Psoriasis vulgaris (PsV) is a common chronic inflammatory skin disease that is characterized by cutaneous inflammation and keratinocyte hyperproliferation, and it is often accompanied by severe complications such as psoriatic arthritis (PsA)1. It is believed to result from pathological immune activation in genetically prone individuals. Substantial evidence supports a central role for T helper (T_H) cells, such are T_H1 and T_H17 cells, in the pathogenesis of PsV1. A variety of cytokines have also been implicated, including the proinflammatory cytokine tumor necrosis factor alpha (TNF-α), the T_H1-axis cytokines, interleukin (IL)-12 and interferon gamma (IFN-γ), and the T_H17-axis cytokines IL-23 and IL-171.

The importance of single nucleotide polymorphisms (SNPs) that affect cytokine production is well established and has been extensively studied, since SNPs may contribute to the susceptibility, severity, and clinical outcome of various immune-mediated chronic inflammatory diseases. One of the most studied SNPs is the polymorphism at position -308 in the promoter region of the TNF gene (rs1800629), which represents a G to A substitution2. It has been shown that transcriptional activators preferentially bind to the A allele, making AA homozygous individuals high TNF-α producers3. A recent meta-analysis identified variant genotypes and alleles of this SNP as protective in PsV4. Another functional SNP is the +1188 A/C polymorphism of the IL12B gene (rs3212227). The IL12B gene encodes the p40 IL-12B subunit that heterodimerizes with the p35 subunit (IL-12A) to form IL-12, or with the p19 subunit (IL-23A) to form IL-23. The rs3212227 SNP is believed to be functional, since the CC genotype confers increased p40 IL-12B expression and mRNA stability5. Recent genome wide association studies (GWASs) identified a significant association between this IL12B SNP and PsV6,7. A third well-defined functional SNP is the polymorphism at +874 T/A in the IFNG gene (rs2430561). For this SNP, a T to A change results in disruption of a nuclear factor kappa-B binding site, which leads to lower IFN-γ production8.

Although all three functional SNPs in the genes encoding TNF-α, the p40 subunit of IL-12/23, and IFN-γ were associated with PsV and/or PsA in several studies4,9,10, there have also been a considerable number of studies that failed to replicate these results11,12,13,14. Obviously, the influence of various genetic variants on PsV risk may be very complex due to the existence of stratification factors, such as ethnic variance and subtype of the disease. As there is a lack of comprehensive studies in the Serbian population, our aim was to analyze the association between these three functional SNPs and PsV in Serbian patients.

The results of genotyping analysis for patients with PsV and healthy controls are presented in Table 2. The genotype distribution of each of the three cytokine genes examined was similar in patients with PsV and the control population (Table 3). In addition, there were no significant differences between patients with either early onset (type 1) or late onset (type 2) PsV and the control subjects (Table 3). When controls were compared with only those patients with PsV who also had PsA, a borderline significance was observed for the rs3212227 AC genotype (conferring protection) and AA genotype (conferring risk) of the IL12B gene SNP (p=0.049 and p=0.062, respectively), while genotypes for other gene SNPs did not differ between the groups (Table 3). Furthermore, TNF, IL12B, and IFNG allele carriage analysis did not reveal any significant differences between controls and patients with PsV or PsA, or between the two types of PsV (data not shown). However, a decreased frequency of TNF G allele carriers among patients with a low PASI score reinforces the possible risk effect associated with this allele or, alternatively, indicates a protective role of the other allele (A) in PsV (Table 4). None of the TNF, IL12B, or IFNG allele combinations was significantly different in patients compared to control subjects (data not shown).

In this study, we investigated the association of PsV in Serbian patients with polymorphisms in the genes encoding TNF-α, the p40 subunit of IL-12/23, and IFN-γ, which had previously been reported to affect gene expression and to have possible roles in the disease's development and severity. The results of our investigation did not identify any significant difference in allele frequencies or genotype distribution between our patients and healthy individuals.

The important role of TNF-α in the pathogenesis of PsV is well established. However, the influences of SNPs located in the TNF promoter region, such as rs1800629, on altered PsV and PsA risk are less clear. Published studies show conflicting results, with some authors reporting association of this SNP with disease susceptibility4 and others failing to find any association11,14, suggesting that this TNF polymorphism may have diverse effects in a population made up of various ethnicities4. Thus, our result could reflect ethnic specificity in the Serbian population, although another study with more patients is needed in order to confirm this. In addition, the most recent meta-analysis revealed a significant protective effect of the TNF -308 A allele (genotypes AA and AG) in PsV4. This is in line with our finding of an association of the G allele with more severe disease, although the number of patients with low PASI score was small in our study.

Data obtained for IL12B genotypes and alleles did not differ in our cohort of patients in comparison with disease-free individuals. This is in contrast with several studies from Europe, Asia, and America that demonstrated the protective effect of the rs3212227 C allele in PsV/PsA10. Such a discrepancy was probably due to a limited sample size in our study. Indeed, the frequency of the risk-conferring AA genotype in our study was clearly higher among PsV patients than the controls (0.74 vs. 0.67), but the difference did not reach statistical significance. Likewise, associations with the AC and AA genotypes were at the limits of significance when patients with PsA were compared with healthy individuals. Assuming the observed genotype frequencies, a total of more than 570 patients with PsV should have been included in our study in order to reach a power of 80% at a probability of p=0.05, which is beyond the number we could gather in our hospital. Interestingly, similar genotype distributions and a lack of significant difference were observed in a recently published study of Spanish patients with PsV12.

In contrast to TNF and IL12B SNPs, studies analyzing the effects of the rs2430561 IFNG polymorphism on PsV development and severity are scarce, yet our results support previous evidence that this SNP does not directly influence susceptibility to PsV11. Lack of association of this IFNG polymorphism was also found in a comprehensive GWAS that looked for correlation between 438,670 SNPs in 1,539 PsV cases and 1,400 controls13. The only exception among these reports was a study by Baran et al.9 that demonstrated, albeit in a limited number of individuals, the predominance of the TA heterozygote in a healthy group of individuals compared to patients with PsV.

The main caveat of our study is the relatively small sample size. Therefore, some associations could have been missed due to an insufficient number of patients and controls, as was probably the case for the IL12B +1188 A/C SNP. Consequently, our results are in apparent contrast to those from several previously published studies that demonstrated association between some of the SNPs and PsV. We believe that the observed differences are primarily due to the low number of patients in our study, since the odds ratios and confidence intervals obtained in our cohort are comparable to those from most other studies4,10,12. On the other hand, ethnic specificity of the Serbian population with regard to some of analyzed SNPs cannot be excluded. Despite these limitations, our study is relevant because it is the first to analyze the association of gene polymorphisms with PsV development and severity in the Serbian population. It is also one of the rare studies that evaluate the relevance of the IFNG +874 T/A polymorphism in patients with PsV. Therefore, the results of this study will expand genetic knowledge regarding the role of three important functional polymorphisms in PsV, which may prove valuable for future meta-analyses and, ultimately, therapy.