Variable Length Poly-C Tract Polymorphisms of the β2-adrenergic Receptor 3′UTR Alter Expression and Agonist Regulation

摘要

β2-adrenergic receptors (β2AR) expressed on airway epithelial and smooth muscle cells regulate mucociliary clearance and relaxation, and are the targets for β-agonists in the treatment of obstructive lung disease. However, the clinical responses display extensive interindividual variability, which is not adequately explained by genetic variability in the 5′-flanking or coding region of the intronless β2AR gene. The nonsynonymous coding polymorphism most often associated with a bronchodilator phenotype (Arg16) is found within three haplotypes that differ by the number of C’s (11, 12 or 13) within a 3′UTR poly-C tract. To examine potential effects of this variability on receptor expression, BEAS-2B cells were transfected with constructs containing the β2AR (Arg16) coding sequence followed by its 3′UTR with the various polymorphic poly-C tracts. β2Arg16-11C had 25% lower mRNA expression and 33% lower β2AR protein expression compared to the other two haplotypes. Consistent with this lower steady-state expression, β2Arg16-11C mRNA displayed more rapid and extensive degradation after actinomycin D treatment compared to β2Arg16-12C and 13C. However, β2Arg16-12C underwent 50% less downregulation of receptor expression during β-agonist exposure compared to the other two haplotypes. Thus, these haplotypes direct a potential low-response phenotype due to decreased steady-state receptor expression combined with wild-type agonist-promoted downregulation (β2Arg16-11C), and, a high-response phenotype due to increased baseline expression combined with decreased agonist-promoted downregulation (β2Arg16-12C). This heterogeneity may contribute to the variability of clinical responses to β-agonist, and genotyping to identify these 3′UTR polymorphisms may improve predictive power within the context of β2AR haplotypes in pharmacogenetic studies.