Abstract P084: Cardiac-Specific Overexpression of Human Histidine-Rich Calcium Binding Protein (HRC-S96A) Genetic Variant Impairs Sarcoplasmic Ca2+ Handling, Resulting in Arrhythmias

Disturbed Ca-homeostasis in the cardiomyocyte is a hallmark of human and experimental heart failure. The histidine-rich Ca binding protein (HRC), an intraluminal SR protein is an additional regulator of SR Ca2+ cycling. Previous biochemical studies have shown that HRC can bind to triadin (a component of ryanodine receptors complex) and SERCA2a, suggesting that HRC may be involved in the regulation of SR Ca2+ release and Ca2+ uptake. We recently identified a human HRC genetic variant (S96A), which appears to correlate with ventricular arrhythmias and sudden death in dilated cardiomyopathy (DCM) patients. To gain insight into the physiological and pathological significance of the S96A-HRC variant, we generated transgenic mice with cardiac-specific overexpression of human HRC wild-type (HRC-WT) and S96A mutant (HRC-S96A) in the HRC null (HRC KO) background. We obtained lines with similar expression levels of HRC-S96A and HRC-WT for further characterization. Overexpression of human HRC-S96A resulted in decreased fractional shortening by 22% (7.7 ± 0.5% in HRC-WTs vs. 6.0 ± 0.4% in HRC-S96A), rates of contraction by 20% (75 ± 5 µm/sec in HRC-WTs vs. 60 ± 4 µm/sec in HRC-S96A) and rates of relaxation by 20% (87 ± 6 µm/sec in HRC-WTs vs. 70 ± 4.5 µm/sec in HRC-S96A) compared with HRC-WT. Myocytes isolated from HRC-S96A mice had diminished Ca2+ transient amplitude and delayed half-decay time of the Ca2+ transient. In addition, the frequency of Ca2+ sparks was significantly higher although caffeine-induced SR Ca2+ release (SR load) was reduced in HRC-S96A cells. To determine the effect of S96A-HRC under stress conditions, 5 Hz field stimulation in the presence of 1 µmol/L Isoproterenol was applied: after-contractions were developed in 46% (16 of 35) of HRC-S96A cardiomyocytes, compared to 12% (4 of 33) of HRC-WTs mice. The findings of the present study demonstrate that the human HRC-S96A variant results in impaired myocytes Ca2+ handling, associated with depressed SR Ca2+-uptake rate and increased rate of SR Ca2+ leak, which may destabilize the cells, promoting aftercontractions under stress conditions. Thus, there appears to be a link between this genetic variant and ventricular arrhythmias in DCM human carriers.