Digeorge syndrome Gene TBX1, regulates heart and carilage morphogenesis in zebrafish by pathways converging on ALCAMA

DiGeorge syndrome, characterized by congenital heart disease, craniofacial and immune system abnormalities, is the most common microdeletion syndrome with an incidence of 1 in 4000 live births. Congenital cardiac defects including conotruncal and ventricular septal defects are the leading cause of mortality. Experiments in mouse have identified Tbx1 as the gene responsible for DiGeorge syndrome, and these findings were subsequently corroborated in human patients. However, the underlying pathways of Tbx1 regulating heart, craniofacial and thymus development remain to be fully characterized. My dissertation focuses on using the zebrafish tbx1-/- mutant with craniofacial and immune defects similar to DiGeorge syndrome patients, to study the role and pathways of tbx1 regulating heart and cartilage development. I found that tbx1-/- mutants have defects in heart morphology, looping, function and differentiation. I showed that tbx1 likely affects heart looping and differentiation by regulating cardiomyocyte proliferation and shape. I further demonstrated that tbx1 regulates heart looping and differentiation via wnt11r and its downstream gene alcama. In addition to heart defects alcama morphants present cartilage abnormalities similar to those obtained by knockdown of edn1, a gene previously identified to act downstream of tbx1 in cartilage development. Further investigation revealed that alcama functions downstream of Edn1 signaling in regulating neural crest differentiation and cartilage formation. In addition, I showed that Alcama on endoderm interacts with Nadl1.1 on iv neural crest to mediate Edn1 signaling and regulate cartilage morphogenesis. Hence, I have demonstrated that Tbx1 regulates Alcama via Wnt11r in the heart and via Edn1 in pharyngeal endoderm.