Expression of foreign DNA is associated with paternal chromosome degradation in intracytoplasmic sperm injection-mediated transgenesis in the mouse

The efficiency of intracytoplasmic sperm injection (ICSI)-mediated transgenesis is often limited by poor embryo development. Because our previous work indicated that impairment of embryo development is frequently related to chromosomal abnormalities, we hypothesized that foreign DNA and/or conditions used to enhance integration of the DNA might induce chromosome damage. Therefore, we examined the chromosomes of mouse embryos produced by transgenesis with the EGFP gene. Spermatozoa were processed with three methods that cause membrane disruption: freeze-thawing, Triton X-100, or Triton X-100 followed by a sucrose wash. Membrane-disrupted spermatozoa were mixed with EGFP plasmids and injected into metaphase 11 oocytes. Three endpoints were evaluated: paternal chromosomes of the zygote, embryo capacity to develop in vitro, and expression of the transgene at the morula/blastocyst stage. In all pretreatments, we observed a significant decrease (approximately 2-fold) in the frequency of normal karyoplates when spermatozoa were incubated with exogenous DNA as compared with the treatment when no DNA was added. As predicted, embryo development was correlated with the integrity of the paternal chromosomes of the zygote. Searching for the possible mechanism of chromosome degradation, we used the ion chelators EGTA and EDTA and found that they neutralize the harmful effect of the transgene and stabilize the paternal chromosomes. In the presence of chelating agents, however, the number of embryos expressing EGFP produced with ICSI-mediated transgenesis decreased significantly. The results suggest that treatment of spermatozoa with exogenous DNA leads to paternal chromosome degradation in the zygote. Furthermore, the mechanisms of disruption of paternal chromosomes and the integration of foreign DNA may be closely related.