Selective repair of methylated purines in regions of chromatin DNA

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Abstract
The distribution of methylated purines in different regions of liver chromatin DNA has been examined after treating rats with [14C]dimethylnitrosamine (2 mg/kg). At different times after administration of the carcinogen, liver nuclei were isolated and fractionated by micrococcal nuclease digestion and low and high salt extractions into an active chromatin fraction, two fractions comprising the bulk of the genome, and a nuclear matrix fraction. Regions of active chromatin and nuclear matrix tended to be methylated more readily than bulk chromatin, with respect to formation of both O6-methyl-guanine and N-methyl purines. Removal of both 7-methyl-guanine and 3-methyladenine (by repair and depurination reactions) occurred at a relatively uniform rate in all chromatin fractions. In contrast, repair of O6-methylguanine proceeded more rapidly from active chromatin than from bulk chromatin, whereas repair of this lesion from nuclear matrix DNA was much slower than for bulk DNA. Pretreat-ment of rats for 4 weeks with non-radioactive dimethylnitros-amine before the administration of [14C]dimethylnitrosamine enhanced the rate of repair of radioactive O6-methylguanine from all chromatin fractions. Nevertheless the rate of loss of the adduct was still faster from active chromatin and slower from matrix DNA than from the bulk of the genome. Since pretreatment also elevated the rate of liver DNA synthesis especially in the nuclear matrix fraction, there is an increased probability of the fixation of mutations due to the presence of O6-methylguanine in this selected region of the genome. The implications of this persistent O6-alkylation of matrix DNA, and rapid repair of O6-alkylguanine in active chromatin for the toxicity and carcinogenicity of alkylating agents are discussed.