Association between interleukin 6 gene haplotype and Alzheimer's disease: a Brazilian case-control study

Alzheimer's disease (AD) is a progressive neurodegenerative disease that takes the form of a local overexpression of cytokines and other inflammatory molecules. We investigated three single-nucleotide polymorphisms (SNP) of the interleukin 6 gene (IL-6) promoter region [-174G/C (rs 1800795), -572C/G (rs 1800796), and -597G/A (rs 1800797)] in 200 patients with late-onset AD and 165 elderly controls in a Brazilian case-control population sample. Genotyping was carried out from blood cells using PCR-RFLP techniques. Statistical analysis was performed to evaluate the Hardy-Weinberg equilibrium and to compare frequencies between groups. No association was found between any IL-6 polymorphism and AD; however the haplotype composed of the -597 A allele and the -174G allele indicated a crude odds ratio (OR) of 0.15 (p = 0.0021) and a significantly adjusted OR (adjusted for the APOE E4 allele value) of 0.15 (p = 0.00294). Linkage disequilibrium was D' = 0.68 among the three SNPs. Our findings revealed a protective effect of AG (-597A, -174G) haplotype, which worked independently of the APOE E4 allele in our Brazilian population sample. Thus, the promoter region of IL-6 gene probably exerts an effect through gene linkage and/or gene interaction.