Chromosome breakage following G2 checkpoint release

DNA double-strand break (DSB) repair and checkpoint control represent distinct mechanisms to reduce chromosomal instability. Ataxia telangiectasia (AT) cells have checkpoint arrest and DSB repair defects. Here, we examine the efficiency and interplay of ATM's G2 checkpoint and repair functions. Artemis cells manifest an identical and epistatic repair defect to AT but show proficient checkpoint responses. Only a few G2 cells enter mitosis within 4 h following irradiation with 1 Gy but manifest multiple chromosome breaks. Most checkpoint-proficient cells arrest at the G2/M checkpoint, with the length of arrest being dependent upon the repair capacity. Strikingly, cells released from checkpoint arrest display 1-2 chromosome breaks. This represents a major contribution to chromosome breakage. The presence of chromosome breaks in cells released from checkpoint arrest suggests that release occurs prior to the completion of DSB repair. Strikingly, we show that checkpoint release occurs at a point when ~3-4 premature chromosome condensation (PCC) breaks and ~20 γH2AX foci remain.