Agents which reduce somatic intrachromosomal recombination below spontaneous frequency in the pKZ1 recombination mutagenesis model

The pKZ1 recombination mutagenesis model contains an E.Coli lacZ transgene in inverse orientation to a promoter-enhancer complex. If an inversion event occurs in the transgene then the lacZ gene is brought into correct transcriptional orientation and the lacZ gene product (β-galactosidase) can be detected in frozen tissues using a chromogenic substrate for the enzyme. The spontaneous frequency of somatic intrachromosomal recombination (SICR) inversion events in spleen tissue is 1-2x10-4. We have previously observed a significant induction in SICR inversions in response to a range of DNA damaging agents which have widely different genotoxic mechanisms. pKZ1 mice were exposed to a single intraperitoneal dose of etoposide or acetamide, or multiple whole body doses of 900 MHz radiofrequency (RF). Three days post-treatment the spleen was removed and frozen tissues analysed for inversion events. A significant 1.3-3.1 fold induction of SICR was observed for the higher doses of etoposide (0.05, 0.5, and 50mg/kg) but a significant 1.5-2.3 fold reduction below spontaneous inversion frequency was observed for the lower doses of etoposide (0.0005 and 0.005 mg/kg). A significant reduction below spontaneous frequency was also observed when animals were treated with 109 mg/ml acetamide (0.6X) or exposed to 25 daily 30 minute exposures of 4W/kg RF (0.4X). This is the first report of recombination suppression in response to any of these agents. The high spontaneous frequency of SICR in spleen of pKZ1 mice may facilitate the detection of agents which reduce mutation frequency below spontaneous compared to other mutation assays. At this stage, the significance of a reduction below spontaneous inversion frequency in treated animals is unclear.