Background: Arrhythmogenic cardiomyopathies (ACM) are inherited genetic disorders characterized by a high risk of life-threatening arrhythmias. ACMs originate from mutations in desmosomal genes that prevent normal myocyte-myocyte adhesion and cause the loss of cardiac function, classically in the right ventricle (RV), due to progressive replacement of the cells by fibrofatty infiltration.
Hypothesis: Our aim was to define the outcome of plakophilin-2 gene mutations on myocyte contractility.
Methods: We created a knock-in mouse model based on a single thymidine insertion in the coding sequence of the plakophilin-2 (PKP2) gene, which resulted in the expression of a truncated PKP2-L404/X5 protein.
Results: Serial echocardiography revealed that RV contractility in 18-week-old PKP2 heterozygous mice (PKP2+/-) was significantly reduced compared to WT and continued to deteriorate with aging (n= 5-7 hearts/group, P<0.05). Meanwhile, the LV of PKP2+/- hearts remained normal. Fibrosis was undetectable in RV and LV of 24-week-old PKP2+/- hearts (n= 4 hearts/group); however, the contractility of isolated RV myocytes, paced from 0.5 to 3Hz, was significantly decreased and slowed compared to 12-week-old PKP2+/- cells or age-matched WT myocytes (1Hz: 5.21±0.6 vs. 9.41±1.1% sarcomere shortening, in PKP2+/- and age-matched WT mice, respectively, n= 6-8 cells/group, P<0.05). In contrast, isolated PKP2+/- LV myocytes exhibited normal contractile parameters independent of the animals age (n= 5-10 cells/age/group). The loading of 24-week-old PKP2+/- myocytes with the intracellular Ca2+ indicator Fura-2 revealed that both RV and LV cells had normal Ca2+ transient characteristics (n= 5-12 cells/ventricle/group) suggesting that this PKP2 mutation directly impacts the sarcomeres. Finally, electrocardiograms of anesthetized mice showed that intraperitoneal injection of the β-adrenergic agonist isoproterenol (1 mg/Kg) and caffeine (120 mg/Kg) increased the susceptibility of the PKP2+/- hearts to arrhythmias (n= 4 hearts/group, P<0.05).
Conclusions: Our data demonstrate that PKP2 (L404/X5)+/- mice exhibit the hallmarks of ACM, and revealed that progressive RV dysfunction proceeds from the reduction in myocyte contractility.