Effects of ionizing radiation on the level of genomic DNA methylation in liver, brain and spleen of mouse as well as in two kinds of cultured cells were examined by high-performance liquid chromatography. Ten Gy of whole body X-radiation reduced the 5-methyldeoxycytidine contents by about 40% within 8 hours after irradiation in liver. Similar effects were observed at 4 or 7 Gy of X-ray irradiation. However, no such change was detected in brain, spleen and cultured cells. The data indicate that radiation-induced alteration in genomic DNA methylation is not ubiquitous among different tissues and cells.

No abstract

In order to understand the possible importance of DNA methylation in ageing, characteristics of its age-associated changes were examined in mouse and man. The total methylated deoxycytidine level in the genome decreased in the senescent period in mouse liver, but not in mouse brain and human liver. The examination of DNA methylation in each individual gene revealed that only a few genes showed alteration in the senescent phase while many genes change in the maturation period. The alterations were gene- and tissue-specific. Comparison of short-living mouse and long-living man for the age-associated changes of the c-myc gene methylation revealed that the rate of change in mouse was about 20 times faster than that in man. This suggests a deep involvement of DNA methylation in ageing. Further investigations into the causes and consequences of the changes would clarify a basic mechanism of ageing.

No abstract