1. Department of Hematology–Oncology, St. Jude Children's Research Hospital, 332 North Lauderdale Street, Memphis, Tennessee 38105, USA
2. Laboratory of Genetics, University of Wisconsin, Genetic Building, 445 Henry Mall, Madison, Wisconsin 53706, USA
3. These authors contributed equally to this work

Correspondence to: Michael B. Kastan¹ Correspondence and requests for materials should be addressed to M.B.K (e-mail: Email: Michael.Kastan@stjude.org).

Abstract

The rare diseases ataxia-telangiectasia (AT), caused by mutations in the ATM gene, and Nijmegen breakage syndrome (NBS), with mutations in the p95/nbs1 gene, share a variety of phenotypic abnormalities such as chromosomal instability, radiation sensitivity and defects in cell-cycle checkpoints in response to ionizing radiation^{1, 2, 3, 4}. The ATM gene encodes a protein kinase that is activated by ionizing radiation or radiomimetic drugs^{5, 6}, whereas p95/nbs1 is part of a protein complex that is involved in responses to DNA double-strand breaks^{3, 7}. Here, because of the similarities between AT and NBS, we evaluated the functional interactions between ATM and p95/nbs1. Activation of the ATM kinase by ionizing radiation and induction of ATM-dependent responses in NBS cells indicated that p95/nbs1 may not be required for signalling to ATM after ionizing radiation. However, p95/nbs1 was phosphorylated on serine 343 in an ATM-dependent manner in vitro and in vivo after ionizing radiation. A p95/nbs1 construct mutated at the ATM phosphorylation site abrogated an S-phase checkpoint induced by ionizing radiation in normal cells and failed to compensate for this functional deficiency in NBS cells. These observations link ATM and p95/nbs1 in a common signalling pathway and provide an explanation for phenotypic similarities in these two diseases.