Figure 7

Increased bleomycin sensitivity and decreased HR-mediated DSB repair of Topo IIβ knockout cells. (A) Sensitivity of Topo IIβ knockout and wild-type cells to bleomycin. Clonogenic survival assays were performed using Topo IIβ knockout and wild-type (WT) Nalm-6 cells. Data are the mean ± SD of three independent experiments. Note that the vertical axis was depicted in a logarithmic scale, and the survival rates of wild-type and knockout cells at 2 μg bleomycin were 16.7% and 8.0%, respectively (p = 0.058). (B) Schematic representation of knock-in of the DR-GFP reporter gene into the HPRT locus. The pHPRT DR-GRP plasmid was knocked-in into exon 3 of the HPRT gene that is located on the X chromosome. Two wild-type and three Topo IIβ knockout clones were isolated. (C) Effect of Topo IIβ knockout on HR-mediated DSB repair. I-SceI was transiently expressed to introduce a DSB in the DR-GFP reporter gene. HR-mediated repair of the DSB in the reporter yielded GFP-positive cells that were scored by flow cytometry. DSB repair activity in a wild type clone (DR1) was set to 1. Relative DSB repair activities in another wild-type clone and three Topo IIβ-knockout clones were indicated. Data represents the mean ± SD of five independent experiments (*p < 0.05; NS, Not significant).