The Role of the X-chromosomal Porcupine Homolog Gene in Mouse Development

WNT ligands are secreted proteins that act as signals between cells. WNTs activate several interconnected signaling pathways that are required for embryonic development as well as tissue homeostasis in adults. The X-chromosomal Porcn gene encodes a membrane-bound O-acyl transferase that is required for the acylation of all 19 WNT ligands encoded in the mammalian genome. Non-acylated WNTs fail to be secreted from the producing cell and thus do not activate downstream signaling targets. In my thesis research, I have investigated the function of Porcn in mouse embryonic development. In vitro, I have shown that Porcn is required for canonical WNT signaling in ES cells and further, for their differentiation into endodermal and mesodermal derivatives. Taking advantage of a mouse line carrying a conditional (floxed) Porcn allele that I have generated, I have focused my studies on the early embryonic roles of Porcn using Cre recombinase-mediated and X chromosome inactivation-based ablation of Porcn function in vivo. I have found that the earliest requirement for Porcn in mouse development is the induction of gastrulation. In contrast to findings from in vitro studies, I have provided evidence that Porcn is not required for pre-implantation development in vivo. Dissecting embryonic and extra- embryonic roles of Porcn, I have been able to show that Porcn is required in the extra-embryonic chorion in order to mediate chorio-allantoic fusion, whereas ablation in the extra-embryonic visceral endoderm had no apparent effects. The extra-embryonic requirement for Porcn results in a parent-of-origin effect in Porcn heterozygous females due to X chromosome inactivation. In contrast to the placentation defect causing embryonic lethality of maternal allele mutants, deletion of the paternal allele caused variable fetal defects resulting in perinatal lethality with only rare survivors to adulthood. Both fetuses and adults represent a mouse model for Focal Dermal Hypoplasia (FDH), the syndrome caused by mutations in the human PORCN gene. My studies highlight the importance of PORCN-mediated WNT signaling for gastrulation, placentation, and fetal development, but suggest that endogenous WNT secretion does not play an essential role in either implantation or blastocyst lineage specification.Ph