Cathy Mendelsohn

Compound null mutations of retinoic acid receptor (RAR) genes lead to lethality in utero or shortly after birth and to numerous developmental abnormalities. In the accompanying paper (Lohnes, D., Mark., M., Mendelsohn, C., Dollé, P., Dierich, A., Gorry, Ph., Gansmuller, A. and Chambon, P. (1994). Development 120, 2723-2748), we describe malformations of the head, vertebrae and limbs which, with the notable exception of the eye defects, were not observed in the offspring of vitamin A-deficient (VAD) dams. We report here abnormalities in the neck, trunk and abdominal regions of RAR double mutant mice, which include: (i) the entire respiratory tract, (ii) the heart, its outlow tract and the great vessels located near the heart, (iii) the thymus, thyroid and parathyroid glands, (iv) the diaphragm, (v) the genito-urinary system, and (vi) the lower digestive tract. A majority of these abnormalities recapitulate those observed in the fetal VAD syndrome described by Joseph Warkany's group more than fourty years ago [Wilson, J. G., Roth, C. B. and Warkany, J. (1953) Am. J. Anat., 92, 189-217; and refs therein]. Our results clearly demonstrate that RARs are essential for vertebrate ontogenesis and therefore that retinoic acid is the active retinoid, which is required at several stages of the development of numerous tissues and organs. We discuss several possibilities that may account for the apparent functional redundancy observed amongst retinoic acid receptors during embryogenesis.

ABSTRACT Numerous congenital malformations have been observed in fetuses of vitamin A-deficient (VAD) dams [Wilson, J. G., Roth, C. B., Warkany, J., (1953), Am. J. Anat. 92, 189-217]. Previous studies of retinoic acid receptor (RAR) mutant mice have not revealed any of these malformations [Li, E., Sucov, H. M., Lee, K.-F., Evans, R. M., Jaenisch, R. (1993) Proc. Natl. Acad. Sci. USA 90, 1590-1594; Lohnes, D., Kastner, P., Dierich, A., Mark, M., LeMeur, M., Chambon, P. (1993) Cell 73, 643-658; Lufkin, T., Lohnes, D., Mark, M., Dierich, A., Gorry, P., Gaub, M. P., LeMeur, M., Chambon, P. (1993) Proc. Natl. Acad. Sci. USA 90, 7225-7229; Mendelsohn, C., Mark, M., Dollé, P., Dierich, A., Gaub, M.P., Krust, A., Lampron, C., Chambon, P. (1994a) Dev. Biol. in press], suggesting either that there is a considerable functional redundancy among members of the RAR family during ontogenesis or that the RARs are not essential transducers of the retinoid signal in vivo. In order to discriminate between these possibilities, we have generated a series of RAR compound null mutants. These RAR double mutants invariably died either in utero or shortly after birth and presented a number of congenital abnormalities, which are reported in this and in the accompanying study. We describe here multiple eye abnormalities which are found in various RAR double mutant fetuses and are similar to those previously seen in VAD fetuses. Interestingly, we found further abnormalities not previously reported in VAD fetuses. These abnormalities affect ocular glands, salivary glands and their associated ducts, the axial and limb skeleton, and all skeletal elements derived from the mesectoderm of the frontonasal mass and of the second and third pharyngeal arches. RAR double mutants also exhibit supernumerary cranial skeletal elements that are present in the ancestral reptilian skull. The role of retinoic acid (RA) and of the RARs in the ontogenesis of the affected structures, particularly of those that are derived from mesenchymal neural crest cells, is discussed.