Abstract 13431: Gain-of-Function Mutations in RNA Binding Motif 20 Result in Cardiac Remodelling and Pathogenesis in Mice

Abstract

Gain-of-function mutations of a gene result in new molecular function. Genetic studies show that mutations in a gene named as RNA binding motif 20 (RBM20) are associated with dilated cardiomyopathy (DCM). To test whether Rbm20 mutations are responsible for cardiac remodelling and potential underlying mechanisms, we selected two mutations (S637A and S639G) located in arginine/serine-rich domain of RBM20 and generated two knock-in mouse models ( Rbm20 S637A and Rbm20 S639G ) respectively. Both mouse lines were subjected to histological and anatomical analysis as well as cardiac function assessment with echocardiography. The results revealed that heterozygous (HT) and homozygous (HM) male and female mice from both lines displayed DCM phenotype with enlarged left ventricular (LV) chamber, thinner LV walls and reduced ejection fraction. Interestingly, we observed that Rbm20 S639G mice manifested more severe phenotype than that in Rbm20 S637A mice. Rbm20 S639G HM mice exhibited higher mortality rate (~50%) than that (~34%) in Rbm20 S637A HM mice at an early age (<100 days). Differentially expressed and spliced genes from RNA-seq data are involved in arrhythmia, cardiomyopathy, and sudden death based on Human Phenotype Ontology enrichment analysis (FDR≤0.05) in both lines. Immunofluorescent analysis in tissues and single cardiomyocytes respectively showed both mutations promote RBM20 nucleocytoplasmic trafficking and protein condensates, suggesting they are gain-of-function mutations, leading to high mortality rate at younger age. Both mutations also interrupted the alternative splicing of TTN and calcium handling genes. Force measurements and Ca 2+ transient evaluation of intact myocytes showed that the reduced cellular diastolic stiffness and impaired contractility in HT and HM mice. Studies suggested that phosphorylation of RS domain is critical for protein trafficking. We then performed mass spectrometry on the in vitro expressed RBM20 and identified phosphorylation on S637 and S639. Phosphomimetic assay showed that phosphorylation was not responsible for RBM20 trafficking and condensates. Together, we reported a new mechanism by which cardiac genetic mutations causes protein trafficking and aggregation in cardiac remodelling.