Datta, K., Weinfeld, M., Neumann, R. D. and Winters, T.A. Determination and Analysis of Site-Specific ¹²⁵I Decay-Induced DNA Double-Strand Break End-Group Structures. Radiat. Res. 167, 152–166 (2007).

End groups contribute to the structural complexity of radiation-induced DNA double-strand breaks (DSBs). As such, end-group structures may affect a cell's ability to repair DSBs. The 3′-end groups of strand breaks caused by γ radiation, or oxidative processes, under oxygenated aqueous conditions have been shown to be distributed primarily between 3′-phosphoglycolate and 3′-phosphate, with 5′-phosphate ends in both cases. In this study, end groups of the high-LET-like DSBs caused by ¹²⁵I decay were investigated. Site-specific DNA double-strand breaks were produced in plasmid pTC27 in the presence or absence of 2 M DMSO by ¹²⁵I-labeled triplex-forming oligonucleotide targeting. End-group structure was assessed enzymatically as a function of the DSB end to serve as a substrate for ligation and various forms of end labeling. Using this approach, we have demonstrated 3′-hydroxyl (3′-OH) and 3′-phosphate (3′-P) end groups and 5′-ends (≥42%) terminated by phosphate. A ³²P postlabeling assay failed to detect 3′-phosphoglycolate in a restriction fragment terminated by the ¹²⁵I-induced DNA double-strand break, and this is likely due to restricted oxygen diffusion during irradiation as a frozen aqueous solution. Even so, end-group structure and relative distribution varied as a function of the free radical scavenging capacity of the irradiation buffer.