Compound mutants for retinoic acid receptor (RAR)β and RARα1 reveal developmental functions for multiple RARβ isoforms

Mice with targeted disruptions in retinoic acid receptor genes have been generated to assess the role of nuclear receptors as transducers of the retinoid signal during vertebrate development. Mice with mutations that disrupt all isoforms of the RARa, RARfl and RAR), genes as well as for the individual RARal, RARfl2 and RARe2 have been described. By breeding the RARal and RARfl strains together we have generated double mutants which have striking phenotypes not discernible in mice homozygous for the individual mutations. Mice lacking both RARal and RARfl died shortly after birth because of hypoxia, although individual RARal and RARfl mutants were phenotypically normal. As previously observed in RAR compound mutants, histological examination of 18.5 dpc fetuses of RARal-/-13 -/double mutants revealed a number of congenital malformations which in many respects were similar to those observed in fetuses of vitamin A-deficient mothers. The regions of congenital defects in RARal/ f l -/double mutants included the eye, the skull, the respiratory tract, the heart, the aortic arch-derived great vessels, and urogenital system. The penetrance of malformations in RARal/ f l -/mutants was greater than that in the reported RARal-/-/32 -/double mutants, Moreover, RARal-/-fl -/mutants exhibited hypoplastic lungs and ossified fusion between basioccipital and exoccipital bones that were not reported in the RARal/ f l2 -lanimals, and displayed ectopic thymus and an unique defect in testis suggesting specific roles for RARfll, 3 and/or 4 isoforms in these structures. The RARa 1 single mutant animals as well as RARa l-/-fl-/double mutant mice were susceptible to the teratogenic effects of RA, demonstrating that RARal and RARfl isoforms singly or in combination do not play a major role in RA-induced craniofacial malformation and limb deformities.