Abstract
Simple SummarySuper-resolution single-molecule localization microscopy (SMLM) allows the detection of cluster formation of phosphorylation sites and recruited repair proteins (e.g., MRE11) around DNA double-strand breaks induced by ionizing radiation. The spatial arrangement of these targets can be described by distance frequency distributions, which can be used as a measure for cluster size and the ratio of cluster numbers per specimen analyzed. For the analysis of the shape of clusters and their topology and similarity, persistent homology was applied and the features were compared along the DNA repair process for two different cell lines (MCF-7 breast cancer cells and CCD-1059SK human skin fibroblasts). The similarity values for components and holes were averaged for each repair time point post-irradiation so that it was possible to compare not only clusters with each other but also cell specimens of different repair periods with each other. It was shown that after X-ray irradiation, these similarity values were high for γH2AX clusters in the early repair phase, while MRE11 clusters revealed an increased similarity at later time points. DNA double-strand breaks (DSBs), known as the most severe damage in chromatin, were induced in breast cancer cells and normal skin fibroblasts by 2 Gy ionizing photon radiation. In response to DSB induction, phosphorylation of the histone variant H2AX to γH2AX was observed in the form of foci visualized by specific antibodies. By means of super-resolution single-molecule localization microscopy (SMLM), it has been recently shown in a first article about these data that these foci can be separated into clusters of about the same size (diameter ~400 nm). The number of clusters increased with the dose applied and decreased with the repair time. It has also been shown that during the repair period, antibody-labeled MRE11 clusters of about half of the γH2AX cluster diameter were formed inside several γH2AX clusters. MRE11 is part of the MRE11–RAD50–NBS1 (MRN) complex, which is known as a DNA strand resection and broken-end bridging component in homologous recombination repair (HRR) and alternative non-homologous end joining (a-NHEJ). This article is a follow-up of the former ones applying novel procedures of mathematics (topology) and similarity measurements on the data set: to obtain a measure for cluster shape and shape similarities, topological quantifications employing persistent homology were calculated and compared. In addition, based on our findings that γH2AX clusters associated with heterochromatin show a high degree of similarity independently of dose and repair time, these earlier published topological analyses and similarity calculations comparing repair foci within individual cells were extended by topological data averaging (2nd-generation heatmaps) over all cells analyzed at a given repair time point; thereby, the two dimensions (0 and 1) expressed by components and holes were studied separately. Finally, these mean value heatmaps were averaged, in addition. For γH2AX clusters, in both normal fibroblast and MCF-7 cancer cell lines, an increased similarity was found at early time points (up to 60 min) after irradiation for both components and holes of clusters. In contrast, for MRE11, the peak in similarity was found at later time points (2 h up to 48 h) after irradiation. In general, the normal fibroblasts showed quicker phosphorylation of H2AX and recruitment of MRE11 to γH2AX clusters compared to breast cancer cells and a shorter time interval of increased similarity for γH2AX clusters. γH2AX foci and randomly distributed MRE11 molecules naturally occurring in non-irradiated control cells did not show any significant topological similarity.
Highlights
DNA damage induced by ionizing radiation and the follow-up repair processes of damage sites depend on the radiation type, the applied dose, the dose rate, and the linear energy transfer (LET) and on the cell radio-sensitivity, its DNA repair capacity, etc
Cells of the human breast cancer cell line MCF-7 and the human skin fibroblast cell line CCD-1059SK were irradiated with 6 MeV photons at a dose of 2 Gy
Most of the MRE11 clusters were found inside γH2AX clusters (Figure 3), which is in agreement with the literature [21,52]
Summary
DNA damage induced by ionizing radiation and the follow-up repair processes of damage sites depend on the radiation type, the applied dose, the dose rate, and the linear energy transfer (LET) and on the cell radio-sensitivity, its DNA repair capacity, etc. (for review, see [1,2]). DNA damage induced by ionizing radiation and the follow-up repair processes of damage sites depend on the radiation type, the applied dose, the dose rate, and the linear energy transfer (LET) and on the cell radio-sensitivity, its DNA repair capacity, etc. In contrast to high-LET particles (e.g., α-particles, heavy ions, etc.) passing and damaging the chromatin in linear tracks [3,4], low-LET photon irradiation induces DNA lesions in a coarse-grain pattern, which locally leads to single-strand breaks (SSBs) and double-strand breaks (DSBs) randomly distributed in the cell nucleus [2,5,6,7]. For low-LET irradiation, the number of γH2AX foci is proportional to the number of DSBs [11], which can be assumed to follow a linear or linear-quadratic dependency to the dose applied [12]. Microscopic counting of labeled γH2AX foci has become a procedure well established in biological dosimetry nowadays [13] with a high degree of standardization and automation [14]
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