A murine model of Nijmegen breakage syndrome

Nijmegen breakage syndrome (NBS) is a rare autosoMal recessive disorder characterized by microcephaly, immunodeficiency, and predisposition to hematopoietic malignancy (1]. The clinical and cellular phenotypes of NBS substantially overlap those of ataxia-telangiectasia (A-T). NBS is caused by mutation of the NBS1 gene, which encodes a member of the Mrel1 complex [2, 3], a trimeric protein complex also containing Mre11 and Rad50 [4]. Several lines of evidence indicate that the ataxia-telangiectasia mutated (ATM) kinase and the Mre11 complex functionally interact [5]. Both NBS and A-T cells exhibit ionizing radiation (IR) sensitivity and defects in the intra S phase checkpoint, resulting in radioresistant DNA synthesis (RDS)-the failure to suppress DNA replication origin firing after IR exposure (6]. NBS1 is phosphorylated by ATM in response to IR, and this event is required for activation of the intra S phase checkpoint (the RDS checkpoint) [7-10]. We derived a murine model of NBS, the Nbs1(DeltaB/DeltaB) mouse. Nbs(DeltaB/DeltaB) cells are phenotypically identical to those established from NBS patients. The Nbs1(DeltaB) allele was synthetically lethal with ATM deficiency. We propose that the ATM-Mre11 complex DNA damage response pathway is essential and that ATM or the Mre11 complex serves as a nexus to additional components of the pathway.