RUNX1-driven endothelial-to-mesenchymal transition contributes to remodelling in LMNA cardiomyopathy

BACKGROUND AND AIMS: LMNA-related dilated cardiomyopathy (LMNA-DCM) is a progressive genetic disorder characterized by conduction disease, malignant arrhythmias, myocardial fibrosis, and heart failure. Although LMNA mutations have traditionally been associated with cardiomyocyte-intrinsic defects, the mechanisms driving fibrotic remodelling remain incompletely understood. METHODS: Spatial transcriptomics and integrated single-nuclei multiomics were performed on explanted human LMNA-DCM hearts to define endothelial transcriptional and epigenomic states associated with fibrosis. Patient-specific induced pluripotent stem cell-derived endothelial cells, engineered cardiac organoids, and the LMNAH222P/H222P mouse model were used to investigate RUNX1-mediated endothelial-to-mesenchymal transition (EndoMT). Genetic and pharmacological RUNX1 inhibition strategies were evaluated in vitro and in vivo. RESULTS: Endothelial populations exhibiting EndoMT-associated transcriptional and epigenomic signatures were identified in human LMNA-DCM hearts. LMNA induced pluripotent stem cell-derived endothelial cells demonstrated endothelial dysfunction, mesenchymal gene activation, and epigenetic activation of RUNX1 following loss of LMNA-mediated repression. Genetic RUNX1 deletion restored endothelial identity, reversed EndoMT-associated transcriptional programmes, and normalized chromatin accessibility at endothelial regulatory loci. In multicellular cardiac organoids, endothelial RUNX1 activation impaired endothelial-cardiomyocyte signalling and cardiomyocyte contractile function, whereas endothelial-specific RUNX1 deletion restored endothelial and myocardial function. Pharmacological RUNX1 inhibition with Ro24-7429 similarly improved endothelial and cardiomyocyte function in vitro and reduced myocardial fibrosis while preserving cardiac function in LMNAH222P/H222P mice, including after disease onset. CONCLUSIONS: RUNX1-driven EndoMT represents a central mechanism linking LMNA mutations to fibrotic remodelling in LMNA cardiomyopathy. These findings support endothelial transcriptional reprogramming and RUNX1 signalling as potential therapeutic targets in fibrotic cardiomyopathy.