Prenatal choline availability modulates brain and liver development in rats via changes in DNA methylation.

摘要

An adequate dietary supply of the essential nutrient, choline, is needed for the normal development of humans and animals. Previous studies have shown that during the second half of gestation there is an increased demand for choline. Adult rats supplemented with choline during embryonic days (E) 11-17 had improved memory performance and did not exhibit age-related memory decline, while prenatally choline-deficient animals had memory deficits. Choline, following its metabolic oxidation to betaine, provides metabolic methyl groups used for the production of S-adenosylmethionine (SAM), a methyl donor utilized by a family of DNA methyltransferases (DNMTs) that methylate the cytosine base in a CpG sequence of DNA. Therefore, we investigated the effects of varied choline supply between E11 and E17 on several indices of methyl group metabolism in fetal liver and brain and found that the levels of SAM were increased by approximately 25% in the liver and brain of E17 rat fetuses whose mothers consumed a choline-supplemented diet. Surprisingly, however, global DNA methylation was increased on E17 in the liver and brain of choline-deficient fetuses. A possible mechanism is upregulation of DNMT expression by choline deficiency and, indeed, we observed overexpression of mRNA for DNMT1, DNMT3A and methyl CpG-binding domain 2 protein (MBD2) associated with choline deficiency in E17 liver and frontal cortex. Previous studies also showed that the prenatal choline supply affects the expression of multiple genes in frontal cortex, including insulin-like growth factor 2 (Igf2). Igf2 expression is regulated by several DNA domains that, when methylated, prevent a repressor from binding and enhance gene expression. We found that the differentially methylated region 2 (DMR2) of Igf2 was hypermethylated and Igf2 mRNA was overexpressed in the liver of E17 choline-deficient embryos correlating with the expression of DNMT1 observed in these animals. These results show that maternal choline intake during pregnancy modulates fetal DNA methylation machinery in a complex fashion. In choline deficiency, there is an apparently compensatory mechanism of upregulation of DNA methylation by the induction of DNMTs.