Suicide Gene Transduction Sensitizes Murine Embryonic and Human Mesenchymal Stem Cells to Ablation on Demand-- a Fail-safe Protection Against Cellular Misbehavior

Stem cells and their progeny constitute a potential resource for replacing damaged tissues or supplying missing functions, but also pose a threat of aberrant behavior, including neoplastic growth or immunopathology. Suicide genes introduced into these cells before transplantation might provide a means of addressing this threat by permitting the ablation of the cells if they subsequently misbehave. Retroviral transduction of the E. coli gpt and herpes thymidine kinase (HSVtk) suicide genes was used to determine the degree to which stem cells could be sensitized to the prodrugs 6-thioxanthine (6TX) and ganciclovir (GCV) respectively, and whether this sensitivity could persist over many cell generations. The ES-E14TG2a murine embryonic stem cell line was rendered sensitive to quantitative ablation at prodrug concentrations well tolerated by untransduced cells (50 microM 6TX, 1 microg/ml GCV). The HSVtk gene also conferred GCV sensitivity on human mesenchymal stem cells and hematopoietic precursors derived from the murine cells, although ablation was not complete. Because ES-E14TG2a cells are deficient in the cellular enzyme HPRT, they are sensitive to hypoxanthine/aminopterin/thymidine (HAT). This property enhanced the persistence of chemosensitivity in gpt-transduced cells by permitting cells that lost 6TX sensitivity to be ablated with HAT.