Expression of methylation pathway enzymes in bovine oocytes and preimplantation embryos

The methylation pathway, which consists of two metabolic cycles of nutrients, i.e., the methionine and folate cycles, generates S-adenosylmethionine, the methyl donor for the methylation of DNA and histones. Using reverse transcription-polymerase chain reaction, we examined the gene expression patterns of the methylation pathway enzymes during bovine oocyte maturation and preimplantation embryonic development up to the blastocyst stage. Bovine oocytes were demonstrated to have the mRNA of all methylation pathway enzymes examined, namely, methionine adenosyltransferase 1A (MAT1A), MAT2A, MAT2B, S-adenosylhomocysteine hydrolase (AHCY), 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR), betaine-homocysteine methyltransferase (BHMT), serine hydroxymethyltransferase 1 (SHMT1), SHMT2, and 5,10-methylenetetrahydrofolate reductase (MTHFR). All the transcripts were consistently expressed throughout all developmental stages, except for MAT1A, which was not detected from the 8-cell stage onward and BHMT, which was not detected in the 8-cell stage. Immunofluorescence analysis of MAT1A protein revealed the relatively higher expression in oocytes and early cleavage stage embryos up to the 8-cell stage compared with the morula and blastocyst stage. Further, to investigate the effects of methylation pathway disruption during the earliest stages of embryonic development, the effects of exogenous homocysteine on preimplantation development and DNA methylation of bovine embryos were investigated in vitro. As results, high concentrations of homocysteine induced hypermethylation of genomic DNA as well as developmental retardation in bovine embryos. These results provide a new insight into nutrient-sensitive epigenetic regulation and perturbation at the earliest stage of our life.