The Role of CHD7 in the Transcriptional Control of Heart Development
Chromatin remodelling provides a key mechanism for the regulation of gene expression through dynamic alterations in nucleosome occupancy at promoters and enhancers. Haploinsufficiency for the ATP-dependent chromatin remodeller chromodomain-helicase-DNA-binding protein 7 (CHD7) causes human CHARGE syndrome. CHARGE is characterised by a distinct pattern of congenital anomalies, including cardiovascular malformations, and has traditionally been considered a neurocristopathy. However, a number of the congenital heart defects associated with CHARGE cannot be attributed to disruption to the neural crest alone. This thesis therefore addresses the tissue-specific requirements and roles for CHD7 during cardiogenesis. CHD7 protein is shown to be present throughout the developing heart until E13.5. Conditional ablation of Chd7 in the early cardiogenic mesoderm results in embryonic lethality due to severe cardiovascular defects. These include haemorrhaging and oedema, major venous and arterial pole malformations, and disruption to cardiac innervation and vascularisation. To further dissect tissue-specific requirements for CHD7, cardiomyocyte-, second heart field- and endothelial-specific knockdowns were also performed. Each cross results in a milder subset of the cardiac defects observed after mesodermal ablation, indicating that CHD7 is required in multiple lineages within the cardiogenic mesoderm. Microarray analysis and validation by in situ hybridisation were used to identify genes dysregulated in the heart following mesodermal Chd7 ablation. These included components of the Semaphorin and Slit-Robo signalling pathways, which have known roles in heart development. Furthermore, aberrant expression of genes involved in calcium handling within cardiomyocytes is seen. Excitation-contraction coupling is disrupted in mutant embryonic cardiomyocytes, demonstrating relevance of the gene expression changes at the cellular level. This work reveals a requirement for CHD7 in mesodermal cardiac progenitors for both inflow and outflow tract development. Novel pathways are identified downstream of CHD7 activity in the developing heart, including the extracellular Semaphorin and Slit-Robo pathways, as well as components of the excitation-contraction coupling machinery.