SERCA2a gene transfer to the failing heart improves its mechanical function and inhibits arrhythmic triggers caused by aberrant calcium waves. The majority of arrhythmias occur before onset of end-stage heart failure when SR function is not compromised. Examination of the electrophysiological (EP) consequences of SERCA2a gene therapy in normal hearts is imperative in determining its potential toxicity as a preventive therapy during early stages of remodeling. We hypothesized that AAV9 mediated gene transfer of SERCA2a to normal rat hearts may promote the incidence of arrhythmic triggers under conditions of intracellular calcium (Ca2+) overload.
Methods: Rats underwent tail vein injection of 5x10E11vg AAV9.SERCA2a (N=8) or AAV9.GFP (N=3). 6 naïve uninjected rats served as controls (Ctl). Using high resolution optical action potential (AP) imaging, we characterized the EP substrate 6 weeks post gene transfer. Ex vivo hearts were examined during normoxic perfusion and challenge with low flow ischemia for 1h followed by reperfusion. Arrhythmia susceptibility was determined under conditions of Ca2+ overload.
Results: Surprisingly, AAV9.GFP hearts exhibited markedly prolonged AP durations (APD) and slower conduction velocities (CV) compared to AAV9.SERCA2a and Ctl hearts. Challenge of AAV9.GFP but not AAV9.SERCA2a or Ctl hearts with burst pacing always resulted in VF. Remarkably, APD & CV rate relationships were identical in Ctl and AAV9.SERCA2a hearts (p=NS). Elevation of pacing rate resulted in a comparable level of AP alternans followed by loss of 1:1 capture in both groups. No evidence of triggered APs was found in AAV9.SERCA2a or Ctl hearts in response to burst stimulation. Ischemia for 1hr produced comparable changes in APD and CV. Challenge with burst stimuli during ischemia elicited arrhythmias in 2/6 Ctl and 6/8 AAV9.SERCA2a hearts (p=0.277, NS). Upon reperfusion, 1/4 & 0/2 remaining Ctl and AAV9.SERCA2a hearts exhibited VF.
Conclusion: Surprisingly, AAV9.GFP but not AAV9.SERCA2a gene transfer to normal rat myocardium alters EP properties and promotes Ca2+ mediated arrhythmias. Lack of an EP phenotype of AAV9.SERCA2a suggests an upper limit of endogenous SERCA2a expression in normal rodent myocardium that is not exceeded by gene transfer.