Endocardial fibroelastosis in hypoplastic left heart syndrome: clinical and experimental correlations

Introduction: Hypoplastic left heart syndrome (HLHS) is the leading cause of mortality among congenital heart diseases. Endocardial fibroelastosis (EFE) is the hallmark of HLHS, but little is known about the underlying mechanisms of fibrogenesis in HLHS. Endothelial to mesenchymal transition (EndMT) is a mechanism by which endothelial cells acquire a mesenchymal phenotype and contribute to the pool of fibroblasts, the main mediators of fibrogenesis.

Methods: We established a mouse model of EFE which allows for endothelial lineage tracing. In this model the hearts of newborn Tie1-cre; R26Rosa-STOP-YFP mice were unloaded by means of heterotopic transplantation onto the abdominal vessels of C57BL/76 wildtype recipient mice. Because endothelial cells in the donor hearts are irreversibly labeled by YFP expression irrespective of their phenotype, this model allows for analysis of EndMT within EFE. Furthermore, we analyzed 12 endocardial tissue samples resected during staged HLHS repair for incidence of EndMT by double immunofluorescence-labeling using antibodies against CD31 (endothelial cell marker) and S100A4 (fibroblast marker).

Results: Unloaded Tie1-cre; R26Rosa-STOP-YFP hearts developed EFE. The fibroblasts contributing to EFE were positive for YFP demonstrating their endothelial origin via EndMT. In addition, confocal analysis revealed presence of CD31+S100A4+ double-positive cells in 10 out of 12 EFE tissues from children with HLHS but not in non-fibrotic control tissues, suggesting that these cells undergo active EndMT with an intermediate phenotype.

Conclusion: Our data suggests that aberrant EndMT is a major contributor to endocardial fibrosis in HLHS and that inhibiting EndMT could be a new therapeutic strategy for fetuses with evolving HLHS.