Presentation description

Congenital Heart Defects (CHD) are the most common birth defects, with Hypoplastic Left Heart Syndrome (HLHS) -underdevelopment of the heart's left side- being the most lethal form. Yet, the cell lineages involved in the development of HLHS remain unclear. Mutations in KMT2D are associated with CHD and Kabuki Syndrome (KS), a rare multi-syndromic disorder. Our Kmt2d null zebrafish mutants recapitulated KS phenotypes, including hypoplastic hearts. The heart is composed of multiple cell layers, including the inner layer (endocardium) and the muscular layer (myocardium). In Kmt2d null mutants, endocardial cells occlude the heart chambers with no change in myocardial cell shape, size, or number. We hypothesize that the endocardial cell lineage is responsible for the hypoplastic heart phenotype observed in Kmt2d null mutants. We engineered a Tol2/CRISPR Cas9 vector system to specifically knock-out Kmt2d in the endocardium of developing zebrafish embryos. Embryos were assessed every 24 hours from 1-day post-fertilization (dpf) to 5dpf for cardiac defects and mortality. At 3dpf, 40.9% of the injected endocardial kmt2d knockout population displayed a cardiac phenotype, compared to 14% of their non-injected siblings. By 5dpf, 53% of the endocardial kmt2d knockout population died, compared to only 4% of their non-injected siblings. Our findings suggest loss of kmt2d in the endocardial lineage leads to increased cardiac defects and mortality, revealing a potential cell lineage involved in hypoplastic hearts. Future experiments will include determining if endocardial Kmt2d knockouts also present a hypoplastic heart. Our findings provide crucial insights into the origins of HLHS, potentially redefining CHD therapies.