Cytokine gene polymorphisms in obstructive sleep apnoea/hypopnoea syndrome

Highlights

• The tumour necrosis factor alpha (TNF-α) (−308A) allele is significantly associated with obstructive sleep apnoea/hypopnoea syndrome (OSAHS).

• Pro-inflammatory cytokine gene polymorphisms are not significantly increased in OSAHS.

• The TNF-α (−863A) allele was significantly associated with obesity in this population.

• The impact of cytokine gene polymorphisms on inflammation in OSAHS is likely to be weak.

Abstract

Objective

The pro-inflammatory cytokines, TNF-α, IL-6, and IL-8 are elevated in obstructive sleep apnoea/hypopnoea syndrome (OSAHS). Cytokine gene interactions are complex and haplotype analysis may be more informative. We hypothesized that the effects of TNF-α in OSAHS might be due to linkage disequilibrium of the TNF-α (−308A) single nucleotide polymorphism (SNP) with other polymorphisms within the TNF-α gene, and that predisposition to elevated IL-6 and IL-8 levels in OSAHS might be attributable to pro-inflammatory IL-6 and IL-8 gene promoter polymorphisms.

Method

173 subjects were classified as having definite OSAHS or not on the basis of apnoea–hypopnoea frequency, sex, age, and symptoms. Population controls comprised 192 random UK blood donors. Genotyping was undertaken for the TNF- α promoter polymorphisms (−1031, −863, −857, −238), two lymphotoxin-α polymorphisms (intron 1 and Thr60Asn), the pro-inflammatory IL-6 gene promoter polymorphism (−174), and IL-8 gene promoter polymorphisms (−251; −781).

Results

There was no significant difference between groups re: genotype/allelic frequency in the genes investigated. Association between disease status and the TNF-α alleles independently (TNF-103, TNF-803, TNF-857, TNF-238) with five haplotypes of TNF-α was not significant (p > 0.05). There was no difference in allelic or genotypic frequencies between obese and non-obese subjects with OSAHS. The TNF- α (−863A) allele alone, was significantly associated with obesity (OR 2.4; CI95% 1.1–5; p = 0.025).

Conclusion

Only the TNF- α (308A) SNP appears to be significantly associated with OSAHS. The impact of cytokine gene polymorphisms on phenotypic expression of inflammation in OSAHS is likely to be complex.

Introduction

There is increasing evidence that obstructive sleep apnoea/hypopnoea syndrome (OSAHS) is associated with hypertension, cardiovascular disease, metabolic derangements, and impaired glucose tolerance [1]. Sleep disruption in OSAHS may contribute to increased susceptibility to cardiovascular diseases. This effect may be exacerbated by an increase in inflammatory activity due to underlying genetic susceptibility.

It has been found that tumour necrosis factor-α (TNF-α) and interleukin-6 (IL-6) are elevated in OSAHS independently of obesity and that the circadian rhythm of TNF-α secretion is disrupted [2], [3]. Other elevated mediators of inflammation include intercellular adhesion molecule-1 (ICAM-1) and C-reactive protein (C-RP) [4]. TNF-α, C-RP, and IL-6 appear to produce harmful effects by inducing endothelial dysfunction. In particular, TNF-α damages endothelial cells, causes apoptosis, and triggers pro-coagulant activity and fibrin deposition. TNF-α also enhances the production of reactive oxygen species including inducible nitric oxide (NO) [5].

There could be a possible genetic propensity towards increased pro-inflammatory cytokine production in OSAHS. Ryan et al. demonstrated in a cell culture model the selective activation of nuclear factor kappa B (NFκB)-dependent inflammatory pathways through intermittent hypoxia and reoxygenation [6]. In another study, reversing OSAHS using continuous positive airway pressure resulted in a drop in the levels of circulating NFκB-dependent cytokines, specifically TNF-α and IL-8 [7]. However, no consistent associations with OSAHS were found with a number of cytokines, including IL-1, IL-6, IL-8, IL-10, IL-12, and interferon gamma (IFN-γ).

These findings are supported by our previously published work [8], which showed an independent association of the pro-inflammatory TNF-α (−308A) allele with the diagnosis of OSAHS. More recently, Bhushan et al. showed significantly higher levels of TNF-α (−308A) and serum TNF-α levels in obese Asian Indians with obstructive sleep apnoea compared to obese controls [9]. Taken together, results from these studies suggest a disease-promoting role for TNF-α in OSAHS.

TNF-α (−308A) is in linkage disequilibrium with human leucocyte antigen (HLA) class I and II alleles; the class III region, which encodes several components of the complement system; and the major histocompatibility (MHC) class IV cluster, which includes lymphotoxin-α (one of five microsatellites within the TNF locus) and lymphotoxin-β [10]. The association of TNF-α (−308 A) with OSAHS may, therefore, be due to the direct influence of the −308 (A-G) single-nucleotide polymorphism (SNP) in question and/or due to linkage disequilibrium with other polymorphisms within the TNF-α gene or other genes within the HLA system.

Since the publication of our paper [8], evidence has emerged that SNP cytokine interactions are complex and that haplotype analysis may be more informative.

The aims of this study were to investigate the hypothesis that inflammatory consequences associated with TNF-α in OSAHS are due to linkage disequilibrium of the TNF-α (−308A) SNP with the TNF-α promoter polymorphisms (−1031, −863, −857 and −238) and the lymphotoxin-α polymorphisms (intron 1 and Thr60Asn). Based on the studies that have found elevated levels of pro-inflammatory cytokines IL-6 and IL-8 in OSAHS [7], [11], we also hypothesized that this might be due to a predisposition attributable to the pro-inflammatory IL-6 gene promoter polymorphism (−174) and IL-8 gene promoter polymorphisms (−251 and −781).