Detection of DNA strand breaks in apoptotic cells by flow- and image-cytometry

Extensive DNA fragmentation that generates a multitude of DNA double-strand breaks (DSBs) is a hallmark of apoptosis. A widely used approach to identify apoptotic cells relies on labeling DSBs in situ with fluorochromes. Flow or image cytometry is then used to detect and quantify apoptotic cells labeled this way. We developed several variants of the methodology that is based on the use of exogenous terminal deoxynucleotidyl transferase (TdT) to label 3'-OH ends of the DSBs with fluorochromes, defined as the TUNEL assay. This chapter describes the variant based on DSBs labeling using 5-Bromo-2'-deoxyuridine-5'-triphosphate (BrdUTP) as a TdT substrate and the incorporated BrdU is subsequently detected immunocytochemically with anti-BrdU antibody. We also describe modifications of the protocol that allow using other than BrdUTP deoxyribonucleotides to label DSBs. Concurrent differential staining of cellular DNA and multiparameter analysis of cells by flow- or image cytometry enables one to correlate the induction of apoptosis with the cell cycle phase. Examples of the detection of apoptotic cells in cultures of human leukemic cell lines treated with TNF-α and DNA topoisomerase I inhibitor topotecan are presented. The protocol can be applied to the cells growing in vitro, treated ex vivo with cytotoxic drugs as well as to clinical samples.