Immunoglobulin M gene association with autoantibody reactivity and type 1 diabetes

Abstract

Several lines of evidence show that autoimmune responses evolving in type 1 diabetes (T1D) patients include the generation of multi-reactive autoantibody (AutoAb) repertoires, but their role in T1D pathogenesis remains elusive. We tested the hypothesis that variants at the immunoglobulin heavy chain (IGH) locus are genetic determinants of AutoAbs against pancreatic antigens and contribute to T1D susceptibility. With this aim, two independent study designs were used: a case-control study and a family-based cohort comprising a total of 240 T1D patients, 172 first-degree relatives (mother and/or father), and 130 unrelated healthy controls living in Portugal. We found that three SNPs in the IGH locus show suggestive association with T1D with the highest nominal association at rs1950942 (in the IGHM-IGHJ gene region) in both the case-control study (P = 9.35E-03) and the family-based cohort (P = 3.08E-03). These SNPs were also associated with IgG AutoAbs against pancreatic antigens and with AutoAb multi-reactivity in T1D patients. Notably, we found that the SNP with the highest association with T1D susceptibility and IgG autoantibody reactivity (rs1950942) was also associated with anti-GAD IgM reactivity in T1D patients (P = 5.98E-03) and in non-affected parents (P = 4.17E-03). This finding implies that IGH association with autoreactive IgM is detectable irrespective of disease status.These results suggest that genetic variants at the IgM gene region of the IGH locus contribute to antibody autoreactivity and are associated with T1D. We propose that the control of autoantibody generation by IGH polymorphisms is a component of the complex architecture of T1D genetic susceptibility.

Keywords: Anti-GAD; Autoantibodies; Genetic association; IgH locus; IgM; Type 1 diabetes.

Fig. 1

Genetic association of immunoglobulin heavy chain (IGH) locus to type 1 diabetes (T1D). a Relative physical distances of nine single nucleotide polymorphisms (SNP) across 430 kb in IGH locus, in chromosome 14. SNP assignment to IGH gene segments is shown (see Table 2 for individual SNP-defined positions). b Nominal P values of association tests in two cohorts of T1D patients. Case control (CC) genotypic analysis under the recessive model was performed in 137 patients and 130 controls and is represented by the inverted shaded triangles. Transmission disequilibrium tests (TDT) were performed in 102 patients and 169 first-degree relatives and are represented by the open circles. Dashed line indicates the threshold of suggestive association (P = 0.05). Linkage disequilibrium (LD) analysis is depicted as a diagram generated by the Haploview software package and values of pair-wise r-squared are represented. The dark shaded area represents a LD block encompassing five analyzed SNPs in the IGHD-M region

Fig. 2

Autoantibodies positivity. a Number of T1D patients of the family-based cohort (represented in black) or their non-affected parents (represented in gray) displaying IgG positivity (filled area) or negativity (striped area) for the indicated antigens. b Number of T1D patients of the family-based cohort showing positivity for the indicated number of antibody specificities against the following antigens: glutamic acid decarboxylase (GAD), islet cell autoantigen (ICA), and protein tyrosine phosphatase (PTP)-like protein (IA-2). Relative frequency of each group is shown as percent of the number of patients analyzed

Fig. 3

Anti-GAD IgM reactivity. Results from 227 T1D patients and 146 non-affected parents are represented as arbitrary units after normalization, as described in the “Materials and methods” section. P < 0.0001, Wilcoxon signed-rank test

Fig. 4

T1D-susceptibility genotype at rs1950942 is associated with increased levels of anti-GAD IgM. Normalized anti-GAD IgM titers in 200 patients (black circles) and 129 relatives (open triangles) stratified by the T1D-aasocited genotyped at rs1950942 (AA versus AG/GG). **P < 0.01, Mann-Whitney test (one-tailed)