Efficient and stable gene transfer following microinjection into nuclei of synchronized animal cells progressing from G1/S boundary to early S phase

We examined the possible phase(s) of the cell cycle in which a foreign gene can be stably transferred to animal cells. DNA of the plasmid pSV2neo containing the neomycin-phosphotransferase gene was microinjected into the nuclei of NIH/3T3 cells synchronized by serum starvation and aphidicolin treatment. The frequency of neo(r)-transformation (expressed as a percentage of microinjected cells) was 6% at the GO phase and increased with progression of the cell cycle to reach a peak of 76% at the G1/S boundary. When the cells started their growth from the G1/S following release from aphidicolin, the frequency increased or decreased in the parallel with the BrdU-labeling index. Furthermore we developed a simplified method in which asynchronously growing cells were treated with aphidicolin at 10 mu g/ml for 16 hrs without serum starvation and subjected to microinjection, and their growth was further induced in aphidicolin-free medium. Using five cell lines (BALB/3T3, BALB/MK-2, NRK, CHO-K1, and HeLa) and one primary culture of chicken embryo fibroblasts (CEF), a 3- to 7-fold increase in the frequency of neo(r)-transformation was consistently detected in aphidicolin-treated cells, compared to non-treated asynchronous cultures. The present study indicates that synchronized animal cells progressing from the G1/S boundary to the early S phase integrate the pSV2neo DNA into their chromosomes with high efficiency.