Background: DNA methylation acts as a mechanism of gene transcription regulation. It has recently gained attention as a possible therapeutic target in cardiac hypertrophy and heart failure. However, its exact role in cardiomyocytes remains controversial. Thus, we knocked out the main de novo DNA methyltransferase in cardiomyocytes, DNMT3A, in human induced pluripotent stem cells (hiPSC). Functional consequences of DNA methylation-deficiency under control and stress conditions were then assessed in human engineered heart tissue (EHT) from knockout hiPSC-derived cardiomyocytes. Methods: DNMT3A was knocked out in hiPSCs by CRISPR/Cas9 gene editing. Fibrin-based EHTs were generated from knockout (KO) and control hiPSC-derived cardiomyocytes. Development and baseline contractility were analyzed by video-optical recording. EHTs were subjected to different stress protocols, including serum starvation, serum variation, and restrictive feeding. Molecular, histological and ultrastructural analyses were performed afterwards. Results: Knockout of DNMT3A in human cardiomyocytes had three main consequences for cardiomyocyte morphology and function: (1) Gene expression changes of contractile proteins such as higher atrial gene expression and lower MYH7/MYH6 ratio correlated with different contraction kinetics in knockout vs. wild-type. (2) Aberrant activation of the glucose/lipid metabolism regulator PPARγ was associated with accumulation of lipid vacuoles within KO cardiomyocytes. (3) HIF-1 protein instability was associated with impaired glucose metabolism and lower glycolytic enzyme expression, rendering KO EHTs sensitive to metabolic stress such as serum withdrawal and restrictive feeding. Conclusions: The results suggest an important role of DNA methylation in the normal homeostasis of cardiomyocytes and during cardiac stress, which could make it an interesting target for cardiac therapy.

A growing number of metabolomic studies have associated high circulating levels of the amphiphilic fatty acid metabolites, long-chain acylcarnitines (LCACs), with cardiovascular disease (CVD) risk. These studies show that plasma LCAC levels can be correlated with the stage and severity of CVD and with indices of cardiac hypertrophy and ventricular function. Complementing these recent clinical associations is an extensive body of basic research that stems mostly from the twentieth century. These works, performed in cardiomyocyte and multicellular preparations from animal and cell models, highlight stereotypical derangements in cardiac electrophysiology induced by exogenous LCAC treatment that promote arrhythmic muscle behavior. In many cases, this is coupled with acute inotropic modulation; however, whether LCACs increase or decrease contractility is inconclusive. Linked to the electromechanical alterations induced by LCAC exposure is an array of effects on cardiac excitation-contraction coupling mechanisms that overload the cardiomyocyte cytosol with Na + and Ca 2+ ions. The aim of this review is to revisit this age-old literature and collate it with recent findings to provide a pathophysiological context for the growing body of metabolomic association studies that link circulating LCACs with CVD.

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