An Ile to Met polymorphism in the catalytic domain of adenylyl cyclase type 9 confers reduced ??2-adrenergic receptor stimulation

Abstract

Adenylyl cyclase (AC) mediates signalling following activation of Gαs-coupled receptors such as the β2-adrenergic receptor (β2AR). Genetic variation in the receptor component of this pathway can alter signal transduction and the response to β-agonists in asthma, but little is known about downstream effectors. Here, we characterize the population genomics and signalling effects of a polymorphism within the coding region of the AC9 gene that results in an Ile to Met substitution at amino acid 772 within the C1b region of the enzyme. Allele frequencies were 0.300 and 0.375 in Caucasians and Asians but were lower in African-Americans (0.163). The functional effects were studied in stably transfected HEK293 cells recombinantly expressing equivalent levels of wild-type (Ile772) and polymorphic (Met772) AC9. The polymorphic substitution results in a loss of function compared to wild-type AC9. Met772 AC9 has lower basal and β2AR-mediated adenylyl cyclase activities compared to Ile772 AC9, as well as reduced activity following stimulation of Gαs by NaF. Direct stimulation of AC9 activity by Mn2± was also depressed in Met772 membranes, indicating decreased catalytic function, consistent with the location of residue 772. AC9 mRNA and protein were expressed in multiple human lung cell-types, including airway smooth muscle and airway epithelium. In the treatment of asthma, there is marked heterogeneity in the response to inhaled β-agonists which is associated with polymorphisms of the β2AR. Identification of a common AC9 variant that confers reduced enzyme activity reveals an additional polymorphism that should be considered in pharmacogenetic studies of β-agonist therapy of asthma.