Assessment of DNA damage produced by 125I-triplex-forming oligonucleotides in cells

Purpose: Triplex-forming oligodeoxyribonucleotides (TFOs) bind specifically to their target sequences by forming hydrogen bonds within the major groove of the target duplex. When labeled with Auger-electron-emitting radioisotopes, TFOs are able to damage the target gene in process named antigene radiotherapy. We compared radiotoxicity and the amount of DNA damage produced within cultured cells by two I-125-labeled TFOs. one with a single target in the genome and another with multiple targets. Materials and methods: Radiotoxicity was measured by clonogenic assay while DNA damage was assessed by the number of histone gamma-H2AX foci formed at the sites of DNA double strand breaks (DSBs). Results: The TFO with multiple nuclear targets was 1.7 fold more radiotoxic and produced on average 1.9 fold more gamma-H2AX foci per cell than the TFO with a single target. Conclusion: Since the two methods gave comparable results, measuring the number of gamma-H2AX foci per decay may be a useful procedure for the assessment of cytotoxic effects and the intranuclear localization of radionuclides when they produce DSBs.