Cytoplasmic RBM20 granules are sufficient to trigger dilated cardiomyopathy

Pathogenic variants in the RS domain of RBM20 cause dilated cardiomyopathy (DCM) by disrupting nuclear import, leading to cytoplasmic accumulation of RBM20 in ribonucleoproteing ranules and loss of splicing function. The relative contribution of RBM20-cytoplasmic granules versus splicing defects to disease pathogenesis remains unclear. Here, we used cardiomyocyte-specific AAV-mediated expression of the Rbm20-R636Q RS-domain variant in juvenile wild-type and Rbm20(R636Q/R636Q) mice to disentangle these mechanisms. The AAV-mediated expression induced cytoplasmic RBM20 granules, and dose-dependently induced ventricular dilation and systolic dysfunction, accompanied by cardiac stress marker induction and mild DCM-like splicingchanges in wild-type mice. Multi-omics profiling positioned treated hearts between wild-type and Rbm20(R636Q/R636Q) and revealed shared programs of extracellular matrix remodeling and mitochondrial protein repression. Notably, overexpression of Rbm20-R636Q had minimal additional impact in Rbm20(R636Q/R636Q) mice. These findings demonstrate that cytoplasmic RBM20 granules are sufficient to drive DCM-like remodeling and dysfunction, even when splicing alterations are only partial, highlighting toxic gain-of-function mechanisms in RBM20 cardiomyopathy.