Genetic test of FAM3B as the causative gene for the heart defects in Down Syndrome

Down Syndrome (DS), or trisomy 21, is the most common human aneuploidy compatible with survival. CHDs are the major inborn defect observed in DS, affecting 40-60% of patients. About half of CHDs in DS are atrioventricular septation defects (AVSDs), which are caused by maldevelopment of the atrioventricular (AV) septum. The causative gene(s) for DS CHDs has yet to be identified. Based on our literature examination and preliminary data, we have identified FAM3B as a candidate causing AVSD in DS. FAM3B is localized within the newly mapped DS CHD critical region on human chromosome 21. Fam3b is expressed in mouse embryonic hearts during AV septum formation. Our cell culture study revealed, for the first time, that FAM3B inhibits activation of Bone Morphogenic Protein (BMP) signaling, which is essential for AV septum formation. Significantly, our transgenic studies showed that overexpression of FAM3B in mouse embryonic hearts led to the AVSD closely resembling the heart defect seen in DS. We hypothesize that the dosage imbalance of FAM3B represses BMP activities during AV septation, causing the AVSD in DS. In this study, we will apply mouse genetic approaches to test this hypothesis. Two aims are proposed. In aim 1, we will test whether elevated expression of FAM3B in embryonic hearts represses BMP activities and causes the AVSD. We recently constructed a Cre-inducible FAM3B overexpression mouse line. This line was crossed with a myocardial-specific Cre line to obtain double transgenic embryos in which overexpression of FAM3B, driven by the cardiac troponin T promoter, was induced in developing hearts. All double transgenic embryos displayed defects in AV septa. We will continue to characterize the double-transgenic embryos to determine their morphological, cellular and molecular defects during AV septation, with the primary focus on BMP-mediated processes. In aim 2, we will test whether a single extra copy of FAM3B is sufficient to cause the AVSD in mice. We will apply the bacterial artificial chromosome transgenic approach to introduce a single extra copy of FAM3B in mice. Accomplishing this study will contribute to the identification of the causative gene for DS CHDs and lead to a better understanding of the molecular/genetic mechanisms governing normal AV septation. Positive results obtained from our mouse models will warrant performing human genetic studies to directly address the etiological role of FAM3B in DS CHDs and other patients with AVSD (AHA Program: Innovative Research Grant)