Heterogeneity in DNA damage using the comet assay.

The single-cell gel electrophoresis or "comet" assay was developed many years ago to analyze DNA damage in individual cells. It is a powerful and versatile technique that relies on microscopic visualization or imaging of DNA after single cells are embedded in agarose, lysed, and electrophoresed. In addition, the basic methodology has been extended to permit the detection of a variety of classes of DNA damage with good sensitivity in virtually any single-cell type. A unique but understudied property of the comet assay is its ability to detect and quantify cellular heterogeneity in response to DNA-damaging agents. This review outlines the considerations in producing and analyzing comet data when heterogeneity in induction of or cellular response to DNA damage is the major consideration. Examples are presented to emphasize the heterogeneity of tumor response to ionizing radiation and cytotoxic drugs.