Hubert Carchon

Carbohydrate deficient glycoprotein syndrome type II: a deficiency in Golgi localised N-acetyl-glucosaminyltransferase II.

The carbohydrate deficient glycoprotein (CDG) syndromes are a family of genetic multisystemic disorders with severe nervous system involvement. This report is on a child with a CDG syndrome that differs from the classical picture but is very similar to a patient reported in 1991. Both these patients are therefore designated CDG syndrome type II. Compared with type I patients they have a more severe psychomotor retardation but no peripheral neuropathy nor cerebellar hypoplasia. The serum transferrin isoform pattern obtained by isoelectric focusing showed disialotransferrin as the major fraction. The serum disialotransferrin, studied in the present patient, contained two moles of truncated monoantennary Sialyl-Gal-GlcNAc-Man(alpha 1-->3)[Man(alpha 1-->6)]Man(beta 1-->4)GlcNAc (beta 1-->4)GlcNAc-Asn per mole of transferrin. A profoundly deficient activity of the Golgi enzyme N-acetylglucosaminyltransferase II (EC 2.4.1.143) was demonstrated in fibroblasts.

3-Phosphoglycerate dehydrogenase deficiency: an inborn error of serine biosynthesis.

Serine concentrations were markedly decreased in the cerebrospinal fluid of two brothers with congenital microcephaly, profound psychomotor retardation, hypertonia, epilepsy, growth retardation, and hypogonadism. The youngest boy also had congenital bilateral cataract. Magnetic resonance imaging of the brain showed evidence of dysmyelination. Plasma serine as well as plasma and cerebrospinal fluid glycine concentrations were also decreased but to a lesser extent. Treatment with oral serine in the youngest patient significantly increased cerebrospinal fluid serine and abolished the convulsions. In fibroblasts of both patients, a decreased activity was demonstrated of 3-phosphoglycerate dehydrogenase, the first step of serine biosynthesis (22% and 13% of the mean control value). This is an unusual disorder as the great majority of aminoacidopathies are catabolic defects. It is a severe but potentially treatable inborn error of metabolism that has not been previously reported in man.

The carbohydrate-deficient glycoprotein syndromes: an overview.

SummaryThe carbohydrate-deficient glycoprotein (CDG) syndromes are a newly recognized family of diseases with autosomal recessive inheritance. The basic defects are probably in the glycosylation pathway (endoplasmic reticulum, Golgi apparatus or post-Golgi). In the present state of our knowledge the central nervous system is always severely affected but nearly all other organs are involved to a variable degree. Like the peroxisomal disorders they also comprise dysmorphic features, the most typical being an abnormal distribution of subcutaneous adipose tissue. A reliable diagnostic test is isoelectric focusing of serum transferrin showing a cathodal shift as a consequence of the partial sialic acid deficiency. Prenatal diagnosis and heterozygote detection are not yet available. These diseases should be differentiated from secondary CDG syndromes such as classical galactosaemia.

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

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

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

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

Carbohydrate-deficient glycoprotein syndrome type II

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

Congenital disorders of glycosylation: the rapidly growing tip of the iceberg.

In addition to many other organs, the brain is affected in 10 of the 11 known congenital disorders of N-linked glycosylation, mostly to a severe degree. Because a large number of enzymes, transporters and other proteins are involved in glycosylation (both N-linked and O-linked), it is expected that the great majority of congenital disorders of glycosylation (CDG) are yet to be identified. Many neurological patients with a CDG escape diagnosis for that reason, but also because existing screening methods fail to detect all patients with a known CDG. These disorders should be looked for in any patient, regardless of age, with an unexplained neurological disorder.

Lysosomal enzyme activities in serum and leukocytes from patients with carbohydrate-deficient glycoprotein syndrome type ia (phosphomannomutase deficiency)

From 10 patients with carbohydrate-deficient glycoprotein (CDG) syndrome due to phosphomannomutase (PMM) deficiency, out of 10 lysosomal enzymes, 7 enzyme activities were measured in serum and 9 in leukocytes. In serum there was a 2-fold to 4-fold increase in activity of β-glucuronidase, β-hexosaminidase, β-galactosidase, and arylsulphatase A. In leukocytes, however, several enzymes had reduced activity, particularly α-fucosidase, β-glucuronidase and α-mannosidase. These abnormalities could result from missorting, defective reuptake and/or reduced stability of the enzymes due to the defective glycosylation.

3-Phosphoglycerate dehydrogenase deficiency and 3-phosphoserine phosphatase deficiency: inborn errors of serine biosynthesis.

Aminoacidopathies are as a rule catabolic defects (Scriver et al 1995). We have discovered two defects in an amino acid synthesis pathway, namely that of serine (Figure 1). One is 3-phosphoglycerate dehydrogenase (EC 1.1.1.95) deficiency found in two brothers with a severe encephalopathy of prenatal onset and very low serine levels in the cerebrospinal fluid. The other is 3-phosphoserine phosphatase (EC 3.1.3.3) deficiency detected as a (compound ?) heterozygous state in a boy with Williams syndrome, providing evidence that the 3-phosphoserine phosphatase locus is close to the elastin locus on chromosome 7q11.23.

Plasma prealbumin in the newborn.

We read with interest the paper of Jacobsen and associates (1). The authors emphasize that in new-born infants with birth weights appropriate for gestation serum prealburnin concentrations increase progressively with gestational age and that in small-for-gestational age babies the levels are significantly lower. Recently, we reported on the same subject and could also find that in pre-term infants plasma prealburnin at birth increases with foetal age (3). However, only one out of nine small-fordates new-born infants had a plasma prealbumin level below the normal range. Also our recent data, shown in Table 1 and presented according to the findings of Jacobsen et al.