Abstract-Preeclampsia is a life-threatening disorder characterized by maternal gestational hypertension and proteinuria that results from placental dysfunction. Placental abnormalities include abnormal syncytiotrophoblast and a 50% reduction in placental expression of the transcription factor Gcm1. In mice, homozygous deletion of Gcm1 prevents syncytiotrophoblast differentiation and is embryonic lethal. We used heterozygous Gcm1 mutants (Gcm1 ϩ/Ϫ ) to test the hypothesis that hypomorphic expression of placental Gcm1 causes defective syncytiotrophoblast differentiation and maternal and placental phenotypes that resemble preeclampsia. We mated wild-type female mice with Gcm1 ϩ/Ϫ fathers to obtain wild-type mothers carrying Ϸ50% Gcm1 ϩ/Ϫ conceptuses. Gcm1 ϩ/Ϫ placentas had syncytiotrophoblast abnormalities including reduced gene expression of Gcm1-regulated SynB, elevated expression of sFlt1, a thickened interhemal membrane separating maternal and fetal circulations, and electron microscopic evidence in syncytiotrophoblast of necrosis and impaired maternal-fetal transfer. Fetoplacental vascularity was quantified by histomorphometry and microcomputed tomography imaging. In Gcm1, it was Ϸ30% greater than wild-type littermates, whereas placental vascular endothelial growth factor A (Vegfa) expression and fetal and placental weights did not differ. Wild-type mothers carrying Gcm1 ϩ/Ϫ conceptuses developed late gestational hypertension (118Ϯ2 versus 109.6Ϯ0.7 mm Hg in controls; PϽ0.05). We next correlated fetoplacental vascularity with placental Gcm1 expression in human control and pathological pregnancies and found that, as in mice, fetoplacental vascularity increased when GCM1 protein expression decreased (R 2 ϭϪ0.45; PϽ0.05). These results support a role for reduced placental Gcm1 expression as a causative factor in defective syncytiotrophoblast differentiation and maternal and placental phenotypes in preeclampsia in humans. (Hypertension. 2012;59:732-739.) • Online Data Supplement Key Words: syncytin Ⅲ placenta Ⅲ preeclampsia Ⅲ angiogenesis Ⅲ VEGFA Ⅲ placental growth factor Ⅲ sFlt1 P lacental dysfunction is believed to be the major cause of one of the most common and serious complications of human pregnancy, preeclampsia (PE). 1 This potentially lifethreatening hypertensive disorder adversely impacts Յ5% of all pregnancies and has no known cure. In PE, growing evidence implicates abnormalities in syncytiotrophoblast (SynT) in the villous exchange region of the placenta. [1][2][3][4][5][6] SynTs are thin, fetal-derived cells that lie between the maternal blood flowing through the villous exchange region of the placenta and the fetal blood flowing through fetal vessels in the highly vascularized villi. This surface is essential for fetomaternal communication and exchange and is formed through the differentiation of SynT from the underlying villous cytotrophoblast cells under the control of the transcription factor GCM1 (glial cells missing 1). 7 Gcm1 in the conceptus is almost exclusively expressed in a subset of trop...