Association of the C8orf13-BLK region with systemic sclerosis in North-American and European populations

Abstract

Objective

Genetic studies in the systemic sclerosis (SSc), an autoimmune disease that clinically manifests with dermal and internal organ fibrosis and small vessel vasculopathy, have identified multiple susceptibility genes including HLA-class II, PTPN22, IRF5, and STAT4 which have also been associated with other autoimmune diseases, such as systemic lupus erythematosus (SLE). These data suggest that there are common autoimmune disease susceptibility genes. The current report sought to determine if polymorphisms in the C8orf13-BLK region (chromosome 8p23.1-B lymphoid tyrosine kinase), which is associated with SLE, are associated also with SSc.

Methods

Two variants in the C8orf13-BLK region (rs13277113 & rs2736340) were tested for association with 1050 SSc cases and 694 controls of North Americans of European descent and replicated in a second series 589 SSc cases and 722 controls from Spain.

Results

The “T” allele at rs2736340 variant was associated with SSc in both the U.S. and Spanish case–control series (P = 6.8 × 10⁻⁵, OR 1.27, 95% CI 1.1–1.4). The “A” allele at rs13277113 variant was associated with SSc in the U.S. series only (P = 3.6 × 10⁻⁴, OR 1.32, 95% CI 1.1–1.6) and was significant in the combined analyses of the two series (P = 2.0 × 10⁻³; OR 1.20, 95% CI 1.1–1.3). Both variants demonstrated an association with the anti-centromere antibody (P = 2.2 × 10⁻⁶ and P = 5.5 × 10⁻⁴, respectively) and limited SSc (P = 3.3 × 10⁻⁵ and P = 2.9 × 10⁻³, respectively) in the combined analysis. Peripheral blood gene expression profiles suggest that B-cell receptor and NFκB signaling are dysregulated based on the risk haplotype of these variants.

Conclusion

We identify and replicate the association of the C8orf13-BLK region as a novel susceptibility factor for SSc, placing it in the category of common autoimmune disease susceptibility genes.

Introduction

Scleroderma (systemic sclerosis, SSc) is a chronic, multisystem autoimmune disease clinically characterized by progressive fibrosis of the skin and internal organs. Pathologically, SSc exhibits three cardinal features: inflammation and autoimmunity, vasculopathy and excessive extracellular matrix production and deposition. How the disease process is triggered, remains to be established, but current paradigms point towards immune dysregulation as a central process in the pathogenesis of SSc.

Multiple lines of evidence, in patients as well as animal models of dermal fibrosis, point to dysregulation of the immune system and autoimmunity in the pathogenesis of SSc. Alterations in T-cell profiles and cytokines have been demonstrated [1]. In addition, the B-cell lineage is implicated [1], [2]. CD19 deficient mice have reduced dermal fibrosis after subcutaneous bleomycin administration [3]. The presence of multiple SSc-associated autoantibodies, most commonly anti-centromere (ACA), anti-topoisomerase Q1 antibodies (ATA), and anti-RNA polymerase III antibodies (ARA), has been well-described in SSc patients and provides indirect evidence for the importance of the B-cell lineage in SSc [4], [5], [6]. Interestingly, the SSc-associated autoantibodies correlate with distinct clinical subsets characterized by extent of cutaneous involvement and pattern of organ involvement [7], [8]. For example, pulmonary arterial hypertension is more common in patients with ACA, pulmonary fibrosis is more common in patients with ATA, and scleroderma renal crisis is more common in patients with ARA [7].

Recently, several genetic polymorphisms have been associated with SSc in susceptibility [9], [10], [11], [12], [13], [14], [15]. Many of these genes are susceptibility factors for other autoimmune diseases suggesting that they are common autoimmune disease susceptibility genes. These data support the paradigm of common dysregulated pathways across multiple autoimmune diseases. Interestingly, some of these genetic variants associated with SSc are associated with susceptibility to developing SSc, while others increase risk only in specific autoantibody subsets seen in SSc [9], [10].

Two genome wide association studies in systemic lupus erythematosus (SLE) have implicated the C8orf13-BLK (B lymphoid tyrosine kinase gene) region of chromosome 8p23.1 as a susceptibility locus for SLE [16], [17]. These findings were further replicated in a Japanese population [18]. B lymphoid kinase (Blk), encoded by the BLK gene is a member of the Src family kinases (SFKs) which includes Blk, Lck, Fyn, Lyn, c-Src, c-Yes, Fgr, and Hck [19]. Blk is the only SFK that is exclusively expressed in B cells and thymocytes and not in mature T cells [20], [21], [22]. Blk transduces signals downstream of the B cell receptor (BCR) and plays a role in BCR signaling and B cell development [23], [24]. B cell development is dependent upon the activation of the transcription factor nuclear factor κB (NFκB) by SFKs (Blk, Fyn, Lyn) [23]. The C8orf13 gene is ubiquitously expressed but its exact function is currently unknown.

Given the importance of B-cells and autoantibodies in the pathogenesis of SSc as well as SLE and the emerging paradigm that autoimmune diseases may share common susceptibility genes, the current study sought to investigate the potential association of the C8orf13-BLK region variants with SSc in two large, independent, and well-described case–control series. Herein we demonstrate a significant association of both C8orf13-BLK region variants with susceptibility to SSc and more strongly with the anti-centromere antibody subset of SSc in both case–control series. We observe a dysregulation of the B-cell receptor and NFκB signaling based on the risk haplotype of these variants in peripheral blood gene expression arrays.