Simultaneous Measurement of Cell Cycle Phase Position and Ionizing Radiation-induced DNA Strand Breakage in Single Human Tumour Cells Using Laser Scanning Confocal Imaging

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Abstract
Techniques for the assessment of DNA damage and repair in individual cells are pertinent to several areas of research, in particular the study of the heterogeneity of tumour cell populations in response to anticancer agents. We describe an adaptation of an in situ alkaline denaturation assay performed on individual nuclei of lysed cells, termed nucleoids, trapped within an agarose film. A novel aspect of the technique described in the application of confocal laser scanning fluorescence microscopy for the measurement of nucleoid relaxation in response to DNA damage. The volumes of spherical nucleoids and their relative DNA contents were determined by ethidium bromide staining and the analysis of confocal sections through the equatorial planes of the nucleoids. Mean nucleoid volume increased as a linear function of X-ray dose (0·5–8 Gy) administered to intact cells prior to lysis. We provide evidence of heterogeneity, in asynchronous cultures, in the DNA unfolding/unwinding characteristics of cells irrespective of cell cycle age. Bivariate plots of relative DNA content versus nucleoid volume allowed the direct assessment of cellular repair capacity with respect to cell cycle position.