Adrenergic Receptor Variants and RBC Adhesion in SCD

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DESCRIPTION (provided by applicant): At least two major adhesion receptors (LU and LW) of sickle red cells (SS RBC) can be activated through epinephrine-responsive and PKA-dependent signaling. Preliminary evidence shows that SS RBC exposed to agents that increase intracellular cAMP demonstrate visible adhesion to post-capillary vessel walls in vivo after such cells are transfused into athymic mice, while SS RBC not exposed to such agents do not adhere. These data suggest that this signaling pathway is physiologically important. In addition, SS RBC from different individuals show variable responsiveness to epinephrine stimulation in vitro. These findings suggest that variable responses to such agonists may depend on adrenergic receptor (AR) polymorphisms and that these may influence the severity of sickle cell disease (SCD). Furthermore, our preliminary evidence implicates beta2ARs in the process of adhesion receptor activation. We therefore hypothesize that beta2AR polymorphisms affect the degree to which SS RBC are adhesive, and that variability in adhesion contributes to the variability of clinical outcomes in SCD. To test these hypotheses, we propose a pilot project to determine whether beta2AR polymorphisms affect SS RBC adhesion and are associated with more or less severe clinical outcomes or different spectra of end-organ damage in SCD. We will use a pre-existing clinical database and DNA bank to identify the beta2AR genotypes of up to 200 patients with SCD (hemoglobin SS or S beta o thalassemia). We will then use this information and additional blood samples from these patients (1) To establish whether beta2AR polymorphisms affect the degree to which SS RBC exhibit an adhesive phenotype in vitro, both with and without epinephrine stimulation; (2) To identify whether the ability of beta2AR inhibitors to block LW- and LU-mediated adhesion in vitro is dependent on beta2AR genotype; and (3) To establish whether a relationship exists between beta2AR phenotypes and clinical sequelae in SCD. We expect that these studies will lead to an improved understanding of why SS RBC from different patients exhibit a variably active adhesive phenotype and how polymorphisms of beta2ARs affect both RBC adhesion and clinical outcomes in SCD. These results will also help us understand the role of beta2AR polymorphisms in red cell biology and in the pathophysiology of SCD and will help direct future therapeutic approaches to ameliorating the vasoocclusive process in SCD.