Pathogenic Role of IL-18 in Sickle Cell Cardiomyopathy and Inducible Ventricular Tachycardia

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ABSTRACT: Individuals with sickle cell disease exhibit cardiovascular manifestations and sudden death as the top causes of premature death. These factors, unfortunately, contribute to the plateauing of the average life expectancy of these patients (in the 4th decade) over the past two decades, yet another profound health disparity observed in African Americans (AAs). Despite expanded understanding of the defining features including systemic vaso-occlusive episodes and hemolysis, there is a paucity of information linking cardiac pathology to premature death. Using system biology approaches, we have generated highly novel information characterizing a previously unrecognized human sickle cardiomyopathy defined by myocardial fibrosis, diastolic dysfunction, prolonged repolarization, and inducible ventricular tachycardia (VT) in the “humanized” sickle mouse model. These studies further demonstrated significant upregulation of circulating IL18 gene expression, an established inflammasome and pro-fibrotic mediator upregulated by free heme, in sickle cardiomyopathy. Additionally, exposure to IL-18 was a key factor in inducing VT in sickle mice. Preliminary data further link decreased expression and activity of cardiac potassium channels (KCND2/KCND3) in sickle mice, which can prolong repolarization, to acute increases in IL-18-mediated NADPH oxidase 4 (Nox4) expression, the latter a key source of reactive oxygenation species (ROS) and induction of cardiac apoptosis. We have further shown that chronic IL-18 inhibition reduces cardiac apoptosis, fibrosis and improves diastolic function in sickle mice coupled with reduced cardiac IL-18 receptor (IL-18R) and Nox4 expression. Finally, our genomic studies have identified novel polymorphisms (SNPs) associated with enhanced IL18 expression and prolonged corrected QT (QTc) interval, an established risk factor for VT. Thus, via three specific aims (SAs), this R01 will interrogate the mechanistic basis for the hypothesis that IL-18/IL- 18R/Nox4 signaling critically downregulates KCND2 and KCND3 function acutely and promotes myocardial fibrosis with sustained activation, exacerbating sickle cardiomyopathy and VT development. SA #1 will functionally validate heme-mediated IL18 promoter regulation including SNPs in a monocyte cell line. SA #2 will define how IL-18/IL-18R/Nox4 signaling acutely downregulates KCND2/KCND3 function leading to prolonged repolarization and chronically, results in cardiac apoptosis and fibrosis. SA #3 will define the therapeutic efficacy of strategies to prevent sickle cell-associated inducible VT. The knowledge gained from this R01 will directly translate into future clinical biomarker studies evaluating risk of sudden cardiac death highlighting those patients with a higher hemolytic burden, pathogenic IL18 SNPs, and circulating IL-18 levels with theoretically higher VT risk. Additionally, the data will test for effective and novel personalized therapies in a poorly recognized and fatal manifestation of sickle cell disease.