Developing with lethal RA levels: genetic ablation ofRargcan restore the viability of mice lackingCyp26a1

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Abstract
We have previously reported that the retinoic acid (RA) catabolizing enzyme CYP26A1 plays an important role in protecting tail bud tissues from inappropriate exposure to RA generated in the adjacent trunk tissues by RALDH2, and that Cyp26a1-null animals exhibit spina bifida and caudal agenesis. We now show that, in the absence of Cyp26a1, retinoic acid receptor gamma (RARγ) mediates ectopic RA-signaling in the tail bud. We also show that activated RARγ results in downregulation ofWnt3a and Fgf8, which integrate highly conserved signaling pathways known for their role in specifying caudal morphogenesis. Ablation of the gene for RARγ (Rarg) rescues Cyp26a1-null mutant animals from caudal regression and embryonic lethality, thus demonstrating that CYP26A1 suppresses the RA-mediated downregulation of WNT3A and FGF8 signaling pathways by eliminating ectopic RA in gastrulating tail bud mesoderm.