Broad phenotypic spectrum and genotype-phenotype correlations in GMPPB-related dystroglycanopathies: an Italian cross-sectional study

MRI for the diagnosis of limb girdle muscular dystrophies

PURPOSE OF REVIEW

In the last 30 years, there have many publications describing the pattern of muscle involvement of different neuromuscular diseases leading to an increase in the information available for diagnosis. A high degree of expertise is needed to remember all the patterns described. Some attempts to use artificial intelligence or analysing muscle MRIs have been developed. We review the main patterns of involvement in limb girdle muscular dystrophies (LGMDs) and summarize the strategies for using artificial intelligence tools in this field.

RECENT FINDINGS

The most frequent LGMDs have a widely described pattern of muscle involvement; however, for those rarer diseases, there is still not too much information available. patients. Most of the articles still include only pelvic and lower limbs muscles, which provide an incomplete picture of the diseases. AI tools have efficiently demonstrated to predict diagnosis of a limited number of disease with high accuracy.

SUMMARY

Muscle MRI continues being a useful tool supporting the diagnosis of patients with LGMD and other neuromuscular diseases. However, the huge variety of patterns described makes their use in clinics a complicated task. Artificial intelligence tools are helping in that regard and there are already some accessible machine learning algorithms that can be used by the global medical community.

Manipulating mannose metabolism as a potential anticancer strategy

Cancer cells acquire metabolic advantages over their normal counterparts regarding the use of nutrients for sustained cell proliferation and cell survival in the tumor microenvironment. Notable among the metabolic traits in cancer cells is the Warburg effect, which is a reprogrammed form of glycolysis that favors the rapid generation of ATP from glucose and the production of biological macromolecules by diverting glucose into various metabolic intermediates. Meanwhile, mannose, which is the C-2 epimer of glucose, has the ability to dampen the Warburg effect, resulting in slow-cycling cancer cells that are highly susceptible to chemotherapy. This anticancer effect of mannose appears when its catabolism is compromised in cancer cells. Moreover, de novo synthesis of mannose within cancer cells has also been identified as a potential target for enhancing chemosensitivity through targeting glycosylation pathways. The underlying mechanisms by which alterations in mannose metabolism induce cancer cell vulnerability are just beginning to emerge. This review summarizes the current state of our knowledge of mannose metabolism and provides insights into its manipulation as a potential anticancer strategy.

Ubiquitination contributes to the regulation of GDP-mannose pyrophosphorylase B activity

GDP-mannose pyrophosphorylase B (GMPPB) loss-of-function is associated with muscular dystrophy and variable additional neurological symptoms. GMPPB facilitates the catalytic conversion of mannose-1-phosphate and GTP to GDP-mannose, which serves as a mannose donor for glycosylation. The activity of GMPPB is regulated by its non-catalytic paralogue GMPPA, which can bind GDP-mannose and interact with GMPPB, thereby acting as an allosteric feedback inhibitor of GMPPB. Using pulldown, immunoprecipitation, turnover experiments as well as immunolabeling and enzyme activity assays, we provide first direct evidence that GMPPB activity is regulated by ubiquitination. We further show that the E3 ubiquitin ligase TRIM67 interacts with GMPPB and that knockdown of TRM67 reduces ubiquitination of GMPPB, thus reflecting a candidate E3 ligase for the ubiquitination of GMPPB. While the inhibition of GMPPB ubiquitination decreases its enzymatic activity, its ubiquitination neither affects its interaction with GMPPA nor its turnover. Taken together, we show that the ubiquitination of GMPPB represents another level of regulation of GDP-mannose supply.

Consequences of GMPPB deficiency for neuromuscular development and maintenance

Guanosine diphosphate-mannose pyrophosphorylase B (GMPPB) catalyzes the conversion of mannose-1-phosphate and GTP to GDP-mannose, which is required as a mannose donor for the biosynthesis of glycan structures necessary for proper cellular functions. Mutations in GMPPB have been associated with various neuromuscular disorders such as muscular dystrophy and myasthenic syndromes. Here, we report that GMPPB protein abundance increases during brain and skeletal muscle development, which is accompanied by an increase in overall protein mannosylation. To model the human disorder in mice, we generated heterozygous GMPPB KO mice using CIRSPR/Cas9. While we were able to obtain homozygous KO mice from heterozygous matings at the blastocyst stage, homozygous KO embryos were absent beyond embryonic day E8.5, suggesting that the homozygous loss of GMPPB results in early embryonic lethality. Since patients with GMPPB loss-of-function manifest with neuromuscular disorders, we investigated the role of GMPPB in vitro. Thereby, we found that the siRNA-mediated knockdown of Gmppb in either primary myoblasts or the myoblast cell line C2C12 impaired myoblast differentiation and resulted in myotube degeneration. siRNA-mediated knockdown of Gmppb also impaired the neuron-like differentiation of N2A cells. Taken together, our data highlight the essential role of GMPPB during development and differentiation, especially in myogenic and neuronal cell types.

Muscle MRI patterns for limb girdle muscle dystrophies: systematic review

Limb girdle muscle dystrophies (LGMDs) are a group of inherited neuromuscular disorders comprising more than 20 genes. There have been increasing efforts to characterize this group with Muscle MRI. However, due to the complexity and similarities, the interpretation of the MRI patterns is usually done by experts in the field. Here, we proposed a step-by-step image interpretation of Muscle MRI in LGDM by evaluating the variability of muscle pattern involvement reported in the literature. A systematic review with an open start date to November 2022 was conducted to describe all LGMDs' muscle MRI patterns. Eighty-eight studies were included in the final review. Data were found to describe muscle MRI patterns for 15 out of 17 LGMDs types. Although the diagnosis of LGMDs is challenging despite the advanced genetic testing and other diagnostic modalities, muscle MRI is shown to help in the diagnosis of LGMDs. To further increase the yield for muscle MRI in the neuromuscular field, larger cohorts of patients need to be conducted.

GDP-Mannose Pyrophosphorylase B (GMPPB)-Related Disorders

GDP-mannose pyrophosphorylase B (GMPPB) is a cytoplasmic protein that catalyzes the formation of GDP-mannose. Impaired GMPPB function reduces the amount of GDP-mannose available for the O-mannosylation of α-dystroglycan (α-DG) and ultimately leads to disruptions of the link between α-DG and extracellular proteins, hence dystroglycanopathy. GMPPB-related disorders are inherited in an autosomal recessive manner and caused by mutations in either a homozygous or compound heterozygous state. The clinical spectrum of GMPPB-related disorders spans from severe congenital muscular dystrophy (CMD) with brain and eye abnormalities to mild forms of limb-girdle muscular dystrophy (LGMD) to recurrent rhabdomyolysis without overt muscle weakness. GMPPB mutations can also lead to the defect of neuromuscular transmission and congenital myasthenic syndrome due to altered glycosylation of the acetylcholine receptor subunits and other synaptic proteins. Such impairment of neuromuscular transmission is a unique feature of GMPPB-related disorders among dystroglycanopathies. LGMD is the most common phenotypic presentation, characterized by predominant proximal weakness involving lower more than upper limbs. Facial, ocular, bulbar, and respiratory muscles are largely spared. Some patients demonstrate fluctuating fatigable weakness suggesting neuromuscular junction involvement. Patients with CMD phenotype often also have structural brain defects, intellectual disability, epilepsy, and ophthalmic abnormalities. Creatine kinase levels are typically elevated, ranging from 2 to >50 times the upper limit of normal. Involvement of the neuromuscular junction is demonstrated by the decrement in the compound muscle action potential amplitude on low-frequency (2-3 Hz) repetitive nerve stimulation in proximal muscles but not in facial muscles. Muscle biopsies typically show myopathic changes with variable degrees of reduced α-DG expression. Higher mobility of β-DG on Western blotting represents a specific feature of GMPPB-related disorders, distinguishing it from other α-dystroglycanopathies. Patients with clinical and electrophysiologic features of neuromuscular transmission defect can respond to acetylcholinesterase inhibitors alone or combined with 3,4 diaminopyridine or salbutamol.

Distinct and Recognisable Muscle MRI Pattern in a Series of Adults Harbouring an Identical GMPPB Gene Mutation

BACKGROUND AND PURPOSE

Mutations in the GMPPB gene affect glycosylation of α-dystroglycan, leading to varied clinical phenotypes. We attempted to delineate the muscle MR imaging spectrum of GMPPB-related Congenital Myasthenic syndrome (CMS) in a single-center cohort study.

OBJECTIVE

To identify the distinct patterns of muscle involvement in GMPPB gene mutations.

METHODS

We analyzed the muscle MR images of 7 genetically proven cases of GMPPB dystroglycanopathy belonging to three families and studied the potential qualitative imaging pattern to aid in clinico -radiological diagnosis in neuromuscular practice. All individuals underwent muscle MRI (T1, T2, STIR/PD Fat sat. sequences in 1.5 T machine) of the lower limbs. Qualitative assessment and scoring were done for muscle changes using Mercuri staging for fibro-fatty replacement on T1 sequence and Borsato score for myoedema on STIR sequence.

RESULTS

All patients were of South Indian origin and presented as slowly progressive childhood to adult-onset fatigable limb-girdle muscle weakness, elevated creatine kinase level, and positive decrement response in proximal muscles. Muscle biopsy revealed features of dystrophy. All patients demonstrated identical homozygous mutation c.1000G >  A in the GMPPB gene. MRI demonstrated early and severe involvement of paraspinal muscles, gluteus minimus, and relatively less severe involvement of the short head of the biceps femoris. A distinct proximo-distal gradient of affliction was identified in the glutei, vasti, tibialis anterior and peronei. Also, a postero-anterior gradient was observed in the gracilis muscle.

CONCLUSION

Hitherto unreported, the distinctive MR imaging pattern described here, coupled with relatively slowly progressive symptoms of fatigable limb-girdle weakness, would facilitate an early diagnosis of the milder form of GMPPB- dystroglycanopathy associated with homozygous GMPPB gene mutation.

[Genomics and proteomics in the research of neuromuscular diseases]

Neurological diseases affect 3-5% of children and, apart from cardiovascular diseases and cancer, represent the most prominent cause of morbidity and mortality in adults and particularly in the aged population of western Europe. Neuromuscular disorders are a subgroup of neurological diseases and often have a genetic origin, which leads to familial clustering. Despite the enormous progress in the analysis of the genome, such as by sequence analysis of coding regions of deoxyribonucleic acid or even the entire deoxyribonucleic acid sequence, in approximately 50% of the patients suffering from rare forms of neurological diseases the genetic cause remains unsolved. The reasons for this limited detection rate are presented in this article. If a treatment concept is available, under certain conditions this can have an impact on the adequate and early treatment of these patients. Considering neuromuscular disorders as a paradigm, this article reports on the advantages of the inclusion of next generation sequencing analysis-based DNA investigations as an omics technology (genomics) and the advantage of the integration with protein analyses (proteomics). A special focus is on the combination of genomics and proteomics in the sense of a proteogenomic approach in the diagnostics and research of these diseases. Along this line, this article presents a proteogenomic approach in the context of a multidisciplinary project aiming towards improved diagnostic work-up and future treatment of patients with neuromuscular diseases; "NMD-GPS: gene and protein signatures as a global positioning system in patients suffering from neuromuscular diseases".

Fondazione Telethon and Unione Italiana Lotta alla Distrofia Muscolare, a successful partnership for neuromuscular healthcare research of value for patients

In 2001, Fondazione Telethon and the Italian muscular dystrophy patient organisation Unione Italiana Lotta alla Distrofia Muscolare joined their efforts to design and launch a call for grant applications specifically dedicated to clinical projects in the field of neuromuscular disorders. This strategic initiative, run regularly over the years and still ongoing, aims at supporting research with impact on the daily life of people with a neuromuscular condition and is centred on macro-priorities identified by the patient organisation. It is investigator-driven, and all proposals are peer-reviewed for quality and feasibility. Over the years, this funding program contributed to strengthening the activities of the Italian neuromuscular clinical network, reaching many achievements in healthcare research. Moreover, it has been an enabling factor for innovative therapy experimentation at international level and prepared the clinical ground to make therapies available to Italian patients. The ultimate scope of healthcare research is to ameliorate the delivery of care. In this paper, the achievements of the funded studies are analysed also from this viewpoint, to ascertain to which extent they have fulfilled the original goals established by the patient organisation. The evidence presented indicates that this has been a highly fruitful program. Factors that contributed to its success, lessons learned, challenges, and issues that remain to be addressed are discussed to provide practical examples of an experience that could inspire also other organizations active in the field of rare disease research.

Expanding the clinical and genetic spectrum of pathogenic variants in STIM1

INTRODUCTION/AIMS

Stromal interaction molecule 1 (STIM1) is a reticular Ca²⁺ sensor composed of a luminal and a cytosolic domain. Autosomal dominant mutations in STIM1 cause tubular aggregate myopathy and Stormorken syndrome or its variant York platelet syndrome. In this study we aimed to expand the features related to new variants in STIM1.

METHODS

We performed a cross-sectional study of individuals harboring monoallelic STIM1 variants recruited at five tertiary centers involved in a study of inherited myopathies analyzed with a multigene-targeted panel.

RESULTS

We identified seven individuals (age range, 26-57 years) harboring variants in STIM1, including five novel changes: three located in the EF-hand domain, one in the sterile α motif (SAM) domain, and one in the cytoplasmatic region of the protein. Functional evaluation of the pathogenic variants using a heterologous expression system and measuring store-operated calcium entry demonstrated their causative role and suggested a link of new variants with the clinical phenotype. Muscle contractures, found in three individuals, showed variability in body distribution and in the number of joints involved. Three patients showed cardiac and respiratory involvement. Short stature, hyposplenism, sensorineural hearing loss, hypothyroidism, and Gilbert syndrome were variably observed among the patients. Laboratory tests revealed hyperCKemia in six patients, thrombocytopenia in two patients, and hypocalcemia in one patient. Muscle biopsy showed the presence of tubular aggregates in three patients, type I fiber atrophy in one patient, and nonspecific myopathic changes in two patients.

DISCUSSION

Our clinical, histological, and molecular data expand the genetic and clinical spectrum of STIM1-related diseases.

A founder mutation in the GMPPB gene [c.1000G > A (p.Asp334Asn)] causes a mild form of limb-girdle muscular dystrophy/congenital myasthenic syndrome (LGMD/CMS) in South Indian patients

Twelve patients from seven unrelated South Indian families with a limb-girdle muscular dystrophy-congenital myasthenic syndrome (LGMD/CMS) phenotype and recessive inheritance underwent deep clinical phenotyping, electrophysiological evaluation, muscle histopathology, and next-generation sequencing/Sanger sequencing-based identification of the genetic defect. Homozygosity mapping was performed using high-throughput genome-wide genotyping for mapping the mutation and to evaluate the founder effect. The age of disease onset among patients ranged from childhood to 40 years of age. The key clinical manifestations observed were progressive fatigable limb-girdle weakness, muscle hypertrophy/atrophy, and preferential weakness in a dystrophic pattern. The ages at last follow-up ranged from 30 to 64 years; nine were independently ambulant, two required assistance, and one was wheelchair-bound. Lower limb muscle MRI showed varying degrees of fat replacement in the glutei, hamstrings, anterior leg muscles, and medial gastrocnemius. All patients showed significant decrement on repetitive nerve stimulation (RNS). Muscle biopsy in 7 patients revealed varying degrees of dystrophic and neurogenic changes. Treatment with pyridostigmine and/or salbutamol resulted in variable improvement in 10 patients. Genetic analysis showed an identical homozygous GMPPB mutation c.1000G > A (p.Asp334Asn) in all affected patients. A region of homozygosity (6Mbp) was observed flanking the c.1000G > A change in carrier chromosomes. This study identifies c.1000G > A in GMPPB as a common founder mutation in an ethnic community of South Indian descent with milder yet variable degree of clinical presentation of GMPPB-associated LGMD-CMS.

GMPPB-congenital disorders of glycosylation associate with decreased enzymatic activity of GMPPB

The congenital disorders of glycosylation (CDG) are a family of metabolic diseases in which glycosylation of proteins or lipids is deficient. GDP-mannose pyrophosphorylase B (GMPPB) mutations lead to CDG, characterized by neurological and muscular defects. However, the genotype-phenotype correlation remains elusive, limiting our understanding of the underlying mechanism and development of therapeutic strategy. Here, we report a case of an individual presenting congenital muscular dystrophy with cerebellar involvement, who presents two heterozygous GMPPB mutations (V111G and G214S). The V111G mutation significantly decreases GMPPB’s enzymatic activity. By measuring enzymatic activities of 17 reported GMPPB mutants identified in patients diagnosed with GMPPB-CDG, we discover that all tested GMPPB variants exhibit significantly decreased enzymatic activity. Using a zebrafish model, we find that Gmppb is required for neuronal and muscle development, and further demonstrate that enzymatic activity of GMPPB mutants correlates with muscular and neuronal phenotypes in zebrafish. Taken together, our findings discover the importance of GMPPB enzymatic activity for the pathogenesis of GMPPB-CDG, and shed light for the development of additional indicators and therapeutic strategy.

Bi-allelic truncating mutations in VWA1 cause neuromyopathy

The von Willebrand Factor A domain containing 1 protein, encoded by VWA1, is an extracellular matrix protein expressed in muscle and peripheral nerve. It interacts with collagen VI and perlecan, two proteins that are affected in hereditary neuromuscular disorders. Lack of VWA1 is known to compromise peripheral nerves in a Vwa1 knock-out mouse model. Exome sequencing led us to identify bi-allelic loss of function variants in VWA1 as the molecular cause underlying a so far genetically undefined neuromuscular disorder. We detected six different truncating variants in 15 affected individuals from six families of German, Arabic, and Roma descent. Disease manifested in childhood or adulthood with proximal and distal muscle weakness predominantly of the lower limbs. Myopathological and neurophysiological findings were indicative of combined neurogenic and myopathic pathology. Early childhood foot deformity was frequent, but no sensory signs were observed. Our findings establish VWA1 as a new disease gene confidently implicated in this autosomal recessive neuromyopathic condition presenting with child-/adult-onset muscle weakness as a key clinical feature.

Congenital myasthenic syndrome in China: genetic and myopathological characterization

OBJECTIVE

We aimed to summarize the clinical, genetic, and myopathological features of a cohort of Chinese patients with congenital myasthenic syndrome, and follow up on therapeutic outcomes.

METHODS

The clinical spectrum, mutational frequency of genes, and pathological diagnostic clues of various subtypes of patients with congenital myasthenic syndrome were summarized. Therapeutic effects were followed up.

RESULTS

Thirty-five patients from 29 families were recruited. Ten genes were identified: GFPT1 (27.6%), AGRN (17.2%), CHRNE (17.2%), COLQ (13.8%), GMPPB (6.9%), CHAT, CHRNA1, DOK7, COG7, and SLC25A1 (3.4% each, respectively). Sole limb-girdle weakness was found in patients with AGRN (1/8) and GFPT1 (7/8) mutations, whereas distal weakness was all observed in patients with AGRN (6/8) mutations. Tubular aggregates were only found in patients with GFPT1 mutations (5/6). The patients with GMPPB mutations (2/2) had decreased alpha-dystroglycan. Acetylcholinesterase inhibitor therapy resulted in no response or worsened symptoms in patients with COLQ mutations, a diverse response in patients with AGRN mutations, and a good response in patients with other subtypes. Albuterol therapy was effective or harmless in most subtypes. Therapy effects became attenuated with long-term use in patients with COLQ or AGRN mutations.

INTERPRETATION

The genetic distribution of congenital myasthenic syndrome in China is distinct from that of other ethnic origins. The appearance of distal weakness, selective limb-girdle myasthenic syndrome, tubular aggregates, and decreased alpha-dystroglycan were indicative of the specific subtypes. Based on the follow-up findings, we suggest cautious evaluation of the long-term efficacy of therapeutic agents in congenital myasthenic syndrome.

Elucidation of the Genetic Cause in Dutch Limb Girdle Muscular Dystrophy Families: A 27-Year's Journey

BACKGROUND

A Dutch cohort of 105 carefully selected limb girdle muscular dystrophy (LGMD) patients from 68 families has been subject to genetic testing over the last 20 years. After subsequent targeted gene analysis around two thirds (45/68) of the families had received a genetic diagnosis in 2013.

OBJECTIVE

To describe the results of further genetic testing in the remaining undiagnosed limb girdle muscular dystrophy families in this cohort.

METHODS

In the families of the cohort for whom no genetic diagnosis was established (n = 23) further testing using Sanger sequencing, next generation sequencing with gene panel analysis or whole-exome sequencing was performed. In one case DNA analysis for facioscapulohumeral dystrophy type 1 was carried out.

RESULTS

In eight families no additional genetic tests could be performed. In 12 of the remaining 15 families in which additional testing could be performed a genetic diagnosis was established: two LGMDR1 calpain3-related families with CAPN3 mutations, one LGMDR2 dysferlin-related family with DYSF mutations, three sarcoglycanopathy families (LGMDR3-5 α-, β- and γ-sarcoglycan-related) with SGCA/SGCB/SGCG mutations, one LGMDR8 TRIM 32-related family with TRIM32 mutations, two LGMDR19 GMPPB-related families with GMPPB mutations, one family with MICU1-related myopathy, one family with FLNC-related myopathy and one family with facioscapulohumeral dystrophy type 1. At this moment a genetic diagnosis has been made in 57 of the 60 families of which DNA was available (95%).

CONCLUSION

A genetic diagnosis is obtained in 95% of the families of the original Dutch LGMD cohort of which DNA was available.

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.

Congenital disorders of glycosylation: A multi-genetic disease family with multiple subcellular locations

This review discusses a selection of congenital disorders of glycosylation that show peculiar features, such as an unusual presentation, different phenotypes, a novel biochemical/genetic mechanism, a relatively high frequency or a relatively efficient treatment.

Advances in the diagnosis of inherited neuromuscular diseases and implications for therapy development

Advances in DNA sequencing technologies have resulted in a near doubling, in under 10 years, of the number of causal genes identified for inherited neuromuscular disorders. However, around half of patients, whether children or adults, do not receive a molecular diagnosis after initial diagnostic workup. Massively parallel technologies targeting RNA, proteins, and metabolites are being increasingly used to diagnose these unsolved cases. The use of these technologies to delineate pathways, biomarkers, and therapeutic targets has led to new approaches entering the drug development pipeline. However, these technologies might give rise to misleading conclusions if used in isolation, and traditional techniques including comprehensive neurological evaluation, histopathology, and biochemistry continue to have a crucial role in diagnostics. For optimal diagnosis, prognosis, and precision medicine, no single ruling technology exists. Instead, an interdisciplinary approach combining novel and traditional neurological techniques with computer-aided analysis and international data sharing is needed to advance the diagnosis and treatment of neuromuscular disorders.

Trouble at the junction: When myopathy and myasthenia overlap

Although myopathies and neuromuscular junction disorders are typically distinct, their coexistence has been reported in several inherited and acquired conditions. Affected individuals have variable clinical phenotypes but typically display both a decrement on repetitive nerve stimulation and myopathic findings on muscle biopsy. Inherited causes include myopathies related to mutations in BIN1, DES, DNM2, GMPPB, MTM1, or PLEC and congenital myasthenic syndromes due to mutations in ALG2, ALG14, COL13A1, DOK7, DPAGT1, or GFPT1. Additionally, a decrement due to muscle fiber inexcitability is observed in certain myotonic disorders. The identification of a defect of neuromuscular transmission in an inherited myopathy may assist in establishing a molecular diagnosis and in selecting patients who would benefit from pharmacological correction of this defect. Acquired cases meanwhile stem from the co-occurrence of myasthenia gravis or Lambert-Eaton myasthenic syndrome with an immune-mediated myopathy, which may be due to paraneoplastic disorders or exposure to immune checkpoint inhibitors.

Toward understanding tissue-specific symptoms in dolichol-phosphate-mannose synthesis disorders; insight from DPM3-CDG

The congenital disorders of glycosylation (CDG) are inborn errors of metabolism with a great genetic heterogeneity. Most CDG are caused by defects in the N-glycan biosynthesis, leading to multisystem phenotypes. However, the occurrence of tissue-restricted clinical symptoms in the various defects in dolichol-phosphate-mannose (DPM) synthesis remains unexplained. To deepen our understanding of the tissue-specific characteristics of defects in the DPM synthesis pathway, we investigated N-glycosylation and O-mannosylation in skeletal muscle of three DPM3-CDG patients presenting with muscle dystrophy and hypo-N-glycosylation of serum transferrin in only two of them. In the three patients, O-mannosylation of alpha-dystroglycan (αDG) was strongly reduced and western blot analysis of beta-dystroglycan (βDG) N-glycosylation revealed a consistent lack of one N-glycan in skeletal muscle. Recently, defective N-glycosylation of βDG has been reported in patients with mutations in guanosine-diphosphate-mannose pyrophosphorylase B (GMPPB). Thus, we suggest that aberrant O-glycosylation of αDG and N-glycosylation of βDG in skeletal muscle is indicative of a defect in the DPM synthesis pathway. Further studies should address to what extent hypo-N-glycosylation of βDG or other skeletal muscle proteins contribute to the phenotype of patients with defects in DPM synthesis. Our findings contribute to our understanding of the tissue-restricted phenotype of DPM3-CDG and other defects in the DPM synthesis pathway.

Next-generation sequencing approach to hyperCKemia: A 2-year cohort study

Objective

Next-generation sequencing (NGS) was applied in molecularly undiagnosed asymptomatic or paucisymptomatic hyperCKemia to investigate whether this technique might allow detection of the genetic basis of the condition.

Methods

Sixty-six patients with undiagnosed asymptomatic or paucisymptomatic hyperCKemia, referred to tertiary neuromuscular centers over an approximately 2-year period, were analyzed using a customized, targeted sequencing panel able to investigate the coding exons and flanking intronic regions of 78 genes associated with limb-girdle muscular dystrophies, rhabdomyolysis, and metabolic and distal myopathies.

Results

A molecular diagnosis was reached in 33 cases, corresponding to a positive diagnostic yield of 50%. Variants of unknown significance were found in 17 patients (26%), whereas 16 cases (24%) remained molecularly undefined. The major features of the diagnosed cases were mild proximal muscle weakness (found in 27%) and myalgia (in 24%). Fourteen patients with a molecular diagnosis and mild myopathic features on muscle biopsy remained asymptomatic at a 24-month follow-up.

Conclusions

This study of patients with undiagnosed hyperCKemia, highlighting the advantages of NGS used as a first-tier diagnostic approach in genetically heterogeneous conditions, illustrates the ongoing evolution of molecular diagnosis in the field of clinical neurology. Isolated hyperCKemia can be the sole feature alerting to a progressive muscular disorder requiring careful surveillance.

Central nervous system involvement in arthrogryposis multiplex congenita: Overview of causes, diagnosis, and care

Arthrogryposis or AMC, arthrogryposis multiplex congenita, is defined as multiple congenital joint contractures in more than two joints and in different body areas. The common cause of all AMC is lack of movement in utero, which in turn can have different causes, one of which is CNS involvement. Intellectual disability/CNS involvement is found in approximately 25% of all AMC. AMC with CNS involvement includes a large number of genetic syndromes. So far, more than 400 genes have been identified as linked to AMC, with and without CNS involvement. A number of neonatally lethal syndromes and syndromes resulting in severe disability due to CNS malfunction belong to this group of syndromes. There are several X-linked disorders with AMC, which are primarily related to intellectual disability. A number of neuromuscular disorders may include AMC and CNS/brain involvement. Careful clinical evaluation by a geneticist and a pediatrician/pediatric neurologist is the first step in making a specific diagnosis. Further investigations may include MRI of the brain and spinal cord, electroencephalogram, blood chemistry for muscle enzymes, other organ investigations (ophtalmology, cardiology, gastrointestinal, and genitourinary systems). Nerve conduction studies, electromyogram, and muscle pathology may be of help when there is associated peripheral nervous system involvement. But most importantly, genetic investigations with targeted or rather whole exome or genome sequencing should be performed. A correct diagnosis is important in planning adequate treatment, in genetic counselling and also for future understanding of pathogenic mechanisms and possible new treatments. A multidiciplinary team is needed both in investigation and treatment.

A homozygous mutation in GMPPB leads to centronuclear myopathy with combined pre- and postsynaptic defects of neuromuscular transmission

Mutations in GMPPB cause a wide spectrum of neuromuscular syndromes, including muscular dystrophies and congenital myasthenic syndrome. The mechanisms by which GMPPB mutations impair neuromuscular transmission however remain incompletely understood. We expand here upon a previous report of one such patient presenting with a myopathy-congenital myasthenic syndrome overlap phenotype. Fatigable proximal muscle weakness developed gradually between 13 and 25 years of age, with subsequent stabilization. Low-frequency repetitive nerve stimulation showed a decrement, while a muscle biopsy demonstrated the presence of a centronuclear myopathy. Genetic testing identified a homozygous c.458C > T (p.Thr153Ile) variant in GMPPB. In-vitro microelectrode recordings and ultrastructural studies showed impairment of both pre- and postsynaptic neuromuscular transmission, thus demonstrating the presence of not only postsynaptic, but also presynaptic pathology in GMPPB-related disorders.

Lysosomal degradation of GMPPB is associated with limb-girdle muscular dystrophy type 2T

Objective

GDP‐mannose pyrophosphorylase B (GMPPB) related phenotype spectrum ranges widely from congenital myasthenic syndrome (CMS), limb‐girdle muscular dystrophy type 2T (LGMD 2T) to severe congenital muscle‐eye‐brain syndrome. Our study investigates the clinicopathologic features of a patient with novel GMPPB mutations and explores the pathogenetic mechanism.

Methods

The patient was a 22‐year‐old woman with chronic proximal limb weakness for 9 years without cognitive deterioration. Weakness became worse after fatigue. Elevated serum creatine kinase and decrements on repetitive nerve stimulation test were recorded. MRI showed fatty infiltration in muscles of lower limbs and shoulder girdle on T1 sequence. Open muscle biopsy and genetic analysis were performed.

Results

Muscle biopsy showed myogenic changes. Two missense mutations in GMPPB gene (c.803T>C and c.1060G>A) were identified in the patient. Western blotting and immunostaining showed GMPPB and α‐dystroglycan deficiency in the patient's muscle. In vitro, mutant GMPPB forming cytoplasmic aggregates completely colocalized with microtubule‐associated protein 1 light chain 3‐II (LC3‐II), a classical marker of autophagosome. Degradation of GMPPB was accompanied by an upregulation of LC3‐II, which could be restored by lysosomal inhibitor leupeptin.

Interpretation

We identified two novel GMPPB mutations causing overlap phenotype between LGMD 2T and CMS. We provided the initial evidence that mutant GMPPB colocalizes with autophagosome at subcellular level. GMPPB mutants degraded by autophagy‐lysosome pathway is associated with LGMD 2T. This study shed the light into the enzyme replacement which could become one of the therapeutic targets in the future study.

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.