Mass spectrometry of transferrin glycoforms to detect congenital disorders of glycosylation: Site-specific profiles and pitfalls.

Mass spectrometry of transferrin is an established method for the detection and diagnosis of congenital disorder of glycosylation (CDG). Transferrin is an 80 kDa glycoprotein and the glycoform at two N-glycosylation sites is comprised of a di-sialylated biantennary oligosaccharide as the major form and minor species with fucosylated or triantennary structures. Rapid CDG screening is carried out by MS of native transferrin. On the other hand, MS of glycopeptides enables site-specific determination of glycoforms, and the affinity-based enrichment of glycopeptides from a complex mixture of proteolytic peptides facilitates efficient analysis. MS of glycopeptides reveals the presence of immature glycoforms even in healthy individuals, indicating that the diagnosis of CDG based on molecular phenotypes requires quantitative evaluation. In this technical note, the aberrant glycosylation profiles of CDG cases are presented to shed light on the MS of native transferrin and glycopeptides from the viewpoint of clinical glycoproteomics.