Differential haplotypic expression of the interleukin-18 gene

Interleukin 18 (IL-18) is suspected to play an important role in atherosclerosis and plaque vulnerability. We had previously shown that haplotypes combining two IL18 gene polymorphisms in complete linkage disequilibrium, C-105T (rs360717) in 5'-untranslated region (UTR) and A+183G (rs5744292) in 3'-UTR, were related to IL-18 circulating levels and cardiovascular outcome, the C(-105) G(+183) haplotype being associated with lower IL-18 levels and lower cardiovascular risk. This study was aimed at investigating the functional role of the two polymorphisms and their haplotypes on IL18 expression levels. Allelic imbalance experiments conducted in 24 and 20 subjects heterozygous for the C-105T and the A+183G polymorphisms did not detect any difference when subjects were considered as a whole (-0.009+/-0.044, P=0.85 and +0.114+/-0.082, P=0.18, respectively). However, when splitting individuals according to their haplo-genotype, the haplotype C(-105) G(+183) was associated with a lower expression level than C(-105) A(+183) (-0.287+/-0.076, P=0.005), but did not differ from T(-105) A(+183) (-0.138+/-0.083, P=0.13). The lower expression associated with C(-105) G(+183) was confirmed by real-time reverse transcription-PCR. Transfection of different haplotypic versions of the 3'-UTR did not show any difference in the expression of an upstream reporter gene. A 10-h study of the mRNA degradation kinetics by allelic imbalance with the A+183G polymorphism did not show any differential allelic degradation. In conclusion, the haplotype associated with lower IL-18 circulating concentrations and a lower cardiovascular risk was consistently associated with decreased IL18 expression levels, although the exact functional mechanisms remain to be elucidated.