Metabolic derangement of methionine and folate metabolism in mice deficient in methionine synthase reductase

Abstract

Hyperhomocyst(e)inemia is a metabolic derangement that is linked to the distribution of folate pools, which provide one-carbon units for biosynthesis of purines and thymidylate and for remethylation of homocysteine to form methionine. In humans, methionine synthase deficiency results in the accumulation of methyltetrahydrofolate at the expense of folate derivatives required for purine and thymidylate biosynthesis. Complete ablation of methionine synthase activity in mice results in embryonic lethality. Other mouse models for hyperhomocyst(e)inemia have normal or reduced levels of methyltetrahydrofolate and are not embryonic lethal, although they have decreased ratios of AdoMet/AdoHcy and impaired methylation. We have constructed a mouse model with a gene trap insertion in the Mtrr gene specifying methionine synthase reductase, an enzyme essential for the activity of methionine synthase. This model is a hypomorph, with reduced methionine synthase reductase activity, thus avoiding the lethality associated with the absence of methionine synthase activity. Mtrr gt/gt mice have increased plasma homocyst(e)ine, decreased plasma methionine, and increased tissue methyltetrahydrofolate. Unexpectedly, Mtrr gt/gt mice do not show decreases in the AdoMet/AdoHcy ratio in most tissues. The different metabolite profiles in the various genetic mouse models for hyperhomocysteinemia may be useful in understanding biological effects of elevated homocyst(e)ine.

Methionine synthase reductase deficiency results in adverse reproductive outcomes and congenital heart defects in mice

“…In our previous study, we reported that adult Mtrr gt/gt males (but not females) weighed significantly less than their wild-type littermates [24]. In this study there were no differences between Mtrr gt/gt males and Mtrr gt/gt females in delay rate, embryonic length, embryonic weight, placental weight or VSD incidence (data not shown).…”

“…It is possible that placental defects contributed to the increased resorptions, as well as to the developmental delay and growth retardation that we observed in the embryos. In mice, Mtrr is highly expressed in the labyrinth layer of the placenta at E9.5 [24]. Appropriate expression of differentially methylated imprinted genes is necessary for proper placental and fetal growth [26], and Mtrr genotype, by altering AdoMet/AdoHcy ratios, may influence the methylation capacity in the placenta.…”

“…Mtrrdeficient mice were generated as reported [24] and backcrossed onto the C57Bl/6J background for 5-6 generations. Genotyping was performed as previously described [24]. Animals were fed commercial mouse chow diet 5001 (Agribrands Purina, St-Hubert, Canada) throughout mating and pregnancy.…”

“…Embryos were fixed in 4% paraformaldehyde in PBS overnight at 4°C and then transferred to 70% ethanol for storage. Yolk sacs were dissected and washed in PBS and used for embryonic Mtrr [24] and gender genotyping [25]. Fifteen litters per maternal genotype were obtained; two litters were removed from the study because of abnormally small litter size (1 embryo) leaving 14 litters in the Mtrr +/+ and Mtrr +/gt groups.…”

“…We recently reported the first characterization of a mouse model with Mtrr deficiency; this mouse has a gene-trap disruption in the Mtrr gene (Mtrr Gt(pGT1Lxf)XG334Byg , hereafter abbreviated as Mtrr gt ) [24]. Compared with Mtrr +/gt and Mtrr +/+ mice, Mtrr gt/gt mice have elevated plasma homocysteine levels and lower plasma methionine levels.…”