Phosphomannose isomerase (PMI) deficiency is the cause of a new type of carbohydrate-deficient glycoprotein syndrome (CDGS). The disorder is caused by mutations in the PMI1 gene. The clinical phenotype is characterized by protein-losing enteropathy, while neurological manifestations prevailing in other types of CDGS are absent. Using standard diagnostic procedures, the disorder is indistinguishable from CDGS type Ia (phosphomannomutase deficiency). Daily oral mannose administration is a successful therapy for this new type of CDG syndrome classified as CDGS type Ib.

Deficiency of GDP-Man:Man 1 GlcNAc 2 -PP-dolichol mannosyltransferase (hALG2), is the cause of a new type of congenital disorders of glycosylation (CDG) designated CDG-Ii. The patient presented normal at birth but developed in the 1st year of life a multisystemic disorder with mental retardation, seizures, coloboma of the iris, hypomyelination, hepatomegaly, and coagulation abnormalities. An accumulation of Man 1 GlcNAc 2 -PP-dolichol and Man 2 GlcNAc 2 -PP-dolichol was observed in skin fibroblasts of the patient. Incubation of patient fibroblast extracts with Man 1 GlcNAc 2 -PP-dolichol and GDP-mannose revealed a severely reduced activity of the mannosyltransferase elongating Man 1 GlcNAc 2 -PP dolichol. Because the Saccharomyces cerevisiae mutant alg2-1 was known to accumulate the same shortened dolichol-linked oligosaccharides as the patient, the yeast ALG2 sequence was used to identify the human ortholog. Genetic analysis revealed that the patient was heterozygous for a single nucleotide deletion and a single nucleotide substitution in the human ortholog of yeast ALG2. Expression of wild type but not of mutant hALG2 cDNA restored the mannosyltransferase activity and the biosynthesis of dolichol-linked oligosaccharides both in patient fibroblasts and in the alg2-1 yeast cells. hALG2 was shown to act as an ␣1,3-mannosyltransferase. The resulting Man␣1,3-ManGlcNAc 2 -PP dolichol is further elongated by a yet unknown ␣1,6-mannosyltransferase.Congenital disorders of glycosylation (CDG) 1 compose a rapidly growing group of inherited multisystemic disorders in man, which are commonly associated with severe psychomotor and mental retardation (1). The characteristic biochemical feature of CDG is defective glycosylation of proteins due to mutations in genes required for the biosynthesis of N-linked oligosaccharides.The attachment of oligosaccharide chains onto newly synthesized proteins is one of the most widespread forms of co-and post-translational modifications and is found in animals, plants, and bacteria. Glycoproteins are located inside cells predominantly in subcellular organelles and in cellular membranes and most abundantly in extracellular fluids and matrices. The oligosaccharide moiety of the glycoproteins can affect their folding, their transport, as well as their biological activity and stability (2, 3). The complex process of protein glycosylation requires more than a hundred glycosyltransferases, glycosidases, and transport proteins. CDG are classified into two groups. Defects of the assembly of lipid-linked oligosaccharides or their transfer onto nascent glycoproteins compose CDG type I, whereas CDG type II includes all defects of trimming and elongation of N-linked oligosaccharides (4). In the past 7 years the molecular nature of eight CDG-I and four CDG-II types could be identified (5-24).Here we describe for the first time a molecular defect in glycoprotein biosynthesis in man which affects at the cytosolic side of the endoplasmic reticulum the transfer of mannosyl residues from GDP-Man to Man 1 Glc...

IntroductionThe congenital disorders of glycosylation (CDGs) comprise a rapidly growing group of inherited multisystemic disorders that are commonly associated with severe psychomotor and mental retardation. The characteristic biochemical feature of CDGs is the defective glycosylation of glycoproteins due to mutations in genes required for the biosynthesis of N-linked oligosaccharides. Defects of the assembly of lipidlinked oligosaccharides or their transfer onto nascent glycoproteins form type I of CDG, whereas CDG type II comprises all defects of trimming and elongating of N-linked oligosaccharides (1). N-glycosylation defects are routinely detected by isoelectric focusing (IEF) of serum transferrin, which normally carries two sialylated biantennary N-linked oligosaccharides. The hyposialylated transferrin from CDG patients shows a cathodic shift, which in CDG-I is due to the loss of either one or both oligosaccharides and in CDG-II is due to the incomplete processing of protein-bound oligosaccharides. In the past six years the molecular nature of six CDG-I and three CDG-II types has been described (2-16).Here we present a new type of CDG-II, designated CDG-IId, in which UDP-Gal:N-acetylglucosamine β-1,4-galactosyltransferase I (β4GalT I) is deficient. β4GalT I belongs to a family of at least six related β-1,4-galactosyltransferases. Although the genes for all β-1,4-galactosyltransferases are located at different chromosomal loci, the evolutionary relationship is indicated by a 30-55% amino acid identity. Moreover, they have in common the localization in the Golgi apparatus, the topology of type II membrane proteins, and the transfer of galactose from UDP-galactose onto the C 4 -hydroxyl group of GlcNAc residues. The enzymes differ in their substrate specificity for glycoproteins and glycolipids and their ability to synthesize lactose in mammals (17). In Chinese hamster ovary cells, which express all six galactosyltransferases, β4GalT I is the main transferase responsible for galactosylation of protein N-linked oligosaccharides (18).The β4GalT deficiency was identified in a 16-monthold boy with mental retardation, a hydrocephalus due to a Dandy-Walker malformation, blood-clotting problems, and myopathy (19). The deficiency of β4GalT I activity is caused by the insertion of a single nucleotide Deficiency of the Golgi enzyme UDP-Gal:N-acetylglucosamine β-1,4-galactosyltransferase I (β4GalT I) (E.C.2.4.1.38) causes a new congenital disorder of glycosylation (CDG), designated type IId (CDG-IId), a severe neurologic disease characterized by a hydrocephalus, myopathy, and bloodclotting defects. Analysis of oligosaccharides from serum transferrin by HPLC, mass spectrometry, and lectin binding revealed the loss of sialic acid and galactose residues. In skin fibroblasts and leukocytes, galactosyltransferase activity was reduced to 5% that of controls. In fibroblasts, a truncated polypeptide was detected that was about 12 kDa smaller in size than wild-type β4GalT I and that failed to localize to the Golgi apparatus. Seq...

IntroductionThe congenital disorders of glycosylation (CDGs) comprise a rapidly growing group of inherited multisystemic disorders that are commonly associated with severe psychomotor and mental retardation. The characteristic biochemical feature of CDGs is the defective glycosylation of glycoproteins due to mutations in genes required for the biosynthesis of N-linked oligosaccharides. Defects of the assembly of lipidlinked oligosaccharides or their transfer onto nascent glycoproteins form type I of CDG, whereas CDG type II comprises all defects of trimming and elongating of N-linked oligosaccharides (1). N-glycosylation defects are routinely detected by isoelectric focusing (IEF) of serum transferrin, which normally carries two sialylated biantennary N-linked oligosaccharides. The hyposialylated transferrin from CDG patients shows a cathodic shift, which in CDG-I is due to the loss of either one or both oligosaccharides and in CDG-II is due to the incomplete processing of protein-bound oligosaccharides. In the past six years the molecular nature of six CDG-I and three CDG-II types has been described (2-16).Here we present a new type of CDG-II, designated CDG-IId, in which UDP-Gal:N-acetylglucosamine β-1,4-galactosyltransferase I (β4GalT I) is deficient. β4GalT I belongs to a family of at least six related β-1,4-galactosyltransferases. Although the genes for all β-1,4-galactosyltransferases are located at different chromosomal loci, the evolutionary relationship is indicated by a 30-55% amino acid identity. Moreover, they have in common the localization in the Golgi apparatus, the topology of type II membrane proteins, and the transfer of galactose from UDP-galactose onto the C 4 -hydroxyl group of GlcNAc residues. The enzymes differ in their substrate specificity for glycoproteins and glycolipids and their ability to synthesize lactose in mammals (17). In Chinese hamster ovary cells, which express all six galactosyltransferases, β4GalT I is the main transferase responsible for galactosylation of protein N-linked oligosaccharides (18).The β4GalT deficiency was identified in a 16-monthold boy with mental retardation, a hydrocephalus due to a Dandy-Walker malformation, blood-clotting problems, and myopathy (19). The deficiency of β4GalT I activity is caused by the insertion of a single nucleotide Deficiency of the Golgi enzyme UDP-Gal:N-acetylglucosamine β-1,4-galactosyltransferase I (β4GalT I) (E.C.2.4.1.38) causes a new congenital disorder of glycosylation (CDG), designated type IId (CDG-IId), a severe neurologic disease characterized by a hydrocephalus, myopathy, and bloodclotting defects. Analysis of oligosaccharides from serum transferrin by HPLC, mass spectrometry, and lectin binding revealed the loss of sialic acid and galactose residues. In skin fibroblasts and leukocytes, galactosyltransferase activity was reduced to 5% that of controls. In fibroblasts, a truncated polypeptide was detected that was about 12 kDa smaller in size than wild-type β4GalT I and that failed to localize to the Golgi apparatus. Seq...