Congenital Disorders of Glycosylation (CDG) are a group of clinically heterogeneous disorders characterized by defects in the Synthesis of Glycans and their association with proteins and lipids during glycosylation. CD BioGlyco has developed a specialized glycoproteomics platform and provides fast and efficient N-glycosylation and O-glycosylation analysis services.
More than 7000 proteins have been reported to be N-glycosylated in humans. N-Glycosylation is a highly conserved glycan modification involved in protein folding, trafficking, and signal transduction. N-Glycosylation is initiated by the stepwise addition of Monosaccharides to N-glycan precursors in the endoplasmic reticulum (ER) through sequential Enzymatic reactions to generate lipid-linked oligosaccharide (LLO). The LLO is then transferred to the nascent protein. The nascent protein enters the folding step and the correctly folded protein is transported to the Golgi apparatus for continuous reaction maturation to produce complex N-Glycans. N-Glycosylation is tightly controlled by many factors. Here, we focus on the biosynthesis process of LLO and the diseases caused by its deficiency.
The biosynthesis of N-glycans in eukaryotes starts with dolichol phosphate (Dol-P) in the ER, which is a highly hydrophobic polyisoprene molecule. N-Acetylglucosamine-1-phosphotransferase (GlcNAc-1-P-transferase, DPAGT1) catalyzes the transfer of GlcNAc-1-P from UDP-GlcNAc to Dol-P on the cytoplasmic side of ER to generate Dol-PP-GlcNAc. In the second step, the ALG13/14 hetero-complex mediates the transfer of N-acetylglucosamine (GlcNAc) to Dol-PP-GlcNAc. Next, mannosyltransferases ALG1, ALG2, and ALG11 stepwise catalyze the transfer of 5 mannose (Man) to generate Dol-PP-GlcNAc2Man5. Dol-PP-GlcNAc2Man5 subsequently flips into the lumen of the ER. ALG3, ALG9, and ALG12 use Dol-P-Man as the donor substrate to transfer Man to Dol-PP-GlcNAc2Man5 to generate Dol-PP-GlcNAc2Man9. Finally, ALG6, ALG8, and ALG10 transfer glucose (Glc) from Dol-P-Glc to Dol-PP-GlcNAc2Man9 to generate a complete N-glycan precursor (Dol-PP-Glc3Man9GlcNAc2).
Fig.1 Biosynthetic pathway of LLO. (Hennet, 2012)
Glycosylation disorders are rare genetic disorders commonly referred to as CDG. Most of the CDGs known so far are N-Glycosylation Deficient, divided into CDG type I (CDG-I) and CDG type II (CDG-II). CDG-I related to LLO biosynthesis will be introduced here.
CDG-Ic is one of the most common types of CDG, clinically manifested by bradykinesia, deformity, muscle hypotonia, and seizures. It is caused by the enzyme ALG6 defect involved in the formation of LLO precursors for N-Linked Glycosylation. This enzyme transfers the covalent α1,3-linked Glc residue to the first Man antenna in Dol-PP-GlcNAc2Man9. At least 30 CDG-Ic patients have been identified. They are affected by one of 21 ALG6 mutations, including sixteen point mutations, one insertion, and four deletions. One of the most common mutations is c.998C>T, resulting in p.A333V substitution.
CDG-Id is caused by the accumulation of the receptor substrate and intermediate glycan Man5GlcNAc2-PP-Dol in mutant fibroblasts due to a defect in the α-1,3-mannosyltransferase encoded by the ALG3 gene. ALG3 catalyzes the transfer of the sixth Man residue onto LLO. The study found that CDG-Id patients all showed slowly progressive encephalopathy, accompanied by microcephaly, severe psychomotor retardation, and seizures.
CDG-Ig is caused by mutations in the ALG12 gene: Dol-P-Man: Man7GlcNAc2-PP-Dol mannosyltransferase. The six patients reported so far all exhibited common CDG symptoms.
CDG-Ih is due to the lack of Dol-P-Glc: Glc1Man9GlcNAc2-PP-Dol α1,3 glucosyltransferase. The enzyme is encoded by the ALG8 gene. In particular, CDG-Ih is the only CDG-I type that is consistently characterized clinically by renal disease and failure.
CDG-Ik is caused by the mutation in the ALG1 gene: GDP-Man: GlcNAc2-PP-Dol β1,4 mannosyltransferase.
CD BioGlyco has developed a reliable and sensitive Glycomics Platform to help researchers better understand the biological attribute of the complex glycosylation mechanisms. Our high-quality scientific services will greatly improve your research efficiency and accelerate research progress. If you are interested in our services, please contact us in time.
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