Interleukin (IL) 6 is an important mediator of inflammatory and immune responses, and IL6 gene polymorphisms are known to play a part in chronic inflammatory and autoimmune disorders.^1,2 Increased IL6 plasma levels and enhanced IL6 mRNA expression have been found in patients with active Behçet’s disease.^3,4 Therefore, this study aimed at investigating the associations between Behçet’s disease in Korean people and two functional IL6 gene polymorphisms—namely, a single nucleotide polymorphism at −174 (G → C) in the IL6 gene promoter (IL6prom) and a variable number of tandem repeat polymorphisms in the 3′ flanking region of the IL6 gene (IL6vntr): the terms IL6prom and IL6vntr were designated as in a previous study.⁵

METHODS AND RESULTS

The study group included 89 Korean patients with Behçet’s disease (36 men, 53 women; mean (SD) age 39.1 (8.5)), fulfilling the International Study Group criteria,⁶ and 123 controls (47 men, 76 women; mean (SD) age 43.1 (13.4)). The cumulative history of severe manifestations was investigated during the disease course.⁷ Analyses of IL6prom and IL6vntr were carried out in all the subjects by polymerase chain reaction (PCR)-restriction fragment length polymorphism and PCR genotyping, respectively.^1,8 Significance was evaluated using Fisher’s exact test or t test and defined as p⩽0.05: p values with Bonferroni’s correction (p_corr) were calculated in certain cases. Haplotype and linkage disequilibrium (LD) analyses were assessed using the estimated haplotype (EH) programme.⁹

There was no evidence of genetic association conferred by the IL6prom polymorphism. In the case of the IL6vntr, four genotypes were identified with the following frequencies in the controls: AB, 2 (1.6%); BB, 117 (95.1%); BC, 3 (2.4%); CC, 1 (0.8); and in the patients with Behçet’s disease: BB, 77 (86.5%); BC, 12 (13.5%). Because the vast majority (98.6%) of the subjects had one of the two common genotypes (BB and BC), comparisons between the groups were made with these major genotypes and alleles. There were significant differences in the frequencies of the IL6vntr genotypes and alleles between patients with Behçet’s disease and controls (genotypes: p = 0.005, p_corr = 0.01; alleles: p = 0.022, p_corr = 0.044) (table 1). The odds ratio for Behçet’s disease associated with the C allele of IL6vntr (IL6vntr*C) was 3.5 (95% confidence interval 1.2 to 10.0).

When the studied subjects were stratified according to the results of HLA-B51 testing, significant differences in the IL6vntr genotype and allele frequencies were found only in the HLA-B51 negative subjects (genotypes: p = 0.005, p_corr = 0.01; alleles: p = 0.02, p_corr = 0.04). Using the EH programme, the distribution of haplotypes between patients with Behçet’s disease and controls differed significantly only in those with the IL6prom*G/IL6vntr*C haplotype (p = 0.001, p_corr = 0.004): the odds ratio for Behçet’s disease in the subjects with this haplotype was 7.3 (95% confidence interval 1.6 to 32.9). In addition, the EH programme revealed a D value of 0.08, suggesting the presence of LD at low level between the two polymorphic sites.

No significant associations were found between the genotypes of the two IL6 polymorphisms (IL6prom and IL6vntr) and clinical variables, including disease duration, mean age at onset, clinical manifestations, severe manifestations, and HLA-B51 positivity, in patients with Behçet’s disease (all p>0.05). However, the distribution of the IL6vntr genotype differed significantly between male and female patients, and the frequency of the BC genotype was much higher in female patients with Behçet’s disease than in male patients (BB: male, 45.5% v female, 54.5%; BC: male, 8.3% v female, 91.7%; p = 0.024, p_corr = 0.048).

DISCUSSION

Our data are consistent with previous investigations showing considerable interethnic variability in the distribution of the IL6prom and IL6vntr genotypes.^1,2,10 There was no evidence for genetic association conferred by the IL6prom polymorphism, whereas significant differences in the IL6vntr genotype and allele frequencies were found between patients with Behçet’s disease and controls. These differences were particularly apparent in the HLA-B51 negative subjects or female patients. In addition, susceptibility to Behçet’s disease was increased significantly in subjects carrying the IL6vntr*C allele and the IL6prom*G/IL6vntr*C haplotype. To confirm these findings, further investigations are required in other ethnic populations.