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

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.