ß-Adrenergic receptor gene polymorphism in dilated cardiomyopathy. Clinical and functional relevance

Sammanfattning: During normal physiological conditions, catecholamines induce positive inotropic and chronotropic responses in the heart through a ß-adrenergic receptor (AR)-activated pathway. The agonist-bound ßAR selectively interacts with the stimulatory Gs-protein, which activates adenylyl cyclase, catalyzing cAMP formation. The adrenergic nervous system is chronically activated in heart failure (HF). This compensatory mechanism helps to preserve cardiac performance temporarily, by increasing contractility and heart rate. However, in due course, this sympathetic drive will be damaging to the failing heart.The aim of this study was to elucidate genetic variance of the ß1AR and further to analyze how possible polymorphisms might affect the pharmacology and function of the receptor and eventually change the prognosis of, or predict the probability for, idiopathic dilated cardiomyopathy (DCM).Sequence analysis of the entire coding region of the ß1AR was performed and two polymorphisms that resulted in amino acid substitutions were identified. At position 49 in the N-terminal domain, either a serine (Ser49-ß1AR) or a glycine (Gly49-ß1AR) was found. At position 389 of the C-terminal region, either a glycine (Gly389) or an arginine (Arg389) was found. Controls and patients with idiopathic HF were genotyped for known polymorphisms in the ß1-, ß2- and ß3ARs. The genetic variants were equally distributed in patients and controls, indicating that neither one is a major determinant for susceptibility for idiopathic HF. However, the Gly49-genotype was associated with an improved long-term survival in patients with idiopathic HF. The pharmacological and functional consequences of the polymorphism in codon 49 of the ß1AR were analyzed using human embryonic kidney 293 cells, expressing the Ser49- or the Gly49-ß1AR. The Gly49-ß1AR demonstrated characteristic features of constitutively activated receptors, displaying increased affinity for agonists, enhanced potency for agonist-activation of adenylyl cyclase and an increased sensitivity to the inhibitory effect of inverse agonists. The Gly49-ß1AR also had a high propensity for desensitization. The stronger regulation of the Gly49-ß1AR could explain the beneficial effect of this genotype on survival, further supporting the concept that desensitization is protective in HF.Furthermore, we aimed to evaluate the influence of the polymorphism in codon 49-ß1AR for MAPK signaling and myocardial apoptosis. We identified myocyte apoptosis in occasional myocardial cells from patients with DCM. However, we did not find any discrepancy in the frequency of apoptotic cells between patients with different genotypes. Cultured cells expressing the Gly49-ß1AR activate ERK1/2, after stimulation with isoproterenol to a significantly higher extent than cells expressing the Ser49-ß1AR. In addition, the Gly49-ß1AR activated the growth and survival signal ERK1/2 to a higher extent than the Ser49-ß1AR in cultured cells. The protective effect of the Gly49-ß1AR in HF patients may, at least in part, be explained by the enhanced protection, at various levels of ß1AR signaling, to high catecholamine levels. These observations may prove useful in follow-up of patients with cardiovascular disease, the selection and timing of pharmacological therapies, and in the prognostic evaluation and selection of such individuals for cardiac transplantation.