haploinsufficient tumour suppressorgene [10]. We have bred these mice with a transgenic mutagenesis mouse harbouring 21 tandem copies of the bacterial lacZ gene, stably integrated on chromosome 11 [11].The mutation frequency and mutation spectrum was deter- mined in DNA extracted from fibroblasts or lymphatic tissues by rescuing the reporter plasmid and transformation of E. coli.
We find that null mutation of Nbn leads to increased DNA dam- age in vitro, even in the absence of an external genotoxic treatment. Distinct mutation spectra indicate that the molecular origin of spontaneous and radiation induced base mutations is different. In vivo we find the basal mutation frequency measured in lym- phatic tissue of the humanised NBS mice is approximately 2.5-fold higher than in control mice. We conclude that spontaneous DNA damage, incurred probably during replication, is a major source of mutations in nibrin deficient cells. Surprisingly, after irradiation, in vivo mutation frequencies were not significantly increased in the humanised NBS mice. This is attributed to efficient apoptosis even in the absence of wild type nibrin and suggests that spontaneous base mutations may have a more significant role in tumorigenesis in NBS patients than previously appreciated.