Common ADRB2 Haplotypes Derived from 26 Polymorphic Sites Direct β2-Adrenergic Receptor Expression and Regulation Phenotypes

Abstract

BackgroundThe β2-adrenergic receptor (β2AR) is expressed on numerous cell-types including airway smooth muscle cells and cardiomyocytes. Drugs (agonists or antagonists) acting at these receptors for treatment of asthma, chronic obstructive pulmonary disease, and heart failure show substantial interindividual variability in response. The ADRB2 gene is polymorphic in noncoding and coding regions, but virtually all ADRB2 association studies have utilized the two common nonsynonymous coding SNPs, often reaching discrepant conclusions.Methodology/Principal FindingsWe constructed the 8 common ADRB2 haplotypes derived from 26 polymorphisms in the promoter, 5′UTR, coding, and 3′UTR of the intronless ADRB2 gene. These were cloned into an expression construct lacking a vector-based promoter, so that β2AR expression was driven by its promoter, and steady state expression could be modified by polymorphisms throughout ADRB2 within a haplotype. “Whole-gene” transfections were performed with COS-7 cells and revealed 4 haplotypes with increased cell surface β2AR protein expression compared to the others. Agonist-promoted downregulation of β2AR protein expression was also haplotype-dependent, and was found to be increased for 2 haplotypes. A phylogenetic tree of the haplotypes was derived and annotated by cellular phenotypes, revealing a pattern potentially driven by expression.Conclusions/SignificanceThus for obstructive lung disease, the initial bronchodilator response from intermittent administration of β-agonist may be influenced by certain β2AR haplotypes (expression phenotypes), while other haplotypes may influence tachyphylaxis during the response to chronic therapy (downregulation phenotypes). An ideal clinical outcome of high expression and less downregulation was found for two haplotypes. Haplotypes may also affect heart failure antagonist therapy, where β2AR increase inotropy and are anti-apoptotic. The haplotype-specific expression and regulation phenotypes found in this transfection-based system suggest that the density of genetic information in the form of these haplotypes, or haplotype-clusters with similar phenotypes can potentially provide greater discrimination of phenotype in human disease and pharmacogenomic association studies.

Untangling asthma phenotypes and endotypes

“…Other haplotypes showed increased agonist-promoted down-regulation of beta2AR protein expression (down-regulating phenotypes), which might be relevant in chronic use of beta2-agonists. Finally, some haplotypes showed the ideal clinical combination of high expression and less down-regulation (73).…”

Pharmacology and Therapeutics of Bronchodilators

“…Recently, Panebra et al (2010) have used the most common ADR␤2 haplotypes (allele frequency of 0.05 or greater) found in a transfection-based system to ascertain expression and agonist-promoted down-regulation. They observed that the initial bronchodilator response from intermittent administration of ␤-AR agonist may be influenced by certain ␤ 2 -AR haplotypes (expression phenotypes), whereas other haplotypes may influence tachyphylaxis during the response to chronic therapy (down-regulation phenotypes).…”

Pharmacogenomics of long-acting β2-agonists

“…ADRB2, located on chromosome 5q31-33, encodes a protein containing 413 amino acids. At least 50 polymorphisms have been identified [89][90][91][92]. Sequencing of ADRB2 in 2006 yielded 49 polymorphisms of which 21 were novel and had frequencies less than 10% in all ethnic groups (except the variant at position --3159 which occurred at greater than 15% in all ethnic groups) ( Figure 4) [11,89].…”

“…This variable-length poly-C tract starts at nucleotide 1266 (or 1269); repeats of 10, 11, 12, 13, and, rarely, 14 C are found in Whites and African American patients with asthma, with the highest frequency of 34 and 64%, respectively, for the 13 poly-C repeat [89,91,99]. This sequence is interrupted by two polymorphisms at the 2nd and 10th C-repeat which yields additional haplotypes [89].…”

“…Conversely, after agonist exposure, the Arg16-12C construct has 50% less receptor expression downregulation [100]. A follow-up study of eight haplotypes constructs from 25 polymorphisms in the promoter, 5¢UTR, coding region, plus variants of the 3¢UTR poly C yielded eight haplotypes for study [91]. Two haplotypes had both increased b 2 -receptor expression and less b 2 -agonist promoted downregulation and had the fewest poly-C repeats; patients with these haplotypes are speculated to have the best overall response to b 2 -agonist treatment.…”