Abstract
Radioresponse at the preimplantation stage was analyzed for mouse embryos fertilized by 6 Gy irradiated sperm. The p53 dependent transactivation of a p53 dependent lacZ reporter microinjected into in female pronucleus was observed in these embryos. DNA synthesis of sperm irradiated zygotes was suppressed p53 dependently in both male and female pronuclei. Even with these p53 activation, the sperm irradiated zygotes did not arrest at G2/M and progressed normally to 2 cell stage. They further cleaved until day 3.5 when they showed a sign of cell cycle arrest. RT-PCR analyses revealed that p21 was not activated in sperm irradiated embryos of day 1.5 to day 3.5. Direct irradiation of embryos also failed to activate p21 until they progressed to day 3.5. Phosphorylation of histone H2AX were inefficient in zygotes and two cell stage embryos. In contrast, prompt phosphorylation of H2AX was found to take place when the embryos were over day 3.5. Apoptosis was only detected in the cells of inner cell mass of the blastcyst stage embryos.
Altogether, our unusual observations demonstrate that the damage response of early embryogenesis does not follow the general pattern conserved throughout eukaryotes. The exceptional damage response is likely to have a protective role in embryogenesis. Indeed, it has long been shown in mice that irradiation of preimplantation stages results in higher rate of embryonal death leading while live born pups are devoid of deleterious outcome such as malformation.
