Forelimb ectrodactyly induced by retinoic acid in the mouse: Gene/environment and environment/environment interactions.

摘要

Retinoids, vitamin A derivatives, are teratogenic in humans. In rodents, exogenous all-trans-retinoic acid (RA) has been shown to cause congenital malformations in many organs depending on dose and time of administration to embryos. RA induces limb dysmorphology in the mouse, with the anatomical type of defect dependent on time of exposure. During early murine limb development, RA induces a forelimb reduction defect initiating at the autopod, with varying susceptibilities for inbred strains. C57BL/6N mice are highly susceptible to this defect, while SWV mice are resistant. In this dissertation, RA-induced forelimb ectrodactyly has been investigated by two approaches. In the first approach, a full genome scan was performed in RA-treated backcross fetuses of F1 (C57BL/6N x SWV) males to C57BL/6N females to identify the genetic basis of the strain difference in this defect. The genome-wide analysis revealed a highly significant linkage to a chromosome 11 locus near D11Mit39 with a maximum logarithm of odds (LOD) score of 9.0. This locus was confirmed in an independent reciprocal backcross. Further, the expression of candidate genes in the region of chromosome 11 near D11Mit39 have been examined in RA- or vehicle-treated forelimb buds from C57BL/6N and SWV mice. The expression analysis showed that Wnt9b is differentially upregulated by RA between the C57BL/6N and SWV forelimb buds, suggesting that Wnt9b might have an important role in the differential sensitivity between C57BL/6N and SWV mice for the induction of RA-induced forelimb defects. These studies have led to hypotheses implicating the Writ pathway and bone density in the induction of forelimb ectrodactyly, as well as identifying a potential locus for Meckel syndrome. In the second approach, the interactive effects of cadmium and RA on the induction of forelimb ectordactyly have been investigated in the C57BL/6N mouse. The results showed that coadministration of cadmium and RA exhibited potentiative and synergistic effects on forelimb ectrodactyly. Further, concurrent exposure to subthreshold doses of cadmium and RA exceeded the threshold for the forelimb ectrodactyly. The implication of this experiment is that, in contrast to what is in the literature, multiple exposures to low level teratogenic agents may be able to induce human congenital malformations.