Targeting double-strand breaks to replicating DNA identifies a subpathway of DSB repair that is defective in ataxia-telangiectasia cells

The critical cellular defect(s) and basis for cell killing by ionizing radi ation in ataxia-telangiectasia (A-T) are unknown. We use the topoisomerase I inhibitor camptothecin (CPT), which kills mainly S-phase cells and induce s DSBs predominantly in replication forks, to show that A-T cells are defec tive in the repair of this particular subclass of DSBs, CPT-treated A-T cel ls reaching G2 have abnormally high levels of chromatid exchanges (viewed a s prematurely condensed G2 chromosomes); aberrations in normal cells are mo stly chromatid breaks. Transfectants of A-T cells with the wild-type ATM cD NA are corrected for CPT sensitivity, chromatid aberrations, and the DSB re pair defect. These data suggest that in normal cells ATM, the A-T protein, probably recognizes DSBs in active replicons and targets the repair machine ry to the breaks; in addition, the ATM protein is involved in the suppressi on of low-fidelity, adventitious rejoining between replication-associated D SBs. The loss of ATM functions therefore leads to genome destabilization, s ensitivity to DSB-inducing agents and to the cancer-promoting illegitimate exchange events that follow. (C) 1999 Academic Press.