Abstract 3408: Folate Suppresses Wnt/Beta-Catenin Inhibition of Cardiogenesis to Protect Against Induction of Congenital Heart Defects

Abstract

Lithium (Li), a mood stabilizing drug, and elevated homocysteine (HCy), a metabolite in the folic acid (FA) cycle, are linked to induction of human congenital heart defects. We determined noninvasively by echocardiography, that exposure of the mouse embryo by an i.p. injection to the dam of Li or HCy during gastrulation on embryonic day E6.75 induces cardiac and valve defects. A single dose of 125 μl Li (25 mM) or HCy (75 μM) on E5.5 to 6.5 results in mouse embryonic lethality; on E6.75 to 7.0, both induce tricuspid and semilunar valve defects (Li: n=131 ; HCy: n=78). The tricuspid valve septal leaflet fails to delaminate, a characteristic of Ebstein’s Anomaly. Use of Li during human pregnancy has been associated with Ebstein’s Anomaly. Li mimics the Wnt/β-catenin (β-cat) signaling pathway by inactivating glycogen synthase kinase-3β. During chick cardiac specification, Li, Wnt 3A, or HCy exposure adversely affects chick cardiogenesis with severity of anomalies based on timing of early exposure. To initiate cardiogenesis, the secreted Wnt antagonist Dickkopf-1 acts extracellularly on the endoderm to upregulate Hex , an inducer of cardiomyogenesis. Exposure of stages 3+/4− chick embryos to Li/Wnt3A/ HCys inhibits Hex and Islet-1 gene expression in the cardiogenic crescent via an intracellular mechanism, thus augmenting inhibitory Wnt/β-cat signaling. FA deficiency leads to elevated HCys levels. We hypothesize that HCys/FA metabolism intersects with Wnt/β-cat signaling and that mechanistically FA supplementation acts by overriding Wnt/β-cat inhibition of gene expression in the embryonic heart fields that leads to cardiac defects. FA, known to protect against HCys-mediated neural tube defects, was tested for protective effects against Li/Wnt3A/HCy during cardiogenesis. With all three experimental exposures, FA addition results in reexpression of the cardiac inducers Hex and Islet-1 at high levels in the chick heart fields. In the mouse, no valve defects were detected in HCys/FA exposed embryos (n=27). Regimen and concentration of FA supplementation necessary to fully rescue Li effects are being tested. In conclusion, folate supplementation potentiates the repression of Wnt/β-cat signaling and protects formation of heart and valve defects. This research has received full or partial funding support from the American Heart Association, AHA Greater Southeast Affiliate (Alabama, Florida, Georgia, Louisiana, Mississippi, Puerto Rico & Tennessee).