Induced Repair of DNA Double-Strand Breaks at the G₁/S-Phase Border

Exposure of human cells to ionizing radiation at the <tex-math>${\rm G}_{1}/{\rm S}\text{-}\text{phase}$</tex-math> border of the cell cycle leads to the production of repair patches of 3 nucleotides, representing the constitutive repair response, and very long repair patches (VLRP) of at least 150 nucleotides, representing an induced response. We examined the type of DNA damage that may signal this induced repair response using two chemicals that produce subsets of the damage induced by ionizing radiation. Treatment of cells at the <tex-math>${\rm G}_{1}/{\rm S}\text{-}\text{phase}$</tex-math> 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 cells were treated with hydrogen peroxide, which produces base modifications and single-strand breaks, no VLRP were observed. Thus it would appear that DNA double-strand breaks are the signal that leads to the induction of the VLRP. We also examined the relationship between the induced repair response and DNA replication. When cells are treated with hydroxyurea, under conditions 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. However, when cells are treated with the DNA polymerase inhibitor aphidicolin in combination with hydroxyurea and cytosine arabinoside, no induced long patches are found. These results indicate that DNA polymerase ?, ? or ? is required for the synthesis of the VLRP.