Interleukin-6 gene polymorphisms and susceptibility to myocardial infarction: the ECTIM study. Etude Cas-Témoin de l'Infarctus du Myocarde.

There is growing evidence that interleukin (IL) 6 plays an important role in the atherosclerotic process because of its role in mediating immune and inflammatory responses and inducing cell proliferation. The present study examined whether molecular variations at the IL-6 locus are involved in the predisposition to myocardial infarction. The entire coding region, 1,158 bp of the 5' flanking region and 237 bp of the 3' flanking region of the IL-6 gene were screened. We detected three nucleotide substitutions in the 5' region at positions -174 (G/C), -572 (G/C), and -596 (G/A) from the transcription start site, and one insertion/deletion in the 3' region at position +528 after the Stop codon. These polymorphisms were genotyped in the Etude Cas-Témoin de l'Infarctus du Myocarde study comparing male patients (n=640) and age-matched controls (n=719) from Northern Ireland and France. The IL-6/G-174C and IL-6/G-596A polymorphisms were in nearly complete association. Carriers of the IL-6/-174 C allele were more frequent in patients than in controls. The population-adjusted odds ratio for myocardial infarction associated with genotype CC+CG vs. GG was estimated as 1.34. In French patients the number of coronary arteries with greater than 50% stenosis was assessed by angiography. The IL-6/-174 C allele was more frequent in patients with two or fewer stenosed vessels than in patients with three-vessel lesions. These results suggest that genetic variation at the IL-6 locus is associated with susceptibility to myocardial infarction, especially events occurring on less extended lesions. These findings would be compatible with a lower IL-6 secretion associated with the IL-6/-174 C allele, itself or in combination with other promoter polymorphisms, leading to more unstable plaques.

Cooperative influence of genetic polymorphisms on interleukin 6 transcriptional regulation.

Interleukin 6 (IL6) plays key roles in hematopoiesis, immune, and acute phase responses. Dysregulated IL6 expression is implicated in diseases such as atherosclerosis and arthritis. We have examined the functional effect of four polymorphisms in the IL6 promoter (-597G-->A, -572G-->C, -373A(n)T(n), -174G-->C) by identifying the naturally occurring haplotypes and comparing their effects on reporter gene expression. The results indicate different transcriptional regulation in the ECV304 cell line compared with the HeLa cell line, suggesting cell type-specific regulation of IL6 expression. The haplotypes showed functional differences in the ECV304 cell line; transcription was higher from the GG9/11G haplotype and lower from the AG8/12G allele. The differences suggest that more than one of the polymorphic sites is functional; the base differences at distinct polymorphic sites do not act independently of one another, and one polymorphism influences the functional effect of variation at other polymorphic sites. These results show that genetic polymorphisms in the promoter influence IL6 transcription not by a simple additive mechanism but rather through complex interactions determined by the haplotype.

Sequence diversity in 36 candidate genes for cardiovascular disorders.

Two strategies involving whole-genome association studies have been proposed for the identification of genes involved in complex diseases. The first one seeks to characterize all common variants of human genes and to test their association with disease. The second one seeks to develop dense maps of single-nucleotide polymorphisms (SNPs) and to detect susceptibility genes through linkage disequilibrium. We performed a molecular screening of the coding and/or flanking regions of 36 candidate genes for cardiovascular diseases. All polymorphisms identified by this screening were further genotyped in 750 subjects of European descent. In the whole set of genes, the lengths explored spanned 53.8 kb in the 5' regions, 68.4 kb in exonic regions, and 13 kb in the 3' regions. The strength of linkage disequilibrium within candidate regions suggests that genomewide maps of SNPs might be efficient ways to identify new disease-susceptibility genes, provided that the maps are sufficiently dense. However, the relatively large number of polymorphisms within coding and regulatory regions of candidate genes raises the possibility that several of them might be functional and that the pattern of genotype-phenotype association might be more complex than initially envisaged, as actually has been observed in some well-characterized genes. These results argue in favor of both genomewide association studies and detailed studies of the overall sequence variation of candidate genes, as complementary approaches.