Cell-cycle dependent response to DNA damage in mouse zygotes

Abstract

Mammalian zygotes were found hypersensitive to radiation exposure, which may be ascribed to unusual DNA damage responses. In the present study, we irradiated mouse zygotes with γ-ray at various phases of the cell cycle and examined the developmental effects of irradiation. Zygotes were irradiated in G1 and G2 phases and their cellular responses were assessed by examining the entry into mitosis, micronucleus formation and development to the blastocyst stage. The zygotes irradiated in G1 phase were arrested for longer time than those irradiated in G2 phase. Besides, although pronuclei formed normally in zygotes irradiated in G1 phase, these zygotes had higher tendency to possess micronuclei when they cleaved into the two-cell stage, and lower potential to develop into the blastocyst stage, compared with the zygotes irradiated in G2 phase. These results indicated that the sensitivity of zygotes to irradiation varied between the two gap periods. To investigate the response to irradiation in M phase, zygotes were arrested at M phase by nocodazole treatment. Although the irradiation did not cause any delay in the cleavage into the 2-cell stage, nucleoplasmic bridges, which form due to inappropriate DNA repair and telomere fusions, were frequently observed. Most of these embryos were permanently arrested in the interphase of the two-cell stage and none of them reached the blastocyst stage. Therefore, these results suggested that mitotic checkpoint was not activated by DNA damage in zygotes, the same as what has been observed in somatic cells. However, DNA repair system, which is shut down during somatic mitosis, seemed to be functional but not appropriate during the first mitosis.