Figure 4. DNA polymeraseβ may protect cells from spontaneous DNA double strand breaks.

WT and polβ^−/− primary fibroblasts were exposed to HU as described in Experimental. A. γ-H2AX foci are amplified by loss of DNA polβ. Overlappping γ-H2AX/53BP1 foci serve as a marker for DNA double strand breaks. Foci were detected by γ-H2AX and 53BP1 immunostaining in primary mouse embryonic fibroblasts as described in Experimental. All four groups of cells either remained untreated or were treated with 300uM HU for 24 hours, fixed and stained with DAPI (blue). Cells were probed with antibody against γ-H2AX (green; anti-γ H2AX antibody) and 53BP1 (red; anti-53BP1) as described in Experimental. Slides were photographed under the Nikon Eclipse 80i microscope (Nikon, CA) and processed using the Nikon Elements built-in software. B. Quantification of DNA double strand breaks. Approximately 100 cells were selected in more than 10 fields of view and the number of foci positive for both γ-H2AX and 53BP1 were counted for each cell. Data are presented as mean ±SEM. *Values significantly different from control at p < 0.01.