Congenital heart defects are the result of abnormal development of mesodermal cells, which form the muscular portion of the heart, or neural crest-derived cells, which populate the cardiac outflow tract and aortic arches. Defects in the two population of cells usually occur in a segmental fashion resulting in abnormalities of distinct regions of the heart with neighboring regions being relatively normal. The long term goal of this proposal is to understand the independent molecular pathways and mechanisms which would control segmental cardiac development. This type of understanding is the first step in identifying the genes which cause heart defects in distinct regions of the heart. Specifically, we focus on elucidating the pathogenesis of isolated cardiac outflow tract defects and those which occur as part of DiGeorge/CATCH-22 (cardiac defects, abnormal facies, thymic hypoplasia, cleft palate, and hypocalcemia associated with chromosome 22 microdeletion) syndrome. In both scenarios, a high percentage of affected individuals harbor a microdeletion of one allele of chromosome 22q11.2 and are thought to have a defect in neural crest-derived cells which populate the branchial and aortic arches and cardiac outflow tract. In contrast, conditions such as hypoplasia of the right or left ventricles have been thought to be the result of flow abnormalities during cardiogenesis. Our recent targeted deletion of the basic helix- loop-helix transcription factor, dHAND, suggests that a subset of cardiac outflow tract defects and hypoplastic right ventricle may be the result of excessive programmed cell death from single gene defects. Although dHAND-null embryos have hypoplasia of the neural crest-derived branchial and aortic arches and right ventricle, dHAND does not map to human chromosome 22. However, by subtraction cloning between wild type and dHAND-null embryos, we have found that a ubiquitin fusion degradation protein (UFD1) is downstream of dHAND and maps to the DiGeorge critical region of ch.22. We have shown that UFD1 is normally expressed in the branchial arches, cardiac outflow tract and right ventricle but is down-regulated in dHAND-null embryos. The UFD family has been studied in yeast where they represent a novel proteolytic pathway for degradation of cellular proteins. Targeted deletion of UFD1 in yeast results in cell death in a dose-dependent fashion, suggesting that the down-regulation of UFD1 may mediate the apoptosis seen in dHAND mutants. Having placed UFD1 as a candidate gene for CATCH-22 syndrome from a molecular pathway and mechanistic approach, we propose three major aims for this proposal: 1) to determine, in mice, the role of UFD1 during embryogenesis and if UFD1 is one of the genes responsible for a DiGeorge/CATCH-22-like phenotype, 2) to determine if mutations and/or deletions of UFD1 in humans contribute to cardiac outflow tract defects or subsets of CATCH-22, 3) to define the role of UFD1 and dHAND in cell survival during embryonic development.
The aims utilize in vivo models of yeast, mice and humans to understand the development of cardiac mesoderm and neural crest in normal and abnormal embryogenesis. In this fashion, we intend to approach the molecular basis for certain congenital heart defects, particularly those affecting the cardiac outflow tract and aortic arch in isolation and in CATCH-22 syndrome.
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