The most significant finding of this study is that PADI4 polymorphisms are associated with RA susceptibility, regardless of anti-CCP as well as erosive joint status. Moreover, significant gene-gene interactions between homozygous PADI4 GTG haplotype and HLA-DRB1 SE alleles were observed for developing anti-CCP-positive and -negative RA. Interestingly, we also observed gene-gene interactions in patients with non-erosive and erosive RA. An additional finding is the lack of gene-environment interaction between PADI4 polymorphisms and smoking. Our findings suggest that homozygous PADI4 GTG haplotype influences RA regardless of joint destruction, and exerts more significant effects on developing RA through interaction with SE alleles.
Several studies and meta-analyses have confirmed the strong association between PADI4
and RA in Asian populations [6-9, 38, 39]. In German and French populations, a weak association between PADI4
and RA was observed [10
]. However, several studies using Caucasian populations have yielded conflicting findings [12
] and it has not yet been demonstrated how the PADI4
polymorphisms influence RA susceptibility. Suzuki et al.
] proposed that a susceptible PADI4
haplotype had significantly increased mRNA stability and half-life compared with a non-susceptibility reference haplotype, and they reported that RA-risk PADI4
haplotype homozygosity was associated with the presence of anti-CCP. Later, it was shown that anti-CCP levels were significantly higher in individuals homozygous for the PADI4
risk haplotype [6
]. Several investigators have speculated that certain PADI4
polymorphisms would enhance citrullination and decrease tolerance for citrullinated proteins, which could lead to the production of anti-CCP and the development of RA [6
]. However, the inconsistent associations between PADI4
polymorphisms and the presence or levels of anti-CCP [6
] raised a question about this hypothesis. In this study, we demonstrated that PADI4
polymorphisms are significantly associated with anti-CCP-positive and -negative RA. Accordingly, the PADI4
gene is more likely to play an important role in another citrullination pathway than its role in anti-CCP formation.
In a recent study, B Hoppe et al.
] performed PADI4 effects on erosive RA in investigation of 373 patients, with non-erosive patients as controls. Interestingly, they found the association of PADI4
SNP with RA was restricted to only patients with joint destruction. However, we also observed that the combination of PADI4
genes and SE alleles increased the risk of developing non-erosive RA as well, which is a result that has not been shown previously. Our results suggest that PADI4
gene is linked to the susceptibility of RA regardless of RA severity, such as erosive joint status. This discrepancy may be due to differences in sample size and the design of the study. Our findings are based on a relatively large size and case-control study, and we think that it might represent a better estimate of results from the risk factors.
Another mechanism proposed for RA association of PADI4
is that PADI4
polymorphisms may interact with an environmental factor, smoking, via citrullinated proteins, resulting in the development of RA. However, the interaction between smoking and PADI4
polymorphisms has not been confirmed, although a possible interaction between only single PADI4
SNP and smoking has been previously reported [30
]. No significant interaction was observed between RA-risk PADI4
haplotype and smoking in this population of Koreans. The number of individuals in our study is fairly large, but the number of smokers with anti-CCP-negative RA is relatively small. This may make conclusions difficult, so additional larger-scale studies need to be performed.
We previously reported that PADI4
SNPs and HLA-DRB1
SE alleles had additive effects in terms of the risk of developing RA, although no significant gene-gene interaction was shown between PADI4
SNPs and SE alleles because of the small sample size [8
]. In this large population, significant interaction was detected between PADI4
risk haplotype homozygotes and SE alleles in both anti-CCP-positive and -negative RA. These results suggest that the homozygous PADI4
risk haplotype contribution to RA pathogenesis may be influenced by HLA-DRB1
SE alleles. These results conflict with a recent finding of no interaction between one PADI4
SNP and SE alleles in a large UK Caucasian population [15
]. The PADI4
polymorphism and SE alleles appear to vary according to ethnicity. This discrepancy between Koreans and Caucasians could be attributed to genetic heterogeneity of RA from ethnic differences. Accordingly, these conflicting results of interaction may be explained by differences in target PADI4
SNP (padi4_89, padi4_90, padi4_92 vs padi4_94) or by differences in the major RA-susceptible SE alleles (for example, *0405 vs *0401) between Korean and Caucasian populations [41