title: Disruption of the folate pathway in zebrafish causes developmental defects.
abstract: BACKGROUND
  Folic acid supplementation reduces the risk of neural tube defects and congenital heart defects. The biological mechanisms through which folate prevents birth defects are not well understood. We explore the use of zebrafish as a model system to investigate the role of folate metabolism during development.
  
  
  RESULTS
  We first identified zebrafish orthologs of 12 human folate metabolic genes. RT-PCR and in situ analysis indicated maternal transcripts supply the embryo with mRNA so that the embryo has an intact folate pathway. To perturb folate metabolism we exposed zebrafish embryos to methotrexate (MTX), a potent inhibitor of dihydrofolate reductase (Dhfr) an essential enzyme in the folate metabolic pathway. Embryos exposed to high doses of MTX exhibited developmental arrest prior to early segmentation. Lower doses of MTX resulted in embryos with a shortened anterior-posterior axis and cardiac defects: linear heart tubes or incomplete cardiac looping. Inhibition of dhfr mRNA with antisense morpholino oligonucleotides resulted in embryonic lethality. One function of the folate pathway is to provide essential one-carbon units for dTMP synthesis, a rate-limiting step of DNA synthesis. After 24 hours of exposure to high levels of MTX, mutant embryos continue to incorporate the thymidine analog BrdU. However, additional experiments indicate that these embryos have fewer mitotic cells, as assayed with phospho-histone H3 antibodies, and that treated embryos have perturbed cell cycles.
  
  
  CONCLUSIONS
  Our studies demonstrate that human and zebrafish utilize similar one-carbon pathways. Our data indicate that folate metabolism is essential for early zebrafish development. Zebrafish studies of the folate pathway and its deficiencies could provide insight into the underlying etiology of human birth defects and the natural role of folate in development.
authors.affiliation: National Human Genome Research Institute, National Institutes of Health, Bethesda, MD, USA.
authors.forename: Marina S
authors.initials: MS
authors.lastname: Lee
authors.forename: Jenna R
authors.initials: JR
authors.lastname: Bonner
authors.forename: David J
authors.initials: DJ
authors.lastname: Bernard
authors.forename: Erica L
authors.initials: EL
authors.lastname: Sanchez
authors.forename: Eric T
authors.initials: ET
authors.lastname: Sause
authors.forename: R Reid
authors.initials: RR
authors.lastname: Prentice
authors.forename: Shawn M
authors.initials: SM
authors.lastname: Burgess
authors.forename: Lawrence C
authors.initials: LC
authors.lastname: Brody
mesh_terms: D000818:Animals; D002453:Cell Cycle / Q000187:drug effects; D002454:Cell Differentiation; D047108:Embryonic Development; D005492:Folic Acid / Q000378:metabolism*; D005493:Folic Acid Antagonists / Q000494:pharmacology; D006801:Humans; D053858:Metabolic Networks and Pathways; D008727:Methotrexate / Q000494:pharmacology; D013762:Tetrahydrofolate Dehydrogenase / Q000378:metabolism; D015027:Zebrafish / Q000196:embryology* / Q000378:metabolism*
chemical_list: D005493:Folic Acid Antagonists; D005492:Folic Acid; D013762:Tetrahydrofolate Dehydrogenase; D008727:Methotrexate