Disruption of the nuclear localization signal in RBM20 is causative in dilated cardiomyopathy.

Human patients carrying genetic mutations in RNA binding motif 20 (RBM20) develop a clinically aggressive dilated cardiomyopathy (DCM). Genetic mutation knock-in (KI) animal models imply that altered function of the arginine-serine-rich (RS) domain is crucial for severe DCM. To test this hypothesis, we generated an RS domain deletion mouse model (Rbm20ΔRS). We show that Rbm20ΔRS mice manifest DCM with mis-splicing of RBM20 target transcripts. We found that RBM20 is mis-localized to the sarcoplasm in Rbm20ΔRS mice, which led to the formation of RBM20 granules similar to those detected in mutation KI animals. In contrast, mice lacking the RNA recognition motif (RRM) show similar mis-splicing of RBM20 target genes, but do not develop DCM or exhibit RBM20 granule formation. Using in vitro studies with immunocytochemical staining, we demonstrate that only DCM-associated mutations in the RS domain facilitate RBM20 nucleocytoplasmic transport and promote granule assembly. Further, we defined the core nuclear localization signal (NLS) within the RS domain. Mutation analysis of phosphorylation sites in the RS domain indicate that this modification is dispensable for RBM20 nucleocytoplasmic transport. Collectively, our findings revealed that disruption of RS domain-mediated nuclear localization is crucial for severe DCM caused by NLS mutations.