EPIGENETICS DURING HEART DEVELOPMENT: THE ROLE OF THE HISTONE METHYLTRANSFERASE DOT1L IN CARDIAC COMMITMENT

The heart is the first organ to form and function in the embryo, and all subsequent events in the life of the organism depend on its function. Cardiac lineage specification and subsequent morphogenesis of the early developing heart are complex processes that rely on networks of interacting DNA-binding transcription factors and targeted activation of cardiac-specific genes. Mutations in cardiac transcription factors, the genes they regulate and the genes that regulate them, result in many inherited congenital heart defects and point to the importance of understanding the molecular basis behind these processes. Chemical alterations on DNA and histones, known as epigenetic modifications, are being increasingly studied for their importance in organogenesis, such as that of the heart. Recently, a dynamic landscape of histone modifications has been reported to occur during cardiac differentiation in vitro: distinct chromatin patterns were associated with stage-specific expression of genes functionally relevant to the heart. However, despite the growing number of reports, the role of the enzymes that catalyze these modifications remains poorly understood in cardiac differentiation in vivo. Here, we show that a definite temporal expression pattern of DOT1-like histone H3 methyltransferase (DOT1L) drives a transitional pattern of H3K79 di-methylation in the genome of differentiating cells, and that the function of this enzyme is obligatory for the correct differentiation of cardiomyocytes. In fact, we found that expression of DOT1L was increased in ex vivo embryonic and neonatal cardiomyocytes with respect to undifferentiated embryonic stem cells and adult cardiomyocytes; moreover, H3K79me2 was highly correlated with transcriptional activation in differentiating cardiomyocytes. We also found that the loci of genes expressed only in later stages of development were enriched in this activating mark, suggesting a role for H3K79me2 in the pre-activation of genes. Our results demonstrate how histone methylation, and in particular H3K79me2, regulates the transcription in developing cardiomyocytes and the central role played by DOT1L in this process. Apart from the increase in our understanding of how epigenetics controls development, our genome-wide data on H3K79me2 could lead to the identification of novel genes and transcriptional regulatory networks involved in cardiac differentiation. Altogether our study illustrates the importance of epigenetic regulation early in development to delineate the fate of a cell and in particular the role of methylation of H3K79 in cardiomyocytes stabilizing the signature for cardiac gene expression. Furthermore, this study add an important information to the intricate process of transcription activation that make a cardiomyocyte; for the first time we built an high resolution map of H3K79me2 in cardiomyocytes that could be the starting point to predict novel transcriptional regulatory networks during cardiomyocyte differentiation, as well provide the opportunity to identify novel genes that might be informative to understand developmental regulatory programs. Indeed we shed light on an additional fundamental enzyme involved in defining the epigenetic code associated with the complex process of heart development and establish a platform useful to identify new mechanisms underlying many congenital heart defects and cardiac developmental malformations.