Differential staining of DNA strand breaks in dried comet assay slides

The comet assay involves embedding cells in agarose on microscope slides. After lysis and electrophoresis, staining is usually performed with a fluorescent DNA-binding dye and observation is carried out on fresh wet slides through an epifluorescence microscope. We present here a simple alternative for preservation of the agarose comet slides and a fluorescent staining that allows fine differential analysis of DNA strand breaks under confocal microscopy. Lymphocytes were processed according to previous published methods. Slides were quickly dehydrated in a hot oven at 50C for 20 min. Once the agarose layer was dried and reduced to a thin film, slides were treated with RNase. Image analysis showed higher tail length, total area, and tail moment. Using confocal microscopic optical sectioning, a thickness of approximately 180 microm for wet slides and 12 microm for dehydrated gels was calculated. Acridine orange, used for DNA differential staining, allowed quantitation of metachromasia and orthochromasia with confocal scanning microscopy. Differences between alkaline and neutral comet assay with AO were clear-cut and, in principle, a metachromatic index can be calculated. (J Histochem Cytochem 49:921-922, 2001)