FIGURE 1:

Increased formation of 53BP1- or p-ATMSer1981–containing TIFs in the MuA-hTR–expressing clones. (A) YCC-B2 parental, vector-containing, and MuA-hTR–expressing cells were coimmunostained with an antibody recognizing 53BP1 (BP13, green) and an antibody recognizing TRF1 (#370, red). 53BP1 is colocalized with TRF1, indicating telomere dysfunction–induced foci (yellow). (B) YCC-B2 cells were also coimmunostained with an antibody against the activated and phosphorylated (Ser1981) form of ATM (10H11.E12, green) and anti-TRF1 (#370, red). Telomere dysfunction–induced foci are shown in yellow. (C) Quantification of TIF-positive cells. Cells with three or more foci colocalizing with TRF1 were counted as TIF positive. For each cell type, 100 cells were counted. Data are mean ± standard error of the mean (SEM) for three independent experiments. Asterisks indicate statistical significance from vector-containing cells (***P < 0.001). (D) Relative telomerase activity was measured by TRAP using serial fivefold dilutions of protein extracts.