Single-Cell RNA-Seq and Patient-Specific iPSCs Reveal Endocardial Abnormalities in Hypoplastic Left Heart Syndrome

Hypoplastic left heart syndrome (HLHS) is one of the most devastating forms of congenital heart defects. Previous studies have only focused on intrinsic defects in the myocardium. However, this does not sufficiently explain the abnormal development of the cardiac valve, septum, and vasculature, which are known to originate from the endocardium. Here, using single-cell RNA profiling, induced pluripotent stem cells, and fetal heart tissue with an underdeveloped left ventricle, we identified a developmentally impaired endocardial cell population in HLHS. The intrinsic endocardial deficits contributed to abnormal endothelial to mesenchymal transition, NOTCH signaling, and extracellular matrix organization, all of which are key factors in valve formation. Consequentially, endocardial abnormalities conferred reduced proliferation and maturation of cardiomyocytes through a disrupted fibronectin-integrin interaction. Several known HLHS de novo mutations all contributed to the abnormal endocardial gene expression through the alteration of promoter/enhancer activities. These mechanistic discoveries provide an alternative angle for early intervention and heart regeneration in HLHS. Overall design: This is the first study providing evidence that a developmentally impaired endocardium can contribute to the ventricular and valvular hypoplasia in hypoplastic left heart syndrome (HLHS), which may provide a novel therapeutic target for early intervention and heart regeneration for this condition.