Steven L. Dresler

DNA Repair Mechanisms and Carcinogenesis

The study of cellular mechanisms for repairing damaged DNA remains as an important element of modern carcinogenesis research. The impetus for investigating DNA repair mechanisms arises from several fundamental observations. First, many carcinogens, both natural and experimental, are known to be DNA-damaging agents,1,2 and cellular removal of DNA damage has been shown to correlate with a diminished incidence of neoplastic transformation in experimental systems.3 Second, human patients genetically deficient in DNA repair have a greatly increased incidence of malignant neoplasms.4 Third, the great majority of known carcinogens have been found to be mutagens as well5,6; it has been inferred from this that the carcinogenic potential of these agents is mediated by interactions with and damage to DNA. These observations have led to the conclusion that DNA repair mechanisms form one of the major anticarcinogenic defenses of the mammalian cell. A key goal of much current research in carcinogenesis is to understand why carcinogens are able to produce cancer in spite of the extensive cellular capacity to repair DNA damage. This review concludes with a consideration of this question.

Biochemsistry of DNA Repair Patch Synthesis in UV-Irradiated Permeable Diploid Human Fibroblasts

Using a well-characterized permeable cell technique and several inhibitors of mammalian DNA polymerases, we have identified polymerase delta as the enzyme responsible for repair patch synthesis in UV-irradiated human fibroblasts. UV-induced repair synthesis appears to be mediated by polymerase delta regardless of (i) the dose of UV administered, (ii) whether the damaged cells are growing or growth-arrested, or (iii) whether repair synthesis is studied immediately or at late times (14 hours or more) after irradiation. The permeable cell technique has also been used to study deoxyribonucleoside triphosphate dNTP) concentration dependences of UV-induced repair synthesis. Apparent Km values for dCTP, dGTP, and dTTP for repair synthesis are 0.11 µM, 0.11 µM, and 0.44 µM, respectively, for AG1518 fibroblasts, and 0.06 µM, 0.07µM, and 0.24 µM, respectively, for IMR-90 fibroblasts. These values are an order of magnitude lower than the Km values for DNA replication. They are also much lower than the Km values for isolated polymerase delta (2.0 µM for dGTP and 3.5 µM for dTTP, suggesting that when the polymerase functions in DNA repair, its characteristics are altered either by association with accessory proteins or by post-translational modification. Also, the Km values for repair synthesis are 5 to 80-fold lower than dNTP concentrations found in intact human fibroblasts, indicating that the “free” dNTP pools of the cell are probably adequate to support high rates of DNA repair in vivo.