Abstract 11: Lamin A/C Mutations Epigenetically Dysregulate Scn5a Gene Expression, Perturbing Action Potential Properties in IPSC-derived Cardiomyocytes

Abstract

Mutations of the LMNA gene, encoding the nuclear lamina proteins Lamin A/C, are a common cause of dilated cardiomyopathy, typically manifesting in association with cardiac conduction defects. LaminA/C regulate various nuclear activities, including maintenance of the nuclear structure, gene transcription and chromatin organization. Most studies on the consequences of Lamin A/C defects were conducted on fibroblasts, while studies on human cardiomyocytes (CMs) are scarce. We therefore generated a cardiac model of laminopathy obtained by differentiation of CMs from induced pluripotent stem cells (iPSCs) of patients carrying the K219T Lamin A/C mutation. In vitro, these cells recapitulate the morphological features of dilated cardiomyopathy, specifically sarcomeric disorganization and increased size. Using this model, we performed a comprehensive analysis of the electrophysiological properties of LMNA-CMs both at single cell level and in a multi-cellular setting. Using patch-clamp technique, results revealed significant changes in maximal upstroke velocity (dV/dt max ), action potential amplitude (APA) and overshoot (OV) in LMNA-CMs compared to those obtained from family-matched healthy controls (CNTR); these defects were associated with a reduction of the peak sodium currents and a diminished conduction velocity, measured in strands of electrically-coupled CMs. Biochemical studies showed a significant reduction of both the sodium channel Nav1.5 protein and its transcript in LMNA-CMs, accompanied by an increased binding of LaminA/C to the promoter of its coding gene, SCN5A. Binding of the Polycomb group protein SUZ12 and of the H3K27me3 histone repressive mark was also increased. Consistently, 3D-FISH experiments also indicated a preferential localization of SCN5A genomic loci at the nuclear periphery in LMNA-CMs. As a whole, our findings support a model in which mutated Lamin A/C perturb SCN5A gene expression by favouring PRC2 (Polycomb Repressive Complex 2) binding to its promoter, leading to decreased sodium current peak and slower conduction velocity. This mechanism may eventually sustain the conduction abnormalities inevitably occurring in patients with LMNA-cardiomyopathy.