Tissue-specific relationship of S-adenosylhomocysteine with allele-specific H19/Igf2 methylation and imprinting in mice with hyperhomocysteinemia.

DNA methylation is linked to homocysteine metabolism through the generation of S-adenosylmethionine (AdoMet) and S-Adenosylhomocysteine (AdoHcy). The ratio of AdoMet/AdoHcy is often considered an indicator of tissue methylation capacity. The goal of this study is to determine the relationship of tissue AdoMet and AdoHcy concentrations to allele-specific methylation and expression of genomically imprinted H19/Igf2. Expression of H19/Igf2 is regulated by a differentially methylated domain (DMD), with H19 paternally imprinted and Igf2 maternally imprinted. F1 hybrid C57BL/6J x Castaneous/EiJ (Cast) mice with (+/-), and without (+/+), heterozygous disruption of cystathionine-beta-synthase (Cbs) were fed a control diet or a diet (called HH) to induce hyperhomocysteinemia and changes in tissue AdoMet and AdoHcy. F1 Cast x Cbs+/- mice fed the HH diet had significantly higher plasma total homocysteine concentrations, higher liver AdoHcy, and lower AdoMet/AdoHcy ratios and this was accompanied by lower liver maternal H19 DMD allele methylation, lower liver Igf2 mRNA levels, and loss of Igf2 maternal imprinting. In contrast, we found no significant differences in AdoMet and AdoHcy in brain between the diet groups but F1 Cast x Cbs+/- mice fed the HH diet had higher maternal H19 DMD methylation and lower H19 mRNA levels in brain. A significant negative relationship between AdoHcy and maternal H19 DMD allele methylation was found in liver but not in brain. These findings suggest the relationship of AdoMet and AdoHcy to gene-specific DNA methylation is tissue-specific and that changes in DNA methylation can occur without changes in AdoMet and AdoHcy.