Nbn−Mre11 interaction is required for tumor suppression and genomic integrity

We derived a mouse model in which a mutant form of Nbn/Nbs1mid8 (hereafter Nbnmid8) exhibits severely impaired binding to the Mre11−Rad50 core of the Mre11 complex. The Nbnmid8 allele was expressed exclusively in hematopoietic lineages (in Nbn−/mid8vav mice). Unlike Nbnflox/floxvav mice with Nbn deficiency in the bone marrow, Nbn−/mid8vav mice were viable. Nbn−/mid8vav mice hematopoiesis was profoundly defective, exhibiting reduced cellularity of thymus and bone marrow, and stage-specific blockage of B cell development. Within 6 mo, Nbn−/mid8 mice developed highly penetrant T cell leukemias. Nbn−/mid8vav leukemias recapitulated mutational features of human T cell acute lymphoblastic leukemia (T-ALL), containing mutations in NOTCH1, TP53, BCL6, BCOR, and IKZF1, suggesting that Nbnmid8 mice may provide a venue to examine the relationship between the Mre11 complex and oncogene activation in the hematopoietic compartment. Genomic analysis of Nbn−/mid8vav malignancies showed focal amplification of 9qA2, causing overexpression of MRE11 and CHK1. We propose that overexpression of MRE11 compensates for the metastable Mre11−Nbnmid8 interaction, and that selective pressure for overexpression reflects the essential role of Nbn in promoting assembly and activity of the Mre11 complex.