The overall objective of my lab's research is to gain insight into the cellular and molecular pathways that establish the early vertebrate body plan. This work couples experimental embryology and molecular techniques in amphibians with genetic analysis in the mouse. The lab's current research focuses on the patterning of the nascent mesoderm and how this initial patterning is eventually translated to the anterior-posterior patterning of the developing heart. To identify the molecular pathways involved in heart development we are studying genes implicated in human congenital heart disease and in particular, the Holt-Oram syndrome. This disease is a highly penetrant autosomal dominant condition that is associated with skeletal and cardiac malformations. The cardiac developmental abnormalities include atrial and ventricular defects and aberrant chamber formation. Recently, it has been shown that patients with Holt-Oram syndrom (HOS) often have mutations within the coding region of the transcription factor TBX5. Thus, we are interested in understanding how TBX5 functions at the molecular level and in defining the role of TBX5 during normal heart development.

In all vertebrates studied the restriction of multipotential cells into the cardiac lineage is one of the first events during gastrulation. To address the role of the TBX5 protein in heart development we are taking three complimentary approaches. First, we are studying the consequences of mis-expressing Tbx5 in Xenopus embryos. Our results demonstrate that the embryo is highly sensitive to changes in levels of Tbx5 with slight increases in Tbx5 leading to an edema in the pericardium and cell defects in the early heart tube. Our analysis of embryos in which Tbx5 is mis-expressed demonstrates the phenotype is due, at least in part, to distinct effects on the two different tissue layers of the developing heart, the endocardium and the myocardium. To study the role of tbx5 in endocardial and myocardial cells we are performing lineage analysis to establish if a lineage relationship exists between the endocardial and myocardial cells and the function of Tbx5 in these two tissue types. The second approach we are using in the lab is to establish the requirement for Tbx5 in heart development by generating an allelic series of Tbx5 by generating Tbx5 mutations in Xenopus tropicalis. Third, we are conducting a mutagenesis screen in Xenopus to detect additional genes involved in anterior-posterior patterning of the early heart tube. Collectively, these approaches will provide insights as to the molecular basis of Holt-Oram disease and eventually lead to clues as the mechanisms of heart development.