Glycosylation defects are a recognized cause of congenital myasthenic syndrome (CMS), affecting the stability and functions of the neuromuscular junction proteins. Mutations in five genes (GFPT1, DPAGT1, GMPPB, ALG2, and ALG14) are currently associated with glycosylation-related CMS. This cohort describes Iranian patients with CMS and variants in these genes. A retrospective study was conducted to examine demographic, clinical, genetic, and histological data from Iranian patients with confirmed CMS-glycosylation defects. Patients were identified and recruited through the Neuromuscular Clinics of Tehran University of Medical Sciences. Only patients with complete clinical and genetic data available were included. Twenty-three genetically confirmed patients with glycosylation-related CMS were enrolled. Genetic analysis revealed the mutations in the GFPT1, GMPPB, and ALG2 genes, with those in GFPT1 and GMPPB being the most common. The median age of onset and diagnosis was 6 and 16 years, respectively. Common clinical features were limb-girdle muscle weakness with minimal ocular involvement. Consanguinity and a positive family history were common, identified in 21 and 14 patients, respectively. Muscle biopsies revealed tubular aggregates in patients with GFPT1 and GMPPB variants. In addition, novel genetic variants were identified, and phenotypic variability was observed even within families sharing identical mutations. This study identifies novel variants and phenotypic variability in glycosylation-related CMS, with GFPT1 and GMPPB as the predominant subtypes in Iran. These findings expand the genotypic and phenotypic spectrum and underscore the importance of early genetic testing in high-consanguinity populations to improve diagnosis and management.

Congenital disorders of glycosylation (CDG) are a complex and heterogeneous family of rare metabolic diseases that affect protein and lipid glycosylation and glycosylphosphatidylinositol synthesis. These disorders can affect multiple organs, leading to a broad spectrum of symptoms that vary among different CDG subtypes and between individuals with same type of CDG. This study aimed to investigate the genetic variants, molecular etiologies, and clinical features of 20 Chinese patients diagnosed with CDG. Using whole-exome sequencing (WES), functional prediction tools, Sanger sequencing, and segregation analysis, we identified variants in several genes: ALG2 (3 patients), DPM2 (3 patients), PMM2 (3 patients), and ALG13 (2 patients). Additionally, variants in COG5, COG6, MOGS, DPM3, ALG1, ALG3, ALG11, SSR4 and SLC35A2 each were observed in single case. In total, 28 distinct variants were identified, 11 of which were previously unreported. Genotype-phenotype correlations revealed notable findings: variants in the N-terminus of ALG2 before the intramembrane domain were associated with congenital myasthenic syndromes (CMS), whereas those in the C-terminus caused ALG2-CDG; DPM2-CDG patients with variants in transmembrane region 1 exhibited more severe phenotypes; male patients with hemizygous variants in SLC35A2 demonstrated milder phenotypes compared to those with mosaic variants. This findings expand the spectrum of known clinical presentations and genetic variants in CDG, and establish possible genotype-phenotype correlations of several pathogenic genes, emphasizing the need for functional studies to unravel the underlying mechanisms.

GFPT1-related congenital myasthenic syndrome (CMS) is a rare, autosomal recessive disorder that impairs neuromuscular transmission due to defective glycosylation of the neuromuscular junction. While typically presenting with limb-girdle weakness, tubular aggregates on biopsy, and a favorable response to acetylcholinesterase inhibitors, the full phenotypic and imaging spectrum remains incompletely defined. We evaluated five Brazilian patients from two unrelated families, all with pathogenic variants in homozygosity in GFPT1 c.41G>A (p.Arg14Gln). Clinical, electrophysiological, and imaging assessments included nerve conduction studies, electromyography, repetitive nerve stimulation (RNS), and muscle ultrasound graded using the modified Heckmatt scale. Functional severity was estimated using the Myasthenia Gravis Foundation of America (MGFA) classification. All patients showed early-onset proximal weakness, distal lower limb weakness, and frequent falls. One patient exhibited atypical features, including neonatal onset epilepsy, and cognitive impairment. RNS revealedmarked decrements in proximal upper-limb muscles (deltoid 43.6%, trapezius 37.3%) and in the distal lower-limb tibialis anterior (36.5%), consistent with foot dorsiflexion weakness. Muscle ultrasound revealed varying degrees of myopathic echogenicity. A strong positive correlation was found between MGFA severity and mean Heckmatt score (p = 0.028), suggesting alignment between functional severity and muscle structural changes. Our findings expand the clinical spectrum of GFPT1-CMS to include possible central nervous system involvement and demonstrate the value of integrating electrophysiology and muscle ultrasound into diagnostic evaluation. Muscle ultrasound may serve as a structural biomarker for phenotypic stratification in CMS, and distal involvement-particularly foot dorsiflexion weakness-represents an additional diagnostic clue for GFPT1-CMS.

Congenital Disorders of Glycosylation (CDG) are severe disruptions in the synthesis of glycoconjugates, resulting in inherited metabolic conditions. These multisystem diseases, typically inherited in an autosomal recessive manner, have an occurrence rate of approximately 1 in 20,000 to 1 in 50,000 live births. The clinical presentation of CDG is highly varied and complex, with neurological symptoms being predominant, affecting multiple organ systems. The process of glycosylation, a critical post-translational modification, is tightly controlled by proteins encoded by over 250 genes, and mutations in any of these genes are known to cause CDG. The discovery of new associated genes over recent years has accelerated; comprehensively characterizing these, especially rare ones, will aid in identifying novel therapeutic targets, improving prognostic evaluations, and developing effective treatments. In vitro models (such as cell lines or patient-derived "clinical-grade" cells) are essential for advancing CDG research. Notably, 60% of defects affecting N- or O-glycosylation impact the eyes, leading to photoreceptor degeneration and cell death. The 661W cell line, derived from immortalized mouse retinal cells and expressing specific ocular markers, serves as a valuable experimental model to study the ocular involvement in CDG. In this study, we utilized the 661W cell line to explore the molecular consequences of a homozygous variant in the ALG2 gene (c.752G>T; p.Arg251Leu), which encodes the enzyme α-1,3-mannosyltransferase. Following transfection with a plasmid carrying the variants of the gene of interest ALG2 p.Arg251/p.Arg251, we carefully evaluated changes in gene expression using RT-PCR and Western blotting. Our results suggest that the 661W cell line may serve as a useful model for examining the potential impact of a specific mutation, supporting a possible link between the mutation's molecular effects and clinical disease progression. These findings could provide valuable insights to inform the development of targeted therapeutic strategies within the framework of personalized medicine.

The neuromuscular junction is a prototypic synapse that has been extensively studied and provides a model for smaller and less accessible central synapses. Central to transmission at the neuromuscular synapse is the muscle acetylcholine receptor cation channel. Studies of the genetic disorders affecting the neuromuscular junction, termed congenital myasthenic syndromes, have illustrated how impaired signal transmission may be caused by a variety of mutations both within the ion channel itself and by the context of the ion channel within the synapse. Thus, multiple pathogenic mutations are also identified in proteins affecting the clustering, location, and density of the receptor within the overall synaptic structure. Disease severity ranges from death in childhood to mild disability throughout life. In addition, in utero, fetal akinesia due to impaired neuromuscular transmission may cause developmental abnormalities. Early studies identified mutations in the genes encoding the acetylcholine receptor subunits that impair ion channel gating or reduce the number of endplate receptors or a combination of the two, giving rise to "slow channel," "fast channel," or deficiency syndromes. Subsequently, it became clear that myasthenic syndromes also stem from mutations in proteins involved in neurotransmitter release, the formation and maintenance of the neuromuscular synapse, or glycosylation. This chapter describes the patient phenotypes, the diverse range of molecular mechanisms for synaptic dysfunction, and the corresponding therapeutic strategies, including drug combinations, that can be tailored to the many subtypes.