Transferrin mutations at the glycosylation site complicate diagnosis of congenital disorders of glycosylation type I
Abstract
Congenital Disorders of Glycosylation (CDG) form a group of metabolic disorders caused by deficient glycosylation of proteins and/or lipids. Isoelectric focusing (IEF) of serum transferrin is the most common screening method to detect abnormalities of protein N-glycosylation.
On basis of the isofocusing profile, patients can be grouped in CDG type I or CDG type II. Secondary causes, such as the presence of a transferrin protein polymorphism can complicate interpretation and must be excluded before time-consuming diagnostics is initiated.
Several protein variants of transferrin are known that result in a shift in pI and thereby result in double bands in IEF. Incubation of the plasma sample with neuraminidase can indicate the presence of a protein polymorphism by showing double bands at the position of asialotransferrin.
Here, we present two cases with a novel protein polymorphism, resulting in a single transferrin isoform after neuraminidase digestion. Mass spectrometric analysis of immunopurified transferrin showed the presence of a non-glycosylated peptide, corresponding to the N-glycan consensus sequence of glycopeptide 2 in case 1 with the heterozygous p.Asn630Thr mutation and of glycopeptide 1 in case 2 with a heterozygous p.Asn432His mutation.
Our results show the importance of mass spectrometry in the diagnostic track of CDG type I patients.