2.1. Mice breeding and irradiation
For the in vivo experiments, the pUR288 lacZ-transgenic mouse line 30 [12], harbouring 21 copies of the bacterial lacZ gene, was crossbred with the humanised NBS mouse which harbours the human NBN gene with the 5 bp founder mutation on a null-mutant murine Nbn background (Nbn−/−NBNdel5; [7]). The lacZ-transgenic mouse was also crossbred with our previously described cre recombinase inducible null mutant mouse (Nbnins6/lox6; [6]) and fibroblasts isolatedfor in vitro experiments in the completeabsence of nibrin. All mice had a mixed C57BL/6/129Sv background. Geno- typing was conducted by PCR analysis on DNA isolated from tail biopsies of the mice. Whole-body irradiation of mice was performed with the Cs137-irradiation device OB29/4 S/N 010 (Gamma-Service Medical GmbH) after sedation with Isoflurane. Mice were irradiated on 5 consecutive days with 0.5 Gy per day. Two days after the last irradiation animals were sacrificed and organs frozen in liquid nitrogen until extraction of DNA for the lacZ mutation assay.
2.2. Cell culture and cre recombinase treatment
Fibroblasts were derived from ear explants of control wild type mice (lacZ-Nbn+/+), humanised mice (lacZ-Nbn−/−NBNdel5) and heterozygous mice (lacZ-Nbnins6/lox6). Cells were cultured in Dul- becco’s modified Eagle’s medium (DMEM; Gibco Life Technologies, Germany) supplemented with 5% glucose and 10% foetal calf serum (FCS). Cell culture conditions were 37 ◦C and 5% CO2. Environ- mental oxygen was reduced to 4%. To generate homozygous null mutant fibroblasts in vitro, lacZ-Nbnins6/lox6 cells were incubated in suspension on two consecutive days with 1 M cre recombinase (Excellgen, Rockville) for 2 h. Loss of nibrin was determined by PCR and western blot analysis as previously described [6]. Fibroblast cultures were irradiated in 75 cm2 flasks on ice with a single dose of 5 Gy and cells subsequently examined in the comet assay or lacZ assay.