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Rapid Repair of O⁶-Methylguanine-DNA Adducts Protects Transgenic Mice from N-Methylnitrosourea-induced Thymic Lymphomas¹

Cancer Center and Department of Medicine, Case Western Reserve University, School of Medicine, Cleveland, Ohio 44106-4937

The methylating agent N-methylnitrosourea (MNU) is biased, surprisingly, in its carcinogenic potential toward the mouse thymus. Previous studies have shown that single doses of MNU administered to adult mice induced thymic lymphomas in over 80% of mice, while transgenic mice expressing high levels of the human O⁶-alkylguanine-DNA alkyltransferase gene in the thymus (MGMT-CD2 transgenics) were protected from developing MNU-induced lymphomas. The mechanism of this protection was examined in this report. In nontransgenic mice given a lymphomagenic dose of 80 mg/kg MNU, depletion of thymic alkyltransferase activity occurred within 3 h and remained undetectable for the subsequent 192 h; whereas in MGMT-CD2-transgenic mice, this dose of MNU did not deplete thymic alkyltransferase, and the lowest level of alkyltransferase was still 10-fold higher than the constitutive level of thymic alkyltransferase in nontransgenic mice. Likewise, the level of O⁶-methylguanine adducts detected in the thymus of nontransgenic mice was 96 pg/µg guanine 3 h after MNU compared to only 8 pg/µg guanine in transgenic mice. By 18 h, the level of O⁶-methylguanine in MGMT-CD2-transgenic mice was below 2 pg/µg guanine, compared to over 70 pg/µg guanine in nontransgenic mice. In contrast, no differences were noted in the liver between groups because the MGMT transgene is not expressed in the liver of this strain of mouse. Our data establish that rapid O⁶-methylguanine-DNA adduct repair due to enhanced levels of alkyltransferase in MGMT-CD2-transgenic mice blocks the initiation of MNU-induced carcinogenesis.

¹ The study was supported by Grant CN-34 from The American Cancer Society and Grants P30 CA43703 and R01 CAE506288.

² Present address: Department of Pathology and Laboratory Medicine, Brown University, School of Medicine, Providence, RI 02912.

³ To whom requests for reprints should be addressed, at Case Western Reserve University, School of Medicine (BRB), 10900 Euclid Ave., Cleveland, OH 44106-4937.

Received 4/ 1/94. Accepted 6/22/94.

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