Rescue of the Limb Deformity in Hammertoe Mutant Mice by Retinoic Acid-induced Cell Death

Retinoids, used therapeutically primarily in the treatment of skin disorders, are potent teratogens. Several craniofacial, neural tube, and limb defects derive from a selective increase in cell death by retinoic acid in sites of spontaneous programmed cell death. Previously we showed that programmed cell death in the limb was apoptotic, and that the webbing of the foot of the Hammertoe mutant mouse correlates with diminished cell death in these regions of webbing. We therefore examined the effect of the induction of cell death by retinoic acid in normal and mutant limbs. Here we report that exogenously administered retinoic acid enhances cell death in the interdigital and marginal regions of the limb. This cell killing is apoptotic by several criteria. We also report that retinoic acid induces cell death in areas of the Hammertoe limb that display a suppression of cell death during development. This induction of cell death ameliorates the mutant phenotype. These results establish that a genetic defect in cell death can be modified by retinoic acid. Retinoic acid, therefore, may be a signal involved in the regulation of cell death during normal limb development. However, neither the effect of retinoic acid on cell death nor the defect of cell death in Hammertoe correlates with an altered expression pattern of the homeobox-containing Msx genes, the retinoic acid receptor beta gene, or the ability of endogenous retinoic acid to bind its receptors. We conclude that retinoic acid may influence pattern formation and cell death through an indirect mechanism.