Abstract 925: CAPON, a Nitric Oxide Synthase Regulator Associated with QT Interval Variation in Humans, Modulates Cardiac Repolarization in Ventricular Myocytes
Background: A common genetic variant in a putative regulatory region of the neuronal nitric oxide synthase regulator CAPON gene has recently been associated with differences in the electrocardiographic QT interval in a human genome-wide study. As CAPON was previously unsuspected to play a role in the heart, we sought to identify CAPON in cardiomyocytes and characterize the biological effects of CAPON overexpression in cardiomyocytes.
Methods and Results: Endogenous expression of CAPON protein was documented by immunofluorescence microscopy of freshly-isolated guinea pig left ventricular (LV) cardiomyocytes and western blot analysis of LV myocardium. For patch-clamp studies, the CAPON cDNA was cloned from guinea pig LV myocardium and then subcloned into an adenoviral vector (AdCAPON-GFP) coexpressing the reporter eGFP. In vivo gene transfer to the guinea pig heart was performed by direct injection of AdCAPON-GFP into the LV apex. Action potential duration (APD) and ionic currents were recorded from cardiomyocytes isolated five days after viral transduction. In CAPON-transduced cardiomyocytes, the APD90 was shortened to 284 ± 21 (n=8) and 240 ± 8 (n=9) ms at 0.05 Hz and 1 Hz, respectively, compared to 383 ± 25 (n=9) and 313 ± 17 (n=12) in non-transduced myocytes (p=0.004 and 0.002, respectively). APD shortening was mediated by a reduction in peak L-type calcium current density in CAPON-transduced cardiomyocytes (−7.2 ± 0.5 pA/pF, n=8), compared to the non-transduced cardiomyocytes (−11.2 ± 1.0 pA/pF, n=12, p=0.006). Interestingly, there was also a reduction of delayed rectifier tail current density at -40 mV (1.1 ± 0.1 pA/pF in CAPON-transduced cardiomyocytes (n=9), compared to 1.8 ± 0.3 pA/pF (p=0.03) in non-transduced cardiomyocytes (n=10)), while current density of the inward rectifier was not changed.
Conclusions: We demonstrate endogenous CAPON protein expression in LV cardiomyocytes, and show that overexpression of this protein accelerates cardiac repolarization via a reduction of L-type calcium current (which prevails over a concomitant reduction in IK). Our findings motivate the hypothesis that CAPON gene variants affect the expression level of CAPON protein in the heart and thereby influence cardiac repolarization.