ATP6V0A2 mutations present in two Mexican Mestizo children with an autosomal recessive cutis laxa syndrome type IIA.

Patients with ARCL-IIA harbor mutations in ATP6V0A2 that codes for an organelle proton pump. The ARCL-IIA syndrome characteristically presents a combined glycosylation defect affecting N-linked and O-linked glycosylations, differentiating it from other cutis laxa syndromes and classifying it as a Congenital Disorder of Glycosylation (ATP6V0A2-CDG). We studied two Mexican Mestizo patients with a clinical phenotype corresponding to an ARCL-IIA syndrome. Both patients presented abnormal transferrin (N-linked) glycosylation but Patient 1 had a normal ApoCIII (O-linked) glycosylation profile. Mutational screening of ATP6V0A2 using cDNA and genomic DNA revealed in Patient 1 a previously reported homozygous nonsense mutation c.187C>T (p.R63X) associated with a novel clinical finding of a VSD. In Patient 2 we found a homozygous c.2293C>T (p.Q765X) mutation that had been previously reported but found that it also altered RNA processing generating a novel transcript not previously identified (r.2176_2293del; p.F726Sfs*10). This is the first report to describe Mestizo patients with molecular diagnosis of ARCL-IIA/ATP6V0A2-CDG and to establish that their mutations are the first to be found in patients from different regions of the world and with different genetic backgrounds.

On the nomenclature of congenital disorders of glycosylation (CDG).

A new nomenclature of CDG is proposed because the current one is too complex for clinicians and provides no added value.

Congenital disorder of glycosylation Ic in patients of Indian origin.

Congenital disorder of glycosylation type Ic (CDG-Ic) is caused by mutations in ALG6, encoding an alpha 1,3-glucosyltransferase. The most frequent mutation found in this gene (C998T resulting in an A333V substitution) has until now been found only in patients of European origin. Here we describe the first occurrence of this CDG-Ic mutation in patients of Indian origin. Of three Indian patients described in this study, patient 1 was homozygous and patient 2 heterozygous for the A333V mutation. In patient 2 we also found a new mutation, IVS3+2_3insT, just 3bp away from the previously described IVS3+5G>A substitution; both mutations resulted in exon 3 skipping. We screened a panel of >350 genomic DNA samples from an ethnically diverse American population to determine the frequency of the A333V mutation. None of the samples carried this mutation, indicating the frequency of patients carrying this homozygous mutation should be <1 in 5x10(5). The discovery of the common CDG-Ic mutation A333V in an Indian population raises questions as to its ethnic origin.

A mutation in the human MPDU1 gene causes congenital disorder of glycosylation type If (CDG-If).

We describe a new congenital disorder of glycosylation, CDG-If. The patient has severe psychomotor retardation, seizures, failure to thrive, dry skin and scaling with erythroderma, and impaired vision. CDG-If is caused by a defect in the gene MPDU1, the human homologue of hamster Lec35, and is the first disorder to affect the use, rather than the biosynthesis, of donor substrates for lipid-linked oligosaccharides. This leads to the synthesis of incomplete and poorly transferred precursor oligosaccharides lacking both mannose and glucose residues. The patient has a homozygous point mutation (221T-->C, L74S) in a semiconserved amino acid of MPDU1. Chinese hamster ovary Lec35 cells lack a functional Lec35 gene and synthesize truncated lipid-linked oligosaccharides similar to the patient's. They lack glucose and mannose residues donated by Glc-P-Dol and Man-P-Dol. Transfection with the normal human MPDU1 allele nearly completely restores normal glycosylation, whereas transfection with the patient's MPDU1 allele only weakly restores normal glycosylation. This work provides a new clinical picture for another CDG that may involve synthesis of multiple types of glycoconjugates.

Congenital disorders of glycosylation and the pediatric liver.

Congenital disorders of glycosylation (CDG) are caused by defects in protein N-glycosylation. These inherited disorders impact multiple organ systems, including the liver, its glycoprotein products, and the gastrointestinal system. Many patients have hypotonia, psychomotor retardation, developmental delay, and failure to thrive. Limited awareness of CDG and the diverse biological functions of glycosylation contribute to underdiagnosis of these disorders. Pediatric hepatologists and gastroenterologists are likely to encounter CDG patients early on in their workups. This review will discuss the clinical pictures, biochemistry, molecular defects, diagnosis, and, for one type, an effective treatment. The broad and diverse CDG presentations within and between the various types indicate that it should be considered in any case of unexplained developmental delay, hepatopathology, especially hepatic fibrosis and/or steatosis, protein-losing enteropathy, coagulopathy, hypoglycemia, and failure to thrive.

Studies of mannose metabolism and effects of long-term mannose ingestion in the mouse.

Dietary mannose is used to treat glycosylation deficient patients with mutations in phosphomannose isomerase (PMI), but there is little information on mannose metabolism in model systems. We chose the mouse as a vertebrate model. Intravenous injection of [2-3H]mannose shows rapid equilibration with the extravascular pool and clearance t(1/2) of 28 min with 95% of the label catabolized via glycolysis in <2 h. Labeled glycoproteins appear in the plasma after 30 min and increase over 3 h. Various organs incorporate [2-3H]mannose into glycoproteins with similar kinetics, indicating direct transport and utilization. Liver and intestine incorporate most of the label (75%), and the majority of the liver-derived proteins eventually appear in plasma. [2-3H]Mannose-labeled liver and intestine organ cultures secrete the majority of their labeled proteins. We also studied the long-term effects of mannose supplementation in the drinking water. It did not cause bloating, diarrhea, abnormal behavior, weight gain or loss, or increase in hemoglobin glycation. Organ weights, histology, litter size, and growth of pups were normal. Water intake of mice given 20% mannose in their water was reduced to half compared to other groups. Mannose in blood increased up to 9-fold (from 100 to 900 microM) and mannose in milk up to 7-fold (from 75 to 500 microM). [2-3H]Mannose clearance, organ distribution, and uptake kinetics and hexose content of glycoproteins in organs were similar in mannose-supplemented and non-supplemented mice. Mannose supplements had little effect on the specific activity of phosphomannomutase (Man-6-P<-->Man-1-P) in different organs, but specific activity of PMI in brain, intestine, muscle, heart and lung gradually increased <2-fold with increasing mannose intake. Thus, long-term mannose supplementation does not appear to have adverse effects on mannose metabolism and mice safely tolerate increased mannose with no apparent ill effects.

Functional significance of PMM2 mutations in mildly affected patients with congenital disorders of glycosylation Ia.

PURPOSE: Congenital disorders of glycosylation (CDG) result from mutations in N-glycan biosynthesis. Mutations in phosphomannomutase (PMM2) cause CDG-Ia. Here, we report four clinically mild patients and their mutations in PMM2. METHODS: Analysis of the PMM2 cDNA and gene revealed the mutations affecting the glycosylation efficiency. RESULTS: The patients have 30% to 50% normal PMM activity in fibroblasts due to different mutations in PMM2, and we studied the effect of each mutation on the PMM activity in a Saccharomyces cerevisiae expression system. CONCLUSIONS: Each patient carried a severe mutation that decreased the PMM activity to less than 10% as well as a relatively mild mutation. A new mutation, deletion of base 24, changed the reading frame. The C9Y, C241S, and L32R mutations showed 27% to 45% activity when expressed in the eukaryotic expression system, and the more severe D148N was shown to be thermolabile.

Balancing N-linked glycosylation to avoid disease.

Complete loss of N-glycosylation is lethal in both yeast and mammals. Substantial deficiencies in some rate-limiting biosynthetic steps cause human congenital disorders of glycosylation (CDG). Patients have a range of clinical problems including variable degrees of mental retardation, liver dysfunction, and intestinal disorders. Over 60 mutations in phosphomannomutase (encoded by PMM2) diminish activity and cause CDG-Ia. The severe mutation R141H in PMM2 is lethal when homozygous, but heterozygous in about 1/70 Northern Europeans. Another disorder, CDG-Ic, is caused by mutations in ALG6, an alpha 1,3glucosyl transferase used for lipid-linked precursor synthesis, yet some function-compromising mutations occur at a high frequency in this gene also. Maintenance of seemingly deleterious mutations implies a selective advantage or positive heterosis. One possible explanation for this is that production of infective viruses such as hepatitis virus B and C, or others that rely heavily on host N-glycosylation, is substantially inhibited when only a tiny fraction of their coat proteins is misglycosylated. In contrast, this reduced glycosylation does not affect the host. Prevalent functional mutations in rate-limiting glycosylation steps could provide some resistance to viral infections, but the cost of this insurance is CDG. A balanced glycosylation level attempts to accommodate these competing agendas. By assessing the occurrence of a series of N-glycosylation-compromising alleles in multi-genic diseases, it may be possible to determine whether impaired glycosylation is a risk factor or a major determinant underlying their pathology.

Functional analysis of novel mutations in a congenital disorder of glycosylation Ia patient with mixed Asian ancestry.

Congenital disorders of glycosylation (CDG) are caused by autosomal recessive mutations in genes affecting N-glycan biosynthesis. Mutations in the PMM2 gene, which encodes the enzyme phosphomannomutase (mannose 6-phosphate <--> mannose 1-phosphate), give rise to the most common form: CDG-Ia. These patients typically present with dysmorphic features and neurological abnormalities, cerebellar hypoplasia, ataxia, hypotonia, and coagulopathy, in addition to feeding problems. However, the clinical symptoms vary greatly. The great majority of known CDG-Ia patients are of European descent where the most common mutant alleles originated. This ethnic bias can also be explained by lack of global awareness of the disorder. Here we report an Asian patient with prominent systemic features that we diagnosed with CDG-Ia resulting from two new mutations in the PMM2 gene (310C --> G resulting in L104V and an intronic mutation IVS1-1G --> A). The latter mutation seems to result in lower mRNA levels, and the L104V has been functionally analyzed in a yeast expression system together with known mutations. The Filipino and Cambodian origins of the parents show that CDG-Ia mutations occur in these ethnic groups as well as in Caucasians.

Genetic and metabolic analysis of the first adult with congenital disorder of glycosylation type Ib: long-term outcome and effects of mannose supplementation.

We report the diagnosis and follow-up of two sibs reported in 1980 with recurrent venous thromboses and protein-losing enteropathy; one sib with biopsy-proven hepatic fibrosis died at age 5. The combination of symptoms was suggestive of the recently characterized congenital disorder of glycosylation type Ib (CDG-Ib), which is caused by a deficiency of the enzyme phosphomannose isomerase (PMI). An abnormal serum transferrin isoelectric focusing (IEF) pattern and a reduced PMI activity confirmed the diagnosis of CDG-Ib. Furthermore, mutational analysis of the MPI gene revealed two missense mutations, 419 T --> C (I140T) and 636 G --> A (R219Q), a single base substitution in intron 5, 670 + 9G --> A, as well as a polymorphism 1131A --> C (V377V) in both sibs. The surviving 33-year-old sib has had no further symptoms following childhood. Short-term low-dose oral mannose supplementation improved her transferrin IEF pattern and normalized her antithrombin III activity, further substantiating the beneficial effect of mannose in CDG-Ib. When her mannose blood level was measured, she showed a lower steady-state level but a faster mannose clearance rate. These results suggest that the clinical manifestations of PMI deficiency, although serious in childhood, can improve with age, even without mannose therapy, and allow for a normal adult life. However, the long-term prognosis may vary from patient to patient.

Two proteins modulating transendothelial migration of leukocytes recognize novel carboxylated glycans on endothelial cells.

We recently showed that a class of novel carboxylated N:-glycans was constitutively expressed on endothelial cells. Activated, but not resting, neutrophils expressed binding sites for the novel glycans. We also showed that a mAb against these novel glycans (mAbGB3.1) inhibited leukocyte extravasation in a murine model of peritoneal inflammation. To identify molecules that mediated these interactions, we isolated binding proteins from bovine lung by their differential affinity for carboxylated or neutralized glycans. Two leukocyte calcium-binding proteins that bound in a carboxylate-dependent manner were identified as S100A8 and annexin I. An intact N terminus of annexin I and heteromeric assembly of S100A8 with S100A9 (another member of the S100 family) appeared necessary for this interaction. A mAb to S100A9 blocked neutrophil binding to immobilized carboxylated glycans. Purified human S100A8/A9 complex and recombinant human annexin I showed carboxylate-dependent binding to immobilized bovine lung carboxylated glycans and recognized a subset of mannose-labeled endothelial glycoproteins immunoprecipitated by mAbGB3.1. Saturable binding of S100A8/A9 complex to endothelial cells was also blocked by mAbGB3.1. These results suggest that the carboxylated glycans play important roles in leukocyte trafficking by interacting with proteins known to modulate extravasation.

A novel anionic modification of N-glycans on mammalian endothelial cells is recognized by activated neutrophils and modulates acute inflammatory responses.

We previously reported an unusual carboxylated modification on N:-glycans isolated from whole bovine lung. We have now raised IgG mAbs against the modification by immunization with biotinylated aminopyridine-derivatized glycans enriched for the anionic species and screening for Abs whose reactivities were abrogated by carboxylate neutralization of bovine lung glycopeptides. One such Ab (mAb GB3.1) was inhibited by carboxylated bovine lung glycopeptides and other multicarboxylated molecules, but not by glycopeptides in which the carboxylate groups were modified. The Ab recognized an epitope constitutively expressed on bovine, human, and other mammalian endothelial cells. Stimulated, but not resting, neutrophils bound to immobilized bovine lung glycopeptides in a carboxylate-dependent manner. The binding of activated neutrophils to immobilized bovine lung glycopeptides was inhibited both by mAb GB3.1 and by soluble glycopeptides in a carboxylate-dependent manner. The Ab also inhibited extravasation of neutrophils and monocytes in a murine model of peritoneal inflammation. This inhibition of cell trafficking correlated with the increased sequestration but reduced transmigration of leukocytes that were found to be adherent to the endothelium of the mesenteric microvasculature. Taken together, these results indicate that these novel carboxylated N:-glycans are constitutively expressed on vascular endothelium and participate in acute inflammatory responses by interaction with activated neutrophils.

Overview of glycoconjugate analysis.

Whereas DNA, RNA, and proteins are linear polymers that can usually be directly sequenced, oligosaccharides show substantially more complexity,having branching and anomeric configurations (alpha and beta linkages). The biosynthesis of oligosaccharides, termed glycosylation, is extremely complex, is not template-driven, varies among different cell types, and cannot be easily predicted from simple rules. This overview discusses the stereochemistry of mono- and oligosaccharides and provides diagrammatic representations of monosaccharides (Fisher projections and Haworth representations) and formulas for representation of oligosaccharide chains. A glossary of terms used in glycobiology is also provided.

Lectin affinity chromatography.

This unit describes the use of lectins for preparative glycoprotein purification. Con A-Sepharose and WGA-agarose are used for convenience and availability. Instructions are given for a small-scale pilot procedure to test for lectin binding and to determine elution conditions. There are many variations on the basic procedure in the literature, but all use the same principles: bind the protein to immobilized lectin through its sugar chain, wash away unbound protein, and elute bound protein with a simple sugar that resembles the sugar ligand of the bound protein.

Release of saccharides from glycoconjugates.

Glycosidases are specific enzymes that can partially or completely remove sugar chains from cell-surface glycoconjugates. The enzymes can be exoglycosidases (which remove a terminal saccharide unit from an oligosaccharide chain), endoglycosidases (which cleave within an oligosaccharide chain, releasing an oligosaccharide fragment), or glycoamidases (which cleave between an oligosaccharide unit and its N-linkage to a protein). Commonly used examples of each enzyme are presented in this unit. Sialidase digestion of purified proteins is described and an Alternate Protocol details the application of the technique to intact cell suspensions. A support protocol describes testing sialidase activity in a variety of buffers. Basic protocols are detailed for the digestion of intact glycoproteins and glycopeptides by the most common endoglycosidases and glycoamidases: Endoglycosidase H (Endo H), Endoglycosidase F2 (Endo F2), and Peptide:N-glycosidase F (PNGase F). The applications described in the second and third support protocols employ these enzymes alone or as part of a sequential digestion. The second support protocol describes how to use partial digestions with one enzyme or sequential digestions with different enzymes to estimate the number or types of N-linked carbohydrate chains on a protein. The third support protocols describes sample preparation and digestion followed by gel-filtration chromatography to recover the released radiolabeled oligosaccharide chains for subsequent analysis.

Use of glycosidases to study protein trafficking.

As proteins transit through the cell secretory pathway, modification of their substituent sugar chains occurs in a stepwise fashion. In the course of this processing (maturation) of oligosaccharide chains, the chains acquire sensitivity or resistance to highly specific glycosidases. Thus it is possible to identify processing mileposts by analyzing the general structure of the carbohydrate chains. This unit describes reaction conditions for the family of glycosidases and analysis of the results of digestion reactions.

Special considerations for glycoproteins and their purification.

This unit begins by describing some properties of glycoproteins (e.g., subcellular location and solubility) that may be useful in determining which purification techniques to try. This discussion is followed by two protocols describing preparative glycoprotein purification using lectin-affinity chromatography, as well as an outline for a small-scale pilot procedure designed to check lectin binding and elution conditions. Lectins are often used for purifying glycoproteins because, in contrast to conventional purification procedures (e.g., gel filtration and ion-exchange chromatography) that exploit general physical properties of glycoproteins, lectins recognize specific three-dimensional structures created by a cluster of sugar residues. Conventional purification procedures are generally tried before applying lectin-affinity chromatography.

Endoglycosidase and glycoamidase release of N-linked oligosaccharides.

Carbohydrate chain modifications are often used to monitor glycoprotein movement through the secretory pathway. This is because stepwise sugar-chain processing is unidirectional and generally corresponds to the forward or anterograde movement of proteins. This unit offers a group of techniques that will help analyze the general structure of carbohydrate chains on a protein and, therefore, oligosaccharide processing mileposts. The sugar chains themselves are not analyzed, but their presence and structure are inferred from gel mobility differences after one or more enzymatic digestions. This approach is most often used in combination with [35S]Met pulse-chase metabolic labeling protocols, but they can be applied to any suitably labeled protein (e.g., biotinylated or 125I-labeled).

Analysis of sulfate esters by solvolysis or hydrolysis.

Sulfate esters are found on N- and O-linked sugar chains or glycosaminoglycan (GAG) chains. Few sulfatases are available that can enzymatically remove them, so chemical procedures must be used. These procedures rely on the differential sensitivity of sulfates located in different linkages on the sugar. In comparison to the conditions used for enzymatic digestion, those used for chemical digestion are very harsh and cannot be used on protein-bound carbohydrates (except for analytical purposes, as described here). With protein-bound carbohydrates, for preparative purposes, the chains must first be released. This unit describes release of sulfate esters by solvolysis along with a method for monitoring the efficiency of the solvolysis reaction. An alternate procedure provides a scale-up method for using the solvolysis reaction with large amounts of material. Also presented are techniques for both acid and basic hydrolysis to release sulfate esters.

Lectin analysis of proteins blotted onto filters.

Lectins are proteins that bind with great specificity to certain carbohydrate structures. Plant lectins are widely used for investigations of carbohydrate structure and for fractionation and purification of individual oligosaccharides and glycopeptides. This unit describes the use of lectins as sensitive indicators for the presence of certain carbohydrate structures linked to proteins blotted onto filters. A tagged lectin is incubated with a blot containing the target protein and binding of the lectin is detected by one of several different procedures. Direct approaches include using lectins labeled with 125I or conjugated to horseradish peroxidase or alkaline phosphatase, which can be detected by chromogenic or luminescent visualization systems. Indirect approaches involve using lectins conjugated to biotin or digoxigenin followed by a second incubation with alkaline phosphatase-conjugated avidin or antibodies specific for the haptenic digoxigenin group and then by visualization. Several commercial kits are available that provide labeled lectins, control proteins, and developing reagents needed for visualization. These systems can also be adapted for use with lectins other than those supplied with kits. The protocol in this unit is easy to perform with or without a kit. However, the results, while suggestive of carbohydrate structure, are not definitive.