Carbohydrate-deficient glycoprotein syndrome: electrophoretic study of multiple serum glycoproteins

Utilising extracellular vesicles for early cancer diagnostics: benefits, challenges and recommendations for the future

To increase cancer patient survival and wellbeing, diagnostic assays need to be able to detect cases earlier, be applied more frequently, and preferably before symptoms develop. The expansion of blood biopsy technologies such as detection of circulating tumour cells and cell-free DNA has shown clinical promise for this. Extracellular vesicles released into the blood from tumour cells may offer a snapshot of the whole of the tumour. They represent a stable and multifaceted complex of a number of different types of molecules including DNA, RNA and protein. These represent biomarker targets that can be collected and analysed from blood samples, offering great potential for early diagnosis. In this review we discuss the benefits and challenges of the use of extracellular vesicles in this context and provide recommendations on where this developing field should focus their efforts to bring future success.

N- and O-glycan analysis for the detection of glycosylation disorders

Background

Congenital disorders of glycosylation (CDGs) are defined as a group of several rare autosomal recessive inborn errors of metabolism that affect the glycosylation of many proteins and/or lipids. Variable clinical presentation is very characteristic for all types of CDGs; symptoms include severe neurological manifestations that usually start in the neonatal period and cause aggressive irreversible neurological damage. These disorders are usually misdiagnosed as other non-inheritable disorders or remain undiagnosed for a long time, leading to severe neurological complications. The diagnosis of CDGs is quite tedious due to their diverse clinical presentation. In Egypt, there is still no available screening programme to detect CDGs in patients at a young age. Therefore, the need for a reliable rapid test that uses a small sample size has emerged.

This study included 50 suspected subjects and 50 healthy controls with matching age and sex. Western blotting and liquid chromatography-tandem mass spectrometry were used for the analysis of N- and O-glycans, respectively.

Results

The study detected 9 patients with hypoglycosylation (18%). Eight of the nine patients showed abnormal separation of N-glycoproteins using Western blotting indicative of reduced glycosylation (16% of the study subjects and 89% of the subjects with hypoglycosylation). Only one of the nine patients showed a decreased level of sialyl-T-antigen with a normal T-antigen level leading to an increased T/ST ratio (2% of study subjects and 11% of the subjects with hypoglycosylation).

Conclusion

Although N- and O-glycan analysis did not determine the underlying type of CDG, it successfully detected hypoglycosylation in 9 clinically suspected patients (18% of the studied subjects). All detected CDG cases were confirmed by molecular analysis results of mutations causing 4 different types of congenital disorders of glycosylation.

Congenital disorder of glycosylation type Ic: report of a Japanese case

Congenital disorders of glycosylation (CDG) are inherited metabolic diseases affecting N-linked glycosylation pathways with variable clinical presentations characterized by psychomotor retardation, seizures, ataxia and hypotonia. CDG-Ic is caused by mutation in the ALG6 gene encoding alpha-1,3-glucosyltransferase. We present a 9-year-old girl diagnosed as having CDG-Ic. She developed severe psychomotor retardation, epileptic seizures, muscle hypotonia, strabismus and some dysmorphic features without inverted nipples or fat pads. She showed a fluctuating serum transaminase level with or without some infection, and a characteristically low level of antithrombin III. MR imaging of the brain at age 2years demonstrated the lower limit of normal myelination, mild atrophy of the cerebrum, and mild hypoplasia of the brainstem and cerebellum. The patient exhibited a CDG type I pattern of serum transferrin on isoelectric focusing and mass spectrometric profiling. Sequence analysis of the ALG6 gene showed two heterozygous mutations, c.998C>T (A333V) and c.1061C>T (P354L). The patient was diagnosed as having CDG-Ic with a novel mutation, making her the first Japanese case. It was suggested that the severe psychomotor retardation in the patient was due to the existence of multiple mutant ALG6 alleles.

A rapid mass spectrometric strategy for the characterization of N- and O-glycan chains in the diagnosis of defects in glycan biosynthesis

Glycosylation of proteins is a very complex process which involves numerous factors such as enzymes or transporters. A defect in one of these factors in glycan biosynthetic pathways leads to dramatic disorders named congenital disorders of glycosylation (CDG). CDG can affect the biosynthesis of not only protein N-glycans but also O-glycans. The structural analysis of glycans on serum glycoproteins is essential to solving the defect. For this reason, we propose in this paper a strategy for the simultaneous characterization of both N- and O-glycan chains isolated from the serum glycoproteins. The serum (20 microL) is used for the characterization of N-glycans which are released by enzymatic digestion with PNGase F. O-glycans are chemically released by reductive elimination from whole serum glycoproteins using 10 microL of the serum. Using strategies based on mass spectrometric analysis, the structures of N- and O-glycan chains are defined. These strategies were applied on the sera from one patient with CDG type IIa, and one patient with a mild form of congenital disorder of glycosylation type II (CDG-II) that is caused by a deficiency in the Cog1 subunit of the complex.

Screening and diagnosis of congenital disorders of glycosylation

The aim of this paper is to review the diagnostics of congenital disorders of glycosylation (CDG), an ever expanding group of diseases. Development delay, neurological, and other clinical abnormalities as well as various non-specific laboratory changes can lead to the first suspicion of the disease. Still common screening test for most CDG types, including CDG Ia, is isoelectric focusing/polyacrylamide gel electrophoresis (IEF). IEF demonstrates the hypoglycosylation of various glycoproteins, usually serum transferrin. Other methods, such as agarose electrophoresis, capillary electrophoresis, high-performance liquid chromatography, micro-column separation combined with turbidimetry, enzyme-(EIA) and radioimmunoassay (RIA) have also been used for screening. However, these methods do not recognize all CDG defects, so other approaches including analysis of membrane-linked markers and urine oligosaccharides should be taken. Confirmation of diagnosis and detailed CDG subtyping starts with thorough structure analysis of the affected lipid-linked oligosaccharide or protein-(peptide)-linked-glycan using metabolic labeling and various (possibly mass-spectrometry combined) techniques. Decreased enzyme activity in peripheral leukocytes/cultured fibroblasts or analysis of affected transporters and other functional proteins combined with identification of specific gene mutations confirm the diagnosis. Prenatal diagnosis, based on enzyme assay or mutation analysis, is also available. Peri-/post-mortem investigations of fatal cases are important for genetic counseling. Evaluation of various analytical approaches and proposed algorithms for investigation complete the review.

Diagnosis of congenital disorders of glycosylation type-I using protein chip technology

A method for the diagnosis of the congenital disorders of glycosylation type I (CDG-I) by SELDI-TOF-MS of serum transferrin immunocaptured on protein chip arrays is described. The underglycosylation of glycoproteins in CDG-I produces glycoforms of transferrin with masses lower than that of the normal fully glycosylated transferrin. Immobilisation of antitransferrin antibodies on reactive-surface protein chip arrays (RS100) selectively enriched transferrin by at least 100-fold and allowed the detection of patterns of transferrin glycoforms by SELDI-TOF-MS using approximately 0.3 microL of serum/plasma. Abnormal patterns of immunocaptured transferrin were detected in patients with known defects in glycosylation (CDG-Ia, CDG-Ib, CDG-Ic, CDG-If and CDG-Ih) and in patients in whom the basic defect has not yet been identified (CDG-Ix). The correction of the N-glycosylation defect in a patient with CDG-Ib after mannose therapy was readily detected. A patient who had an abnormal transferrin profile by IEF but a normal profile by SELDI-TOF-MS analysis was shown to have an amino acid polymorphism by sequencing transferrin by quadrupole-TOF MS. Complete agreement was obtained between analysis of immunocaptured transferrin by SELDI-TOF-MS and the IEF profile of transferrin, the clinical severity of the disease and the levels of aspartylglucosaminidase activity (a surrogate marker for the diagnosis of CDG-I). SELDI-TOF-MS of transferrin immunocaptured on protein chip arrays is a highly sensitive diagnostic method for CDG-I, which could be fully automated using microtitre plates and robotics.

Congenital disorder of glycosylation type Id: clinical phenotype, molecular analysis, prenatal diagnosis, and glycosylation of fetal proteins

Congenital disorder of glycosylation type Id is an inherited glycosylation disorder based on a defect of the first mannosyltransferase involved in N-glycan biosynthesis inside the endoplasmic reticulum. Only one patient with this disease has been described until now. In this article, a second patient and an affected fetus are described. The patient showed abnormal glycosylation of several plasma proteins as demonstrated by isoelectric focusing and Western blot. Lipid-linked oligosaccharides in the endoplasmic reticulum, reflecting early N-glycan assembly, revealed an accumulation of immature Man(5)GlcNAc(2)-glycans in fibroblasts of the patient. Chorion cells of the affected fetus showed the same characteristic lipid-linked oligosaccharides pattern. However, the fetus had a normal glycosylation of several plasma proteins. Some fetal glycoproteins are known to be derived from the mother, but even glycoproteins that do not cross the placenta were normally glycosylated in the affected fetus. Maternal or placental factors that partially compensate for the glycosylation defect in the fetal stage must be proposed and may be relevant for the therapy of these disorders in the future.

Pitfalls and drawbacks in screening of congenital disorders of glycosylation

Congenital disorders of glycosylation include a group of diseases, each of them caused by different protein (mostly enzyme) impairment due to a specific gene defect. The many subtypes are classified according to clinical features, enzymology and molecular genetic analyses. Problems in diagnostics arise from the great diversity in clinical presentation, usually age-related, and different severities of individual types of these, by far underdiagnosed, diseases. Also the biochemical findings tend to vary, even within a single type. No one screening test, common for all types, is available so far. Several methods of choice may be used in the first approach; other procedures must follow for detailed typing of the defect. Possible drawbacks and pitfalls in the diagnostics from the viewpoint of our 3-year studies and practical screening experience are presented.

Detailed glycan analysis of serum glycoproteins of patients with congenital disorders of glycosylation indicates the specific defective glycan processing step and provides an insight into pathogenesis

The fundamental importance of correct protein glycosylation is abundantly clear in a group of diseases known as congenital disorders of glycosylation (CDGs). In these diseases, many biological functions are compromised, giving rise to a wide range of severe clinical conditions. By performing detailed analyses of the total serum glycoproteins as well as isolated transferrin and IgG, we have directly correlated aberrant glycosylation with a faulty glycosylation processing step. In one patient the complete absence of complex type sugars was consistent with ablation of GlcNAcTase II activity. In another CDG type II patient, the identification of specific hybrid sugars suggested that the defective processing step was cell type-specific and involved the mannosidase III pathway. In each case, complementary serum proteome analyses revealed significant changes in some 31 glycoproteins, including components of the complement system. This biochemical approach to charting diseases that involve alterations in glycan processing provides a rapid indicator of the nature, severity, and cell type specificity of the suboptimal glycan processing steps; allows links to genetic mutations; indicates the expression levels of proteins; and gives insight into the pathways affected in the disease process.

Western blotting with diaminobenzidine detection for the diagnosis of congenital disorders of glycosylation

Congenital disorders of glycosylation (CDG) are a growing group of genetic disorders caused by a deficient assembly or processing of glycoproteins. Our aim was to improve a western blotting detection procedure previously described and to assess the efficiency of this procedure for CDG screening, using isoelectric focusing (IEF) as the reference method. We analysed transferrin and haptoglobin in serum from 12 patients with CDG-Ia, 3 patients with CDG-X and 95 healthy paediatric controls. These proteins were also studied in dried blood spot samples. Reference values for our paediatric population were established. No differences (Mann-Whitney test) were observed in the percentage of low molecular weight transferrin and haptoglobin fractions according to sex and age of the controls. Densitometric analysis showed a high percentage of the less sialylated fractions of glycoproteins in all CDG-Ia patients and normal values in the CDG-X patients. In conclusion, western blotting with diaminobenzidine detection is a simple and sensitive procedure to screen for CDG, either in serum or blood spot samples. Densitometric analysis and the establishment of reference values might improve the detection of subtle changes in the glycosylation of proteins.

Defect in N-glycosylation of proteins is tissue-dependent in congenital disorders of glycosylation Ia

The biochemical hallmark of Congenital Disorders of Glycosylation (CDG) including type Ia is a defective N-glycosylation of serum glycoproteins. Hypoglycosylated forms of alpha1-antitrypsin have been detected by Western blot in serum from CDG Ia patients. In contrast we were not able to detect hypoglycosylation in alpha1-antitrypsin synthesized by fibroblasts, keratinocytes, enterocytes, and leukocytes. Similarly no hypoglycosylation was detectable in a membrane-associated N-linked glycoprotein, the facilitative glucose transporter GLUT-1 and also in serum immunoglobulin G isolated from sera of CDG Ia patients. We conclude that the phenotypic expression of CDG Ia is tissue-dependent.

[Carbohydrate-deficient blood glycoprotein syndrome]

Carbohydrate-deficient glycoprotein syndrome (CDGS) is a newly delineated group of inherited multisystemic disorders associated with abnormal glycosylation of a number of serum glycoproteins. Several types have been described on the basis of clinical presentation and biochemical changes of the glycosylation of serum transferrin and attributed to different enzymatic defects; their clinical presentations are fully different and a clinical heterogeneity is observed within a same type of CDGS. Patients with CDGS type la usually present with neurologic (hypotonia, strabismus and cerebellar hypoplasia) and cutaneous (inverted nipples, abnormal distribution of adipose tissue) abnormalities, together with multivisceral involvement (digestive, hepatic, cardiac, renal). However, neurologic and cutaneous symptoms may be absent, so that CDGS must be looked for in case of unexplained organ failure such as isolated liver insufficiency, cardiomyopathy, pericarditis, tubulopathy, nephrotic syndrome, vascular accident or retinitis pigmentosa. Patients with CDGS type Ib present with liver disease, enteropathy and hypoglycemia without neurologic involvement. These patients are successfully treated with oral mannose administration emphasizing the importance of making the diagnosis. Patients with CDGS type Ic present with mild psychomotor retardation and seizures. Patients with CDGS type II have psychomotor retardation association with severe gastrointestinal disorder, dysmorphic features and abnormal electroretinogram. Other types (III, IV) are less clearly defined and the clinical presentation includes convulsive encephalopathy. Biological abnormalities such as mild hepatic cytolysis, hematologic and hormonal abnormalities are consistently observed in CDGS type I, as well as renal hyperechogeneity, leading one to look for this syndrome when they are associated. Until now, only four enzymatic deficiencies have been identified (types Ia, Ib, Ic, II).

Screening for "prelysosomal disorders": carbohydrate-deficient glycoprotein syndromes

Physicians have become accustomed to thinking of certain inborn errors of metabolism (e.g., lysosomal, peroxisomal, and mitochondrial diseases) as being associated with specific subcellular organelles. In recent years, a family of disorders of N-glycosylation has been recognized, in which the metabolic defect is expressed in the cytosol, endoplasmic reticulum, and Golgi apparatus. These could be conveniently thought of as "prelysosomal" disorders. At least six of these entities are characterized by hypoglycosylation of many glycoconjugates, and have been designated as the carbohydrate-deficient glycoprotein syndromes. Given the ubiquity of the products of N-glycosylation in the cellular economy, it is not surprising that these defects in metabolism have protean clinical manifestations. Delayed development and other neurologic symptoms are wedded to variable dysfunctions of the heart, liver, and endocrine and coagulation systems. Patients can have dysmorphic features or cerebellar hypoplasia, attesting to the antenatal expression of these disorders. The most frequently recognized phenotype (several hundred cases worldwide) has been designated carbohydrate-deficient glycoprotein syndrome type la, and results from mutations in phosphomannomutase, a cytosolic enzyme involved in the synthesis of the lipid-linked oligosaccharide that is eventually attached to nascent glycoproteins through the amide group of asparagine residues. All forms of carbohydrate-deficient glycoprotein syndrome express an excess of hypoglycosylated isoforms of circulating transferrin, which serves as a useful screening tool. Physicians should consider screening for carbohydrate-deficient glycoprotein syndrome in individuals with delayed development, seizures, strokelike episodes, cerebellar hypoplasia, and demyelinating neuropathy with or without other signs of multisystem disease.

Carbohydrate-deficient glycoprotein syndrome type IA (phosphomannomutase-deficiency)

The carbohydrate-deficient glycoprotein or CDG syndromes (OMIM 212065) are a recently delineated group of genetic, multisystem diseases with variable dysmorphic features. The known CDG syndromes are characterized by a partial deficiency of the N-linked glycans of secretory glycoproteins, lysosomal enzymes, and probably also membranous glycoproteins. Due to the deficiency of terminal N-acetylneuraminic acid or sialic acid, the glycan changes can be observed in serum transferrin or other glycoproteins using isoelectrofocusing with immunofixation as the most widely used diagnostic technique. Most patients show a serum sialotransferrin pattern characterized by increased di- and asialotransferrin bands (type I pattern). The majority of patients with type I are phosphomannomutase deficient (type IA), while in a few other patients, deficiencies of phosphomannose isomerase (type IB) or endoplasmic reticulum glucosyltransferase (type IC) have been demonstrated. This review is an update on CDG syndrome type IA.

Carbohydrate-deficient glycoprotein syndrome type II

The carbohydrate-deficient glycoprotein syndromes (CDGS) are a group of autosomal recessive multisystemic diseases characterized by defective glycosylation of N-glycans. This review describes recent findings on two patients with CDGS type II. In contrast to CDGS type I, the type II patients show a more severe psychomotor retardation, no peripheral neuropathy and a normal cerebellum. The CDGS type II serum transferrin isoelectric focusing pattern shows a large amount (95%) of disialotransferrin in which each of the two glycosylation sites is occupied by a truncated monosialo-monoantennary N-glycan. Fine structure analysis of this glycan suggested a defect in the Golgi enzyme UDP-GlcNAc:alpha-6-D-mannoside beta-1,2-N-acetylglucosaminyltransferase II (GnT II; EC 2.4.1.143) which catalyzes an essential step in the biosynthetic pathway leading from hybrid to complex N-glycans. GnT II activity is reduced by over 98% in fibroblast and mononuclear cell extracts from the CDGS type II patients. Direct sequencing of the GnT II coding region from the two patients identified two point mutations in the catalytic domain of GnT II, S290F (TCC to TTC) and H262R (CAC to CGC). Either of these mutations inactivates the enzyme and probably also causes reduced expression. The CDG syndromes and other congenital defects in glycan synthesis as well as studies of null mutations in the mouse provide strong evidence that the glycan moieties of glycoproteins play essential roles in the normal development and physiology of mammals and probably of all multicellular organisms.

Determination of carbohydrate-deficient transferrin separated by lectin affinity chromatography for detecting chronic alcohol abuse

Carbohydrate-deficient transferrin (CDT) has been established as a valuable biological marker for detecting chronic alcohol abuse. To improve the diagnostic efficiency, we studied new CDT determination procedures involving the use of lectin affinity chromatography with Allomyrina dichotoma agglutinin (allo A) and Trichosanthes japonica agglutinin I (TJA-I) to isolate the CDT isoforms CDT-allo A and CDT-TJA, respectively. These procedures, based on detection of the CDT-allo A and CDT-TJA isoforms in sera, showed high sensitivity (100% and 98%, respectively) and high specificity (93% and 85%, respectively). These results demonstrate that the new procedures involving the use of lectin affinity chromatography are more useful for isolating markers in the CDT test than the conventional charge-based separation method.

The sugar moiety of Tamm-Horsfall protein is affected by the carbohydrate-deficient glycoprotein type I syndrome. A case study

As the sugar moiety of Tamm-Horsfall protein (THP) is affected by many pathological conditions, the aim of this study was to examine the influence of carbohydrate-deficient glycoprotein syndrome (CDG) on THP glycans. THP was isolated from urine of one patient with CDG type I and N-glycan profiling, analysis of monosaccharide content, determination of THP reactivity with specific lectins and with anti-THP antibodies were performed. THP of the CDG patient showed markedly lower amounts of all monosaccharides. Diminished amounts of lactosamine-type chains, galactose and alpha2,3 linked sialic acid were expressed in lower reactivity with PHA-L, DSA and MAA, respectively. These modifications were reflected in altered proportions of tetrasialylated and disialylated oligosaccharide chains. THP of the CDG patient reacted slightly more with anti-THP antibodies. Our results indicate that the CDG type I affects the THP sugar moiety and slightly enhances the THP immunoreactivity.

Missense mutations in the phosphomannomutase 2 gene of two Japanese siblings with carbohydrate-deficient glycoprotein syndrome type I

Carbohydrate-deficient glycoprotein syndrome type I (CDG1) is an autosomal recessive disorder characterized by severe nervous system involvement and a carbohydrate moiety deficiency in N-linked glycoproteins. Clinical symptoms are psychomotor retardation, stroke-like episodes or hemorrhagic episodes, hepatic dysfunction, polyneuropathy, and cerebellar ataxia. Marked atrophy of the cerebellar hemispheres and pons is recognizable on CT scan or MRI. CDGI has been mapped to human chromosome 16p by linkage studies. Recently, missense mutations in the gene for phosphomannomutase (PMM2) have been detected in Caucasian patients with CDG1. We studied DNA mutations in PMM2 in a Japanese family with CDG1. DNA sequencing of PMM2 in the siblings showed missense mutations of maternal origin in exon 5 and of paternal origin in exon 8. No such mutations were detected in 50 unrelated healthy Japanese. These findings suggest that the PMM2 is responsible for CDG1 in the Japanese as well as in Caucasians, and CDG1 may be the diagnosis in OPCA of neonatal onset, more often than currently thought.

Missense mutations in phosphomannomutase 2 gene in two Japanese families with carbohydrate-deficient glycoprotein syndrome type 1

Carbohydrate-deficient glycoprotein syndrome type 1 (CDG1) (MIM: 212065) is an autosomal recessive disorder with psychomotor retardation, strokelike episodes, ataxia, and olivopontocerebellar atrophy (OPCA) of neonatal onset. Recently, DNA substitutions in a gene for phosphomannomutase 2 (PMM2), mapped to 16p13, were identified in patients with CDG1. Biochemical findings in previously reported Japanese patients with CDG1 were slightly different from those of Caucasians, suggesting genetic heterogeneity of CDG1 in Japanese patients. We investigated the DNA sequence of PMM2 in two unrelated Japanese families with CDG1. Missense mutations in exon 5 (Phe144Leu) and exon 8 (Tyr229Ser, Arg238Pro) of the PMM2 gene were present in two families, but they were not present in 72 unrelated healthy Japanese individuals. One of the missense mutations, Phe144Leu in exon 5, was common to two families with CDG1. Our findings confirm that mutations in the PMM2 gene account for at least some Japanese patients with CDG1 similar to that seen in Caucasians and that exons 5 and 8 are hot spots of mutations of CDG1 caused by the PMM2 gene.

Carbohydrate-deficient glycoprotein syndromes: inborn errors of protein glycosylation

The carbohydrate-deficient glycoprotein (CDG) syndromes (CDGS) are a series of autosomal recessive enzyme deficiencies which result in incomplete glycosylation of plasma proteins. CDGS types Ia and Ib have been related to deficiencies of phosphomannomutase and phosphomannose isomerase, respectively, while CDGS type II results from a deficiency of N-acetylglucosaminyltransferase II. Secondary CDG syndromes are associated with galactosaemia and hereditary fructose intolerance. The diagnosis of CDGS is most easily made by studying the glycoforms of suitable marker proteins using either electrophoresis or isoelectric focusing. This paper reviews the structure of the glycan chains of proteins and structural alterations in CDGS. It also outlines analytical techniques which are useful in the laboratory study of protein glycoforms and the diagnosis of CDGS.

Increased expression of beta-hexosaminidase alpha chain in cultured skin fibroblasts from patients with carbohydrate-deficient glycoprotein syndrome type I

Carbohydrate-deficient glycoprotein (CDG) syndrome type I is an autosomal recessive multisystem disorder characterized by multiple serum glycoproteins with deficient oligosaccharide chains. This characteristic under-glycosylation is found in several serum glycoproteins. We studied secreted forms of lysosomal enzymes, beta-hexosaminidase and alpha-fucosidase, in serum from the patients and media of cultured fibroblasts. Both beta-hexosaminidase and alpha-fucosidase activities were increased in sera from three CDG patients. The enzyme activity staining using the fluorogenic substrate-4-methylumbelliferyl-alpha-L-fucopyranoside after polyacrylamide gel isoelectric focusing revealed abnormal cathodal bands in sera from CDG patients. On the other hand, no abnormal secreted forms of beta-hexosaminidase and alpha-fucosidase were detected in media from cultured CDG fibroblasts by isoelectric focusing and sodium-dodecyl sulfate-polyacrylamide gel electrophoresis. However, SDS-polyacrylamide gel electrophoresis and Western blotting analysis of beta-hexosaminidase using anti-beta-hexosaminidase A (anti-alpha + beta chains) antibody, showed an increase of a 55-kDa mature form of the alpha chain. Northern blotting analysis identified an increase in mRNA levels of beta-hexosaminidase alpha chain in CDG fibroblasts. Although under-glycosylated fractions of these lysosomal enzymes were not detected in cultured fibroblasts, it was suggested that intracellular processing of these lysosomal enzymes in CDG patients might be altered.

The heart and pericardial effusions in CDGS-I (carbohydrate-deficient glycoprotein syndrome type I)

Pericardial effusions were found in 6 of 10 children with carbohydrate-deficient glycoprotein syndrome type I (CDGS-I). In three cases pericardectomy was necessary. Blood concentrations of several glycoproteins and albumin were low. Similar abnormal isoforms of four glycoproteins were found in blood (B) and pericardial fluid (PF). There was a significant negative correlation between the mean concentration ratio PF/B and the molecular mass (MW) of 11 proteins. For proteins with MW < 100 kDa there were significant correlations in the controls, but not in the patients, between the PF/B ratio and both the MW and the sialic acid contents in the (glyco-)proteins. The pericardium exhibited focal mixed inflammatory changes with mesothelial proliferation, with widened endoplasmic reticulum and flocculent and/or lamellated material. Damage to a pericardial protein barrier is suggested to be involved in pericardial effusion in CDGS-I.

Isoforms and levels of transferrin, antithrombin, alpha(1)-antitrypsin and thyroxine-binding globulin in 48 patients with carbohydrate-deficient glycoprotein syndrome type I

Carbohydrate-deficient glycoprotein syndrome type I (CDGS I) is an autosomal recessive disease with multiple organ manifestations. The diagnostic biochemical marker has been typical carbohydrate-deficient isoforms of transferrin (Tf). Many other glycoproteins in blood may show similar defects, but have not been systematically studied before. Forty-eight CDGS I patients and 22 controls were examined for total concentrations and isoform distribution of Tf, antithrombin (AT), alpha(1)-antitrypsin (alpha(1)-AT) and thyroxine-binding globulin (TBG), and for the level of carbohydrate-deficient transferrin (CDT). The absolute values varied with age. The most frequent persistent quantitative changes were reduced levels of AT (97%) and elevated CDT values (100%). Isoforms lacking one to eight of four to eight possible sialic acid residues were found in AT, TBG and Tf in all cases, with variable intensity and frequency, and in all except one patient in alpha(1)-AT. The isoform changes were most constant and pronounced in Tf. The other three glycoproteins showed more abnormal heterogeneity in the youngest than in the older patients. The results indicated that the biochemical defect stabilizes with age, and suggested partial hypoglycosylation rather than non-glycosylation of these glycoproteins. Analysis of Tf isoforms is still the safest diagnostic marker of CDGS I from full-term birth and over the ages.

Human fibroblasts prefer mannose over glucose as a source of mannose for N-glycosylation. Evidence for the functional importance of transported mannose

Mannose in N-linked oligosaccharides is assumed to be derived primarily from glucose through phosphomannose isomerase (PMI). The discovery of mammalian mannose-specific transporters that function at physiological concentrations suggested that mannose might directly contribute to oligosaccharide synthesis. To determine the relative contribution of glucose and mannose, human fibroblasts were labeled with either [2-3H]mannose or [1,5,6-3H]glucose at the same specific activity, and the N-linked chains were released by PNGase F digestion. Most of the trichloroacetic acid-precipitable [3H]mannose label was released by this digestion, but only about 10% of the trichloroacetic acid-precipitable material was released from cells labeled with [1,5,6-3H]glucose. Both sugars labeled a similar array of oligosaccharides, and acid hydrolysis of these chains showed that [2-3H]mannose contributed 65-75% of the [3H]mannose in cells labeled for 1 h, despite the 100-fold higher concentration of exogenous glucose. Mannose consumption and [2-3H]mannose utilization were within the range of rates expected for mannose transport via the mannose-specific transporter. About 7-14% of the [2-3H]mannose is used for glycosylation, while the rest (86-93%) is catabolized to 3H2O via PMI. Increasing the exogenous mannose concentration beyond mannose transporter saturation results in the conversion of >99% of [2-3H]mannose into 3H2O. Long term labeling of cells with [2-3H]mannose showed that the specific activity of mannose in glycoproteins reached 77% of the specific activity of [2-3H]mannose added to the medium. These results show that when fibroblasts are provided with physiological concentrations of mannose, they use the mannose-specific transporter to supply the majority of mannose needed for glycoprotein synthesis. PMI may normally be used to catabolize excess mannose rather than to primarily supply Man-6-P for glycoprotein synthesis.

Microheterogeneity of serum glycoproteins and their liver precursors in patients with carbohydrate-deficient glycoprotein syndrome type I: apparent deficiencies in clusterin and serum amyloid P

Serum and liver protein patterns were studied, respectively, in 5 patients (serum) and 1 patient (liver) with carbohydrate-deficient glycoprotein syndrome (CDGS) type I by high-resolution two-dimensional electrophoresis (2-DE) and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The pattern of serum glycoproteins in all 5 patients presented abnormal trains of isoforms with decreased mass (delta molecular weight 3000) and all showed a cathodal shift. Two-dimensional electrophoresis and SDS-PAGE mass analysis of transferrin, alpha1 -antitrypsin, haptoglobin beta-chain, and alpha1-acid glycoprotein after neuraminidase and N-glycosidase F treatments demonstrated that the additional trains of the isoforms found in CDGS type I contain homologous species of isoforms. Some of them still showed charge differences, and all still contained glycans except for transferrin, with some unusual nonglycosylated isoforms. In addition, deficiencies in clusterin and serum amyloid P, not described so far, have been found in all 5 patients. The two-dimensional pattern of immunodetected precursors of serum proteins in liver cells from 1 patient with CDGS showed abnormal low-mass precursors and the absence of the precursors normally found in controls. These results suggest that these abnormal precursors accumulate during the early oligosaccharide processing of the nascent protein-bound oligosaccharides and that glycoprotein precursors undergo an altered intracellular transport while the post-translational processing along the normal pathway is still apparently functioning in patients with CDGS.

Influence of transferrin glycans on receptor binding and iron-donation

Human bi-bi-antennary transferrin (Tf) was partially deglycosylated by subsequently incubating with one or more of the following exoglycosidases: neuraminidase, beta-galactosidase or N-Acetyl-beta-D-glucosaminidase. Aglyco-Tf obtained from serum of a patient suffering from the Carbohydrate Deficient Glycoprotein syndrome was isolated. Receptor binding and the Tf and iron uptake capacities of the fully glycosylated-, partially deglycosylated- and aglyco-Tf were compared using the human hepatoma cell line PLC/PRF/5. No difference in binding capacity between the iso-Tf fractions could be demonstrated, however, the Tf and iron uptake capacity of aglyco-Tf was clearly reduced compared with the other Tf fractions.

Diagnostic value of Western blotting in carbohydrate-deficient glycoprotein syndrome

Carbohydrate-deficient glycoprotein syndrome (CDGS) is a newly recognized family of diseases characterized by the absence from the transferrin molecule of at least one glycan chain (type I) or an antenna of the glycan chain (type II). CDGS is currently diagnosed by studies of serum transferrin sialylation. We have developed an alternative Western blot-based method to detect serum transferrin species with reduced molecular masses due to altered glycosylation. Two additional bands are observed in type I CDGS, while a single lower band is observed in type II CDGS, relative to healthy subjects. N-glycanase treatment of serum from type I CDGS patients and normal subjects yields a single band of the same mass in the two cases, confirming that the glycan is the only moiety involved in the differential Western blot pattern. Similar results were found with serum alpha 1-acid glycoprotein, haptoglobin and alpha 1-antitrypsin. Western-blot analysis of one or more serum glycoproteins permits the differential diagnosis of CDGS.