title: An inducible null mutant murine model of Nijmegen breakage syndrome proves the essential function of NBS1 in chromosomal stability and cell viability.
abstract: The human genetic disorder, Nijmegen breakage syndrome, is characterized by radiosensitivity, immunodeficiency, chromosomal instability and an increased risk for cancer of the lymphatic system. The NBS1 gene codes for a protein, nibrin, involved in the processing/repair of DNA double strand breaks and in cell cycle checkpoints. Most patients are homozygous for a founder mutation, a 5 bp deletion, which might not be a null mutation, as functionally relevant truncated nibrin proteins are observed, at least in vitro. In agreement with this hypothesis, null mutation of the homologous gene, Nbn, is lethal in mice. Here, we have used Cre recombinase/loxP technology to generate an inducible Nbn null mutation allowing the examination of DNA-repair and cell cycle-checkpoints in the complete absence of nibrin. Induction of Nbn null mutation leads to the loss of the G2/M checkpoint, increased chromosome damage, radiomimetic-sensitivity and cell death. In vivo, this particularly affects the lymphatic tissues, bone marrow, thymus and spleen, whereas liver, kidney and muscle are hardly affected. In vitro, null mutant murine fibroblasts can be rescued from cell death by transfer of human nibrin cDNA and, more significantly, by a cDNA carrying the 5 bp deletion. This demonstrates, for the first time, that the common human mutation is hypomorphic and that the expression of a truncated protein is sufficient to restore nibrin's vital cellular functions.
authors.affiliation: Institut für Humangenetik, Charité Universitätsmedizin Berlin, Campus Virchow-Klinikum, Augustenburger Platz 1, 13353 Berlin, Germany.
authors.forename: Ilja
authors.initials: I
authors.lastname: Demuth
authors.forename: Pierre-Olivier
authors.initials: PO
authors.lastname: Frappart
authors.forename: Gabriele
authors.initials: G
authors.lastname: Hildebrand
authors.forename: Anna
authors.initials: A
authors.lastname: Melchers
authors.forename: Stephan
authors.initials: S
authors.lastname: Lobitz
authors.forename: Lars
authors.initials: L
authors.lastname: Stöckl
authors.forename: Raymonda
authors.initials: R
authors.lastname: Varon
authors.forename: Zdenko
authors.initials: Z
authors.lastname: Herceg
authors.forename: Karl
authors.initials: K
authors.lastname: Sperling
authors.forename: Zhao-Qi
authors.initials: ZQ
authors.lastname: Wang
authors.forename: Martin
authors.initials: M
authors.lastname: Digweed
mesh_terms: D000818:Animals; D002453:Cell Cycle / Q000235:genetics; D018797:Cell Cycle Proteins / Q000235:genetics* / Q000502:physiology*; D002470:Cell Survival / Q000235:genetics; D002478:Cells, Cultured; D043171:Chromosomal Instability / Q000235:genetics*; D019457:Chromosome Breakage / Q000235:genetics; D025063:Chromosome Disorders / Q000235:genetics*; D004260:DNA Repair / Q000235:genetics; D018076:DNA, Complementary / Q000235:genetics; D004268:DNA-Binding Proteins; D004195:Disease Models, Animal; D005347:Fibroblasts / Q000378:metabolism; D018390:Gene Targeting; D006801:Humans; D007153:Immunologic Deficiency Syndromes / Q000235:genetics; D019426:Integrases / Q000235:genetics / Q000378:metabolism; D051379:Mice; D008817:Mice, Mutant Strains; D009687:Nuclear Proteins / Q000235:genetics* / Q000502:physiology*; D017384:Sequence Deletion / Q000235:genetics; D013577:Syndrome; D014764:Viral Proteins / Q000235:genetics / Q000378:metabolism
chemical_list: D018797:Cell Cycle Proteins; D018076:DNA, Complementary; D004268:DNA-Binding Proteins; C112156:NBN protein, human; C466108:Nijmegen breakage syndrome 1 protein, mouse; D009687:Nuclear Proteins; D014764:Viral Proteins; C045073:Cre recombinase; D019426:Integrases