Functional analysis of a CASZ1/CHD5 interaction in cardiac development and disease

Early cardiac development involves coordination of a number of processes including the specification and differentiation of cardiomyocytes, the ventral migration of cardiac cell populations, the formation of a linear heart tube, and the subsequent morphogenic movements required for cardiac looping and chamber formation. These early processes require the precise spatial and temporal regulation of a number of genetic pathways, which coordinate the activation and repression of various cardiac-specific transcription factors and their targets. These transcription factors are in part regulated by the formation of transcriptional complexes via interaction with regionally expressed cofactors. In this dissertation, I explore the role of protein-protein interactions in regulating the novel para zinc finger transcription factor CASTOR (CASZ1).;We carried out a yeast two-hybrid screen to identify cardiac specific CASZ1 interacting partners and identified congenital heart disease protein 5 (CHD5), a small coiled coil protein expressed within the putative critical region of chromosome 21associated with congenital heart disease in Down syndrome patients. We observe that CASZ1 and CHD5 co-localize in the nuclei of cardiomyocytes and interact in vivo. We demonstrate that the interaction between CASZ1 and CHD5 is necessary for early cardiogenesis in Xenopus. We find that the absence of CASZ1 or CHD5 results in a failure of cardiac looping and early chamber formation. We observe that CASZ1 and CHD5-depleted cardiomycytes fail to undergo the cell shape changes associated with cardiomyocyte maturation, and that cardiomyocytes display defects in cell adhesion and apical-basal polarity. Thus CASZ1 and CHD5 are collectively required for the maintenance of early cardiomyocyte integrity. Overall this work provides a mechanism by which CASZ1 functions in early cardiac development and implicates CASZ1 and CHD5 collectively as potential causal factors for congenital heart disease in not only Down syndrome patients but in the general population as well.