Shmt1 and de novo thymidylate biosynthesis underlie folate-responsive neural tube defects in mice

Abstract

Background: Folic acid supplementation prevents the occurrence and recurrence of neural tube defects (NTDs), but the causal metabolic pathways underlying folic acid-responsive NTDs have not been established. Serine hydroxymethyltransferase (SHMT1) partitions folate-derived one-carbon units to thymidylate biosynthesis at the expense of cellular methylation, and therefore SHMT1-deficient mice are a model to investigate the metabolic origin of folate-associated pathologies. Objectives: We examined whether genetic disruption of the Shmt1 gene in mice induces NTDs in response to maternal folate and choline deficiency and whether a corresponding disruption in de novo thymidylate biosynthesis underlies NTD pathogenesis. Design: Shmt1 wild-type, Shmt1 +/2 , and Shmt1 2/2 mice fed either folate-and choline-sufficient or folate-and choline-deficient diets were bred, and litters were examined for the presence of NTDs. Biomarkers of impaired folate metabolism were measured in the dams. In addition, the effect of Shmt1 disruption on NTD incidence was investigated in Pax3 Sp mice, an established folate-responsive NTD mouse model. Results: Shmt1 +/2 and Shmt1 2/2 embryos exhibited exencephaly in response to maternal folate and choline deficiency. Shmt1 disruption on the Pax3 Sp background exacerbated NTD frequency and severity. Pax3 disruption impaired de novo thymidylate and purine biosynthesis and altered amounts of SHMT1 and thymidylate synthase protein.Conclusions: SHMT1 is the only folate-metabolizing enzyme that has been shown to affect neural tube closure in mice by directly inhibiting folate metabolism. These results provide evidence that disruption of Shmt1 expression causes NTDs by impairing thymidylate biosynthesis and shows that changes in the expression of genes that encode folate-dependent enzymes may be key determinates of NTD risk.Am J Clin Nutr 2011;93:789-98.

Association between SNPs in genes involved in folate metabolism and preterm birth risk

“…One-carbon metabolism can be impaired by genetic variation, nutrient deficiencies, or both, which can simultaneously disrupt de novo nucleotide biosynthesis and S-adenosylmethionine synthesis to result in reduced proliferative capacity, increased uracil in DNA, elevated plasma homocysteine, and reduced cellular methylation (Stover, 2004;Beaudin et al, 2011).…”

Deletion of Mthfd1l causes embryonic lethality and neural tube and craniofacial defects in mice

“…Homozygous knockout of Shmt1, which encodes a cytoplasmic folate-metabolizing enzyme (Fig. 1, reaction 4), gives rise to a low frequency of NTDs in embryos from Shmt1 −/− dams fed a folate-deficient diet (18,19). Amt encodes an aminomethyltransferase that is a subunit of the mitochondrially localized glycine cleavage system (GCS), which processes glycine to donate 1C units to THF, forming CH 2 -THF (Fig.…”

Nuclear Localization of de Novo Thymidylate Biosynthesis Pathway Is Required to Prevent Uracil Accumulation in DNA

“…Shmt1 Ϫ/Ϫ mice are viable and fertile (3) and retain about 25% de novo thymidylate biosynthesis capacity in isolated hepatic nuclei because of redundant function of the enzyme SHMT2␣, encoded by Shmt2 through alternative promoter usage (8). Despite this functional redundancy between SHMT1 and SHMT2␣ in the cytoplasm and nucleus, SHMT1 is the major provider of methyleneTHF; mice are sensitive to reductions in SHMT1 expression, as indicated by elevated levels of uracil in nuclear DNA in Shmt1 ϩ/Ϫ mice (3, 9), and their increased susceptibility to neural tube defects (10) and Apc min/ϩ -mediated intestinal tumors (9). Interestingly, a common SHMT1 human variant, L474F, is not an effective substrate for UBC-9-catalyzed sumoylation in vitro and exhibits impaired nuclear translocation at S phase (8), suggesting that its association with cardiovascular disease (11,12) and lung cancer (13) risk may be due to impaired nuclear de novo thymidylate synthesis.…”