Vitamin D-binding protein gene polymorphism association with IA-2 autoantibodies in type 1 diabetes

https://doi.org/10.1016/j.clinbiochem.2004.12.013Get rights and content

Abstract

Background:

Vitamin D-binding protein (DBP) is the main systemic transporter of 1.25(OH)2D3 and is essential for its cellular endocytosis. There are two known polymorphisms in exon 11 of the DBP gene resulting in amino acid variants: GAT→GAG substitution replaces aspartic acid by glutamic acid in codon 416; and ACG→AAG substitution in codon 420 leads to an exchange of threonine for lysine. These DBP variants lead to differences in the affinity for 1.25(OH)2D3. Correlations between DBP alleles and type 1 diabetes have been described in different populations. Therefore, we investigated the polymorphism in codon 416 of the DBP gene for an association with autoimmune markers of type 1 diabetes.

Design and methods:

The present analysis was a case control study. 110 patients, 68 controls, and 115 first-degree relatives were genotyped for the DBP polymorphism in codon 416. DNA typing of DBP locus was performed by the PCR-restriction fragment length polymorphism method (RFLP).

Results:

The frequencies of the Asp/Glu and Glu/Glu were significantly increased in diabetic subjects with detectable IA-2 antibodies (P < 0.01). On the contrary, the DBP Glu-containing genotype was not accompanied by differences in the prevalence of GAD65 antibodies. These finding supports a role of the vitamin D endocrine system in the autoimmune process of type 1 diabetes.

Introduction

Type 1 diabetes is a late consequence of an autoimmune process directed against β-cells related to genetic susceptibility background. Immune changes, including the production of antibodies against islet cell proteins, can be detected many months or years before the onset of diabetes. Among multiple islet antigens identified, the 65-kDa isoform of glutamic acid decarboxylase (GAD65) and the tyrosine phosphatase-like insulinoma associated protein-2 (IA-2) are the major markers of the disease present in the bloodstream [1], [2], [3]. These autoantibodies allow the prediction of disease in nondiabetic individuals [4], [5]. This chronic disease is heterogeneous in its clinical expression [6], and many evidences indicate that inherited genetic factors influenced both susceptibility and resistance to the disease. Many loci have been linked to diabetes suggesting that type 1 diabetes is a polygenic disorder. The major genetic determinants of type 1 diabetes reside in the HLA region within the major histocompatibility complex (MHC) on the short arm of chromosome 6. A close association between HLA-DQ alleles and type 1 diabetes has been demonstrated [7].

There is evidence for a role of the vitamin D endocrine system in the pathogenesis of autoimmune diseases. 1.25-dihydroxyvitamin D3 prevents autoimmune thyroiditis in animal models [8]. In humans, serum levels of 1.25(OH)2D3 were found to be significantly lower in autoimmune hyperthyroidism than in healthy subjects [9].

1.25(OH)2D3 exerts its immunomodulatory effects by downregulating the expression of HLA class II molecules [10] and inhibiting lymphocyte proliferation as well as secretion of inflammatory cytokines [11]. Correlations between Vitamin D receptor (VDR) polymorphisms but also vitamin D-binding protein (DBP) alleles and type 1 diabetes have been described in various populations. However, the meaning of these correlations in type 1 diabetes is unclear.

The vitamin D-binding protein (DBP), also called Group Specific Component (Gc), is the main systemic transporter of 1.25(OH)2D3 and is essential for its cellular endocytosis [12]. Serum levels of 1.25(OH)2D3 were shown to correlate with those of DBP [13]. The DBP gene on chromosome 4q contains three polymorphisms: a variable (TAAA)n repeat is located on intron 8 [14], a T→G substitution in exon 11 replaces aspartic acid by glutamic acid in codon 416 and a C→G substitution in codon 420 leads to an exchange of threonine for lysine [15]. These DBP variants might be responsible for differences in the affinity for 1.25(OH)2D3 [16].

Having pointed out the correlation between vitamin D-binding protein allele in codon 416 and type 1 diabetes in a previous study [17], we, therefore, investigated the polymorphism in codon 416 of the DBP gene for an association with autoimmune markers of type 1 diabetes.

Section snippets

Patients, first-degree relatives, and control subjects

Type 1 diabetic patients of the diabetology department at the civil hospital of Strasbourg constituted the reference population. The patients recruited and included over 2 consecutive calendar years (2001–2003) were: 48 males and 62 females. The mean age at diagnosis was 13.3 years (range = 5–38 years) and the mean duration of the disease was 32.2 years (range 1–37). The mean age of patients at inclusion was 25.1 years (range = 2–52 years). Type 1 diabetes was diagnosed according to the WHO

Autoantibodies

Out of the 110 diabetic patients, 64 (58.2%) had detectable levels of GAD antibodies (mean level 25 U/mL, range 2.5–45 U/mL) and 38 (34.5%) IA-2 had detectable level of IA-2 antibodies (mean level 22 U/mL, range 1.3–28 U/mL). Out of the 68 healthy control subjects, 1 (1.5%) was tested positive for GAD antibodies; all of the healthy control subjects were negative for IA-2 antibodies. IA-2 antibodies were detected in the initial sample of 7 relatives (6.1%), GAD in 9 (7.8%), and both antibodies

Discussion

The present study confirmed the correlation between DBP polymorphism and type 1 diabetes [17]. The frequency of the distribution of DBP alleles in codon 416 differed between diabetic subjects and controls, the frequency of Glu variant being elevated to 61.9% (P < 0.01) in diabetic patients. The allelic combination represented by the heterozygous Asp/Glu was not different in the two groups and Glu/Glu genotype was significantly increased in type 1 diabetic patients. Therefore, we noted DBP

Acknowledgments

This work was supported by the European Center of Diabetes Research (CeeD). We gratefully thank the members of CeeD and the volunteers who took part in the study.

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