[Tissue-specific decrease and change in the character of DNA methylation in cattle with aging].

The content of 5-methylcytosine (m5C) in DNA of different cow organs is found to decrease in ontogenesis. No other age changes were found in DNA structure and molecular population (GC content, the content of different in the length and the base composition pyrimidine clusters, hyperchromicity value etc.). It is suggested that the age tissue-specific decrease of the m5C content is due to a decrease of the level of DNA methylation, which takes place in heart, kidneys, spleen and brain DNAs and is not observed in lungs. Maximal level of DNA methylation was observed at embryogenesis, when the m5C content in heart DNA decreases (from 1.8 to 1.4 mol. %), then it remains stable up to 1 year old and then sharply decreases with age and reaches 0.94 mol. % at the age of 10 years. The m5C content in liver DNA decreases (from 1.6 to 1.2 mol. %) at the end of embryogenesis and then it does not change. A pronounced decrease of the m5C content was observed in DNAs of cerebral hemispheres, cerebellum, hyppocampus, neurohypophysis and thymus. The decrease of m5C content in thymus DNA is non-random, it concerns only Pu-m5C-Pu, but not long pyrimidine clusters Pu-Pyn-Pu (n greater than or equal to 2). Tissue-specific decrease and the change in pattern of DNA methylation observed are suggested to be a possible mechanism for the age change (distortion) of transcription and cell functioning.