TFAP2 transcription factors in midface development, evolution, and dysplasia

Despite the diverse facial shapes, vertebrate species remarkably share a common craniofacial blueprint spatially marked by a combination of patterning transcription factor genes. These genes are found within cranial neural crest cells (NCCs), which give rise to the bulk of the bone and cartilage tissues of the face during embryonic development. Fine-tuning of the conserved genetic programs orchestrating cranial NCC patterning leads to considerable breadth in facial shapes across and within species; such molecular complexity makes them sensitive to perturbations, as indicated by the high prevalence of craniofacial birth defects like orofacial clefts. Despite our understanding of the genes and pathways defining each cranial NCC positional identity, how their programs are activated and reinforced is poorly understood. In this thesis, we hypothesize that after their acquisition, AP-2 transcription factors (TFAP2) reinforce the cranial NCC midface identity to shape the forehead, nose, cheeks. Using mouse genetics, we first show that combined loss of both Tfap2a and Tfap2b in NCCs results in a fully penetrant midfacial cleft typifying frontonasal dysplasia and that such pathology arises from defects at late migratory stages of cranial NCC development. Transcriptomic analyses revealed dysregulation of midface positional genes like the frontonasal dysplasia-associated ALX transcription factor genes Alx1, Alx3, and Alx4. Epigenome analyses (transcription factor binding, histone modifications, chromatin accessibility) suggest that dysregulated midface gene expression in absence of TFAP2 genes is partly due to a failure to maintain permissive chromatin architecture. Deploying epistasis strategies, we show that ALX is a direct output of TFAP2, while complementary approaches in zebrafish support the notion that the TFAP2-ALX axis is conserved in vertebrates. Leveraging our epigenome datasets and mouse model, we identify a transcription factor candidate to cooperate with TFAP2 in the midface. This work advances our understanding of mechanisms underlying midfacial neural crest development and frontonasal dysplasia.