Abstract
Epigenetic regulation is implicated in embryonic development and the control of gene expression in a cell-specific manner. However, little is known about the role of histone methylation changes on human cardiac differentiation and maturation. Using human embryonic stem cells (hESCs) and their derived ventricular (V) cardiomyocytes (CMs) as a model, we examined trimethylation of histone H3 lysine 4 (H3K4me3) and lysine 27 (H3K27me3) on promoters of genes associated with cardiac electrophysiology, contraction, and Ca(2+) handling. To avoid ambiguities due to heterogeneous chamber-specific types, hESC-derived ventricular cardiomyocytes (VCMs) were selected by dual zeocin-GFP expression under the transcriptional control of the MLC2v promoter and confirmed electrophysiologically by its signature action potential phenotype. High levels of H3K4me3 are present on pluripotency genes in hESCs with an absence of H3K27me3. Human ESC-VCMS, relative to hESCs, were characterized by a profound loss of H3K27me3 and an enrichment of H3K4me3 marks on cardiac-specific genes, including MYH6, MYH7, MYL2, cTNT, and ANF. Gene transcripts encoding key voltage-gated ion channels and Ca(2+)-handling proteins in hESC-VCMs were significantly increased, which could be attributed to a distinct pattern of differential H3K4me3 and H3K27me3 profiles. Treatment of hESC-VCMs with the histone deacetylase inhibitor valproic acid increased H3K4me3 on gene promoters, induced hypertrophic growth (as gauged by cell volume and capacitance), and augmented cardiac gene expression, but it did not affect electrophysiological properties of these cells. Hence, cardiac differentiation of hESCs involves a dynamic shift in histone methylation, which differentially affects VCM gene expression and function. We conclude that the epigenetic state of hESC-VCMs is dynamic and primed to promote growth and developmental maturation, but that proper environmental stimuli with chromatin remodeling will be required to synergistically trigger global CM maturation to a more adult-like phenotype.