Abstract
Immunoglobulin G (IgG), a glycoprotein secreted by plasma B-cells, plays a major role in the human adaptive immune response and are associated with a wide range of diseases. Glycosylation of the Fc binding region of IgGs, responsible for the antibody’s effector function, is essential for prompting a proper immune response. This study focuses on the general genetic impact on IgG glycosylation as well as corresponding subclass specificities. To identify genetic loci involved in IgG glycosylation, we performed a genome-wide association study (GWAS) on liquid chromatography electrospray mass spectrometry (LC–ESI-MS)—measured IgG glycopeptides of 1,823 individuals in the Cooperative Health Research in the Augsburg Region (KORA F4) study cohort. In addition, we performed GWAS on subclass-specific ratios of IgG glycans to gain power in identifying genetic factors underlying single enzymatic steps in the glycosylation pathways. We replicated our findings in 1,836 individuals from the Leiden Longevity Study (LLS). We were able to show subclass-specific genetic influences on single IgG glycan structures. The replicated results indicate that, in addition to genes encoding for glycosyltransferases (i.e., ST6GAL1, B4GALT1, FUT8, and MGAT3), other genetic loci have strong influences on the IgG glycosylation patterns. A novel locus on chromosome 1, harboring RUNX3, which encodes for a transcription factor of the runt domain-containing family, is associated with decreased galactosylation. Interestingly, members of the RUNX family are cross-regulated, and RUNX3 is involved in both IgA class switching and B-cell maturation as well as T-cell differentiation and apoptosis. Besides the involvement of glycosyltransferases in IgG glycosylation, we suggest that, due to the impact of variants within RUNX3, potentially mechanisms involved in B-cell activation and T-cell differentiation during the immune response as well as cell migration and invasion involve IgG glycosylation.
Highlights
Glycosylation is among the most abundant post-translational protein modifications [1] and defects therein can lead to severe diseases [2,3,4], and aberrant glycosylation patterns are likewise associated with different types of cancer [5,6,7,8,9,10,11,12]
Using QQ-plots to compare the associations obtained with initial immunoglobulin G (IgG) glycan traits versus within-subclass ratios, we clearly demonstrate a gain in power for the latter analytical approach (Figure 3)
With the new analytical method, LC–ESI-MS, we confirmed the association of IgG glycosylation to six of the loci previously identified with UPLC [21, 23] and, detect a novel locus, RUNX3, on chromosome 1p36.11
Summary
Glycosylation is among the most abundant post-translational protein modifications [1] and defects therein can lead to severe diseases [2,3,4], and aberrant glycosylation patterns are likewise associated with different types of cancer [5,6,7,8,9,10,11,12]. A first genome-wide association study (GWAS) by Lauc et al [21]., including 2,247 individuals from four European cohorts (CROATIA-Vis, CROATIA-Korcula, Orkney Complex Disease Study and Northern Swedish Population Health Study), identified four loci encoding glycosyltransferases associated with IgG N-glycans. The authors likewise propose that five additional loci are involved in IgG glycosylation showing that a GWAS can be used to identify genetic loci controlling glycosylation of a single plasma protein [21]. They replicated the association of two of their loci, MGAT3 and B4GALT1, in a cohort of MALDI–TOF MS (matrix-assisted laser desorption/ionization time-of-flight mass spectrometry) measured glycan data from the Leiden Longevity Study (LLS) [22]. The authors examine IgG glycan structures measured by ultra-performance liquid chromatography [(UPLC) [24]] in a multivariate way and thereby detect five novel genetic loci that are associated with combinations of IgG glycan traits
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