Congenital disorders of glycosylation, analytical aspects

Abstract

Molecular diagnosis of congenital disorders of glycosylation (CDG) has long relied on electrophoresis (isoelectric focusing) performed to detect the absence of N-acetylneuraminic acid, a marker saccharide of mature oligosaccharides, in serotransferrin. Mass spectrometry (MS) is now replacing electrophoresis and playing an essential role in molecular diagnosis. Electrospray ionization (ESI) MS allows detection of the entire missing N-linked oligosaccharide chain from serotransferrin in CDG-I and detailed characterization of oligosaccharide structures when operated with sufficient resolving power. ESI MS generates multiply charged ions, and the mass spectrum may be transformed/deconvoluted into the spectrum of singly charged species. On the other hand, matrix-assisted laser desorption/ionization (MALDI) MS of glycopeptides, which are enriched from tryptic digests by hydrophilic interaction, allows detailed profiling of oligosaccharides attached to each glycosylation site. The current CDG concept covers disorders affecting other types of glycosylation, among which mucintype O-glycosylation is assessed by MALDI MS of serum apolipoprotein C-III without any purification steps.