History
The first CDG patients (twin sisters) were described in 1980 by Jaeken et al. Their main features were psychomotor retardation, cerebral and cerebellar atrophy and fluctuating hormone levels (e.g.prolactin, FSH and GH). During the next 15 years the underlying defect remained unknown but since the plasmaprotein transferrin was underglycosylated (as shown by e.g. isoelectric focusing), the new syndrome was named carbohydrate-deficient glycoprotein syndrome (CDGS) Its "classical" phenotype included psychomotor retardation, ataxia, strabismus, anomalies (fat pads and inverted nipples) and coagulopathy.In 1994, a new phenotype was described and named CDGS-II. In 1995, Van Schaftingen and Jaeken showed that CDGS-I (now PMM2-CDG) was caused by the deficiency of the enzyme phosphomannomutase. This enzyme is responsible for the interconversion of mannose-6-phosphate and mannose-1-phosphate, and its deficiency leads to a shortage in GDP-mannose and dolichol (Dol)-mannose (Man), two donors required for the synthesis of the lipid-linked oligosaccharide precursor of N-linked glycosylation.In 1998, Niehues described a new CDG syndrome, MPI-CDG, which is caused by mutations in the enzyme metabolically upstream of PMM2, phosphomannose isomerase (PMI). A functional therapy for MPI-CDG, alimentary mannose was also described. The characterization of new defects took increased and several new Type I and Type II defects were delineated.In 2012, Need described the first case of a congenital disorder of deglycosylation, NGLY1 deficiency. A 2014 study of NGLY1 deficient patients found similarities with traditional congenital disorders of glycosylation.
A vast amount of carbohydrate binding molecules (lectins) depend on correct glycosylation for appropriate binding; the selectins, involved in leukocyte extravasation, is a prime example. Their binding depends on a correct fucosylation of cell surface glycoproteins. Lack thereof leads to leukocytosis and increase sensitivity to infections as seen in SLC35C1-CDG(CDG-IIc); caused by a GDP-fucose (Fuc) transporter deficiency.All N-linked oligosaccharides originate from a common lipid-linked oligosaccharide (LLO) precursor, synthesized in the ER on a dolichol-phosphate (Dol-P) anchor. The mature LLO is transferred co-translationally to consensus sequence Asn residues in the nascent protein, and is further modified by trimming and re-building in the Golgi. Deficiencies in the genes involved in N-linked glycosylation constitute the molecular background to most of the CDGs. Type I defects involve the synthesis and transfer of the LLO
Type II defects impair the modification process of protein-bound oligosaccharides. Type I
Type II
The mature LLO chain is next transferred to the growing protein chain, a process catalysed by the oligosaccharyl transferase (OST) complex. Once transferred to the protein chain, the oligosaccharide is trimmed by specific glycosidases. This process is vital since the lectin chaperones calnexin and calreticulin, involved in protein quality, bind to the Glc1Man9GlcNAc-structure and assure proper folding. Lack of the first glycosidase (GCS1) causes CDG-IIb. Removal of the Glc residues and the first Man residue occurs in the ER. The glycoprotein then travels to the Golgi, where a multitude of different structures with different biological activities are formed.