Nat Rev Mol Cell Biol. in an accumulation of persistent chromosome aberrations and SBE13 increased cell death. Using reversible SBE13 inhibitors of DNA-PK (DNA-dependent protein kinase), another kinase involved in responding to DNA damage, and ATM, we showed that these two kinases acted through unique DNA repair mechanisms: ATM resolved DNA damage through a mechanism including sister chromatid exchange (SCE), whereas DNA-PK acted through nonhomologous end joining. Furthermore, because DNA damageCinduced SCE occurred in A-T fibroblasts that lack functional ATM protein, and the inhibitors of ATM kinase experienced no effect on DNA damageCinduced SCE in A-T fibroblasts, we showed that the consequences of short-term inhibition of the kinase activity of ATM and adaptation to ATM protein disruption were unique. This suggests that A-T fibroblasts have adapted to the loss of ATM and have alternative mechanisms to initiate SCE. INTRODUCTION Ataxia telangiectasia (A-T) is usually a child years disorder characterized by neurodegeneration, predisposition to cancers, and profound, lethal sensitivity to ionizing radiation (radiosensitivity). A-T is usually caused by either compound heterozygosity or homozygosity for truncating mutations (frameshift or nonsense mutations) in the (encodes a protein kinase that is critical for the initiation of DNA damage responses in mammalian cells exposed to ionizing radiation (IR) or to other brokers that introduce double-strand breaks (DSBs) into DNA (1, 3, 4). Cells derived from A-T patients exhibit defective cell cycle checkpoint responses, increased chromosome aberrations, and increased cell death after IR, thus revealing the importance of ATM-dependent signaling in irradiated cells (5). ATM belongs to a family of kinases, the phosphoinositide 3-kinaseCrelated protein kinases, that function in DNA damage responses. The kinase activity of ATM is extremely sensitive to DNA damage and is activated in cells within seconds of exposure to doses as low as 0.1-gray (Gy) IR (6). The kinase activity of ATM is essential for the activation of downstream effector kinases, such as checkpoint kinase 2 (CHK2) (7), and the phosphorylation of numerous substrates that impede origin firing (the initiation of DNA replication at a Rabbit Polyclonal to CBLN1 particular origin) during S phase (8) and that halt the progression of the cell cycle at the G1-S phase (9) and G2-M phase (10) transitions. Such cell cycle checkpoints were envisioned as transient delays of the cell cycle that allow sufficient time for chromosome repair and that prevent cell cycle progression in the presence of chromosome damage (11). However, the chromosomal instability of A-T cells may not be entirely due to defective cell cycle SBE13 checkpoints. Chromosome aberrations accumulated in irradiated A-T cells arrested in G0 for up to 48 hours, indicating this damage is not a consequence of defective cell cycle checkpoints (12, 13). Similarly, when aphidicolin was used to block the G1-S phase transition in A-T cells, no decrease in cell death was observed after IR (14). Because increased chromosome aberrations and cell death were obvious in cells that were not progressing through the cell cycle, these data are indicative of a DNA repair defect in A-T cells that is impartial of cell cycle checkpoints. The repair of DSBs can occur through nonhomologous end joining (NHEJ) or homologous recombination (HR) and the kinase activity of ATM has been implicated in both mechanisms. HR is usually a high-fidelity DSB repair mechanism that is generally restricted to the S and G2 phases of the cell cycle when a sister chromatid is usually available as a repair template (15). ATM promotes the HR-mediated repair of DSBs in various systems, including in DT40 chicken cells in response to IR (16) and in Chinese hamster cells in response to inhibition of poly(adenosine diphosphate ribose) polymerase (PARP) (17). Furthermore, the kinase activity of ATM participates in DSB end resection, which is a key step in HR (18). Nevertheless, sister chromatid exchange (SCE), which occurs through HR-mediated repair, is usually normal in A-T cells (19C21). NHEJ operates throughout the cell cycle but is particularly important in G1 when a sister chromatid is not available as.