Molecular evidence for the induction of large interstitial deletions on mouse chromosome 8 by ionizing radiation

The P19H22 mouse embryonal carcinoma cell line is characterized by a hemizygous deficiency for the chromosome 8 encoded aprt (adenine phosphoribosyltransferase) gene and heterozygosity for many chromosome 8 loci. We have previously demonstrated that this cell line is suitable for mutational studies because it is permissive of events ranging in size from base-pair substitutions at the aprt locus to apparent loss of chromosome 8. Large mutational events, defined by loss of the remaining aprt allele, were found to predominate in spontaneous mutants and those induced by ionizing radiation (Turker et al., Mutation Res., 329, 97-105, 1995). In this study we have used a PCR based assay to screen for loss of heterozygosity at microsatellite loci both proximal and distal to aprt in ¹³⁷Cs-induced and spontaneous aprt mutants. This approach allowed us to distinguish apparent interstitial deletional events from apparent recombinational events. Significantly, 32.5% (26 of 80) of the mutational events induced by ¹³⁷Cs appeared to be interstitial deletions as compared with 7.7% (6 of 78) in the spontaneous group. This difference was statistically significant (p < 0.0001) suggesting that exposure to ¹³⁷Cs caused a significant number of deletion mutations. Most ¹³⁷Cs-induced interstitial deletions were larger than 6 cM, whereas none of the spontaneous deletions were larger than 6 cM. These results provide further support for the notion that ionizing radiation induces deletion mutations and validate the use of the P19H22 cell line for the study of events induced by ionizing radiation.