In this manuscript, we identified a non-coding SNP (rs892940) located 5’ of the THRB
gene that is associated with response to β2
-agonists in the childhood asthma trial (FBAT p value = 0.001, population-based p value = 0.09) and replicated this association in two adult asthma populations (combined p value of 0.0012 in three replication populations and 0.0007 in all populations). Previous work by our group demonstrated that the expression of this gene is altered by exposure to a β2
-agonist in human airway epithelial and smooth muscle cells, co-treated with pro-inflammatory cytokines and LTD4, which are known to be elevated in asthmatic patients.6
Taken together, this thyroid hormone receptor gene is a novel candidate for regulation of variable response to a common asthma therapy. Further studies are necessary to determine if the associated SNP or any variant in LD with it may regulate the expression or activity of the THRB
gene in response to bronchodilators. Genetic variants associated with BDR may facilitate genetic tests for predicting individual asthma therapy outcomes.
gene is located on chromosome 3p24.2, encoding for the β subunit of the thyroid hormone receptor, which is one of 2 genes (α and β) that code for several isoforms.18
The thyroid hormone receptor is located in the nucleus and upon binding to the thyroid hormone, regulates (both repress and activate) transcription through binding to T3 response elements either as a homodimer or heterodimer with retinoid X receptor beta (RXRB
). The thyroid hormone, mediated through activation of its receptor, has been implicated in the growth and development of the lung as well as other organs in pre- and post-natal stages.19, 20
In a study of rats treated with this hormone, one group showed increased relaxation of the renal artery smooth muscle along with elevated cyclic AMP, nitric oxide synthase (NOS) and NO, which is a potent vasodilator.21
Thus, genetic variants in THRB
may affect the expression of this receptor and have wide-spread downstream effects on transcription regulation that may contribute to inflammation, constriction of the bronchial smooth muscle and obstruction of the airways. However, given the multiple protein isoforms, an earlier knockout mouse study demonstrated biological redundancy of the receptor activity.2222
In addition, the biological effect of a potentially regulatory mutation, which may alter the level of the wild-type protein in specific cells depending on the available transcription machinery, likely differs from a non-synonymous variant that alters the protein function in all cells expressing the gene. Thus, variable expression of the thyroid hormone receptor beta isoform may be cell specific and may not have the detrimental effects of a coding variant or another gene without functional redundancy. The mechanism by which THRB
modulates BDR is unknown and further investigations are necessary to determine its role in β2
A limitation of our study was the sample sizes of the asthma trials, especially for LOCCS (n = 159) and LODO (n=155), which may have reduced the power to detect genetic associations. To compensate for the reduced power, we selected only those SNPs associated with BDR in both family-based and population-based analyses in CAMP to carry forward for replication. In addition, there were ascertainment biases of the replication populations, which may have contributed to heterogeneity across the cohorts. Specifically, participants in the LOCCS trial were previously treated with glucocorticoids and consequently, had well-controlled asthma compared to the other trials. Glucocorticoid treatment has been shown to alter arginine metabolism by inhibiting the induction of NOS by cytokines, thereby reducing NO production, resulting in improved lung function.23
This may explain, in part, for the lower mean BDR and more normalized BDR distribution observed in the LOCCS trial compared to the other populations. Also, approximately 60% of LODO participants were taking a controller medication such as a long-acting β2
-agonist that could modify BDR. Finally, the Sepracor trial recruited only high responders to albuterol (BDR ≥15%). As a result of the heterogeneity in BDR distributions across these studies, we dichotomized the phenotype using the median value of each study to distinguish responders from non-responders, which differ from the conventional thresholds for classifying responders from non-responders.16, 17
The reproducibility of our association results across the three replication trials, given the population heterogeneity, makes our study more robust. Moreover, whereas the initial association analyses were conducted in a childhood asthma population, the replication trials were composed primarily of adults, but each included some childhood cases.
The fact that multiple SNPs across a number of genes were only modestly associated with BDR and no stronger haplotype effect within these genes were found suggests that the genetic associations identified in this manuscript are likely due to linkage disequilibrium (LD) with the causative variant(s). Further studies are necessary to determine the functional role, if any, of the associated SNP in THRB on the expression of this gene or if it is in LD with other potentially functional variants.
The identification of TFs which modulate BDR provides a better understanding of the inter-individual variability in response to β2-agonists, the most common class of asthma medications, as well as novel therapeutic targets for better symptom control. For example, antagonists, inhibitors or small interfering RNAs may be used to alter the expression of a specific TF gene. However, to date, few general TFs have been associated with asthma and asthma pharmacogenetics (that is, vitamin D receptor) since over-expression or suppression of such proteins are expected to result in wide-spread adverse effects. Therapeutic interventions to regulate the expression of TFs (e.g. antisense oligonucleotides, TF decoys) would have to be cell-specific such as via aerosol or intra-tracheal administration, which specifically targeting TF expression in human lung cells such as airway epithelial and ASMCs only, without affecting gene expression in other cell types or organs. Further studies are necessary to improve the administration of such therapies in humans to minimize adverse effects and optimize therapeutic benefits.