Interspecific variation in one-carbon metabolism within the ovarian follicle, oocyte and pre-implantation embryo: consequences for epigenetic programming of DNA methylation

One-carbon (1C) metabolism provides methyl groups for the synthesis and/or methylation of purines and pyrimidines, biogenic amines, proteins and phospholipids. Our understanding of how 1C pathways operate, however, pertains mostly to the (rat) liver. Here we report that transcripts for all bar two genes (i.e., BHMT, MAT1A) encoding enzymes in the linked methionine-folate cycles are expressed in all cell types within the ovarian follicle, oocyte and blastocyst in the cow, sheep and pig; as well as in rat granulosa cells (GCs) and human KGN cells. BHMT protein was absent in bovine theca and GCs, as was activity of this enzyme in GCs. Mathematical modelling predicted that absence of this enzyme would lead to more volatile S-adenosylmethionine-mediated transmethylation in response to 1C substrate (e.g., methionine) or cofactor provision. We tested the sensitivity of bovine GCs to reduced methionine (from 50 to 10µM) and observed a diminished flux of 1C units through the methionine cycle. We then used Reduced-Representation Bisulfite Sequencing to demonstrate that this reduction in methionine during bovine embryo culture leads to genome-wide alterations to DNA methylation in >1,600 genes, including a cohort of imprinted genes linked to an abnormal fetal-overgrowth phenotype. Bovine ovarian and embryonic cells are acutely sensitive to methionine, but further experimentation is required to determine the significance of interspecific variation in BHMT expression.