Abnormal N-glycosylation Research Articles

Functional Characterization and In Silico Prediction Tools Improve the Pathogenicity Prediction of Novel Bile Acid Transporter Variants.

The pathogenicity of cholestatic liver diseases (CLDs) remains insufficiently characterized, hindering definitive diagnosis and timely treatment. The aim of this study was to improve the pathogenicity prediction of novel bile acid (BA) transporter variants in patients with CLDs. We analyzed the clinical characteristics and genetic profiles of a CLD cohort (n = 57) using multiple in silico tools and invitro functional assays. We identified 78 unique variants in four BA transporter genes. The predominant defects were associated with ABCC2 (57/78, 73.1%), with the most frequent being missense variants (39/78, 50.0%). Using in silico tools, we identified 47 novel variants: 12 mis-splicing, 21 deleterious missense, and 23 with altered protein stability. Of the 34 novel variants in ABCC2 identified through invitro functional assays, seven incurred aberrant splicing, 11 missense variants resulted in MRP2 reduction, 9 missense variants resulted in abnormal N-glycosylation, 18 variants altered MRP2 localization, and 26 variants reduced organic anion transport activity. These findings indicate that a multidisciplinary approach, integrating bioinformatics and experimental data, significantly enhances the accuracy of genetic-based CLD diagnosis. It serves as a foundational study for BA transport variants pathogenicity reclassification and expands the mutation spectrum of CLDs in China.

ALG13-Related Epilepsy: Current Insights and Future Research Directions.

The ALG13 gene encodes a subunit of the uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc) transferase enzyme, which plays a key role in the N-linked glycosylation pathway. This pathway involves the attachment of carbohydrate structures to asparagine (Asn) residues in proteins within the endoplasmic reticulum, by which N-glycosylated proteins produced participate a wide range of processes such as electrical gradients formation and neurotransmission. Mutations in the ALG13 gene have been identified as a causative factor for congenital disorders of glycosylation (CDG) and have been frequently associated with epilepsy in affected individuals. Several studies have demonstrated a strong correlation between abnormal N-glycosylation due to ALG13 deficiency and the onset of epilepsy. Despite these findings, the precise role of ALG13 in the pathogenesis of epilepsy remains unclear. This review provides a comprehensive overview of the current literature on ALG13-related disorders, with a focus on recent evidence regarding its role in epilepsy development and progression. Future research directions are also proposed to further elucidate the molecular mechanisms underlying this association.

Acceptors stability modulates the efficiency of post-translational protein N-glycosylation.

N-glycosylation is the most common protein modification in the eukaryotic secretory pathway. It involves the attachment a high mannose glycan to Asn residues in the context of Asn-X-Ser/Thr/Cys, a motif known as N-glycosylation sequon. This process is mediated by STT3A and STT3B, the catalytic subunits of the oligosaccharyltransferase complexes. STT3A forms part of complexes associated with the SEC61 translocon and functions co-translationally. Vacant sequons have another opportunity for glycosylation by complexes carrying STT3B. Local sequence information plays an important role in determining N-glycosylation efficiency, but non-local factors can also have a significant impact. For instance, certain proteins associated with human genetic diseases exhibit abnormal N-glycosylation levels despite having wild-type acceptor sites. Here, we investigated the effect of protein stability on this process. To this end, we generated a family of 40 N-glycan acceptors based on superfolder GFP, and we measured their efficiency in HEK293 cells and in two derived cell lines lacking STT3B or STT3A. Sequon occupancy was highly dependent on protein stability, improving as the thermodynamic stability of the acceptor proteins decreases. This effect is mainly due to the activity of the STT3B-based OST complex. These findings can be integrated into a simple kinetic model that distinguishes local information within sequons from global information of the acceptor proteins.

Presenilin Deficiency Results in Cellular Cholesterol Accumulation by Impairment of Protein Glycosylation and NPC1 Function.

Presenilin proteins (PS1 and PS2) represent the catalytic subunit of γ-secretase and play a critical role in the generation of the amyloid β (Aβ) peptide and the pathogenesis of Alzheimer disease (AD). However, PS proteins also exert multiple functions beyond Aβ generation. In this study, we examine the individual roles of PS1 and PS2 in cellular cholesterol metabolism. Deletion of PS1 or PS2 in mouse models led to cholesterol accumulation in cerebral neurons. Cholesterol accumulation was also observed in the lysosomes of embryonic fibroblasts from Psen1-knockout (PS1-KO) and Psen2-KO (PS2-KO) mice and was associated with decreased expression of the Niemann-Pick type C1 (NPC1) protein involved in intracellular cholesterol transport in late endosomal/lysosomal compartments. Mass spectrometry and complementary biochemical analyses also revealed abnormal N-glycosylation of NPC1 and several other membrane proteins in PS1-KO and PS2-KO cells. Interestingly, pharmacological inhibition of N-glycosylation resulted in intracellular cholesterol accumulation prominently in lysosomes and decreased NPC1, thereby resembling the changes in PS1-KO and PS2-KO cells. In turn, treatment of PS1-KO and PS2-KO mouse embryonic fibroblasts (MEFs) with the chaperone inducer arimoclomol partially normalized NPC1 expression and rescued lysosomal cholesterol accumulation. Additionally, the intracellular cholesterol accumulation in PS1-KO and PS2-KO MEFs was prevented by overexpression of NPC1. Collectively, these data indicate that a loss of PS function results in impaired protein N-glycosylation, which eventually causes decreased expression of NPC1 and intracellular cholesterol accumulation. This mechanism could contribute to the neurodegeneration observed in PS KO mice and potentially to the pathogenesis of AD.

Functional classification of DDOST variants of uncertain clinical significance in congenital disorders of glycosylation

Congenital disorders of glycosylation (CDG) are rare genetic disorders with a spectrum of clinical manifestations caused by abnormal N-glycosylation of secreted and cell surface proteins. Over 130 genes are implicated and next generation sequencing further identifies potential disease drivers in affected individuals. However, functional testing of these variants is challenging, making it difficult to distinguish pathogenic from non-pathogenic events. Using proximity labelling, we identified OST48 as a protein that transiently interacts with lysyl oxidase (LOX), a secreted enzyme that cross-links the fibrous extracellular matrix. OST48 is a non-catalytic component of the oligosaccharyltransferase (OST) complex, which transfers glycans to substrate proteins. OST48 is encoded by DDOST, and 43 variants of DDOST are described in CDG patients, of which 34 are classified as variants of uncertain clinical significance (VUS). We developed an assay based on LOX N-glycosylation that confirmed two previously characterised DDOST variants as pathogenic. Notably, 39 of the 41 remaining variants did not have impaired activity, but we demonstrated that p.S243F and p.E286del were functionally impaired, consistent with a role in driving CDG in those patients. Thus, we describe a rapid assay for functional testing of clinically relevant CDG variants to complement genome sequencing and support clinical diagnosis of affected individuals.

Complex-Type N-Glycans Are Associated with Cartilage Degeneration within Different Loading Sites of the Tibial Plateau for Knee Osteoarthritis Patients.

Abnormal N-glycosylation has been shown to play an important role in the pathogenesis of multiple diseases. However, little is known about the relationship between N-glycosylation and knee osteoarthritis (KOA) progression at the tissue level. Thus, the aim of this study was to quantify the cartilage histomorphometric changes in formalin-fixed paraffin-embedded (FFPE) tissue collected from the lateral and medial compartments of the tibial plateau KOA patients (n = 8). Subsequently, N-glycans were analyzed by matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) followed by in situ MS/MS fragmentation. Overall, the Osteoarthritis Research Society International (OARSI) histological grade and cartilage surface fibrillation index were significantly higher, and chondrocyte size in the superficial zone was much larger, for the medial high-loaded cartilage compared to the lateral less-loaded cartilage. Among 92 putative N-glycans observed by MALDI-MSI, 3 complex-type N-glycans, (Hex)4(HexNAc)3, (Hex)4(HexNAc)4, and (Hex)5(HexNAc)4, and 1 oligomannose-type N-glycan, (Hex)9(HexNAc)2, were significantly higher in intensity in the medial cartilage compared to the lateral cartilage, whereas 2 tetra-antennary fucosylated-type N-glycans, (Hex)3(HexNAc)6(Fuc)2 and (Hex)3(HexNAc)6(Fuc)3, were significantly higher in intensity in the lateral cartilage than the medial cartilage. Our findings indicate that complex-type N-glycans are associated with higher severity of cartilage degeneration and may influence the cellular processes of KOA.

Proteomic and N-glycoproteomic analyses of total subchondral bone protein in patients with primary knee osteoarthritis

N-glycosylation is an important post-translational modification necessary to maintain the structural and functional properties of proteins. Impaired N-glycosylation has been observed in several diseases. It is significantly modified by the state of cells and is used as a diagnostic or prognostic indicator for multiple human diseases, including cancer and osteoarthritis (OA). Aim of the study was to explore the N-glycosylation levels of subchondral bone proteins in patients with primary knee OA (KOA) and screen for potential biological markers for the diagnosis and treatment of primary KOA. A comparative analysis of total protein N-glycosylation under the cartilage was performed in medial subchondral bone (MSB, N = 5) and lateral subchondral bone (LSB, N = 5) specimens from female patients with primary KOA. To analyse the N-glycosylation sites of the proteins, non-labelled quantitative proteomic and N-glycoproteomic analyses were performed based on liquid chromatography-tandem mass spectrometry (LC-MS/MS) data. Parallel reaction monitoring (PRM) validation experiments were carried out on differential N-glycosylation sites of proteins in selected specimens, including MSB (N = 5) and LSB (N = 5), from patients with primary KOA. In total, 1149 proteins with 1369 unique N-chain glycopeptides were detected, and 1215 N-glycosylation sites were found, in which ptmRS scores for 1163 N-glycosylation sites were ≥ 0.9. In addition, N-glycosylation of the total protein in MSB compared to that in LSB was identified, in which 295 N-glycosylation sites were significantly different, including 75 upregulated and 220 downregulated N-glycosylation sites in MSB samples. Importantly, Gene Ontology (GO) and Kyoto Encyclopaedia of Genes and Genomes (KEGG) pathway enrichment analyses of proteins with differential N-glycosylation sites showed that they were primarily associated with metabolic pathways including ECM-receptor interactions, focal adhesion, protein digestion and absorption, amoebiasis, and complement and coagulation cascades. Finally, PRM experiments confirmed the N-glycosylation sites of collagen type VI, alpha 3 (COL6A3, VAVVQHAPSESVDN[+3]ASMPPVK), aggrecan core protein (ACAN, FTFQEAAN[+3]EC[+57]R, TVYVHAN[+3]QTGYPDPSSR), laminin subunit gamma-1 (LAMC1, IPAIN[+3]QTITEANEK), matrix-remodelling-associated protein 5 (MXRA5, ITLHEN[+3]R), cDNA, FLJ92775, highly similar to Homo sapiens melanoma cell adhesion molecule (MCAM), mRNA(B2R642, C[+57]VASVPSIPGLN[+3]R), and aminopeptidase fragment (Q59E93, AEFN[+3]ITLIHPK) in the array data of the top 20 N-glycosylation sites. These abnormal N-glycosylation patterns provide reliable insights for the development of diagnostic and therapeutic methods for primary KOA.

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.

Novel Urinary Glycan Biomarkers Predict Cardiovascular Events in Patients With Type 2 Diabetes: A Multicenter Prospective Study With 5-Year Follow Up (U-CARE Study 2).

Background: Although various biomarkers predict cardiovascular event (CVE) in patients with diabetes, the relationship of urinary glycan profile with CVE in patients with diabetes remains unclear.Methods: Among 680 patients with type 2 diabetes, we examined the baseline urinary glycan signals binding to 45 lectins with different specificities. Primary outcome was defined as CVE including cardiovascular disease, stroke, and peripheral arterial disease.Results: During approximately a 5-year follow-up period, 62 patients reached the endpoint. Cox proportional hazards analysis revealed that urinary glycan signals binding to two lectins were significantly associated with the outcome after adjustment for known indicators of CVE and for false discovery rate, as well as increased model fitness. Hazard ratios for these lectins (+1 SD for the glycan index) were UDA (recognizing glycan: mixture of Man5 to Man9): 1.78 (95% CI: 1.24–2.55, P = 0.002) and Calsepa [High-Man (Man2–6)]: 1.56 (1.19–2.04, P = 0.001). Common glycan binding to these lectins was high-mannose type of N-glycans. Moreover, adding glycan index for UDA to a model including known confounders improved the outcome prediction [Difference of Harrel's C-index: 0.028 (95% CI: 0.001–0.055, P = 0.044), net reclassification improvement at 5-year risk increased by 0.368 (0.045–0.692, P = 0.026), and the Akaike information criterion and Bayesian information criterion decreased from 725.7 to 716.5, and 761.8 to 757.2, respectively].Conclusion: The urinary excretion of high-mannose glycan may be a valuable biomarker for improving prediction of CVE in patients with type 2 diabetes, and provides the rationale to explore the mechanism underlying abnormal N-glycosylation occurring in patients with diabetes at higher risk of CVE.Trial Registration: This study was registered with the University Hospital Medical Information Network on June 26, 2012 (Clinical trial number: UMIN000011525, URL: https://upload.umin.ac.jp/cgi-open-bin/ctr_e/ctr_view.cgi?recptno=R000013482).

Abnormal N-Glycosylation of Human Lens Epithelial Cells in Type-2 Diabetes May Contribute to Cataract Progression.

PurposeIn order to better understand cataract development, we analyzed the glycosylation profile of human lens epithelial cells (HLECs) from anterior lens capsules of type 2 diabetes mellitus (T2DM) and non-diabetic (ND) patients undergoing routine cataract surgery.SettingResearch Department of the Asociación para Evitar la Ceguera, Hospital “Dr. Luis Sánchez Bulnes”, Mexico.DesignExperimental study.MethodsEvaluation of anterior lens capsules from T2DM and ND patients undergoing phacoemulsification and free from other ocular diseases.ResultsHematoxylin-eosin staining revealed HLECs alterations in T2DM samples. From lectins with different sugar specificities used, concanavalin A showed significant differences, labeling homogeneously both in the cytoplasm and in cell membranes in ND capsules, while in T2DM capsules, in addition to membrane and cytoplasm labeling, there were perinuclear vesicles with high concanavalin A labeling. Two-dimensional gel electrophoresis showed that T2DM patients have a ~65-kDa spot with an isoelectric point of 5.5 with a higher density compared to ND capsules, and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis showed 62% homology with type-1 cytokeratin. Immunohistochemistry using anti-pan cytokeratin antibody revealed co-localization with concanavalin A, and a lectin blot revealed with concanavalin A showed a band of ~65 kDa, a molecular weight that corresponds to human type 1 cytokeratin.ConclusionThese results suggest that over-expression of N-glycosidically linked human type 1 cytokeratin may induce capsule disruption and affect selective permeability, allowing the entry of different molecules to the lens that facilitate cataract progression.

Advances in the discovery of novel biomarkers for cancer: spotlight on protein N-glycosylation.

Progress on glycosylation and tumor markers has not been extensively reported. Glycosylation plays an important part in post-translational modification. Previous research on glycosylation-modified biomarkers has lagged behind due to insufficient understanding of glycosylation-related regulations. However, some new methods and ideas illustrated in recent research may provide new inspirations in the field. This article aims to review current advances in revealing relationship between tumors and abnormal N-glycosylation and discuss leading-edge applications of N-glycosylation in developing novel tumor biomarkers.

Novel model of secreted human tau protein reveals the impact of the abnormal N-glycosylation of tau on its aggregation propensity

Alzheimer’s disease (AD) is the most common neurodegenerative disorder and has no disease-modifying treatment yet. The hallmarks of AD are two amyloidogenic proteins: tau and amyloid β (Aβ). Tau undergoes several posttranslational modifications, including N-glycosylation. Tau was reported to be N-glycosylated in AD brains, but not in healthy counterparts, which may affect AD etiology. Here, we aimed to examine the effect of N-glycosylation on aggregation propensity of tau. To that end, a novel SH-SY5Y cell-based model was generated in which recombinant human tau (htau) is forced to be secreted from the cells. Secreted htau was found to localize in the secretory pathway compartments and to undergo N-glycosylation. Following N-glycan cleavage of the secreted htau, various biophysical results collectively indicated that the untreated N-glycosylated secreted htau is markedly less aggregative, contains thinner and shorter fibrils, as compared to treated de-glycosylated secreted htau. This finding shows that N-glycans attached to htau may affect its aggregation. This could help to better understand the effect of N-glycosylated htau on AD progression.

A recurrent missense variant in SLC9A7 causes nonsyndromic X-linked intellectual disability with alteration of Golgi acidification and aberrant glycosylation.

We report two unrelated families with multigenerational nonsyndromic intellectual disability (ID) segregating with a recurrent de novo missense variant (c.1543C>T:p.Leu515Phe) in the alkali cation/proton exchanger gene SLC9A7 (also commonly referred to as NHE7). SLC9A7 is located on human X chromosome at Xp11.3 and has not yet been associated with a human phenotype. The gene is widely transcribed, but especially abundant in brain, skeletal muscle and various secretory tissues. Within cells, SLC9A7 resides in the Golgi apparatus, with prominent enrichment in the trans-Golgi network (TGN) and post-Golgi vesicles. In transfected Chinese hamster ovary AP-1 cells, the Leu515Phe mutant protein was correctly targeted to the TGN/post-Golgi vesicles, but its N-linked oligosaccharide maturation as well as that of a co-transfected secretory membrane glycoprotein, vesicular stomatitis virus G (VSVG) glycoprotein, was reduced compared to cells co-expressing SLC9A7 wild-type and VSVG. This correlated with alkalinization of the TGN/post-Golgi compartments, suggestive of a gain-of-function. Membrane trafficking of glycosylation-deficient Leu515Phe and co-transfected VSVG to the cell surface, however, was relatively unaffected. Mass spectrometry analysis of patient sera also revealed an abnormal N-glycosylation profile for transferrin, a clinical diagnostic marker for congenital disorders of glycosylation. These data implicate a crucial role for SLC9A7 in the regulation of TGN/post-Golgi pH homeostasis and glycosylation of exported cargo, which may underlie the cellular pathophysiology and neurodevelopmental deficits associated with this particular nonsyndromic form of X-linked ID.

Abnormal N-glycosylation pattern for brain nucleotide pyrophosphatase-5 (NPP-5) in Mecp2-mutant murine models of Rett syndrome

Neurological disorders can be associated with protein glycosylation abnormalities. Rett syndrome is a devastating genetic brain disorder, mainly caused by de novo loss-of-function mutations in the methyl-CpG binding protein 2 (MECP2) gene. Although its pathogenesis appears to be closely associated with a redox imbalance, no information on glycosylation is available. Glycoprotein detection strategies (i.e., lectin-blotting) were applied to identify target glycosylation changes in the whole brain of Mecp2 mutant murine models of the disease. Remarkable glycosylation pattern changes for a peculiar 50kDa protein, i.e., the N-linked brain nucleotide pyrophosphatase-5 were evidenced, with decreased N-glycosylation in the presymptomatic and symptomatic mutant mice. Glycosylation changes were rescued by selected brain Mecp2 reactivation. Our findings indicate that there is a causal link between the amount of Mecp2 and the N-glycosylation of NPP-5.

Partial IFN-γR2 deficiency is due to protein misfolding and can be rescued by inhibitors of glycosylation

AbstractWe report a molecular study of the two known patients with autosomal recessive, partial interferon-γ receptor (IFN-γR)2 deficiency (homozygous for mutations R114C and G227R), and three novel, unrelated children, homozygous for S124F (P1) and G141R (P2 and P3). IFN-γR2 levels on the surface of the three latter patients' cells are slightly lower than those on control cells. The patients' cells also display impaired, but not abolished, response to IFN-γ. Moreover, the R114C, S124F, G141R and G227R IFNGR2 hypomorphic alleles all encode misfolded proteins with abnormal N-glycosylation. The mutants are largely retained in the endoplasmic reticulum, although a small proportion reach and function at the cell surface. Strikingly, the IFN-γ response of the patients' cells is enhanced by chemical modifiers of N-glycosylation, as previously shown for patients with gain-of-glysosylation T168N and misfolding 382-387dup null mutations. All four in-frame IFNGR2 hypomorphic mutant alleles encoding surface-expressed receptors are thus deleterious by a mechanism involving abnormal N-glycosylation and misfolding of the IFN-γR2 protein. The diagnosis of partial IFN-γR2 deficiency is clinically useful, as affected patients should be treated with IFN-, unlike patients with complete IFN-γR2 deficiency. Moreover, inhibitors of glycosylation might be beneficial in patients with complete or partial IFN-γR2 deficiency due to misfolding or gain-of-glycosylation receptors.

Abrogation of Complex Glycosylation by Swainsonine Results in Strain- and Cell-specific Inhibition of Prion Replication

Neuroblastoma-derived N2a-PK1 cells, fibroblastic LD9 cells, and CNS-derived CAD5 cells can be infected efficiently and persistently by various prion strains, as measured by the standard scrapie cell assay. Swainsonine, an inhibitor of Golgi α-mannosidase II that causes abnormal N-glycosylation, strongly inhibits infection of PK1 cells by RML, 79A and 22F, less so by 139A, and not at all by 22L prions, and it does not diminish propagation of any of these strains in LD9 or CAD5 cells. Misglycosylated PrP(C) formed in the presence of swainsonine is a good substrate for conversion to PrP(Sc), and misglycosylated PrP(Sc) is fully able to trigger infection and seed the protein misfolding cyclic amplification reaction. Distinct subclones of PK1 cells mediate swainsonine inhibition to very different degrees, implicating misglycosylation of one or more host proteins in the inhibitory process. The use of swainsonine and other glycosylation inhibitors described herein enhances the ability of the cell panel assay to differentiate between prion strains. Moreover, as shown elsewhere, the susceptibility of prions to inhibition by swainsonine in PK1 cells is a mutable trait.

Normal glycosylation screening does not rule out SRD5A3-CDG

By applying next-generation sequencing in three affected siblings showing mental retardation, cataract, coloboma, dysmorphic features and kyphosis, Kahrizi et al.1 reported a novel mutation in the recently discovered 5α-reductase type 3 (SRD5A3) gene.2 The authors suggest that Kahrizi syndrome (MIM 612713) is allelic to the novel type of congenital disorder of glycosylation (SRD5A3-CDG), presenting with variable degree of cerebellar abnormalities, ataxia and eye malformations and caused by different SRD5A3 mutations.2, 3 They emphasize the need for direct genetic testing for SRD5A3 in patients with coloboma or cataract in combination with mental retardation. However, the potential for biochemical screening of glycosylation abnormalities was underestimated on the basis of insufficient information. Screening for CDG in the index family (CVH385) clearly showed abnormal N-glycosylation of transferrin, as described by Cantagrel et al.2 In addition, the CDG screening of the original patient described by Al-Gazali et al.4 was only reported to be normal initially, but was clearly abnormal in three different samples at different ages (6 months, 1 year and 10 years) using radioimmuneassay, transferrin isoelectric focusing (TIEF) or mass spectrometry.2 Furthermore, all patients, as described by Cantagrel et al.2 and by Morava et al.,3 show abnormal CDG type I glycosylation profiles on multiple occasions. We also would like to emphasize that the biochemical mechanism, underlying the somewhat clinically similar Peters' Plus syndrome (MIM 261540), is completely different. The abnormal Glcβ1-3Fuc O-linked disaccharides on thrombospondin type 1 repeats of Peters' Plus syndrome5 are not present in serum transferrin and, thus, TIEF screening is not indicated in the diagnostics of this disease. It is important to note that so far all other patients reported with SRD5A3 mutations were children, and Kahrizi's paper1 is the first report on adults. Abnormal glycosylation evaluated in plasma might be less evident with aging, and TIEF might become normal, especially in adults.6 These variable and age-dependent changes in serum glycosylation pattern (eg, normal results at old or very young age) are known for other types of CDG as well.7 Owing to the indicative cerebello-ocular presentation in SRD5A3-CDG, we agree with the authors' remark to continue with direct testing for SRD5A3 mutations in highly suspected cases, even when CDG screening is normal.

Glycoproteomic analysis of abnormal N-glycosylation on the kappa chain of cryocrystalglobulin in a patient of multiple myeloma

Crystalglobulinemia (cryocrystalglobulinemia) is a rare complication of multiple myeloma. Crystallization of immunoglobulin in blood circulation causes systemic vasculopathy especially in skin and kidney. We found a rare case of crystalglobulinemia in which the light chain was N-glycosylated. The abnormal N-glycosylation was primarily detected as the molecular mass shift on SDS-PAGE by PNGase F treatment. The cryocrystalglobulin was shown to be composed of 55-kDa heavy and 32-kDa light chains on SDS-PAGE. However, the apparent molecular masses of them shifted to 51 kDa and 28 kDa, respectively by PNGase-F treatment. The cryocrystalglobulin was identified as an IgG κ type by peptide mass fingerprinting. The N-glycans on the κ light chain were assigned to non-fucosylated biantennary oligosaccharides and their bisected forms by MALDI-TOF MS/MS analysis of glycopeptides. Sialylation of the abnormal N-glycans was suggested by linear-mode MS and confirmed by HPLC analysis. The N-glycosylation consensus Asn (Asn-Xxx-Ser/Thr) was found in the glycopeptide at the N-glycosylation site determined as “EIVMTQSPANLSVLPGER” by MALDI-TOF MS/MS, in which the consensus Asn (N) was converted to Asp (D) in the enzymatically deglycosylated peptide.

Translation attenuation by PERK balances ER glycoprotein synthesis with lipid-linked oligosaccharide flux

Endoplasmic reticulum (ER) homeostasis requires transfer and subsequent processing of the glycan Glc3Man9GlcNAc2 (G3M9Gn2) from the lipid-linked oligosaccharide (LLO) glucose3mannose9N-acetylglucosamine2-P-P-dolichol (G3M9Gn2-P-P-Dol) to asparaginyl residues of nascent glycoprotein precursor polypeptides. However, it is unclear how the ER is protected against dysfunction from abnormal accumulation of LLO intermediates and aberrant N-glycosylation, as occurs in certain metabolic diseases. In metazoans phosphorylation of eukaryotic initiation factor 2α (eIF2α) on Ser51 by PERK (PKR-like ER kinase), which is activated by ER stress, attenuates translation initiation. We use brief glucose deprivation to simulate LLO biosynthesis disorders, and show that attenuation of polypeptide synthesis by PERK promotes extension of LLO intermediates to G3M9Gn2-P-P-Dol under these substrate-limiting conditions, as well as counteract abnormal N-glycosylation. This simple mechanism requires eIF2α Ser51 phosphorylation by PERK, and is mimicked by agents that stimulate cytoplasmic stress-responsive Ser51 kinase activity. Thus, by sensing ER stress from defective glycosylation, PERK can restore ER homeostasis by balancing polypeptide synthesis with flux through the LLO pathway.

High Residual Activity of PMM2 in Patients’ Fibroblasts: Possible Pitfall in the Diagnosis of CDG-Ia (Phosphomannomutase Deficiency)

Congenital disorders of glycosylation (CDGs) are a rapidly enlarging group of inherited diseases with abnormal N-glycosylation of glycoconjugates. Most patients have CDG-Ia, which is due to a phosphomannomutase (PMM) deficiency. In this article, we report that a significant portion (9 of 54) of patients with CDG-Ia had a rather high residual PMM activity in fibroblasts included in the normal range (means of the controls +/- 2 SD) and amounting to 35%-70% of the mean control value. The clinical diagnosis of CDG-Ia was made difficult by the fact that most (6 of 9) of these patients belong to a subgroup characterized by a phenotype that is milder than classical CDG-Ia. These patients lack some of the symptoms that are suggestive for the diagnosis, such as inverted nipples and abnormal fat deposition, and, as a mean, had higher residual PMM activities in fibroblasts (2.05+/-0.61 mU/mg protein, n=9; vs. controls 5.34+/-1.74 mU/mg protein, n=22), compared with patients with moderate (1.32+/-0.86 mU/mg protein, n=18) or severe (0.63+/-0.56 mU/mg protein, n=27, P<.001) cases. Yet they all showed mild mental retardation, hypotonia, cerebellar hypoplasia, and strabismus. All of them had an abnormal serum transferrin pattern and a significantly reduced PMM activity in leukocytes. Six of the nine patients with mild presentations were compound heterozygotes for the C241S mutation, which is known to reduce PMM activity by only approximately 2-fold. Our results indicate that intermediate PMM values in fibroblasts may mask the diagnosis of CDG-Ia, which is better accomplished by measurement of PMM activity in leukocytes and mutation search in the PMM2 gene. They also indicate that there is some degree of correlation between the residual activity in fibroblasts and the clinical phenotype.