Mass spectrometry proves under-O-glycosylation of glomerular IgA1 in IgA nephropathy

Background. The IgA1 molecule, which is predominantly deposited in glomerul i in IgA nephropathy (IgAN), is a unique serum glycoprotein because it has O-glycan side chains in its hinge region. Our study was conducted to invest igate the O-glycan structure in the glomerular IgA1 in IgAN. Methods. The IgA1 was separated from 290 renal biopsy specimens of 278 IgAN patients and from four serum IgA1 samples (IgAN, 2; control, 2). The varie ty of O-glycan glycoform was determined by estimating the precise molecular weights of the IgA1 hinge glycopeptides using matrix-assisted laser desorp tion ionization time of flight mass spectrometry. Results. The peak distribution of IgA1 hinge glycopeptides clearly shifted to lesser molecular weights in both glomerular and serum IgA1 in IgAN compa red with the serum IgA1 of controls. In the five major peaks of IgA1 hinge glycopeptides in each sample, the numbers of carbohydrates composing O-glyc ans (GalNAc, Gal, and NANA) in the deposited and serum IgA1 in IgAN patient s were significantly fewer than those in the serum IgA1 in the control grou ps. Conclusion. The O-glycan side chains in the hinge of the glomerular IgA1 we re highly underglycosylated in IgAN. These results indicate that the decrea sed sialylation and galactosylation of the IgA1 hinge glycopeptides play a crucial role in its glomerular deposition in IgAN.