O-GlcNAcylation Affects the Pathway Choice of DNA Double-Strand Break Repair

Exposing cells to DNA damaging agents, such as ionizing radiation (IR) or cytotoxic chemicals, can cause DNA double-strand breaks (DSBs), which are crucial to repair to maintain genetic integrity. O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) is a post-translational modification (PTM), which has been reported to be involved in the DNA damage response (DDR) and chromatin remodeling. Here, we investigated the impact of O-GlcNAcylation on the DDR, DSB repair and chromatin status in more detail. We also applied charged particle irradiation to analyze differences of O-GlcNAcylation and its impact on DSB repair in respect of spatial dose deposition and radiation quality. Various techniques were used, such as the γH2AX foci assay, live cell microscopy and Fluorescence Lifetime Microscopy (FLIM) to detect DSB rejoining, protein accumulation and chromatin states after treating the cells with O-GlcNAc transferase (OGT) or O-GlcNAcase (OGA) inhibitors. We confirmed that O-GlcNAcylation of MDC1 is increased upon irradiation and identified additional repair factors related to Homologous Recombination (HR), CtIP and BRCA1, which were increasingly O-GlcNAcyated upon irradiation. This is consistent with our findings that the function of HR is affected by OGT inhibition. Besides, we found that OGT and OGA activity modulate chromatin compaction states, providing a potential additional level of DNA-repair regulation.

Impact of dimethyl sulfoxide on irradiation-related DNA double-strand-break induction, -repair and cell survival

Dimethyl sulfoxide (DMSO) is an effective radical scavenger and, when added to cells, reduces the initial number of radiation-induced DNA double-strand breaks (DSB). The aim of this study was to investigate modification by DMSO of both DSB induction and DSB repair by means of pulsed-field gel electrophoresis (PFGE) as well as gamma-H2AX immunofluorescence staining. WiDr cells (human colon carcinoma provided by DKFZ) were incubated with 2% DMSO for 2 h (or mock-treated) prior to irradiation with varying X-ray doses and subsequent incubation for repair. Sample processing for PFGE analysis or counting of γ-H2AX foci was performed according to standard protocols. Effects on apoptosis induction and cell survival were investigated additionally by standard protocols. DMSO reduced DSB yield after 20-80 Gy measured by PFGE. A qualitatively similar result was found after low-dose irradiation (1 Gy) using γ-H2AX immunofluorescence staining. During incubation for repair, both DNA fragment rejoining (PFGE) as well as γ-H2AX foci removal occurred at a reduced rate when cells had been pre-treated with DMSO. But this effect was clearly more pronounced for the PFGE-analyzed double-strand breakage, particularly at early repair times. WiDr cells treated with DMSO (2%) showed a significantly increased clonogenic survival after irradiation doses above 8 Gy. Apoptosis rates were not changed by DMSO. The radio-protective effect of DMSO, well known from other PFGE studies, could be confirmed for the formation of γ-H2AX foci. DSB generated in the presence of DMSO were less rapidly repaired. DMSO showed radio-protective effects on clonogenic survival but not on apoptosis.