Suicide gene therapy of human breast cancer in SCID mice model by the regulation of Tet-On

Background RevTet-On gene expression system was used to deliver the suicide gene tk to human breast cancer cell line MCF-7 and control the tk gene expression level. The animal model of human breast cancer on severe combined immune deficiency (SCID) mice was set up to explore the suicid egene therapy by the regulation of Tet-On. Methods Herpes simplex virus-thymidine kinase (HSVtk) gene was inserted into the plasmid pRevTRE and the recombinant retroviral vector pRevTRE/HSVtk was constructed. Using modified calcium phosphate co-precipitation method, two transfections, pRevTRE/HSVtk and pRevTet-On were performed for MCF-7 cell line and selected by hygromycin B and G418. MCF-7 cell line that stably expressed Tet-regulated tk gene was established. HSVtk gene expression in the MCF/TRE/tk/Tet-On cell line was under the control of Doxycycline (Dox). Cell viability was also determined by MTT assay, whereas HSVtk gene expression was analyzed by reverse transcription-PCR (RT-PCR). Results MCF/TRE/tk/Tet-On cell survival rate was decreased from 100% to less than 20% when ganciclovir (GCV) concentration was increased from 0 to 1000 μg/ml at 1 μg/ml of Dox after 72 hours of GCV administration. At 1 μg/ml of GCV concentration, the cell numbers decreased from 7×10^4 cells/ml to 2×10^4 cells/ml when Dox concentration was increased from 0 to 1500 ng/ml after 72 hours culture. In addition, bystander effects were generated in vitro when 10%-25% of transduced MCF-7 cells were mixed in untransduced MCF-7 cells. On the other hand, the human breast cancer models in SCID mice were set up. The tk gene was expressed with the regulated character after MCF/TRE/tk/Tet-On cells were implanted into the female SCID mice 7 days after Dox induction followed by intraperitoneally administration of GCV for 23 days. Subcutaneous tumors in SCID mice that were implanted with MCF/TRE/tk/Tet-On cells shrank remarkably after Dox and GCV administration as compared with the control. Conclusion The human breast tumor cells (MCF-7) expressing HSVtk gene can be eradicated by administration of GCV and induced with tetracycline or its derivative Dox in vitro and in vivo.