Transfection of the inner cell mass and lack of a unique DNA sequence affecting the uptake of exogenous DNA by sperm as shown by dideoxy sequencing analogues

PURPOSE

The purpose of this study was to determine whether exogenous DNA internalized into blastocysts after transference from DNA-carrier sperm are localized at the inner cell mass or trophoblast cells and to identify differences in uptake of exogenous DNA fragments by sperm due to unique DNA sequences.

METHODS

Mouse blastocysts at the hatching stage were exposed to migrating human sperm cells carrying exogenous DNA fragments synthesized from the E6-E7 conserved gene regions of human papillomavirus (HPV) types 16 and 18. After an interaction period of 2 hr, the transfected blastocysts were washed several times to remove extraneous sperm and the blastocysts were dissected into groups of cells derived from the inner cell mass and trophoblasts. The cells were analyzed by polymerase chain reaction (PCR) for the presence of HPV DNA fragments. In the second part of the experiment, thawed donor (N = 10) sperm cells were pooled, washed, and divided into two fractions. The first (control) fraction was added with formalin and further divided and added with a 35S-radiolabeled G, A, T, or C sequencing mixture. The second fraction was similarly treated but the formalin step was omitted from the treatment. After an hour of incubation at 37 degrees C, the sperm specimens were washed several times by centrifugation and DNA extracted by the GeneReleaser method. The extracted DNA were processed on sequence gels, and the autoradiographs analyzed.

RESULTS

Mouse blastocysts transfected by carrier sperm with DNA from HPV types 16 and 18 showed localization of the HPV DNA to both the inner cell mass and trophoblast cells. Negative controls consisting of untreated human sperm and untreated mouse blastocysts did not reveal any evidence of HPV DNA. The positive sperm control generated expected DNA fragments from HPV types 16 and 18. In the second experiment, the intensities of the DNA fragments in the G, A, T, and C columns from low to high molecular weights were not different from the positive control bands. Band intensities of the four sequencing columns were similar. Formalin pretreatment of the sperm inhibited uptake of the DNA fragments from the smallest to the largest DNA molecules.

CONCLUSIONS

Exogenous DNA taken into blastocysts are localized to both the inner cell mass and trophoblast cells. Only live sperm exhibited the capacity to carry various sizes of exogenous DNA, suggesting the involvement of active cell membrane mechanism in the transference process. The results showed that DNA fragments terminating in any of the four nucleotides were equally taken up by the sperm cell. Fragments of DNA produced by the sequencing reaction failed to identify a unique DNA sequence that would facilitate or inhibit the sperm from taking up exogenous DNA.