RBM20 Mutations Induce an Arrhythmogenic Dilated Cardiomyopathy Related to Disturbed Calcium Handling
Background—Mutations in RBM20 cause a clinically aggressive form of dilated cardiomyopathy (DCM), with an increased risk of malignant ventricular arrhythmias. RBM20 is a splicing factor that targets multiple pivotal cardiac genes, such as Titin (TTN) and Calcium/calmodulin-dependent kinase II delta (CAMK2D). Aberrant TTN splicing is thought to be the main determinant of RBM20-induced DCM, but is not likely to explain the increased risk of arrhythmias. Here, we investigated the extent at which RBM20 mutation carriers have an increased risk of arrhythmias and explore the underlying molecular mechanism.
Methods—We compared clinical characteristics of RBM20 and TTN mutation carriers and used our previously generated Rbm20 knockout (KO) mice to investigate downstream effects of Rbm20-dependent splicing. Cellular electrophysiology and Ca2+ measurements were performed on isolated cardiomyocytes from Rbm20 KO mice to determine the intracellular consequences of reduced Rbm20 levels.
Results—Sustained ventricular arrhythmias were more frequent in human RBM20 mutation carriers than in TTN mutation carriers (44% vs 5%, respectively, p=0.006). Splicing events that affected Ca2+ and ion handling genes were enriched in Rbm20 KO mice, most notably in the genes CamkIIδ and RyR2. Aberrant splicing of CamkIIδ in Rbm20 KO mice resulted in a remarkable shift of CamkIIδ towards the δ-A isoform which is known to activate the L-type Ca2+ current (ICa,L). In line with this, we found an increased ICa,L, intracellular Ca2+ overload and increased sarcoplasmic reticulum (SR) Ca2+ content in Rbm20 KO myocytes. Additionally, not only complete loss of Rbm20, but also heterozygous loss of Rbm20 increased spontaneous SR Ca2+ releases, which could be attenuated by treatment with the ICa,L antagonist verapamil.
Conclusions—We show that loss of Rbm20 disturbs Ca2+ handling and leads to more pro-arrhythmic Ca2+ releases from the SR. Patients that carry a pathogenic RBM20 mutation have more ventricular arrhythmias despite a similar LV function, compared to patients with a TTN mutation. Our experimental data suggests that RBM20 mutation carriers may benefit from treatment with an ICa,L blocker to reduce their arrhythmia burden.
- Received October 3, 2017.
- Revision received March 14, 2018.
- Accepted March 26, 2018.