent fetal vasculature (PFV) is a congenital developmental anomaly of the eye that accounts for about 5% of childhood blindness. The molecular mechanism of PFV remains unclear. As a glycosyltransferase of α-dystroglycan, LARGE mutations have been found in congenital muscular dystrophy patients with brain abnormalities. Spontaneous Large mutant mice displayed similar symptoms of human muscle-eye-brain disorders. However, the detailed roles of Large in ocular vasculature development still need to be uncovered.

RESULTS

In this paper, we report that a novel Large mutation generated by the piggyBac transposon insertion leads to PFV and abnormal retinal vasculature in mice. Glycosylation of α-DG, an essential component of the extracellular matrix, was significantly impaired in these Large mutants, leading to broken inner limiting membrane (ILM). As a guide of the retinal vasculature development, the distribution of retinal astrocytes became irregular within the retina, and many astrocytes abnormally migrated into the vitreous along with the hyaloid vessels in Large mutants.

CONCLUSIONS

Large is essential for ILM formation and retinal astrocyte migration. The novel Large mutant mouse can serve as a new PFV model to further dissect LARGE functions in ocular vasculature development.

Mutations in INPP5K Cause a Form of Congenital Muscular Dystrophy Overlapping Marinesco-Sjögren Syndrome and Dystroglycanopathy

Congenital muscular dystrophies display a wide phenotypic and genetic heterogeneity. The combination of clinical, biochemical, and molecular genetic findings must be considered to obtain the precise diagnosis and provide appropriate genetic counselling. Here we report five individuals from four families presenting with variable clinical features including muscular dystrophy with a reduction in dystroglycan glycosylation, short stature, intellectual disability, and cataracts, overlapping both the dystroglycanopathies and Marinesco-Sjögren syndrome. Whole-exome sequencing revealed homozygous missense and compound heterozygous mutations in INPP5K in the affected members of each family. INPP5K encodes the inositol polyphosphate-5-phosphatase K, also known as SKIP (skeletal muscle and kidney enriched inositol phosphatase), which is highly expressed in the brain and muscle. INPP5K localizes to both the endoplasmic reticulum and to actin ruffles in the cytoplasm. It has been shown to regulate myoblast differentiation and has also been implicated in protein processing through its interaction with the ER chaperone HSPA5/BiP. We show that morpholino-mediated inpp5k loss of function in the zebrafish results in shortened body axis, microphthalmia with disorganized lens, microcephaly, reduced touch-evoked motility, and highly disorganized myofibers. Altogether these data demonstrate that mutations in INPP5K cause a congenital muscular dystrophy syndrome with short stature, cataracts, and intellectual disability.

Compound heterozygous POMT1 mutations in a Chinese family with autosomal recessive muscular dystrophy-dystroglycanopathy C1

Muscular dystrophy-dystroglycanopathy (MDDG) is a genetically and clinically heterogeneous group of muscular disorders, characterized by congenital muscular dystrophy or later-onset limb-girdle muscular dystrophy accompanied by brain and ocular abnormalities, resulting from aberrant alpha-dystroglycan glycosylation. Exome sequencing and Sanger sequencing were performed on a six-generation consanguineous Han Chinese family, members of which had autosomal recessive MDDG. Compound heterozygous mutations, c.1338+1G>A (p.H415Kfs*3) and c.1457G>C (p.W486S, rs746849558), in the protein O-mannosyltransferase 1 gene (POMT1), were identified as the genetic cause. Patients that exhibited milder MDDG manifested as later-onset progressive proximal pelvic, shoulder girdle and limb muscle weakness, joint contractures, mental retardation and elevated creatine kinase, without structural brain or ocular abnormalities, were further genetically diagnosed as MDDGC1. The POMT1 gene splice-site mutation (c.1338+1G>A) which leads to exon 13 skipping and results in a truncated protein may contribute to a severe phenotype, while the allelic missense mutation (p.W486S) may reduce MDDG severity. These findings may expand phenotype and mutation spectrum of the POMT1 gene. Clinical diagnosis supplemented with molecular screening may result in more accurate diagnoses of, prognoses for, and improved genetic counselling for this disease.

Comparison of brain MRI findings with language and motor function in the dystroglycanopathies

OBJECTIVE

To describe the spectrum of brain MRI findings in a cohort of individuals with dystroglycanopathies (DGs) and relate MRI results to function.

METHODS

All available brain MRIs done for clinical indications on individuals enrolled in a DG natural history study (NCT00313677) were reviewed. Reports were reviewed when MRI was not available. MRIs were categorized as follows: (1) cortical, brainstem, and cerebellar malformations; (2) cortical and cerebellar malformations; or (3) normal. Language development was assigned to 1 of 3 categories by a speech pathologist. Maximal motor function and presence of epilepsy were determined by history or examination.

RESULTS

Twenty-five MRIs and 9 reports were reviewed. The most common MRI abnormalities were cobblestone cortex or dysgyria with an anterior-posterior gradient and cerebellar hypoplasia. Seven individuals had MRIs in group 1, 8 in group 2, and 19 in group 3. Language was impaired in 100% of those in MRI groups 1 and 2, and degree of language impairment correlated with severity of imaging. Eighty-five percent of the whole group achieved independent walking, but only 33% did in group 1. Epilepsy was present in 8% of the cohort and rose to 37% of those with an abnormal MRI.

CONCLUSIONS

Developmental abnormalities of the brain such as cobblestone lissencephaly, cerebellar cysts, pontine hypoplasia, and brainstem bowing are hallmarks of DG and should prompt consideration of these diagnoses. Brain imaging in individuals with DG helps to predict outcomes, especially language development, aiding clinicians in prognostic counseling.

Congenital muscular dystrophy: from muscle to brain

Congenital muscular dystrophies (CMDs) are a wide group of muscular disorders that manifest with very early onset of muscular weakness, sometime associated to severe brain involvement.The histologic pattern of muscle anomalies is typical of dystrophic lesions but quite variable depending on the different stages and on the severity of the disorder.Recent classification of CMDs have been reported most of which based on the combination of clinical, biochemical, molecular and genetic findings, but genotype/phenotype correlation are in constant progression due to more diffuse utilization of the molecular analysis.In this article, the Authors report on CMDs belonging to the group of dystroglycanopathies and in particular on the most severe forms represented by the Fukuyama CMD, Muscle-Eye-Brain disease and Walker Walburg syndrome.Clinical diagnosis of infantile hypotonia is particularly difficult considering the different etiologic factors causing the lesions, the difficulty in localizing the involved CNS area (central vs. peripheral) and the limited role of the diagnostic procedures at this early age.The diagnostic evaluation is not easy mainly in differentiating the various types of CMDs, and represents a challenge for the neonatologists and pediatricians. Suggestions are reported on the way to reach a correct diagnosis with the appropriate use of the diagnostic means.

Mutations in POMGNT1 cause non-syndromic retinitis pigmentosa

A growing number of human diseases have been linked to defects in protein glycosylation that affects a wide range of organs. Among them, O-mannosylation is an unusual type of protein glycosylation that is largely restricted to the muscular and nerve system. Consistently, mutations in genes involved in the O-mannosylation pathway result in infantile-onset, severe developmental defects involving skeleton muscle, brain and eye, such as the muscle-eye-brain disease (MIM no. 253280). However, the functional importance of O-mannosylation in these tissues at later stages remains largely unknown. In our study, we have identified recessive mutations in POMGNT1, which encodes an essential component in O-mannosylation pathway, in three unrelated families with autosomal recessive retinitis pigmentosa (RP), but without extraocular involvement. Enzymatic assay of these mutant alleles demonstrate that they greatly reduce the POMGNT1 enzymatic activity and are likely to be hypomorphic. Immunohistochemistry shows that POMGNT1 is specifically expressed in photoreceptor basal body. Taken together, our work identifies a novel disease-causing gene for RP and indicates that proper protein O-mannosylation is not only essential for early organ development, but also important for maintaining survival and function of the highly specialized retinal cells at later stages.

Neuronal migration disorders: Focus on the cytoskeleton and epilepsy

A wide spectrum of focal, regional, or diffuse structural brain abnormalities, collectively known as malformations of cortical development (MCDs), frequently manifest with intellectual disability (ID), epilepsy, and/or autistic spectrum disorder (ASD). As the acronym suggests, MCDs are perturbations of the normal architecture of the cerebral cortex and hippocampus. The pathogenesis of these disorders remains incompletely understood; however, one area that has provided important insights has been the study of neuronal migration. The amalgamation of human genetics and experimental studies in animal models has led to the recognition that common genetic causes of neurodevelopmental disorders, including many severe epilepsy syndromes, are due to mutations in genes regulating the migration of newly born post-mitotic neurons. Neuronal migration genes often, though not exclusively, code for proteins involved in the function of the cytoskeleton. Other cellular processes, such as cell division and axon/dendrite formation, which similarly depend on cytoskeletal functions, may also be affected. We focus here on how the susceptibility of the highly organized neocortex and hippocampus may be due to their laminar organization, which involves the tight regulation, both temporally and spatially, of gene expression, specialized progenitor cells, the migration of neurons over large distances and a birthdate-specific layering of neurons. Perturbations in neuronal migration result in abnormal lamination, neuronal differentiation defects, abnormal cellular morphology and circuit formation. Ultimately this results in disorganized excitatory and inhibitory activity leading to the symptoms observed in individuals with these disorders.