Fig. 8
ATM counteracts toxic NHEJ of seDSBs in the S phase. a, b ATM is not required for BIR-mediated repair of collapsed replication forks. Representative images (a) and quantification (b) of sister chromatid exchanges (SCEs) in cells treated with 50 nM topotecan (n = 100/genotype). Quantifications of chromosome numbers and SCEs in untreated cells are presented in Supplementary Figure 8c, d. Scatter dot plots showing mean ± s.d. number of SCEs across the representative genotypes. Data from n = 3 technical replicates. The graph was generated by using GraphPad Prism 7; NS = not significant (p > 0.05); one-way ANOVA; total df = 11. c ATM is required to prevent toxic fusions upon formation of seDSBs. Representative images of metaphase spreads depicting multicolor fluorescent in situ hybridization (M-FISH) using mouse 21-color painting chromosome probes. White arrows indicate fusions. Representative karyotypes are presented in Supplementary Figure 9a. d Contingency graphs showing the percentages of chromosome rearrangements from chromosomal spreads of untreated cells and cells treated with 50 nM topotecan, generated by using GraphPad Prism 7. n = 3 technical replicates measuring n ≥ 20 metaphases/genotype in each experiment. Statistical analysis is presented in Supplementary Figure 9c. e Model for the role of ATM in the repair of seDSBs resulting from collapsed replication forks in S phase of the cell cycle. Column 1, ATM promotes resection of seDSBs, thereby speeding up their repair by HRR and minimizing the time-window during which toxic NHEJ might take place. As shown, ATM also counteracts NHEJ by other mechanisms (see main text). Column 2, in the absence of ATM, seDSB resection is delayed and NHEJ is not suppressed, leading to some seDSBs being subject to illegitimate NHEJ, causing chromosome fusions and ensuing cell death. Column 3, inactivating NHEJ alleviates the hypersensitivity of ATM-null cells to agents that generate seDSBs because toxic, illegitimate NHEJ is absent. Column 4, modifying seDSB end-resection dynamics by loss of BRCA1-A complex components alleviates (rebalances) the seDSB resection defect of ATM-deficient cells, thereby minimizing the potential for illegitimate NHEJ. Source data are provided as a Source Data file
