ENHANCED EXPRESSION OF LIMB MALFORMATIONS AND AXIAL SKELETON ALTERATIONS IN LEGLESS MUTANTS BY TRANSPLACENTAL EXPOSURE TO RETINOIC ACID

This manuscript reports on the limb malformations and axial skeleton a lterations found in legless fetuses and their heterozygote and wild-ty pe littermates transplacentally exposed to all-trans-retinoic acid via a single intraperitoneal injection on Day 7, 8, 9, 9.5, 10, 10.5, or 11 of gestation. The most surprising aspect of the results was the tem poral sensitivity of the legless mouse limb to exogenous retinoic acid . On Day 11, when both fore- and hindlimbs of nonmutant embryos can be made abnormal by retinoic acid and other teratogens, retinoic acid di d not increase the frequency or severity of legless hindlimb defects a nd forelimb malformations were only slightly enhanced. On the other ha nd, retinoic acid administration on Day 7 exacerbated forelimb malform ations in legless fetuses at a time when visible emergence of the affe cted structure is still 48 hr away. Heterozygote and wild-type fetuses had no limb malformations at this time point. A similar phenomenon wa s observed with hindlimb malformations after retinoic acid exposure on Day 8 except for a few mild limb malformations in heterozygotes at a high dose of retinoic acid. This early hypersensitivity of fore- and h indlimbs was followed by a period of reduced sensitivity (Day 8 foreli mb; Day 9 hindlimb) when even very high doses (50 mg/kg) induced minim al changes in the typical legless malformation pattern. Subsequently, at the time of visible limb bud emergence (Day 9 forelimbs; Day 10 hin dlimbs), sensitivity to exogenous retinoic acid was again detected. Su rprisingly, the altered malformation patterns induced by retinoic acid in lgl mutants were nearly identical to those from earlier, preemerge nce exposure. A number of axial skeleton alterations were induced in l egless fetuses by retinoic acid, especially after exposure on Days 7, 8, or 9. Posterior truncations were particularly noteworthy, showing a graded response in which frequency and severity of truncation were wo rst in lgl/lgl fetuses; heterozygotes gave an intermediate response, a nd wild-type fetuses were least affected. This exacerbation of the leg less phenotype by exogenous retinoic acid coupled with the similarity between legless and retinoid malformations suggest that the legless mu tation has altered endogenous retinoid homeostasis or a downstream ret inoid-responsive gene. (C) 1994 Academic Press, Inc.