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Luminescent double-strand breaks

The impacts which low radiation doses have on human cells can now be observed "live" with PTB's microbeam. The double-strand breaks in the DNA generated by the particles and the incipient repair mechanisms of each single cell can be made visible by a fluorescence process. These responses, which are induced within seconds and minutes, can be observed in real time under the microscope. This "Live Cell Imaging" is the result of an interdisciplinary cooperation between PTB physicists and molecular biologists from Braunschweig and Düsseldorf.

At the microbeam, human cells (here: fibroblasts) were irradiated in α pattern of lines with a distance of 10 µm and approx. 1 µm between α-particle hits (symbolized by yellow dots). Along the particle track, double-strand breaks of the DNA occur. In the figure, they appear as bright foci, because fluorescence- marked repair proteins accumulate at the damage spots.

Up to now, only uncertain extrapolations exist for the risk caused to health by low radiation doses of less than 50 mSv (for example from natural radon exposure). For a better assessment of this risk, the fundamental radiobiological mechanisms must be investigated. For these studies, the microbeam is an important instrument. It allows to precisely target components of living cells with single or counted particles and, thus, to selectively adjust the radiation dose. Along the particle track, double-strand breaks and other radiation damages occur which trigger reactions and DNA repair processes in the cells within seconds and minutes.

These initial responses can now be observed "live" as the appearance of fluorescent foci, because the cells have been genetically modified by fusing, for example, the green fluorescent protein (GFP) to a selected reporter or repair protein, which then accumulates at the DNA double-strand break. Meanwhile it is even possible to modify different proteins with different fluorescences in one single cell and to observe them separately. The partners have developed these new and versatile cell systems by stable gene transfers at the Deutsche Sammlung von Mikroorganismen und Zellkulturen (DSMZ), Braunschweig and the Dusseldorf University Hospital. The effects of different radiation qualities can also be studied at the microbeam. The effect of densely ionizing radiation (e. g. natural radon exposure) can, for example, be investigated with α-particles. With high-energy protons, the effect of loosely ionizing X-radiation can be simulated.

Whereas in earlier studies, mostly late biological effects had been analyzed, the new cell systems now allow the initial radiation responses to be observed. Furthermore, it can now, for example, be investigated how therapeutic agents used in addition to the microbeam irradiation modify the processes of DNA repair.

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