Identification of IRAK1 as a risk gene with critical role in the pathogenesis of systemic lupus erythematosus

Abstract

A combined forward and reverse genetic approach was undertaken to test the candidacy of IRAK1 (interleukin-1 receptor associated kinase-1) as an X chromosome-encoded risk factor for systemic lupus erythematosus (SLE). In studying approximately 5,000 subjects and healthy controls, 5 SNPs spanning the IRAK1 gene showed disease association (P values reaching 10(-10), odds ratio >1.5) in both adult- and childhood-onset SLE, in 4 different ethnic groups, with a 4 SNP haplotype (GGGG) being strongly associated with the disease. The functional role of IRAK1 was next examined by using congenic mouse models bearing the disease loci: Sle1 or Sle3. IRAK1 deficiency abrogated all lupus-associated phenotypes, including IgM and IgG autoantibodies, lymphocytic activation, and renal disease in both models. In addition, the absence of IRAK1 reversed the dendritic cell "hyperactivity" associated with Sle3. Collectively, the forward genetic studies in human SLE and the mechanistic studies in mouse models establish IRAK1 as a disease gene in lupus, capable of modulating at least 2 key checkpoints in disease development. This demonstration of an X chromosome gene as a disease susceptibility factor in human SLE raises the possibility that the gender difference in SLE may in part be attributed to sex chromosome genes.

Fig. 1.

Association of IRAK1 SNPs with SLE in 4 ethnic groups (EA, European Americans; AA, African Americans; AsA, Asian Americans; HA, Hispanic Americans) in childhood- and adult-onset SLE cases. The position of exons (green rectangles) and introns (connecting lines) are indicated in the bottom plot. The dotted horizontal line corresponds to P = 0.05. The exact numbers of subjects studied are detailed in Table S1.

Fig. 2.

Reduced serum IgM and IgG autoantibodies in B6.Sle1^z.IRAK1^−/Y mice. B6.Sle1^z mice (homozygous for the z allele of Sle1) either sufficient or deficient in IRAK1 (n = 15–20) were examined at the age of 9–12 months for serum levels of IgM (A–C) and IgG (D–F) autoantibodies to various nuclear antigens. Shown data are drawn from 2 independent experiments. The composite results are plotted as box and whisker plots. IRAK1 knockouts (labeled as −/−) are indicated with gray filled boxes; Sle1 sufficient for IRAK1 (+/+) are unfilled. The box contains the interquartile range (Q1–Q3) with the median indicated as a thick black line; the whiskers contain the observations within 1.5 times the interquartile range, and observations outside this range are indicated with open circles.

Fig. 3.

Cellular phenotypes in B6.Sle1^z.IRAK1^−/Y mice. B6.Sle1^z mice either sufficient or deficient in IRAK1 (n = 7–10) were examined at the age of 4–6 months for spleen weight (A) and cellularity (B–D), as well as the mean B cell size (as assessed from the forward scatter channel) (E). Data shown are drawn from 2 independent experiments and are presented as in Fig. 2.

Fig. 4.

B6.Sle3^z.IRAK^−/Y mice exhibit reduced serum autoantibodies and nephritis. (A, B, D, and E) B6.Sle3^z mice (homozygous for the z allele of Sle3) either sufficient or deficient in IRAK1 were examined at the age of 9–12 months for serum levels of IgM and IgG autoantibodies to various nuclear antigens (n = 17). (C) The 24-hr proteinuria as a measure of glomerulonephritis is assessed in both strains (n = 9–15). Shown data are drawn from 2 independent experiments. Data shown in A–E are presented as in Fig. 2. (F) Representative H&E staining (400× magnification) of kidney sections from an IRAK1-sufficient Sle3^z mouse showing World Health Organization grade 3 glomerulonephritis and an IRAK1-deficient Sle3^z mouse with World Health Organization grade 1 glomerulonephritis.

Fig. 5.

Cellular phenotypes in B6.Sle3^z.IRAK1^−/Y mice. (A–D) B6.Sle3^z mice either sufficient (shown in pink) or deficient (shown in green) in IRAK1 were examined at the age of 4–6 months for the surface expression of activation/maturation marker CD80 in comparison to an isotype control antibody (gray) (n = 8) F4/80^hi, CD11b^hi/medium, CD11c^low splenic macrophages (A) and CD11b⁺ CD11c^hi splenic myeloid DCs (B). (C and D) Bone-marrow-derived DCs from B6.Sle3^z.IRAK1-sufficient and B6.Sle3^z.IRAK1-deficient mice, stimulated with TLR ligand poly(I·C) (C) or CpG oligonucleotide (D). (E) TNF-α production by bone-marrow-derived DCs 24 hr after stimulation with TLR ligand. (F) Ratio of CD4 to CD8 spleen T cells in B6.Sle3^z.IRAK1-sufficient vs. -deficient mice. A–D are representative normalized histograms of flow cytometry data. E and F are box and whisker plots as described for Fig. 2.