Gene-environment interactions in the causation of neural tube defects: folate deficiency increases susceptibility conferred by loss of Pax3 function

Abstract

Risk of neural tube defects (NTDs) is determined by genetic and environmental factors, among which folate status appears to play a key role. However, the precise nature of the link between low folate status and NTDs is poorly understood, and it remains unclear how folic acid prevents NTDs. We investigated the effect of folate level on risk of NTDs in splotch (Sp(2)(H)) mice, which carry a mutation in Pax3. Dietary folate restriction results in reduced maternal blood folate, elevated plasma homocysteine and reduced embryonic folate content. Folate deficiency does not cause NTDs in wild-type mice, but causes a significant increase in cranial NTDs among Sp(2)(H) embryos, demonstrating a gene-environment interaction. Control treatments, in which intermediate levels of folate are supplied, suggest that NTD risk is related to embryonic folate concentration, not maternal blood folate concentration. Notably, the effect of folate deficiency appears more deleterious in female embryos than males, since defects are not prevented by exogenous folic acid. Folate-deficient embryos exhibit developmental delay and growth retardation. However, folate content normalized to protein content is appropriate for developmental stage, suggesting that folate availability places a tight limit on growth and development. Folate-deficient embryos also exhibit a reduced ratio of s-adenosylmethionine (SAM) to s-adenosylhomocysteine (SAH). This could indicate inhibition of the methylation cycle, but we did not detect any diminution in global DNA methylation, in contrast to embryos in which the methylation cycle was specifically inhibited. Hence, folate deficiency increases the risk of NTDs in genetically predisposed splotch embryos, probably via embryonic growth retardation.

Folic Acid Remodels Chromatin on Hes1 and Neurog2 Promoters during Caudal Neural Tube Development

“…Lack of dietary FA produces profound retardation in embryonic growth and developmental progression, suggesting that cell proliferation is compromised (21). In our previous work (13), we found that neural crest cells from open neural tubes of Sp Ϫ/Ϫ embryos failed to form neurospheres in culture.…”

Association of genomic instability, and the methylation status of imprinted genes and mismatch-repair genes, with neural tube defects

“…In addition, folate deficiency increases uracil mis-incorporation into DNA in mice, which could lead to genomic instability. 7 Genomic instability and changes in DNA methylation status may, therefore, have significant effects on the development of NTDs.…”

Maternal Diet Supplementation with Methyl Donors and Increased Parity Affect the Incidence of Craniofacial Defects in the Offspring of Twisted gastrulation Mutant Mice

“…Twsg1 2/2 mice do not show NTDs, indicating the dispensability of TWSG1 for neural tube development. The protective effect of methyl donors on NTD development was previously established in both NTD-susceptible mice (17,71,72) and humans (2,3,7). In contrast to the effect of methyl donor supplementation, increased parity may act to favor NTDs, based on work in humans (33).…”

“…In each stage, 3 factors are considered for their inferred effects: deficiency of TWSG1, MDS, and increased parity. Grayed factors in the first neural tube step indicate this portion of the model is derived from literature review and not from the results of this study: MDS references (2,3,7,17,71,72); parity reference (33). MDS, methyl donor supplemented.…”