Relationship Between Chromatin Structure and Sensitivity to Molecularly Targeted Auger Electron Radiation Therapy

Abstract

The open structure of euchromatin renders it susceptible to DNA damage by ionizing radiation (IR) compared with compact heterochromatin. The effect of chromatin configuration on the efficacy of Auger electron radiotherapy was investigated. Chromatin structure was altered in MDA-MB-468 and 231-H2N human breast cancer cells by suberoylanilide hydroxamic acid (SAHA), 5-aza-2-deoxycytidine, or hypertonic treatment. The extent and duration of chromatin structural changes were evaluated using the micrococcal nuclease assay. DNA damage (γH2AX assay) and clonogenic survival were evaluated after exposure to (111)In-DTPA-hEGF, an Auger electron-emitting radiopharmaceutical, or IR. The intracellular distribution of (111)In-DTPA-hEGF after chromatin modification was investigated in cell fractionation experiments. Chromatin remained condensed for up to 20 minutes after NaCl and in a relaxed state 24 hours after SAHA treatment. The number of γH2AX foci per cell was greater in MDA-MB-468 and 231-H2N cells after IR (0.5 Gy) plus SAHA (1 μM) compared with IR alone (16 ± 0.6 and 14 ± 0.3 vs. 12 ± 0.4 and 11 ± 0.2, respectively). More γH2AX foci were observed in MDA-MB-468 and 231-H2N cells exposed to (111)In-DTPA-hEGF (6 MBq/μg) plus SAHA vs. (111)In-DTPA-hEGF alone (11 ± 0.3 and 12 ± 0.7 vs. 9 ± 0.4 and 7 ± 0.3, respectively). 5-aza-2-deoxycytidine enhanced the DNA damage caused by IR and (111)In-DTPA-hEGF. Clonogenic survival was reduced in MDA-MB-468 and 231-H2N cells after IR (6 Gy) plus SAHA (1 μM) vs. IR alone (0.6% ± 0.01 and 0.3% ± 0.2 vs. 5.8% ± 0.2 and 2% ± 0.1, respectively) and after (111)In-DTPA-hEGF plus SAHA compared to (111)In-DTPA-hEGF alone (21% ± 0.4% and 19% ± 4.6 vs. 33% ± 2.3 and 32% ± 3.7). SAHA did not affect (111)In-DTPA-hEGF nuclear localization. Hypertonic treatment resulted in fewer γH2AX foci per cell after IR and (111)In-DTPA-hEGF compared to controls but did not significantly alter clonogenic survival. Chromatin structure affects DNA damage and cell survival after exposure to Auger electron radiation.