No abstract

).Retinoic acid, the active derivative of vitamin A (retinol), is a hormonal signaling molecule that acts in developing and adult tissues 1 . The Cyp26a1 (cytochrome p450, 26) protein metabolizes retinoic acid into more polar hydroxylated and oxidized derivatives 2,3 . Whether some of these derivatives are biologically active metabolites has been debated 4,5 . Cyp26a1 -/-mouse fetuses have lethal morphogenetic phenotypes mimicking those generated by excess retinoic acid administration, indicating that human CYP26A1 may be essential in controlling retinoic acid levels during development 6,7 . This hypothesis suggests that the Cyp26a1 -/-phenotype could be rescued under conditions in which embryonic retinoic acid levels are decreased. We show that Cyp26a1 -/-mice are phenotypically rescued by heterozygous disruption of Aldh1a2 (also known as Raldh2), which encodes a retinaldehyde dehydrogenase responsible for the synthesis of retinoic acid during early embryonic development 8,9 . Aldh1a2 haploinsufficiency prevents the appearance of spina bifida and rescues the development of posterior structures (sacral/caudal vertebrae, hindgut, urogenital tract), while partly preventing cervical vertebral transformations and hindbrain pattern alterations in Cyp26a1 -/-mice. Thus, some of these double-mutant mice can reach adulthood. This study is the first report of a mutation acting as a dominant suppressor of a lethal morphogenetic mutation in mammals. We provide genetic evidence that ALDH1A2 and CYP26A1 activities concurrently establish local embryonic retinoic acid levels that must be finely tuned to allow posterior organ development and to prevent spina bifida.

Targeted inactivation of the mouse retinaldehyde dehydrogenase 2(RALDH2/ALDH1a2), the enzyme responsible for early embryonic retinoic acid synthesis, is embryonic lethal because of defects in early heart morphogenesis. Transient maternal RA supplementation from E7.5 to (at least)E8.5 rescues most of these defects, but the supplemented Raldh2–/– mutants die prenatally, from a lack of septation of the heart outflow tract (Niederreither, K., Vermot, J.,Messaddeq, N., Schuhbaur, B., Chambon, P. and Dollé, P.(2001). Development 128, 1019-1031). We have investigated the developmental basis for this defect, and found that the RA-supplemented Raldh2–/– embryos exhibit impaired development of their posterior (3rd-6th) branchial arch region. While the development of the first and second arches and their derivatives, as well as the formation of the first branchial pouch, appear to proceed normally, more posterior pharyngeal pouches fail to form and the pharyngeal endoderm develops a rudimentary, pouch-like structure. All derivatives of the posterior branchial arches are affected. These include the aortic arches, pouch-derived organs(thymus, parathyroid gland) and post-otic neural crest cells, which fail to establish segmental migratory pathways and are misrouted caudally. Patterning and axonal outgrowth of the posterior (9th-12th) cranial nerves is also altered. Vagal crest deficiency in Raldh2–/–mutants leads to agenesis of the enteric ganglia, a condition reminiscent of human Hirschprung's disease. In addition, we provide evidence that: (i)wildtype Raldh2 expression is restricted to the posteriormost pharyngeal mesoderm; (ii) endogenous RA response occurs in both the pharyngeal endoderm and mesoderm, and extends more rostrally than Raldh2expression up to the 2nd arch; (iii) RA target genes (Hoxa1, Hoxb1)are downregulated in both the pharyngeal endoderm and mesoderm of mutant embryos. Thus, RALDH2 plays a crucial role in producing RA required for pharyngeal development, and RA is one of the diffusible mesodermal signals that pattern the pharyngeal endoderm.