A truncating mutation in B3GNT1 causes severe Walker-Warburg syndrome

B4GAT1 Gene Associated Congenital Muscular Dystrophy Presenting with Recurrent Severe Ventriculomegaly: Case Report and Review of Literature

BACKGROUND

Congenital muscular dystrophies (CMD) range in phenotype from an antenatal presentation with brain and eye anomalies to isolated muscular weakness. B4GAT1 gene has recently been associated with muscular dystrophy-dystroglycanopathy, type A, 13 and two families have been reported.

CASE REPORT

We report the third family with B4GAT1 associated CMD presenting as recurrent severe ventriculomegaly, cerebellar and vermian hypoplasia in fetal life, which was identified after the second affected pregnancy. The mutations identified were similar to those reported in a previously reported Indian family, homozygous, p.Asn390Asp, and p. Ala406Val, suggesting founder mutation.

CONCLUSION

B4GAT1 mutations are associated with CMD and may present in fetal life as severe ventriculomegaly. The homozygous B4GAT1 mutations, p.Asn390Asp, and p. Ala406Val, described in two Indian families (including this case) might represent a founder mutation.

Neuroimaging manifestations and genetic heterogeneity of Walker-Warburg syndrome in Saudi patients

BACKGROUND

Walker-Warburg syndrome (WWS), an autosomal recessive disease, is the most severe phenotype of congenital muscular dystrophies. Its diagnosis remains primarily clinical and radiological. Identification of its causative variants will assist genetic counseling. We aim to describe genetic and neuroimaging findings of WWS and investigate the correlation between them.

METHODS

We retrospectively reviewed the clinical, genetic and neuroimaging findings of eleven Saudi neonates diagnosed with WWS between April 2012 and December 2018 in a single tertiary care center. Correlation between neuroimaging and genetic findings was investigated.

RESULTS

All patients had macrocephaly except one who had intrauterine growth restriction. Dysmorphic features were identified in nearly half of the patients. Creatine kinase levels were available in nine patients and were always elevated. Homozygous pathogenic variants were identified in all patients spanning POMT1 (n = 5), TMEM5 (n = 3), ISPD (n = 2) and POMT2 (n = 1) including one patient who had a dual molecular diagnosis of ISPD and PGAP2. On neuroimaging, all patients showed cobblestone cortex, classical infratentorial findings, and hydrocephalus. Other cerebral cortical malformations included subependymal heterotopia, polymicrogyria and open-lip schizencephaly in four, two and one patients, respectively. Buphthalmos and microphthalmia were the most prevalent orbital findings and found in all patients either unilaterally or bilaterally.

CONCLUSION

WWS is a genetically heterogeneous disorder among Saudis. The case with an additional PGAP2-related phenotype exemplifies the increased risk of dual autosomal recessive disorders in consanguineous populations. MRI is excellent in demonstrating spectrum of WWS brain and orbital malformations; however, no definite correlation could be found between the MRI findings and the genetic variant.

Extracellular Control of Radial Glia Proliferation and Scaffolding During Cortical Development and Pathology

During the development of the cortex, newly generated neurons migrate long-distances in the expanding tissue to reach their final positions. Pyramidal neurons are produced from dorsal progenitors, e.g., radial glia (RGs) in the ventricular zone, and then migrate along RG processes basally toward the cortex. These neurons are hence dependent upon RG extensions to support their migration from apical to basal regions. Several studies have investigated how intracellular determinants are required for RG polarity and subsequent formation and maintenance of their processes. Fewer studies have identified the influence of the extracellular environment on this architecture. This review will focus on extracellular factors which influence RG morphology and pyramidal neuronal migration during normal development and their perturbations in pathology. During cortical development, RGs are present in different strategic positions: apical RGs (aRGs) have their cell bodies located in the ventricular zone with an apical process contacting the ventricle, while they also have a basal process extending radially to reach the pial surface of the cortex. This particular conformation allows aRGs to be exposed to long range and short range signaling cues, whereas basal RGs (bRGs, also known as outer RGs, oRGs) have their cell bodies located throughout the cortical wall, limiting their access to ventricular factors. Long range signals impacting aRGs include secreted molecules present in the embryonic cerebrospinal fluid (e.g., Neuregulin, EGF, FGF, Wnt, BMP). Secreted molecules also contribute to the extracellular matrix (fibronectin, laminin, reelin). Classical short range factors include cell to cell signaling, adhesion molecules and mechano-transduction mechanisms (e.g., TAG1, Notch, cadherins, mechanical tension). Changes in one or several of these components influencing the RG extracellular environment can disrupt the development or maintenance of RG architecture on which neuronal migration relies, leading to a range of cortical malformations. First, we will detail the known long range signaling cues impacting RG. Then, we will review how short range cell contacts are also important to instruct the RG framework. Understanding how RG processes are structured by their environment to maintain and support radial migration is a critical part of the investigation of neurodevelopmental disorders.

Muscular, Ocular and Brain Involvement Associated with a De Novo 11q13.2q14.1 Duplication: Contribution to the Differential Diagnosis of Muscle-Eye-Brain Congenital Muscular Dystrophy

Muscular weakness and hypotonia may be associated with multisystem involvement giving rise to complex phenotypes, many of which are uncharacterized. We report a patient presenting with congenital hypotonia and severe ocular and brain abnormalities, evoking a Muscle Eye Brain disease (MEB). She had global muscular weakness, hypotonia and amyotrophy, joint hyperlaxity, kyphoscoliosis, respiratory insufficiency, dysmorphic features and severe intellectual disability. Brain MRI showed cortical atrophy and hypoplasia of the corpus callosum. Normal CK levels, non-progressive course and absence of dystrophic features or α-dystroglycan abnormalities on the muscle biopsy were not typical of MEB. CGH array identified a large de novo duplication in chromosome 11, including regions partially duplicated in three other patients with common clinical features. This report adds to the differential diagnosis of complex phenotypes characterized by muscular, ocular and CNS involvement and highlights the potential contribution of still unrecognized chromosomal abnormalities to these phenotypes.

Phosphotyrosine profiling of human cerebrospinal fluid

Background

Cerebrospinal fluid (CSF) is an important source of potential biomarkers that affect the brain. Biomarkers for neurodegenerative disorders are needed to assist in diagnosis, monitoring disease progression and evaluating efficacy of therapies. Recent studies have demonstrated the involvement of tyrosine kinases in neuronal cell death. Thus, neurodegeneration in the brain is related to altered tyrosine phosphorylation of proteins in the brain and identification of abnormally phosphorylated tyrosine peptides in CSF has the potential to ascertain candidate biomarkers for neurodegenerative disorders.

Methods

In this study, we used an antibody-based tyrosine phosphopeptide enrichment method coupled with high resolution Orbitrap Fusion Tribrid Lumos Fourier transform mass spectrometer to catalog tyrosine phosphorylated peptides from cerebrospinal fluid. The subset of identified tyrosine phosphorylated peptides was also validated using parallel reaction monitoring (PRM)-based targeted approach.

Results

To date, there are no published studies on global profiling of phosphotyrosine modifications of CSF proteins. We carried out phosphotyrosine profiling of CSF using an anti-phosphotyrosine antibody-based enrichment and analysis using high resolution Orbitrap Fusion Lumos mass spectrometer. We identified 111 phosphotyrosine peptides mapping to 66 proteins, which included 24 proteins which have not been identified in CSF previously. We then validated a set of 5 tyrosine phosphorylated peptides in an independent set of CSF samples from cognitively normal subjects, using a PRM-based targeted approach.

Conclusions

The findings from this deep phosphotyrosine profiling of CSF samples have the potential to identify novel disease-related phosphotyrosine-containing peptides in CSF.

Electronic supplementary material

The online version of this article (10.1186/s12014-018-9205-1) contains supplementary material, which is available to authorized users.

Untangling the complexity of limb-girdle muscular dystrophies

The limb-girdle muscular dystrophies (LGMDs) are a group of genetically heterogeneous, autosomal inherited muscular dystrophies with a childhood to adult onset, manifesting with hip- and shoulder-girdle muscle weakness. When the term LGMD was first conceptualized in 1954, it was thought to be a single entity. Currently, there are 8 autosomal dominant (LGMD1A-1H) and 26 autosomal recessive (LGMD2A-2Z) variants according to the Online Mendelian Inheritance in Man database. In addition, there are other genetically identified muscular dystrophies with an LGMD phenotype not yet classified as LGMD. This highlights the entanglement of LGMDs, which represents an area in continuous expansion. Herein we aim to simplify the complexity of LGMDs by subgrouping them on the basis of the underlying defective protein and impaired function. Muscle Nerve 58: 167-177, 2018.

Noncompaction cardiomyopathy in an infant with Walker-Warburg syndrome

Walker-Warburg syndrome (WWS) is a rare autosomal recessive, congenital muscular dystrophy that is associated with brain and eye anomalies. Several genes encoding proteins involved in α-dystroglycan glycosylation have been implicated in the aetiology of WWS. We describe a patient with nonclassical features of WWS presenting with heart failure related to noncompaction cardiomyopathy resulting in death at 4 months of age. Muscle biopsy revealed absent α-dystroglycan on immunostaining and genetic testing confirmed the diagnosis with two previously described POMT2 mutations. This is the first reported case of WWS syndrome associated with noncompaction cardiomyopathy.

Evidence of early defects in Cajal-Retzius cell localization during brain development in a mouse model of dystroglycanopathy

AIMS

The secondary dystroglycanopathies represent a heterogeneous group of congenital muscular dystrophies characterized by the defective glycosylation of alpha dystroglycan. These disorders are associated with mutations in at least 17 genes, including Fukutin-related protein (FKRP). At the severe end of the clinical spectrum there is substantial brain involvement, and cobblestone lissencephaly is highly suggestive of these disorders. The precise pathogenesis of this phenotype has, however, remained unclear with most attention focused on the disruption to the radial glial scaffold. Here, we set out to investigate whether lesions are apparent prior to the differentiation of the radial glia.

METHODS

A detailed investigation of the structural brain defects from embryonic day 10.5 (E10.5) up until the time of birth (P0) was undertaken in the Fkrp-deficient mice (FKRP^KD ). Reelin, and downstream PI3K/Akt signalling pathways were analysed using Western blot.

RESULTS

We show that early basement membrane defects and neuroglial ectopia precede radial glial cell differentiation. Furthermore, we identify mislocalization of Cajal-Retzius cells which nonetheless is not associated with any apparent disruption to the reelin, and downstream PI3K/Akt signalling pathways.

CONCLUSIONS

These observations identify Cajal-Retzius cell mislocalization as an early event during the development of cortical defects thereby identifying an earlier onset and more complex pathogenesis than originally reported for the secondary dystroglycanopathies. Overall this study provides new insight into central nervous system involvement in this group of diseases.

Comprehensive target capture/next-generation sequencing as a second-tier diagnostic approach for congenital muscular dystrophy in Taiwan

PURPOSE

Congenital muscular dystrophy (CMD) is a heterogeneous disease entity. The detailed clinical manifestation and causative gene for each subgroup of CMD are quite variable. This study aims to analyze the phenotypes and genotypes of Taiwanese patients with CMD as the epidemiology of CMD varies among populations and has been scantly described in Asia.

METHODS

A total of 48 patients suspected to have CMD were screened and categorized by histochemistry and immunohistochemistry studies. Different genetic analyses, including next-generation sequencing (NGS), were selected, based on the clinical and pathological findings.

RESULTS

We identified 17 patients with sarcolemma-specific collagen VI deficiency (SSCD), 6 patients with merosin deficiency, two with reduced alpha-dystroglycan staining, and two with striking lymphocyte infiltration in addition to dystrophic change on muscle pathology. Fourteen in 15 patients with SSCD, were shown to have COL6A1, COL6A2 or COL6A3 mutations by NGS analysis; all showed marked distal hyperlaxity and normal intelligence but the overall severity was less than in previously reported patients from other populations. All six patients with merosin deficiency had mutations in LAMA2. They showed relatively uniform phenotype that were compatible with previous studies, except for higher proportion of mental retardation with epilepsy. With reduced alpha-dystroglycan staining, one patient was found to carry mutations in POMT1 while another patient carried mutations in TRAPPC11. LMNA mutations were found in the two patients with inflammatory change on muscle pathology. They were clinically characterized by neck flexion limitation and early joint contracture, but no cardiac problem had developed yet.

CONCLUSION

Muscle pathology remains helpful in guiding further molecular analyses by direct sequencing of certain genes or by target capture/NGS as a second-tier diagnostic tool, and is crucial for establishing the genotype-phenotype correlation. We also determined the frequencies of the different types of CMD in our cohort which is important for the development of a specific care system for each disease.

Accelerating Gene Discovery by Phenotyping Whole-Genome Sequenced Multi-mutation Strains and Using the Sequence Kernel Association Test (SKAT)

Forward genetic screens represent powerful, unbiased approaches to uncover novel components in any biological process. Such screens suffer from a major bottleneck, however, namely the cloning of corresponding genes causing the phenotypic variation. Reverse genetic screens have been employed as a way to circumvent this issue, but can often be limited in scope. Here we demonstrate an innovative approach to gene discovery. Using C. elegans as a model system, we used a whole-genome sequenced multi-mutation library, from the Million Mutation Project, together with the Sequence Kernel Association Test (SKAT), to rapidly screen for and identify genes associated with a phenotype of interest, namely defects in dye-filling of ciliated sensory neurons. Such anomalies in dye-filling are often associated with the disruption of cilia, organelles which in humans are implicated in sensory physiology (including vision, smell and hearing), development and disease. Beyond identifying several well characterised dye-filling genes, our approach uncovered three genes not previously linked to ciliated sensory neuron development or function. From these putative novel dye-filling genes, we confirmed the involvement of BGNT-1.1 in ciliated sensory neuron function and morphogenesis. BGNT-1.1 functions at the trans-Golgi network of sheath cells (glia) to influence dye-filling and cilium length, in a cell non-autonomous manner. Notably, BGNT-1.1 is the orthologue of human B3GNT1/B4GAT1, a glycosyltransferase associated with Walker-Warburg syndrome (WWS). WWS is a multigenic disorder characterised by muscular dystrophy as well as brain and eye anomalies. Together, our work unveils an effective and innovative approach to gene discovery, and provides the first evidence that B3GNT1-associated Walker-Warburg syndrome may be considered a ciliopathy.

Model organisms are useful tools for uncovering new genes involved in a biological process via genetic screens. Such an approach is powerful, but suffers from drawbacks that can slow down gene discovery. In forward genetics screens, difficult-to-map phenotypes present daunting challenges, and whole-genome coverage can be equally challenging for reverse genetic screens where typically only a single gene’s function is assayed per strain. Here, we show a different approach which includes positive aspects of forward (high-coverage, randomly-induced mutations) and reverse genetics (prior knowledge of gene disruption) to accelerate gene discovery. We paired a whole-genome sequenced multi-mutation C. elegans library with a rare-variant associated test to rapidly identify genes associated with a phenotype of interest: defects in sensory neurons bearing sensory organelles called cilia, via a simple dye-filling assay to probe the form and function of these cells. We found two well characterised dye-filling genes and three genes, not previously linked to ciliated sensory neuron development or function, that were associated with dye-filling defects. We reveal that disruption of one of these (BGNT-1.1), whose human orthologue is associated with Walker-Warburg syndrome, results in abrogated uptake of dye and cilia length defects. We believe that our novel approach is useful for any organism with a small genome that can be quickly sequenced and where many mutant strains can be easily isolated and phenotyped, such as Drosophila and Arabidopsis.

New era in genetics of early-onset muscle disease: Breakthroughs and challenges

Early-onset muscle disease includes three major entities that present generally at or before birth: congenital myopathies, congenital muscular dystrophies and congenital myasthenic syndromes. Almost exclusively there is weakness and hypotonia, although cases manifesting hypertonia are increasingly being recognised. These diseases display a wide phenotypic and genetic heterogeneity, with the uptake of next generation sequencing resulting in an unparalleled extension of the phenotype-genotype correlations and "diagnosis by sequencing" due to unbiased sequencing. Perhaps now more than ever, detailed clinical evaluations are necessary to guide the genetic diagnosis; with arrival at a molecular diagnosis frequently occurring following dialogue between the molecular geneticist, the referring clinician and the pathologist. There is an ever-increasing blurring of the boundaries between the congenital myopathies, dystrophies and myasthenic syndromes. In addition, many novel disease genes have been described and new insights have been gained into skeletal muscle development and function. Despite the advances made, a significant percentage of patients remain without a molecular diagnosis, suggesting that there are many more human disease genes and mechanisms to identify. It is now technically- and clinically-feasible to perform next generation sequencing for severe diseases on a population-wide scale, such that preconception-carrier screening can occur. Newborn screening for selected early-onset muscle diseases is also technically and ethically-achievable, with benefits to the patient and family from early management of these diseases and should also be implemented. The need for world-wide Reference Centres to meticulously curate polymorphisms and mutations within a particular gene is becoming increasingly apparent, particularly for interpretation of variants in the large genes which cause early-onset myopathies: NEB, RYR1 and TTN. Functional validation of candidate disease variants is crucial for accurate interpretation of next generation sequencing and appropriate genetic counseling. Many published "pathogenic" variants are too frequent in control populations and are thus likely rare polymorphisms. Mechanisms need to be put in place to systematically update the classification of variants such that accurate interpretation of variants occurs. In this review, we highlight the recent advances made and the challenges ahead for the molecular diagnosis of early-onset muscle diseases.

Transgenic Rescue of the LARGEmyd Mouse: A LARGE Therapeutic Window?

LARGE is a glycosyltransferase involved in glycosylation of α-dystroglycan (α-DG). Absence of this protein in the LARGE_myd mouse results in α-DG hypoglycosylation, and is associated with central nervous system abnormalities and progressive muscular dystrophy. Up-regulation of LARGE has previously been proposed as a therapy for the secondary dystroglycanopathies: overexpression in cells compensates for defects in multiple dystroglycanopathy genes. Counterintuitively, LARGE overexpression in an FKRP-deficient mouse exacerbates pathology, suggesting that modulation of α-DG glycosylation requires further investigation. Here we demonstrate that transgenic expression of human LARGE (LARGE-LV5) in the LARGE_myd mouse restores α-DG glycosylation (with marked hyperglycosylation in muscle) and that this corrects both the muscle pathology and brain architecture. By quantitative analyses of LARGE transcripts we also here show that levels of transgenic and endogenous LARGE in the brains of transgenic animals are comparable, but that the transgene is markedly overexpressed in heart and particularly skeletal muscle (20–100 fold over endogenous). Our data suggest LARGE overexpression may only be deleterious under a forced regenerative context, such as that resulting from a reduction in FKRP: in the absence of such a defect we show that systemic expression of LARGE can indeed act therapeutically, and that even dramatic LARGE overexpression is well-tolerated in heart and skeletal muscle. Moreover, correction of LARGE_myd brain pathology with only moderate, near-physiological LARGE expression suggests a generous therapeutic window.

Milder forms of muscular dystrophy associated with POMGNT2 mutations

Objective:

To determine the genetic variants in patients with dystroglycanopathy (DGP) and assess the pathogenicity of these variants.

Methods:

A total of 20 patients with DGP were identified by immunohistochemistry or Western blot analysis. Whole-exome sequencing (WES) was performed using patient samples. The pathogenicity of the variants identified was evaluated on the basis of the phenotypic recovery in a knockout (KO) haploid human cell line by transfection with mutated POMGNT2 cDNA and on the basis of the in vitro enzymatic activity of mutated proteins.

Results:

WES identified homozygous and compound heterozygous missense variants in POMGNT2 in 3 patients with the milder limb-girdle muscular dystrophy (LGMD) and intellectual disability without brain malformation. The 2 identified variants were located in the putative glycosyltransferase domain of POMGNT2, which affected its enzymatic activity. Mutated POMGNT2 cDNAs failed to rescue the phenotype of POMGNT2-KO cells.

Conclusions:

Novel variants in POMGNT2 are associated with milder forms of LGMD. The findings of this study expand the clinical and pathologic spectrum of DGP associated with POMGNT2 variants from the severest Walker-Warburg syndrome to the mildest LGMD phenotypes. The simple method to verify pathogenesis of variants may allow researchers to evaluate any variants present in all of the known causative genes and the variants in novel candidate genes to detect DGPs, particularly without using patients' specimens.

Identification of a novel MKS locus defined by TMEM107 mutation

Meckel-Gruber syndrome (MKS) is a perinatally lethal disorder characterized by the triad of occipital encephalocele, polydactyly and polycystic kidneys. Typical of other disorders related to defective primary cilium (ciliopathies), MKS is genetically heterogeneous with mutations in a dozen genes to date known to cause the disease. In an ongoing effort to characterize MKS clinically and genetically, we implemented a gene panel and next-generation sequencing approach to identify the causal mutation in 25 MKS families. Of the three families that did not harbor an identifiable causal mutation by this approach, two mapped to a novel disease locus in which whole-exome sequencing revealed the likely causal mutation as a homozygous splicing variant in TMEM107, which we confirm leads to aberrant splicing and nonsense-mediated decay. TMEM107 had been independently identified in two mouse models as a cilia-related protein and mutant mice display typical ciliopathy phenotypes. Our analysis of patient fibroblasts shows marked ciliogenesis defect with an accompanying perturbation of sonic hedgehog signaling, highly concordant with the cellular phenotype in Tmem107 mutants. This study shows that known MKS loci account for the overwhelming majority of MKS cases but additional loci exist including MKS13 caused by TMEM107 mutation.

Glycobiology of α-dystroglycan and muscular dystrophy

Most proteins are modified by glycans, which can modulate the biological properties and functions of glycoproteins. The major glycans can be classified into N-glycans and O-glycans according to their glycan-peptide linkage. This review will provide an overview of the O-mannosyl glycans, one subtype of O-glycans. Originally, O-mannosyl glycan was only known to be present on a limited number of glycoproteins, especially α-dystroglycan (α-DG). However, once a clear relationship was established between O-mannosyl glycan and the pathological mechanisms of some congenital muscular dystrophies in humans, research on the biochemistry and pathology of O-mannosyl glycans has been expanding. Because α-DG glycosylation is defective in congenital muscular dystrophies, which also feature abnormal neuronal migration, these disorders are collectively called α-dystroglycanopathies. In this article, I will describe the structure, biosynthesis and pathology of O-mannosyl glycans.

B4GAT1 is the priming enzyme for the LARGE-dependent functional glycosylation of α-dystroglycan

Recent studies demonstrated that mutations in B3GNT1, an enzyme proposed to be involved in poly-N-acetyllactosamine synthesis, were causal for congenital muscular dystrophy with hypoglycosylation of α-dystroglycan (secondary dystroglycanopathies). Since defects in the O-mannosylation protein glycosylation pathway are primarily responsible for dystroglycanopathies and with no established O-mannose initiated structures containing a β3 linked GlcNAc known, we biochemically interrogated this human enzyme. Here we report this enzyme is not a β-1,3-N-acetylglucosaminyltransferase with catalytic activity towards β-galactose but rather a β-1,4-glucuronyltransferase, designated B4GAT1, towards both α- and β-anomers of xylose. The dual-activity LARGE enzyme is capable of extending products of B4GAT1 and we provide experimental evidence that B4GAT1 is the priming enzyme for LARGE. Our results further define the functional O-mannosylated glycan structure and indicate that B4GAT1 is involved in the initiation of the LARGE-dependent repeating disaccharide that is necessary for extracellular matrix protein binding to O-mannosylated α-dystroglycan that is lacking in secondary dystroglycanopathies.

The glucuronyltransferase B4GAT1 is required for initiation of LARGE-mediated α-dystroglycan functional glycosylation

Dystroglycan is a cell membrane receptor that organizes the basement membrane by binding ligands in the extracellular matrix. Proper glycosylation of the α-dystroglycan (α-DG) subunit is essential for these activities, and lack thereof results in neuromuscular disease. Currently, neither the glycan synthesis pathway nor the roles of many known or putative glycosyltransferases that are essential for this process are well understood. Here we show that FKRP, FKTN, TMEM5 and B4GAT1 (formerly known as B3GNT1) localize to the Golgi and contribute to the O-mannosyl post-phosphorylation modification of α-DG. Moreover, we assigned B4GAT1 a function as a xylose β1,4-glucuronyltransferase. Nuclear magnetic resonance studies confirmed that a glucuronic acid β1,4-xylose disaccharide synthesized by B4GAT1 acts as an acceptor primer that can be elongated by LARGE with the ligand-binding heteropolysaccharide. Our findings greatly broaden the understanding of α-DG glycosylation and provide mechanistic insight into why mutations in B4GAT1 disrupt dystroglycan function and cause disease.

DOI:http://dx.doi.org/10.7554/eLife.03941.001

Dystroglycan is a protein that is critical for the proper function of many tissues, especially muscles and brain. Dystroglycan helps to connect the structural network inside the cell with the matrix outside of the cell. The extracellular matrix fills the space between the cells to serve as a scaffold and hold cells together within a tissue. It is well established that the interaction of cells with their extracellular environments is important for structuring tissues, as well as for helping cells to specialize and migrate. These interactions also play a role in the progression of cancer.

As is the case for many proteins, dystroglycan must be modified with particular sugar molecules in order to work correctly. Enzymes called glycosyltransferases are responsible for sequentially assembling a complex array of sugar molecules on dystroglycan. This modification is essential for making dystroglycan ‘sticky’, so it can bind to the components of the extracellular matrix. If sugar molecules are added incorrectly, dystroglycan loses its ability to bind to these components. This causes congenital muscular dystrophies, a group of diseases that are characterized by a progressive loss of muscle function.

Willer et al. use a wide range of experimental techniques to investigate the types of sugar molecules added to dystroglycan, the overall structure of the resulting ‘sticky’ complex and the mechanism whereby it is built. This reveals that a glycosyltransferase known as B3GNT1 is one of the enzymes responsible for adding a sugar molecule to the complex. This enzyme was first described in the literature over a decade ago, and the name B3GNT1 was assigned, according to a code, to reflect the sugar molecule it was thought to transfer to proteins. However, Willer et al. (and independently, Praissman et al.) find that this enzyme actually attaches a different sugar modification to dystroglycan, and so should therefore be called B4GAT1 instead.

Willer et al. find that the sugar molecule added by the B4GAT1 enzyme acts as a platform for the assembly of a much larger sugar polymer that cells use to anchor themselves within a tissue. Some viruses–including Lassa virus, which causes severe fever and bleeding–also use the ‘sticky’ sugar modification of dystroglycan to bind to and invade cells, causing disease in humans. Understanding the structure of this complex, and how these sugar modifications are added to dystroglycan, could therefore help to develop treatments for a wide range of diseases like progressive muscle weakening and viral infections.

DOI:http://dx.doi.org/10.7554/eLife.03941.002