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

The genetic basis of hydrocephalus: genes, pathways, mechanisms, and global impact

Hydrocephalus (HC) is a heterogenous disease characterized by alterations in cerebrospinal fluid (CSF) dynamics that may cause increased intracranial pressure. HC is a component of a wide array of genetic syndromes as well as a secondary consequence of brain injury (intraventricular hemorrhage (IVH), infection, etc.) that can present across the age spectrum, highlighting the phenotypic heterogeneity of the disease. Surgical treatments include ventricular shunting and endoscopic third ventriculostomy with or without choroid plexus cauterization, both of which are prone to failure, and no effective pharmacologic treatments for HC have been developed. Thus, there is an urgent need to understand the genetic architecture and molecular pathogenesis of HC. Without this knowledge, the development of preventive, diagnostic, and therapeutic measures is impeded. However, the genetics of HC is extraordinarily complex, based on studies of varying size, scope, and rigor. This review serves to provide a comprehensive overview of genes, pathways, mechanisms, and global impact of genetics contributing to all etiologies of HC in humans.

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

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

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.

Congenital muscular dystrophy-dystroglycanopathy, type A, featuring bilateral retinal dysplasia and vertical angle kappa

Muscular dystrophy-dystroglycanopathy type A (MDDGA3), one of a group of diseases collectively known as congenital muscular dystrophies, is an alpha-dystroglycanopathy with characteristic brain and ocular abnormalities. We report the case of a 9-month-old boy with developmental delay whose family sought evaluation for esotropia. Subsequent examination, imaging, and testing revealed significant motor and cognitive delay, marked weakness with appendicular spasticity, and a diffuse brain malformation. In addition, the patient had poor visual acuity, nystagmus, optic nerve hypoplasia, bilateral retinal dysplasia and retinal dragging with a large vertical angle kappa, and an avascular peripheral retina. Genetic testing revealed two known heterozygous mutations in the POMGnT1 gene confirming MDDGA3. He was treated with botulinum toxin injections for his strabismus and continues to be followed, with planned laser ablation of the peripheral avascular retina.

Parent-Child Interaction Therapy in a Case of Global Developmental Delay and Leukoencephalopathy

Parent-child interaction therapy (PCIT) is an evidence-based, behavioral dyadic treatment for caregivers and their children aged 2-7 years old with emotional and behavioral disorders. Here we present a treatment course of a 3-years-old girl with leukoencephalopathy, dysgenesis of the brainstem, and associated global developmental delay who was diagnosed with muscular dystrophy after PCIT completion. At the beginning of PCIT she had the developmental level of an 18 months old with language skills of a 12-18 months old; both her vocabulary and verbal expression were very limited. She had slow, unco-ordinated gait with limited fine motor skills. She was referred to Psychiatry with concerns regarding disruptive behaviors including severe self-injury. PCIT was started with a focus on PRIDE skills (Praise, Reflection, Imitation, behavioral Description and Enjoyment); particularly behavioral description and reflection with simple developmentally appropriate labeled praise. Modifications to treatment included using non-verbal actions (e.g., "high fives" as praises), sign language and using only one-step basic commands, which greatly improved compliance. In a matter of weeks, the patient demonstrated remarkable improvement in her disruptive behavior as evidenced by parent/daycare report and clinical observation. Surprisingly her vocabulary more than doubled and her ability of self-expression also increased; she was able to point to things and ask for them. This clinical experience suggests that PCIT principles are effective treatment interventions for other clinical presentations outside of the usual inclusion criteria. Implementation of targeted PCIT interventions greatly benefited the development of language skills and communication in a young child with global developmental delay.

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.

Expression pattern in retinal photoreceptors of POMGnT1, a protein involved in muscle-eye-brain disease

PURPOSE

The POMGNT1 gene, encoding protein O-linked-mannose β-1,2-N-acetylglucosaminyltransferase 1, is associated with muscle-eye-brain disease (MEB) and other dystroglycanopathies. This gene's lack of function or expression causes hypoglycosylation of α-dystroglycan (α-DG) in the muscle and the central nervous system, including the brain and the retina. The ocular symptoms of patients with MEB include retinal degeneration and detachment, glaucoma, and abnormal electroretinogram. Nevertheless, the POMGnT1 expression pattern in the healthy mammalian retina has not yet been investigated. In this work, we address the expression of the POMGNT1 gene in the healthy retina of a variety of mammals and characterize the distribution pattern of this gene in the adult mouse retina and the 661W photoreceptor cell line.

METHODS

Using reverse transcription (RT)-PCR and immunoblotting, we studied POMGNT1 expression at the mRNA and protein levels in various mammalian species, from rodents to humans. Immunofluorescence confocal microscopy analyses were performed to characterize the distribution profile of its protein product in mouse retinal sections and in 661W cultured cells. The intranuclear distribution of POMT1 and POMT2, the two enzymes preceding POMGnT1 in the α-DG O-mannosyl glycosylation pathway, was also analyzed.

RESULTS

POMGNT1 mRNA and its encoded protein were expressed in the neural retina of all mammals studied. POMGnT1 was located in the cytoplasmic fraction in the mouse retina and concentrated in the myoid portion of the photoreceptor inner segments, where the protein colocalized with GM130, a Golgi complex marker. The presence of POMGnT1 in the Golgi complex was also evident in 661W cells. However, and in contrast to retinal tissue, POMGnT1 additionally accumulated in the nucleus of the 661W photoreceptors. Colocalization was found within this organelle between POMGnT1 and POMT1/2, the latter associated with euchromatic regions of the nucleus.

CONCLUSIONS

Our results indicate that POMGnT1 participates not only in the synthesis of O-mannosyl glycans added to α-DG in the Golgi complex but also in the glycosylation of other yet-to-be-identified proteins in the nucleus of mouse photoreceptors.

Prenatal diagnosis of congenital myopathies and muscular dystrophies

Congenital myopathies and muscular dystrophies constitute a genetically and phenotypically heterogeneous group of rare inherited diseases characterized by muscle weakness and atrophy, motor delay and respiratory insufficiency. To date, curative care is not available for these diseases, which may severely affect both life-span and quality of life. We discuss prenatal diagnosis and genetic counseling for families at risk, as well as diagnostic possibilities in sporadic cases.

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.

Generalization of Rare Variant Association Tests for Longitudinal Family Studies

Given the functional relevance of many rare variants, their identification is frequently critical for dissecting disease etiology. Functional variants are likely to be aggregated in family studies enriched with affected members, and this aggregation increases the statistical power to detect rare variants associated with a trait of interest. Longitudinal family studies provide additional information for identifying genetic and environmental factors associated with disease over time. However, methods to analyze rare variants in longitudinal family data remain fairly limited. These methods should be capable of accounting for different sources of correlations and handling large amounts of sequencing data efficiently. To identify rare variants associated with a phenotype in longitudinal family studies, we extended pedigree-based burden (BT) and kernel (KS) association tests to genetic longitudinal studies. Generalized estimating equation (GEE) approaches were used to generalize the pedigree-based BT and KS to multiple correlated phenotypes under the generalized linear model framework, adjusting for fixed effects of confounding factors. These tests accounted for complex correlations between repeated measures of the same phenotype (serial correlations) and between individuals in the same family (familial correlations). We conducted comprehensive simulation studies to compare the proposed tests with mixed-effects models and marginal models, using GEEs under various configurations. When the proposed tests were applied to data from the Diabetes Heart Study, we found exome variants of POMGNT1 and JAK1 genes were associated with type 2 diabetes.

O-linked mannose β-1,2-N-acetylglucosaminyltransferase 1 correlated with the malignancy in glioma

O-linked mannose β-1,2-N-acetylglucosaminyltransferase 1 (PomGnT1) constitutes one third of the O-linked glycoproteins in brain tissue. However, its functions have been seldom investigated in brain cancers. In this study, immunohistochemistry was used for the detection of the PomGnT1 protein in 133 cases of glioma tissues. Spearman correlation analysis was used for the relationship between PomGnT1 staining and the glioma grade. Receiver operating characteristic curve was used to measure the diagnostic value of PomGnT1 protein in the degree of glioma malignance. We found that PomGnT1 expression was correlated with glioma grade, and it could be used as a marker to distinguish low- and high-grade gliomas. Stably transfected U87 cells were constructed to overexpress short hairpin RNA of PomGnT1. Immunofluorescence test detected that this protein also could restrain the generation of U87 cells' pseudopodia. Western blotting further showed that the PomGnT1 protein had an impact on the c-myc protein level. In conclusion, our data suggest that PomGnT1 protein was correlated with the malignance of glioma progression, the mechanism involved in glioma cell's pseudopodium formation, and the expression of c-myc protein.

A novel homozygous ISPD gene mutation causing phenotype variability in a consanguineous family

Within the group of muscular dystrophies, dystroglycanopathies represent an important subgroup of recessively inherited disorders. Their severity varies from the relatively mild forms of adult-onset limb-girdle muscular dystrophy (LGMD), to the severe congenital muscular dystrophies (CMD) with cerebral and ocular involvement. We describe 2 consanguineous children of Pakistani origin, carrying a new homozygous missense mutation c.367G>A (p.Gly123Arg) in the ISPD gene. Mutations in this gene have been recently reported as a common cause of congenital and limb-girdle muscular dystrophy. Patient 1 is an 8-year-old female with an intermediate phenotype between CMD and early LGMD; patient 2 is a 20-month-old male and second cousin of patient 1, showing a CMD phenotype. Cognitive development, brain MRI, eye examination, electrocardiogram and echocardiogram were normal in both patients. To our knowledge, this is the first report on the co-occurrence of both a CMD/early LGMD intermediate phenotype and a CMD within the same family carrying a homozygous ISPD mutation.

Clinical, radiological, and genetic survey of patients with muscle-eye-brain disease caused by mutations in POMGNT1

BACKGROUND

To evaluate clinical, genetic, and radiologic features of our patients with muscle-eye-brain disease.

METHODS

The data of patients who were diagnosed with muscle-eye-brain disease from a cohort of patients with congenital muscular dystrophy in the Division of Pediatric Neurology of Dokuz Eylül University School of Medicine and Gaziantep Children's Hospital between 2005 and 2013 were analyzed retrospectively.

RESULTS

From a cohort of 34 patients with congenital muscular dystrophy, 12 patients from 10 families were diagnosed with muscle-eye-brain disease. The mean age of the patients was 9 ± 5.5 years (2-19 years). Mean serum creatine kinase value was 2485.80 ± 1308.54 IU/L (700-4267 IU/L). All patients presented with muscular hypotonia at birth followed by varying degrees of spasticity and exaggerated deep tendon reflexes in later stages of life. Three patients were able to walk. The most common ophthalmologic and radiologic abnormalities were cataracts, retinal detachment, periventricular white matter abnormalities, ventriculomegaly, pontocerebellar hypoplasia, and multiple cerebellar cysts. All of the patients had mutations in the POMGNT1 gene. The most common mutation detected in 66% of patients was c.1814 G > A (p.R605H). Two novel mutations were identified.

CONCLUSIONS

We suggest that muscle-eye-brain disease is a relatively common muscular dystrophy in Turkey. It should be suspected in patients with muscular hypotonia, increased creatine kinase, and structural eye and brain abnormalities. The c.1814 G > A mutation in exon 21 of the POMGNT1 gene is apparently a common mutation in the Turkish population. Individuals with this mutation show classical features of muscle-eye-brain disease, but others may exhibit a milder phenotype and retain the ability to walk independently. Congenital muscular dystrophy patients from Turkey carrying the clinical and radiologic features of muscle-eye-brain disease should be evaluated for mutations in POMGNT1 gene.

The o-mannosylation pathway: glycosyltransferases and proteins implicated in congenital muscular dystrophy

Several forms of congenital muscular dystrophy, referred to as dystroglycanopathies, result from defects in the protein O-mannosylation biosynthetic pathway. In this minireview, I discuss 12 proteins involved in the pathway and how they play a role in the building of glycan structures (most notably on the protein α-dystroglycan) that allow for binding to multiple proteins of the extracellular matrix.