53BP1 promotes telomere fusions in cells treated with a DNA-PKcs inhibitor.

<p>(<b>A</b>) Schematic of types of telomere fusions observed in this study. Left panel: most fusion in DNA-PKcs null mouse cells (<i>Prkdc</i>^−/−) are “chromosome-type” (two telomere signals at the fusion point). Right panel: in contrast, most fusions in wt cells treated with the DNA-PKcs inhibitor NU7026 occur in replicative phases of the cell cycle and are therefore “chromatid-type” (three telomere signals). (<b>B</b>) Examples of “chromatid-type” telomere fusions in wt and 53BP1-deficient cells treated with the DNA-PKcs inhibitor NU7026 for 24 hours. Normal chromosomes in cells treated with vehicle (DMSO) are shown for comparison. (<b>C–E</b>) Quantification of NU7026-induced telomere fusions in wt and <i>Trp53bp1</i>^−/− MEFs. Exponentially growing cells were incubated with 40 µM NU7026 or vehicle (DMSO) for 24 hours. Colcemid was added during the last four hours of the incubation period (diagrammed in C). Cells were fixed and metaphases analyzed by telomere FISH. The number of telomere fusions per metaphase in NU7026-treated cells is shown in D. Bars represent the average and standard deviation of two independent experiment (n = 2 MEF lines per experiment). The distribution of the number of telomere fusions per metaphase in the same experiments is shown in E.