Abstract Body (Do not enter title and authors here): Introduction: Trisomy 21 (T21) is the most common genetic cause of congenital heart disease (CHD) with a preponderance of atrioventricular canal defects (AVCD), a type of malformation that is uncommon in euploid CHD patients. Mechanisms that cause CHD and AVCD in T21 are unknown. We recently reported an 11-fold increase in levels of SOST which resides on chromosome 17 and encodes sclerostin, a secreted Wnt inhibitor, in T21 CHD tissues compared to euploid CHD controls. Further investigations revealed that SOST is primarily expressed in cardiac endothelial cells and genetic inactivation of DSCAM on chromosome 21 in T21 endothelial cells derived from human induced pluripotent stem cells (iPSC-ECs) diminishes sclerostin secretion.

Methods and Results: DSCAM is a transmembrane cell adhesion molecule that is involved in neuronal synapse formation. Recent studies suggest that the DSCAM intracellular domain (ICD) can translocate to the cell nucleus to regulate gene expression in HEK293 cells. To test whether the DSCAM ICD can function as a transcription regulator in endothelial cells, we overexpressed DSCAM-ICD-GFP fusion protein in T21 iPSC-ECs and euploid human aortic endothelial cells (HAECs) using lentiviral vectors. We observed a discrete nuclear localization pattern of DSCAM-ICD-GFP compared to a more diffused cytoplasmic localization of GFP alone. Overexpression of DSCAM-ICD-GFP also resulted in a modest increase in sclerostin secretion in HAECs as compared to GFP controls. Ongoing experiments will measure sclerostin secretion in T21 iPSC-ECs that overexpress DSCAM-ICD-GFP. To test whether increased sclerostin secretion can lead to CHD, we obtained mice carrying a human SOST transgene (SOST^Tg) that exhibit osteopenia, consistent with Wnt inhibition by sclerostin. As cardiac phenotype has not been assessed, we will report cardiac structure and function using echocardiography and histology to determine the consequences of SOST overexpression.

Conclusions: An increase in DSCAM gene dosage in T21 leads to increased sclerostin secretion in endothelial cells. The localization of DSCAM ICD in the cell nucleus indicates potential direct or indirect regulatory activities on SOST and other genes. Defining these effects and the consequences of sclerostin-inhibition of Wnt signaling may indicate the mechanisms for CHD in approximately half of patients with T21.