Topoisomerase II Mediated Breaks in Spermatozoa Cause the Specific Degradation of Paternal DNA in Fertilized Oocytes.

We have demonstrated that mouse spermatozoa can cleave their DNA into 50 kb fragments when treated with TritonX-100, MnCl2, and CaCl2. This cleavage, termed "Sperm Chromatin Fragmentation" (SCF), is mediated by Topoisomerase IIB (TOP2B) when stimulated by a factor in the epididymal fluid, most likely a nuclease, and can be at least partially religated by EDTA. When the protamines are removed, this DNA breakage is followed by digestion of all the DNA by a nuclease(s). We tested whether the oocyte could repair TOP2B induced sperm DNA breaks, or whether partial religation by EDTA would allow spermatozoa to fertilize the oocytes normally. Oocytes injected with untreated spermatozoa developed normally. However, oocytes injected with spermatozoa treated with MnCl2 and CaCl2 to induce SCF, with or without subsequent EDTA treatment, failed to develop. In both of these treatment groups, the maternal pronuclei developed normally and replicated their DNA. However the paternal pronuclei did not replicate their DNA, and the DNA began to disappear at 6 hours after injection-- at about the same time that maternal DNA replication was initiated. These data suggest that when TOP2B is induced to cleave the sperm DNA before fertilization, the paternal DNA is subsequently degraded by a highly regulated mechanism that does not affect the maternal chromatin. Furthermore, partial religation of the TOP2B breaks by EDTA does not prevent either the inhibition of DNA synthesis or the DNA degradation.