DNA double-strand break and chromosomal rejoining defects with misrejoining in Nijmegen breakage syndrome cells.

NBS1-deficient cells exhibit pronounced radiosensitivity and defects in chromosome integrity after ionizing radiation (IR) exposure, yet show only a minor defect in DNA double-strand break (DSB) rejoining, leaving an as yet unresolved enigma as to the nature of the radiosensitivity of these cells. To further investigate the relationship between radiosensitivity, DSB repair, and chromosome stability, we have compared cytological and molecular assays of DSB misrejoining and repair in NBS1-defective, wild type, and NBS1-complemented cells after IR damage. Our findings suggest a subtle defect in overall DSB rejoining in NBS1-defective cells and uniquely also reveal reduced ability of NBS1-defective cells to rejoin correct ends of DSBs. In agreement with published results, one of two different NBS1-defective cell lines showed a slight defect in overall rejoining of DSBs compared to its complemented counterpart, whereas another NBS line did not show any difference from wild type cells. Significant defects in the correct rejoining of DSBs compared to their respective controls were observed for both NBS1-defective lines. The defect in DSB rejoining and the increased misrejoining detected at the molecular level were also reflected in higher levels of fragments and translocations, respectively, at the chromosomal level. This work provides both molecular and cytological evidence that NBS1-deficient cells have defects in DSB processing and reveals that these molecular events can be manifest cytologically.