O- and N-Glycosylation of Serum Immunoglobulin A is Associated with IgA Nephropathy and Glomerular Function.

Background: Immunoglobulin A (IgA) nephropathy (IgAN) is the most common primary glomerular disease worldwide, and is a leading cause of renal failure. The disease mechanisms are not completely understood, but a higher abundance of galactose-deficient IgA is recognized to play a crucial role in IgAN pathogenesis. While both types of human IgA (IgA1 and IgA2) have several N-glycans as posttranslational modification, only IgA1 features extensive hinge-region O-glycosylation. IgA1 galactose-deficiency on the O-glycans is commonly detected by a lectin-based method. To date, limited detail is known about IgA O- and N-glycosylation in IgAN. Methods: To gain insights into the complex O- and N-glycosylation of serum IgA1 and IgA2 in IgAN, we employed liquid chromatography-mass spectrometry (LC-MS) for the analysis of tryptic glycopeptides of serum IgA from 83 IgAN patients and 244 age and sex-matched healthy controls. Results: Multiple structural features of N-glycosylation of IgA1 and IgA2 were associated with IgAN and glomerular function in our cross-sectional study. These features included differences in galactosylation, sialylation, bisection, fucosylation, and N-glycan complexity. Moreover, IgA1 O-glycan sialylation was associated with both disease risk and glomerular function. Finally, glycopeptides were a better predictor of IgAN and glomerular function than galactose-deficient IgA1 levels measured by lectin-based ELISA. Conclusions: Our high-resolution data suggest that IgA O- and N-glycopeptides are promising targets for future investigations on the pathophysiology of IgAN and as potential noninvasive biomarkers for disease prediction and deteriorating kidney function.