Abstract Body: Introduction: Mutations in RNA-binding motif protein 20 (RBM20) cause a severe form of dilated cardiomyopathy (DCM) and sudden death. The arginine/serine-rich domain of RBM20 is a “hotspot” for mutations that lead to re-localization of RBM20 from the nucleus to the cytoplasm and abnormal splicing of cardiac mRNAs. To understand the molecular mechanisms underlying RBM20 cardiomyopathies and to design a generalizable therapeutic strategy, we analyzed the RNA-seq data of RBM20^R636Q/R636Q mice and noted that the expression of CaMKIIδ, a key driver of heart disease, was increased in the mutant mice, resulting in its overactivation. We hypothesize that inhibition of CaMKIIδ in RBM20 mutant mice rescues RBM20 cardiomyopathies.
Methods: We crossed RBM20^R636Q/R636Q mice with a CaMKIIδ knockout model to generate RBM20^R636Q/R636Q; CaMKIIδ KO mice. We evaluated survival, cardiac function via echocardiography, and heart remodeling by H&E and Picrosirius red staining. RBM20 localization was visualized by immunofluorescent staining and splicing variants of RBM20-regulated cardiac mRNAs were assessed via RT-PCR. Ca²⁺ transients in adult cardiomyocytes were detected. Additionally, we profiled the proteome and phosphoproteome of these mice.
Results: We showed that deleting CaMKIIδ in RBM20 ^R636Q/R636Q mice partially rescued heart function, significantly ameliorated cardiac dilation and fibrosis and extended lifespan, although RBM20 cytoplasmic localization and splicing defects were not affected. Adult cardiomyocytes from RBM20 ^R636Q/R636Q mice displayed aberrant Ca²⁺ signaling, which was mitigated in double mutant mice. Omics data showed that expression of proteins involved in extracellular matrix assembly and muscle structure, and phosphorylation of peptides associated with the sarcomere and stress fibers were restored close to WT level in double mutant mice.
Conclusion: This study demonstrates that CaMKIIδ is a critical downstream target of RBM20 that contributes to RBM20 cardiomyopathies. CaMKIIδ inhibition rescues cardiac defects and prolongs lifespan of RBM20 ^R636Q/R636Q mice. These findings provide a comprehensive understanding of the molecular basis of RBM20 cardiomyopathies and pinpoint a general therapeutic target for this disease.