Both the folate cycle and betaine‐homocysteine methyltransferase contribute methyl groups for DNA methylation in mouse blastocysts

Abstract

The embryonic pattern of global DNA methylation is first established in the inner cell mass (ICM) of the mouse blastocyst. The methyl donor S-adenosylmethionine (SAM) is produced in most cells through the folate cycle, but only a few cell types generate SAM from betaine (N,N,N-trimethylglycine) via betaine-homocysteine methyltransferase (BHMT), which is expressed in the mouse ICM. Here, mean ICM cell numbers decreased from 18-19 in controls to 11-13 when the folate cycle was inhibited by the antifolate methotrexate and to 12-14 when BHMT expression was knocked down by antisense morpholinos. Inhibiting both pathways, however, much more severely affected ICM development (7-8 cells). Total SAM levels in mouse blastocysts decreased significantly only when both pathways were inhibited (from 3.1 to 1.6 pmol/100 blastocysts). DNA methylation, detected as 5-methylcytosine (5-MeC) immunofluorescence in isolated ICMs, was minimally affected by inhibition of either pathway alone but decreased by at least 45-55% when both BHMT and the folate cycle were inhibited simultaneously. Effects on cell numbers and 5-MeC levels in the ICM were completely rescued by methionine (immediate SAM precursor) or SAM. Both the folate cycle and betaine/BHMT appear to contribute to a methyl pool required for normal ICM development and establishing initial embryonic DNA methylation.

Epigenetic imprinting during assisted reproductive technologies: The effect of temporal and cumulative fluctuations in methionine cycling on the DNA methylation state

“…Indeed, several studies point toward beneficial effects of inducing a well‐balanced folate and methionine cycle in both males (Dattilo et al, ) and females (Haggarty et al, ; Boxmeer et al, ,; Gaskins et al, ; Murto et al, ) for pregnancy success in ART. Furthermore, a recent study in mouse blastocysts showed the necessity of folate and BHMT homocysteine recycling pathways to ensure that sufficient levels of S‐adenosylmethionine enable a sufficient DNA methylation level (Zhang et al, ).…”

Role of methionine adenosyltransferase 2A in bovine preimplantation development and its associated genomic regions

“…MAT contributes to OCM, whose modulation during the periconceptional period (direct or via maternal diets) can have long-lasting phenotypic effects on (1) pre- and post-natal growth with alterations of body composition, (2) metabolic processes (including glucose homeostasis, lipid metabolism, and cardiovascular functions), and (3) immune functions, and these phenotypes closely overlap with the symptoms of human metabolic syndrome or syndrome X 12, 37, 38 . In terms of growth and metabolic processes, GO terms such as ‘anatomical structure development’, ‘multicellular organismal development’, and ‘metabolic process’ (encompassing ‘lipid metabolic process’) were enriched by the ChIP peak-associated genes.…”

“…For example, disruption of methionine metabolism by ethionine, an antimetabolite of methionine, impairs blastocyst development of mammalian preimplantation embryos in vitro 9, 10, 13 . Analogously, inhibition of the folate cycle 11, 12 and/or betaine-homocysteine methyltransferase knockdown 12 also causes defective preimplantation development. In addition, the involvement of OCM in epigenetic modification of gene expression is also of interest, not only in terms of successful embryonic development, but also in relation to its possible association with nutrition-dependent phenotypic programming during the periconceptional period 14–16 .…”

“…Recently, mammalian oocytes and preimplantation embryos have been shown to express OCM-related enzymes, including MAT 8, 11, 12, 17 . In mammals, three MAT genes ( MAT1A , MAT2A , and MAT2B ) encode three MAT isozymes; the MATI and MATIII isozymes are a tetramer and dimer, respectively, of an identical catalytic subunit encoded by the MAT1A gene, whereas the MATII isozyme is a hetero-oligomer consisting of catalytic subunit(s) encoded by MAT2A and regulatory subunit(s) encoded by MAT2B 3 .…”

Activation of the methylation cycle in cells reprogrammed into a stem cell-like state

“…The observed increase of endogenous choline and decrease in betaine might be consistent with the rapid metabolism of betaine by BHMT at the iPS stage, suggesting a stem-cell phenotype-related activation of liver-specific BHMT. Interestingly, two studies have revealed that BHMT is unexpectedly active in the mouse blastocyst and promotes the development of the inner cell mass [30, 31], where ES cells reside. Additionally, gene expression profiling in mouse ES cells highlighted the activation of Bhmt1 and Bhmt2 genes [32], both encoding for BHMT.…”