Abstract 2666: Relationship between chromatin structure and sensitivity to molecularly-targeted radiation therapy

Abstract

Abstract The open structure of euchromatin renders it more susceptible to DNA damage by ionising radiation (IR) compared to compact heterochromatin. The goal was to investigate whether chromatin accessibility also affects the toxicity of Auger electron-emitting radiopharmaceuticals. SAHA is a histone deacetylase inhibitor that results in chromatin remodelling to an open structure and is used to treat cutaneous T-cell lymphoma. Epidermal growth factor receptor (EGFR)-overexpressing MDA-MB-468 and MDA-MB-231/H2N breast cancer cells were treated with SAHA and irradiated (137Cs source, 1Gy/min, 0.5Gy) or incubated with 111Indium-DTPA-hEGF (111In-EGF) for one hour. Cells were stained for γH2AX, a marker of DNA double-strand breaks, 30min after γ-irradiation or after exposure to 111In-EGF. Nuclear uptake of 111In following SAHA was evaluated in cellular fractionation experiments. Total cellular H2AX protein expression after SAHA treatment was analysed by western blot. Cells treated with SAHA and consequently irradiated (up to 6Gy) or incubated with 111In-EGF (24h), were plated and incubated for 7-10 days in clonogenic survival experiments. Cells were exposed to calyculin A (50nM) or NaCl (0.5M), which result in chromatin condensation, irradiated (1Gy) or incubated with 111In-EGF (1h) and stained for γH2AX as before. In both MDA-MB-468 and MDA-MB-231/H2N cells, SAHA resulted in an increase in the number of γH2AX foci (16 and 14 per cell respectively) compared to IR alone (12 and 11 foci/cell respectively). SAHA plus 111In-EGF also increased DNA damage (22 and 12 foci/cell respectively) compared to 111In-EGF alone (15 and 7 foci/cell respectively). SAHA did not alter the extent of nuclear uptake of 111In in either cell line (0.4-0.6% of total administered), nor did it alter total cellular H2AX protein expression. Preliminary clonogenic data suggest that SAHA decreases cell survival after either irradiation or incubation with 111In-EGF in both cell lines. MDA-MB-468 cells treated with calyculin A accumulated less damage after γ-irradiation (11 foci/cell) compared to IR alone (20 foci/cell). Treatment of both cell lines with NaCl before irradiation (11 and 9 foci/cell) also protected against DNA damage compared to IR alone (19 and 16 foci/cell). Preliminary data suggests that cells treated with NaCl prior to incubation with 111In-EGF also acquired less DNA damage. Chromatin density modification alters radiosensitivity and DNA damage after γ-irradiation. The results of this study indicate that this is also the case for Auger electron-emitting molecularly-targeted radiation therapeutic agents. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2666. doi:10.1158/1538-7445.AM2011-2666