Clinical and biochemical footprints of congenital disorders of glycosylation: Proposed nosology

We have identified 200 congenital disorders of glycosylation (CDG) caused by 189 different gene defects and have proposed a classification system for CDG based on the mode of action. This classification includes 8 categories: 1. Disorders of monosaccharide synthesis and interconversion, 2. Disorders of nucleotide sugar synthesis and transport, 3. Disorders of N-linked protein glycosylation, 4. Disorders of O-linked protein glycosylation, 5. Disorders of lipid glycosylation, 6. Disorders of vesicular trafficking, 7. Disorders of multiple glycosylation pathways and 8. Disorders of glycoprotein/glycan degradation. Additionally, using information from IEMbase, we have described the clinical involvement of 19 organs and systems, as well as essential laboratory investigations for each type of CDG. Neurological, dysmorphic, skeletal, and ocular manifestations were the most prevalent, occurring in 81%, 56%, 53%, and 46% of CDG, respectively. This was followed by digestive, cardiovascular, dermatological, endocrine, and hematological symptoms (17-34%). Immunological, genitourinary, respiratory, psychiatric, and renal symptoms were less frequently reported (8-12%), with hair and dental abnormalities present in only 4-7% of CDG. The information provided in this study, including our proposed classification system for CDG, may be beneficial for healthcare providers caring for individuals with metabolic conditions associated with CDG.

Fast screening of N-glycosylation disorders by sialotransferrin profiling with capillary zone electrophoresis

Background Congenital disorders of glycosylation (CDG) are a growing group of rare genetic disorders. The most frequently used screening method is sialotransferrin profiling using isoelectric focusing (IEF). Capillary zone electrophoresis (CZE) may be a simple and fast alternative. We investigated the Capillarys™ CDT assay (Sebia, France) to screen for N-glycosylation disorders, using IEF as gold standard. Methods Intra- and inter-assay precision were established, and analyses in heparin-anticoagulated plasma and serum were compared. Accuracy was assessed by comparing IEF and CZE profiles of 153 samples, including 49 normal, 53 CDG type I, 2 CDG type II, 1 combined CDG type I and type II and 48 samples with a Tf-polymorphism. Neuraminidase-treated plasma was analysed to discriminate CDG and Tf-polymorphisms using samples of 52 subjects (25 had a confirmed Tf-polymorphism). Age-dependent reference values were established using profiles of 312 samples. Results Heparin-plasma is as suitable as serum for CDG screening with the Capillarys™ CDT assay. The precision of the method is high, with a limit of quantification (LOQ) of 0.5%. All profiles, including CDG and Tf-polymorphisms, were correctly identified with CZE. Forty-nine of 52 neuraminidase-treated samples correctly identified the presence/absence of a Tf-polymorphism. Interferences in 3/52 samples hampered interpretation. Sialo-Tf profiles were dependent of age, in particular in the first three months of age. Conclusions CZE analysis with the Capillarys™ CDT kit (Sebia) is a fast and reliable method for screening of N-glycosylation defects. Tf-polymorphisms could be excluded after overnight incubation with neuraminidase.

Phenotypic and genotypic spectrum of congenital disorders of glycosylation type I and type II

BACKGROUND

Congenital disorders of glycosylation (CDG) are inborn defects of glycan metabolism. They are multisystem disorders. Analysis of transferrin isoforms is applied as a screening test for CDG type I (CDG-I) and type II (CDG-II). We performed a retrospective cohort study to determine spectrum of phenotype and genotype and prevalence of the different subtypes of CDG-I and CDG-II.

MATERIAL AND METHODS

All patients with CDG-I and CDG-II evaluated in our institution's Metabolic Genetics Clinics were included. Electronic and paper patient charts were reviewed. We set-up a high performance liquid chromatography transferrin isoelectric focusing (TIEF) method to measure transferrin isoforms in our Institution. We reviewed the literature for the rare CDG-I and CDG-II subtypes seen in our Institution.

RESULTS

Fifteen patients were included: 9 with PMM2-CDG and 6 with non-PMM2-CDG (one ALG3-CDG, one ALG9-CDG, two ALG11-CDG, one MPDU1-CDG and one ATP6V0A2-CDG). All patients with PMM2-CDG and 5 patients with non-PMM2-CDG showed abnormal TIEF suggestive of CDG-I or CDG-II pattern. In all patients, molecular diagnosis was confirmed either by single gene testing, targeted next generation sequencing for CDG genes, or by whole exome sequencing.

CONCLUSION

We report 15 new patients with CDG-I and CDG-II. Whole exome sequencing will likely identify more patients with normal TIEF and expand the phenotypic spectrum of CDG-I and CDG-II.

Congenital disorders of glycosylation: new defects and still counting

Almost 50 inborn errors of metabolism have been described due to congenital defects in N-linked glycosylation. These phenotypically diverse disorders typically present as clinical syndromes, affecting multiple systems including the central nervous system, muscle function, transport, regulation, immunity, endocrine system, and coagulation. An increasing number of disorders have been discovered using novel techniques that combine glycobiology with next-generation sequencing or use tandem mass spectrometry in combination with molecular gene-hunting techniques. The number of "classic" congenital disorders of glycosylation (CDGs) due to N-linked glycosylation defects is still rising. Eight novel CDGs affecting N-linked glycans were discovered in 2013 alone. Newly discovered genes teach us about the significance of glycosylation in cell-cell interaction, signaling, organ development, cell survival, and mosaicism, in addition to the consequences of abnormal glycosylation for muscle function. We have learned how important glycosylation is in posttranslational modification and how glycosylation defects can imitate recognizable, previously described phenotypes. In many CDG subtypes, patients unexpectedly presented with long-term survival, whereas some others presented with nonsyndromic intellectual disability. In this review, recently discovered N-linked CDGs are described, with a focus on clinical presentations and therapeutic ideas. A diagnostic approach in unsolved N-linked CDG cases with abnormal transferrin screening results is also suggested.

Congenital disorders of glycosylation. Part I. Defects of protein N-glycosylation

Glycosylation is the most common chemical process of protein modification and occurs in every living cell. Disturbances of this process may be either congenital or acquired. Congenital disorders of glycosylation (CDG) are a rapidly growing disease family, with about 50 disorders reported since its first clinical description in 1980. Most of the human diseases have been discovered recently. CDG result from defects in the synthesis of the N- and O-glycans moiety of glycoproteins, and in the attachment to the polypeptide chain of proteins. These defects have been found in the activation, presentation, and transport of sugar precursors, in the enzymes responsible for glycosylation, and in proteins that control the traffic of component. There are two main types of protein glycosylation: N-glycosylation and O-glycosylation. Most diseases are due to defects in the N-glycosylation pathway. For the sake of convenience, CDG were divided into 2 types, type I and II. CDG can affect nearly all organs and systems. The considerable variability of clinical features makes it difficult to recognize patients with CDG. Diagnosis can be made on the basis of abnormal glycosylation display. In this paper, an overview of CDG with a new nomenclature limited to the group of protein N-glycosylation disorders, clinical phenotype and diagnostic approach, have been presented. The location, reasons for defects, and the number of cases have been also described. This publication aims to draw attention to the possibility of occurrence of CDG in each multisystem disorder with an unknown origin.

A novel congenital disorder of glycosylation type without central nervous system involvement caused by mutations in the phosphoglucomutase 1 gene

Recent years have seen great advances in our knowledge of congenital disorders of glycosylation (CDG), a clinically and biochemically heterogeneous group of genetic diseases caused by defects in the synthesis (CDG-I) or processing (CDG-II) of glycans that form glycoconjugates. This paper reports a new subtype of non-neurological CDG involving the impaired cytoplasmic biosynthesis of nucleotide sugars needed for glycan biosynthesis. A patient presented with muscle fatigue, elevated creatine kinase, growth hormone deficiency, and first branchial arch syndrome. These findings, together with the abnormal type II plasma transferrin isoform profile detected, was compatible with a CDG. Functional testing and clinical analyses suggested a deficiency in the interconversion of glucose-1-phosphate and glucose-6-phosphate catalyzed by phosphoglucomutase (PGM1), a defect previously described as glycogenosis type XIV (GSDXIV, MIM 612934). PGM1 activity in patient-derived fibroblasts was significantly reduced, as was the quantity of immunoreactive PGM1 protein (Western blot assays). Mutation analysis of PGM1 and subsequent functional analysis investigating transient expression of PGM1 in immortalized patient fibroblasts, followed by ex vivo splicing assays using minigenes, allowed the characterization of two novel pathogenic mutations: c.871G>A (p.Gly291Arg) and c.1144 + 3A>T. The latter represents a severe splicing mutation leading to the out-of-frame skipping of exon 7 and the formation of a truncated protein (p.Arg343fs). MALDI mass spectra of permethylated protein N-glycans from the patient's serum suggested a marked hypoglycosylation defect. The present findings confirm that, in addition to a rare muscular glycolytic defect, PGM1 deficiency causes a non-neurological disorder of glycosylation.

From glycosylation disorders to dolichol biosynthesis defects: a new class of metabolic diseases

Polyisoprenoid alcohols are membrane lipids that are present in every cell, conserved from archaea to higher eukaryotes. The most common form, alpha-saturated polyprenol or dolichol is present in all tissues and most organelle membranes of eukaryotic cells. Dolichol has a well defined role as a lipid carrier for the glycan precursor in the early stages of N-linked protein glycosylation, which is assembled in the endoplasmic reticulum of all eukaryotic cells. Other glycosylation processes including C- and O-mannosylation, GPI-anchor biosynthesis and O-glucosylation also depend on dolichol biosynthesis via the availability of dolichol-P-mannose and dolichol-P-glucose in the ER. The ubiquity of dolichol in cellular compartments that are not involved in glycosylation raises the possibility of additional functions independent of these protein post-translational modifications. The molecular basis of several steps involved in the synthesis and the recycling of dolichol and its derivatives is still unknown, which hampers further research into this direction. In this review, we summarize the current knowledge on structural and functional aspects of dolichol metabolites. We will describe the metabolic disorders with a defect in known steps of dolichol biosynthesis and recycling in human and discuss their pathogenic mechanisms. Exploration of the developmental, cellular and biochemical defects associated with these disorders will provide a better understanding of the functions of this lipid class in human.

Congenital disorders of glycosylation (CDG): it's (nearly) all in it!

Congenital disorders of glycosylation (CDG) is a booming class of metabolic diseases. Its number has increased nearly fourfold (to 45) since 2003, the year of the Komrower lecture, entitled 'Congenital disorders of glycosylation CDG): It's all in it!'. This paper presents an overview of recently discovered CDG and CDG phenotypes, of a diagnostic approach, of (the lack of) treatment, of CDG genetics, of a novel CDG nomenclature and classification, and of some future directions in the CDG field.

Nijmegen paediatric CDG rating scale: a novel tool to assess disease progression

Congenital disorders of glycosylation (CDG) are a group of clinically heterogeneous inborn errors of metabolism. At present, treatment is available for only one CDG, but potential treatments for the other CDG are on the horizon. It will be vitally important in clinical trials of such agents to have a clear understanding of both the natural history of CDG and the corresponding burden of disability suffered by patients. To date, no multicentre studies have attempted to document the natural history of CDG. This is in part due to the lack of a reliable assessment tool to score CDG's diverse clinical spectrum. Based on our earlier experience evaluating disease progression in disorders of oxidative phosphorylation, we developed a practical and semi-quantitative rating scale for children with CDG. The Nijmegen Paediatric CDG Rating Scale (NPCRS) has been validated in 12 children, offering a tool to objectively monitor disease progression. We undertook a successful trial of the NPCRS with a collaboration of nine experienced physicians, using video records of physical and neurological examination of patients. The use of NPCRS can facilitate both longitudinal and natural history studies that will be essential for future interventions.

Metabolic cutis laxa syndromes

Cutis laxa is a rare skin disorder characterized by wrinkled, redundant, inelastic and sagging skin due to defective synthesis of elastic fibers and other proteins of the extracellular matrix. Wrinkled, inelastic skin occurs in many cases as an acquired condition. Syndromic forms of cutis laxa, however, are caused by diverse genetic defects, mostly coding for structural extracellular matrix proteins. Surprisingly a number of metabolic disorders have been also found to be associated with inherited cutis laxa. Menkes disease was the first metabolic disease reported with old-looking, wrinkled skin. Cutis laxa has recently been found in patients with abnormal glycosylation. The discovery of the COG7 defect in patients with wrinkled, inelastic skin was the first genetic link with the Congenital Disorders of Glycosylation (CDG). Since then several inborn errors of metabolism with cutis laxa have been described with variable severity. These include P5CS, ATP6V0A2-CDG and PYCR1 defects. In spite of the evolving number of cutis laxa-related diseases a large part of the cases remain genetically unsolved. In metabolic cutis laxa syndromes the clinical and laboratory features might partially overlap, however there are some distinct, discriminative features. In this review on metabolic diseases causing cutis laxa we offer a practical approach for the differential diagnosis of metabolic cutis laxa syndromes.

The impact of mass spectrometry in the diagnosis of congenital disorders of glycosylation

Contribution of mass spectrometry (MS) in the diagnosis and characterization of congenital disorders of glycosylation (CDG) has long been known. CDG type I diseases are characterized by the under-occupancy of protein N-glycosylation sites. Electrospray (ESI) MS and matrix assisted laser desorption ionization (MALDI) MS are effective for underglycosylation analyses of intact serum Transferrin (Tf) in CDG-I patients by mass determination of individual component glycoforms. Thus, high-throughput methods developed to speed-up analytical times found increasing application in clinical testing for CDG detection. ESI MS recognizable glycoform profiles of serum Tf have been reported in CDG-I different from PMM2-CDG and in individual CDG-II defects. MALDI MS analysis of acidic and neutral N-linked glycans released from total plasma or targeted glycoproteins, is the mainstream tool to explore abnormal oligosaccharide structure and changes in the relative amount of individual oligosaccharides in CDG-II patients. Here we briefly review state-of-the-art and updates of MS-based applications for the diagnosis of CDG with special emphasis to detectable glycosylation profiles reported in different CDG types.

Congenital disorder of glycosylation type Ix: review of clinical spectrum and diagnostic steps

Congenital disorder of glycosylation type I (CDG I) represent a rapidly growing group of inherited multisystem disorders with 13 genetically established subtypes (CDG Ia to CDG Im), and a high number of biochemically unresolved cases (CDG Ix). Further diagnostic effort and prognosis counselling are very challenging in these children. In the current study, we reviewed the clinical records of 10 CDG Ix patients and compared the data with 13 CDG Ix patients published in the literature in search for specific symptoms to create clinical subgroups. The most frequent findings were rather nonspecific, including developmental delay and axial hypotonia. Several features were found that are uncommon in CDG syndrome, such as elevated creatine kinase or arthrogryposis. Distinct ophthalmological abnormalities were observed including optic nerve atrophy, cataract and glaucoma. Two subgroups could be established: one with a pure neurological presentation and the other with a neurological-multivisceral form. The first group had a significantly better prognosis. The unique presentation of microcephaly, seizures, ascites, hepatomegaly, nephrotic syndrome and severe developmental delay was observed in one child diagnosed with CDG Ik. Establishing clinical subgroups and increasing the number of patients within the subgroups may lead the way towards the genetic defect in children with a so far unsolved type of the congenital disorders of glycosylation. Raising awareness for less common, non-CDG specific clinical features such as congenital joint contractures, movement disorders or ophthalmological anomalies will encourage clinicians to think of CDG in its more unusual presentation. Clinical grouping also helps to determine the prognosis and provide better counselling for the families.

Clinical features in adults with congenital disorders of glycosylation type Ia (CDG-Ia)

Congenital disorders of glycosylation (CDG) are a group of metabolic disorders resulting from defective synthesis of N-linked oligosaccharides. CDG-Ia is the most common of the 21 known types defined by defects in different steps of the synthetic pathway. An increasing number of American adults with CDG-Ia are being recognized but little is documented on the morbidity and mortality in this population. These adults have moderate mental retardation, ataxia, retinitis pigmentosa, peripheral neuropathy, kyphoscoliosis, and endocrinopathies. Four adults with CDG-Ia, ages 19-36 years old are presented. All are active, dysarthric conversant adults with moderate cognitive impairment. They are ataxic and wheelchair dependent, however, only the oldest man shows significant muscle atrophy. All have diagnosed peripheral neuropathy. Three of four remain on anticonvulsants with only occasional seizures, none have had stroke-like episodes since their teen years. Their skeletal issues include significant kyphoscoliosis, joint contractures, and osteopenia. Retinitis pigmentosa and myopia complicate their functional vision. The women do not menstruate and the men have small testes resulting from hypogonadotropic hypogonadism. Documentation of clinical complications and successful management strategies in adults with CDG will improve their quality of life and allow more informed prognostic discussions with families of younger affected individuals.

Gastrointestinal and other clinical manifestations in 17 children with congenital disorders of glycosylation type Ia, Ib, and Ic

OBJECTIVES

The typical signs and symptoms of congenital disorders of glycosylation (CDG) include dysmorphy, failure to thrive, and neurologic abnormalities. However, more and more children diagnosed at a young age are not dysmorphic and do not have neurologic involvement. The authors studied the gastrointestinal and other clinical manifestations of CDG type Ia, Ib, and Ic.

METHODS

As of January 2003, 17 children were identified with CDG at the authors' institution. The medical records of the patients were reviewed.

RESULTS

Five children had CDG Ia, three children CDG Ib, and nine children CDG Ic. Age at diagnosis ranged from 2 months to 15 years. Failure to thrive was present in 80% of patients with CDG Ia, in 66% of those with CDG Ib, and in 11% of those with CDG Ic. Five children had protein-losing enteropathy (two CDG Ia, two CDG Ib, and one CDG Ic). Hepatomegaly was present in 40% of patients with CDG Ia, in 66% of those with CDG Ib, and in 11% of those with CDG Ic. In CDG Ic, hepatomegaly was transient. In CDG Ia, histologic analysis of the liver showed swollen hepatocytes, steatosis, and fibrosis. In CDG Ib, hamartomatous collections of bile ducts were seen. In one patient with CDG Ib, the clinical picture was restricted to congenital hepatic fibrosis for more than a decade.

CONCLUSIONS

The study confirms the heterogeneity of the clinical picture in children with CDG type Ia, Ib, and Ic. Children with protein-losing enteropathy should be tested for CDG. Protein-losing enteropathy can be caused, not only by CDG Ia and Ib, but also by type Ic. Children with congenital hepatic fibrosis should be tested for CDG, even in the absence of other symptoms. In CDG Ib, histologic analysis of the liver showed hamartomatous collections of bile ducts (Meyenburg complex).

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.

[Congenital disorders of glycosylation]

Congenital disorders of glycosylation are group of hereditary diseases resulting in severe psychomotor retardation and multiorgan failure. So far eleven different defects were identified on the pathway of N-glycan biosynthesis. Seven of them belong to CDG type I and result in incomplete occupation of potential N-glycosylation sites. Four defects were found in N-glycan processing. Molecular background of CDG and potential perspectives of therapy are summarised and updated in this review.

Affinity capture and elution/electrospray ionization mass spectrometry assay of phosphomannomutase and phosphomannose isomerase for the multiplex analysis of congenital disorders of glycosylation types Ia and Ib

We report a new application of affinity capture-elution electrospray mass spectrometry (ACESI-MS) to assay the enzymes phosphomannomutase (PMM) and phosphomannose isomerase (PMI), which when deficient cause congenital disorders of glycosylation CDG-type Ia and type Ib, respectively. The novel feature of this mass-spectrometry-based assay is that it allows one to distinguish and quantify enzymatic products that are isomeric with their substrates that are present simultaneously in complex mixtures, such as cultured human cell homogenates. This is achieved by coupled assays in which the PMM and PMI primary products are in vitro subjected to another enzymatic reaction with yeast transketolase that changes the mass of the products to be detected by mass spectrometry. The affinity purification procedure is fully automated, and the mass spectrometric analysis is multiplexed in a fashion that is suitable for high-throughput applications.

Glycoforms of six serum glycoproteins in a patient with congenital disorder of glycosylation type I

In this paper the occurrence and relative content of defectively glycosylated serum glycoforms in transferrin (Tf), alpha1-acid glycoprotein (AGP), haptoglobin (Hp), alpha1-antitrypsin (alpha1-AT), alpha2-macroglobulin (alpha2-MG) and ceruloplasmin (Cpl) in the serum of a patient with congenital disorder of glycosylation type I are reported. Blood samples were taken when the patient was 14 years old and then after a one-year interval. The patterns of glycoforms in both samples were compared. In 4 out of 6 examined glycoproteins, glycoforms lacking one and two oligosaccharide chains occurred. "Underglycosylated" glycoforms of alpha2-MG and Cpl were not clearly detectable. Tf was shown to be affected with this defect to a higher extent than other glycoproteins, containing only 30% properly glycosylated molecules and also as much as 30% of the molecules lacking two glycan units. In Hp and alpha1-AT the proportions of properly and defectively glycosylated forms were similar. This properly glycosylated form comprised 47% of the Hp and 51-55% of the alpha1-AT molecules. As in AGP and Tf, about 30% the of molecules lacked one glycan unit. Twenty-one percent of the Hp molecules were devoid of two glycans, and this amount slightly increased in the course of the year. In alpha1-AT, 19 and 17% of the molecules lacked two glycans in both samples, respectively. Only in AGP we did find a substantial difference between the two blood samples. In the course of the year, the amount of the form lacking 2-chains decreased from 12 to 3%, resulting in a simultaneous increase in the forms lacking one chain and the properly glycosylated. Our work also indicates, that applying a simple method of biochemical analysis such as SDS-PAGE/Western-blotting could be helpful in preliminary diagnosis and could improve the identification of congenital disorders of glycosylation.

Congenital disorders of glycosylation

Congenital disorders of glycosylation (CDG) are a rapidly growing group of genetic diseases that are due to defects in the synthesis of glycans and in the attachment of glycans to other compounds. Most CDG are multisystem diseases that include severe brain involvement. The CDG causing sialic acid deficiency of N-glycans can be diagnosed by isoelectrofocusing of serum sialotransferrins. An efficient treatment, namely oral D-mannose, is available for only one CDG (CDG-Ib). In many patients with CDG, the basic defect is unknown (CDG-x). Glycan structural analysis, yeast genetics, and knockout animal models are essential tools in the elucidation of novel CDG. Eleven primary genetic glycosylation diseases have been discovered and their basic defects identified: six in the N-glycan assembly, three in the N-glycan processing, and two in the O-glycan (glycosaminoglycan) assembly. This review summarizes their clinical, biochemical, and genetic characteristics and speculates on further developments in this field.

A new subtype of a congenital disorder of glycosylation (CDG) with mild clinical manifestations

A boy with an unspecific symptomatology consisting of mental retardation, strabismus, hypotonia and mild ataxia was diagnosed with a congenital disorder of glycosylation (CDG). Neither cerebellar atrophy nor dysmorphic features were present. The serum transferrin band pattern obtained by isoelectric focusing(IEF) showed a strongly elevated disialotransferrin band together with only slightly elevated asialotransferrin, thus a type I pattern. This is a new CDG classified CDG-x since CDG-la, -b, -c, -d and -e were excluded. Quantitative differences to the type 1 pattern of a CDG-la patient with a moderate to severe course were confirmed by densitometric evaluation of the gels and by SDS gel electrophoresis. Liver biopsy showed lysosomal inclusions suggesting a pre-Golgi defect. This patient's case supports the approach to include isoelectric focusing of serum transferrin in the diagnostic work-up of patients with unexplained symptoms.

Complementation cloning identifies CDG-IIc, a new type of congenital disorders of glycosylation, as a GDP-fucose transporter deficiency

Congenital disorders of glycosylation (CDG) comprise a rapidly growing group of inherited disorders in which glycosylation of glycoproteins is defective due to mutations in genes required for the assembly of lipid-linked oligosaccharides, their transfer to nascent glycoproteins (CDG-I) or the processing of protein-bound glycans (CDG-II). Previously' a defect in the GDP-fucose import into the lumen of the Golgi was identified in a person with CDG (A.C.) with a general deficiency of fucosyl residues in glycoproteins. This patient presents the clinical features of leukocyte adhesion deficiency type II (LAD II) including mental retardation, short stature, facial stigmata, and recurrent bacterial peripheral infections with persistently elevated peripheral leukocytes. Using a fucose-specific, lectin-staining procedure for detection of fucosylated glycoproteins and a retroviral cDNA library, we isolated a cDNA complementing the fucosylation defect in the patient's fibroblasts. The cDNA encodes a highly hydrophobic protein of 364 amino acids with multiple putative transmembrane domains. Restoration of GDP-fucose import activity in Golgi-enriched vesicles from the patient's fibroblasts verified the GDP-fucose transporter activity of this protein. We identified two missense mutations in the GDP-fucose transporter cDNA of patient A.C. and of two other people with LAD II. Thus complementation cloning allowed us to identify the human GDP-fucose transporter cDNA and GDP-fucose transporter deficiency as a cause for a new type of CDG. Following the recent recommendations for the nomenclature for CDG, this new type is classified as CDG-IIc (formerly LAD II).

Band 3 glycoprotein and glycophorin A from erythrocytes of children with congenital disorder of glycosylation type-Ia are underglycosylated

Band 3 and PAS-1 (a dimer of glycophorin A) from erythrocyte membranes of three children with congenital disorder of glycosylation, type Ia (CDG-Ia), aged 1 month, 3 years and 10 years respectively, were examined by a new technique that allowed determination of carbohydrate molar composition of glycoproteins separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis. In CDG children a single N-glycan of band 3 glycoprotein was hypoglycosylated and its mannose content was normal or elevated. Glycophorin A which is the major carrier of erythrocyte sialic acid, was deficient in N-acetylgalactosamine, and sialic acid residues. This finding indicated a partial unglycosylation of O-glycans in glycophorin A. In keeping with the results of PAS-1 analysis, total sialic acid in erythrocyte membranes from CDG children was reduced to 40-56% of normal values. A possible molecular mechanism of hypo- and unglycosylation of band 3 and glycophorin A, respectively, in CDG is discussed.

A broad spectrum of clinical presentations in congenital disorders of glycosylation I: a series of 26 cases

INTRODUCTION

Congenital disorders of glycosylation (CDG), or carbohydrate deficient glycoprotein syndromes, form a new group of multisystem disorders characterised by defective glycoprotein biosynthesis, ascribed to various biochemical mechanisms.

METHODS

We report the clinical, biological, and molecular analysis of 26 CDG I patients, including 20 CDG Ia, two CDG Ib, one CDG Ic, and three CDG Ix, detected by western blotting and isoelectric focusing of serum transferrin.

RESULTS

Based on the clinical features, CDG Ia could be split into two subtypes: a neurological form with psychomotor retardation, strabismus, cerebellar hypoplasia, and retinitis pigmentosa (n=11), and a multivisceral form with neurological and extraneurological manifestations including liver, cardiac, renal, or gastrointestinal involvement (n=9). Interestingly, dysmorphic features, inverted nipples, cerebellar hypoplasia, and abnormal subcutaneous fat distribution were not consistently observed in CDG Ia. By contrast, the two CDG Ib patients had severe liver disease, enteropathy, and hyperinsulinaemic hypoglycaemia but no neurological involvement. Finally, the CDG Ic patient and one of the CDG Ix patients had psychomotor retardation and seizures. The other CDG Ix patients had severe proximal tubulopathy, bilateral cataract, and white matter abnormalities (one patient), or multiorgan failure and multiple birth defects (one patient).

CONCLUSIONS

Owing to the remarkable clinical variability of CDG, this novel disease probably remains largely underdiagnosed. The successful treatment of CDG Ib patients with oral mannose emphasises the paramount importance of early diagnosis of PMI deficiency.

Mutations in PMM2 that cause congenital disorders of glycosylation, type Ia (CDG-Ia)

The PMM2 gene, which is defective in CDG-Ia, was cloned three years ago [Matthijs et al., 1997b]. Several publications list PMM2 mutations [Matthijs et al., 1997b, 1998; Kjaergaard et al., 1998, 1999; Bjursell et al., 1998, 2000; Imtiaz et al., 2000] and a few mutations have appeared in case reports or abstracts [Crosby et al., 1999; Kondo et al., 1999; Krasnewich et al., 1999; Mizugishi et al., 1999; Vuillaumier-Barrot et al., 1999, 2000b]. However, the number of molecularly characterized cases is steadily increasing and many new mutations may never make it to the literature. Therefore, we decided to collate data from six research and diagnostic laboratories that have committed themselves to a systematic search for PMM2 mutations. In total we list 58 different mutations found in 249 patients from 23 countries. We have also collected demographic data and registered the number of deceased patients. The documentation of the genotype-phenotype correlation is certainly valuable, but is out of the scope of this molecular update. The list of mutations will also be available online (URL: http://www.kuleuven. ac.be/med/cdg) and investigators are invited to submit new data to this PMM2 mutation database.

PMM2 mutation spectrum, including 10 novel mutations, in a large CDG type 1A family material with a focus on Scandinavian families

Carbohydrate-deficient glycoprotein syndrome type IA (CDG IA) is an autosomal recessive disease characterized clinically by severe involvement of the central and peripheral nervous system, and biochemically by complex defects in carbohydrate residues in a number of serum glycoproteins. CDG IA is caused by mutations in the PMM2 gene located in chromosome region 16p13. In this study, 61 CDG type IA patients (122 chromosomes) were screened for mutations in the PMM2 gene using a combination of SSCP and sequence analysis. More than 95% of the mutations could be detected. All of them were missense mutations. Mutations 422G>A and 357C>A were strikingly more common in the material and comprised 58% of mutations detected. Of the 20 mutations found, 10 were not reported previously. Seven mutations, e.g. 26G>A (five alleles) and 548T>C (seven alleles), were found only in Scandinavian families. The most common genotype was 357C>A/422G>A (36%). Three patients were homozygous, 357C>A/357C>A (two cases), and 548T>C/548T>C (one case). No patients homozygous for the most common mutation 422G>A were detected. The different mutations were clustered e.g., in that most were located in exon 5 (five) and exon 8 (six), while no mutation was detected in exon 2. When the frequencies of each mutation were included, exon 5 comprised 61% (65 chromosomes) of the mutations; in Scandinavian patients the frequency of these mutations was 72%. Thus, analysis of exon five in these patients enables both reliable and time-saving first screening in prenatal diagnostic cases. This could be followed by a second step of additional strategies for the detection of other mutations.

Budd-Chiari syndrome associated with coagulation abnormalities in a child with carbohydrate deficient glycoprotein syndrome type Ix

A 6-year-old male patient presented with Budd-Chiari syndrome and glycoprotein abnormalities associated with carbohydrate deficient glycoprotein syndrome type I with yet unidentified molecular defect (type Ix). Budd-Chiari syndrome most likely developed after hepatic venous thrombosis caused by coagulation abnormalities resulting from hypoglycosylation and functional impairment of anticoagulant proteins.

Congenital disorder of glycosylation-Ic: case report and genetic defect

The clinical phenotype and the molecular defect of a patient with a new subtype of congenital disorders of glycosylation (CDG-Ic, formerly designated as CDGS type V) characterized by a deficiency of Dol-P-Glc: Man9GlcNAc2-PP-Dol glucosyltransferase is described. The clinical picture presents with several features similar to CDG-Ia (phosphomannomutase 2 deficiency) such as hypotonia and atactic-dystonic movements. In contrast to CDG-Ia, the course of the disease appears milder. The head growth, the functioning of the peripheral nerves and the initial cerebellar development were normal. Sequencing of the patient's Dol-P-Glc: Man9GlcNAc2-PP-Dol glucosyltransferase cDNA revealed an in-frame deletion of three nucleotides leading to the loss of isoleucine 299.

Carbohydrate-deficient glycoprotein syndrome type 1a: a variant phenotype with borderline cognitive dysfunction, cerebellar hypoplasia, and coagulation disturbances

An 8-year-old boy is described with borderline cognitive impairment, cerebellar hypoplasia, a stroke-like episode, and venous thrombosis of the left leg after a period of immobilization. The pattern of multiple abnormalities in blood coagulation suggested carbohydrate-deficient glycoprotein syndrome type 1a. Isoelectric focusing of serum transferrin was abnormal. The activity of phosphomannomutase in leukocytes and fibroblasts was decreased. Mutation analysis of the PMM2 gene revealed the R141H/E151G genotype. These results confirm the presence of carbohydrate-deficient glycoprotein syndrome type 1a without severe psychomotor retardation.

[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.

Haptoglobin glycoforms in a case of carbohydrate-deficient glycoprotein syndrome

Alterations in haptoglobin (Hp) glycosylation were examined in the plasma of the first patient with carbohydrate-deficient glycoprotein syndrome (CDGS) who was described in Poland. Hp concentration in the CDGS patient plasma was low (240 mg/l) and the Hp phenotype was shown to be 2-2. Three glycoforms of the Hp beta subunit were observed in SDS-PAGE in CDGS. The densitometric analysis and molecular weight determinations suggested that 50% of glycoforms were fully glycosylated; 30% contained three out of four and 20% only two out of four glycan units compared to those that are present in Hp derived from healthy people. Results with lectins (concanavalin A and Sambucus nigra, Maackia amurensis and Alleuria aurantia agglutinins) indicate that all three glycoforms of beta subunit of CDGS-Hp contained biantennary complex glycans terminated with alpha2,6 bound sialic acid, but without fucose or alpha2,3 linked sialic acid. Hp glycosylation abnormalities described in this work suggest that this case was a type I carbohydrate-deficient glycoprotein syndrome.

Hyperinsulinemic hypoglycemia as a presenting sign in phosphomannose isomerase deficiency: A new manifestation of carbohydrate-deficient glycoprotein syndrome treatable with mannose

We report the case of a patient with carbohydrate-deficient glycoprotein syndrome type Ib who developed normally until 3 months of age, when she was referred to the hospital for evaluation of hypoglycemia that was found to be related to hyperinsulinism. She also had vomiting episodes, hepatomegaly, and intractable diarrhea, which evoked the diagnosis of carbohydrate-deficient glycoprotein syndrome. Oral mannose treatment at a dose of 0.17 g/kg body weight 6 times/d was followed by a clinical improvement and normalization of blood glucose, aminotransferases, and coagulation factor levels. Hyperinsulinemic hypoglycemia should be considered as a leading sign of carbohydrate-deficient glycoprotein syndrome type Ib, especially when it is associated with enteropathy and abnormal liver tests.

A neurodystrophic syndrome resembling carbohydrate-deficient glycoprotein syndrome type III

A 10-month old girl is described with a serum transferrin isoform abnormality of the same kind as in two previously reported girls with carbohydrate-deficient glycoprotein syndrome type III. This patient presented with joint abnormalities and rapidly developing hypsarrhythmia, hypotonia, psychomotor delay and growth retardation. Fingers, toes, nails and local skin were dysmorphic. She had pale optic discs, thoracic syringomyelia and frontal lobe atrophy at three months. The CDT value in serum was greatly elevated. Several carbohydrate-deficient isoforms were found in transferrin (four), alpha1-antitrypsin (three), antithrombin (two) and thyroxine-binding globulin (four). Mutations in the CDGS 1-gene were excluded. The CDGS III glycoprotein abnormality most probably represents a distinct disorder of glycoprotein metabolism, and needs to be considered in unclear hypsarrhythmia with developmental delay. Dysmorphic features may be added to this syndrome.

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.

Carbohydrate-deficient glycoprotein syndrome type V: deficiency of dolichyl-P-Glc:Man9GlcNAc2-PP-dolichyl glucosyltransferase

Deficiency of dolichyl-P-Glc:Man9GlcNAc2-PP-dolichyl glucosyltransferase is the cause of an additional type of carbohydrate-deficient glycoprotein syndrome (CDGS type V). Clinically this type resembles the classical type Ia of CDGS caused by the deficiency of phosphomannomutase. As a result of the glucosyltransferase deficiency in CDGS type V nonglucosylated lipid-linked oligosaccharides accumulate. The defect is leaky and glucosylated oligosaccharides are found on nascent glycoproteins. The limited availability of glucosylated lipid-linked oligosaccharides explains the incomplete usage of N-glycosylation sites in glycoproteins. This finding is reflected in the presence of transferrin forms in serum that lack one or both of the two N-linked oligosaccharides and the reduction of mannose incorporation to about one-third of control in glycoproteins of fibroblasts.

A novel carbohydrate-deficient glycoprotein syndrome characterized by a deficiency in glucosylation of the dolichol-linked oligosaccharide

Carbohydrate-deficient glycoprotein syndromes (CDGS) type I are a group of genetic diseases characterized by a deficiency of N-linked protein glycosylation in the endoplasmic reticulum. The majority of these CDGS patients have phosphomannomutase (PMM) deficiency (type A). This enzyme is required for the synthesis of GDP-mannose, one of the substrates in the biosynthesis of the dolichol-linked oligosaccharide Glc3Man9GlcNAc2. This oligosaccharide serves as the donor substrate in the N-linked glycosylation process. We report on the biochemical characterization of a novel CDGS type I in fibroblasts of four related patients with normal PMM activity but a strongly reduced ability to synthesize glucosylated dolichol-linked oligosaccharide leading to accumulation of dolichol-linked Man9GlcNAc2. This deficiency in the synthesis of dolichol-linked Glc3Man9GlcNAc2 oligosaccharide explains the hypoglycosylation of serum proteins in these patients, because nonglucosylated oligosaccharides are suboptimal substrates in the protein glycosylation process, catalyzed by the oligosaccharyltransferase complex. Accordingly, the efficiency of N-linked protein glycosylation was found to be reduced in fibroblasts from these patients.

A novel disorder of N-glycosylation due to phosphomannose isomerase deficiency

Three siblings suffered from an unusual disorder of cyclic vomiting and congenital hepatic fibrosis. Serum transferrin isoelectric focusing showed increased asialo- and disialotransferrin isoforms as seen in the carbohydrate-deficient glycoprotein (CDG) syndrome type I. Phosphomannomutase, which is deficient in most patients with type I CDG syndrome, was found to be normal in all three patients. Structural analysis of serum transferrin revealed nonglycosylated, hypoglycosylated, and normoglycosylated transferrin molecules. These findings suggested a defect in the early glycosylation pathway. Phosphomannose isomerase was found to be deficient and the defect was present in leucocytes, fibroblasts, and liver tissue. Phosphomannose isomerase deficiency appears to be a novel glycosylation disorder, which is biochemically indistinguishable from CDG syndrome type I. However, the clinical presentation is entirely different.

Immunoglobulin levels in patients with carbohydrate-deficient glycoprotein syndrome type I

BACKGROUND

The characteristic feature of carbohydrate-deficient glycoprotein syndrome (CDGS) type I, a multisystemic disease, is underglycosylation of many serum glycoproteins, such as transferrin. A few cases of severe infections during childhood have been reported and an underlying immunodeficiency has been suggested. Because of this and the fact that all immunoglobulin (Ig) isotypes are glycoproteins we analysed the Ig levels in patients with CDGS I.

METHODS

The serum concentrations of IgG1, IgG2, IgG3, IgG4, IgA, IgM, IgD and IgE, and the frequency of the G2m(23) allotype were measured by enzyme immunoassay in 15 patients with CDGS type I.

RESULTS

Ten (67%) patients had an elevated level of at least one Ig, when compared to age-related reference ranges. No particular isotype was involved although a tendency towards high IgE levels was registered. The frequency of homozygous G2m(23)-negative CDGS patients (33%) was not different from that of blood donors (34%).

CONCLUSION

We conclude that CDGS I patients have no major changes in the serum levels of any specific Ig isotype. The severe infections observed in some CDGS patients are therefore unlikely to involve any Ig deficiency. Our results do not exclude that Ig of patients with CDGS may have altered physiological functions because of abnormal glycosylation.

Carbohydrate-deficient glycoprotein syndrome

Carbohydrate-deficient glycoprotein syndrome consists of a group of disorders with multisystemic involvement and prominent neurologic symptoms. The full clinical spectrum continues to evolve, with four types currently recognized; type I is by far the most common. The clinical presentation of CDGS appears more severe in infants than in adults. Diagnosis is based on the clinical findings of characteristic fat distribution, neurologic impairment, and developmental delay, combined with the biochemical finding of cathodally migrating serum glycoproteins, transferrin in particular, on isoelectric focusing. Scientific evidence supports the hypothesis that abnormal synthesis of N-linked oligosaccharides is the basic metabolic defect in CDGS. The complex, multistep nature of the N-linked oligosaccharide pathway and the clinical heterogeneity of CDGS suggest that several different defects in the pathway can result in this disorder. Two specific enzyme defects have been reported: phosphomannomutase deficiency in some type I patients and N-acetylglucosamine transferase II deficiency in type II patients. Investigations continue into other metabolic bases of CDGS. The discovery of some of the enzyme defects paves the way for cloning, mutational analysis, and eventually prenatal diagnosis in appropriate families. No known treatment exists for CDGS; pallintive care and support is the most that can be offered. Family support systems are blossoming both in the United States and abroad, giving families the ability to communicate with each other and with workers in the field. As more cases are diagnosed and scientific research continues, advances in clinical definition, supportive care, nutrition, and prenatal diagnosis of CDGS are inevitable.

Phosphomannomutase deficiency is a cause of carbohydrate-deficient glycoprotein syndrome type I

Carbohydrate-deficient glycoprotein (CDG) syndromes are genetic multisystemic disorders characterized by defective N-glycosylation of serum and cellular proteins. The activity of phosphomannomutase was markedly deficient (< or = 10% of the control activity) in fibroblasts, liver and/or leucocytes of 6 patients with CDG syndrome type I. Other enzymes involved in the conversion of glucose to mannose 1-phosphate, as well as phosphoglucomutase, had normal activities. Phosphomannomutase activity was normal in fibroblasts of 2 patients with CDG syndrome type II. Since this enzyme provides the mannose 1-phosphate required for the initial steps of protein glycosylation, it is concluded that phosphomannomutase deficiency, which is first reported here for higher organisms, is a cause, and most likely the major one, of CDG syndrome type I.

[Carbohydrate-deficient glycoprotein syndrome]

Carbohydrate-deficient glycoprotein syndrome is characterized by mental retardation, ataxia, hepatopathy during infancy, cerebellar hypoplasia, peripheral neuropathy, internal strabismus, growth retardation and stroke-like episodes. Since the description of female siblings with unique clinical and biochemical features by Jaeken (1980) and the discovery of unique isoforms of serum transferrin in the patients by Jaeken (1984), more than 120 patients have been diagnosed. The biochemical marker is asialo- and disialo-transferrin. We have found the first Japanese patients and, through analysing serum glycoproteins from these patients, we was noted that multiple serum glycoproteins contain abnormal fractions, on isoelectric focusing. By analysing the sugar chain of transferrin, we have found that the abnormality is caused by a defect in the transfer of asparagine-N-linked oligosaccharide. Recently, two clinical and biochemical variants have been reported. One, characterized by severe mental retardation, no cerebellar hypoplasia, no peripheral neuropathy, diasirotransferrin dominancy, has proven to have a deficiency of N-acetylglucosaminyltransferase II, by Jaeken (1993).

Carbohydrate-deficient glycoprotein syndrome--a fourth subtype

Two infants are described, who, we suggest, represent a fourth subtype of carbohydrate-deficient glycoprotein (CDG) syndrome. Both patients showed microcephaly and severe epilepsy with absent psychomotor development and similar minor dysmorphic features. There were no signs of liver dysfunction. Several glycoproteins in blood, including transferrin, alpha 1-antitrypsin, antithrombin and thyroxine-binding globulin, demonstrated abnormal isoforms suggesting a partial deficiency of mainly one or two sialic acid residues. Both the clinical picture and the glycoprotein abnormalities were different from previously defined types of CDG syndrome.