FIG. 1.

POT1a and -b repress HDR in parallel with Ku70. (A) Examples of T-SCE events in POT1 DKO cells deficient for Ku70. CO-FISH analysis of POT1a^S/F POT1b^S/F Ku70^−/− MEFs was performed 92 h after treatment with adenoviral Cre. Fluorescence signals of the TelG and TelC probes are depicted separately and merged. The enlargements depict chromosomes exhibiting T-SCEs. (B) Comparable induction of T-SCEs upon the simultaneous removal of POT1a and POT1b or the removal of TRF2 in Ku70^−/− cells (POT1 DKO Ku70^−/−, P = 0.006 by Student's t test). Shown is the quantification of three independent CO-FISH experiments; at least 6,000 chromosome ends were analyzed for the POT1a^S/F POT1b^S/F cell lines and at least 4,000 for the TRF2^FLOX/− Ku70^−/− cell lines. The increase in T-SCE frequency after POT1a/b deletion in Ku70-proficient cells is not statistically significant (P = 0.184). (C) Both POT1a and POT1b alone are capable of suppressing T-SCEs in Ku70^−/− MEFs. Shown is the quantification of three independent CO-FISH experiments. At least 1,000 chromosome ends were analyzed for each cell line in each experiment. Error bars in panels B and C indicate standard deviations.