Loss of myocardial Hey2/Hrt2 function disrupts rightward shift of atrioventricular cushion tissue and causes tricuspid atresia.

Abstract

Endocardial cushion tissue is primordia of the valves and septa of the adult heart, and its malformation causes various congenital heart diseases (CHDs). Tricuspid atresia (TA) is defined as congenital absence or agenesis of the tricuspid valve caused by endocardial cushion defects. However, little is known about what type of endocardial cushion defect causes TA. Using three-dimensional volume rendering image analysis, we demonstrated morphological changes of endocardial cushion tissue in developing Hey2/Hrt2 KO mouse embryos that showed malformation of the tricuspid valve, which resembled human TA at neonatal period. In control embryos, atrioventricular (AV) endocardial cushion tissues showed rightward shift to form a tricuspid valve. However, the rightward shift of endocardial cushion tissue was disrupted in Hey2/Hrt2 KO embryos, leading to the misalignment of AV cushions. We also found that muscular tissue filled up the space between the right atrium and ventricle, resulting in the absence of the tricuspid valve. Moreover, analysis using tissue-specific conditional KO mice showed that HEY2/HRT2-expressing myocardium may physically regulate the AV shift. Disruption of rightward cushion movement is an initial cue of TA phenotype, and myocardial HEY2/HRT2 is necessary for the regulation of proper alignment of AV endocardial cushion tissue.