"ATR activation in response to ionizing radiation: still ATM territory"

Abstract

Unrepaired DNA double-strand breaks (DSBs) are a major cause for genomic instability. Therefore, upon detection of a DSB a rapid response must be assembled to coordinate the proper repair/signaling of the lesion or the elimination of cells with unsustainable amounts of DNA damage. Three members of the PIKK family of protein kinases -ATM, ATR and DNA-PKcs- take the lead and initiate the signaling cascade emanating from DSB sites. Whereas DNA-PKcs activity seems to be restricted to the phosphorylation of targets involved in DNA repair, ATM and ATR phosphorylate a broad spectrum of cell cycle regulators and DNA repair proteins. In the canonical model, ATM and ATR are activated by two different types of lesions and signal through two independent and alternate pathways. Specifically, ATR is activated by various forms of DNA damage, including DSBs, arising at stalled replication forks ("replication stress"), and ATM is responsible for the signaling of DSBs that are not associated with the replication machinery throughout the cell cycle. Recent evidence suggests that this model might be oversimplified and that coordinated crosstalk between ATM and ATR activation routes goes on at the core of the DNA damage response.

Figure 1

Sequence of events in ATM→ ATR dependent signaling of DSBs in S/G2 cells. (1) Recuitment of the MRN complex to the DSB. (2) Recuitment/activation of ATM to DSBs. (3) ATM dependent stimulation of the Mre11-dependent 5' to 3' nuclease activity. (4) Generation of ssDNA. (5) Recruitment of RPA to the generated ssDNA. (6) Recruitment of the ATRIP/ATR complex to ssDNA-bound RPA. (7) ATR dependent activation of Chk1. (8) Inhibition of the specific CDK activity responsible for S and/or G2/M progression; in other words, execution of the checkpoint. (9) Control of the ssDNA/RPA/ATR pathway by CDK activity. The numbers followed by an asterisk (3,4 and 9) point to the less understood concepts of the model: (3)* How does ATM stimulate Mre11 nuclease activity? (4)* To what extent is Mre11 nuclease activity responsible for the ssDNA generation in response to IR-induced DSBs? And, most importantly; (9)* how does CDK activity "preactivate" the ATR/Chk1 pathway?