Altered glycan structures: the molecular basis of congenital disorders of glycosylation.

Congenital disorders of glycosylation (CDG) are a group of diseases that affect glycoprotein biogenesis. Eighteen different types of CDG have been defined genetically. They result from deficiencies in either the biosynthesis of oligosaccharide precursors or specific steps of N-glycan assembly, resulting in the absence or structural alteration of N-glycan chains. These diseases have a broad range of clinical phenotypes and affect nearly every organ system, with special emphasis on normal brain development and the multiple functions of the nervous, hepatic, gastrointestinal and immune systems. Although most of the deficiencies observed in CDG patients are only partial, the severity of the clinical manifestations signifies the relevance of protein N-glycosylation and shows the importance of defined glycan structures.