Primary IgA nephropathy (IgAN) is the most common glomerular disease in children and adolescents who undergo renal biopsy because of isolated microscopic hematuria or hematuria associated with non-nephrotic proteinuria (1). Even though the exact pathogenesis is not yet known, IgAN is characterized by glomerular mesangial proliferation in response to the deposition of IgA. While the disease is characterized by a single histopathological finding, patients with IgAN show variable clinical and histopathological features. Polymorphisms of candidate genes influence this individual variability. To date, there has been evidence suggesting that abnormal immune responses play an important role in the pathogenesis of IgAN (2,3).

Interleukin 7 (IL7) is a well-known essential factor for the development and maintenance of the immune system. IL7 works through its receptor, IL7R (also known as CD127). IL7R, a member of the hematopoietin receptor family, is a type 1 membrane glycoprotein capable of binding α-helical cytokines (4). Signaling via IL7R induces somatic recombination of the T-cell receptor and immunoglobulin genes, promoting both the proliferation and differentiation of developing B and T lymphocytes. The IL7R is heterodimeric and consists of the IL7R α chain (IL7Rα, encoded by IL7R) and the common cytokine-receptor γ chain (γc, encoded by IL2RG; also known as CD132). IL7Rα transduces transmembrane signals through the recruitment of intracellular messengers to its cytoplasmic tale. IL7Rα is expressed on immature B and T cells, and genetic ablation or the presence of neutralizing antibodies has been found to block lymphocyte development (5). In addition, mutation of IL7Rα has been implicated in the development of a subtype of severe combined immunodeficiency. Functionally, IL7Rα is important in the body's innate and adaptive inflammatory and immunological response. This signaling is critical for T-cell differentiation of CD4⁻CD8⁻ thymocytes and has a role in the survival of CD4⁺CD8⁺ cells after positive selection. Moreover, IL7 signaling modulates peripheral memory or naïve T-cell homeostasis (6) and production of several proinflammatory (IFNr, IL2) and anti-inflammatory cytokines (IL4) (7). Even though the downstream signaling pathway of IL7R remains poorly understood, previous studies have shown that it leads to activation of the phosphatidylinositol-3-kinase pathway, the Ras/Raf signaling cascade and Janus kinase/STAT pathways, which have all been suggested to be important for the pathogenesis of various diseases, including IgAN (8,9). In addition, the IL7/IL7R pathway has been studied as part of the pathogenesis of various autoimmune diseases, such as multiple sclerosis (MS), rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), sarcoidosis and psoriasis (10,11).

Accordingly, the IL7/IL7R pathway is a very attractive target for inflammatory disorders, and its genetic variations have been proposed to be involved in the pathogenesis of various autoimmune diseases (10,12). However, no prior study has investigated the association between SNPs of IL7R and renal diseases, especially in children. Thus, the present study was conducted to investigate the association between polymorphisms of the IL7R gene and childhood IgAN.

In the analysis of demographics of IgAN patients, we found that pathological grade was significantly associated with proteinuria and podocyte foot process effacement (P=0.002 and <0.001, respectively) (Table I).

The genotypic distributions of all SNPs in this study were consistent with HWE (P>0.05). The genotyping data of the 198 cases and 288 controls showed a significant association between the presence of IgAN and both of the candidate missense SNPs by logistic regression analysis [rs1494558 (codominant model, OR=1.63, 95% CI 1.24–2.14, P=0.0003; dominant model, OR=2.01, 95% CI 1.36–2.97, P=0.0003; recessive model, OR=1.70, 95% CI 1.03–2.82, P=0.0390) and rs1494555 (codominant model, OR=1.48, 95% CI 1.13–1.94, P=0.0038; dominant model, OR=1.70, 95% CI 1.13–2.56, P=0.0099; recessive model, OR=1.63, 95% CI 1.03–2.60, P=0.0380)] (Table II). Upon comparison of the allelic frequencies, the G allele of rs1494558 (OR=1.59, 95% CI 1.22–2.06, P=0.0010) and T allele of rs1494555 (OR=1.43, 95% CI 1.11–1.85, P=0.0060) were found to be risk alleles for IgAN. In the measurement of pair-wise LD, one LD block was identified by the Gabriel method (18), and the block consisted of both rs1494558 and rs1494555 (data not shown). The haplotype analysis showed a statistical significance with the haplotype AC (codominant model, OR=0.69, 95% CI 0.53–0.90, P=0.0066; recessive model, OR=0.59, 95% CI 0.40–0.89, P=0.0124) and the haplotype GT (codominant model, OR=1.62, 95% CI 1.24–2.12, P=0.0005; dominant model, OR=1.97, 95% CI 1.34–2.90, P=0.0006; recessive model, OR=1.72, 95% CI 1.04–2.85, P=0.0362) (Table III).

In order to verify our data, we calculated the sample power for each SNP using a genetic power calculator; the sample power was 0.9641 for rs1494558 (number of cases for 80% power = 109) and 0.9662 for rs1494555 (number of cases for 80% power = 108) with α=0.05 and genotype relative risk = 2-fold. Accordingly, our case-control study was sufficiently powered to detect a positive association.

When the genotypic association between the SNPs and subgroups of IgAN patients was assessed, nephrotic range proteinuria (>40 mg/m²/h) that developed during the course of the disease was found to be associated with rs1494558 (codominant model, OR=1.92, 95% CI 1.06–3.46, P=0.0270; recessive model, OR=2.81, 95% CI 1.18–6.67, P=0.0230) (Table IV). The allelic frequency analysis showed that the G allele of rs1494558 (OR=1.83, 95% CI 1.07–3.15, P=0.0270) and T allele of rs1494555 (OR=1.74, 95% CI 1.01–3.00, P=0.0460) were associated with the risk of developing nephrotic range proteinuria in IgAN patients. No significant difference was shown in clinical subgroup analysis in podocyte foot process effacement or pathological progression (data not shown).

The relationship between the IL7/IL7R pathway and markers of inflammation has been observed in RA patients; influence on the clinical disease activity and the production of IL6, CRP, IL1b, IFN-γ and TNF-α was noted (7,11). In addition, IL7 may induce matrix metalloproteinase (MMP)-13 and matrix degradation (12,19). A study of patients with SLE reported abnormal bone marrow-derived mesenchymal stem cells showing IL7 down-regulation (10). In addition, an inhibitory effect of IL7R has been reported in allergic inflammation processes (20).

Recently, an association of IL7R polymorphisms with certain target diseases was reported in several studies. MS is the most studied disease, with well-replicated results (21–23). In these studies, the SNPs and their haplotypes of IL7R were suggested to be susceptibility and/or progression markers for MS, and the authors even speculated that overexpression of IL7R possibly results in increased numbers of autoantigen-specific T cells (21,22). Other studies have shown that SNPs of IL7 and/or IL7R are associated with susceptibility to RA, sarcoidosis and type 1 diabetes (24,25). These reports suggest that the IL7R locus may be involved in the pathogenesis of several diseases with an immunologic basis.

In the present study, we found a strong association between IL7R SNPs and IgAN susceptibility. Haplotype (AC and TG) showed a strong association with IgAN susceptibility, as well. The calculated sample power was sufficient to detect this association. Furthermore, there was a weaker but significant association with the development of nephrotic range proteinuria in IgAN patients. When the allelic frequencies were compared to other populations, both SNPs had a similar distribution in the Japanese population from the HapMap database (http://www.hapmap.org). However, the A allele of rs1494558 and C allele of rs1494555 showed slightly higher frequencies than those of the other three populations used for comparison. Both alleles were investigated as protective alleles in the present analysis.

Two SNPs found to be significantly associated with IgAN pathogenesis, rs1494558 and rs1494555, are missense SNPs in the coding region that result in changes of amino acid sequences. Thus, there is a potential of affecting protein function or expression; consequently, the SNPs may be involved in the pathogenesis of IgAN.

However, we could not demonstrate the association between pathological progression of IgAN and IL7R SNPs, despite their possible effects on the fibrotic process as reflected by a previously proposed function on MMP-13 and fibroblasts. In addition, even though an association with proteinuria was observed, no significant relationship with podocyte foot process effacement was found. These discrepancies could be a result of limitations of our study. Patients were enrolled by school screening urinalysis and, as reflected by the relatively mild pathological findings of our patients, they were in an early stage of disease. As the nephrotic proteinuria included late onset proteinuria that developed during the follow-up period, it may not correspond to the early stage biopsy findings.

In summary, there is evidence suggesting that IL7R may be involved in the pathogenesis of IgAN, but IL7R-related polymorphisms have not previously been studied in patients with IgAN. In the present study, two missense SNPs of IL7R, rs1494558 and rs1494555, were found to be significantly associated with both susceptibility to IgAN and nephrotic range proteinuria in IgAN patients.