Maternal Protein Deficiency Causes Hypermethylation of DNA in the Livers of Rat Fetuses

Abstract

Maternal protein deficiency during pregnancy is associated with changes in glucose tolerance and hypertension in the offspring of rats. In this study the growth of rat fetuses was examined when the dams were fed diets containing 18% casein, 9% casein or 8% casein supplemented with threonine. The extra threonine was added to reverse the decrease in circulating threonine concentrations that occurs when pregnant rats are fed protein-deficient diets. The fetuses of the group fed the low protein diet supplemented with threonine were significantly smaller than those of the control group and not significantly different from those fed low protein. Homogenates prepared from the livers of dams fed the diet containing 9% casein oxidized threonine at approximately twice the rate of homogenates prepared from dams fed the diet containing 18% casein. We conclude that circulating levels of threonine fall as a consequence of an increase in the activity of the pathway that metabolizes homocysteine produced by the transulfuration of methionine. Serum homocysteine was unaffected in the dams fed low protein diets compared with controls, but was significantly greater in dams fed the low protein diet supplemented with threonine. Elevated levels of homocysteine are associated with changes in the methylation of DNA. The endogenous methylation of DNA was greater than that of controls in the livers of fetuses from dams fed the 9% protein diets and increased further when the diet was supplemented with threonine. Our results suggest that changes in methionine metabolism increase homocysteine production, which leads to changes in DNA methylation in the fetus. An increase in maternal homocysteine may compromise fetal development, leading to the onset of glucose intolerance and hypertension in adult life.

The effect of dietary protein on the amino acid supply and threonine metabolism in the pregnant rat

“…The activity of threonine dehydrogenase (upper panel of Fig. 4) remained constant throughout pregnancy and The samples from day 21 were taken from an experiment described previously [7]. Error bars + sd (n= 4).…”

“…The rates of threonine oxidation by homogenates of liver were taken from an experiment described previously [7]. Bars sharing a common subscript are not significantly different from one another.…”

“…On the basis of measurements made late in gestation (day 21), we had initially hypothesised that the falling concentrations in the circulation were the result of an increase in threonine oxidation [7]. These new data show that hepatic oxidation cannot account for the changes in plasma threonine concentrations.…”

“…Maternal tissues were removed, washed in saline, blotted dry and weighed. One portion of the maternal liver was homogenised immediately and used for the estimation of threonine oxidation by the method described previously [7]. The remainder of the maternal liver and samples of other tissues was divided into portions, frozen in liquid nitrogen and then stored at -70 • C until required.…”

“…The remainder of the maternal liver and samples of other tissues was divided into portions, frozen in liquid nitrogen and then stored at -70 • C until required. Some samples at d21 of gestation were derived from animals used in a previous study which followed a similar protocol [7]. All experimental procedures were approved and conducted in accordance with the UK Animals (Scientific Procedures) Act, 1986.…”