Induced repair of DNA double-strand breaks at the G(1)/S-phase border

Exposure of human cells to ionizing radiation at the G(1)/S-phase border of the cell cycle leads to the production of repair patches of 3 nucleotides, representing the constitutive repair response, and very long repair patche s (VLRP) of at least 150 nucleotides, representing an induced response. We examined the type of DNA damage that may signal this induced repair respons e using two chemicals that produce subsets of the damage induced by ionizin g radiation, Treatment of cells at the G(1)/S-phase border with bleomycin, which produces a high proportion of DNA double-strand breaks, also leads to the production of VLRP of at least 130 nucleotides. In contrast, when cell s were treated with hydrogen peroxide, which produces base modifications an d single-strand breaks, no VLRP were observed. Thus it would appear that DN A double-strand breaks are the signal that leads to the induction of the VL RP, We also examined the relationship between the induced repair response a nd DNA replication. When cells are treated with hydroxyurea, under conditio ns that inhibit more than 98% of the DNA synthesis, prior to exposure to 5 Gy, repair patches of 3 and 150 nucleotides are found. This indicates that the longer repair patches are not a result of aberrant DNA replication, How ever, when cells are treated with the DNA polymerase inhibitor aphidicolin in combination with hydroxyurea and cytosine arabinoside, no induced long p atches are found. These results indicate that DNA polymerase alpha, delta o r epsilon is required for the synthesis of the VLRP. (C) 1999 by Radiation Research Society.