To understand the basis of axial specification and neural development, mouse models are commonly utilized. Bent tail (Bn) is a spontaneous mutation on the mouse X chromosome that produces tail deformities and open neural tube defects (NTDs). Analysis of progeny from an intraspecific backcross places the mutation between the microsatellites DXMit166 and DXMit140. Refined genetic and physical mapping of the Bn critical region demonstrate that the mutation is associated with a <170 kb submicroscopic deletion, including the entire Zic3 locus. Human mutations in ZIC3 are associated with left-right axis malformations. The presence of anal and spinal abnormalities in some patients and deletion of Zic3 in Bn mice support a key role for this gene in neural tube development and closure. However, mutations in the ZIC3 gene have yet to be identified in families with X-linked NTDs.
Holoprosencephaly (HPE) results from abnormal development of the forebrain. One gene associated with HPE in humans, TG interacting factor (TGIF), was identified by its ability to bind to the retinoid X receptor response element, and has been shown to play a role in regulating TGF-β signaling. HPE is not evident in mice carrying the targeted null allele of Tgif. To elucidate whether Tgif in conjunction with reductions in TGF-β signaling can cause HPE, mice that have mutations in both Smad2 and Tgif were generated. Results show that of the Smad2+/-; Tgif+/- and Smad2+/-; Tgif-/- embryos, one third display HPE. Molecular characterization at E9.5 reveals that Shh, Fgf8, Six3 and Zic2 expression are not affected. The forebrain domain of Otx2 expression shows a modest to nearly complete reduction in affected embryos. Published data has shown that introduction of retinoic acid (RA) to cultured embryos or pregnant dams can induce HPE in embryos. Some of these studies have further shown a reduction in rostral Otx2 expression. The known interaction of Tgif and RA signaling led to the evaluation of the possibility that the HPE resulted from altering RA pathways. Analysis of developing embryos provided evidence that genetic deficiencies of Smad2 and Tgif allows for increased sensitivity to RA, suggesting a unique link between genetics and environmental teratogens.