BACKGROUND: Mutations in RBM20 cause a clinically aggressive form of dilated cardiomyopathy, with an increased risk of malignant ventricular arrhythmias. RBM20 is a splicing factor that targets multiple pivotal cardiac genes, such as Titin and CAMK2D. We have recently shown that genetic deletion of Rbm20 induces an arrhythmogenic dilated cardiomyopathy which we found to be related to disturbed calcium handling. Rbm20 knockout (KO) mice present with an intracellular calcium overload, and an increased occurrence of pro-arrhythmic spontaneous calcium releases from the SR. We hypothesize that these effects are caused by misspliced CaMKIIδ.

METHODS: Here, we further investigated the in vivo arrhythmia susceptibility of Rbm20 KO mice after injection of epinephrine/caffeine using telemetry. In addition, to investigate the causal relationship between misspliced CaMKIIδ and the observed phenotype in Rbm20 cardiomyopathy, we employ a double knockout (DKO) mouse model, where we knocked out CaMKIIδ in an Rbm20 KO background. Lastly, to unravel the mechanism through which misspliced CaMKIIδ induces the observed phenotype, we are using molecular biology techniques, including proteomic and transcriptomic approaches, to investigate the target specificity and functional differences of the cardiac splice variants of CaMKIIδ.

RESULTS AND CONCLUSIONS: Rbm20 KO mice are more susceptible to sudden cardiac death after arrhythmia induction, suggesting that also in vivo we can recapitulate the arrhythmogenic phenotype of RBM20-mutation carriers. Interestingly, DKO mice are protected from cardiac dysfunction, even though they have similar splicing abnormalities. Phosphoproteomics of wildtype and Rbm20 KO hearts revealed several proteins, including CaMKIIδ, Ttn, and Obscn, to be hyperphosphorylated. Furthermore, while we could show that all cardiac CaMKIIδ splice variants can phosphorylate and translocate HDAC4 out of the nucleus, we are currently investigating whether they also phosphorylate distinct targets. These data suggest that missplicing and potentially increased activation of CaMKIIδ cause cardiac dysfunction in RBM20 cardiomyopathy.