Isotopic Tracing of Nucleotide Sugar Metabolism in Human Pluripotent Stem Cells

Metabolism not only produces energy necessary for the cell but is also a key regulator of several cellular functions, including pluripotency and self-renewal. Nucleotide sugars (NSs) are activated sugars that link glucose metabolism with cellular functions via protein N-glycosylation and O-GlcNAcylation. Thus, understanding how different metabolic pathways converge in the synthesis of NSs is critical to explore new opportunities for metabolic interference and modulation of stem cell functions. Tracer-based metabolomics is suited for this challenge, however chemically-defined, customizable media for stem cell culture in which nutrients can be replaced with isotopically labeled analogs are scarcely available. Here, we established a customizable flux-conditioned E8 (FC-E8) medium that enables stem cell culture with stable isotopes for metabolic tracing, and a dedicated liquid chromatography mass-spectrometry (LC-MS/MS) method targeting metabolic pathways converging in NS biosynthesis. By 13C6-glucose feeding, we successfully traced the time-course of carbon incorporation into NSs directly via glucose, and indirectly via other pathways, such as glycolysis and pentose phosphate pathways, in induced pluripotent stem cells (hiPSCs) and embryonic stem cells. Then, we applied these tools to investigate the NS biosynthesis in hiPSC lines from a patient affected by deficiency of phosphoglucomutase 1 (PGM1), an enzyme regulating the synthesis of the two most abundant NSs, UDP-glucose and UDP-galactose.

In Vitro Skeletal Muscle Model of PGM1 Deficiency Reveals Altered Energy Homeostasis

Phosphoglucomutase 1 (PGM1) is a key enzyme for the regulation of energy metabolism from glycogen and glycolysis, as it catalyzes the interconversion of glucose 1-phosphate and glucose 6-phosphate. PGM1 deficiency is an autosomal recessive disorder characterized by a highly heterogenous clinical spectrum, including hypoglycemia, cleft palate, liver dysfunction, growth delay, exercise intolerance, and dilated cardiomyopathy. Abnormal protein glycosylation has been observed in this disease. Oral supplementation with D-galactose efficiently restores protein glycosylation by replenishing the lacking pool of UDP-galactose, and rescues some symptoms, such as hypoglycemia, hepatopathy, and growth delay. However, D-galactose effects on skeletal muscle and heart symptoms remain unclear. In this study, we established an in vitro muscle model for PGM1 deficiency to investigate the role of PGM1 and the effect of D-galactose on nucleotide sugars and energy metabolism. Genome-editing of C2C12 myoblasts via CRISPR/Cas9 resulted in Pgm1 (mouse homologue of human PGM1, according to updated nomenclature) knockout clones, which showed impaired maturation to myotubes. No difference was found for steady-state levels of nucleotide sugars, while dynamic flux analysis based on ¹³C6-galactose suggested a block in the use of galactose for energy production in knockout myoblasts. Subsequent analyses revealed a lower basal respiration and mitochondrial ATP production capacity in the knockout myoblasts and myotubes, which were not restored by D-galactose. In conclusion, an in vitro mouse muscle cell model has been established to study the muscle-specific metabolic mechanisms in PGM1 deficiency, which suggested that galactose was unable to restore the reduced energy production capacity.

Synergistic use of glycomics and single-molecule molecular inversion probes for identification of congenital disorders of glycosylation type-1

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.

Structure-Activity Relationship of Fluorinated Sialic Acid Inhibitors for Bacterial Sialylation

Bacterial pathogens such as Nontypeable Haemophilus influenzae (NTHi) can evade the immune system by taking up and presenting host-derived sialic acids. Herein, we report a detailed structure-activity relationship of sialic acid-based inhibitors that prevent the transfer of host sialic acids to NTHi. We report the synthesis and biological evaluation of C-5, C-8, and C-9 derivatives of the parent compound 3-fluorosialic acid (SiaNFAc). Small modifications are tolerated at the C-5 and C-9 positions, while the C-8 position does not allow for modification. These structure-activity relationships define the chemical space available to develop selective bacterial sialylation inhibitors.

Screening for abnormal glycosylation in a cohort of adult liver disease patients

Congenital disorders of glycosylation (CDG) are a rapidly expanding group of rare genetic defects in glycosylation. In a novel CDG subgroup of vacuolar-ATPase (V-ATPase) assembly defects, various degrees of hepatic injury have been described, including end-stage liver disease. However, the CDG diagnostic workflow can be complex as liver disease per se may be associated with abnormal glycosylation. Therefore, we collected serum samples of patients with a wide range of liver pathology to study the performance and yield of two CDG screening methods. Our aim was to identify glycosylation patterns that could help to differentiate between primary and secondary glycosylation defects in liver disease. To this end, we analyzed serum samples of 1042 adult liver disease patients. This cohort consisted of 567 liver transplant candidates and 475 chronic liver disease patients. Our workflow consisted of screening for abnormal glycosylation by transferrin isoelectric focusing (tIEF), followed by in-depth analysis of the abnormal samples with quadruple time-of-flight mass spectrometry (QTOF-MS). Screening with tIEF resulted in identification of 247 (26%) abnormal samples. QTOF-MS analysis of 110 of those did not reveal glycosylation abnormalities comparable with those seen in V-ATPase assembly factor defects. However, two patients presented with isolated sialylation deficiency. Fucosylation was significantly increased in liver transplant candidates compared to healthy controls and patients with chronic liver disease. In conclusion, a significant percentage of patients with liver disease presented with abnormal CDG screening results. However, the glycosylation pattern was not indicative for a V-ATPase assembly factor defect. Advanced glycoanalytical techniques assist in the dissection of secondary and primary glycosylation defects.

Phosphoglucomutase-1 deficiency: Early presentation, metabolic management and detection in neonatal blood spots

Phosphoglucomutase 1 deficiency is a congenital disorder of glycosylation (CDG) with multiorgan involvement affecting carbohydrate metabolism, N-glycosylation and energy production. The metabolic management consists of dietary D-galactose supplementation that ameliorates hypoglycemia, hepatic dysfunction, endocrine anomalies and growth delay. Previous studies suggest that D-galactose administration in juvenile patients leads to more significant and long-lasting effects, stressing the urge of neonatal diagnosis (0-6 months of age). Here, we detail the early clinical presentation of PGM1-CDG in eleven infantile patients, and applied the modified Beutler test for screening of PGM1-CDG in neonatal dried blood spots (DBSs). All eleven infants presented episodic hypoglycemia and elevated transaminases, along with cleft palate and growth delay (10/11), muscle involvement (8/11), neurologic involvement (5/11), cardiac defects (2/11). Standard dietary measures for suspected lactose intolerance in four patients prior to diagnosis led to worsening of hypoglycemia, hepatic failure and recurrent diarrhea, which resolved upon D-galactose supplementation. To investigate possible differences in early vs. late clinical presentation, we performed the first systematic literature review for PGM1-CDG, which highlighted respiratory and gastrointestinal symptoms as significantly more diagnosed in neonatal age. The modified Butler-test successfully identified PGM1-CDG in DBSs from seven patients, including for the first time Guthrie cards from newborn screening, confirming the possibility of future inclusion of PGM1-CDG in neonatal screening programs. In conclusion, severe infantile morbidity of PGM1-CDG due to delayed diagnosis could be prevented by raising awareness on its early presentation and by inclusion in newborn screening programs, enabling early treatments and galactose-based metabolic management.

Cytidine Diphosphate-Ribitol Analysis for Diagnostics and Treatment Monitoring of Cytidine Diphosphate-l-Ribitol Pyrophosphorylase A Muscular Dystrophy

BACKGROUND

Many muscular dystrophies currently remain untreatable. Recently, dietary ribitol has been suggested as a treatment for cytidine diphosphate (CDP)-l-ribitol pyrophosphorylase A (CRPPA, ISPD), fukutin (FKTN), and fukutin-related protein (FKRP) myopathy, by raising CDP-ribitol concentrations. Thus, to facilitate fast diagnosis, treatment development, and treatment monitoring, sensitive detection of CDP-ribitol is required.

METHODS

An LC-MS method was optimized for CDP-ribitol in human and mice cells and tissues.

RESULTS

CDP-ribitol, the product of CRPPA, was detected in all major human and mouse tissues. Moreover, CDP-ribitol concentrations were reduced in fibroblasts and skeletal muscle biopsies from patients with CRPPA myopathy, showing that CDP-ribitol could serve as a diagnostic marker to identify patients with CRPPA with severe Walker-Warburg syndrome and mild limb-girdle muscular dystrophy (LGMD) phenotypes. A screen for potentially therapeutic monosaccharides revealed that ribose, in addition to ribitol, restored CDP-ribitol concentrations and the associated O-glycosylation defect of α-dystroglycan. As the effect occurred in a mutation-dependent manner, we established a CDP-ribitol blood test to facilitate diagnosis and predict individualized treatment response. Ex vivo incubation of blood cells with ribose or ribitol restored CDP-ribitol concentrations in a patient with CRPPA LGMD.

CONCLUSIONS

Sensitive detection of CDP-ribitol with LC-MS allows fast diagnosis of patients with severe and mild CRPPA myopathy. Ribose offers a readily testable dietary therapy for CRPPA myopathy, with possible applicability for patients with FKRP and FKTN myopathy. Evaluation of CDP-ribitol in blood is a promising tool for the evaluation and monitoring of dietary therapies for CRPPA myopathy in a patient-specific manner.

Activity of N-acylneuraminate-9-phosphatase (NANP) is not essential for de novo sialic acid biosynthesis

BACKGROUND

Sialylation of glycoproteins and glycolipids is important for biological processes such as cellular communication, cell migration and protein function. Biosynthesis of CMP-sialic acid, the essential substrate, comprises five enzymatic steps, involving ManNAc and sialic acid and their phosphorylated forms as intermediates. Genetic diseases in this pathway result in different and tissue-restricted phenotypes, which is poorly understood.

METHODS AND RESULTS

We aimed to study the mechanisms of sialic acid metabolism in knockouts (KO) of the sialic acid pathway in two independent cell lines. Sialylation of cell surface glycans was reduced by KO of GNE (UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase), NANS (sialic acid synthase) and CMAS (N-acylneuraminate cytidylyltransferase) genes, but was largely unaffected in NANP (N-acylneuraminate-9-phosphatase) KO, as studied by MAA and PNA lectin binding. NANP is the third enzyme in sialic acid biosynthesis and dephosphorylates sialic acid 9-phosphate to free sialic acid. LC-MS analysis of sialic acid metabolites showed that CMP-sialic acid was dramatically reduced in GNE and NANS KO cells and undetectable in CMAS KO. In agreement with normal cell surface sialylation, CMP-sialic acid levels in NANP KO were comparable to WT cells, even though sialic acid 9-phosphate, the substrate of NANP accumulated. Metabolic flux analysis with ¹³C₆-labelled ManNAc showed a lower, but significant conversion of ManNAc into sialic acid.

CONCLUSIONS

Our data provide evidence that NANP activity is not essential for de novo sialic acid production and point towards an alternative phosphatase activity, bypassing NANP.

GENERAL SIGNIFICANCE

This report contributes to a better understanding of sialic acid biosynthesis in humans.

Combined sialic acid and histone deacetylase (HDAC) inhibitor treatment up-regulates the neuroblastoma antigen GD2

Neuroblastoma cells highly express the disialoganglioside GD2, a tumor-associated carbohydrate antigen, which is only sparsely expressed on healthy tissue. GD2 is a primary target for the development of immunotherapy for neuroblastoma. Immunotherapy with monoclonal anti-GD2 antibodies has proven safety and efficacy in clinical trials and is included in the standard treatment for children with high-risk neuroblastoma. Strategies to modulate GD2 expression in neuroblastoma could further improve anti-GD2-targeted immunotherapy. Here, we report that the cellular sialylation pathway, as well as epigenetic reprogramming, strongly modulates GD2 expression in human and mouse neuroblastoma cell lines. Recognition of GD2 by the 14G2a antibody is sialic acid-dependent and was blocked with the fluorinated sialic acid mimetic Ac₅3F_axNeu5Ac. Interestingly, sialic acid supplementation using a cell-permeable sialic acid analogue (Ac₅Neu5Ac) boosted GD2 expression without or with minor alterations in overall cell surface sialylation. Furthermore, sialic acid supplementation with Ac₅Neu5Ac combined with various histone deacetylase (HDAC) inhibitors, including vorinostat, enhanced GD2 expression in neuroblastoma cells beyond their individual effects. Mechanistic studies revealed that Ac₅Neu5Ac supplementation increased intracellular CMP-Neu5Ac concentrations, thereby providing higher substrate levels for sialyltransferases. Furthermore, HDAC inhibitor treatment increased mRNA expression of the sialyltransferases GM3 synthase (ST3GAL5) and GD3 synthase (ST8SIA1), both of which are involved in GD2 biosynthesis. Our findings reveal that sialic acid analogues and HDAC inhibitors enhance GD2 expression and could potentially be employed to boost anti-GD2 targeted immunotherapy in neuroblastoma patients.

Sialic acid catabolism by N-acetylneuraminate pyruvate lyase is essential for muscle function

Sialic acids are important components of glycoproteins and glycolipids essential for cellular communication, infection, and metastasis. The importance of sialic acid biosynthesis in human physiology is well illustrated by the severe metabolic disorders in this pathway. However, the biological role of sialic acid catabolism in humans remains unclear. Here, we present evidence that sialic acid catabolism is important for heart and skeletal muscle function and development in humans and zebrafish. In two siblings, presenting with sialuria, exercise intolerance/muscle wasting, and cardiac symptoms in the brother, compound heterozygous mutations [chr1:182775324C>T (c.187C>T; p.Arg63Cys) and chr1:182772897A>G (c.133A>G; p.Asn45Asp)] were found in the N-acetylneuraminate pyruvate lyase gene (NPL). In vitro, NPL activity and sialic acid catabolism were affected, with a cell-type-specific reduction of N-acetyl mannosamine (ManNAc). A knockdown of NPL in zebrafish resulted in severe skeletal myopathy and cardiac edema, mimicking the human phenotype. The phenotype was rescued by expression of wild-type human NPL but not by the p.Arg63Cys or p.Asn45Asp mutants. Importantly, the myopathy phenotype in zebrafish embryos was rescued by treatment with the catabolic products of NPL: N-acetyl glucosamine (GlcNAc) and ManNAc; the latter also rescuing the cardiac phenotype. In conclusion, we provide the first report to our knowledge of a human defect in sialic acid catabolism, which implicates an important role of the sialic acid catabolic pathway in mammalian muscle physiology, and suggests opportunities for monosaccharide replacement therapy in human patients.

Intact transferrin and total plasma glycoprofiling for diagnosis and therapy monitoring in phosphoglucomutase-I deficiency

Phosphoglucomutase 1 (PGM1) deficiency results in a mixed phenotype of a Glycogen Storage Disorder and a Congenital Disorder of Glycosylation (CDG). Screening for abnormal glycosylation has identified more than 40 patients, manifesting with a broad clinical and biochemical spectrum which complicates diagnosis. Together with the availability of D-galactose as dietary therapy, there is an urgent need for specific glycomarkers for early diagnosis and treatment monitoring. We performed glycomics profiling by high-resolution QTOF mass spectrometry in a series of 19 PGM1-CDG patients, covering a broad range of biochemical and clinical severity. Bioinformatics and statistical analysis were used to select glycomarkers for diagnostics and define glycan-indexes for treatment monitoring. Using 3 transferrin glycobiomarkers, all PGM1-CDG patients were diagnosed with 100% specificity and sensitivity. Total plasma glycoprofiling showed an increase in high mannose glycans and fucosylation, while global galactosylation and sialylation were severely decreased. For treatment monitoring, we defined 3 glycan-indexes, reflecting normal glycosylation, a lack of complete glycans (LOCGI) and of galactose residues (LOGI). These indexes showed improved glycosylation upon D-galactose treatment with a fast and near-normalization of the galactose index (LOGI) in 6 out of 8 patients and a slower normalization of the LOCGI in all patients. Total plasma glycoprofiling showed improvement of the global high mannose glycans, fucosylation, sialylation, and galactosylation status on D-galactose treatment. Our study indicates specific glycomarkers for diagnosis of mildly and severely affected PGM1-CDG patients, and to monitor the glycan-specific effects of D-galactose therapy.

Clinical glycomics for the diagnosis of congenital disorders of glycosylation

Clinical glycomics comprises a spectrum of different analytical methodologies to analyze glycan structures, which provides insights into the mechanisms of glycosylation. Within clinical diagnostics, glycomics serves as a functional readout of genetic variants, and can form a basis for therapy development, as was described for PGM1-CDG. Integration of glycomics with genomics has resulted in the elucidation of previously unknown disorders of glycosylation, namely CCDC115-CDG, TMEM199-CDG, ATP6AP1-CDG, MAN1B1-CDG, and PGM1-CDG. This review provides an introduction into protein glycosylation and presents the different glycomics methodologies ranging from gel electrophoresis to mass spectrometry (MS) and from free glycans to intact glycoproteins. The role of glycomics in the diagnosis of congenital disorders of glycosylation (CDG) is presented, including a diagnostic flow chart and an overview of glycomics data of known CDG subtypes. The review ends with some future perspectives, showing upcoming technologies as system wide mapping of the N- and O-glycoproteome, intact glycoprotein profiling and analysis of sugar metabolism. These new advances will provide additional insights and opportunities to develop personalized therapy. This is especially true for inborn errors of metabolism, which are amenable to causal therapy, because interventions through supplementation therapy can directly target the pathogenesis at the molecular level.

Unraveling the unknown areas of the human metabolome: the role of infrared ion spectroscopy

The identification of molecular biomarkers is critical for diagnosing and treating patients and for establishing a fundamental understanding of the pathophysiology and underlying biochemistry of inborn errors of metabolism. Currently, liquid chromatography/high-resolution mass spectrometry and nuclear magnetic resonance spectroscopy are the principle methods used for biomarker research and for structural elucidation of small molecules in patient body fluids. While both are powerful techniques, several limitations exist that often make the identification of unknown compounds challenging. Here, we describe how infrared ion spectroscopy has the potential to be a valuable orthogonal technique that provides highly-specific molecular structure information while maintaining ultra-high sensitivity. Here, we characterize and distinguish two well-known biomarkers of inborn errors of metabolism, glutaric acid for glutaric aciduria and ethylmalonic acid for short-chain acyl-CoA dehydrogenase deficiency, using infrared ion spectroscopy. In contrast to tandem mass spectra, in which ion fragments can hardly be predicted, we show that the prediction of an IR spectrum allows reference-free identification in the case that standard compounds are either commercially or synthetically unavailable. Finally, we illustrate how functional group information can be obtained from an IR spectrum for an unknown and how this is valuable information to, for example, narrow down a list of candidate structures resulting from a database query. Early diagnosis in inborn errors of metabolism is crucial for enabling treatment and depends on the identification of biomarkers specific for the disorder. Infrared ion spectroscopy has the potential to play a pivotal role in the identification of challenging biomarkers.

Mutations in ATP6V1E1 or ATP6V1A Cause Autosomal-Recessive Cutis Laxa

Defects of the V-type proton (H⁺) ATPase (V-ATPase) impair acidification and intracellular trafficking of membrane-enclosed compartments, including secretory granules, endosomes, and lysosomes. Whole-exome sequencing in five families affected by mild to severe cutis laxa, dysmorphic facial features, and cardiopulmonary involvement identified biallelic missense mutations in ATP6V1E1 and ATP6V1A, which encode the E1 and A subunits, respectively, of the V₁ domain of the heteromultimeric V-ATPase complex. Structural modeling indicated that all substitutions affect critical residues and inter- or intrasubunit interactions. Furthermore, complexome profiling, a method combining blue-native gel electrophoresis and liquid chromatography tandem mass spectrometry, showed that they disturb either the assembly or the stability of the V-ATPase complex. Protein glycosylation was variably affected. Abnormal vesicular trafficking was evidenced by delayed retrograde transport after brefeldin A treatment and abnormal swelling and fragmentation of the Golgi apparatus. In addition to showing reduced and fragmented elastic fibers, the histopathological hallmark of cutis laxa, transmission electron microscopy of the dermis also showed pronounced changes in the structure and organization of the collagen fibers. Our findings expand the clinical and molecular spectrum of metabolic cutis laxa syndromes and further link defective extracellular matrix assembly to faulty protein processing and cellular trafficking caused by genetic defects in the V-ATPase complex.

Galectin-3-Binding Glycomimetics that Strongly Reduce Bleomycin-Induced Lung Fibrosis and Modulate Intracellular Glycan Recognition

Discovery of glycan-competitive galectin-3-binding compounds that attenuate lung fibrosis in a murine model and that block intracellular galectin-3 accumulation at damaged vesicles, hence revealing galectin-3-glycan interactions involved in fibrosis progression and in intracellular galectin-3 activities, is reported. 3,3'-Bis-(4-aryltriazol-1-yl)thiodigalactosides were synthesized and evaluated as antagonists of galectin-1, -2, -3, and -4 N-terminal, -4 C-terminal, -7 and -8 N-terminal, -9 N-terminal, and -9 C-terminal domains. Compounds displaying low-nanomolar affinities for galectins-1 and -3 were identified in a competitive fluorescence anisotropy assay. X-ray structural analysis of selected compounds in complex with galectin-3, together with galectin-3 mutant binding experiments, revealed that both the aryltriazolyl moieties and fluoro substituents on the compounds are involved in key interactions responsible for exceptional affinities towards galectin-3. The most potent galectin-3 antagonist was demonstrated to act in an assay monitoring galectin-3 accumulation upon amitriptyline-induced vesicle damage, visualizing a biochemically/medically relevant intracellular lectin-carbohydrate binding event and that it can be blocked by a small molecule. The same antagonist administered intratracheally attenuated bleomycin-induced pulmonary fibrosis in a mouse model with a dose/response profile comparing favorably with that of oral administration of the marketed antifibrotic compound pirfenidone.

ATP6AP1 deficiency causes an immunodeficiency with hepatopathy, cognitive impairment and abnormal protein glycosylation

The V-ATPase is the main regulator of intra-organellar acidification. Assembly of this complex has extensively been studied in yeast, while limited knowledge exists for man. We identified 11 male patients with hemizygous missense mutations in ATP6AP1, encoding accessory protein Ac45 of the V-ATPase. Homology detection at the level of sequence profiles indicated Ac45 as the long-sought human homologue of yeast V-ATPase assembly factor Voa1. Processed wild-type Ac45, but not its disease mutants, restored V-ATPase-dependent growth in Voa1 mutant yeast. Patients display an immunodeficiency phenotype associated with hypogammaglobulinemia, hepatopathy and a spectrum of neurocognitive abnormalities. Ac45 in human brain is present as the common, processed ∼40-kDa form, while liver shows a 62-kDa intact protein, and B-cells a 50-kDa isoform. Our work unmasks Ac45 as the functional ortholog of yeast V-ATPase assembly factor Voa1 and reveals a novel link of tissue-specific V-ATPase assembly with immunoglobulin production and cognitive function.

High-resolution mass spectrometry glycoprofiling of intact transferrin for diagnosis and subtype identification in the congenital disorders of glycosylation

Diagnostic screening of the congenital disorders of glycosylation (CDG) generally involves isoelectric focusing of plasma transferrin, a robust method easily integrated in medical laboratories. Structural information is needed as the next step, as required for the challenging classification of Golgi glycosylation defects (CDG-II). Here, we present the use of high-resolution nano liquid chromatography-chip (C8)-quadrupole time of flight mass spectrometry (nanoLC-chip [C8]-QTOF MS) for protein-specific glycoprofiling of intact transferrin, which allows screening and direct diagnosis of a number of CDG-II defects. Transferrin was immunopurified from 10 μL of plasma and analyzed by nanoLC-chip-QTOF MS. Charge distribution raw data were deconvoluted by Mass Hunter software to reconstructed mass spectra. Plasma samples were processed from controls (n = 56), patients with known defects (n = 30), and patients with secondary (n = 6) or unsolved (n = 3) cause of abnormal glycosylation. This fast and robust method, established for CDG diagnostics, requires only 2 hours analysis time, including sample preparation and analysis. For CDG-I patients, the characteristic loss of complete N-glycans could be detected with high sensitivity. Known CDG-II defects (phosphoglucomutase 1 [PGM1-CDG], mannosyl (α-1,6-)-glycoprotein β-1,2-N-acetylglucosaminyltransferase [MGAT2-CDG], β-1,4-galactosyltransferase 1 [B4GALT1-CDG], CMP-sialic acid transporter [SLC35A1-CDG], UDP-galactose transporter [SLC35A2-CDG] and mannosyl-oligosaccharide 1,2-alpha-mannosidase [MAN1B1-CDG]) resulted in characteristic diagnostic profiles. Moreover, in the group of Golgi trafficking defects and unsolved CDG-II patients, distinct profiles were observed, which facilitate identification of the specific CDG subtype. The established QTOF method affords high sensitivity and resolution for the detection of complete glycan loss and structural assignment of truncated glycans in a single assay. The speed and robustness allow its clinical diagnostic application as a first step in the diagnostic procedure for CDG defects.

A common sugar-nucleotide-mediated mechanism of inhibition of (glycosamino)glycan biosynthesis, as evidenced by 6F-GalNAc (Ac3)

Glycosaminoglycan (GAG) polysaccharides have been implicated in a variety of cellular processes, and alterations in their amount and structure have been associated with diseases such as cancer. In this study, we probed 11 sugar analogs for their capacity to interfere with GAG biosynthesis. One analog, with a modification not directly involved in the glycosidic bond formation, 6F-N-acetyl-d-galactosamine (GalNAc) (Ac3), was selected for further study on its metabolic and biologic effect. Treatment of human ovarian carcinoma cells with 50 μM 6F-GalNAc (Ac3) inhibited biosynthesis of GAGs (chondroitin/dermatan sulfate by ∼50-60%, heparan sulfate by ∼35%), N-acetyl-d-glucosamine (GlcNAc)/GalNAc containing glycans recognized by the lectins Datura stramonium and peanut agglutinin (by ∼74 and ∼43%, respectively), and O-GlcNAc protein modification. With respect to function, 6F-GalNAc (Ac3) treatment inhibited growth factor signaling and reduced in vivo angiogenesis by ∼33%. Although the analog was readily transformed in cells into the uridine 5'-diphosphate (UDP)-activated form, it was not incorporated into GAGs. Rather, it strongly reduced cellular UDP-GalNAc and UDP-GlcNAc pools. Together with data from the literature, these findings indicate that nucleotide sugar depletion without incorporation is a common mechanism of sugar analogs for inhibiting GAG/glycan biosynthesis.

Successful liver transplantation and long-term follow-up in a patient with MPI-CDG

Hepatopathy is the most common feature in the Congenital Disorders of Glycosylation (CDG). More than 70 subtypes have been identified in this growing group of inborn errors. Most defects present as multisystem disease, whereas phosphomannose isomerase deficiency (MPI-CDG) presents with exclusive hepato-intestinal phenotype. MPI-CDG has been considered as one of the very few treatable disorders of glycosylation; several patients showed significant improvement of their life-threatening protein-losing enteropathy and coagulation disorder on oral mannose supplementation therapy. However, patients who have MPI-CDG develop progressive liver insufficiency during a later course of disease. A patient who had MPI-CDG developed progressive liver fibrosis, despite oral mannose supplementation and repeated fractionated heparin therapy. She showed mannose therapy-associated hemolytic jaundice. She developed severe dyspnea and exercise intolerance owing to pulmonary involvement, necessitating liver transplant. After transplantation her physical exercise tolerance, pulmonary functions, and metabolic parameters became fully restored. She is still doing well 2 years after transplantation now. In conclusion, we here report on the first successful liver transplantation in CDG.

Multiple phenotypes in phosphoglucomutase 1 deficiency

BACKGROUND

Congenital disorders of glycosylation are genetic syndromes that result in impaired glycoprotein production. We evaluated patients who had a novel recessive disorder of glycosylation, with a range of clinical manifestations that included hepatopathy, bifid uvula, malignant hyperthermia, hypogonadotropic hypogonadism, growth retardation, hypoglycemia, myopathy, dilated cardiomyopathy, and cardiac arrest.

METHODS

Homozygosity mapping followed by whole-exome sequencing was used to identify a mutation in the gene for phosphoglucomutase 1 (PGM1) in two siblings. Sequencing identified additional mutations in 15 other families. Phosphoglucomutase 1 enzyme activity was assayed on cell extracts. Analyses of glycosylation efficiency and quantitative studies of sugar metabolites were performed. Galactose supplementation in fibroblast cultures and dietary supplementation in the patients were studied to determine the effect on glycosylation.

RESULTS

Phosphoglucomutase 1 enzyme activity was markedly diminished in all patients. Mass spectrometry of transferrin showed a loss of complete N-glycans and the presence of truncated glycans lacking galactose. Fibroblasts supplemented with galactose showed restoration of protein glycosylation and no evidence of glycogen accumulation. Dietary supplementation with galactose in six patients resulted in changes suggestive of clinical improvement. A new screening test showed good discrimination between patients and controls.

CONCLUSIONS

Phosphoglucomutase 1 deficiency, previously identified as a glycogenosis, is also a congenital disorder of glycosylation. Supplementation with galactose leads to biochemical improvement in indexes of glycosylation in cells and patients, and supplementation with complex carbohydrates stabilizes blood glucose. A new screening test has been developed but has not yet been validated. (Funded by the Netherlands Organization for Scientific Research and others.).

A compound heterozygous mutation in DPAGT1 results in a congenital disorder of glycosylation with a relatively mild phenotype

Congenital disorders of glycosylation (CDG) are a large group of recessive multisystem disorders caused by impaired protein or lipid glycosylation. The CDG-I subgroup is characterized by protein N-glycosylation defects originating in the endoplasmic reticulum. The genetic defect is known for 17 different CDG-I subtypes. Patients in the few reported DPAGT1-CDG families exhibit severe intellectual disability (ID), epilepsy, microcephaly, severe hypotonia, facial dysmorphism and structural brain anomalies. In this study, we report a non-consanguineous family with two affected adults presenting with a relatively mild phenotype consisting of moderate ID, epilepsy, hypotonia, aggressive behavior and balance problems. Exome sequencing revealed a compound heterozygous missense mutation, c.85A>T (p.I29F) and c.503T>C (p.L168P), in the DPAGT1 gene. The affected amino acids are located in the first and fifth transmembrane domains of the protein. Isoelectric focusing and high-resolution mass spectrometry analyses of serum transferrin revealed glycosylation profiles that are consistent with a CDG-I defect. Our results show that the clinical spectrum of DPAGT1-CDG is much broader than appreciated so far.

Detection of galectin-3 by novel peptidic photoprobes

Photoprobes were prepared with specificity for binding, labeling, and visualizing galectin-3 in a mixture of proteins. The probes were derived from a galectin-3 binding 15-mer peptide sequence in which a benzophenone photolabel was incorporated at the N-terminus and in another case as a phenyl alanine replacement in the middle of the sequence. Detection of galectin-3 was possible in Escherichia coli lysates that were spiked with various amounts of galectin-3.

A new chemical probe for the detection of the cancer-linked galectin-3

A chemical probe was developed for the detection of the emerging cancer marker galectin-3. The probe contains a benzophenone moiety which covalently attaches itself to the protein upon binding and irradiation. Introduction of a fluorescent label via'click' chemistry allows the labelled proteins to be visualized in a gel. With the probe, selective visualization of galectin-3 in protein mixtures was shown and remarkably even in cell lysates.