Date Published: February 24, 2017
Publisher: Public Library of Science
Author(s): Melissa E. Munroe, Nathan Pezant, Michael A. Brown, Dustin A. Fife, Joel M. Guthridge, Jennifer A. Kelly, Graham Wiley, Patrick M. Gaffney, Judith A. James, Courtney G. Montgomery, Jose C. Crispin.
Antiviral defenses are inappropriately activated in systemic lupus erythematosus (SLE) and association between SLE and the antiviral helicase gene, IFIH1, is well established. We sought to extend the previously reported association of pathogenic soluble mediators and autoantibodies with mouse Mda5 to its human ortholog, IFIH1. To better understand the role this gene plays in human lupus, we assessed association of IFIH1 variants with soluble mediators and autoantibodies in 357 European-American SLE patients, first-degree relatives, and unrelated, unaffected healthy controls. Association between each of 135 genotyped SNPs in IFIH1 and four lupus-associated plasma mediators, IL-6, TNF-α, IFN-β, and IP-10, were investigated via linear regression. No significant associations were found to SNPs orthologous to those identified in exon 13 of the mouse. However, outside of this region there were significant associations between IL-6 and rs76162067 (p = 0.008), as well as IP-10 and rs79711023 (p = 0.003), located in a region of IFIH1 previously shown to directly influence MDA-5 mediated IP-10 and IL-6 secretion. SLE patients and FDRs carrying the minor allele for rs79711023 demonstrated lower levels of IP-10, while only FDRs carrying the minor allele for rs76162067 demonstrated an increased level of IL-6. This would suggest that the change in IP-10 is genotypically driven, while the change in IL-6 may be reflective of SLE transition status. These data suggest that IFIH1 may contribute to SLE pathogenesis via altered inflammatory mechanisms.
Systemic lupus erythematosus (SLE) is an autoimmune disease marked by immune dysregulation and chronic inflammation resulting from reduced immunologic tolerance to nuclear self-antigens. One mechanism by which autoimmunity may develop is through an innate immune response to viral infection that results in cross-reactive recognition of self-antigen via molecular mimicry . One component of antiviral immune defense is the family of retinoic acid-inducible gene-I-like receptors (RIG-I like receptors [RLRs]). RLRs are encoded by a number of IFN-inducible genes . One such IFN-inducible gene, Interferon induced with helicase C domain 1 (IFIH1), has previously been associated with multiple autoimmune conditions, including SLE [3, 4]. IFIH1 has been associated in SLE with increased sensitivity to serum Type I IFN and the presence of anti-dsDNA autoantibodies .
We sought to determine if the soluble mediators that significantly associated with mutant Mda5 , also associated with the human ortholog, IFIH1. Plasma levels of select soluble mediators and the presence of lupus-associated autoantibody specificities in 357 individuals (SLE patients, FDRs of SLE patients not included in this study, and unrelated, unaffected controls with no family history of SLE were analyzed along with 135 quality controlled SNPs in the region of IFIH1 (Fig 1). The zoom plots presented demonstrate no significant associations (p<0.01) between variants within IFIH1 exon 13 (red dots) and plasma levels of IL-6, TNF-α, IFN-β, IP-10, or anti-dsDNA (Fig 1A–1E). The number of positive lupus-associated autoantibody specificities was also not significantly associated with variants in exon 13 (Fig 1F). However, significant associations were found with SNPs outside of exon 13 (blue dots). IL-6 was significantly associated with rs76162067 (p = 0.008; Fig 1A), while IP-10 was significantly associated with rs79711023 (p = 0.003; Fig 1D), both of which are upstream of previously identified SLE-associated SNPs, rs13023380, rs10930046, and rs1990760 ([4, 5], Fig 1G). Results from LDlink show that neither rs76162067 nor rs79711023 are in strong linkage disequilibrium with any of the previously identified SLE-associated SNPs mentioned above (S1 Table). Of the 357 individuals assessed, 14 carried the minor (TC) allele for rs73162067, an intronic SNP located between exons 1 and 2, while 19 individuals carried the minor (TC) allele for rs79711023, an intronic SNP located between exon 2 and 3 (Fig 2A). Those individuals carrying the minor allele for rs76162067 and rs79711023 exhibited higher plasma levels of IL-6 (p = 0.0086) and lower plasma levels of IP-10 (p = 0.0250), respectively (Fig 2B and 2C). These data confirm in humans the presence of significant associations between IFIH1 and distinct pro-inflammatory mediators. We sought to determine if pro-inflammatory mediators previously shown to be associated with Mda5, resulting in a lupus-like phenotype in mice , also associated with IFIH1 in humans. Contrary to the results of the motivating study in mice, we see no association of soluble mediators with SNPs in exon 13 of IFIH1. When we expanded our analyses, however, to include 135 SNPs across IFIH1 we found two promising effects associated with IL-6 and IP-10. Although previous investigations have shown IFIH1, IL-6, and IP-10 to be associated with SLE [4, 19, 20], none have shown a direct connection between human IFIH1 and these two soluble mediators. SNPs significantly associated with IL-6 (rs76162067) and IP-10 (rs79711023) are located within the region of exons 1–3 that encodes the CARD1/CARD2 domain of the MDA5 protein . The CARD1/CARD2 domain of IFIH1-encoded MDA5 has been shown to signal through the adaptor protein, interferon-β promoter stimulator (IPS-1) , necessary for MDA-5 mediated secretion of IL-6 (via NFκB) and IP-10 (via NFκB and IRF3) . Source: http://doi.org/10.1371/journal.pone.0171193