Overexpression of Calcineurin in Mouse Causes Arrhythmic Cardiac Death

Deli Dong MSc, LM Semeniuk , Yanjun Duan MD, Jeffery D. Molkentin PhD,
Henry J Duff MD

BACKGROUND AND RATIONALE

Calcineurin, a calcium-activated phosphatase, dephosphorylates a cytosolic protein, NF-AT3, which subsequently translocates into the nucleus where it interacts with GATA-4 to activate a series of hypertrophic genes. Overexpression of calcineurin in transgenic mice (TG) results in cardiac hypertrophy and unexpected deaths. Since the onset of sudden cardiac death is a time-dependent process, this study relates the functional abnormalities to the development of unexpected deaths. Cyclosporin A has been reported to reverse cardiac hypertrophy but not cardiac fibrosis.  This study assessed the effects of chronic in vivo treatment with cyclosporin A on time-dependent development of sudden deaths, and cardiac systolic and diastolic dysfunction.  Since depolarization-activated K⁺ channels contribute dominantly to repolarization in mouse, we also hypothesized that this TG would decrease these K⁺ currents and that in vivo cyclosporin A (CsA), the calcineurin inhibitor, would reverse this. 

RESULTS

None of TG survived beyond 24 weeks (n=38) whereas none of the wildtype (WT, n=47) died. Paradoxically, while cyclosporin A reversed hypertrophy, it exaggerated propensity to sudden death (p<0.05; Kaplan Meier).  Prolongation of repolarization preceded the development of sustained pleomorphic ventricular tachycardia and high-degree atrioventricular block, which occurred during spontaneous sudden deaths.  Both LV internal dimension and LV mass index were significantly increased in TG at day 14. After day 28, relative dilation of the left ventricle occurred as mass index decreased while internal dimension progressively increased.  Sudden deaths developed during progressive dilation and were associated with systolic and diastolic dysfunction. Cyclosporin A treatment reversed the increase in mass index and the dilation. Capacitance measurements of myocyte size showed significant increases in the TG, which are substantially reversed by CsA.  However, CsA did not reverse the systolic and diastolic dysfunction.  Electrophysiologic correlates of these echocardiographic changes were assessed.  Independent of whether the data fits a bi- or a tri-exponential model, the density of I_tof was significantly reduced in TG vs. WT and CsA reversed this effect.  While I_tos and I_Kslow were also reduced in TG, this change was not reversed by CsA.  To assess whether the decreased "repolarization reserve" contributed to arrhythmogenesis, the residual I_Kr was blocked by dofetilide precipitating pleomorphic ventricular tachycardias. 

CONCLUSION

Calcineurin overexpression causes arrhythmic sudden deaths generally temporally associated with systolic and diastolic dysfunction but not with hypertrophy alone.  Arrhythmias are associated with prolonged repolarization and downregulation of all components of I_to.  Cyclosporin A treatment reversed cardiac hypertrophy but did not reverse systolic and diastolic dysfunction and was associated with exaggerated propensity to sudden cardiac death.  CsA reversed downregulation of I_tof but not I_tos or I_Kslow.  The residual I_Kr takes on a more important physiologic role since block of this remaining current precipitates ventricular pleomorphic ventricular tachycardia.