Finding the genetic mechanisms of folate deficiency and neural tube defects-Leaving no stone unturned

Abstract

Neural tube defects (NTDs) occur secondary to failed closure of the neural tube between the third and fourth weeks of gestation. The worldwide incidence ranges from 0.3 to 200 per 10,000 births with the United States of American NTD incidence at around 3-6.3 per 10,000 dependent on race and socioeconomic background. Human NTD incidence has fallen by 35-50% in North America due to mandatory folic acid fortification of enriched cereal grain products since 1998. The US Food and Drug Administration has approved the folic acid fortification of corn masa flour with the goal to further reduce the incidence of NTDs, especially among individuals who are Hispanic. However, the genetic mechanisms determining who will benefit most from folate enrichment of the diet remains unclear despite volumes of literature published on studies of association of genes with functions related to folate metabolism and risk of human NTDs. The advances in omics technologies provides hypothesis-free tools to interrogate every single gene within the genome of NTD affected individuals to discover pathogenic variants and methylation targets throughout the affected genome. By identifying genes with expression regulated by presence of folate through transcriptome profiling studies, the genetic mechanisms leading to human NTDs due to folate deficiency may begin to be more efficiently revealed.

Keywords: folate one-carbon metabolism; genetic association; genetic variants; human neural tube defects; methylation.

Figure 1. Folate one-carbon metabolism and interacting pathways components

Folate one-carbon metabolism (FOCM) and methionine cycle (Met cycle) is a major metabolic pathway involving many biomolecules involved in multiple metabolic pathways vital to cell survival, cell growth and proliferation, and cell differentiation. The participating biomolecules include different forms of folates, B-vitamins, betaine, choline, serine, glycine, threonine, and methionine for FOCM/Met metabolism, and glucose to genearate energy for the metabolic processes. Biomolecules breaking down from food by maternal digestive enzymes will be absorbed by maternal enterocytes. Absorbed biomolecules will be transported into maternal circulation to supply maternal cells and organs and also to the developing embryo via the placenta during pregnancy. FOCM biomolecules transported into cells will be utilized by the FOCM/Met cycle enzymes producing important substrates for the downstream network of metabolic pathways including purine and pyrimidine synthesis, glycine cleavage, NADPH/NADP synthesis, peroxides transsulfuration, protein synthesis, phospholipid synthesis, and transmethylation of DNA, RNA and protein. Ser – serine, Gly – glycine, Thr –threonine, Met – methionine, Hcy – homocysteine, SAM – S-adenosyl-methionine, THF –tetrahydrofolate, fTHF– formyl-THF, CH2THF – methylene-THF, CH3THF- methyl-THF. B-vitamins: B2- riboflavin, B6 - pyridoxine, B9 – folic acid or THF, B12 – cobalamins, PE - phosphatidylethanolamine, PS - phosphatidylserine, PC - phosphatidylcholine.