CTLA4, expressed on activated T-cells negatively regulates T-cell activation [3
]. Functional CTLA4
variants could contribute to the pathogenesis of disorders characterized by abnormal T-cell responses, including JIA. CTLA4
variants are associated with T1D, autoimmune thyroiditis, and RA [7
]. Variants tested in our study have functional consequences. For instance, the T allele at C-318T
is associated with up-regulation of CTLA4
]. The A49G
SNP, which changes threonine to alanine at position 17 in the leader sequence of CTLA4, affects the CTLA4-driven down-regulation of T-cell activation [15
]. The strongest signal of association in the CTLA4
locus is with the CT60 SNP [7
]. In a comprehensive study of CTLA4 variants, Ueda et al. demonstrated that the G allele at CT60 was associated with susceptibility to autoimmunity. The ratio of soluble CTLA4 to full length CTLA4 in CD4 T-cells was 50% lower in individuals with the susceptible genotype (G/G) compared with individuals who had the A/A genotype (p < 0.002). Thus, CTLA4 variants investigated in our study have functional consequences and known disease associations.
To date three studies have investigated associations between CTLA4
variants and juvenile arthritis[4
]. These studies were primarily focused on adult RA or asthma patients and included relatively small numbers of children with JIA. Only limited phenotypic information about JIA/JRA was available in these studies. Miterski et al. reported an association of C318T with JRA in a study of 200 subjects from Germany [4
]. An association with A49G was not observed. In another study, Suppiah et al. tested 72 children with JIA, 289 adults with RA, and 475 controls from Northern Ireland for CTLA4 variants[6
]. In contrast to other studies on RA, Suppiah et al found an increased frequency of the A allele of the A49G SNP in the RA group and in the combined arthritis group, although an association was not found with the JIA subgroup alone. That study did not find any associations with the CT60 SNP, although they found differences in frequencies of A49G-CT60 haplotypes between arthritic cases and controls. Finally, Schubert et al. studied C-318T and A49G SNPs in a cohort of 321 children with asthma, 86 children with JIA, and 270 controls from Germany[5
]. That study found no associations between JIA and either the C-318T or A49G variants.
We did not find an association between C–318T, A49G, or CT60 variants and JIA in a large cohort. We also did not replicate the association described between C-318T and JRA by Miterski et al. Lack of replication of genetic studies occurs due to myriad reasons, including type-1 statistical errors or hidden population stratification in the original study, inadequate power in replication studies, and population differences. The discrepancies in the reported results for C-318T in JIA could be due to true population differences or co-occurrence of other autoimmune disorders in the cohort studied by Miterski et al [4
]. The mean age of disease onset (10.3 years) was higher in the cohort studied by Miterski et al. than in our study, raising the possibility that there were more individuals with RF-positive JIA in their cohort.
Inadequate power can cause failure to replicate, but our combined cohort is among the largest used for JIA genetic studies. Our study had 80% power to detect OR of 1.8, 1.5 and 1.4 for C-318T, A49G and CT60 respectively. Furthermore, our meta-analysis failed to find an association between either variant and JIA. The meta-analysis had 80% power to detect an OR of 1.53 and 1.27 for C-318T and A49G respectively, but an association of lower magnitude cannot be ruled out, especially for C-318T. We did observe an uncorrected P < 0.05 for C-318T when we combined our cohort and the cohort reported by Schubert et al. [5
], but this result suggests that the T allele is protective, in contrast to the finding by Miterski et al that it increases susceptibility. Although this finding might reflect a statistical fluctuation, it would be interesting if this observation can be replicated in other JIA cohorts because the T allele at C-318T has been suggested to protect against autoimmune diseases by increasing CTLA4 transcriptional activity [14
variants are associated with adult RA. However, a meta-analysis found that the association between the CT60-G
allele and RA was relatively modest, with an OR of ~1.1 [13
]. When RA patients were stratified, only patients positive for anti-citrullinated cyclic peptide (CCP) antibodies demonstrated a significant association with the CT60 variant, while CCP-negative patients did not show an association [13
]. Only a small proportion of children with JIA have RF or anti-CCP antibodies, and this could explain the lack of association of CTLA4
variants with JIA. Although our RF-positive polyarticular JIA cohort was small, borderline associations were observed between this subtype and both the C-318T and A49G polymorphisms. Furthermore, FBAT revealed an association between the CT60-G/G genotype and RF-positive polyarticular JIA. These observations suggest that while RF-positive polyarticular JIA might share genetic associations with adult RA, the other JIA subtypes are likely distinct phenotypes and consequently have different genetic associations. It is conceivable that genetic factors associated with RF-negative RA are associated with JIA.