The inherited chromosomal instability disorder Nijmegen breakage syndrome (NBS) results from truncating mutations in the NBS1 gene, which encodes the protein nibrin. Nibrin is part of a nuclear multiprotein complex that also contains the DNA repair proteins Mre11 and Rad50. Upon irradiation, this complex redistributes within the nucleus, forming distinct foci that have been implicated as sites of DNA repair. In NBS cells, nibrin is absent and Mre11 and Rad50 are cytoplasmic. In this study, the interacting domains on nibrin and Mre11 were mapped using the yeast two-hybrid system and expression of epitope-tagged constructs in NBS fibroblasts. Deletion of the carboxy-terminal 101 amino acids of nibrin eliminated its ability to interact with Mre11 and to complement the radiation sensitivity of NBS cells. However, this truncated form of nibrin could localize to the nucleus and form radiation-inducible foci. Expression of a carboxy-terminal 354-amino-acid fragment of nibrin was sufficient to direct the nuclear localization of nibrin, as well as that of Mre11 and Rad50. Despite providing some partial complementation of the radiation-sensitive phenotype, the nibrin-Mre11-Rad50 complexes in these cells were unable to form foci. These results indicate that nibrin directs not only the nuclear localization of the nibrin-Mre11-Rad50 complexes but also radiation-induced focus formation. However, direct interaction between nibrin and Mre11 is required for normal cellular survival postirradiation. Distinct domains of nibrin are required for each of these functions, focus formation, nuclear localization, and Mre11 interaction.

Distinct functional domains of nibrin mediate Mre11 binding, focus formation, and nuclear localization.

A Murine Model of Nijmegen Breakage Syndrome

Eukaryotic meiotic recombination requires numerous biochemical processes, including break initiation, end resection, strand invasion and heteroduplex formation, and, finally, crossover resolution. In this review, we discuss primarily those proteins involved in the initial stages of homologous recombination, including SPO11, MRE11, RAD50, NBS1, DMC1, RAD51, RAD51 paralogs, RAD52, RPA, RAD54, and RAD54B. Focusing on the mouse as a model organism, we discuss what is known about the conserved roles of these proteins in vertebrate somatic cells and in mammalian meiosis. We consider such information as gene expression in gonadal tissue, protein localization patterns on chromosomal cores in meiocyte nuclei, and information gleaned from mouse models.

Homologous recombinational repair proteins in mouse meiosis

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.

An inducible null mutant murine model of Nijmegen breakage syndrome proves the essential function of NBS1 in chromosomal stability and cell viability

Meiosis is an essential process for sexually reproducing organisms, leading to the formation of specialized generative cells. This review intends to highlight current knowledge of early events during meiosis derived from various model organisms, including plants. It will particularly focus on cis- and trans-requirements of meiotic DNA double strand break (DSB) formation, a hallmark event during meiosis and a prerequisite for recombination of genetic traits. Proteins involved in DSB formation in different organisms, emphasizing the known factors from plants, will be introduced and their functions outlined. Recent technical advances in DSB detection and meiotic recombination analysis will be reviewed, as these new tools now allow analysis of early meiotic recombination in plants with incredible accuracy. To anticipate future directions in plant meiosis research, unpublished results will be included wherever possible.

Have a break: determinants of meiotic DNA double strand break (DSB) formation and processing in plants

. The rare autosomal recessive disorder Nijmegen breakage syndrome (NBS) results from mutations in the NBS1 gene on human chromosome 8q21. A mouse homolog of the NBS1 gene was iso

J. Woessner

Papers

Identification, characterization, and mapping of a mouse homolog of the gene mutated in Nijmegen breakage syndrome