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

Plasma glycoproteomics delivers high-specificity disease biomarkers by detecting site-specific glycosylation abnormalities

INTRODUCTION

The human plasma glycoproteome holds enormous potential to identify personalized biomarkers for diagnostics. Glycoproteomics has matured into a technology for plasma N-glycoproteome analysis but further evolution towards clinical applications depends on the clinical validity and understanding of protein- and site-specific glycosylation changes in disease.

OBJECTIVES

Here, we exploited the uniqueness of a patient cohort of genetic defects in well-defined glycosylation pathways to assess the clinical applicability of plasma N-glycoproteomics.

METHODS

Comparative glycoproteomics was performed of blood plasma from 40 controls and 74 patients with 13 different genetic diseases that impact the protein N-glycosylation pathway. Baseline glycosylation in healthy individuals was compared to reference glycome and intact transferrin protein mass spectrometry data. Use of glycoproteomics data for biomarker discovery and sample stratification was evaluated by multivariate chemometrics and supervised machine learning. Clinical relevance of site-specific glycosylation changes were evaluated in the context of genetic defects that lead to distinct accumulation or loss of specific glycans. Integrated analysis of site-specific glycoproteome changes in disease was performed using chord diagrams and correlated with intact transferrin protein mass spectrometry data.

RESULTS

Glycoproteomics identified 191 unique glycoforms from 58 unique peptide sequences of 34 plasma glycoproteins that span over 3 magnitudes of abundance in plasma. Chemometrics identified high-specificity biomarker signatures for each of the individual genetic defects with better stratification performance than the current diagnostic standard method. Bioinformatic analyses revealed site-specific glycosylation differences that could be explained by underlying glycobiology and protein-intrinsic factors.

CONCLUSION

Our work illustrates the strong potential of plasma glycoproteomics to significantly increase specificity of glycoprotein biomarkers with direct insights in site-specific glycosylation changes to better understand the glycobiological mechanisms underlying human disease.

"Hide and seek": Misleading transferrin variants in PMM2-CDG complicate diagnostics

PURPOSE

Congenital disorders of glycosylation (CDG) are one of the fastest growing groups of inborn errors of metabolism. Despite the availability of next-generation sequencing techniques and advanced methods for evaluation of glycosylation, CDG screening mainly relies on the analysis of serum transferrin (Tf) by isoelectric focusing, HPLC or capillary electrophoresis. The main pitfall of this screening method is the presence of Tf protein variants within the general population. Although reports describe the role of Tf variants leading to falsely abnormal results, their significance in confounding diagnosis in patients with CDG has not been documented so far. Here, we describe two PMM2-CDG cases, in which Tf variants complicated the diagnostic.

EXPERIMENTAL DESIGN

Glycosylation investigations included classical screening techniques (capillary electrophoresis, isoelectric focusing and HPLC of Tf) and various confirmation techniques (two-dimensional electrophoresis, western blot, N-glycome, UPLC-FLR/QTOF MS with Rapifluor). Tf variants were highlighted following neuraminidase treatment. Sequencing of PMM2 was performed.

RESULTS

In both patients, Tf screening pointed to CDG-II, while second-line analyses pointed to CDG-I. Tf variants were found in both patients, explaining these discrepancies. PMM2 causative variants were identified in both patients.

CONCLUSION AND CLINICAL RELEVANCE

We suggest that a neuraminidase treatment should be performed when a typical CDG Tf pattern is found upon initial screening analysis.

Development of tools to facilitate the diagnosis of hereditary fructose intolerance

Although hereditary fructose intolerance (HFI) is an inborn error of fructose metabolism that classically presents at infancy, the diagnosis is often missed or delayed. In this study, we aimed to develop tools to facilitate the diagnosis of HFI. The intake of fructose-containing food products, that is, fruit, fruit juice and sugar-sweetened beverages, was assessed by a 3-day food diary in adult HFI patients (n = 15) and age, sex, and BMI-matched controls (n = 15). Furthermore, glycosylation of transferrin was examined using high-resolution mass spectrometry and abnormally glycosylated transferrin was expressed as ratio of normal glycosylated transferrin. We found that the sensitivity and specificity of the 3-day food diary for the intake of at least one fructose-containing food product were both 100%. Both mono-glyco:diglyco transferrin and a-glyco+mono-glyco:di-glyco transferrin were greater in HFI patients and had a high-discriminatory power (area under the receiver operating characteristic curve: 0.97 and 0.94, respectively). In this well-characterized cohort of adult HFI patients, the 3-day food questionnaire and the glycosylation pattern of transferrin are valuable tools to facilitate the recognition and diagnosis of HFI in adult patients.

The role of clinical glyco(proteo)mics in precision medicine

Glycoproteomics reveals site-specific O- and N-glycosylation that may influence protein properties including binding, activity and half-life. The increasingly mature toolbox with glycomic- and glycoproteomic strategies is applied for the development of biopharmaceuticals and discovery and clinical evaluation of glycobiomarkers in various disease fields. Notwithstanding the contributions of glycoscience in identifying new drug targets, the current report is focused on the biomarker modality that is of interest for diagnostic and monitoring purposes. To this end it is noted that the identification of biomarkers has received more attention than corresponding quantification. Most analytical methods are very efficient in detecting large numbers of analytes but developments to accurately quantify these have so far been limited. In this perspective a parallel is made with earlier proposed tiers for protein quantification using mass spectrometry. Moreover, the foreseen reporting of multimarker readouts is discussed to describe an individual's health or disease state and their role in clinical decision-making. The potential of longitudinal sampling and monitoring of glycomic features for diagnosis and treatment monitoring is emphasized. Finally, different strategies that address quantification of a multimarker panel will be discussed.

N-Glycoprofiling of SLC35A2-CDG: Patient with a Novel Hemizygous Variant

Congenital disorders of glycosylation (CDG) are a group of rare inherited metabolic disorders caused by a defect in the process of protein glycosylation. In this work, we present a comprehensive glycoprofile analysis of a male patient with a novel missense variant in the SLC35A2 gene, coding a galactose transporter that translocates UDP-galactose from the cytosol to the lumen of the endoplasmic reticulum and Golgi apparatus. Isoelectric focusing of serum transferrin, which resulted in a CDG type II pattern, was followed by structural analysis of transferrin and serum N-glycans, as well as the analysis of apolipoprotein CIII O-glycans by mass spectrometry. An abnormal serum N-glycoprofile with significantly increased levels of agalactosylated (Hex3HexNAc4-5 and Hex3HexNAc5Fuc1) and monogalactosylated (Hex4HexNAc4 ± NeuAc1) N-glycans was observed. Additionally, whole exome sequencing and Sanger sequencing revealed de novo hemizygous c.461T > C (p.Leu154Pro) mutation in the SLC35A2 gene. Based on the combination of biochemical, analytical, and genomic approaches, the set of distinctive N-glycan biomarkers was characterized. Potentially, the set of identified aberrant N-glycans can be specific for other variants causing SLC35A2-CDG and can distinguish this disorder from the other CDGs or other defects in the galactose metabolism.

Glycoproteomics in Cerebrospinal Fluid Reveals Brain-Specific Glycosylation Changes

The glycosylation of proteins plays an important role in neurological development and disease. Glycoproteomic studies on cerebrospinal fluid (CSF) are a valuable tool to gain insight into brain glycosylation and its changes in disease. However, it is important to consider that most proteins in CSFs originate from the blood and enter the CSF across the blood-CSF barrier, thus not reflecting the glycosylation status of the brain. Here, we apply a glycoproteomics method to human CSF, focusing on differences between brain- and blood-derived proteins. To facilitate the analysis of the glycan site occupancy, we refrain from glycopeptide enrichment. In healthy individuals, we describe the presence of heterogeneous brain-type N-glycans on prostaglandin H2-D isomerase alongside the dominant plasma-type N-glycans for proteins such as transferrin or haptoglobin, showing the tissue specificity of protein glycosylation. We apply our methodology to patients diagnosed with various genetic glycosylation disorders who have neurological impairments. In patients with severe glycosylation alterations, we observe that heavily truncated glycans and a complete loss of glycans are more pronounced in brain-derived proteins. We speculate that a similar effect can be observed in other neurological diseases where a focus on brain-derived proteins in the CSF could be similarly beneficial to gain insight into disease-related changes.

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.

Identification and quantification of sialylated and core-fucosylated N-glycans in human transferrin by UPLC and LC-MS/MS

Sialylated and core-fucosylated N-glycans in human transferrin (HTF) are used as glycan biomarkers due to their increased or decreased characteristics in certain diseases. However, their absolute quantities remain unclear. In this study, N-glycans of HTF were identified by UPLC and LC-MS/MS using fluorescence tags [2-aminobenzamide (AB) and procainamide (ProA)] and columns [HILIC and anion exchange chromatography-HILIC (AXH)]. The structures of 14 (including five core-fucosylated) N-glycans in total comprising two non-, six mono-, four di-, and two tri-sialylated N-glycans were identified. The quantities (%) of each N-glycan relative to the total N-glycans (100%) were obtained. HILIC and AXH were better for peak identification and separability except for desialylation, respectively. Specifically, sialylated (in ProA-HILIC and ProA-AXH by UPLC or LC-MS/MS) and core-fucosylated (in AB-HILIC and ProA-AXH by UPLC) N-glycans were efficiently identified. Seven neuraminidase-treated (including three core-fucosylated) N-glycans were efficiently identified in ProA-AXH, even their poor separation. Additionally, ProA-AXH was more efficient for the estimation of the absolute quantities of N-glycans from the results of fluorescence intensity (by UPLC) and relative quantity (by LC-MS/MS). These results first demonstrate that ProA is useful for identifying and quantifying sialylated, core-fucosylated, and neuraminidase-treated desialylated N-glycans in HTF using AXH by UPLC and LC/MS.

Mass Spectrometry of Transferrin and Apolipoprotein CIII from Dried Blood Spots for Congenital Disorders of Glycosylation

Dried blood spot (DBS) is the standard specimen for the newborn screening of inborn errors of metabolism (IEM) by tandem mass spectrometry. Availability of DBS for the mass spectrometric analysis of the diagnostic marker proteins, transferrin (Tf) and apolipoprotein CIII (apoCIII), of congenital disorders of glycosylation (CDG) was examined. Recovery of Tf from DBS was only slightly reduced compared with fresh serum. Although oxidation of the core polypeptides was observed, glycans of Tf and apoCIII were unaffected by storage of DBS in the ambient environment for at least 1 month. The combination of DBS and the triple quadrupole mass spectrometer used for IEM screening was sufficient to characterize the aberrant glycoprofiles of Tf and apoCIII in CDG. DBS or dried serum spot on filter paper can reduce the cost of sample transportation and potentially promote mass spectrometric screening of CDG.

Electrospray Ionization Mass Spectrometry of Transferrin: Use of Quadrupole Mass Analyzers for Congenital Disorders of Glycosylation

Electrospray ionization (ESI) mass spectrometry of transferrin can be used to diagnose congenital disorders of glycosylation (CDG) by detecting abnormal N-glycosylation due to reduced site occupancy or processing failure. Time-of-flight mass spectrometers are widely used to separate 25–45 charged ions in the m/z 1,700–3,000 range, and a summed zero-charge mass distribution is generated despite the risk of improper deconvolution. In this study, the low m/z region of the multiply-charged ion mass spectrum enabled a robust analysis of CDG. A triple quadrupole mass spectrometer, the standard instrument for newborn screening for inborn errors of metabolism, permitted the identification of the key ions characteristic of different types of CDG affecting PMM2, ALG14, SLC35A1, SLC35A2, MAN1B1 and PGM1 in the m/z 1,970–2,000 region. Charge deconvolution was used as a complementary tool for validating the findings. It was necessary to set a cutoff level for the evaluation, since small peaks indicating glycosylation failure or reduced sialylation were observed, even in control subjects. This method and workflow facilitates the implementation of MS-based analyses and the screening of CDG in clinical laboratories.

Active site variants in STT3A cause a dominant type I congenital disorder of glycosylation with neuromusculoskeletal findings

Congenital disorders of glycosylation (CDGs) form a group of rare diseases characterized by hypoglycosylation. We here report the identification of 16 individuals from nine families who have either inherited or de novo heterozygous missense variants in STT3A, leading to an autosomal-dominant CDG. STT3A encodes the catalytic subunit of the STT3A-containing oligosaccharyltransferase (OST) complex, essential for protein N-glycosylation. Affected individuals presented with variable skeletal anomalies, short stature, macrocephaly, and dysmorphic features; half had intellectual disability. Additional features included increased muscle tone and muscle cramps. Modeling of the variants in the 3D structure of the OST complex indicated that all variants are located in the catalytic site of STT3A, suggesting a direct mechanistic link to the transfer of oligosaccharides onto nascent glycoproteins. Indeed, expression of STT3A at mRNA and steady-state protein level in fibroblasts was normal, while glycosylation was abnormal. In S. cerevisiae, expression of STT3 containing variants homologous to those in affected individuals induced defective glycosylation of carboxypeptidase Y in a wild-type yeast strain and expression of the same mutants in the STT3 hypomorphic stt3-7 yeast strain worsened the already observed glycosylation defect. These data support a dominant pathomechanism underlying the glycosylation defect. Recessive mutations in STT3A have previously been described to lead to a CDG. We present here a dominant form of STT3A-CDG that, because of the presence of abnormal transferrin glycoforms, is unusual among dominant type I CDGs.

Congenital disorder of glycosylation caused by starting site-specific variant in syntaxin-5

The SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein syntaxin-5 (Stx5) is essential for Golgi transport. In humans, the STX5 mRNA encodes two protein isoforms, Stx5 Long (Stx5L) from the first starting methionine and Stx5 Short (Stx5S) from an alternative starting methionine at position 55. In this study, we identify a human disorder caused by a single missense substitution in the second starting methionine (p.M55V), resulting in complete loss of the short isoform. Patients suffer from an early fatal multisystem disease, including severe liver disease, skeletal abnormalities and abnormal glycosylation. Primary human dermal fibroblasts isolated from these patients show defective glycosylation, altered Golgi morphology as measured by electron microscopy, mislocalization of glycosyltransferases, and compromised ER-Golgi trafficking. Measurements of cognate binding SNAREs, based on biotin-synchronizable forms of Stx5 (the RUSH system) and Förster resonance energy transfer (FRET), revealed that the short isoform of Stx5 is essential for intra-Golgi transport. Alternative starting codons of Stx5 are thus linked to human disease, demonstrating that the site of translation initiation is an important new layer of regulating protein trafficking.

MAN1B1-CDG: Three new individuals and associated biochemical profiles

Congenital disorders of glycosylation (CDG) constitute an ever-growing group of genetic diseases affecting the glycosylation of proteins. CDG individuals usually present with severe multisystem disorders. MAN1B1-CDG is a CDG with nonspecific clinical symptoms such as intellectual deficiency and developmental delay. Although up to 40 affected individuals were described so far, its final diagnosis is not straightforward using common biochemical methods due to the trace-level accumulation of defective glycan structures. In this study, we present three unreported MAN1B1-CDG individuals and propose a decision tree to reach diagnosis using a panel of techniques ranging from exome sequencing to gel electrophoresis and mass spectrometry. The occurrence of MAN1B1-CDG in patients showing unexplained intellectual disability and development delay, as well as a particular transferrin glycosylation profile, can be ascertained notably using matrix assisted laser desorption/ionization – time of flight (MALDI-TOF) mass spectrometry analysis of endo-β-acetylglucosaminidase H-released serum N-glycans. In addition to reporting new pathogenic variants and additional clinical signs such as hypersialorrhea, we highlight particular biochemical features of MAN1B1-CDG with potential glycoprotein-specific glycosylation defects.

Galactose in human metabolism, glycosylation and congenital metabolic diseases: Time for a closer look

Galactose is an essential carbohydrate for cellular metabolism, as it contributes to energy production and storage in several human tissues while also being a precursor for glycosylation. Galactosylated glycoconjugates, such as glycoproteins, keratan sulfate-containing proteoglycans and glycolipids, exert a plethora of biological functions, including structural support, cellular adhesion, intracellular signaling and many more. The biological relevance of galactose is further entailed by the number of pathogenic conditions consequent to defects in galactosylation and galactose homeostasis. The growing number of rare congenital disorders involving galactose along with its recent therapeutical applications are drawing increasing attention to galactose metabolism. In this review, we aim to draw a comprehensive overview of the biological functions of galactose in human cells, including its metabolism and its role in glycosylation, and to provide a systematic description of all known congenital metabolic disorders resulting from alterations of its homeostasis.

Glycomic-Based Biomarkers for Ovarian Cancer: Advances and Challenges

Ovarian cancer remains one of the most common causes of death among gynecological malignancies afflicting women worldwide. Among the gynecological cancers, cervical and endometrial cancers confer the greatest burden to the developing and the developed world, respectively; however, the overall survival rates for patients with ovarian cancer are worse than the two aforementioned. The majority of patients with ovarian cancer are diagnosed at an advanced stage when cancer has metastasized to different body sites and the cure rates, including the five-year survival, are significantly diminished. The delay in diagnosis is due to the absence of or unspecific symptoms at the initial stages of cancer as well as a lack of effective screening and diagnostic biomarkers that can detect cancer at the early stages. This, therefore, provides an imperative to prospect for new biomarkers that will provide early diagnostic strategies allowing timely mitigative interventions. Glycosylation is a protein post-translational modification that is modified in cancer patients. In the current review, we document the state-of-the-art of blood-based glycomic biomarkers for early diagnosis of ovarian cancer and the technologies currently used in this endeavor.

D-galactose supplementation in individuals with PMM2-CDG: results of a multicenter, open label, prospective pilot clinical trial

PMM2-CDG is the most prevalent congenital disorder of glycosylation (CDG) with only symptomatic therapy. Some CDG have been successfully treated with D-galactose. We performed an open-label pilot trial with D-galactose in 9 PMM2-CDG patients. Overall, there was no significant improvement but some milder patients did show positive clinical changes; also there was a trend toward improved glycosylation. Larger placebo-controlled studies are required to determine whether D-galactose could be used as supportive treatment in PMM2-CDG patients.Trial registration ClinicalTrials.gov Identifier: NCT02955264. Registered 4 November 2016, https://clinicaltrials.gov/ct2/show/NCT02955264.

International consensus guidelines for phosphoglucomutase 1 deficiency (PGM1-CDG): Diagnosis, follow-up, and management

Phosphoglucomutase 1 (PGM1) deficiency is a rare genetic disorder that affects glycogen metabolism, glycolysis, and protein glycosylation. Previously known as GSD XIV, it was recently reclassified as a congenital disorder of glycosylation, PGM1-CDG. PGM1-CDG usually manifests as a multisystem disease. Most patients present as infants with cleft palate, liver function abnormalities and hypoglycemia, but some patients present in adulthood with isolated muscle involvement. Some patients develop life-threatening cardiomyopathy. Unlike most other CDG, PGM1-CDG has an effective treatment option, d-galactose, which has been shown to improve many of the patients' symptoms. Therefore, early diagnosis and initiation of treatment for PGM1-CDG patients are crucial decisions. In this article, our group of international experts suggests diagnostic, follow-up, and management guidelines for PGM1-CDG. These guidelines are based on the best available evidence-based data and experts' opinions aiming to provide a practical resource for health care providers to facilitate successful diagnosis and optimal management of PGM1-CDG patients.

Mass spectrometry in clinical glycomics: The path from biomarker identification to clinical implementation

Over the past decades, the genome and proteome have been widely explored for biomarker discovery and personalized medicine. However, there is still a large need for improved diagnostics and stratification strategies for a wide range of diseases. Post-translational modification of proteins by glycosylation affects protein structure and function, and glycosylation has been implicated in many prevalent human diseases. Numerous proteins for which the plasma levels are nowadays evaluated in clinical practice are glycoproteins. While the glycosylation of these proteins often changes with disease, their glycosylation status is largely ignored in the clinical setting. Hence, the implementation of glycomic markers in the clinic is still in its infancy. This is for a large part caused by the high complexity of protein glycosylation itself and of the analytical techniques required for their robust quantification. Mass spectrometry-based workflows are particularly suitable for the quantification of glycans and glycoproteins, but still require advances for their transformation from a biomedical research setting to a clinical laboratory. In this review, we describe why and how glycomics is expected to find its role in clinical tests and the status of current mass spectrometry-based methods for clinical glycomics.

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.

Dissecting Total Plasma and Protein-Specific Glycosylation Profiles in Congenital Disorders of Glycosylation

Protein N-glycosylation is a multifactorial process involved in many biological processes. A broad range of congenital disorders of glycosylation (CDGs) have been described that feature defects in protein N-glycan biosynthesis. Here, we present insights into the disrupted N-glycosylation of various CDG patients exhibiting defects in the transport of nucleotide sugars, Golgi glycosylation or Golgi trafficking. We studied enzymatically released N-glycans of total plasma proteins and affinity purified immunoglobulin G (IgG) from patients and healthy controls using mass spectrometry (MS). The applied method allowed the differentiation of sialic acid linkage isomers via their derivatization. Furthermore, protein-specific glycan profiles were quantified for transferrin and IgG Fc using electrospray ionization MS of intact proteins and glycopeptides, respectively. Next to the previously described glycomic effects, we report unprecedented sialic linkage-specific effects. Defects in proteins involved in Golgi trafficking (COG5-CDG) and CMP-sialic acid transport (SLC35A1-CDG) resulted in lower levels of sialylated structures on plasma proteins as compared to healthy controls. Findings for these specific CDGs include a more pronounced effect for α2,3-sialylation than for α2,6-sialylation. The diverse abnormalities in glycomic features described in this study reflect the broad range of biological mechanisms that influence protein glycosylation.

A new strategy implementing mass spectrometry in the diagnosis of congenital disorders of N-glycosylation (CDG)

Objectives

Congenital disorders of N-glycosylation (CDG) are a large group of rare metabolic disorders caused by defects in the most common post-translational modification of proteins. CDGs are often difficult to diagnose as they are manifested with non-specific symptoms and signs. Analysis of serum transferrin (TRF) isoforms, as the classical procedure used to identify a CDG patient, enables to predict pathological steps in the N-linked glycosylation process.

Methods

We devised a new strategy based on liquid chromatography-mass spectrometry (LC-MS) for the analysis of TRF isoforms by combining a simple and fast sample preparation with a specific chromatographic cleanup/separation step followed by mass-spectrometric measurement. Single TRF isoform masses were obtained through reconstruction of multiply charged electrospray data collected by quadrupole-MS technology. Hereby, we report the first analyzed serum samples obtained from 20 CDG patients and 100 controls.

Results

The ratio of desialylated isoforms to total TRF was calculated for patients and controls. CDG-Type I patients showed higher amounts of bi-sialo isoform (range: 6.7-29.6%) compared to controls (<5.5%, mean percentage 3.9%). CDG-Type II pattern showed an increased peak of tri-sialo isoforms. The mean percentage of tri-sialo-TRF was 9.3% (range: 2.9-12.9%) in controls, which was lower than that obtained from two patients with COG5-CDG and MAN1B1-CDG (18.5 and 24.5%). Intraday and between-day imprecisions were less than 9 and 16%, respectively, for bi-sialo- and less than 3 and 6% for tri-sialo-TRF.

Conclusions

This LC-MS-based approach provides a simple, sensitive and fast analytical tool for characterizing CDG disorders in a routine clinical biochemistry while improving diagnostic accuracy and speeding clinical decision-making.

Novel role for the Golgi membrane protein TMEM165 in control of migration and invasion for breast carcinoma

The TMEM165 gene encodes for a multiple pass membrane protein localized in the Golgi that has been linked to congenital disorders of glycosylation. The TMEM165 protein is a putative ion transporter that regulates H⁺/Ca⁺⁺/Mn⁺⁺ homeostasis and pH in the Golgi. Previously, we identified TMEM165 as a potential biomarker for breast carcinoma in a glycoproteomic study using late stage invasive ductal carcinoma tissues with patient- matched adjacent normal tissues. The TMEM165 protein was not detected in non-malignant matched breast tissues and was detected in invasive ductal breast carcinoma tissues by mass spectrometry. Our hypothesis is that the TMEM165 protein confers a growth advantage to breast cancer. In this preliminary study we have investigated the expression of TMEM165 in earlier stage invasive ductal carcinoma and ductal carcinoma in situ cases. We created a CRISPR/Cas9 knockout of TMEM165 in the human invasive breast cancer cell line MDAMB231. Our results indicate that removal of TMEM165 in these cells results in a significant reduction of cell migration, tumor growth, and tumor vascularization in vivo. Furthermore, we find that TMEM165 expression alters the glycosylation of breast cancer cells and these changes promote the invasion and growth of breast cancer by altering the expression levels of key glycoproteins involved in regulation of the epithelial to mesenchymal transition such as E-cadherin. These studies illustrate new potential functions for this Golgi membrane protein in the control of breast cancer growth and invasion.

Consensus guideline for the diagnosis and management of mannose phosphate isomerase-congenital disorder of glycosylation

Mannose phosphate isomerase-congenital disorder of glycosylation (MPI-CDG) deficiency is a rare subtype of congenital disorders of protein N-glycosylation. It is characterised by deficiency of MPI caused by pathogenic variants in MPI gene. The manifestation of MPI-CDG is different from other CDGs as the patients suffer dominantly from gastrointestinal and hepatic involvement whereas they usually do not present intellectual disability or neurological impairment. It is also one of the few treatable subtypes of CDGs with proven effect of oral mannose. This article covers a complex review of the literature and recommendations for the management of MPI-CDG with an emphasis on the clinical aspect of the disease. A team of international experts elaborated summaries and recommendations for diagnostics, differential diagnosis, management, and treatment of each system/organ involvement based on evidence-based data and experts' opinions. Those guidelines also reveal more questions about MPI-CDG which need to be further studied.

Sugary Logistics Gone Wrong: Membrane Trafficking and Congenital Disorders of Glycosylation

Glycosylation is an important post-translational modification for both intracellular and secreted proteins. For glycosylation to occur, cargo must be transported after synthesis through the different compartments of the Golgi apparatus where distinct monosaccharides are sequentially bound and trimmed, resulting in increasingly complex branched glycan structures. Of utmost importance for this process is the intraorganellar environment of the Golgi. Each Golgi compartment has a distinct pH, which is maintained by the vacuolar H+-ATPase (V-ATPase). Moreover, tethering factors such as Golgins and the conserved oligomeric Golgi (COG) complex, in concert with coatomer (COPI) and soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-mediated membrane fusion, efficiently deliver glycosylation enzymes to the right Golgi compartment. Together, these factors maintain intra-Golgi trafficking of proteins involved in glycosylation and thereby enable proper glycosylation. However, pathogenic mutations in these factors can cause defective glycosylation and lead to diseases with a wide variety of symptoms such as liver dysfunction and skin and bone disorders. Collectively, this group of disorders is known as congenital disorders of glycosylation (CDG). Recent technological advances have enabled the robust identification of novel CDGs related to membrane trafficking components. In this review, we highlight differences and similarities between membrane trafficking-related CDGs.

Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry to Detect Diagnostic Glycopeptide Markers of Congenital Disorders of Glycosylation

Congenital disorders of glycosylation (CDG), an increasingly recognized group of diseases that affect glycosylation, comprise the largest known subgroup of approximately 100 responsible genes related to N-glycosylation. This subgroup presents various molecular abnormalities, of either the CDG-I or the CDG-II type, attributable to a lack of glycans or abnormal glycoform profiles, respectively. The most effective approach to identifying these N-glycosylation disorders is mass spectrometry (MS) using either released glycans, intact glycoproteins or proteolytic peptides as analytes. Among these, MS of tryptic peptides derived from transferrin can be used to reliably identify signature peptides that are characteristic of CDG-I and II. In the present study, matrix-assisted laser desorption/ionization (MALDI) MS was applied to various N-glycosylation disorders including ALG1-CDG, B4GALT1-CDG, SLC35A2-CDG, ATP6V0A2-CDG, TRAPPC11-CDG and MAN1B1-CDG. This method does not require the prior enrichment of glycopeptides or chromatographic separation, and thus serves as a practical alternative to liquid chromatography-electrospray ionization MS. The signature peptides are biomarkers of CDG.

N-Glycosylation profiling of intact target proteins by high-resolution mass spectrometry (MS) and glycan analysis using ion mobility-MS/MS

Glycosylation characterization could lead to the discovery of biomarkers and is crucial in quality control of biopharmaceuticals. Here we present a method to quantify glycoforms on intact proteins, with parallel glycan identification by IMS-MS/MS.

Patients With Aldolase B Deficiency Are Characterized by Increased Intrahepatic Triglyceride Content

CONTEXT

There is an ongoing debate about whether and how fructose is involved in the pathogenesis of nonalcoholic fatty liver disease (NAFLD). A recent experimental study showed an increased intrahepatic triglyceride (IHTG) content in mice deficient for aldolase B (aldo B-/-), the enzyme that converts fructose-1-phosphate to triose phosphates.

OBJECTIVE

To translate these experimental findings to the human situation.

DESIGN

Case-control study.

SETTING

Outpatient clinic for inborn errors of metabolism.

PATIENTS OR OTHER PARTICIPANTS

Patients with hereditary fructose intolerance, a rare inborn error of metabolism caused by a defect in aldolase B (n = 15), and healthy persons matched for age, sex, and body mass index (BMI) (n =15).

MAIN OUTCOME MEASURE

IHTG content, assessed by proton magnetic resonance spectroscopy.

RESULTS

IHTG content was higher in aldo B-/- patients than controls (2.5% vs 0.6%; P = 0.001) on a background of lean body mass (median BMI, 20.4 and 21.8 kg/m2, respectively). Glucose excursions during an oral glucose load were higher in aldo B-/- patients (P = 0.043). Hypoglycosylated transferrin, a surrogate marker for hepatic fructose-1-phosphate concentrations, was more abundant in aldo B-/- patients than in controls (P < 0.001). Finally, plasma β-hydroxybutyrate, a biomarker of hepatic β-oxidation, was lower in aldo B-/- patients than controls (P = 0.009).

CONCLUSIONS

This study extends previous experimental findings by demonstrating that aldolase B deficiency also results in IHTG accumulation in humans. It suggests that the accumulation of fructose-1-phosphate and impairment of β-oxidation are involved in the pathogenesis.

High-resolution capillary zone electrophoresis and mass spectrometry for distinction of undersialylated and hypoglycosylated transferrin glycoforms in body fluids

High-resolution capillary zone electrophoresis is used to distinguish transferrin glycoforms present in human serum, cerebrospinal fluid, and serum treated with neuraminidase and N-glycosidase F. The obtained data are compared to mass spectrometry data from the literature. The main focus is on the analysis of the various asialo-transferrin, monosialo-transferrin, and disialo-transferrin molecules found in these samples. The features of capillary zone electrophoresis and mass spectrometry are reviewed and highlighted in the context of the analysis of undersialylated and hypoglycosylated transferrin molecules. High-resolution capillary zone electrophoresis represents an effective tool to assess the diversity of transferrin patterns whereas mass spectrometry is the method of choice to elucidate structural identification about the glycoforms. Hypoglycosylated transferrin glycoforms present in sera of alcohol abusers and normal subjects are structurally identical to those in sera of patients with a congenital disorder of glycosylation type I. Asialo-transferrin, monosialo-transferrin and disialo-transferrin observed in sera of patients with a type II congenital disorder of glycosylation or a hemolytic uremic syndrome, in cerebrospinal fluid and after treatment of serum with neuraminidase are undersialylated transferrin glycoforms with two N-glycans of varying structure. Undersialylated disialo-transferrin is also observed in sera with high levels of trisialo-transferrin.

Separation methods hyphenated to mass spectrometry for the characterization of the protein glycosylation at the intact level

Glycosylation is one of the most common post-translational modifications of proteins that affects their biological activity, solubility, and half-life. Therefore, its characterization is of great interest in proteomic, particularly from a diagnostic and therapeutic point of view. However, the number and type of glycosylation sites, the degree of site occupancy and the different possible structures of glycans can lead to a very large number of isoforms for a given protein, called glycoforms. The identification of these glycoforms constitutes an important analytical challenge. Indeed, to attempt to characterize all of them, it is necessary to develop efficient separation methods associated with a sensitive and informative detection mode, such as mass spectrometry (MS). Most analytical methods are based on bottom-up proteomics, which consists in the analysis of the protein at the glycopeptides level after its digestion. Even if this approach provides essential information, including the localization and composition of glycans on the protein, it is also characterized by a loss of information on macro-heterogeneity, i.e. the nature of the glycans present on a given glycoform. The analysis of glycoforms at the intact level can overcome this disadvantage. The aim of this review is to detail the state-of-the art of separation methods that can be easily hyphenated with MS for the characterization of protein glycosylation at the intact level. The different electrophoretic and chromatographic approaches are discussed in detail. The miniaturization of these separation methods is also discussed with their potential applications. While the first studies focused on the development and optimization of the separation step to achieve high resolution between isoforms, the recent ones are much more application-oriented, such as clinical diagnosis, quality control, and glycoprotein monitoring in formulations or biological samples.

Clinical, neuroradiological, and biochemical features of SLC35A2-CDG patients

SLC35A2-CDG is caused by mutations in the X-linked SLC35A2 gene encoding the UDP-galactose transporter. SLC35A2 mutations lead to hypogalactosylation of N-glycans. SLC35A2-CDG is characterized by severe neurological symptoms and, in many patients, early-onset epileptic encephalopathy. In view of the diagnostic challenges, we studied the clinical, neuroradiological, and biochemical features of 15 patients (11 females and 4 males) with SLC35A2-CDG from various centers. We describe nine novel pathogenic variations in SLC35A2. All affected individuals presented with a global developmental delay, and hypotonia, while 70% were nonambulatory. Epilepsy was present in 80% of the patients, and in EEG hypsarrhythmia and findings consistent with epileptic encephalopathy were frequently seen. The most common brain MRI abnormality was cerebral atrophy with delayed myelination and multifocal inhomogeneous abnormal patchy white matter hyperintensities, which seemed to be nonprogressive. Thin corpus callosum was also common, and all the patients had a corpus callosum shorter than normal for their age. Variable dysmorphic features and growth deficiency were noted. Biochemically, normal mucin type O-glycosylation and lipid glycosylation were found, while transferrin mass spectrometry was found to be more specific in the identification of SLC35A2-CDG, as compared to routine screening tests. Although normal glycosylation studies together with clinical variability and genetic results complicate the diagnosis of SLC35A2-CDG, our data indicate that the combination of these three elements can support the pathogenicity of mutations in SLC35A2.

Desialylation of platelets induced by Von Willebrand Factor is a novel mechanism of platelet clearance in dengue

Thrombocytopenia and platelet dysfunction are commonly observed in patients with dengue virus (DENV) infection and may contribute to complications such as bleeding and plasma leakage. The etiology of dengue-associated thrombocytopenia is multifactorial and includes increased platelet clearance. The binding of the coagulation protein von Willebrand factor (VWF) to the platelet membrane and removal of sialic acid (desialylation) are two well-known mechanisms of platelet clearance, but whether these conditions also contribute to thrombocytopenia in dengue infection is unknown. In two observational cohort studies in Bandung and Jepara, Indonesia, we show that adult patients with dengue not only had higher plasma concentrations of plasma VWF antigen and active VWF, but that circulating platelets had also bound more VWF to their membrane. The amount of platelet-VWF binding correlated well with platelet count. Furthermore, sialic acid levels in dengue patients were significantly reduced as assessed by the binding of Sambucus nigra lectin (SNA) and Maackia amurensis lectin II (MAL-II) to platelets. Sialic acid on the platelet membrane is neuraminidase-labile, but dengue virus has no known neuraminidase activity. Indeed, no detectable activity of neuraminidase was present in plasma of dengue patients and no desialylation was found of plasma transferrin. Platelet sialylation was also not altered by in vitro exposure of platelets to DENV nonstructural protein 1 or cultured DENV. In contrast, induction of binding of VWF to glycoprotein 1b on platelets using the VWF-activating protein ristocetin resulted in the removal of platelet sialic acid by translocation of platelet neuraminidase to the platelet surface. The neuraminidase inhibitor oseltamivir reduced VWF-induced platelet desialylation. Our data demonstrate that excessive binding of VWF to platelets in dengue results in neuraminidase-mediated platelet desialylation and platelet clearance. Oseltamivir might be a novel treatment option for severe thrombocytopenia in dengue infection.

Dengue is the most common arbovirus infection in the world. A decrease in the number of blood platelets is an almost universal finding in severe dengue. Binding of the coagulation protein von Willebrand factor (VWF) and loss of sialic acid residues from the platelet membrane are two main mechanisms of clearance of senescent platelets under non-pathological conditions. Here, we show that platelets from patients with acute dengue have bound more VWF and have lost sialic acid from their membrane. Sialic acid can be cleaved by the enzyme neuraminidase. We show that neuraminidase activity in the plasma is not increased and that neither dengue virus itself nor nonstructural protein 1, a protein secreted by dengue virus, cleave sialic acid from the platelet membrane. In contrast, binding of VWF to platelets results in translocation of neuraminidase to the platelet membrane and subsequent cleavage of sialic acid. This process could be inhibited by the neuraminidase inhibitor oseltamivir, a commonly used anti-influenza drug. Altogether, our results indicate that VWF binding to platelets is increased in dengue infection, leading to the removal of sialic acid and platelet clearance. Oseltamivir may prevent this process and thus represent a novel treatment option for low platelet numbers in dengue infection.

Relative quantification of plasma N-glycans in type II congenital disorder of glycosylation patients by mass spectrometry

BACKGROUND

Type II Congenital Disorders of Glycosylation (CDG-II) are a group of diseases with challenging diagnostics characterized by defects in the processing of glycans in the Golgi apparatus. Mass Spectrometry (MS) has been a valuable tool in the definition of CDG-II subtypes. While some CDG-II subtypes are associated with specific N-glycan structures, others only produce changes in relative levels, reinforcing the demand for quantification methods.

METHODS

Plasma samples from control individuals were pooled, derivatized with deuterated iodomethane (I-CD₃), and used as internal standards for controls and patients whose glycans were derivatized with iodomethane (I-CH₃), followed by MALDI MS, LC-MS and -MS/MS analyses.

RESULTS

Total N-glycans from fifteen CDG-II patients were evaluated, and 4 cases with molecular diagnosis were considered in detail: 2ATP6V0A2-CDG siblings, and 2 MAN1B1-CDG patients, one of them carrying a previously undescribed p.Gly536Val mutation.

CONCLUSIONS

Our methodology offers a feasible alternative to the current methods for CDG-II diagnosis by MS, which quantify glycan structures as fractions of the total summed signal across a mass spectrum, a strategy that lowers the variability of minor components. Moreover, given its sensitivity for less concentrated yet biologically relevant structures, it might assist the uncovering of novel diagnostic glycans in other CDG-II subtypes.

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.

Mass spectrometry for protein sialoglycosylation

Sialic acids are a family of structurally unique and negatively charged nine-carbon sugars, normally found at the terminal positions of glycan chains on glycoproteins and glycolipids. The glycosylation of proteins is a universal post-translational modification in eukaryotic species and regulates essential biological functions, in which the most common sialic acid is N-acetyl-neuraminic acid (2-keto-5-acetamido-3,5-dideoxy-D-glycero-D-galactononulopyranos-1-onic acid) (Neu5NAc). Because of the properties of sialic acids under general mass spectrometry (MS) conditions, such as instability, ionization discrimination, and mixed adducts, the use of MS in the analysis of protein sialoglycosylation is still challenging. The present review is focused on the application of MS related methodologies to the study of both N- and O-linked sialoglycans. We reviewed MS-based strategies for characterizing sialylation by analyzing intact glycoproteins, proteolytic digested glycopeptides, and released glycans. The review concludes with future perspectives in the field.

Use of Endoglycosidase H as a diagnostic tool for MAN1B1-CDG patients

Congenital disorders of glycosylation (CDG) are heterogeneous group of genetic protein and lipid glycosylation abnormalities. With some 33 reported patients, MAN1B1-CDG belongs to the more frequent causes of CDG-II. MAN1B1 encodes an α1,2-mannosidase that removes the terminal mannose residue from the middle branch. Several methods have been proposed to characterize the glycosylation changes. In MAN1B1-CDG, the abnormal accumulating N-glycan structures are mostly absent or found in trace amounts in total human serum. To overcome this issue, in this study, we present a straightforward procedure based on the use of Endo-β-N-acetylglucosaminidase H to easily diagnose MAN1B1-CDG patients and mannosidase defects.

Proteoform Analysis to Fulfill Unmet Clinical Needs and Reach Global Standardization of Protein Measurands in Clinical Chemistry Proteomics

In clinical testing of protein markers, structure variants of the measurand are often not taken into account. This heterogeneous character of protein measurands in immunoassays often renders test standardization impossible. Consequently, test results from different methods can lead to underdiagnosis or overdiagnosis and, thus, undertreatment or overtreatment of patients. The systematic structural analysis of protein isoforms has been coined proteoform profiling and is performed through mass spectrometry-based proteomics strategies. Knowledge on proteoforms allows refining existing uni-marker tests and moreover has great potential to contribute to the urgent need for new tests to predict prognosis and severity of diseases.

High-resolution capillary zone electrophoresis for transferrin glycoform analysis associated with congenital disorders of glycosylation

High-resolution capillary zone electrophoresis is used to assess the transferrin profile in serum of patients with eight different congenital disorders of glycosylation that represent type I, type II, and mixed type I/II disorders. Capillary zone electrophoresis data are compared to patterns obtained by gel isoelectric focusing. The high-resolution capillary zone electrophoresis method is shown to represent an effective tool to assess the diversity of transferrin patterns. Hypoglycosylated disialo-, monosialo-, and asialo-transferrin in type I cases can be distinguished from the corresponding underdesialylated transferrin glycoforms present in type II disorders. The latter can be separated from and detected ahead of their corresponding hypoglycosylated forms of type I patients. Both types of glycoforms are detected in sera of mixed type I/II patients. The assay has the potential to be used as screening method for congenital disorders of glycosylation. It can be run with a few microliters of serum when microvials are used.

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.

Complex Phenotypes in Inborn Errors of Metabolism: Overlapping Presentations in Congenital Disorders of Glycosylation and Mitochondrial Disorders

Congenital disorders of glycosylation (CDG) and mitochondrial disorders have overlapping clinical features, including central nervous system, cardiac, gastrointestinal, hepatic, muscular, endocrine, and psychiatric disease. Specific abnormalities orienting the clinician toward the right diagnostic approach include abnormal fat distribution, coagulation abnormalities, together with anticoagulation abnormalities, hyperinsulinism, and congenital malformations in CDG. Diabetes, sensorineural deafness, and depression are very rare in CDG but common in mitochondrial disease. Chronic lactic acidosis is highly suggestive of mitochondrial dysfunction. Serum transferrin isoform analysis is specific for glycosylation abnormalities but not abnormal in all types of CDG.

Mass Spectrometry Approaches to Glycomic and Glycoproteomic Analyses

Glycomic and glycoproteomic analyses involve the characterization of oligosaccharides (glycans) conjugated to proteins. Glycans are produced through a complicated nontemplate driven process involving the competition of enzymes that extend the nascent chain. The large diversity of structures, the variations in polarity of the individual saccharide residues, and the poor ionization efficiencies of glycans all conspire to make the analysis arguably much more difficult than any other biopolymer. Furthermore, the large number of glycoforms associated with a specific protein site makes it more difficult to characterize than any post-translational modification. Nonetheless, there have been significant progress, and advanced separation and mass spectrometry methods have been at its center and the main reason for the progress. While glycomic and glycoproteomic analyses are still typically available only through highly specialized laboratories, new software and workflow is making it more accessible. This review focuses on the role of mass spectrometry and separation methods in advancing glycomic and glycoproteomic analyses. It describes the current state of the field and progress toward making it more available to the larger scientific community.

UPLC-MS/MS based diagnostics for epithelial ovarian cancer using fully sialylated C4-binding protein

Serum levels of fully sialylated C4-binding protein (FS-C4BP) are remarkably elevated in patients with epithelial ovarian cancer (EOC) and can be used as a marker to distinguish ovarian clear cell carcinoma from endometrioma. This study aimed to develop a stable, robust and reliable liquid chromatography–hybrid mass spectrometry (UPLC-MS/MS) based diagnostic method that would generalize FS-C4BP as a clinical EOC biomarker. Glycopeptides derived from 20 μL of trypsin-digested serum glycoprotein were analyzed via UPLC equipped with an electrospray ionization time-of-flight mass spectrometer. This UPLC-MS/MS-based diagnostic method was optimized for FS-C4BP and validated using sera from 119 patients with EOC and 127 women without cancer. A1958 (C4BP peptide with two fully sialylated biantennary glycans) was selected as a marker of FS-C4BP because its level in serum was highest among FS-C4BP family members. Preparation and UPLC-MS/MS were optimized for A1958, and performance and robustness were significantly improved relative to our previous method. An area under the curve analysis of the FS-C4BP index receiver operating characteristic curve revealed that the ratio between A1958 and A1813 (C4BP peptide with two partially sialylated biantennary glycans) reached 85%. A combination of the FS-C4BP index and carbohydrate antigen-125 levels further enhanced the sensitivity and specificity.