Our data confirmed, replicated and localized the genetic association between the IL2/IL21 haplotype block and susceptibility to lupus. We observed genetic association in both IL2 and IL21 genes in the European-derived participants with the most significant association residing in and around IL21. In the European-derived sample set there is also a smaller but independent association with one marker contained within the IL2 gene. Analysis in the African-American participants revealed far fewer significant SNPs associated with lupus than in the European-derived sample set, likely reflecting smaller haplotype blocks in African-Americans. Interestingly, the two most significantly associated markers in the European-derived sample set (contained in and around IL21) were also associated with lupus in the African-American participants, while polymorphisms in IL2 were not associated with lupus in African-Americans (). Although rs2069779 located in IL2, did not meet the inclusion criterion for minor allele frequency in African-Americans (rs2069779, MAF= 0.9%), no difference in allele frequency was observed between cases and controls (MAFCase= 0.009, MAFControl= 0.009). While the genetic association with rs2069779 in IL2 appears to be independent of the main genetic association in this locus in IL21 in the European-derived sample set, this effect was not reproducible in the African-American lupus patients. The failure to confirm this IL2 association in African-Americans further supports a stronger role for IL21 polymorphisms in lupus susceptibility at the IL2/IL21 locus.
We and others have demonstrated the genetic association of polymorphisms within the IL2
locus at 4q27 with lupus and multiple other autoimmune and inflammatory disorders. (3
) Our data show a modest association between lupus and the commonly studied IL2
inter-genic marker rs6822844 in the European-derived but not the African-American lupus sample set (P= 0.014, 0.41, respectively). These data, together with conditional analysis in the European-derived participants, suggest that the observed association between lupus and rs6822844 is explained by the association with rs6835457 and rs907715, which shows an independent genetic effect in our studies. Neither rs6835457 nor rs907715 were included in commonly used genome-wide association platforms and therefore were not evaluated in genome-wide associated studies of other autoimmune or inflammatory diseases.
The association of polymorphisms at 4q27 has been demonstrated in type I diabetes in human studies.(4
) Significant evidence for involvement of IL2
in type 1 diabetes also comes from the NOD mouse in which the most highly associated non-MHC locus is the Idd3
region of the murine genome which contains the IL2
genes. In one study, the overexpression of IL-21 was observed to correlate with the number of Idd3
alleles, and this change in expression was shown to occur in response to polymorphisms which establish an Sp1 binding site upstream of IL21
) This study observed no difference in the expression levels of IL-2. In contrast to these findings, a previous study reported the underexpression of IL-2 at the NOD Idd3
locus, leading to the loss of stability in peripheral Treg cell cohorts, and these changes were unaccompanied by changes in IL-21 expression. (14
) This reduction in Treg cell numbers was in turn shown to lead to an increase in presentation of beta cell antigens by dendritic cells.
While these studies and others present compelling evidence for the involvement of both IL2
in type 1 diabetes, the associations observed in other autoimmune diseases are illustrative as well. Significant epistasis has been reported between rs6822844 and polymorphisms in IL23R
in ulcerative colitis.(35
) The association of these distal variants suggests a role for polymorphisms at 4q27 in the establishment of a Th17 defect.(36
) Furthermore, these data suggest that IL-21, a driver of Th17 differentiation (37
), likely accounts for the genetic effect observed at 4q27 in ulcerative colitis.
We have previously reported the association of polymorphisms in IL21R
with lupus susceptibility (rs3093301, Pmeta
=0.0001, OR=1.16 [95% confidence interval 1.08–1.25]) (38
). The association of polymorphisms in the IL21R
gene located at 16p11 further implicates the IL-21/IL-21R pathway signaling in lupus risk. IL-21/IL-21R signaling plays a pathogenic role in multiple models of murine lupus. Blockade of IL-21R signaling with IL-21R.Fc attenuates the severity of disease in the MRL/lpr
lupus mouse model. (39
) The deletion of IL-21R in BXSB-Yaa
mice ameliorates antibody-mediated disease manifestations, while IL-21R competent BXSB-Yaa
mice produce high levels of IgG1, IgG2b, and IgG3.(10
) These data further support a role for an IL-21/IL-21R signaling defect in lupus pathogenesis.
In conclusion, using two large ethnically-diverse lupus sample sets, conditional analysis, and trans-ethnic genotyping, we fine-mapped and localized the genetic association with lupus in the IL2/IL21 LD block to IL21. These data might be relevant to a number of other autoimmune and inflammatory diseases with a reported genetic association in the same region.