This research aims to analyze the molecular genetics of faciogenital dysplasia (FGDY), an X-linked developmental disorder that adversely affects the formation of multiple skeletal structures including elements of the face, spine, and distal extremities. The gene responsible for this disorder, FGD1, encodes a guanine nucleotide exchange factor (GEF) that specifically activates Cdc42, a member of the Rho (Ras-homology) family of the p21 GTPases. By activating Cdc42, FGD1 stimulates fibroblasts to form filopodia, cytoskeletal elements involved in cellular signaling, adhesion and migration. Through Cdc42, FGD1 also activates the stress-activated protein kinase (SAPK) signaling cascade, a pathway that regulates cell growth and differentiation. Preliminary studies show that FGD1 is primarily expressed in skeletal progenitors including vertebral precartilagenous condensations, ossifying cervical vertebrae, and ossifying craniofacial bones, bones affected in FGDY. However, data also shows that mammalian genomes contain multiple FGD1 homologues, a result that suggests that FGD1 may be a component of a partially redundant Cdc42 activation system. Together, the accumulated data indicates that FGD1 is a component of a developmentally regulated signaling pathway involved in cellular morphology and mammalian skeletogenesis. The overall goals of this application are to develop a comprehensive model to further elucidate the role of the FGD/Cdc42 signaling cascade in mammalian morphogenesis. To this end, cross-hybridization and degenerate PCR amplification techniques will be used to isolate additional FGD1 homologues; mouse and zebrafish FGD cDNA clones will be used to determine the spatial and temporal patterns of gene expression during embryogenesis. To provide precise cellular localization, FGD1-directed antibodies will be used to study embryonic tissue by immunocytochemistry. Microinjection and in vitro assays will be used to study the molecular biology and to molecularly dissect the functional domains of the FGD proteins. Two-hybrid and other molecular techniques will be used to isolate and characterize additional components of the FGD1/Cdc42 signal transduction pathway. Transgenic mice that express recombinant FGD1 proteins will be used to study the developmental consequences of dysregulated FGD1 expression. FGD-deficient mice will be generated to study the developmental consequences of null FGD genotypes. Compound FGD-deficient mice will be generated to examine FGD homologues for functional redundancy. These analyses will provide a means to study the molecular genetics of the FGD/Cdc42 signal transduction pathway in mammalian morphogenesis. These analyses should result in a more thorough understanding of skeletogenesis, signal transduction, and the general principles responsible for the diversity of skeletal form.
