The Effects of Folate Deficiency and Arsenic Exposure on Thymidylate Synthesis in Serine Hydroxymethyltransferase 1‐Deficient Mouse Embryonic Fibroblasts

Abstract

Low maternal folate is associated with the failure of the neural tube to close during development, leading to a class of developmental anomalies referred to as Neural Tube Defects (NTDs). NTDs are observed in mouse embryos with impaired de novo thymidylate (dTMP) biosynthesis resulting from decreased SHMT1 expression when the mother is fed a folate‐deficient diet. 5,10‐methylenetetrahydrofolate is a cofactor for thymidylate synthase (TYMS) in the conversion of deoxyuridylate (dUMP) to dTMP via one‐carbon metabolism. De novo thymidylate synthesis is critical to prevent uracil misincoporation, which can lead to double strand breaks (DSBs). Tetrahydrofolate (THF) is converted into 5,10‐methyleneTHF by serine hydroxymethyltransferase 1 and 2 (SHMT1, SHMT2) and the methylenetetrahydrofolate dehydrogenase 1 (MTHFD1) at the replication fork. Arsenic is also shown to induce NTDs in animal models. Reduced SHMT1, SHMT2, and MTHFD1 expression, either alone or in combination, lead to a greater extent of γH2AX phosphorylation in HeLa cells, indicating that these proteins contribute to genomic stability. Additionally, we observed that As2O3 exposure increased genome instability and changed the distribution of cells across the cell cycle in HeLa cells. Here, we present the extent of DNA DSBs, apoptosis, and p53 expression in mouse embryonic fibroblasts in response to arsenic exposure and folate deficiency. Shmt1 −/− MEFs were isolated from embryos derived from dams fed either a folate‐deficient or control diet and cultured in folate‐replete or folate‐depleted media with or without 5μM As2O3. Folate deficiency and As2O3 exposure result in the formation of DNA DSBs, with the greatest effect seen with folate deficiency and As2O3 exposure in combination. These data indicate that folate deficiency and As2O3 exposure affect genome stability in SHMT1‐deficient MEFs.Support or Funding InformationPublic Health Services grant HD059120