p53 dependent S checkpoint in early mouse embryos and radiation induction of genomic instability

在Ms 6 hm小卫星基因座和粉眼不稳定基因座的母系等位基因的突变水平升高，发现在精子阶段照射的雄性小鼠所生的F1小鼠。这些突变证明了由于基因组不稳定性而导致的非靶向和延迟重组突变的操作，所述基因组不稳定性是在小鼠早期发育期间由辐射精子引入的辐射DNA损伤诱导的。为了阐明基因组不稳定性的分子机制，我们研究了精子辐射小鼠胚胎的损伤反应，将p53报告质粒显微注射到受精卵中，当受精卵与辐射精子受精时，p53报告质粒被转录激活。研究发现，这种原核串扰会抑制精子照射受精卵中雌性原核和雄性原核的DNA合成。在p53-/-受精卵中没有观察到这种DNA合成的抑制，当精子照射p53-/-受精卵用p53蛋白显微注射时，这种抑制被恢复，这表明了一种新的S检查点途径。将突变型p53蛋白显微注射到p53-/-受精卵中，证实了依赖于p53的S检查点需要p53的DNA结合结构域，而不需要反式激活结构域，并在原代小鼠中分析了依赖于p53的S检查点，发现其在低于2戈伊的剂量下起作用。此外，通过用Id U和Cld U双标记细胞分析DNA纤维延伸，表明p53依赖的S检查点通过减慢复制叉进展来抑制DNA合成。这种复制的减慢与姐妹染色单体之间更高的重组率有关，这可以解释小卫星的非靶向重组。在粉红色眼睛的不稳定等位基因上观察到的延迟重组需要另一种损伤记忆的机制，对此还有更多的研究。

Elevated levels of mutations were found at the maternally derived alleles of the Ms6hm minisatellite locus and the pink-eyed unstable locus in F1 mice born to male mice irradiated at the spermatozoa stage. These mutations demonstrated the operation of untargeted and delayed recombinational mutations due to genomic instability which was induced during early mouse development by radiation DNA damage introduced by the irradiated sperm. In order to elucidate the molecular mechanism of genomic instability, we have undertaken studies of damage response in sperm irradiated mouse embryos.A p53 responsible reporter plasmid microinjected into zygotes was transcriptionally activated when they zygotes were fertilized with irradiated sperm. This pronuclear cross-talk was found to suppress DNA synthesis of female pronucleus as well as of male pronucleus in sperm irradiated zygotes. This suppression of DNA synthesis was not observed in p53-/-zygotes which was restored when the sperm irradiated p53-/-zygotes were microinjected with p53 protein, suggesting a novel pathway of S checkpoints. Microinjection of mutant p53 proteins into sperm irradiated p53-/-zygotes demonstrated that this p53 dependent S checkpoint required DNA binding domain of p53,but the transactivation domain.The p53 dependent S checkpoint was analyzed in primary mouse and found operate at doses below 2 Gy. In addition, analyses of DNA fiber elongation by double labeling the cells with Id U and Cld U showedthatthe p53 dependent S checkpoint suppressed DNA synthesis by slowing down of replication fork progression. This slowing down of replication was associated with a higher rate of recombination between sister chromatids which can explain the untargeted recombination of minisatellite. Delayed recombination observed at the pink-eyed unstable allele requires yet another mechanism of damage memory for which much more to be studied yet.