Knockout of maternal SMC2 enhances DNA damage in zygotes
Since the chromatin bridge and micronuclei are linked with genome injury, we then studied the DNA damage inSmc2flox /flox Gdf9Cre females embryos by staining of γ-H2AX, a bio-label for DNA damage (McManus & Hendzel, 2005). We revealed that DNA damage signal intensity was increased in zygotes with loss of maternal SMC2. At the PN-3 stage, SMC2+/+ zygotes showed less γH2AX signal; then γH2AX was detected in WT zygotes at the PN-4 stage when DNA replication began but was repaired in PN-5 (Fig. 5A). By contrast, the SMC2-/+ zygotes showed stronger γH2AX signals than the WT zygotes even since the PN-3 stage, and failed to be repaired at PN-5 (Fig. 5A). The γH2AX foci in female pronuclei at the PN-3 and PN-5 stage were counted, and the numbers are shown in Fig. 5B and C. The accumulation of DNA damage may be the reason for the developmental arrest of SMC2-/+ zygotes.
On the other hand, the zygotic histone methylation was not affected by the maternal SMC2 deletion. SMC2-/+ zygotes showed normal distribution patterns of H3K4me3, which was more abundant in the female pronucleus than in the male pronucleus. (Fig. S1).