Cystic fibrosis (CF) is an autosomal recessive, genetic disorder that affects approximately 85,000 individuals worldwide . This multisystemic disorder is caused by mutations affecting the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) in the epithelial membrane of exocrine glands, which lead to dysfunctional fluid and ion-transport causing a production of thickened mucus . Pathological mucociliary clearance leads to CF lung disease, which over time becomes the major life-shortening factor for CF patients . Chronic plugging of bronchioles with secretions, recurring bacterial infections and pulmonary exacerbations instigate the development and retention of a hostile inflammatory environment in the lungs, leading to tissue breakdown and irreversible lung damage . The most relevant microorganism in CF lungs, Pseudomonas aeruginosa (Pa), provokes a vigorous inflammatory response with neutrophilic infiltration of airways and subsequent damage by the release of proteases and oxidants . This dysregulated chronic state of inflammation in CF airways is sustained by a variety of proinflammatory mediators including TNF-α, IL-1β, IL-6 and IL-8 and leads to a decline in lung function caused by bronchiectasis and irreversible fibrotic remodeling of lung tissue . 98% of all CF patients die of progressive respiratory insufficiency .
Whilst prevalent CFTR mutations are an important determinator for the severity of CF lung disease, the genotype-phenotype correlation between the genetically determined loss of CFTR function and lung function decline is approximately 60% [6, 7]. This suggests that other non-CFTR related factors, such as genetic modifiers with a regulatory effect on the inflammatory response in CF lungs, may also have a significant impact on lung function decline in CF patients.
TGF-β1 has been identified as such a genetic modifier for CF lung disease . Produced by bronchial epithelial cell, this growth factor acts with a localized, modulatory role in the recruitment and activation of neutrophilic granulocytes within a complex network of inflammatory and anti-inflammatory cytokines, thereby regulating inflammatory processes, specifically in context of chronic pulmonary disease . TGF-β1 inhibits the degradation of extracellular matrix by stimulating protease-inhibitors leading to fibrotic reconstruction of lung tissue . Furthermore, it promotes smooth muscle cell hypertrophy and hyperplasia [4, 10].
In a recent study, Sagwal et al have shown that levels of serum TGF-β1 were increased in pulmonary exacerbation phases, in infection with Pa and in subjects with a ΔF508 mutation . TGF-β1 levels decreased significantly after antibiotic treatment of pulmonary exacerbations .
Moreover, it has been shown that TGF-β1 has an inhibitory effect on the biogenesis of CFTR and prevents the functional rescue of delF508-CFTR . In a recent study by Mitash et al., TGF-β1 has been associated with degradation of CFTR mRNA in human bronchial epithelial cells via recruitment of microRNAs to an RNA-induced silencing complex . Snodgrass et al. have shown that TGF-β1 was associated with CFTR inhibition and prevention of functional rescue in human epithelial cells .
However, in vivo levels of TGF-β1 are dependent on specific polymorphisms in the TGF-β1 gene . So far, few studies have investigated the effects of genetic polymorphisms of TGF-β1 on lung function. In the context of CF, three single nucleotide polymorphisms (= SNPs) have been investigated. Each of these polymorphisms, i.e. rs1800469 located in the promotor region and both rs1800470 and rs1800471 located in Exon 1 of the TGF-β1 gene, result in a change in the primary amino acid sequence of the TGF-β1 [6, 13, 14].
In previous studies it was shown that some of these TGF-β1 polymorphic genotypes are associated with higher TGF-β1 expression, a steeper decline in pulmonary function (FEV1) as well as increased pulmonary fibrosis [6, 13, 15, 16]. However, some of the results among these studies are contradictory, as different genotypes were associated with a decrease in pulmonary function and worse clinical status. Furthermore, very little is known about the impact of a TGF-β1 polymorphism-related dysregulation of the signal pathway of TGF-β1 on the complex inflammatory response of the CF airways. It has to be noted, however, that immunological factors contributing to or perhaps even enabling the onset of bacterial infection with Pa, one of the major predictors for mortality and morbidity for CF patients, could not yet be identified .
The primary aim of this study was to investigate whether TGF-β1 SNP genotypes, as modifiers of CF lung disease, can be associated with a faster decline in pulmonary function. To our knowledge, there is no data correlating TGF-β1 phenotypes with the individual FEV1 slopes of CF patients. FEV1 correlates with morbidity and mortality of CF-patients and is a gold standard outcome parameter in routine diagnostics to assess disease progression as well as in clinical studies to investigate the efficacy of new drugs . Furthermore, we wanted to investigate whether TGF-β1 polymorphisms are associated with higher TGF-β1 expression, higher Pa infection rates and elevated levels of proinflammatory cytokines in sputum.