In order to terminate our system at the end of the therapy, we designed our therapeutic cells to produce the enzyme thymidine kinase.

The addition of the prodrug ganciclovir, which is converted into a toxic compound in cells producing thymidine kinase, efficiently initiates the apoptosis of therapeutic cells, making this mechanism virtualy free of leaky apoptosis.

Expression of thymidine kinase is not deleterious to cell growth.

The Thymidine kinase/Ganciclovir system in detail

Suicide gene therapy or gene-directed enzyme prodrug therapy (GDEPT) is widely used in cancer treatment. One of the most used GDEPT systems is the herpes simplex virus thymidine kinase (HSV-TK) with purine nucleoside analog ganciclovir (GCV) as a prodrug. Systemic administration of the prodrug ganciclovir induces apoptosis only in cells transfected with HSV-thymidine kinase while the untransfected cells survive. Unlike human thymidine kinase, HSV-thymidine kinase is able to phosphorylate ganciclovir to form ganciclovir-monophosphate, which is then phosphorylated to ganciclovir-diphosphate followed by ganciclovir-triphosphate. Ganciclovir-triphosphate is then incorporated into the DNA, which causes inhibition of DNA synthesis and subsequently leads to apoptosis (Ardiani et al., 2010). We used the HSV-thymidine kinase fused to mouse guanylate kinase (mGMK:TK30, BioBrick BBa_K404113, prepared by the Freiburg_Bioware team in 2010) in our system because it improves phosphorylation of ganciclovir-monophosphate to ganciclovir-diphosphate, thus preventing accumulation of ineffective intermediate products (i.e. GCV-MP, GCV-DP) due to the limited ability of the endogenous guanylate kinase (Figure 1).

We successfully implemented HSV-TK/GCV system to our cellular device to function as a controllable “safety switch”. This means that we can inactivate our cellular device whenever we want, after or even in the middle of therapy, simply by administering ganciclovir to the patient.

Following the BioBrick Standard Assembly technique we inserted the mouse guanylate kinase – thymidine kinase fusion gene (mGMK:TK30, (BBa_K404113)) under the control of the CMV promoter (BBa_K782063; Figure 2). HSV-TK mutant (TK30) contains six amino acid substitutions and shows enhanced sensitivity to ganciclovir (Kokoris et al., 1999).

We transfected HEK293 cells with the pPCMV_mGMK:TK30 plasmid and monitored cell survival after the addition of ganciclovir. We daily observed cells a with light microscope and monitored the morphological signs of decreased viability. Additionally we stained cells with Hoechst and SytoxGreen514 dye to distinguish between live and dead cells with the help of a confocal microscope. We quantified viable cell numbers at different time points by cell counting and flow cytometry. For detailed protocols please refer to the Experimental methods section.


We tested different amounts of transfected pPCMV_mGMK:TK30 DNA, different concentrations of ganciclovir and different time periods after the addition of ganciclovir to find the optimal conditions where our pCMV_mGMK:TK30-transfected cells would be most efficiently killed while the untransfected cells would be left unharmed. We monitored pPCMV_mGMK:TK30-transfected ganciclovir-treated cells under a light microscope and discovered that as little as 40 hours of incubation with higher concentrations of ganciclovir (>30 µg/ml) is enough to kill over 50 % of our pPCMV-mGMK:TK30-transfected cells. After 65 hours of incubation with ganciclovir there was a decrease in cell number and survival already at low concentrations of ganciclovir (1 µg/ml). We showed that the HSV-TK/GCV system successfully kills cells even at low amounts of transfected mGMK:TK30 and upon addition of relatively low concentrations of ganciclovir (5 µg/ml).

We determined the number of viable cells after ganciclovir treatment by staining the cells with Trypan Blue (to distinguish between living and dead cells) and counting them. After 6 days of incubation with ganciclovir, the cytotoxic effect could be seen when only 1 µg/ml ganciclovir was added to pPCMV_mGMK:TK30 transfected cells while untransfected cells remained unharmed. The number of viable cells decreased to <10 % when 10 µg/ml ganciclovir was added. These results demonstrate that our safety mechanism is effective even at low doses of ganciclovir, where only therapeutic cells are killed while other cells are left unharmed.

Cells were stained with Hoechst and SytoxGreen514 dye that differentially stain live and dead cells (Figure 4) and were imaged with a confocal microscope. Cell permeable Hoechst (depicted blue) binds to nucleic acids and therefore stains all cells in a culture. On the other hand cell impermeable SytoxGreen (depicted pink) cannot cross the membrane of live cells and therefore stains only dead cells. The cytotoxic effect of ganciclovir is evident at concentrations as low as 10 µg/ml and only a few viable cells were present upon addition of 100 µg/ml ganciclovir. On the other hand untransfected cells are not affected by ganciclovir. Moreover, cells expressing mGMK:TK30 (under the control of the CMV promoter) had no effect on cell survival if no ganciclovir was added.

The bystander effect for mGMK:TK30 was demonstrated by Ardiani et al. (2010) who showed substantial ganciclovir-induced cell death although only a fraction of cells were transfected. There are several different mechanisms of a bystander effect: the transfer of toxic metabolite ganciclovir-triphosphate through gap junctions (Elshami et al., 1996), endocytosis of apoptotic bodies generated from HSV-TK expressing cells by neighbouring untransfected cells (Freeman et al., 1993) and release of soluble factors (Drake et al., 2000). We also demonstrated that even if some cells do not express mGMK:TK30 (GFP was used as a control of transfection), ganciclovir affects practically all cells in a culture (Figure 5). If transiently transfected therapeutic cells would be microencapsulated and used for therapy, the bystander effect ensures that all cells in a microcapsule would be killed upon addition of ganciclovir even if some of them would not express the HSV-thymidine kinase.

Trypan Blue and SytoxGreen514 only stain cells that are already dead. We also wanted to determine the number of cells that are still in the early stages of apoptosis. To do this we stained the cells with Annexin V and analyzed them with flow cytometry. Annexin V binds phosphatidylserine, which is located on the inner side of the plasma membrane. When apoptosis is initiated phosphatidylserine is translocated to the extracellular side of the plasma membrane which enables staining with Annexin V.

Figure 6 shows the percent of Annexin V positive cells 65 hours after ganciclovir addition. The percent of apoptotic cells increases with higher ganciclovir concentrations and higher amounts of transfected pPCMV_mGMK:TK30. Even though only around 25 % of cells stain with Annexin V we must keep in mind that after 65 hours ganciclovir already killed a large amount of cells which were therefore not included in the analysis.


Ardiani, A., Sanchez-bonilla, M., and Black, M.E. (2010) Fusion Enzymes Containing HSV-1 Thymidine Kinase Mutants and Guanylate Kinase Enhance Prodrug Sensitivity In Vitro and In Vivo. Cancer 17, 86-96.

Drake, R.R., Pitlyk, K., McMasters, R.A., Mercer, K.E., Young, H., Moyer, M.P. (2000) Connexin-independent ganciclovir-mediated killing conferred on bystander effect-resistant cell lines by a herpes simplex virus-thymidine kinase-expressing colon cell line. Molecular therapy: the journal of the American Society of Gene Ther. 2, 515-523.

Elshami, A.A., Saavedra, A., Zhang, H., Kucharczuk, J.C., Spray, D.C., Fishman, G.I., Amin, K.M., Kaiser, L.R., Albelda, S.M. (1996) Gap junctions play a role in the ‘bystander effect’ of the herpes simplex virus thymidine kinase/ganciclovir system in vitro. Gene Ther. 3, 85-92.

Freeman, S.M., Abboud, C.N., Whartenby, K.A., Packman, C.H., Koeplin, D.S., Moolten, F.L., Abraham, G.N. (1993) The Bystander Effect : Tumor Regression When a Fraction of the Tumor Mass Is Genetically Modified. In Vitro 53, 5274-5283.

Kokoris, M.S., Sabo P., Adman E.T., Black, M.E. (1999) Enhancement of tumor ablation by a selected HSV-1 thymidine kinase mutant. Gene Ther. 6, 1415-1426.

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