PROJECT SUMMARY Dental and craniofacial abnormalities are some of the most common congenital conditions in the United States, and tooth abnormalities occur in 3-4% of adults. For several dental and craniofacial disorders, mutations in regulatory genes have been identified. For example, mutations in PITX2 and EDA are both associated with missing or misshapen primary and adult teeth. Understanding how these genes are regulated and function in normal and disease contexts is thus important to better understand how teeth form and regenerate. However, because developing teeth are dynamic, heterogeneous tissues, and the initiation of and response to molecular signals is often cell-type-specific, we currently lack a clear understanding of how Pitx2 and Eda are spatially and temporally regulated during tooth initiation, and which gene network(s) they affect to promote tooth placode proliferation. Therefore, the overall goal of this work is to determine the cell-type and tissue-specific gene regulation and signaling networks that underlie tooth placode initiation and morphogenesis. This goal will be achieved through the completion of several objectives in the threespine stickleback, a tractable fish model for developmental biology that, like humans but unlike mice, has the ability to replace teeth. First, genomic enhancers that regulate the expression of Pitx2 and Eda in developing teeth will be identified and analyzed to improve our ability to predict enhancers from genomic sequence (Specific Aim 1). Next, the effects of Pitx2 and Eda mutant alleles on early tooth germ cell proliferation and morphology will be quantified, both in initial and replacement teeth (Specific Aim 2). Finally, the downstream transcriptional effects of Pitx2 and Eda mutant alleles will be quantified using several complementary techniques, specifically in situ hybridization and cell proliferation assays (Specific Aim 2). This work will directly test the hypothesis that Pitx2 and Eda regulate dental epithelial cell proliferation of neighboring cells through distinct downstream targets. In addition to improving our functional understanding of two important master regulator genes that cause human tooth disorders, this work will provide training in traditional and cutting-edge developmental biology techniques in a dynamic and supportive scientific environment. The acquisition of these skills, interactions within the local scientific community, and training through courses at the research institute will facilitate the ability of the applicant to conduct rigorous, independent research of the genetic and developmental basis of tooth formation.