DMA Repair in Normal and Preneoplastic Mammary Tissues1

The purpose of this investigation was to obtain a com parative measure of DMA repair synthesis in hormonedependent normal mammary tissue and hormone-inde pendent D hyperplastic alveolar nodule (HAN) outgrowth. Treatment of mammary fragments in culture with 5 mm hydroxyurea inhibits 98% of the semiconservative DNA synthesis. Treatment of mammary fragments with methyl methanesulfonate in the presence of hydroxyurea results in a 4to 7-fold higher incorporation of [3H]deoxythymidine into the mammary cell DNA than does treatment with hydroxyurea alone. This hydroxyurea-resistant alkylating agent-induced [3H]deoxythymidine incorporation was studied by CsCI density gradient centrifugation and has been found to represent DNA repair replication. Similar levels of repair replication were found in both normal and preneoplastic D -HAN outgrowth. Autoradiographic anal ysis of mammary fragments and D -HAN outgrowth treated with methyl methanesulfonate plus hydroxyurea revealed that 30 to 50% of the epithelial cell nuclei were lightly labeled. No detectable repair synthesis was found in fat or stromal cells. The average number of grains per labeled nuclei was the same for both expiants. These results suggest that a reduced DNA repair capacity is not associated with the increased sensitivity of D -HAN out growth to the tumorigenic effect of chemical carcinogens.