Production of the proinflammatory cytokine interleukin 1β (IL1β) increases in several acute or chronic diseases.1 The IL1β gene is a member of the IL1 gene family clustered on chromosome 2.1 A C/T polymorphism in the promoter region of the IL1β gene at position −511 influences the protein production, with the highest levels in T allele carriers.2
Data suggest that inflammation plays a role in the pathogenesis of subarachnoid haemorrhage (SAH) from ruptured aneurysm.3 We hypothesise that individuals genetically predisposed to an exaggerated cytokine production may be at risk of aneurysmal SAH. We studied the significance of IL1β –511 C/T polymorphism in patients with aneurysmal SAH and in healthy controls.
We included 231 unrelated patients with aneurysmal SAH, of a total of 401 patients with SAH admitted to the Institute of Neurology, Krakow, Poland between 2003 and 2005. Patients with dissecting and fusiform aneurysms (n = 10), arteriovenous malformations (n = 30), SAH of unknown origin (n = 39), those who were comatose on admission (n = 51), and those who did not agree to participate in the study (n = 40) were excluded.
The diagnosis of aneurysmal SAH was confirmed by cranial computed tomography or lumbar puncture, or both, and in each case by digital subtraction angiography. We also included 231 unrelated controls, matched for age (±1 years) and sex with the patients, without any stroke history. Both the cases and the controls were white and came from southern Poland. Before inclusion in the study all participants gave their informed consent. The study was approved by the university ethics committee and was carried out in accordance with the Helsinki Declaration of 1975, as revised in 1983.
We collected demographic data and risk factor profiles for all subjects, as described elsewhere.4 The procedure for genotyping the polymorphism studied has already been reported.5
Data on continuous characteristics are expressed as means (SD) and on categorical characteristics as per cent values or absolute numbers. Comparisons between groups were made with the χ2 test or Student's t test. A probability (p) value of <0.05 was considered significant. Hardy–Weinberg equilibrium was tested for by the χ2 method. The association of the polymorphism studied with the risk of aneurysmal SAH was investigated by logistic regression analysis. For multivariate risk predictors, adjusted odds ratios (OR) and 95% confidence intervals (CI) are reported. The calculations were undertaken using the commercial statistical package “STATISTICA for Windows”, v.6.0 (StatSoft Inc, Tulsa, Oklahoma, USA).
Genotype frequencies in the patients and controls were in Hardy–Weinberg equilibrium. The TT genotype was more often present in patients with aneurysmal SAH than in their controls. There was no difference between the studied groups when the frequencies of CC genotypes were compared with CT and TT genotypes taken together. The T allele was overrepresented in the cases. Patients with aneurysmal SAH presented significantly more often with hypertension, smoking, and excessive alcohol intake (table 1).
Table 1 Risk factors, interleukin (IL) 1β genotypes and alleles in patients with aneurysmal subarachnoid haemorrhage and their controls.
| Controls (n = 231) | Cases (n = 231) | p Value | |
|---|---|---|---|
| Age (years) (mean (SD)) | 50.4 (12.4) | 49.9 (12.7%) | *0.66 |
| Female | 132 (57.1%) | 132 (57.1%) | †1.00 |
| Hypertension | 80 (34.6%) | 125 (54.1%) | †<0.00001 |
| Smoking | 98 (42.4%) | 147 (63.6%) | †<0.00001 |
| Excessive alcohol intake | 10 (4.3%) | 35 (15.2%) | †0.0001 |
| IL1β genotype | |||
| CC | 111 (48.1%) | 100 (43.3%) | †CC v CT+TT, p = 0.3 |
| CT | 106 (45.8%) | 99 (42.85) | †CC+CT v TT, p = 0.006 |
| TT | 14 (6.1%) | 32 (13.9%) | |
| IL1β alleles | |||
| T allele frequency | 134 (29.0%) | 163 (35.3%) | †C v T, p = 0.041 |
Values are n (%) unless stated otherwise.
*Student t test.
†χ2 test.
Logistic regression analysis showed that the TT genotype (OR = 1.98 (95% CI, 1.001 to 3.99), p = 0.049), hypertension (OR = 2.58 (1.69 to 3.94), p = 0.00001), smoking (OR = 2.11 (1.42 to 3.17), p = 0.0002), and excessive alcohol intake (OR = 3.56 (1.62 to 7.8), p = 0.001) independently affected the risk for aneurysmal SAH.
We found that the TT genotype of IL1β –511 C/T polymorphism increased the risk of aneurysmal SAH in a Polish population. The p value in the multivariate analysis, in contrast to the univariate analysis, is at the limit of significance, what may reflect the correlation between hypertension and the polymorphism studied (TT carriers in the control group were more likely to have hypertension than the other genotype carriers (57.1% v 33.2%, p<0.05)).
It is suggested that inflammation is involved in the pathogenesis of ruptured intracranial aneurysms. The endothelium of ruptured aneurysms is invaded with macrophages and leucocytes.3 It is possible that inflammation is not only a direct response to bleeding after aneurysm rupture, but could have existed before the rupture. Inflammatory and immunological reactions are also present in unruptured aneurysms.3 The increased risk of aneurysmal SAH in TT carriers of the IL1β –511 polymorphism may be a result of either local or peripheral inflammation.
The increased local production of IL1β, more pronounced in TT carriers, may exaggerate the fragility of the aneurysm wall by stimulating the production of other proinflammatory molecules1 or by a decrease in collagen production, which has been shown to occur in the aneurysm wall.3 It cannot be excluded that chronic systemic inflammation, which is more pronounced in TT carriers, increases the risk of aneurysmal SAH. However, this has not been examined so far in this type of stroke.
Genetic association studies have an important risk of false positive findings. Our results should be replicated in an independent population to confirm the association.
Footnotes
Competing interests: none declared
References
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