Abstract

Defects in nine genes of the N-linked glycosylation pathway cause congenital disorders of glycosylation (CDGs) and serious medical consequences. Although glycobiology is seldom featured in a general medical education, an increasing number of physicians are becoming acquainted with the field because it directly impacts patient diagnosis and care. Medical practice and attitudes will change in the postgenomic era, and glycobiology has an opportunity to be a cornerstone of part of that new perspective. This review of recent developments in the CDG field describes the biochemical and molecular basis of these disorders, describes successful experimental approaches, and points out a few perspectives on current problems. The broad, multisystemic presentations of these patients emphasize that glycobiology is very much a general medical science, cutting across many traditional medical specialties. The glycobiology community is well poised to provide novel perspectives for the dedicated clinicians treating both well-known and emerging human diseases.

Accepted on September 13, 2001;

Introduction

Congenital disorders of glycosylation (CDGs) were previously called carbohydrate-deficient glycoprotein syndromes (Participants at First International Workshop on CDGS, 2000), but the fast-paced discovery of defective genes that cause CDGs required a consistent nomenclature and classification criteria. The current system will undoubtedly undergo revision in the future, because 2%–3% of the active genome is probably used for glycoconjugate synthesis or recognition and defects in only some of those genes have been described. For now, the simplified CDG nomenclature, which applies primarily to N-linked oligosaccharide biosynthesis, will be used. Group I CDG defects are defined as those altering synthesis and transfer of the dolichyl-pyrophophosphate–linked precursor oligosaccharide to recipient proteins and includes about 30 known genes. Group II CDG defects affect subsequent processing steps, mostly on N-linked sugar chains, requiring at least another 20 genes. Different types of disorders in each group are defined by a small letter code (a, b, c, etc.) indicating the chronological order in which the defective gene was identified. Currently, Group I includes types a–f, and Group II includes types a–c. Obviously, this system is not comprehensive and does not yet address important disorders in the biosynthesis of proteoglycans (Duncan et al., 2001), O-linked oligosaccharides, or glycosylphosphatidylinositol anchors.

This review will cover developments of the past few years, newly discovered types of CDGs, methods and experimental systems that identified them, advantages and limitations of diagnostic indicators, and exceptions to the general rules.

Diagnosis of CDG

Physicians are becoming increasingly aware of the broad, diverse, and multisystemic clinical presentations of CDGs (de Praeter et al., 2000; Etzioni and Tonetti, 2000; Grünewald et al., 2000; van Ommen et al., 2000; de Lonlay et al., 2001; Jaeken et al., 2001). Typically, a patient presenting with characteristic symptoms is tested for abnormal glycosylation by an isoelectric focusing analysis (IEF), ion-exchange analysis, or agarose gel electrophoresis of serum transferrin (Tf) (Stibler and Hultcrantz, 1987; Stibler and Jaeken, 1990; Wada et al., 1992; Lof et al., 1993; Stibler and Cederberg, 1993; Yamashita et al., 1993a; Landberg et al., 1995; Keir et al., 1999). The Tf analysis (Figure 1) biochemically indicates abnormal glycosylation, but does not indicate its cause or whether it is congenital or acquired. So far, only uncontrolled fructosemia, galactosemia, recent alcoholism, and liver disease yield false positive indicators of inherited glycosylation defects by this test (Stibler and Hultcrantz, 1987; Stibler et al., 1988a,b, 1997; Jaeken et al., 1996a; Charlwood et al., 1998a; Cottalasso et al., 1998; Arndt, 2001; Sillanaukee et al., 2001).

Tf is only one of many misglycosylated proteins in CDGs (Harrison et al., 1992; Stibler et al., 1998; Henry et al., 1999), but it is a sensitive and convenient marker. Tf normally has two N-linked disialylated biantennary chains. Insufficient amount or poor transfer of oligosaccharide chains from the dolichyl-PP-oligosaccharide precursor leads to unoccupied Asn-X-Ser/Thr sequons, producing some molecules that lack one or two entire chains, and and two or four sialic acids, respectively (Wada et al., 1992; Yamashita et al., 1993a,b; Lacey et al., 2001), raising the isoelectric point. This pattern is typical of Group I CDGs. Altered processing of normally transferred N-linked chains is also reflected in altered Tf IEF; some Group II disorders give a series of abnormal Tf bands depending on the specific enzymatic defect (Stibler et al., 1993, 1999; Coddeville et al., 1998). Amino acid polymorphisms can also change pI, and therefore sialidase treatment is used to confirm that abnormal patterns are based on glycan heterogeneity.

Tf IEF is currently the most widely used method for CDG diagnosis, although automated electrospray ionization mass spectrometry (ESI-MS) with in-line immunoaffinity purification of Tf may provide another option (Lacey et al., 2001). ESI-MS has the advantage of differentiating the absence of an entire chain from differences in oligosaccharide processing, whereas simple IEF can only suggest such differences. Even though Tf analysis is the best test available, it has limitations. Not all types of CDGs can be detected, for example, CDG-IIb and -IIc and some CDG-Ia patients with proven genetic and enzymatic defects nevertheless either gradually or spontaneously develop a nearly normal Tf pattern (Fletcher et al., 2000; Dupre et al., 2001) (discussed below). Altered glycosylation in CDG patients can also be detected by analysis of other plasma or serum glycoproteins (Harrison et al., 1992; Krasnewich et al., 1995; Krasnewich and Gahl, 1997) and by β-trace protein in the cerebrospinal fluid (CSF) (Pohl et al., 1997; Grünewald et al., 1999). There is a need and opportunity to develop different biochemical CDG diagnostic tests. Lectin blot analysis may offer another option for detecting CDG alterations. This has recently been applied in a few cases (Ferrari et al., 2001; Wang et al., 2001).

Carriers may show slightly altered Tf patterns, and prenatal Tf IEF testing alone in at-risk families is not useful (Clayton et al., 1993; Stibler and Skovby, 1994; Charlwood et al., 1998b).

Identifying CDG defects

All of the CDG disorders identified so far are autosomal recessive (Schachter, 2000, 2001; Aebi and Hennet, 2001; Jaeken et al., 2001; Marquardt and Freeze, 2001). Tracking them down has relied on biochemical clues gleaned by tried-and-true methods of metabolic disease research: Find the accumulation or depletion of a critical substrate in the pathway as an indicator of the likely enzymatic defect. This approach is familiar for the analysis of yeast and mammalian cell glycosylation mutants (Stanley, 1984; Burda and Aebi, 1999; Freeze and Aebi, 1999; Aebi and Hennet, 2001; Marquardt and Freeze, 2001). Most of these were isolated following chemical mutagenesis resulting in altered sensitivity to cytotoxic lectins, antibody binding, or failure to incorporate lethal amounts of radiolabels such as [2-3H]mannose (reviewed in Burda and Aebi, 1999; Aebi and Hennet, 2001; Marquardt and Freeze, 2001).

The clinical-glycobiology connection is shown in Figure 2. Once the astute clinician obtains a positive result from a Tf IEF or similar diagnostic test, enzyme assays are done on fibroblasts or leukocytes. In addition, fibroblasts are biosynthetically labeled with exogenous [2-3H]mannose to determine glycosylation efficiency, structure of the lipid- and protein-bound oligosaccharide chains, or the levels of various metabolic intermediates, for example, Man-1-P (Körner et al., 1998b) or GDP-Man (Rush et al., 2000). Altered levels of key intermediates or oligosaccharide structures suggest possible enzymatic defects analogous to yeast and Chinese hamster ovary (CHO) mutants characterized over the past two decades. An enzymatic deficiency may lead to detection of specific defects in known human genes (de Koning et al., 1998b; Jaeken et al., 1998; Niehues et al., 1998). If the human gene was not known at the time, EST searches for homologs identified the human gene (Matthijs et al., 1997a; Imbach et al., 1999, 2000b; Körner et al., 1999; Kim et al., 2000; Luhn et al., 2001). Of course, it is essential to show that the putative mutation is not simply a polymorphism, and this requires functional expression of the putative mutated gene. Fortunately, established yeast and mammalian cell mutants provide the ideal test system to express normal human and mutated alleles (Aebi and Hennet, 2001; Marquardt and Freeze, 2001). In most instances, this approach has worked quite well. Once the functional mutations are found, they are confirmed at the genomic level in the afflicted patient, in the parents, and sometimes in siblings. Clearly this approach applies to mutations in the coding sequence, but mutations in promoters, introns, splice junctions, and so on, that decrease the amount of functional mRNA are also seen. Genomic sequencing for these other types of mutations has not been developed, although a few frequent intronic variants have been identified (Matthijs et al., 2000).

CDGs: causes, clinical features, and a few treatments

The nine known causes of CDG (as of September 2001), the year they were discovered, and the approximate number of patients affected are listed in Table I. Figure 3 shows their locations in the N-linked oligosaccharide biosynthetic pathway. The clinical features are presented in Table II. Except for CDG-Ib—phosphomannose isomerase deficiency—all of the disorders show various degrees of mental and psychomotor retardation and many have various gastrointestinal disorders (Westphal et al., 2000c; Jaeken et al., 2001; Leonard et al., 2001; Marquardt and Freeze, 2001; Schachter, 2001).

CDG-Ia—phosphomannomutase deficiency

This is by far the most common form of CDG, with over 300 patients diagnosed mostly in Europe (Eeg-Olofsson and Wahlström, 1991; Winchester et al., 1995; Freeze, 1998; Jaeken et al., 2001; Schachter, 2001), although an increasing number of patients have been found in North America. European ancestry is most common, but a few cases have been identified in African American, Filipino/Cambodian, and Chinese/Malaysian populations. Cases have also been reported in Japan, South America, and the Middle East (Jaeken et al., 2001; Schachter, 2001). The clinical picture is variable (van Ommen et al., 2000; de Lonlay et al., 2001; Grünewald et al., 2001; Leonard et al., 2001), but nearly always includes mental retardation, although this can sometimes be surprisingly mild. Cerebellar hypoplasia, hepatomegaly, liver dysfunction, and coagulopathy caused by decreased antithrombin III, Factor XI, Protein C, and Protein S are often seen (Van Geet and Jaeken, 1993; Stibler et al., 1998). Severe infections and pericardial effusions sometimes occur in the first several years, and the mortality can be as high as 20% (Matthijs et al., 2000; Jaeken et al., 2001; Kjaergaard et al., 2001; Schachter, 2001). The children are ataxic, with long limbs and short torsos. They generally do not walk, but can crawl and may stand with support. They have pleasant dispositions. Although most patients are children, an increasing number of adults have been diagnosed in their late 20s. One 31-year-old American patient declares his “eclectic taste in music goes from Carmen to heavy metal.” He does well in an assisted living environment. Other adults with mild forms of the disease can even run and draw.

Jaeken diagnosed the first patients in 1980 (Jaeken et al., 1980) and Van Schaftigen and Jaeken discovered the defect in 1995 (Van Schaftingen and Jaeken, 1995). Over 60 mutations are now known to occur in PMM2 (Matthijs et al., 2000), which encodes phosphomannomutase. In the cytosol, this enzyme converts Man-6-P to Man-1-P (Pirard et al., 1999a,b), which is then used to form GDP-Man (see Figure 3). Mutations in the PMM2 encoded enzyme decrease catalytic activity and/or enzyme stability, which decreases the GDP-Man pool (Kjaergaard et al., 1999; Pirard et al., 1999b; Vuillaumier-Barrot et al., 2000; Westphal et al., 2001a). Another gene, PMM1, occurs in humans, but its expression appears mostly limited to the lung and brain (Matthijs et al., 1997b; Schollen et al., 1998). No CDG patients have mutations in this gene. A pseudogene has also been identified (Schollen et al., 1998). A few severe mutations are especially common in northern Europe. One, R141H, appears to be lethal in the homozygous state (Matthijs et al., 1998); however, 1/70 normal individuals are heterozygous for the mutation, suggesting a nonobvious advantage for the heterozygous mutation (Matthijs et al., 2000; Schollen et al., 2000b). Based on various mutation frequencies in Europe, Matthijs (Schollen et al., 2000b) estimates that 200 new CDG-Ia cases occur each year in Europe and North America. Most of these are probably not being diagnosed. Because the 20% mortality decreases sharply after the first few years, it is very likely that there are several thousand CDG-Ia cases on each continent.

Several of the mutations in PMM2 are extremely mild, and some patients have phosphomannomutase (PMM) enzymatic activities in fibroblasts not far below heterozygous levels (Grünewald et al., 2001; Westphal et al., 2001c). In mildly affected patients, there is some correlation between residual PMM enzymatic activity and clinical severity, but this does not hold true for most patients (Matthijs et al., 1998; Imtiaz et al., 2000; de Lonlay et al., 2001; Grünewald et al., 2001). Those with high activity still have strabismus (inward crossed eyes) and hypotonia, but do not have the multiorgan involvement, retinopathy, or severe mental retardation. PMM assays in patient leukocytes tend to give lower (15%–25%) residual activity in this group of patients, probably because these cells had been stored for some time before assay (Grünewald et al., 2001). The mutant proteins are usually thermolabile (Kjaergaard et al., 1999; Pirard et al., 1999b; Vuillaumier-Barrot et al., 2000). Human PMM2 has been expressed in both Escherichia coli and Saccharomyces cerevisiae (Pirard et al., 1999b; Grünewald et al., 2001). The normal human allele can rescue the lethal temperature sensitive phenotype of sec53S. cerevisiae cells, but severe mutations do not (Westphal et al., 2001a). Direct enzymatic assays of expressed PMM2 alleles give comparable results in both host systems.

Fibroblasts from CDG-Ia patients synthesize truncated lipid-linked oligosaccharide (LLO) (containing four to five, rather than nine Man residues), incorporate less [2-3H]mannose into proteins (Powell et al., 1994; Krasnewich et al., 1995; Panneerselvam and Freeze, 1996a), and have a decreased amount of [3H]Man-1-P (Körner et al., 1998b) and GDP-[3H]Man (Rush et al., 2000). The actual size of the GDP-Man pool is about 10-fold lower in CDG cells (2.5 pmoles/106 cells) compared to control (23 pmole/106 cells). Adding 200–1000 µM mannose to the growth medium normalizes the GDP-Man pool size, the size of the LLO molecule, and incorporation of labeled mannose into glycoproteins (Panneerselvam and Freeze, 1996a). Mannose presumably enters the cells through the mannose-specific transporter (Ogier-Denis et al., 1990, 1994; Panneerselvam and Freeze, 1996b; Panneerselvam et al., 1997). Mannose uptake by CDG cells also appears to be decreased (Dupre et al., 1999).

Small uncontrolled trials giving oral mannose to CDG-Ia patients of various ages for up to two years did not improve any biochemical or clinical feature (Marquardt et al., 1997; Mayatepek et al., 1997; Kjaergaard et al., 1998; Mayatepek and Kohlmüller, 1998). However, some parents insist that their children’s behavior and clinical condition improved on mannose. Because these studies were uncontrolled, it is impossible to evaluate the parent’s positive impressions. Although this therapy has not appeared promising, suitably modified derivatives of Man-1-P, such as nontoxic acetoxymethyl esters that enter cells (Schultz et al., 1993, 1994), offer another avenue to provide the missing substrate. Gene therapy is not a likely option for CDG-Ia patients in the near future, given the current controversies and cautious climate.

PMM2 knockout mice have not been developed yet. Based on the lethality of the R141H mutation in humans, it is unlikely that a complete ablation of the gene would produce viable offspring. Engineering selected mutations in the gene are more likely to have a positive outcome, which will depend on selecting appropriate point mutations that are compatible with some residual enzymatic activity, rather than a total absence. Another important consideration is that the mutation is expressed in a suitable genetic background. The importance of the genetic background is well illustrated in the mouse model of CDG-IIa (Wang et al., 2001). Conditional knockouts, such as the Cre-Loxp system, are another option (Orban et al., 1992; Hennet and Ellies, 1999).

CDG-Ib—phosphomannose isomerase deficiency

The metabolic step immediately preceeding the PMM-catalyzed reaction leading to GDP-Man is the conversion of Fructose-6-P into Man-6-P using phosphomannose isomerase (PMI) encoded by MPI (Gracy and Noltmann, 1968; Schultz et al., 1994; Proudfoot et al., 1994a,b). About 20 patients are known, and the clinical features include coagulopathy, hypoglycemia, protein-losing enteropathy (loss of plasma proteins through the intestine), diarrhea, cyclic vomiting, liver dysfunction, and hepatic fibrosis; surprisingly there is no mental retardation, neuropathy, or ataxia (de Koning et al., 1998a,b, 2000; de Lonlay et al., 1998; Jaeken et al., 1998; Niehues et al., 1998; Babovic-Vuksanovic et al., 1999). Several patients died before genetic diagnosis, and the oldest known patient is 34 years old and currently in good health, although her sibling died from complications of the disease at age 5 (Pedersen and Tygstrup, 1980; Westphal et al., 2001b).

The cause of CDG-Ib was discovered nearly simultaneously in three different laboratories (de Koning et al., 1998b; Jaeken et al., 1998; Niehues et al., 1998). In one case, coagulopathy prompted a Tf IEF test, which indicated CDG, despite normal intelligence. Later the patient nearly died of intestinal bleeding and was placed on “compassionate use” mannose (Alper, 2001a) and recovered over the course of a few months. Protein-losing enteropathy (PLE), hypoglycemia, coagulopathy, and abnormal Tf were all corrected, but the defect was unknown at the time. Shortly afterward, direct assays of fibroblasts showed a deficiency in PMI, and molecular studies confirmed mutations in conserved amino acids (Niehues et al., 1998). Other patients had point mutations, deletions, insertions, or unstable mRNA species (Schollen et al., 2000a). Several reports indicated that other patients, especially young ones, also responded well to mannose (Babovic-Vuksanovic et al., 1999; de Lonlay et al., 1999). On the other hand, those close to adult age have not required (or complied with) mannose therapy and yet seem to do relatively well (de Koning et al., 2000; Westphal et al., 2001b). This result suggests that the stress of growth in childhood may tax and overwhelm the glycosylation system, and that the metabolic challenges faced by mature adults are less acute than those children face.

Free mannose does not exist in foods to any appreciable extent, but protein hydrolysates typically contain about 0.2% mannose in glycoconjugates (Panneerselvam et al., 1997; Alton et al., 1998). Indigestible galactomannans have β-linked mannose, which is mostly unavailable for direct use by the body (Davis and Lewis, 1975; Reid, 1985). Rats (Alton et al., 1998), mice (Davis and Freeze, 2001) and humans (Alton et al., 1997) take up mannose well from the gut and at least a portion of it is used for glycoprotein synthesis in all organs (Davis and Freeze, 2001). The liver and the intestine are by far the biggest consumers of mannose for glycoprotein synthesis. Not surprisingly, these organs are most affected in CDGs. Most of the mannose in the blood appears to be consumed by the glycolytic pathway once it enters the cell.

PMI is a dimer in the cytosol and is not known to be associated with any membranous structure; however PMI does not leak out of streptolysin O-permeabilized cells as readily as lactate dehydrogenase (Kim and Freeze, unpublished data). Glycolytic enzymes have been variably described as part of a supermolecular complex (Clegg and Jackson, 1990; Clegg, 1991). It is possible that PMI is complexed with other proteins for more efficient utilization of the substrates.

PMI-deficient yeast (pmi40) can be complemented by the normal human allele, but not by alleles carrying mutations that substantially compromise activity (Westphal et al., 2001b).

PMI knockout mice have been created by gene trap methods. Heterozygotes have 50% of normal PMI activity, but no viable –/– progeny arose from heterozygous matings, even if mannose-supplemented drinking water was supplied during gestation. Further work is required to determine the reason for the lethal outcome (unpublished data). Like PMM mutants, it may be necessary to use homologous recombination techniques to introduce mutant alleles into the mouse to create a model of the human disease.

CDG-Ic—α1,3 glucosyltransferase deficiency

About 20 patients have been shown to have a deficiency in α1,3 glucosyl transferase (Burda et al., 1998; Körner et al., 1998a; Grünewald et al., 2000; Hanefeld et al., 2000; Imbach et al., 2000a; Westphal et al., 2000a,b). Eight mutations have been identified in human ALG6 that encodes the α1,3 glucosyltransferase, which adds the first glucose to the dolichyl-PP-oligosaccharide, as shown in Figure 3. Nonglucosylated oligosaccharides are poor substrates for the oligosaccharyl transferase complex, resulting in unoccupied glycosylation sites on many glycoproteins. Clinically, CDG-Ic patients resemble mild PMM-deficient patients. Although they are mentally retarded and have seizures, there is no cerebellar hypoplasia, typical dysmorphic appearance, or strabismus (crossed eyes). Coagulopathy is mild and they are considerably less neurologically affected than usually seen in CDG-Ia.

[2-3H]Mannose labeling of fibroblasts produces a variable increase in the amount of truncated LLO, Man9GlcNAc2, and reduced Glc3Man9GlcNAc2. Anywhere from 20% to 90% of the LLO may occur in the truncated form, even in patients with the identical homozygous point mutations in ALG6 (Imbach et al., 2000a). Reasons for this broad heterogeneity may be genetic background or subtle differences in labeling conditions or cell growth.

The human ALG6 gene was cloned by its homology to the S. cerevisiae gene (Reiss et al., 1996; Imbach et al., 1999). The similarity of the early stages of the N-linked oligosaccharide pathway in S. cerevisiae and mammals has been a boon to progress in this field (Hennet and Ellies, 1999; Aebi and Hennet, 2001; Marquardt and Freeze, 2001; Wang et al., 2001). Labs studying yeast N-linked glycosylation have obtained mutant strains in nearly all of the steps of the pathway (Huffaker and Robbins, 1983; Beck et al., 1990; Herscovics and Orlean, 1993; Burda and Aebi, 1999; Burda et al., 1999; Freeze and Aebi, 1999; Aebi and Hennet, 2001; Schenk et al., personal communication). ALG6-deficient strains are ideal systems to test the functional activity of mutated human alleles of α1,3glucosyltransferase. This is usually done by analysis of the vacuolar protein carboxypeptidase Y (CPY), which has four N-linked oligosaccharide chains. alg6 strains produce stable CPY species lacking one or two chains. The impact of this mutation can be enhanced by placing it in a strain carrying a mutated WBP1, one of the subunits of the oligosaccharyl transferase complex. Neither mutation alone is lethal, but they are conditionally lethal when combined. Rescuing the conditional lethality with various human genes provides a sensitive growth-based assay, which can be accurately titrated based on degree of CPY glycosylation (Imbach et al., 1999, 2000a; Westphal et al., 2000a,b). Alternatively, the ability of human ALG6 alleles to restore normal CPY glycosylation can be tested in the alg6 strain with normal WBP1. In this case the growth rate is varied to stress glycosylation capacity. At slow growth rates, mutations in ALG6 have only modest impact on CPY glycosylation, but at rapid growth rates, the effect is substantial (Westphal et al., 2000a). Figure 4 illustrates this for a series of different point mutations detected in one CDG patient.

The effect of growth rate on the impact of ALG6 mutations on glycosylation found a correlation in one CDG-Ic patient who experienced nearly fatal PLE during intestinal viral infections (Westphal et al., 2000a). During infection, enterocytes in the small intestine increase their growth rate to compensate for the cytopathic infection. During PLE, the heparan sulfate proteoglycan (HSPG) and core protein syndecan-1 that normally coat the basolateral surface of the small intestine decline, and only rare scattered intracellular deposits of HSPG remain. When PLE resolves, many of the small intestinal villi were again populated with normal HSPG. Glycosylation of either syndecan 1 or key heparan sulfate biosynthetic enzymes may have been insufficient during times of rapid growth in this patient. The environmental stress appeared to overwhelm this young child’s compromised glycosylation system. The burden was location-selective because heparan sulfate was normal at all times in the more slowly turning over epithelial cells of the colon, stomach, and esophagus. These observations illustrate an important point: Fever, infectious agents, or poor diet may combine with other compromised genes to overwhelm the compromised glycosylation machinery.

CDG-Id and CDG-Ie

CDG-Id was previously called carbohydrate-deficient glycoprotein syndromes type IV (Stibler et al., 1995; Körner et al., 1999). Only a few CDG-Id and Ie patients have been described. In those cases, patients with both disorders have severe neurological abnormalities, including intractable seizures, hypotonia, and severe psychomotor retardation. Optic atrophy (CDG-Id) associated with coloboma of the iris (Bewley and Reid, 1985) and cortical blindness are seen (CDG-Ie) (Kim et al., 2000; Imbach et al., 2000b). CDG-Id is caused by a deficiency in Dol-P-Man:Dol-PP-Man5GlcNAc2 α-1,3-mannosyltransferase (α-1,3-Man-T) which transfers Man from Dol-P-Man to Dol-PP-Man5GlcNAc2 to form Man6GlcNAc2 (Bewley and Reid, 1985). This enzyme is specific for LLO synthesis and causes accumulation of Dol-PP-Man5GlcNAc2, due to poor transfer to protein. Since this defect is incomplete, some sugar chains are still synthesized normally. The gene responsible for CDG-Id is the ortholog of yeast ALG3 (Imbach et al., 2000a). Disialo Tf is increased, but asialotransferin is nearly completely absent.

CDG-Ie patients also produce an abnormal Tf pattern very similar to that seen in CDG-Id. In CDG-Ie, the lesion occurs in the catalytic subunit of Dol-P-Man synthase, (GDP-Man:Dol-P mannosyltransferase, the DPM1 gene) (Imbach et al., 2000b; Kim et al., 2000). Yeast provided important clues for solving this defect, but Man-P-Dol synthesis in mammalian cells is different. In S. cerevisiaeDPM1 encodes a single transmembrane, endoplasmic reticulum (ER)–resident enzyme (Orlean et al., 1988; Beck et al., 1990). Yeast DPM1 complements two Man-P-Dol synthesis-deficient mammalian cell lines, BW5147 Thy–1E, and CHO Lec15, which synthesize and accumulate a truncated Man5GlcNAc2-P-P-Dol LLO. However, in mammals, DPM1 lacks a transmembrane domain or an ER retention signal (Tomita et al., 1998), but mammalian DPM1 associates with another small transmembrane protein encoded by DPM2, which has an ER retention signal and may bind Dol-P (Maeda et al., 1998). Cell line BW5147 Thy–1E is defective in DPM1 while Lec15 is defective in DPM2. A third component of the complex, DPM3, was recently identified (Maeda et al., 2000), but mutations have not yet been identified in either DPM2 or DPM3. Synthesis of Man6GlcNAc2 is impaired, but in CDG-Ie the substrate for subsequent LLO mannose elongation reactions and other Man-containing glycoconjugates including GPI-anchored proteins is limited.

CDG-If—Man-P-Dol utilization gene deficiency

A small group of patients is emerging with this disorder (Kranz et al., 2001; Schenk et al., 2001). Early clinical features include muscular hypotonia, optic atrophy, frequent seizures, and general cerebral atrophy. Dry, hyperkeratotic skin with scaling may occur and eventually disappear. Gastrointestinal problems can include decreased food intake, abdominal pain, and frequent vomiting, without diarrhea. Severe failure to thrive, ataxia, and profound psychomotor retardation are seen. Slightly reduced AT III may occur, but without elevated transminases (Kranz et al., personal communication; Schenk et al., personal communication).

These patients have a defect in a gene named MPDU1, which is required for the utilization but not for the synthesis of Man-P-Dol and Glc-P-Dol (Anand et al., 2001). The expression of this gene suppresses a defect in CHO Lec35 cells, which are deficient in LLO, glycosylphosphatidylinositol anchor synthesis, and C-mannosylation (Slonina et al., 1993; Ware and Lehrman, 1996, 1998; Doucey et al., 1998). The pLec35 appears to be more selective for facilitating utilization of Man-P-Dol than for Glc-P-Dol (Anand et al., 2001). The protein itself is hydrophobic and probably inserted into the ER membrane, but its sequence gives no clue to its function or mechanism, so it is unclear how this protein “facilitates” precursor utilization. It may help present or orient the monosaccharide donor for the respective transferases.

CDG-IIa—Mgat2 deficiency

Only about five patients are known with this disorder, which is caused by the loss of UDP-GlcNAc:α-6-D-mannoside β1,2-N-acetylglucosaminyltransferase II (GlcNAc-TII, encoded by Mgat2) (Jaeken et al., 1994, 1996b; Tan et al., 1996; Schachter and Jaeken, 1999; Schachter, 2000). LLO synthesis and transfer are normal, but only monosialylated chains are made during subsequent processing due to the complete absence of the GlcNAc-β1,2-Man-α1,6- antenna. The Tf IEF pattern is different from that seen in CDG-I. There is a complete absence of fully sialylated Tf and a large amount of disialylated Tf, resulting from two monosialylated truncated chains (Coddeville et al., 1998).

This was the first CDG defect identified and the first to produce a viable mouse strain lacking the relevant gene (Wang et al., 2001). Up to 60% of the Mgat2 null mouse embryos develop fully, but 99% of newborns die within the first week after birth. The phenotypes and defects between the murine model and patients are quite remarkable and nearly identical (Wang et al., 2001). Both show abnormal Tf IEF patterns and loss of E-/L-phytohemagglutinin lectin binding to plasma proteins. The surviving mice are small and have kyphoscoliosis, facial dysmorphia, muscular atrophy, tremors, and osteopenia resulting from increased osteoclast activity (Wang et al., 2001). Ossification at the growth plates is reduced and the calcified bone density is reduced >30%. One of the survivors showed a ventricular septal defect in the heart. Female mice are fertile, but spermatogenesis is blocked in males. Coagulation is prolonged; various clotting factors are similarly decreased deficiencies. The population of B and T lymphocytes are reduced in the mice, and IgG levels are low in patients. A surprising point is that the majority of Mgat2 deficient mice die from gastrointestinal blockage. Hemorrhaging and abnormal microbial colonization are seen in some gastrointestinal tracts. These results would suggest that humans with CDG-IIa are likely to die in gestation or shortly after birth from any number of developmental or physiological abnormalities. Thus, the true incidence of mutations in this gene and the disorder are probably underestimated.

Genetic background plays a major role in the survival of Mgat2 null mice. When the homozygous null allele was expressed in a C57/BL6 background, all mice died by 4 weeks of age. When the null mutation was expressed in the outbred ICR strain, the proportion of surviving mice remained low, but about 2% of the offspring survived for many months. The survivors showed the same disease signs described above, but individual variation was greater and the severity was generally reduced.

CDG-IIb—α-glucosidase deficiency

CDG-IIb is caused by a severe deficiency in glucosidase I that removes the terminal α1,2 glucose from the oligosaccharide after its transfer to protein (de Praeter et al., 2000). Only one patient has been reported (de Praeter et al., 2000). She was dysmorphic with a broad nose, high arched palate, overlapping fingers, together with hypotonia and edema. Seizures developed around 3 weeks that required artificial ventilation. Hepatomegaly developed and congenital hepatic fibrosis occurred with proliferation and dilatation of the bile ducts, fat accumulation, and excessive iron storage. Liver cells had multilamellar inclusion bodies. The girl died at 2.5 months of age.

An interesting feature about this patient was that her serum Tf and β-CSF trace proteins were both normal. Diagnosis was based on the structural analysis of an abnormal urinary tetrasaccharide, identified as Glc3Man. This fragment very likely resulted from the endo-α1,2-mannosidase cleavage of unprocessed oligosaccharides (Moore and Spiro, 1990, 1992; Zuber et al., 2000). Once these residues are removed, further normal processing can occur. Two separate point mutations were identified in this child from a consanguineous marriage. This finding suggests that mutations in the α-glucosidase gene may be frequent, but the consanguinity also cautions that some clinical symptoms may have resulted from mutations in other genes.

CDG-IIc—GDP-fucose transporter deficiency

This disorder has usually been called leukocyte adhesion deficiency type II (LAD-II), because the patients have increased leukocyte counts (Etzioni et al., 1995; Becker and Lowe, 1999; Etzioni and Tonetti, 2000). For historical reasons, this alias will probably persist. Only three male patients have been described all having mental retardation, short limbs and stature, and a flat face with a broad, depressed nasal bridge (Etzioni et al., 1992; Frydman et al., 1992; Marquardt et al., 1999a). The defect was recently shown to result from reduced GDP-fucose import into the Golgi (Lübke et al., 1999, 2001; Luhn et al., 2001) resulting in a global decrease in fucosylated glycans. Because fucosylation is critical for expression of sialyl-Lewis X on neutrophils, this explains the faulty, selectin-mediated leukocyte adhesion (Etzioni et al., 1992; Marquardt et al., 1999a). Two simultaneous papers identified the gene and the mutations responsible for the deficiency (Lübke et al., 2001; Luhn et al., 2001).

Addition of fucose to the culture medium of LAD-II fibroblasts restores fucosylated ligand expression (Karsan et al., 1998; Marquardt et al., 1999b). In one case, the faulty leukocyte adhesion was rapidly corrected by oral fucose supplements (Marquardt et al., 1999b) that have been maintained for more than 2 years. Impaired fucosylation of IgM also appeared to improve, however, fucose did not restore all fucosylation, because the fucose-containing blood group H-antigen does not appear on the erythrocyte surface (Marquardt and Freeze, 2001). This is fortunate because the presence of H antigen on LAD-II red cells would cause a lethal hemolysis reaction from anti-H present in the LAD-II sera. A plasma membrane fucose-specific transporter was previously reported to occur in cells (Wiese et al., 1994). This presumably carries fucose into the cells where fucose kinase directly produces Fuc-1-P (Park et al., 1998), the precursor for GDP-fucose. Presumably, exogenous fucose increases the effective cytosolic concentration of GDP-Fuc seen by the transporter in some cells. It is interesting that the physiological function of neutrophils is restored very quickly. P-selectin binding appeared first, followed by E-selectin binding, whereas other fucosylated glycoconjugates never appeared. Other LAD-II patients with a similar biochemical defect did not respond to fucose therapy, apparently because the mutations did not affect the Km (Sturla et al., 2001).

Finding cell-type and protein preferential restoration with fucose therapy illustrates a surprising diversity rather than “all-or-none” responses. A similar situation occurs in the studies of mutant cells with an impaired UDP-Gal Golgi transporter. Synthesis of abundant, galactose-rich N- and O-glycans are severely affected, while synthesis of chondroitin and heparan sulfates, which require only two Gal residues in the core linkage region, are relatively unaffected (Toma et al., 1996). This may be due to a smaller quantitative demand for glycosaminoglycan synthesis or that these galactosyltransferases have a lower Km for UDP-Gal. Similar effects may arise when different steps in the N-linked pathway are affected and show temporal or cell-type selective expression. As illustrated for CDG-Ic, the same mutation has variable effects depending on the cell type and environmental demands.

CDG-X—defects unknown

About 20% of CDG patients with abnormal Tf IEF patterns remain untyped. Their clinical presentations are quite broad and probably represent a spectrum of other defects in the synthesis and transfer of LLO to protein, and in the subsequent processing. As awareness of the disorders increases among physicians, more patients who may show only one or a few of the classical clinical features are tested by Tf IEF. In some cases, positive outcomes lead to search for a novel defective gene.

Emerging problems await emerging solutions

Several laboratories actively pursue the approaches outlined in Figure 2. These include the analysis of glycan structures on transferrin, other serum glycoproteins, and metabolic labeling. In some cases, the metabolic analysis of fibroblasts is uninformative; it gives no clues to the defect. Without an altered metabolite (e.g., LLO), it is difficult to pick a fruitful direction for further investigation. Defects could occur in scores of different enzymes, “structural” proteins, or novel genes with unknown functions. Development of metabolic labeling conditions that maximize labeling differences between known CDG cells and comparable controls might be applicable to CDG cells where the defects are not known. Subtle variability in labeling conditions may create confusion because it is now appreciated that fibroblasts in particular are extremely sensitive to their growth conditions in inducing the unfolded protein response that controls expression of chaperone levels (Doerrler and Lehrman, 1999; Benedetti et al., 2000). This in turn can result in altered glycosylation of specific glycoproteins. Because most glycosyltransferases are themselves glycoproteins and some require proper N-glycosylation for full activity, defects in early glycosylation steps (Group I disorders) may also influence subsequent oligosaccharide processing (Seatter et al., 1998) steps and could be mistaken for Group II disorders.

Another problem is that some patients with CDG signs and symptoms have normal Tf (Fletcher et al., 2000; Dupre et al., 2001). A negative result at this point usually eliminates CDG from further consideration. In some instances, however, persistent investigators were convinced that CDG was the culprit and carried out PMM assays, thereby finding reduced activity and identifying corresponding mutations in PMM2. On the other side, confirmed PMM-deficient patients with abnormal Tf patterns at a young age can attain a nearly normal IEF pattern later in life. It is not known how often this occurs, but it suggests that some older patients may be more difficult to diagnose. How can these differences be explained?

Important considerations must involve “genetic background” and environmental stressors (Dipple and McCabe, 2000; Rao, 2001). The effects of environmental stress, whether viral infections leading to PLE or insufficient mannose in the diet, are obvious examples from the cases discussed above. Pinpointing specific stress is not easy or straightforward, and of course single patients provide inadequate statistics. The question of genetic background is conceptually easy to grasp and well illustrated in the mouse model of CDG-IIa, but is not easy to demonstrate on an individual basis. However, candidate “background” alleles, probably some variants (single nucleotide polymorphisms), will hopefully be identified and shown to have a functional impact on the development of the disease. If so, populations can be studied to determine if there is an increased frequency of a particular allele associated with that group of patients. Thus, we may link mildly defective glycosylation as a risk factor in diseases that are not simply due to a monogenic CDG (Freeze, 2001; Freeze and Westphal, 2001).

It is unlikely that the same reaction in the pathway leading to oligosaccharide transfer to protein is the rate-limiting step in all cell types under all environmental conditions. Rather, glycosylation capacity is likely to be limited by the expression levels of different genes at different times, under different conditions, and in distinct cell-types. The control, coordination, and interaction of over 30 gene products are all dedicated to a single process confined in time and space—transfer of oligosaccharides from the lipid carrier to the nascent protein. It is indeed surprising that various mutations in different genes contributing to this process should generate such clinical variability in CDG-I patients. This fact suggests that some of these glycosylation genes have other unknown functions or that the contribution of individual genes to the rate-limiting final step can be additive and/or synergistic (Vockley et al., 2000).

We cannot yet give biochemical and cellular explanations for how the glycosylation defects produce the myriad of symptoms or their diversity. Continued searching is clearly needed for specific proteins whose misglycosylation leads to specific pathologies, and more mouse models are also necessary.

Beyond monogenic N-linked disorders

It is surprising that the clinical spectrum of the CDG patients is so diverse because all of the Group I defects ultimately affect the transfer of LLO to protein and there are only minor effects on subsequent oligosaccharide processing (Ferrari et al., 2001). This outcome suggests that multiple genes contribute to these “monogenic” defects. Mild mutations in other genes in the N-linked pathway could have a detrimental effect in some situations (Vockley et al., 2000) (see Glycoforum, in this issue). Viewed from another perspective, glycosylation-compromising mutations (in the genetic background) could play important roles in the occurrence of disease. A good example is galactosemia, which primarily results from a loss of GALT, whose product catalyzes Gal-1-P + UDP-Glc→UDP-Gal + Glc-1-P (Segal, 1995b; Elsas and Lai, 1998; Novelli and Reichardt, 2000). Ingestion of excessive galactose creates severe pathology and death. Even when galactose intake is strictly controlled, a unique pathology develops. However, mice with a GALT null allele and fed high galactose-containing diets are essentially normal, and exhibit none of the signs of galactosemia (Ning et al., 2000). Other genes besides GALT are obviously needed to produce the pathology. Some suggest pathology results from the toxicity of metabolized Gal-1-P, but others point out that the alteration in Tf galactosylation and sialylation may indicate that altered glycosylation could be one of the complications in this disorder (Ornstein et al., 1992; Segal, 1995a; Charlwood et al., 1998a). Galactosemia can also be caused by defects in two other genes in the pathway, UDP-Gal epimerase and galactokinase (Koscielak, 1995; Elsas and Lai, 1998; Novelli and Reichardt, 2000).

Much of the pathology associated with cystic fibrosis results from altered glycosylation (Scanlin and Glick, 1999, 2000), although the critical defect occurs in chloride ion transporter, CFTR (Kopito and Ron, 2000; Nadeau, 2001). The most frequent point mutation in this disorder, Δ508, causes protein misfolding and ER retention. The clinical severity of patients with this mutation is highly variable. One explanation for the variability of this disease could be the sum of N-glycosylation reactions. It is conceivable that the severity of both mono- and multigenetic disorders are strongly influenced by their glycosylation background.

The current narrow definition of CDGs—mostly centering on the N-linked pathway—includes <10% of the genes involved in glycosylation. Limited space does not allow a discussion of other disorders that affect the N-linked pathway, but these are listed in Table III along with some disorders in proteoglycan synthesis.

Enormous strides made in understanding the importance of proteoglycans to embryonic development and intracellular signaling, promise to offer molecular explanations for other complex metabolic and developmental disorders (Bulik et al., 2000; Selleck, 2000, 2001). These include the discovery that EXT genes responsible for multiple hereditary exostoses, in fact, encode heparan sulfate co-polymerases (McCormick et al., 2000; Wuyts and Van Hul, 2000). Ehlers-Danlos syndrome progeroid form results from a deficiency in the first galactosyl transferase needed to assemble the proteoglycan linkage oligosaccharide (Quentin et al., 1990; Almeida et al., 1999; Okajima et al., 1999). A keratan sulfate GlcNAc-6-sulfotransferase was recently identified as the cause of macular corneal dystrophy (Akama et al., 2000). These examples offer only a few hints of the future directions. Recent commentaries on many of these issues in “high-impact” journals emphasize the importance of glycobiology (Blair, 2000; Alper, 2001a,b; Axford, 2001; Leonard et al., 2001; Lowe, 2001).

The muscular dystrophies may be a fertile area for glycobiology as well. The mouse myodystrophy (myd) mutation produces an autosomal recessive neuromuscular condition that includes progressive myopathy and acute focal necrosis. The myd gene encodes a glycosyltransferase that is thought to modify α-dystroglycan (Grewal et al., 2001), which conains O-mannose-linked oligosaccharides (Endo, 1999). In addition, hereditary inclusion body myopathy, an adult-onset autosomal recessive disorder that occurs predominantly in Jews of Persian descent, is caused by point mutations in UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase. Different point mutations in the same gene cause sialuria because of the loss of allosteric regulation of endogenous sialic acid synthesis. Glycosylation differences have not been documented in hereditary inclusion body myopathy (Eisenberg et al., 2001).

Glycobiology and medicine: an evolving partnership

Completion of the human genome project will offer “new medicine,” new directions, and further challenges. The coemergence of glycosylation disorders can be considered part of this new direction. Obviously, the first key steps rely on physician awareness, orientation, and training. The medical community has responded well to these newly defined disorders, but glycobiology is seldom included for physicians in training. Because the spectrum of glycosylation phenotypes can be very broad, CDG pateients are often seen by different pediatric subspecialists: gastroenterologists, neurologists, cardiologists, endocrinologists, or hepatologists. Without a doubt, interdisciplinary teams will be called into service to solve these cases, and unless there is evidence of genetic involvement, that is, the second child with similar problems, most of the initial tests will be employed to eliminate (or confirm) lesions in metabolic pathways other than glycosylation. This is appropriate, but altered glycosylation needs to be considered more often as causing these disorders. Ultimately this means that the physician needs to be aware of glycosylation disorders, the appropriate tests, and their limitations. Glycobiology has an opportunity to become a major contributor to the development of new attitudes in medical training, practice, and the basic understanding of human disease.

CDG diagnostic and contact information

Parents and family of CDG patients have active support groups in the United States (CDG Family Network Foundation, www.cdgs.com/cdgsfn.htm) and Europe (www.cdg-syndrom.de/ or www.emu.lu.se/cdg/indexeng.shtml). Diagnosis of CDG by mass spectrometry is now being done at the Mayo Clinic (Rochester, MN). IEF analysis is also available there, at UCSD Biochemical Genetics Laboratory (biochemgen.ucsd.edu/), Greenwood Genetic Center, Metabolic Laboratory (www.ggc.org/), and University Hospital Leuven-Gasthuisberg (www.kuleuven.ac.be/med/cdg/).

Acknowledgments

The author wants to thank Harry Schachter for insightful and critical commentary on this review, and Jamey Marth for permission to cite results on MGATII-deficient mice. Members of the author’s laboratory, especially Geetha Srikrishna and Vibeke Westphal, contributed many valuable discussions, and Vibeke Westphal kindly provided Figure 1. Supported by R01 GM55695 and R01 DK55615, March of Dimes grant 1-FY00–671 and the Cure Autism Now Foundation.

Abbreviations

CDG, congenital disorders of glycosylation; CHO, Chinese hamster ovary; CPY, carboxypeptidase Y; CSF, cerebrospinal fluid; ER, endoplasmic reticulum; ESI-MS, electrospray ionization mass spectrometry; HSPG, heparan sulfate proteoglycan; IEF, isoelectric focusing; LAD-II, leukocyte adhesion deficiency II; LLO, lipid-linked oligosaccharide; PLE, protein-losing enteropathy; PMI, phosphomannose isomerase; PMM, phosphomannomutase; Tf, transferrin.

Fig. 1. Examples of isoelectric focusing patterns of serum Tf in control and CDG patients. Controls show predominently tetrasialo bands represented schematically in the diagram at the lower left showing Tf with two disialylated biantennary chains. CDG patients with Group I disorders make some Tf molecules that lack an entire N-linked chain and therefore also two sialic acids, leading to the appearance of the disialo form, as shown in the center schematic example. Absence of both chains may also occur leading to an asialo form. The Tf patterns of patients with various defects can give variable amounts of sialyated forms. For example disialo-Tf can also arise from other mutations that alter the processing of N-linked chains, as shown in the lower right schematic.

Fig. 1. Examples of isoelectric focusing patterns of serum Tf in control and CDG patients. Controls show predominently tetrasialo bands represented schematically in the diagram at the lower left showing Tf with two disialylated biantennary chains. CDG patients with Group I disorders make some Tf molecules that lack an entire N-linked chain and therefore also two sialic acids, leading to the appearance of the disialo form, as shown in the center schematic example. Absence of both chains may also occur leading to an asialo form. The Tf patterns of patients with various defects can give variable amounts of sialyated forms. For example disialo-Tf can also arise from other mutations that alter the processing of N-linked chains, as shown in the lower right schematic.

Fig. 2. The physician-glycobiologist interface. As described in the text, the physician trained to recognize glycosylation disorders notes critical signs in patients and orders a Tf IEF test. If the results indicate a likely glycosylation deficiency, the physician hands this over to the glycobiologist for analysis of the plasma protein glycan structures, enzymatic analysis of leukocytes or fibroblasts, and metabolic labeling. Altered LLO structures or protein N-glycans provide clues to likely defects, which then lead to further enzymatic analysis and sequencing of candidate genes. Mutations are tested for functional consequences in either yeast or mammalian cells deficient in the candidate defective gene.

Fig. 2. The physician-glycobiologist interface. As described in the text, the physician trained to recognize glycosylation disorders notes critical signs in patients and orders a Tf IEF test. If the results indicate a likely glycosylation deficiency, the physician hands this over to the glycobiologist for analysis of the plasma protein glycan structures, enzymatic analysis of leukocytes or fibroblasts, and metabolic labeling. Altered LLO structures or protein N-glycans provide clues to likely defects, which then lead to further enzymatic analysis and sequencing of candidate genes. Mutations are tested for functional consequences in either yeast or mammalian cells deficient in the candidate defective gene.

Fig. 3. N-linked oligosaccharide biosynthetic pathway and location of the known defects. The defects are marked in bold red arrows along with their abbreviated names. Refer to Tables I and II for further information. Reactions above the horizontal pink line occur in the cytoplasm. Those disorders above the green horizontal belong to Group I (types Ia–If) and those below to Group II (types IIa–IIc). Symbols for monosaccharides are: (upright triangles) glucose, (right triangles) fructose, (closed circles, red and blue) mannose, (squares) N-acetylglucosamine, (open circles, yellow) galactose, (diamonds) sialic acid, and (upside-down triangles) fucose.

Fig. 3. N-linked oligosaccharide biosynthetic pathway and location of the known defects. The defects are marked in bold red arrows along with their abbreviated names. Refer to Tables I and II for further information. Reactions above the horizontal pink line occur in the cytoplasm. Those disorders above the green horizontal belong to Group I (types Ia–If) and those below to Group II (types IIa–IIc). Symbols for monosaccharides are: (upright triangles) glucose, (right triangles) fructose, (closed circles, red and blue) mannose, (squares) N-acetylglucosamine, (open circles, yellow) galactose, (diamonds) sialic acid, and (upside-down triangles) fucose.

Fig. 4. Effects of various ALG6 mutations on CPY glycosylation in S. cerevisiae. Upper panel shows the effects on fast growing cells generation time = 2.6 ± 0.3 h: An Δalg6 strain of S. cerevisiae was harvested at exponential growth phase. The strain is unable to fully glycosylate CPY and makes molecules lacking one or two N-linked chains (–ALG6). Transformation with control ALG6 restores glycosylation to normal (+ALG6), whereas transformation with the allele encoding the A333V protein has a limited ability to rescue glycosylation. The other allele encoding the Y131H, S308R protein cannot rescue glycosylation of CPY. When each of the latter mutations is expressed separately, the S308R mutation seems more severe than Y131H. The lower panel shows the same rescue experiment when the cells are growing with a generation time 18 ± 2 h. Notice that the impact of the same mutations is considerably less severe at this slower growth rate. (Taken from Westphal et al., 2000a)

Fig. 4. Effects of various ALG6 mutations on CPY glycosylation in S. cerevisiae. Upper panel shows the effects on fast growing cells generation time = 2.6 ± 0.3 h: An Δalg6 strain of S. cerevisiae was harvested at exponential growth phase. The strain is unable to fully glycosylate CPY and makes molecules lacking one or two N-linked chains (–ALG6). Transformation with control ALG6 restores glycosylation to normal (+ALG6), whereas transformation with the allele encoding the A333V protein has a limited ability to rescue glycosylation. The other allele encoding the Y131H, S308R protein cannot rescue glycosylation of CPY. When each of the latter mutations is expressed separately, the S308R mutation seems more severe than Y131H. The lower panel shows the same rescue experiment when the cells are growing with a generation time 18 ± 2 h. Notice that the impact of the same mutations is considerably less severe at this slower growth rate. (Taken from Westphal et al., 2000a)

Table I.

Congenital disorders of glycosylation: biochemical and molecular defects, chromosomal location, number of patients, and year identified

 CDG type Enzymatic or protein defect Gene OMIM* Location Patients Year 
Group I: Defects in the assembly and transfer of lipid-linked oligosaccharide (LLO) to proteins in the ER 
 Ia Phosphomannomutase 2 (PMM) PMM2 212065 16p13 ∼300 1995 
    601785    
 Ib Phosphomannose isomerase (PMI) MPI 602579 15q22-qter ∼20 1998 
    154550    
 Ic Dolichyl-P-Glc:Man9GlcNAc2-PP-dolichyl α-1,3-glucosyltransferase ALG6 603147 1p22.3 ∼30 1999 
    604566    
 Id Dolichyl-P-Man:Man5GlcNAc2-PP-dolichyl α-1,3-mannosyltransferase  ALG3 601110 1999 
   NOT56L     
 Ie Dolichol-P-Man synthase 1 DPM1 603503 20q13 2000 
 If Dolichol-P-Man utilization defect 1 (suppressor of Lec35) MPDU1 604041 17p12-13 2001 
 Ix Multiple Defects: causes unknown — 603585 —  
    212067    
Group II: Defects in the processing of N-glycans 
 IIa UDP-GlcNAc: α-6-d-mannoside β-1,2-N-acetylglucosaminyltransferase II (GnT II) MGAT2 212066 14q21 1994 
    602616    
 IIb α-1,2-glucosidase I GCS1 601336 2p12-13 2000 
 IIc (LAD-II) GDP-fucose transporter (cytosol→Golgi) —— None None 2001 
 CDG type Enzymatic or protein defect Gene OMIM* Location Patients Year 
Group I: Defects in the assembly and transfer of lipid-linked oligosaccharide (LLO) to proteins in the ER 
 Ia Phosphomannomutase 2 (PMM) PMM2 212065 16p13 ∼300 1995 
    601785    
 Ib Phosphomannose isomerase (PMI) MPI 602579 15q22-qter ∼20 1998 
    154550    
 Ic Dolichyl-P-Glc:Man9GlcNAc2-PP-dolichyl α-1,3-glucosyltransferase ALG6 603147 1p22.3 ∼30 1999 
    604566    
 Id Dolichyl-P-Man:Man5GlcNAc2-PP-dolichyl α-1,3-mannosyltransferase  ALG3 601110 1999 
   NOT56L     
 Ie Dolichol-P-Man synthase 1 DPM1 603503 20q13 2000 
 If Dolichol-P-Man utilization defect 1 (suppressor of Lec35) MPDU1 604041 17p12-13 2001 
 Ix Multiple Defects: causes unknown — 603585 —  
    212067    
Group II: Defects in the processing of N-glycans 
 IIa UDP-GlcNAc: α-6-d-mannoside β-1,2-N-acetylglucosaminyltransferase II (GnT II) MGAT2 212066 14q21 1994 
    602616    
 IIb α-1,2-glucosidase I GCS1 601336 2p12-13 2000 
 IIc (LAD-II) GDP-fucose transporter (cytosol→Golgi) —— None None 2001 

*OMIM = online Mendelian inheritance in man (www.ncbi.nlm.nih.gov/).

Table II.

Clinical features of the congenital disorders of glycosylation

Type Features 
Ia Quite variable psychomotor retardation, hypotonia, peripheral neuropathy, stroke-like episodes, seizures, cerebellar hypoplasia, internal strabismus, abnormal eye movements, subcutaneous fat distribution, inverted nipples, cardiomyopathy, proteinuria 
Ib Normal development, hypoglycemia, coagulopathy, hepatomegaly, protein-losing enteropathy, hepatic fibrosis, cyclic vomiting, diarrhea 
Ic Hypotonia, psychomotor retardation, internal strabismus, feeding problems, coagulopathy, seizures, normal cerebellar development 
Id Hypotonia, intractable seizures, severe psychomotor retardation, microcephaly, reduced responsiveness, optic atrophy, adducted thumbs, high-arched palate 
Ie Hypotonia, intractable seizures, delayed myelination, cortical blindness, severe psychomotor retardation, high-arched palate 
If Hypotonia, frequent seizures, blindness, dry skin, severe psychomotor retardation, severe failure to thrive, decreased food intake, frequent vomiting 
IIa Hypotonia, severe psychomotor retardation, frequent infections, normal cerebellum, coarse facies, widely spaced nipples, low set ears, ventricular septal defect 
IIb Hypotonia, generalized edema, hypoventilation, apnea, hepatomegaly, demyelinating polyneuropathy 
IIc (LAD-II) Elevated peripheral leukocytes, absence of CD15, Bombay blood group phenotype, failure to thrive, hypotonia, psychomotor retardation, short arms and legs, simian crease 
Type Features 
Ia Quite variable psychomotor retardation, hypotonia, peripheral neuropathy, stroke-like episodes, seizures, cerebellar hypoplasia, internal strabismus, abnormal eye movements, subcutaneous fat distribution, inverted nipples, cardiomyopathy, proteinuria 
Ib Normal development, hypoglycemia, coagulopathy, hepatomegaly, protein-losing enteropathy, hepatic fibrosis, cyclic vomiting, diarrhea 
Ic Hypotonia, psychomotor retardation, internal strabismus, feeding problems, coagulopathy, seizures, normal cerebellar development 
Id Hypotonia, intractable seizures, severe psychomotor retardation, microcephaly, reduced responsiveness, optic atrophy, adducted thumbs, high-arched palate 
Ie Hypotonia, intractable seizures, delayed myelination, cortical blindness, severe psychomotor retardation, high-arched palate 
If Hypotonia, frequent seizures, blindness, dry skin, severe psychomotor retardation, severe failure to thrive, decreased food intake, frequent vomiting 
IIa Hypotonia, severe psychomotor retardation, frequent infections, normal cerebellum, coarse facies, widely spaced nipples, low set ears, ventricular septal defect 
IIb Hypotonia, generalized edema, hypoventilation, apnea, hepatomegaly, demyelinating polyneuropathy 
IIc (LAD-II) Elevated peripheral leukocytes, absence of CD15, Bombay blood group phenotype, failure to thrive, hypotonia, psychomotor retardation, short arms and legs, simian crease 
Table III.

Other diseases involving abnormal glycosylation

Name of disorder Features Enzymatic defect Gene OMIM* 
HEMPAS (congenital dyserythropoietic anemia Type II, CDA II) Mild disease with anemia and hemosiderosis α-mannosidase II deficiency; other genes MAN2A1 224100154582 
I-Cell disease Severe developmental abnormalities N-acetylglucosamine-1- phosphotransferase for synthesis of Man-6-phosphate targeting signal GNPTA 252500 
Multiple hereditary exostoses Bony outgrowths Heparan sulfate co-polymerase EXT1, EXT2 133701 
Macular corneal dystrophy  Progressive corneal opacity GlcNAc-6-sulfotransferase CHST6 605294 
Ehlers-Danlos syndrome progeroid form Hypotonia, delayed development, connective tissue abnormalities, loose skin Xylosylprotein β-1,4-galactosyltransferase used for dermatan sulfate proteoglycan synthesis XGPT1B4GALT7 130070604327 
Hereditary inclusion body myopathy Adult onset, localized myopathy UDP-GlcNAc-2 epimerase/ManNAc kinase GNE 600737 
Name of disorder Features Enzymatic defect Gene OMIM* 
HEMPAS (congenital dyserythropoietic anemia Type II, CDA II) Mild disease with anemia and hemosiderosis α-mannosidase II deficiency; other genes MAN2A1 224100154582 
I-Cell disease Severe developmental abnormalities N-acetylglucosamine-1- phosphotransferase for synthesis of Man-6-phosphate targeting signal GNPTA 252500 
Multiple hereditary exostoses Bony outgrowths Heparan sulfate co-polymerase EXT1, EXT2 133701 
Macular corneal dystrophy  Progressive corneal opacity GlcNAc-6-sulfotransferase CHST6 605294 
Ehlers-Danlos syndrome progeroid form Hypotonia, delayed development, connective tissue abnormalities, loose skin Xylosylprotein β-1,4-galactosyltransferase used for dermatan sulfate proteoglycan synthesis XGPT1B4GALT7 130070604327 
Hereditary inclusion body myopathy Adult onset, localized myopathy UDP-GlcNAc-2 epimerase/ManNAc kinase GNE 600737 

*OMIM: online Mendelian inheritance in man (www.ncbi.nlm.nih.gov/).

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