Unrepairable DNA Double Strand Breaks Initiate Cytotoxicity with HSV-TK/Ganciclovir

The herpes simplex virus thymidine kinase (HSV-TK) is the most widely used suicide gene in cancer gene therapy due to its superior anticancer activity with ganciclovir compared to other HSV-TK substrates, such as 1-β-D-arabinofuranosyl thymine (araT). We have evaluated the role of DNA damage as a mechanism for the superiority of GCV. Using γ-H2AX foci as an indicator of DNA damage, GCV induced ≥ 7-fold more foci than araT at similarly cytotoxic concentrations. The number of foci decreased after removal of either drug, followed by an increase in Rad51 foci indicating that homologous recombination repair (HRR) was used to repair this damage. Notably, only GCV produced a late and persistent increase in γ-H2AX foci demonstrating the induction of unrepairable DNA damage. Both drugs induced the ATR damage response pathway, as evidenced by Chk1 activation. However, GCV resulted in greater activation of ATM, which coincided with the late induction of γ-H2AX foci, demonstrating the presence of DNA double strand breaks (DSBs). The increase in DSBs after Rad51 induction suggested that they occurred as a result of a failed attempt at HRR. These data demonstrate that the late and unrepairable DSBs observed uniquely with GCV account for its superior cytotoxicity and further suggest that inhibition of HRR will enhance cytotoxicity with HSV-TK/GCV.