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The glycome describes the complete repertoire of glycoconjugates composed of carbohydrate chains, or glycans, that are covalently linked to lipid or protein molecules. Glycoconjugates are formed through a process called glycosylation and can differ in their glycan sequences, the connections between them and their length. Glycoconjugate synthesis is a dynamic process that depends on the local milieu of enzymes, sugar precursors and organelle structures as well as the cell types involved and cellular signals. Studies of rare genetic disorders that affect glycosylation first highlighted the biological importance of the glycome, and technological advances have improved our understanding of its heterogeneity and complexity. Researchers can now routinely assess how the secreted and cell-surface glycomes reflect overall cellular status in health and disease. In fact, changes in glycosylation can modulate inflammatory responses, enable viral immune escape, promote cancer cell metastasis or regulate apoptosis; the composition of the glycome also affects kidney function in health and disease. New insights into the structure and function of the glycome can now be applied to therapy development and could improve our ability to fine-tune immunological responses and inflammation, optimize the performance of therapeutic antibodies and boost immune responses to cancer. These examples illustrate the potential of the emerging field of ‘glycomedicine’.Glycosylation refers to the addition of carbohydrate chains to proteins and lipids. In this Review, the authors discuss the broad role of glycans in immunity, cancer, xenotransplantation and glomerular filtration and the potential of ‘glycomedicine’.

The COVID-19 pandemic is caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Similar to other coronaviruses, its particles are composed of four structural proteins: spike (S), envelope (E), membrane (M), and nucleocapsid (N) proteins. S, E, and M proteins are glycosylated, and the N protein is phosphorylated. The S protein is involved in the interaction with the host receptor human angiotensin-converting enzyme 2 (hACE2), which is also heavily glycosylated. Recent studies have revealed several other potential host receptors or factors that can increase or modulate the SARS-CoV-2 infection. Interestingly, most of these molecules bear carbohydrate residues. While glycans acquired by the viruses through the hijacking of the host machinery help the viruses in their infectivity, they also play roles in immune evasion or modulation. Glycans play complex roles in viral pathobiology, both on their own and in association with carrier biomolecules, such as proteins or glycosaminoglycans (GAGs). Understanding these roles in detail can help in developing suitable strategies for prevention and therapy of COVID-19. In this review, we sought to emphasize the interplay of SARS-CoV-2 glycosylated proteins and their host receptors in viral attachment, entry, replication, and infection. Moreover, the implications for future therapeutic interventions targeting these glycosylated biomolecules are also discussed in detail.

SLC35A2-CDG is an X-linked congenital disorder of glycosylation (CDG), characterized by defective UDP-galactose transport into the Golgi and endoplasmic reticulum and consequent insufficient galactosylation of glycans. Clinically, this translates into a range of predominantly neurological symptoms. Although the pathomechanism of this disorder is not fully understood, oral galactose supplementation has led to clinical and biochemical improvement in some patients. Here, we show that protein glycosylation (N- and O-linked) was only minimally disturbed in SLC35A2-CDG patient-derived fibroblasts. However, lipid glycosylation was significantly impaired, with accumulation of glucosylceramide and deficiency of digalactosylated glycosphingolipids (GSLs) and complex gangliosides. Galactose supplementation increased UDP-galactose, its transport into the Golgi, and improved deficient GSL synthesis through direct incorporation of the provided galactose. This improved GSL homeostasis in all patient-derived fibroblasts and in another SLC35A2 deficient cell model (CHO-Lec8). Additionally, SLC35A2-CDG serum analysis identified hydroxylated GSLs, particularly GM3, as potential disease biomarkers. Given the essential role of gangliosides in central nervous system function, their deficiency is likely a key factor in the neurological involvement of this disorder. These findings pave the way for new nutritional therapies with GSL supplements and highlight the importance of studying lipid glycosylation to better understand the complex pathophysiology of CDG.

SLC35A1‐CDG is a very rare type of congenital disorders of glycosylation (CDG) with only five cases known to date. Here, we review the literature and present new data from a sixth patient carrying the uncharacterized variant c.133A>G; p.Thr45Ala in the SLC35A1 gene. In addition to known clinical symptoms of SLC35A1‐CDG, the patient presents with failure to thrive, short stature, café‐au‐lait spot, and preauricular ear tag. Even though examination of CDG markers transferrin (Tf), alpha‐1‐antitrypsin (A1AT), and apolipoprotein CIII (ApoCIII) revealed no abnormalities in serum, the patient’s fibroblasts showed significant alterations of protein expression or glycosylation of ICAM1, GP130, and TGN46 as well as differences in staining signals of lectins MAL‐I, RCAI, and SNA and deviations in LC‐MS analysis of total cellular N‐glycans. Transfection of CRISPR/Cas9 generated SLC35A1 HEK293 knockout cells with either wild‐type SLC35A1 or the c.133A>G variant restored the cellular CMP‐Neu5Ac to wild‐type levels, making a direct effect of p.Thr45Ala on the function of the transporter unlikely. Instead, our results imply that the residual transporter activity of 65% is caused by a decreased stability of the mutated SLC35A1 protein. Since O‐GlcNAcylation was affected as well, energy and lipid homeostasis were analyzed and found to be significantly altered. Notably, proliferation and glycosylation of the SLC35A1‐deficient patient fibroblasts were enhanced by supplementation of the cell culture medium with 10 mM GlcNAc.

Congenital disorders of glycosylation (CDG) are a group of genetic disorders that affect protein and lipid glycosylation and glycosylphosphatidylinositol synthesis. More than 100 different disorders have been reported and the number is rapidly increasing. Since glycosylation is an essential post-translational process, patients present a large range of symptoms and variable phenotypes, from very mild to extremely severe. Only for few CDG, potentially curative therapies are being used, including dietary supplementation (e.g., galactose for PGM1-CDG, fucose for SLC35C1-CDG, Mn2+ for TMEM165-CDG or mannose for MPI-CDG) and organ transplantation (e.g., liver for MPI-CDG and heart for DOLK-CDG). However, for the majority of patients, only symptomatic and preventive treatments are in use. This constitutes a burden for patients, care-givers and ultimately the healthcare system. Innovative diagnostic approaches, in vitro and in vivo models and novel biomarkers have been developed that can lead to novel therapeutic avenues aiming to ameliorate the patients’ symptoms and lives. This review summarizes the advances in therapeutic approaches for CDG.

BackgroundPhosphomannomutase 2-congenital disorder of glycosylation (PMM2-CDG) is a multisystem inborn error of metabolism.ObjectivesTo better characterise the natural history of PMM2-CDG.MethodsMedical charts of 96 patients with PMM2-CDG (86 families, 41 males, 55 females) were retrospectively reviewed. Data on clinical, laboratory and molecular parameters at diagnosis were analysed. Follow-up data at last examination were reported for 25 patients.ResultsThe patients were born between 1963 and 2011. Diagnosis of PMM2-CDG was made at a mean (SD) age of 6.8 (8.5) years. The presenting signs were mostly neurological (hypotonia, intellectual disability, cerebellar syndrome) and observed in almost all the patients. A total of 38 patients (14 males, 24 females) exhibited, in addition to neurological signs, visceral features including at least one of these: feeding difficulty requiring a nutritional support (n=23), cardiac features (n=20; pericarditis: 14, cardiac malformation: 9, cardiomyopathy: 2), hepato-gastrointestinal features (n=12; chronic diarrhoea: 7, protein-losing enteropathy: 1, ascites: 3, liver failure: 1, portal hypertension: 1), kidney features (n=4; nephrotic syndrome: 2, tubulopathy: 2) and hydrops fetalis (n=1). Twelve patients died at a mean age of 3.8 years (especially from pericarditis and other cardiac issues). Laboratory abnormalities mostly included elevated transaminases and abnormal coagulation parameters. High thyreostimulin levels, hypocholesterolemia, hypoalbuminemia and elevated transaminases were associated with the visceral phenotype. Besides the common Arg141His PMM2 variant harboured by half of the patients, 45 different variants were observed.ConclusionsPMM2-CDG clinical phenotype is heterogeneous in terms of clinical course, with no clear division between neurological and visceral presentations.

Congenital disorders of glycosylation (CDG) are a group of clinically and genetically heterogeneous metabolic disorders. Over 150 CDG types have been described. Most CDG types are ultrarare disorders. CDG types affecting N-glycosylation are the most common type of CDG with emerging therapeutic possibilities. This review is an update on the available therapies for disorders affecting the N-linked glycosylation pathway. In the first part of the review, we highlight the clinical presentation, general principles of management, and disease-specific therapies for N-linked glycosylation CDG types, organized by organ system. The second part of the review focuses on the therapeutic strategies currently available and under development. We summarize the successful (pre-) clinical application of nutritional therapies, transplantation, activated sugars, gene therapy, and pharmacological chaperones and outline the anticipated expansion of the therapeutic possibilities in CDG. We aim to provide a comprehensive update on the treatable aspects of CDG types involving N-linked glycosylation, with particular emphasis on disease-specific treatment options for the involved organ systems; call for natural history studies; and present current and future therapeutic strategies for CDG.

Congenital disorders of glycosylation (CDG) are a group of rare metabolic diseases, due to impaired protein and lipid glycosylation. We identified two patients with defective serum transferrin glycosylation and mutations in the MAGT1 gene. These patients present with a phenotype that is mainly characterized by intellectual and developmental disability. MAGT1 has been described to be a subunit of the oligosaccharyltransferase (OST) complex and more specifically of the STT3B complex. However, it was also claimed that MAGT1 is a magnesium (Mg2+) transporter. So far, patients with mutations in MAGT1 were linked to a primary immunodeficiency, characterized by chronic EBV infections attributed to a Mg2+ homeostasis defect (XMEN). We compared the clinical and cellular phenotype of our two patients to that of an XMEN patient that we recently identified. All three patients have an N-glycosylation defect, as was shown by the study of different substrates, such as GLUT1 and SHBG, demonstrating that the posttranslational glycosylation carried out by the STT3B complex is dysfunctional in all three patients. Moreover, MAGT1 deficiency is associated with an enhanced expression of TUSC3, the homolog protein of MAGT1, pointing toward a compensatory mechanism. Hence, we delineate MAGT1-CDG as a disorder associated with two different clinical phenotypes caused by defects in glycosylation.

Context:TMEM165 deficiency is a severe multisystem disease that manifests with metabolic, endocrine, and skeletal involvement. It leads to one type of congenital disorders of glycosylation (CDG), a rapidly growing group of inherited diseases in which the glycosylation process is altered. Patients have decreased galactosylation by serum glycan analysis. There are >100 CDGs, but only specific types are treatable.Objective:Galactose has been shown to be beneficial in other CDG types with abnormal galactosylation. The aim of this study was to characterize the effects of galactose supplementation on Golgi glycosylation in TMEM165-depleted HEK293 cells, as well as in 2 patients with TMEM165-CDG and in their cultured skin fibroblast cells.Design and Setting:Glycosylation was assessed by mass spectrometry, western blot analysis, and transferrin isoelectrofocusing.Patients and Interventions:Both unrelated patients with TMEM165-CDG with the same deep intronic homozygous mutation (c.792+182G>A) were allocated to receive d-galactose in a daily dose of 1 g/kg.Results:We analyzed N-linked glycans and glycolipids in knockout TMEM165 HEK293 cells, revealing severe hypogalactosylation and GalNAc transfer defects. Although these defects were completely corrected by the addition of Mn2+, we demonstrated that the observed N-glycosylation defect could also be overcome by galactose supplementation. We then demonstrated that oral galactose supplementation in patients with TMEM165-deficient CDG improved biochemical and clinical parameters, including a substantial increase in the negatively charged transferrin isoforms, and a decrease in hypogalactosylated total N-glycan structures, endocrine function, and coagulation parameters.Conclusion:To our knowledge, this is the first description of abnormal glycosylation of lipids in the TMEM165 defect and the first report of successful dietary treatment in TMEM165 deficiency. We recommend the use of oral d-galactose therapy in TMEM165-CDG.We studied the effects of oral D-galactose therapy in patients with TMEM165-CDG. We found that abnormal glycosylation and clinical parameters could be improved by galactose treatment.

Transepithelial migration (TEM) of large numbers of neutrophils (PMN) is a histopathological hallmark of many mucosal inflammatory diseases, yet the mechanisms controlling TEM are poorly defined. We have recently reported a role for shedding of CD44v6 during late events in PMN TEM that are associated with detachment from the luminal (apical) membrane. CD44v6 is a highly glycosylated protein, and our studies have revealed that GM35 binding is dependent not only on expression of CD44v6 protein, but also on its appropriate glycosylation. Here we report that PMN-dependent shedding of CD44v6 specifically requires O-linked glycosylation and this shedding event regulates PMN detachment. Furthermore, glycoepitope screening through the Consortium for Functional Glycomics identified sialyl Lewis A (sLeA) as the glycan specifically expressed on CD44v6 that is critical for GM35 function. Such findings were subsequently confirmed in that antibody-mediated masking of sLeA blocked PMN TEM. ELISA analyses revealed that this inhibition of PMN TEM was also accompanied by a blockade of PMN-dependent shedding of epithelial CD44v6. In summary, we have identified a role for sLeA in PMN-associated shedding of the CD44v6 extracellular domain as well as in PMN detachment from the apical intestinal epithelium as a late event in TEM. Increased understanding of these events could provide potential targets for therapeutic interventions. This work is supported by R01 funding (CAP), a CCFA Senior Research Award, and Emory University Research Council seed grant (NAL).