Paternal exposure to cyclophosphamide induces DNA damage and alters the expression of DNA repair genes in the rat preimplantation embryo.

Chronic low dose treatment of male rats with cyclophosphamide, an anticancer alkylating agent, damages male germ cells, resulting in greater than 80% peri-implantation progeny loss. Little transcription or repair takes place in the DNA of post-meiotic male germ cells. The spermatozoal genome regains its transcriptional capacity in the fertilized oocyte. We hypothesized that as a consequence of exposure of male rats to cyclophosphamide DNA damage to the male genome is transmitted to the conceptus; furthermore, this damage leads to alterations in the expression profiles of DNA repair genes during preimplantation development. Male rats were treated with either saline or cyclophosphamide (6mg/kg/day, 4-6 weeks) and mated to control females; 1-8 cell stage embryos were collected. The alkaline comet assay was used to assess DNA damage in 1-cell embryos. A significantly higher percentage (68%) of the embryos fertilized by cyclophosphamide-exposed spermatozoa displayed a comet indicative of DNA damage, compared to those sired by control males (18%). The in situ transcription/antisense RNA approach was used to determine if DNA damage alters the expression of DNA repair genes in early embryos. Dramatic increases in the transcripts for selected members of the nucleotide excision repair family (XPC, XPE and PCNA), mismatch repair family (PMS1), and recombination repair family (RAD50) were found in 1-cell stage embryos sired by cyclophosphamide-treated males compared to controls, while decreases in the expression of base excision repair family members (UNG1, UNG2, and XRCC1) and in recombination repair transcripts (RAD54) were observed. By the 8-cell stage, transcripts for specific members of the nucleotide excision repair family (XPC) and mismatch repair family (MSH2, PMS2) were elevated greatly in control embryos compared to embryos sired by drug-treated males; in contrast, transcripts for other members of the nucleotide excision repair family (XPE, PCNA), as well as some of the base excision repair family (UNG1), were elevated in embryos sired by drug-treated males. Therefore, DNA damage incurred in spermatozoa, following cyclophosphamide exposure is associated with alterations in the expression profiles of DNA repair genes in preimplantation embryos as early as the 1-cell stage. Genotoxic stress may disturb the nuclear remodeling and reprogramming events that follow fertilization and precede zygotic genome activation.