Malling H V, Newbold R R, Lewis S, Barnett L, Weaver R P
Mammalian Mutagenesis Group, Laboratory of Toxicology, Environmental Toxicology Program, National Institute Of Environmental Health Sciences, P.O. Box 12233, Research Triangle Park, NC 27709-2233, USA.
Mutat Res. 1999 Jul 21;444(1):85-95. doi: 10.1016/s1383-5718(99)00088-1.
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 to be fixed by replication during brain development. Spontaneous and ethylnitrosourea (ENU)-induced reverse mutations at a single AT base pair were measured in brain tissue by using mice transgenic for PhiX174 am3, cs70. The line (am54) has 50 PhiX genomes per haploid genome integrated in a tandem array and is maintained by random breeding on a C57BL/6 background. For mutagenesis 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 concurrent 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-injection 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 frequency in the treated group was significantly lower than controls. In a second study animals were euthanized 3, 10 and 74 days after treatment. Two groups (3 and 74 days post-injection) also showed a significant decrease in the revertant frequency as compared to controls. Additional sets of adult animals 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 detected 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 frequency. Moreover, under certain conditions, ENU-produced a significantly lower mutant frequency than was observed in the control animals. This decrease in the revertant frequency among the treated animals was likely due to selective killing of cells with a higher spontaneous revertant frequency than cells with lower frequency.
小鼠大脑中的大多数细胞分裂在出生后14天就已停止。因此，可以假定脑细胞中发生的自发突变在大脑发育过程中通过复制而固定下来。通过使用PhiX174 am3、cs70转基因小鼠，对脑组织中单个AT碱基对处的自发突变和乙基亚硝基脲（ENU）诱导的回复突变进行了测量。品系（am54）每个单倍体基因组中有50个PhiX基因组以串联阵列形式整合，并在C57BL/6背景下通过随机交配维持。为了进行诱变研究，将纯合的am54雄性小鼠与非转基因的C57BL6/J雌性小鼠交配。该杂交产生的4日龄后代用50 mg/kg ENU处理，并在68 - 80日龄时安乐死。经ENU处理的动物大脑中am3回复突变体的频率明显高于同期对照。在第二个实验中，半合子雄性后代（85至94日龄）用150 mg/kg ENU处理，并在注射后的不同时间间隔（3、10和110天）安乐死。处理后3天和10天的回复突变频率与对照值无显著差异。然而，在注射后110天的时间间隔，处理组的平均回复突变频率明显低于对照组。在第二项研究中，动物在处理后3、10和74天安乐死。两组（注射后3天和74天）与对照组相比，回复突变频率也显著降低。另外几组成年动物用50和150 mg/kg ENU处理，并在处理后195至201天安乐死。用50或150 mg/kg ENU处理的动物的平均回复突变频率与对照值无显著差异。因此，尽管在用ENU处理新生小鼠时，PhiX174系统中检测到突变频率增加，但用ENU处理成熟小鼠并未导致突变频率增加。此外，在某些条件下，ENU产生的突变频率明显低于对照动物中观察到的频率。处理动物中回复突变频率的降低可能是由于选择性杀死了自发回复突变频率较高的细胞，而不是频率较低的细胞。
