a Metaphase telomere FISH detection of telomere loss or multiple telomeres signals at ends of chromosomes. NC or siTCOF1 transfected U2OS cells were treated with scrambled LNA gapmer or TERRA targeting LNA gapmer for 24 h, then treated with nocodazole for 6 h, and subjected to FISH. b Quantification of (a). The percentage of chromosomes with MTSs was calculated. For each group, 1000 or more chromosomes were examined. c Quantification of (a). The percentage of chromosomes with one or more telomere-free ends was calculated. For each group, 1000 or more chromosomes were examined. All values are the average ±SEM of three independent experiments. The unpaired Student’s two-tailed t-test was used to determine the statistical significance (*P < 0.05; **P < 0.01). d Immunofluorescence and FISH detection of γH2AX foci at telomeres in U2OS cells transfected with NC, siTCOF1-1 or siTCOF1-2. e Quantification of (d). The average number of telomeric γH2AX foci per cell was determined (n ≥ 100 cells). All values are the average ±SEM of three independent experiments. The unpaired Student’s two-tailed t-test was used to determine the statistical significance (**P < 0.01; ***P < 0.001). f Proposed working model.