Methylation of DNA of the brain and liver of young and old rats.

In vitro methylation of purified DNA and chromatin-DNA in nuclei of the liver and brain of young (18 week) and old (120 week) female rats has been studied using 3H-SAM as the -CH3 group donor. Incorporation of -CH3 group is higher in old liver and brain, but it is far higher in the latter. 5 mC is 11% lower in the old brain, but there is no difference in the liver. Methylation by Hpa II methylase does not show any difference in the incorporation of -CH3 group into DNA of the liver of the two ages. However, its incorporation is lower in the old brain. Methylation by Msp I methylase causes slightly higher incorporation of -CH3 groups in the old brain. This shows a higher percentage of unmethylated external cytosines in the 5'-CCGG-3' sequences. On the contrary, methylation by Eco RI methylase is considerably higher in the old brain. These studies show alterations in the methylation status of the DNA during ageing which may cause changes in the expression of genes.

Methylation, acetylation and phosphorylation of the bases of DNA of young and old rats.

The possibility of methylation, acetylation and phosphorylation of the bases of DNA has been studied in vitro by incubating nuclei of the liver and cerebral hemisphere of young (18 wk) and old (120 wk) rats with radioactive donors, [3H]S approximately adenosyl methylmethionine, [3H]-acetyl approximately CoA and [32P]-gamma-ATP for methylation, acetylation and phosphorylation of the bases, respectively. Nuclei were also incubated with S approximately adenosyl homocysteine to inhibit methylation with sodium butyrate to stimulate acetylation and with alkaline phosphatase to remove phosphate groups incorporated into the bases. DNA was then extensively purified and incorporation of each type of label was estimated. The data show that both methylation and acetylation of DNA of old rats were significantly higher than those of young rats, and phosphorylation is lower in old rats. Such modifications may prevent base pairing between the two strands of DNA, alter its conformation and binding of trans-acting factors at specific sites, and thereby alter gene expression.