Variation in Uteroglobin-Related Protein 1 (UGRP1) gene is associated with Allergic Rhinitis in Singapore Chinese

Allergic diseases such as asthma, allergic rhinitis and atopic dermatitis are global health problems affecting 10-25% of the world's population. Allergic rhinitis (AR) is an IgE-mediated inflammatory disease of the nasal mucosa which is caused by exposure to allergens. AR is characterized by hyper-responsiveness, high levels of Th2 cells [1] and manifestation of symptoms such as rhinorrhea, sneezing, nasal congestion and rhino-conjunctivitis. AR affects approximately 500 million people worldwide [2, 3]. In Singapore, the prevalence of AR was reported to be 13.1% [4]. AR is not a life threatening disease; however its impact on the quality of life and productivity is significant [3, 5]. In addition, AR is known to be associated with other conditions such as asthma, sinusitis, anosmia, otitis media, nasal polyps, lower airway infections and dental malocclusion [2, 6–8].

Many reports support a genetic basis for atopy and allergy [9, 10]. The underlying pathogenic mechanism of allergic diseases is not fully elucidated and may be the result of complex interactions between genetic and environmental factors [2, 6]. Studies looking at twins have provided convincing evidence for a genetic influence, as observed by the greater concordance of allergic manifestations in monozygotic compared to dizygotic twins [11–14]. Non-genetic factors such as an increase in exposure to irritants and allergens, changes in lifestyle, nutrition, pollution and stress, may also influence the onset and development of atopic diseases [7, 15–17]. Individuals with a strong family history of allergic disease are more likely to develop allergic symptoms, irrespective of the varying environmental risk factors across societies [15, 18].

The gene coding for secretoglobulin (SCGB3A2), also known as UGRP1, is located within chromosome 5q31-33. This region of the human chromosome 5 also contains many other candidate genes for allergic diseases such as interleukins - IL-3, IL-4, IL-5, IL-9, IL-13; macrophage colony stimulating factor (CSF) and β₂-adrenergic receptor (ADRB2) [19]. Genetic variations in UGRP1 have been associated with autoimmune diseases such as Hashimoto thyroiditis (HT) and Graves' disease (GD) [20], and allergic diseases such as asthma [21]. A promoter polymorphism in the gene (G-112A) was found to increase the risk of asthma in a Japanese population [19]. However, the association was not replicated in studies involving Indians [22] and German Caucasians [23]. There has not been any association study evaluating the role of these polymorphisms in AR to date. In Singapore a family based linkage study had previously identified the 5q31-33 region to be significantly linked to atopy and asthma in the local ethnic Chinese population [24]. As part of a larger study to fine-map candidate genes for atopy and asthma in this chromosome region, resequencing of the UGRP1 gene was performed, with the aim of identifying novel polymorphisms in Singapore Chinese and evaluating their association to allergic rhinitis and asthma.

UGRP1 was originally suggested to have an anti-inflammatory function due to its similarity to uteroglobin/Clara cell secretory protein [21]. UGRP1 is localized in the chromosome 5q31-32 region where asthma susceptibility locus has been assigned [35] and the high expression of this protein in epithelial cells of the airway [21] suggests a possible role in allergic airway inflammation. Further evidence of the role of UGRP1 is seen in its regulation by various T-regulatory cytokines such as IL-10 [36], IL-5 and IL-9 [37, 38], suggesting its involvement in allergic response. Claire et al., have shown that there is an increase in UGRP1 in the induced sputum of patients with asthma and rhinitis, further suggesting a possible role in inflammatory diseases [39].

The SNP rs7726552 showed significant association with AR in our population. Haplotype analysis revealed a single haplotype block which, when tested for association, was significantly different in cases as compared to controls. This haplotype block, a 4.13 kb region, includes the 5' upstream, promoter and the first intron. The SNP rs7726552 is present in the 5' upstream region and hence could affect the regulation of gene expression levels. Previous reports have shown that such polymorphisms in the UGRP1 gene could predispose an individual to allergic inflammation by reducing the levels of UGRP1 in the airway epithelial cells [19]. Interestingly, the promoter polymorphism G-122A, which was previously associated with asthma in Japanese people [19] was also identified in our population and was designated as UGRP1-G-1351A. However, similar to studies on Indians and German Caucasians, the association to asthma for this SNP was not replicated in the Singapore Chinese population. This could be attributed to underlying differences in the genetic makeup of the populations evaluated. The evaluation of the influence of rs7726552 on UGRP1 expression and regulation would be helpful in identifying the role it may play in allergy. Much is known as to how a gene is regulated and this may involve multiple potential mechanisms such as differential gene splicing or binding of transcription factors to regulatory elements. The presence of SNPs in these regulatory regions might then predispose an individual to disease [40, 41]. The in silico analysis revealed potential binding sites for Oct-1, NF-1 and GATA-1 at the 5' upstream locus where rs7726552 was identified. Oct-1 has been previously shown to be important in regulating the expression of IL13 which is a key regulator of Th2-mediated inflammation in allergic diseases. Kiesler et al., demonstrated that a polymorphism in the regulatory element affects the transcription of the IL13 gene by creating a binding site for Oct-1 [42]. Similarly, Hasegawa et al., demonstrated that the T allele on the Fc epsilon RI alpha-chain promoter introduced an additional binding motif for GATA-1 compared to the C allele and hence the transcription activity of the T allele was enhanced because of the higher affinity for the transcription factor [43]. In a similar fashion, the A to T change in rs7726552 is predicted to result in the loss of the binding site for Oct-1 and conversely introduce an additional binding site for GATA-1. This prediction suggests why the T allele confers protection (OR 0.81) with a higher proportion of the controls (17%) having the allele as compared to the cases (14%). The genotyping results showed more evidence for a dominant model of association producing a P_dominant = 0.0048 and OR = 0.79. The functional significance and the mechanism of the causal variant in UGRP1 leading to the onset of allergic rhinitis needs to be confirmed and validated with further studies.

Haplotype association analysis revealed a common haplotype on UGRP1 to have significant different frequencies between cases and controls. This haplotype block contains 7 SNPs from the promoter region and first intron. Previous reports on the UGRP1 gene have also confirmed the functionality of haplotype to be associated to the promoter region [36] and the regulation of UGRP1 expression in asthmatic patients [19]. Srisodsai et al., have shown that in a mouse model, administration of IL10 increased constitutive UGRP1 mRNA expression and suggested UGRP1 as a potential target for IL-10 anti-inflammatory activities in the lung [36]. Chiba et al., demonstrated that UGRP1 can suppress inflammation in a mouse model for allergic airway inflammation and also proposed UGRP1 as a therapeutic candidate for treating lung inflammation [35]. These reports highlight a potential role for UGRP1 in treatment of allergic airway diseases.

In summary, our study reveals the association of UGRP1 polymorphisms, specifically rs7726552, with allergic rhinitis in the Singapore Chinese population. Haplotype analysis suggests that multiple polymorphisms in the gene could be contributing collectively to the pathogenesis of AR. Further functional characterization of these variants would be important in determining if these variants could be used as risk factors for AR.