A novel truncating variant c.1222dupC in RBM20 causes cardiomyopathy consistent with haploinsufficiency

RBM20 is a cardiac splicing factor responsible for splicing of several cardiac genes such as TTN, TRDN, RyR2, PDLIM1, and CAMK2D. Mutations in RBM20 are a major cause of familial dilated cardiomyopathy (DCM), and lead to missplicing of RBM20 target genes. Here, we describe a novel pathogenic truncating mutation, RBM20 c.1222dupC, identified in a patient with mitral valve prolapse and late onset familial DCM. This mutation introduces a premature termination codon and generates a truncated protein of ~55 kDa in vitro. Splicing assays demonstrated complete loss of activity and no dominant-negative effect on WT RBM20. Overexpression in NRCMs revealed that the truncated protein localized to both cytoplasm and nucleus, partially co-localizing with WT RBM20, despite lacking the RS and RRM domains. To model the patient’s condition, we generated heterozygous c.1222dupC iPSC-derived cardiomyocytes. Western blot analysis of endogenous RBM20 revealed a strong reduction in RBM20 protein level. RT-PCR revealed splicing defects in canonical RBM20 targets, and RNA-sequencing identified widespread splicing abnormalities, including in established RBM20 targets (TTN, RyR2, CAMK2D, and CACNA1G). Together, these findings establish RBM20 c.1222dupC as a pathogenic truncating variant that causes DCM primarily through haploinsufficiency.