Title of Research Grant

Identifying transcriptional changes in the defective tissue underlying the major congenital heart defect of Down syndrome

Researcher

John Klingensmith, PhD
Applicant’s Primary Department: Cell Biology

Summary of Projected Project

Forty percent of individuals with Down syndrome (DS) have congenital heart defects, the most common of which is failure of the heart to properly divide into four chambers (atrioventricular septal defect, or AVSD). Recent studies of AVSD in trisomy 21 fetuses, which develop DS, demonstrate a failure to form the dorsal mesenchymal protrusion (DMP), an essential element of the atrial septum. The Ts16 mouse model of DS also exhibits AVSD at high penetrance, again due to a failure to form the DMP. The embryological and molecular basis of this defect has not been addressed. We have identified a new mouse model of AVSD in which disruption of Hedgehog signaling to the dorsal mesocardium results in absence of the DMP, and consequently leads to AVSD. We propose to use microarray screening to identify the transcriptional changes in the dorsal mesocardium, from which the DMP arises, in both models of AVSD. Analyzing disrupted transcription in each model alone will produce useful lists of misregulated genes potentially involved in the pathogenic mechanisms. Especially useful will be the identification of genes coordinately misregulated in both models, as they are likely to be fundamental to the normal development of the AV septal complex and mechanism of AVSD in DS. We will use quantitative PCR and in situ hybridization to confirm results, and analyze the known functions of the misregulated genes. These studies will shed new light on the molecular changes mediating the pathogenesis of AVSD and potentially of other manifestations of DS.

Lay Summary of Projected Project

Forty percent of individuals with Down syndrome (DS) have congenital heart defects, many of which require immediate surgical intervention at birth. The most common heart defect in DS is failure of the heart to properly divide into four chambers (atrioventricular septal defect, or AVSD). In patients with DS the AVSD, a piece of tissue called the dorsal mesenchymal protrusion (DMP) fails to form. We have found that in mouse embryos, when the DMP fails to form, the walls between the four chambers do not form correctly and blood mixes between the left and right sides of the heart. As a result, blood leaving the heart to the rest of the body does not have enough oxygen. We have recently identified a mouse mutant that lacks the DMP and develops AVSD, similar to the case in patients with DS. By comparing the genes misexpressed in this mutant and a well-characterized mouse model of DS with the same heart defect, we will begin to identify the genes important for formation of the DMP and correct compartmentalization of the developing heart – and that are likely to be among the downstream mediators of the genetic defect causing Down syndrome. Using microarray gene chips, we can simultaneously screen 40,000 genes to find those implicated in DMP formation and its failures in these mice. Identifying which genes are misregulated in the development of AVSD is a key step toward understanding why patients with DS frequently present with this serious heart defect.