Effect of distance between decaying 125I and DNA on Auger-electron induced double-strand break yield

Abstract

Purpose: To determine the possible effects of ¹²⁵I-to-DNA distance on the magnitude and mechanism of Auger-electron induced-double-strand break (DSB) production.

Materials and methods: We have synthesized a series of ¹²⁵I-labeled Hoechst (H) derivatives (¹²⁵IE–H, ¹²⁵IB–H, ¹²⁵I-C₈–H and ¹²⁵I-C₁₂–H). While all four molecules share a common DNA minor groove binding bis-benzimidazole motif, they are designed to position ¹²⁵I at varying distances from the DNA helix. Each Hoechst derivative was incubated at 4°C in phosphate buffered saline (PBS) together with supercoiled (SC) ³H-pUC19 plasmid DNA (ratio 3:1) ± the ?OH scavenger dimethyl sulfoxide (DMSO) (0.2 M). Aliquots were analyzed on agarose gels over time and DSB yields per decay of ¹²⁵I atom were determined. Docking of the iodinated compounds on a DNA molecule was carried out to determine the distance between the iodine atom and the central axis of DNA.

Results: In the absence of DMSO, the results show that the DSB yields decrease monotonically as the ¹²⁵I atom is distanced – by 10.5 Å to 13.9 Å – from the DNA helix (¹²⁵IEH: 0.52 ± 0.01; ¹²⁵IB–H: 0.24 ± 0.03; ¹²⁵I-C₈–H: 0.18 ± 0.02; ¹²⁵I-C₁₂–H: 0.10 ± 0.00). In the presence of DMSO, DSB yields for ¹²⁵IEH (0.49 ± 0.02) and ¹²⁵IB-H (0.26 ± 0.04) remain largely unchanged indicating that DSB are entirely produced by direct effects. Strikingly, ¹²⁵I-C₈–H or ¹²⁵I-C₁₂–H, did not produce detectable DSB in the presence of DMSO under similar conditions suggesting when ¹²⁵I atom is positioned >?12 Å from the DNA, DSB are entirely produced by indirect effects.

Conclusion: These results suggest that at a critical distance between the ¹²⁵I atom and the DNA helix, DSB production switches from an ‘all’ direct to an ‘all’ indirect mechanism, the latter situation being comparable to the decay of ¹²⁵I free in solution. These experimental findings were correlated with theoretical expectations based on microdosimetry.