Structure-function analysis of human protein O-linked mannose beta1,2-N-acetylglucosaminyltransferase 1, POMGnT1

Variable Ophthalmologic Phenotypes Associated with Biallelic Loss-of-Function Variants in POMGNT1

O-mannosylation is a post-translational modification required for the proper function of various proteins and critical for development and growth. POMGNT1 encodes the enzyme O-linked-mannose β-1,2-N-acetylglucosaminyltransferase 1, which catalyzes the second step in the synthesis of α-dystroglycan O-mannosyl glycans. Among POMGNT1-related α-dystroglycanopathies, muscle-eye-brain (MEB) disease presents with congenital muscular dystrophy, structural brain abnormalities, and retinal dystrophy. Defects in protein O-mannosylation due to biallelic loss-of-function POMGNT1 mutations produce disturbances in assembling and organizing the basal membrane in the neuroretinal system, involving both the central and peripheral nervous systems. In the retina, POMGNT1 is expressed in photoreceptors and is localized near the photoreceptor cilium basal body, a structure critical for protein transport. Recent studies have reported an isolated degenerative ocular phenotype without any involvement of muscular or neuronal tissues. Here, we report on a family with three siblings affected by an apparently isolated clinically variable retinal disease and sharing biallelic inactivating POMGNT1 variants. Notably, the rod-cone dystrophy phenotype in the three siblings varied significantly in onset, presentation, and severity. These findings provide further evidence of the clinical variability associated with defective POMGNT1 function.

The homozygous pathogenic variant of the POMGNT1 gene identified using whole-exome sequencing in Iranian family with congenital hydrocephalus

Background

Hydrocephalus is one of the most common pathophysiological disabilities with a high mortality rate, which occurs both congenitally and acquired. It is estimated that genetic components are the etiology for up to 40% of hydrocephalus cases; however, causal mutations identified until now could only explain approximately 20% of congenital hydrocephalus (CH) patients, and most potential hydrocephalus-associated genes have yet to be determined. This study sought to find causal variations in a consanguineous family with four affected children diagnosed with hydrocephalus.

Material and methods

In this study, we evaluated twenty-five members of an extended family consisting of a nuclear family with four affected children resulting from a consanguineous couple and eighteen of their relatives, including one hydrocephalus case. The mother of this family was experiencing her 15th week of pregnancy, and cytogenetic evaluation was performed using amniocentesis to identify fetal chromosomal abnormalities. We conducted whole-exome sequencing (WES) on the genomic DNA of the proband to detect the CH-causing variants, followed by confirmation and segregation analysis of the detected variant in the proband, fetus, and family members through Sanger sequencing.

Results

Following the bioinformatic analysis and data filtering, we found a homozygous variant [NM_001243766.2:c.74G>A:p.W25X] within the protein O-mannose beta-1,2-N-acetylglucosaminyltransferase 1 (POMGNT1) gene confirmed by Sanger sequencing in the proband and segregated with the hydrocephalus in the family. The variant was described as pathogenic and regarded as a nonsense-mediated mRNA decay (NMD) due to the premature stop codon, which results in a truncated protein.

Conclusion

The results of the current study broadened the mutational gene spectrum of CH and our knowledge of the hydrocephalus etiology by introducing a novel homozygous variant within the POMGNT1 gene, which had never been previously reported solitary in these patients.

Uniparental disomy for chromosome 1 with POMGNT1 splice-site variant causes muscle-eye-brain disease

POMGNT1, encoding protein O-mannose beta-1,2-N-acetylglucosaminyltransferase 1, is one of the genes responsible for dystroglycanopathy (DGP), which includes multiple phenotypes such as muscle-eye-brain disease (MEB), congenital muscular dystrophy with intellectual disability, and limb-girdle muscular dystrophy Here, we report a case of MEB that is the result of a homozygous variant of POMGNT1 that is revealed through uniparental disomy (UPD). An 8-month-old boy was admitted with mental and motor retardation, hypotonia, esotropia, early onset severe myopia, and structural brain abnormalities. A panel testing of genetic myopathy-related genes was used to identify a homozygous c.636C>T (p.Phe212Phe) variant in exon 7 of POMGNT1 in the patient, a heterozygous c.636C>T variant in the father, and the wild type in the mother. Quantitative polymerase chain reaction (q-PCR) revealed no abnormal copy numbers in exon 7. Trio-based whole-exome sequencing (trio-WES) revealed a possible paternal UPD on chromosome 1 of the patient. Chromosomal microarray analysis (CMA) revealed a 120,451 kb loss of heterozygosity (LOH) on 1p36.33-p11.2, encompassing POMGNT1, and a 99,319 kb loss of heterozygosity on 1q21.2-q44, which indicated UPD. Moreover, RNA sequencing (RNA-seq) verified that the c.636C>T variant was a splice-site variant, leading to skipping of exon 7 (p.Asp179Valfs*23). In conclusion, to the best of our knowledge, we present the first case of MEB caused by UPD, providing valuable insights into the genetic mechanisms underlying this condition.

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

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

Molecular Mechanisms and Risk Factors for the Pathogenesis of Hydrocephalus

Hydrocephalus is a neurological condition due to the aberrant circulation and/or obstruction of cerebrospinal fluid (CSF) flow with consequent enlargement of cerebral ventricular cavities. However, it is noticed that a lot of patients may still go through symptomatic progression despite standard shunting procedures, suggesting that hydrocephalus is far more complicated than a simple CSF circulative/obstructive disorder. Growing evidence indicates that genetic factors play a fundamental role in the pathogenesis of some hydrocephalus. Although the genetic research of hydrocephalus in humans is limited, many genetic loci of hydrocephalus have been defined in animal models. In general, the molecular abnormalities involved in the pathogenesis of hydrocephalus include brain development and ependymal cell dysfunction, apoptosis, inflammation, free radical generation, blood flow, and cerebral metabolism. Moreover, recent studies have indicated that the molecular abnormalities relevant to aberrant cerebral glymphatic drainage turn into an attractive subject in the CSF circulation disorder. Furthermore, the prevalent risk factors could facilitate the development of hydrocephalus. In this review, we elicited some possible fundamental molecular mechanisms and facilitating risk factors involved in the pathogenesis of hydrocephalus, and aimed to widen the diagnosis and therapeutic strategies for hydrocephalus management. Such knowledge could be used to improve patient care in different ways, such as early precise diagnosis and effective therapeutic regimens.

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

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

FAM3B/PANDER-Like Carbohydrate-Binding Domain in a Glycosyltransferase, POMGNT1

Protein O-mannose β1,2-N-acetylglucosaminyltransferase 1 (POMGNT1) is one of the gene products responsible for α-dystroglycanopathy, which is a type of congenital muscular dystrophy caused by O-mannosyl glycan defects. The originally identified function of POMGNT1 was as a glycosyltransferase that catalyzes the formation of the GlcNAcβ1-2Man linkage of O-mannosyl glycan, but the enzyme function is not essential for α-dystroglycanopathy pathogenesis. Our recent study revealed that the stem domain of POMGNT1 has a carbohydrate-binding ability, which recognizes the GalNAcβ1-3GlcNAc structure. This carbohydrate-binding activity is required for the formation of the ribitol phosphate (RboP)-3GalNAcβ1-3GlcNAc structure by fukutin. This protocol describes methods to assess the carbohydrate-binding activity of the POMGNT1 stem domain.

PomGnT1 enhances temozolomide resistance by activating epithelial-mesenchymal transition signaling in glioblastoma

Temozolomide (TMZ) is commonly used in glioblastoma (GBM) chemotherapy. However, a great challenge for TMZ treatment is the rapid development of resistance and subsequent tumor recurrence and poor outcome. In the present study we established TMZ-resistant GBM cells (U87-TR and U251-TR) and found that the expression of PomGnT1 was significantly upregulated in TMZ-resistant GBM cells compared with the TMZ-sensitive counterparts. Furthermore, overexpression of PomGnT1 in U87-MG and U251-MG cells led to increased IC50 values for TMZ and reduced apoptosis of cells. Knockdown of PomGnT1 in both U87-TR and U251-TR cells led to decreased IC50 values for TMZ and enhanced apoptosis. Biochemical analysis revealed that PomGnT1 regulates the expression of factors in epithelial-mesenchymal transition signaling including TCF8, vimentin, β-catenin and Slug in GBM cells. These findings demonstrate that PomGnT1 might be a new focus of GBM research for treatment of recurrent TMZ-resistant GBM.

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.

Glycosylation with ribitol-phosphate in mammals: New insights into the O-mannosyl glycan

BACKGROUND

O-mannosyl glycans have been found in a limited number of glycoproteins of the brain, nerves, and skeletal muscles, particularly in α-dystroglycan (α-DG). Defects in O-mannosyl glycan on α-DG are the primary cause of a group of congenital muscular dystrophies, which are collectively termed α-dystroglycanopathy. Recent studies have revealed various O-mannosyl glycan structures, which can be classified as core M1, core M2, and core M3 glycans. Although many dystroglycanopathy genes are involved in core M3 processing, the structure and biosynthesis of core M3 glycan remains only partially understood.

SCOPE OF REVIEW

This review presents recent findings about the structure, biosynthesis, and pathology of O-mannosyl glycans.

MAJOR CONCLUSIONS

Recent studies have revealed that the entire structure of core M3 glycan, including ribitol-5-phosphate, is a novel structure in mammals; its unique biosynthetic pathway has been elucidated by the identification of new causative genes for α-dystroglycanopathies and their functions.

GENERAL SIGNIFICANCE

O-mannosyl glycan has a novel, unique structure that is important for the maintenance of brain and muscle functions. These findings have opened up a new field in glycoscience. These studies will further contribute to the understanding of the pathomechanism of α-dystroglycanopathy and the development of glycotherapeutics. This article is part of a Special Issue entitled Neuro-glycoscience, edited by Kenji Kadomatsu and Hiroshi Kitagawa.

Carbohydrate-binding domain of the POMGnT1 stem region modulates O-mannosylation sites of α-dystroglycan

The dystrophin glycoprotein complex, which connects the cell membrane to the basement membrane, is essential for a variety of biological events, including maintenance of muscle integrity. An O-mannose-type GalNAc-β1,3-GlcNAc-β1,4-(phosphate-6)-Man structure of α-dystroglycan (α-DG), a subunit of the complex that is anchored to the cell membrane, interacts directly with laminin in the basement membrane. Reduced glycosylation of α-DG is linked to some types of inherited muscular dystrophy; consistent with this relationship, many disease-related mutations have been detected in genes involved in O-mannosyl glycan synthesis. Defects in protein O-linked mannose β1,2-N-acetylglucosaminyltransferase 1 (POMGnT1), a glycosyltransferase that participates in the formation of GlcNAc-β1,2-Man glycan, are causally related to muscle-eye-brain disease (MEB), a congenital muscular dystrophy, although the role of POMGnT1 in postphosphoryl modification of GalNAc-β1,3-GlcNAc-β1,4-(phosphate-6)-Man glycan remains elusive. Our crystal structures of POMGnT1 agreed with our previous results showing that the catalytic domain recognizes substrate O-mannosylated proteins via hydrophobic interactions with little sequence specificity. Unexpectedly, we found that the stem domain recognizes the β-linked GlcNAc of O-mannosyl glycan, an enzymatic product of POMGnT1. This interaction may recruit POMGnT1 to a specific site of α-DG to promote GlcNAc-β1,2-Man clustering and also may recruit other enzymes that interact with POMGnT1, e.g., fukutin, which is required for further modification of the GalNAc-β1,3-GlcNAc-β1,4-(phosphate-6)-Man glycan. On the basis of our findings, we propose a mechanism for the deficiency in postphosphoryl modification of the glycan observed in POMGnT1-KO mice and MEB patients.

The Genetic Basis of Hydrocephalus

Studies of syndromic hydrocephalus have led to the identification of >100 causative genes. Even though this work has illuminated numerous pathways associated with hydrocephalus, it has also highlighted the fact that the genetics underlying this phenotype are more complex than anticipated originally. Mendelian forms of hydrocephalus account for a small fraction of the genetic burden, with clear evidence of background-dependent effects of alleles on penetrance and expressivity of driver mutations in key developmental and homeostatic pathways. Here, we synthesize the currently implicated genes and inheritance paradigms underlying hydrocephalus, grouping causal loci into functional modules that affect discrete, albeit partially overlapping, cellular processes. These in turn have the potential to both inform pathomechanism and assist in the rational molecular classification of a clinically heterogeneous phenotype. Finally, we discuss conceptual methods that can lead to enhanced gene identification and dissection of disease basis, knowledge that will potentially form a foundation for the design of future therapeutics.

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.

The Classification, Natural History and Treatment of the Limb Girdle Muscular Dystrophies

Over sixty years ago John Walton and Frederick Nattrass defined limb girdle muscular dystrophy (LGMD) as a separate entity from the X-linked dystrophinopathies such as Duchenne and Becker muscular dystrophies. LGMD is a highly heterogeneous group of very rare neuromuscular disorders whose common factor is their autosomal inheritance. Sixty years later, with the development of increasingly advanced molecular genetic investigations, a more precise classification and understanding of the pathogenesis is possible.To date, over 30 distinct subtypes of LGMD have been identified, most of them inherited in an autosomal recessive fashion. There are significant differences in the frequency of subtypes of LGMD between different ethnic populations, providing evidence of founder mutations. Clinically there is phenotypic heterogeneity between subtypes of LGMD with varying severity and age of onset of symptoms. The first natural history studies into subtypes of LGMD are in process, but large scale longitudinal data have been lacking due to the rare nature of these diseases. Following natural history data collection, the next challenge is to develop more effective, disease specific treatments. Current management is focussed on symptomatic and supportive treatments. Advances in the application of new omics technologies and the generation of large-scale biomedical data will help to better understand disease mechanisms in LGMD and should ultimately help to accelerate the development of novel and more effective therapeutic approaches.

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.

Novel POMGNT1 point mutations and intragenic rearrangements associated with muscle-eye-brain disease

Congenital muscular dystrophies due to defects in genes encoding proteins involved in α-dystroglycan (α-DG) glycosylation are a heterogeneous group of muscle disorders variably associated with central nervous system and eye abnormalities. One of the more severe is muscle-eye-brain disease (MEB). Mutations in genes coding for proven or putative glycosyltransferases (POMT1, POMT2, POMGnT1, fukutin, FKRP, and LARGE), the DPM3 gene encoding a DOL-P-Man synthase subunit, and the DAG1 gene encoding α-dystroglycan, have been associated with altered α-DG glycosylation.

We report new POMGnT1 mutations and evaluate protein expression in 3 patients and 2 foetuses with variably severe MEB features. We identify two new point mutations (c.643 C > T, c.1863delC), one new intragenic rearrangement (deletion of exons 2–8), and a new intron retention (between exons 21 and 22) resulting from a known point mutation c.1895 + 1 G > T. Our study provides further evidence that rearrangements of the POMGnT1 gene are relatively common. Importantly, if heterozygous, they can be missed on standard genomic DNA sequencing. POMGNT1 protein analysis in 3 patients showed that the severity of the phenotype does not correlate with protein expression. Cerebral MRI is important for identifying MEB and α-dystroglycanopathy phenotypes in children and foetuses, and hence for directing the genetic analysis.

Biochemical correlation of activity of the α-dystroglycan-modifying glycosyltransferase POMGnT1 with mutations in muscle-eye-brain disease

Congenital muscular dystrophies have a broad spectrum of genotypes and phenotypes and there is a need for a better biochemical understanding of this group of diseases in order to aid diagnosis and treatment. Several mutations resulting in these diseases cause reduced O-mannosyl glycosylation of glycoproteins, including α-dystroglycan. The enzyme POMGnT1 (protein-O-mannose N-acetylglucosaminyltransferase 1; EC 2.4.1.-) catalyses the transfer of N-acetylglucosamine to O-linked mannose of α-dystroglycan. In the present paper we describe the biochemical characterization of 14 clinical mutants of the glycosyltransferase POMGnT1, which have been linked to muscle-eye-brain disease or similar conditions. Truncated mutant variants of the human enzyme (recombinant POMGnT1) were expressed in Escherichia coli and screened for catalytic activity. We find that three mutants show some activity towards mannosylated peptide substrates mimicking α-dystroglycan; the residues affected by these mutants are predicted by homology modelling to be on the periphery of the POMGnT1 surface. Only in part does the location of a previously described mutated residue on the periphery of the protein structure correlate with a less severe disease mutant.

Protein O-mannosylation in animal development and physiology: from human disorders to Drosophila phenotypes

Protein O-mannosylation has a profound effect on the development and physiology of mammalian organisms. Mutations in genes affecting O-mannosyl glycan biosynthesis result in congenital muscular dystrophies. The main pathological mechanism triggered by O-mannosylation defects is a compromised interaction of cells with the extracellular matrix due to abnormal glycosylation of alpha-dystroglycan. Hypoglycosylation of alpha-dystroglycan impairs its ligand-binding activity and results in muscle degeneration and failure of neuronal migration. Recent experiments revealed the existence of compensatory mechanisms that could ameliorate defects of O-mannosylation. However, these mechanisms remain poorly understood. O-mannosylation and dystroglycan pathway genes show remarkable evolutionary conservation in a wide range of metazoans. Mutations and downregulation of these genes in zebrafish and Drosophila result in muscle defects and degeneration, also causing neurological phenotypes, which suggests that O-mannosylation has similar functions in mammals and lower animals. Thus, future studies in genetically tractable model organisms, such as zebrafish and Drosophila, should help to reveal molecular and genetic mechanisms of mammalian O-mannosylation and its role in the regulation of dystroglycan function.

Reduced proliferative activity of primary POMGnT1-null myoblasts in vitro

Protein O-linked mannose beta1,2-N-acetylglucosaminyltransferase 1 (POMGnT1) is an enzyme that transfers N-acetylglucosamine to O-mannose of glycoproteins. Mutations of the POMGnT1 gene cause muscle-eye-brain (MEB) disease. To obtain a better understanding of the pathogenesis of MEB disease, we mutated the POMGnT1 gene in mice using a targeting technique. The mutant muscle showed aberrant glycosylation of alpha-DG, and alpha-DG from mutant muscle failed to bind laminin in a binding assay. POMGnT1(-/-) muscle showed minimal pathological changes with very low-serum creatine kinase levels, and had normally formed muscle basal lamina, but showed reduced muscle mass, reduced numbers of muscle fibers, and impaired muscle regeneration. Importantly, POMGnT1(-/-) satellite cells proliferated slowly, but efficiently differentiated into multinuclear myotubes in vitro. Transfer of a retrovirus vector-mediated POMGnT1 gene into POMGnT1(-/-) myoblasts completely restored the glycosylation of alpha-DG, but proliferation of the cells was not improved. Our results suggest that proper glycosylation of alpha-DG is important for maintenance of the proliferative activity of satellite cells in vivo.

Muscular dystrophies due to glycosylation defects

In the last few years, muscular dystrophies due to reduced glycosylation of alpha-dystroglycan (ADG) have emerged as a common group of conditions, now referred to as dystroglycanopathies. Mutations in six genes (POMT1, POMT2, POMGnT1, Fukutin, FKRP and LARGE) have so far been identified in patients with a dystroglycanopathy. Allelic mutations in each of these genes can result in a wide spectrum of clinical conditions, ranging from severe congenital onset with associated structural brain malformations (Walker Warburg syndrome; muscle-eye-brain disease; Fukuyama muscular dystrophy; congenital muscular dystrophy type 1D) to a relatively milder congenital variant with no brain involvement (congenital muscular dystrophy type 1C), and to limb-girdle muscular dystrophy (LGMD) type 2 variants with onset in childhood or adult life (LGMD2I, LGMD2L, and LGMD2N). ADG is a peripheral membrane protein that undergoes multiple and complex glycosylation steps to regulate its ability to effectively interact with extracellular matrix proteins, such as laminin, agrin, and perlecan. Although the precise composition of the glycans present on ADG are not known, it has been demonstrated that the forced overexpression of LARGE, or its paralog LARGE2, is capable of increasing the glycosylation of ADG in normal cells. In addition, its overexpression is capable of restoring dystroglycan glycosylation and laminin binding properties in primary cell cultures of patients affected by different genetically defined dystroglycanopathy variants. These observations suggest that there could be a role for therapeutic strategies to overcome the glycosylation defect in these conditions via the overexpression of LARGE.

Novel POMGnT1 mutations define broader phenotypic spectrum of muscle-eye-brain disease

Muscle-eye-brain disease (MEB, OMIM 253280) is an autosomal recessive disorder characterized by a distinct triad of congenital muscular dystrophy, structural eye abnormalities, and cobblestone lissencephaly. Clinically, MEB patients present with early onset muscular hypotonia, severely compromised motor development, and mental retardation. Magnetic resonance imaging reveals a lissencephaly type II with hypoplasia of the brainstem and cerebellum. MEB is associated with mutations in the gene for protein O-mannose beta-1,2-N-acetylglucosaminyltransferase (POMGnT1, OMIM 606822). In this paper, we report the clinical findings of nine MEB patients from eight families. Eight of the nine patients presented typical features of MEB. However, a broad phenotypic variability was observed, ranging from two patients with severe autistic features to another patient with an unusually mild phenotype, initially diagnosed as congenital muscular dystrophy. Furthermore, severe hydrocephalus was reported in two families during a previous pregnancy, emphasizing the phenotypic overlap with Walker-Warburg syndrome. In addition to three previously reported mutations, we identified six novel POMGnT1 mutations (one missense, five truncating) in the present patient cohort. Our data suggest mutational hotspots within the minimal catalytic domain at arginine residue 442 (exon 16) and in intron 17. It is interesting to note that all mutations analyzed so far result in a complete loss of enzyme activity. Therefore, we conclude that the type and position of the POMGnT1 mutations are not of predictive value for the clinical severity. This supports the notion that additional environmental and/or genetic factors may contribute to the observed broad spectrum of POMGnT1-associated phenotypes.

Carriers and patients with muscle–eye–brain disease can be rapidly diagnosed by enzymatic analysis of fibroblasts and lymphoblasts

We report a new fibroblast and lymphoblast based protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 enzymatic assay, which allows rapid and accurate diagnosis of carriers and patients with muscle-eye-brain type of congenital muscular dystrophy. Seven patients with genetically confirmed muscle-eye-brain disease were assayed for protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 enzyme activity. In three patients and their heterozygous parents, the assays were done on EBV-transformed lymphoblasts, in another three patients they were done on cultured fibroblasts and in the last patient on both fibroblasts and lymphoblasts. Cultured fibroblasts and lymphoblasts from the muscle-eye-brain patients showed a highly significant decrease in protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 activity relative to controls. The residual protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 level in fibroblasts (average 0.11 nmoles/h per mg) was about 13% of normal controls. The ratio of protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 activity to the activity of a glycosyltransferase control (N-acetylglucosaminyltransferase 1; GnT1) in fibroblasts was on average 0.006 in muscle-eye-brain patients and 0.045 in controls. The average residual protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 level in lymphoblasts was 15% of normal controls. The average ratio of protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1/GnT1 activity was 0.007 in muscle-eye-brain patients, 0.026 in heterozygous carriers and 0.046 in normal controls. Assay of protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 activity in fibroblasts and lymphoblasts from muscle-eye-brain carriers and patients provides a rapid and relatively simple diagnostic test for this disease and could be used as a screening test in carriers and patients with complex congenital muscular dystrophy.

Molecular etiopathogenesis of limb girdle muscular and congenital muscular dystrophies: boundaries and contiguities

The muscular dystrophies are a heterogeneous group of inherited disorders characterized by progressive muscle wasting and weakness. These disorders present a large clinical variability regarding age of onset, patterns of skeletal muscle involvement, heart damage, rate of progression and mode of inheritance. Difficulties in classification are often caused by the relatively common sporadic occurrence of autosomal recessive forms as well as by intrafamilial clinical variability. Furthermore recent discoveries, particularly regarding the proteins linking the sarcolemma to components of the extracellular matrix, have restricted the gap existing between limb girdle (LGMD) and congenital muscular dystrophies (CMD). Therefore a renewed definition of boundaries between these two groups is required. Molecular genetic studies have demonstrated different causative mutations in the genes encoding a disparate collection of proteins involved in all aspects of muscle cell biology. These novel skeletal muscle genes encode highly diverse proteins with different localization within or at the surface of the skeletal muscle fibre, such as the sarcolemmal muscle membrane (dystrophin, sarcoglycans, dysferlin, caveolin-3), the extracellular matrix (alpha2 laminin, collagen VI), the sarcomere (telethonin, myotilin, titin, nebulin and ZASP), the muscle cytosol (calpain-3, TRIM32), the nucleus (emerin, lamin A/C) and the glycosilation pathway enzymes (fukutin and fukutin related proteins). The accumulating knowledge about the role of these different proteins in muscle pathology has led to a profound change in the original phenotype-based classification and shed new light on the molecular pathogenesis of these disorders.