The Role of Interleukin 18 Gene Polymorphism in the Development of Atherosclerosis

Abstract

Väitöskirjatutkimuksessa on tutkittu tulehdusreaktiossa välittäjäaineena toimivan sytokiinin (Interleukiini-18) geenin polymorfian eli monimuotoisuuden vaikutusta valtimokovettumataudin kehittymisessä.

Tutkimuksessa havaittiin, että Interleukiini 18 geenin monimuotoisuus vaikuttaa pääosin miesten keskuudessa valtimokovettumataudin varhaiseen kehittymiseen ja sepelvaltimotaudin riskiin. Lisäksi havaittiin, että keski-ikäisten miesten keskuudessa geenin polymorfia vaikuttaa yhdessä verenpainetaudin kanssa selkeästi sydänäkkikuoleman ilmenemiseen.

Background: Atherosclerosis is an inflammatory disease. The process starts in childhood with the accumulation of lipid-laden macrophages (foam cells) onto the intima of the arteries. Atherosclerosis can be asymptomatic for decades, and the subsequent clinical manifestations of the disease vary from lower limb ischemia, myocardial infarction and stroke to sudden cardiac death (SCD). The subclinical development of atherosclerosis can be measured by carotid artery intima media thickness (IMT), carotid arterial elasticity (CAC) and brachial artery flow-mediated dilatation (FMD) which depicts the endothelial function. Within atherosclerotic plaques, antigen-presenting cells (i.e. macrophages) and T lymphocytes are the main players modifying the progression of the disease. Macrophages express interleukin 18 (IL-18) in response to inflammatory stimuli, and IL-18 further activates other cells. The main targets of IL-18 are the CD4+ T lymphocytes. IL-18 in combination with IL-12 stimulates the expression of the highly pro-atherosclerotic interferon-γ (IFN-γ). Both IL-18 and IFN-? have been found to be expressed in human atherosclerotic lesions and linked with the development of atherosclerosis and unstable vulnerable atherosclerotic plaques in murine models and in humans.

Aims: The present study aimed to investigate whether the variation of the IL-18 gene (SNPs rs1946519, rs360717, rs549908, rs4937100, rs5744292 and rs187238) affects 1) the development of subclinical atherosclerosis among a healthy Finnish population (Study I); 2) the expression of angiography-verified coronary artery disease (CAD) and the mortality for cardiovascular causes among a Finnish patient population (Study II); 3) the occurrence of SCD and the artery wall areas of autopsy-verified coronary atherosclerotic lesions (Studies III and IV). Finally, the purpose was to examine 4) whether the traditional risk factors interact with IL-18 gene polymorphism and modify the risk of SCD (Study IV).

Subjects and Methods: The study was based on three clinical and one autopsy study series comprising a total of over 5,000 subjects. In order to explore the frequencies of the most common haplotypes and 5 tag SNPs in the Finnish population, and to investigate the possible association between IL-18 gene polymorphism and the development of subclinical atherosclerosis, we used a study population of the Cardiovascular Risk in Young Finns Study comprising 2,282 young Finnish Caucasian adults selected randomly from the national population registry (Study I). In order to investigate whether the same 5 IL-18 tag SNPs would affect the expression of CAD, we used a subpopulation of patients (n=461) who had undergone coronary angiography in the Finnish Cardiovascular Study. Follow-up data on the whole study population (n=2152) was used to investigate if these IL-18 gene polymorphisms associate with an altered risk of death due to cardiovascular causes (Study II). Furthermore, the material of an autopsy study of 663 men (The Helsinki Sudden Death Study, HSDS) was used to assess whether a known promoter region -137 G/C SNP (rs187238) affects the occurrence of SCD and CAD, and whether this effect is modulated by known risk factors (Studies III and IV). The Tampere Vascular Study, comprising 21 atherosclerotic and 5 control samples obtained from live patients, was further utilized to verify the functional interaction in reference to the expression of IL-18 and IFN-γ (Study IV).

Results: In the Cardiovascular Risk in Young Finns Study, one major haplotype (frequency, 0.24) associated significantly with lower IMT among young healthy Caucasian men. Among women there was no similar association. The sex-by-genotype interaction in relation to IMT was significant. Other polymorphisms or haplotypes were not seen to have any effect on IMT, CAC or FMD values (Study I). Among the patient population of the Finnish Cardiovascular Risk Study, the variation of the IL-18 gene was not observed to associate significantly with cardiovascular mortality. However, among patients who had undergone coronary angiography, one major haplotype (frequency, 0.27) had a different sex-dependent impact on the expression of CAD. Among men, the carriers of this haplotype had a lower occurrence of left main branch CAD, whereas among women the haplotype was not seen to associate with the expression of CAD (Study II). This major haplotype is also the only one carrying the minor T allele of the +127 C/T tag SNP. It is also the only haplotype carrying the minor C allele of the functional promoter region -137 G/C SNP (rs187238).

In the HSDS, the minor C allele of the -137 G/C SNP (rs187238) associated with a lower occurrence of SCD (Study III). Furthermore, hypertension interacts with the -137 G/C SNP, affecting the risk of SCD due to CHD and the development of coronary atherosclerosis. Hypertension was a major risk factor for both the risk of SCD and the development of coronary atherosclerosis among GG homozygotes. Among C allele carriers, we did not find any significant effect. Hypertension also affected the mRNA expression of IL-18 and IFN-γ in human atherosclerotic plaques obtained from live patients. This effect was also seen to be modulated by IL-18 genotype (Study IV).

Conclusions: IL-18 gene polymorphism was observed to affect the development of subclinical atherosclerosis as well as the occurrence of angiography-verified CAD among men but not among women.

Furthermore, a functional IL-18 gene polymor-phism (rs187238) was found to associate with the risk of SCD among men. Hyperten-sion interacted with this promoter region polymorphism and modulated the artery wall areas of coronary atherosclerotic plaques and the risk of SCD due to CHD.