Increased Endothelial Sclerostin Caused by Elevated DSCAM Mediates Multiple Trisomy 21 (Down Syndrome) Phenotypes

Trisomy 21 (T21), a recurrent aneuploidy occurring in 1:800 births, predisposes to congenital heart disease (CHD) and multiple extra-cardiac phenotypes. Despite a definitive genetic etiology, the mechanisms by which T21 perturbs development and maintenance remains poorly understood. We compared the transcriptome of CHD tissues from 49 T21 and 226 euploid (eCHD) patients. We resolved cell lineages that mis-expressed T21 transcripts by cardiac single-nucleus RNA-sequencing and RNA in situ hybridization. Compared to eCHD, T21 had increased chr21 gene expression, 11-fold greater levels (p=1.2E-8) of SOST (chr17), encoding the Wnt-inhibitor sclerostin, and 1.4-fold higher levels (p=8.7E-8) of the SOST transcriptional activator ZNF467 (chr7). Cardiac endothelial cells co-expressed SOST and ZNF467. T21 endothelial cells had 6.9-fold higher SOST (p=2.7E-27) with downregulation of Wnt pathway genes. Within the chr21 CHD critical region, the expression of DSCAM was correlated with SOST (p=1.9E-5) and ZNF467 (p=2.9E-4). Deletion of DSCAM in T21 endothelial cells derived from human induced pluripotent stem cells resulted in decreased sclerostin secretion. As Wnt signaling is critical for atrioventricular canal formation, bone health, and pulmonary vascular homeostasis, we conclude that T21-mediated increased sclerostin levels inappropriately inhibits Wnt activities and promotes Down syndrome phenotypes. These findings imply therapeutic potential for anti-sclerostin antibodies in T21.