Secretoglobin 1A1 gene and asthma pre‐disposition: what is the evidence?

Asthma afflicts about 10% of children and 5% of adults in Western populations and its prevalence is rising in developing world [1]. It is a complex heterogeneous disease, caused by an unknown number of interacting genes with critical environmental factors. Genetic factors contribute 30–60% of pre-disposition to asthma development [2]. It is likely that different asthma-related phenotypes are caused by different sets of genes; therefore, the pre-disposition to atopy, total serum IgE, eosinophilia, bronchial hyperreactivity, airway remodelling and epithelial dysfunction may be encoded by different genes [3]. Numerous genes and genetic loci have been implicated in asthma, but the magnitude of effect of each gene is unknown, while independent validation of many of the linked loci proved challenging. The fact that the same gene may have different effects in different settings such as different environments or through interacting with different genes, or through effect of age or sex complicates our ability to fully establish the role played by asthma genes. The secretoglobin 1A1 (SCGB1A1) gene encoding a 16 kDa protein (also known as CC16, CC10, CCSP and uteroglobin) has been implicated as a potential candidate gene for asthma because of its genetic location and biological function. SCGB1A1 (uteroglobin) is the founding member of the secretoglobin family of small, secreted, disulphide-bridged dimeric proteins found only in mammals [4]. It is expressed in various tissues, but constitutes one of the major proteins secreted by the respiratory epithelium (making up to 5% of the total protein recovered from bronchoalveolar lavage). It is the predominant product of the non-ciliated (Clara) cells based at the epithelial lining of the pulmonary bronchioles. The Clara cell also secretes several other proteins such as Clara cell 55 kDa protein; surfactant proteins A, B and D; Clara cell tryptase and β-galactoside binding lectine [5]. SCGB1A1 inhibits the chemotaxis of neutrophils and monocytes and controls fibroblasts proliferation and migration [6]. It also inhibits the activity of both secreted and intracellular phospholipase A2, a key regulatory enzyme in the synthesis of prostaglandins and leukotrienes [7]. SCGB1A1 also inhibits in vitro both the production and the biological activity of IFN-gamma, while conversely IFN-gamma itself has been found to increase expression of SCGB1A1 mRNA in mouse lung and the SCGB1A1 protein levels in cultured human airways epithelial cells [8]. SCGB1A1 may play a protective role against the pro-inflammatory response in the lung, and low activity variants of the SCGB1A1 protein may be associated with increased susceptibility to inflammatory lung disorders such as asthma. This view is supported by data showing that asthmatic individuals demonstrate significantly decreased levels of SCGB1A1 protein in the bronchoalveolar lavage and serum when compared with normal controls [9, 10]. The production of SCGB1A1 seems to peak early in life, and SCGB1A1 has an inhibitory effect on Th2 cell differentiation from human infants through an effect on dendritic cells [11]. The role of SCGB1A1 in protection against oxidant stress is also suggested by studies demonstrating a significant increase in lung inflammation and expression of bronchial airway inflammatory markers in SCGB1A1-defficient mice during acute viral infection, or when exposed to oxidant stress factors (ozone, hyperoxia), as compared with SCGB1A1-competent mice [12]. In addition to asthma, an anti-inflammatory role for SCGB1A1 has been reported in other respiratory diseases such as pulmonary fibrosis and chronic obstructive pulmonary disease (COPD). Exploitation of SCGB1A1 as potential therapeutic target in asthma and airway disease does not seem feasible. However, effort has been made to use SCGB1A1 as biomarker of irritant exposure that might precipitate or exacerbate asthma [13]. Patients with allergic rhinitis have lower level of SCGB1A1 in nasal secretions when compared with healthy controls [10]. Frequent infant wheezers also displayed reduced serum SCGB1A1 regardless of their atopic status [14]. Immunohistochemical expression of SCGB1A1 in epithelial cells in small airways (<6 mm) in asthmatics as compared with control non-smokers was significantly reduced [15]. Acute exposure to specific airways irritants such as chlorine seems to induce transient increase in serum SCGB1A1 levels [16]. Serum SCGB1A1 level is also influenced by smoking, sex and age. Variation in methods used to measure SCGB1A1 led to varied reported levels [13]. Individuals carrying specific SCGB1A1 gene polymorphisms, which alter either the production of the coded protein or its activity, maybe at increased risk of developing airway inflammation and asthma. The SCGB1A1 gene in humans has been localized to chromosome 11q12-13, a region which has been linked to atopy and asthma [17]. The gene is 4995 bp long and comprises a 5′ untranslated region of 550 bp and three short exons (118, 187, 135 bp). The first intron contains a polymorphic microsatellite at +2921 to +3110 and intron 2 contains Alu insert, though neither appears to affect the splicing of the mRNA transcript. Mutational screening of the three exons detected an adenine to guanine substitution at position 38 downstream from exon1 (A38G) [18]. Though A38G is an exonic polymorphism, it is not yet known if it has functional implication. Previous data have shown that this polymorphism is associated with altered plasma levels of the SCGB1A1 gene product, though the mechanism for this association remains unclear [19]. Laing et al studied the A38G polymorphism in an unmatched case-control of 67 asthmatics and 46 unaffected children and reported a 6.9-fold increased risk to developing asthma in the SCGB1A1*A allele homozygotes and 4.2-fold increased risk in those with heterozygous AG genotype [20]. A further study of 813 Taiwanese infants reported reduced SCGB1A1 serum level and increased frequency of AA homozygotes in infants with frequent wheezing (>3) as compared with controls. This study could not identify an effect from atopy or smoking on wheezing frequency [15]. Haplotypic analysis involving A38G alleles and an intragenic repeat polymorphism in an adult asthmatic Indian population comprised unrelated individuals and family trios demonstrated positive association with asthma, but individual polymorphism analysis was not significant [21]. A further study in two German paediatric populations (872 including 94 asthmatics and 112 allergic asthmatic children) did not find association with asthma but observed increased prevalence of bronchial hyperreactivity in individuals homozygous or heterozygous for the SCGB1A1*38A [22]. Other studies failed to establish a relation between asthma and SCGB1A markers. Campbell et al. [23] observed no difference in the frequency of microsatellite +2921 to +3110 alleles in adult asthmatics vs. controls in Caucasian population. Mao et al. [24] reported negative association between asthma and the same microsatellite or A38G polymorphism in Caucasian and Japanese adult populations. Mansur et al. [25] found no association between A38G and asthma or bronchial hyperreactivity in 217 unrelated adult Caucasian populations. In this issue of Clinical and Experimental Allergy, Laing et al. [26] report results of cross-sectional and longitudinal association study of SCGB1A1 A38G polymorphism and asthma phenotypes of doctor-diagnosed asthma, airway responsiveness, specific and total IgE, and skin prick testing. A cohort of 231 individuals was recruited from parents unselected for asthma or atopy, before birth and followed up at ages of 1 month, 6 years and 11 years. When compared with wild genotype, individuals expressing AA or AG genotypes (pooled together) had increased risk to develop airway responsiveness at 1 month of age (1.72-fold), raised specific IgE (OR=6.95) and house dust mite (HDM) sensitization at age of 6 years, and doctor-diagnosed asthma (OR=3.93) and cat dander skin sensitization (OR=4.34) at 11 years of age. Longitudinal analysis of paired data at ages of 6 and 11 years indicated that children with AA genotype had increased risk of expressing persistent high serum-specific IgE (OR=11.87) and skin test demonstrated HDM sensitization (OR=7.84). This study adds to the body of evidence implicating SCGB1A1 in childhood asthma and allergy. Arguably, the strength of association demonstrated in this study is not unequivocal to confirm the relationship. This study population is small in terms of genetic studies, but its power is enhanced by its longitudinal design that allows improved phenotyping by minimizing erroneous phenotyping and reducing the potential of wrong labelling of transient symptoms. The study population was recruited pre-birth from unselected and unrelated individuals regardless of parental family history of allergy or asthma. The prevalence of atopy (up to 50.3% at age of 11 years) and asthma (33.6% at age of 6 years) is high and consistent with previous reports [1]. The observation of consistent association with AA and AG genotypes and the biological plausibility of SCGB1A1 strengthen the evidence for this gene's role in childhood asthma. Six phenotypes analysed at three different time intervals in addition to longitudinal analysis is bound to increase the possibility of false positive results (not corrected for multiple testing). However, this study examined a priori hypothesis aimed at replicating previous findings, where adjustment for multiple testing would be considered conservative. Several studies implicated SCGB1A1 gene in asthma, while others failed to do so. Important differences in terms of studied populations heterogeniety, phenotypes and environmental triggers exposure as well as study methodology are likely to be critical. Replication of association and linkage studies in complex diseases such as asthma in general proved difficult, where the power to replicate linkage or association is limited due to poor understanding of the number of genes interacting with non-specified environmental trigger to express the phenotype. It is noticeable that most positive studies of SCGB1A1 gene were conducted on children while adult studies were largely negative, suggesting an association with childhood rather than adult asthma. The reporting of this association by more than one group in different studies strengthens the case for this gene's role in asthma. To establish this unequivocally, however, the following issues need to be addressed. SCGB1A1 gene is located in an important genetic region (11q12-13) where other genes have already been implicated in asthma pre-disposition. The possibility that the observed association between SCGB1A1 and asthma is caused by a nearby gene/s in linkage disequilibrium with SCGB1A1 is not entirely excluded. Polymorphisms within the beta-subunit of the high affinity receptor for IgE (FCERIB) [27] and the glutathione S-transferase P1 (GSTP1) have been associated with atopy and asthma [28]. It is also feasible that more than one gene accounts for the reported linkage between asthma and this genetic region and these may interact. One study reported 22-fold increase in asthma occurrence in individuals carrying SCGB1A1*A38 allele and non-functioning variants of GSTP1, compared with 1.9 odds ratio for SCGB1A1*A38 carriers alone [29]. Laing et al. argues in this issue of Clinical and Experimental Allergy that SCGB1A1 is not in linkage disequilibrium with these genes. To definitely exclude an effect from a nearby gene, collective study of all candidate genes in 11q13 region with emphasis on functional polymorphisms using haplotypic analysis in large well-defined populations is required to confirm the SCGB1A1 role and explore the role of the nearby genes. Additionally, the magnitude of effect played by this gene in asthma pre-disposition (i.e. minor vs. major asthma gene) will remain to be determined. The recent genome screen of childhood asthma did not implicate SCGB1A1 [30]. In conclusion, early identification of individuals at risk and identification of causes may assist in designing effective preventative approach to asthma and allergy development. Establishing the association between this SCGB1A1 gene variant and asthma is a step towards this direction. There is currently accumulating body evidence to support this association but as ever in complex disease genetics, the evidence is patchy. To progress this, further more studies are required to address the issue of linkage disequilibirium across this genetic region on chromosome 11, using ideally functional haplotypic analysis approach incorporating gene–environment model interaction in well-characterized populations. The magnitude of effect of this gene to asthma pre-disposition is currently unknown and requires further study.