The DNA demethylase TET1 modifies the impact of maternal folic acid status on embryonic brain development

Folic acid (FA) prevents neural tube defects (NTDs), but it remains unclear why many cases of human NTDs still occur despite FA supplementation. Here, we investigate how the DNA demethylase TET1 interacts with maternal dietary FA status to regulate embryonic brain development. We determined that cranial NTDs in Tet1-/- embryos display higher penetrance in non-inbred than in inbred genetic backgrounds and are resistant to FA supplementation across strains. Maternal diets that are either too rich or deficient in FA are linked to an increased incidence of mild cranial deformities in wild type and Tet1+/- offspring and altered DNA hypermethylation in Tet1-/- embryos primarily at neurodevelopmental loci. Excess FA in Tet1-/- embryos results in phospholipid metabolite loss and reduced expression of multiple membrane solute carriers, including a FA transporter gene which exhibits increased promoter DNA methylation, thus mimicking a pseudo-FA deficiency. FA deficiency reveals an impact of Tet1 haploinsufficiency that contributes to DNA hypermethylation and susceptibility to NTDs. Overall, our study reveals that epigenetic dysregulation may underlie the resistance of NTDs to FA supplementation.