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.

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.

This report outlines the case of a child affected by a type of congenital disorder of glycosylation (CDG) known as ALG2-CDG (OMIM 607906), presenting as a congenital myasthenic syndrome (CMS) caused by variants identified in ALG2, which encodes an α1,3-mannosyltransferase (EC 2.4.1.132) involved in the early steps of N-glycosylation. To date, fourteen cases of ALG2-CDG have been documented worldwide. From birth, the child experienced perinatal asphyxia, muscular weakness, feeding difficulties linked to an absence of the sucking reflex, congenital hip dislocation, and hypotonia. Over time, additional complications emerged, such as inspiratory stridor, gastroesophageal reflux, low intake, recurrent seizures, respiratory infections, an inability to maintain the head upright, and a global developmental delay. Whole genome sequencing (WGS) revealed the presence of two ALG2 variants in compound heterozygosity: a novel variant c.1055_1056delinsTGA p.(Ser352Leufs*3) and a variant of uncertain significance (VUS) c.964C>A p.(Pro322Thr). Additional studies, including determination of carbohydrate-deficient transferrin (CDT) revealed a mild type I CDG pattern and the presence of an abnormal transferrin glycoform containing a linear heptasaccharide consisting of one sialic acid, one galactose, one N-acetyl-glucosamine, two mannoses and two N-acetylglucosamines (NeuAc-Gal-GlcNAc-Man2-GlcNAc2), ALG2-CDG diagnostic biomarker, confirming the pathogenicity of these variants.

Phosphomannomutase 2 (PMM2) converts mannose-6-phospahate to mannose-1-phosphate; the substrate for GDP-mannose, a building block of the glycosylation biosynthetic pathway. Pathogenic variants in the PMM2 gene have been shown to be associated with protein hypoglycosylation causing PMM2-congenital disorder of glycosylation (PMM2-CDG). While mannose supplementation improves glycosylation in vitro, but not in vivo, we hypothesized that liposomal delivery of mannose-1-phosphate could increase the stability and delivery of the activated sugar to enter the targeted compartments of cells. Thus, we studied the effect of liposome-encapsulated mannose-1-P (GLM101) on global protein glycosylation and on the cellular proteome in skin fibroblasts from individuals with PMM2-CDG, as well as in individuals with two N-glycosylation defects early in the pathway, namely ALG2-CDG and ALG11-CDG. We leveraged multiplexed proteomics and N-glycoproteomics in fibroblasts derived from different individuals with various pathogenic variants in PMM2, ALG2 and ALG11 genes. Proteomics data revealed a moderate but significant change in the abundance of some of the proteins in all CDG fibroblasts upon GLM101 treatment. On the other hand, N-glycoproteomics revealed the GLM101 treatment enhanced the expression levels of several high-mannose and complex/hybrid glycopeptides from numerous cellular proteins in individuals with defects in PMM2 and ALG2 genes. Both PMM2-CDG and ALG2-CDG exhibited several-fold increase in glycopeptides bearing Man6 and higher glycans and a decrease in Man5 and smaller glycan moieties, suggesting that GLM101 helps in the formation of mature glycoforms. These changes in protein glycosylation were observed in all individuals irrespective of their genetic variants. ALG11-CDG fibroblasts also showed increase in high mannose glycopeptides upon treatment; however, the improvement was not as dramatic as the other two CDG. Overall, our findings suggest that treatment with GLM101 overcomes the genetic block in the glycosylation pathway and can be used as a potential therapy for CDG with enzymatic defects in early steps in protein N-glycosylation.

Congenital disorders of glycosylation type 1 (CDG-I) comprise a group of 27 genetic defects with heterogeneous multisystem phenotype, mostly presenting with nonspecific neurological symptoms. The biochemical hallmark of CDG-I is a partial absence of complete N-glycans on transferrin. However, recent findings of a diagnostic N-tetrasaccharide for ALG1-CDG and increased high-mannose N-glycans for a few other CDG suggested the potential of glycan structural analysis for CDG-I gene discovery. We analyzed the relative abundance of total plasma N-glycans by high resolution quadrupole time-of-flight mass spectrometry in a large cohort of 111 CDG-I patients with known (n = 75) or unsolved (n = 36) genetic cause. We designed single-molecule molecular inversion probes (smMIPs) for sequencing of CDG-I candidate genes on the basis of specific N-glycan signatures. Glycomics profiling in patients with known defects revealed novel features such as the N-tetrasaccharide in ALG2-CDG patients and a novel fucosylated N-pentasaccharide as specific glycomarker for ALG1-CDG. Moreover, group-specific high-mannose N-glycan signatures were found in ALG3-, ALG9-, ALG11-, ALG12-, RFT1-, SRD5A3-, DOLK-, DPM1-, DPM3-, MPDU1-, ALG13-CDG, and hereditary fructose intolerance. Further differential analysis revealed high-mannose profiles, characteristic for ALG12- and ALG9-CDG. Prediction of candidate genes by glycomics profiling in 36 patients with thus far unsolved CDG-I and subsequent smMIPs sequencing led to a yield of solved cases of 78% (28/36). Combined plasma glycomics profiling and targeted smMIPs sequencing of candidate genes is a powerful approach to identify causative mutations in CDG-I patient cohorts.