Photons bring light into DNA repair: The comet assay and laser microbeams for studying photogenotoxicity of drugs and ageing

This contribution reviews recent applications of micro-manipulation; by UV photons in DNA repair and ageing research as well as in the evaluation of the phototoxicity of drugs. In some case micromanipulation is combined with the comet assay a technique, which allows a direct view on DNA damages. It is shown that in humans, the sensitivity of DNA to UV induced damage and its subseqent repair is surprisingly stable up to high age and that drugs which are usally non-toxic-induce DNA damage when irradiated in parallel by UV irradiation. Using the immune fluorescent comet assay, IFCA a variant of the comet assay direct comparison of the effects of ionizing Cs-137 radiation with those of localized UV radiation is possible. When a laser microbeam is used to damage DNA in a cell nucleus with high temporal and spatial resolution, it can be observed directly how repair moleculate (are recruited) at the Cells with different degrees of DNA damage after comet assay in its alkaline version, which detects single plus double strand breaks. site of damage. Comparison of the recruitment speed allows establishing an order of DNA repair events.