Mutagenesis of a single AT basepair in mice transgenic for PhiX174 am3, cs70 II. Brain

Most cell divisions in the mouse brain have ceased by 14 days after birth. Therefore, spontaneous mutations that occur in brain cells can be assumed t o be fixed by replication during brain development. Spontaneous and ethylni trosourea (ENU)-induced reverse mutations at a single AT base pair were mea sured in brain tissue by using mice transgenic for PhiX174 am3, cs70. The l ine (am54) has 50 PhiX genomes per haploid genome integrated in a tandem ar ray and is maintained by random breeding on a C57BL/6 background. For mutag enesis studies, homozygous am54 males were mated to non-transgenic C57BL6/J females. Four-day old offspring from this cross were treated with 50 mg/kg ENU and were euthanized at 68-80 days of age. The ENU-treated animals had a significantly higher frequency of am3 revertants in brain than did concur rent controls. In a second experiment, hemizygous male offspring (85 to 94 days old) were treated with 150 mg/kg ENU and euthanized at various post-in jection intervals (3, 10 and 110 days). The revertant frequencies 3 and 10 days after treatment were not significantly different from control values. At the 110 days post-injection interval, however, the average revertant fre quency in the treated group was significantly lower than controls. In a sec ond study animals were euthanized 3, 10 and 74 days after treatment. Two gr oups (3 and 74 days post-injection) also showed a significant decrease in t he revertant frequency as compared to controls. Additional sets of adult an imals were treated with 50 and 150 mg/kg ENU and were euthanized 195 to 201 days after treatment. The average revertant frequency of the animals that were treated with 50 or 150 mg/kg ENU was not significantly different from the control value. Thus, although an increase in mutant frequency is detect ed in the PhiX174 system when neonatal mice are treated with ENU, treatment of mature mice with ENU did not result in an increase in the mutant freque ncy. Moreover, under certain conditions, ENU-produced a significantly lower mutant frequency than was observed in the control animals. This decrease i n the revertant frequency among the treated animals was Likely due to selec tive killing of cells with a higher spontaneous revertant frequency than ce lls with lower frequency. (C) 1999 Elsevier Science B.V. All rights reserve d.