Eukaryotic cells have two pathways for DNA repair - the first, homologous recombination, depends upon the presence of homologous DNA, such as sister chromatids, whereas the second, non-homologous end joining, is capable of fusing any broken DNA ends. Deficiencies in a cell’s ability to carry out DNA repair can cause susceptibility to cancer. Two human DNA-repair syndromes that fall under this umbrella are ataxia telangiectasia (AT) and Nijmegen breakage syndrome (NBS). In the mid-to-late 1990s, the genes responsible for these disorders were identified as ATM and NBS1, respectively. The ATM protein has been shown to be a serine/threonine kinase, but the function of NBS1/p95 is less clear. In a recent issue of Nature (404, 613–617; 2000), Lim and colleagues report the finding that the products of ATM and NBS1 are in fact sequential components of a pathway that repairs DNA damage caused by ionizing radiation.

A downstream target of the ATM pathway is the p53 tumour-suppressor protein. Lim and colleagues found that ionizing-radiation-dependent activation of p53 is unaffected in NBS cells, indicating that NBS1/p95 may not act upstream of ATM. Irradiation of normal cells resulted in phosphorylation of NBS1/p95; this was abrogated in AT cells, but not in cells lacking DNA-PKcs, a kinase involved in non-homologous DNA repair. The ability of ATM to directly phosphorylate NBS1/p95 was therefore examined. One site in the NBS1/p95 sequence, S343, was found to be phosphorylated by ATM in vitro, which is consistent with the finding that this site is phosphorylated in vivo in response to ionizing radiation.

The obvious next question is this – what effect does phosphorylation of NBS1/p95 have on its function? NBS1/p95 forms a complex with hMre11 and hRad50 that is thought to be involved in the identification and repair of DNA double-strand breaks. Formation of this complex can be followed by the appearance of foci after cells are irradiated. Rather disappointingly, in both AT cells and cells containing the NBS1/p95(S343A) phosphorylation mutant, focus formation was not affected (see picture; IR, ionizing radiation).

One more property of NBS cells remained to be tested, however. In normal cells, exposure to ionizing radiation induces S-phase arrest, but in both AT and NBS cells the S-phase checkpoint is lost, resulting in ‘radioresistant’ DNA synthesis in the presence of DNA damage. Cells expressing the NBS1/p95(S343A) mutant also exhibited reduced S-phase arrest, indicating that phosphorylation of NBS1/p95 may be required for maintenance of the S-phase checkpoint.

These results contribute to the increasingly complex model of how ATM controls different checkpoints – its regulation of p53 phosphoryation influences G1 arrest; its phosphorylation of NBS1/p95 regulates S-phase arrest; and it may also inhibit the G2/M transition through interactions with hChk2 kinase. The findings of Lim and colleagues offer an intriguing glimpse of the function of the NBS1-hRad50-hMre11 complex in the control of cell-cycle checkpoints, although the details of its activity remain to be revealed.