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.