Strain-dependent alterations in the expression of folate pathway genes following teratogenic exposure to valproic acid in a mouse model.

The molecular basis for the well-established hierarchy of susceptibility to valproic acid-induced neural tube defects in inbred mouse strains was examined using in situ transcription and anti-sense RNA amplification methodologies with both univariate and multivariate analyses of the resulting gene expression data. The highly sensitive SWV strain demonstrated a significant reduction in the expression of the folate binding protein (FBP-1) following the teratogenic insult at gestational day 8:18, while the more resistant LM/Bc embryos were up-regulating this gene in response to valproic acid treatment. More importantly, at all 3 gestational timepoints spanning the period of murine neural tube closure examined in this study, the LM/Bc embryos had significantly higher MTHFR (5,10-methylenetetrahydrofolate reductase) gene expression levels compared to the SWV embryos. As this folate pathway enzyme is important in homocysteine and methionine metabolism, it suggests that the SWV embryos may be hypomethylated, and essential gene expression during critical periods of neural tube closure is compromised by the teratogenic exposure to valproic acid. This study represents the first evidence of a strain difference in transcriptional activity in response to a teratogenic exposure that might be causally related to the development of the teratogen-induced congenital malformations.

Valproic acid-induced changes in gene expression during neurulation in a mouse model.

The teratogenic potential of valproic acid has been well established both in experimental models and in human clinical studies. As with all human teratogens, there are genetically determined differences in individual susceptibility to the induction of congenital defects. Using a mouse model of valproate-induced neural tube defects, a study was undertaken to examine differential changes in gene expression for selected transcription factor (Pax-3, Emx-1, Emx-2, c-fos, c-jun, creb) and cell cycle checkpoint genes (bcl-2, p53, wee-1) during neural tube closure. In general, exposure to teratogenic concentrations of valproic acid elicited GD 9:12 control levels of transcription factor mRNA expression in GD 9:0 embryos of both strains. This accelerated developmental profile is marked by significant elevation of Emx-1, Emx-2, c-fos, c-jun, and creb expression. There was also a significant over expression of the cell cycle genes p53 and bcl-2 in the LM/Bc embryos in response to the teratogenic insult. Examination of the ratio of expression of these genes clearly favored bcl-2, which supports the hypothesis that altered neuroepithelial cell proliferation rates, rather than increased apoptosis, is the underlying mechanism by which valproic acid alters normal neural tube morphogenesis. An investigation into interactive effects of these genes on the molecular profile of GD 9:0 embryos further validated this observation. That is, the overall proliferative state among the control embryos was prematurely modified into a more differentiated state following teratogenic insult. These results suggest that alterations in the expression of multiple genes are most likely responsible for valproic acid-induced neural tube defects.