<i>trithorax</i> is an essential regulator of cardiac <i>Hox</i> gene expression and anterior-posterior patterning of the <i>Drosophila</i> embryonic heart tube

The precise regulation of transcription required for embryonic development is partially controlled by the actions of the Trithorax group (TrxG) and Polycomb group (PcG) proteins. The genes trithorax (trx), trithorax-related (trr), and SET domain containing 1 (Set1) encode COMPASS-like histone methyltransferases, a subgroup of TrxG proteins that impart H3K4 methylation modifications onto chromatin in order to activate and maintain transcription. In this study, we identify the role of these genes in the development of the embryonic heart of the fruit fly Drosophila melanogaster. trx, trr, and Set1 independently ensure proper cardiac cell divisions. Additionally, trx regulation of collinear Hox expression is necessary for the anterior-posterior cardiac patterning of the linear heart tube. trx inactivation in Drosophila results in a remarkable homeotic transformation of the posterior heart-proper segment into an aorta-like fate due to the loss of posterior abdominal A expression. Furthermore, cardiac expression of Antennapedia, Ultrabithorax, and Abdominal B is also deregulated in trx mutants. Together, these data suggest that the COMPASS-like histone methyltransferases are essential developmental regulators of cardiogenesis, being necessary for both cardiac cell divisions and heart patterning.