in this case a reversion from ATM−/− to ATM+/−. All available evidence, including at least the 1.4 kb spanning the c.217_218 delGA position to the location of rs2066734, supports the conclusion that gene conversion of ATM occurred, replacing the maternal allele with a copy of the paternal allele to create an allele encoding full-length ATM protein capable of phosphorylating pCHK2 and γH2A.X upon XR. This likely occurred between initial reprogramming of the iPSC line (P0) and P10, as DNA prepared from P4 had not reverted, or at least not detectably. One puzzling aspect was that the ATM protein levels detected by western blotting appeared to change over passaging (Figure 1D). It is not clear if this is meaningful in the context of our study, as ATM protein remained detectable after the reversion event, even if levels were reduced. Alternatively, ATM gene reversion could have occurred in circulating T cells prior to reprogramming. Even if this were the case, Figure S2B demonstrates that independent SNP variants accumulate after reprogramming, proving that iPSC cultures accumulate genetic variants during culture.
