Histone deacetylase (HDAC) inhibition enhances olaparib-induced DNA damage accumulation. (A) Cells were treated with olaparib and suberoylanilide hydroxamic acid (SAHA) alone or in combination for 5 d. DNA double-strand breaks in the individual cells were measured with a comet assay. The percentage of tail-moment was calculated and is presented in bar graphs with error bars (± SE); *P <0.001. (B) The expression of DNA damage-responsive proteins was measured by western blot analysis following treatment with olaparib and SAHA alone or in combination. (C) The cells were treated with 1 μmol/L olaparib and/or 1 μmol/L SAHA and the immunofluorescence analysis were conducted with the indicated antibodies. Confocal microscopy was used to observe the signals corresponding to RAD51 (red) and ɣ-H2AX (green). The DNA was counterstained with 4',6-diamidino-2-phenylindole (DAPI) (blue). The percentage of cells containing more than 10 foci of RAD51 and ɣ-H2AX over three experiments is presented in a bar graph. At least 100 nuclei were analyzed for each experiment (right). Columns, the mean of three independent experiments; bars, ± SE; *P <0.001.