Because the results suggested that gene correction may have occurred during the early passaging of Q3SC, we asked whether any of the ATM−/− iPSCs could be reverted in culture. To increase the chances of observing rare events, 96-well plates containing wells with CAR3, Q1SA, Q3SA, or Q3SC iPSC were X-irradiated at 1 Gy to promote DNA damage and induce genetic recombination or repair events. One week later, the cultures were irradiated again to activate ATM, and after 30 min of response time, all cultures were fixed, stained for phospho-ATM (pATM) and γH2A.X, and then imaged (Figure 3B). A parallel plate with no radiation treatment served as negative control. Automated counting of the fraction of pATM-containing nuclei indicated XR-induced ATM autophosphorylation. The ATM+/− lines, CAR3 and Q3SC, each had robust increases in pATM following XR. Q1SA, which encodes a full-length ATM protein with a S2394L variation, appeared to induce phosphorylation at S1981, the epitope for the phospho-specific ATM antibody, but this increase was similar to non-irradiated (NR) background. Similarly, the ATM−/− Q3SA appeared to show a weaker trend of XR-induced pATM
