Mouse embryos lacking RXR alpha are resistant to retinoic-acid-induced limb defects

Embryonic exposure to the vitamin A metabolite retinoic acid (RA) causes malformations in numerous developing tissues, including the limbs, which serves as a model system of retinoic acid action. RA treatment of wild-type mouse embryos results in digit truncations and long bone reductions. These effects are mediated by products of the retinoic acid and retinoid X receptor genes (RARs and RXRs), members of the nuclear receptor family of ligand-dependent transcription factors. Mouse embryos homozygous for a mutation in the RXR alpha gene appear normal in limb development, although such embryos are phenotypically affected in other tissues. We now describe resistance to limb malformations normally induced by teratogenic RA exposure in the RXR alpha-/- background. RA treatments that cause limb defects in 100% of wild-type embryos fail to elicit malformations in RXR alpha homozygotes, implicating RXR alpha as a component in the teratogenic process in the limbs. Heterozygous embryos are intermediate in sensitivity to RA, suggesting the importance of RXR alpha gene dosage in limb teratogenesis. Expression of the RA-inducible gene RAR beta 2 was equivalent between wild-type and homozygous embryos after RA treatment. RA treatment also did not distinguish between wild-type and RXR alpha -/- embryos in the spatial expression of sonic hedgehog (Shh) and hoxd-12, two other genes implicated in limb development. However, the quantitative level of hoxd-12 expression was elevated in RXR alpha -/- embryos. These observations indicate that transcriptional processes which are inappropriately regulated in the mouse limb by exogenous RA require RXR alpha for their execution, and that specific teratogenic processes, as well as specific normal developmental processes under vitamin A control, occur through individual members of the RXR and RAR families.