Aim To study the association between smoking and IL13 gene polymorphisms with psoriatic arthritis (PsA) and psoriasis.

Methods The authors genotyped three groups of Caucasians: those with PsA, those with psoriasis without arthritis (PsC) and healthy controls for the rs20541 and rs848 IL13 single nucleotide polymorphisms (SNPs). An additional SNP, rs1800925, was genotyped only in the PsA and PsC groups. The differences in allelic distributions were compared by χ² test. The prevalence of smoking was compared between people with PsA and those with PsC. The combined effect of genotype and smoking was tested by comparing the frequencies of different combinations of rs1800925 genotype and smoking status in PsA and PsC.

Results 555 PsA patients, 342 PsC patients and 217 healthy controls were included in the study. Smoking was less prevalent in patients with PsA compared with PsC (47.4% vs 59.4%, p<0.0006). rs20541*G and rs848*C alleles were associated with PsA compared with controls (OR 1.64, p=0.0005, OR 1.61, p=0.0007 respectively). The association between these alleles and PsC compared with controls was only of borderline significance (OR 1.33, p=0.06, OR 1.26, p=0.11 respectively). Two major alleles, rs1800925*C (OR 1.28, p=0.045) and rs848*C (OR 1.30, p=0.047) were increased in PsA compared with PsC. The combination of non-smoking and the genotype rs1800925*CC was associated with increased susceptibility to PsA compared with PsC. Among smokers, rs1800925*CC was not associated with PsA compared with PsC.

Conclusions IL13 gene polymorphism is associated with increased susceptibility to PsA in psoriasis patients.