The recognition of primary dysregulation of the transforming growth factor beta (TGF-2) axis in human vascular disease has revolutionized the pathogenetic understanding of thoracic aortic aneurysm (TAA). Initial insight into TGF-2 signaling was discovered by the study of the Marfan syndrome (MFS), a common, autosomal dominant condition caused by mutations in the gene encoding fibrillin-1 (1). Our current model of pathology in MFS posits that decreased expression of the fibrillin-1 protein leads to decreased binding of and therefore increased bioavailability of the large latent complex (LLC) of TGF-2 (2). Increased signaling through the TGF-2 pathway deleteriously influences cellular performance and phenotype, driving in the case of the cardiovascular system, aortic aneurysm. The majority of observed clinical phenotypes in this syndrome, including cardiovascular phenotypes, have commonly been considered as postnatally acquired. However, TGF-2 is a well-described developmental cytokine of profound importance in the cardiovascular system and perturbation of this system would a priori be expected to disrupt multiple aspects of systemic organogenesis. Additionally, fibrillin-1 has recently been shown to bind to an extended repertoire of TGF-2 family members including BMPs and GDFs, further expanding the possibilities for developmental cytokine dysregulation in fibrillin deficiency (3). Data will be presented of interrogation of mouse models of MFS, which demonstrate abnormalities of myocardial and arterial development. We believe these abnormalities in cardiac development are related to pathogenic proliferative signaling in the second heart field (SHF), a recently identified developmental field encompassing the conotruncus and right ventricle (4-6). The major hypothesis to be tested in this application is that disruption of TGF-2 superfamily member signaling caused by an absence of fibrillin-1 leads to pathologic proliferative signaling within the developing SHF and therefore contributes to pathologic cardiac and proximal aortic development in MFS. This hypothesis will be interrogated in three specific aims. In the first, a complete analysis of aberrant SHF development wil be undertaken in murine models of severe MFS. In the second aim, SHF cells from fibrillin-1 deficient embryos will be isolated and characterized, to identity aberrant signaling events driving pathologic cellular proliferation. In the final aim, manipulation of aberrant SHF expansion will be explored as a therapeutic strategy in MFS, with initial experimentation in a murine embryonic stem cell (mESC) model of SHF development and with subsequent in vivo validation. Importantly, we believe this early SHF dysregulation may initiate the pathogenic sequence as well as define the anatomic susceptibility to aneurysm in the proximal aorta in MFS.