Testing for Congenital Disorders of Glycosylation by HPLC Measurement of Serum Transferrin Glycoforms

Congenital disorders of glycosylation (CDG), formerly named carbohydrate-deficient glycoprotein syndrome, are rare hereditary disorders caused by mutations in the genes coding for enzymes involved in the biosynthesis of glycoproteins and other glycoconjugates (1). The clinical characteristics are variable, but often include psychomotor, growth, and mental retardation from early childhood. CDG are the result of defects in the assembly and transfer (type I) or processing (type II) of the glycan moieties, and as a result, the carbohydrate chains are either completely missing (type I) or structurally abnormal (type II). CDG-Ia is the most frequent subtype, with ∼500 patients reported, whereas <100 patients have been diagnosed with types Ib–Ik and only ∼10 with type II (IIa–IId) (1)(2)(3)(4)(5). However, it is assumed that the limited awareness of CDG, together with their variable and unspecific clinical symptoms, contribute to underdiagnosis of these disorders (6)(7)(8).Another reason for underdiagnosis could be that methods for CDG testing are not generally available in clinical laboratories. Testing for CDG focuses mainly on the abnormal pattern of N-linked glycans of serum transferrin. The type I pattern is characterized by an increase of transferrin glycoforms missing one or both of the entire biantennary N-glycans (9)(10) (traditionally named disialo- and asialotransferrin, respectively, based on the number of terminal sialic acid residues), whereas type II is characterized by increased trisialo- and monosialotransferrin, indicating the presence of truncated glycans (11)(12)(13). A variety of laboratory techniques have been evaluated for this purpose, with isoelectric focusing being the most common and serving as the reference procedure to date. A sensitive HPLC method is widely used for detection of alcohol-induced changes in serum transferrin glycoforms \[i.e., carbohydrate-deficient transferrin (CDT)\] (14). This study …