The study of DNA damage holds a wide interest within both basic and
applied fields of research. Elucidating the mechanisms involved in the generation
of DNA damage, and the consequences of this damage, will have an enormous
impact on multiple fields of scientific research and will ultimately lead
to a better understanding of human disease. One of the most widely used methods
for detecting DNA damage in situ is TdT-mediated dUTP-biotin nick end
labeling (TUNEL) staining (1). TUNEL staining was initially described as a
method for staining cells that have undergone programmed cell death, or
apoptosis, and exhibit the biochemical hallmark of apoptosis—internucleosomal
DNA fragmentation (2–6). TUNEL staining relies on the ability of the
enzyme terminal deoxynucleotidyl transferase to incorporate labeled dUTP into
free 3'-hydroxyl termini generated by the fragmentation of genomic DNA into
low molecular weight double-stranded DNA and high molecular weight single
stranded DNA (1). While TUNEL staining has nearly universally been adopted
as the method of choice for detecting apoptosis in situ, it should be recognized
that TUNEL staining is not limited to the detection of apoptotic cells. TUNEL
staining may also be used to detect DNA damage associated with non-apoptotic
events such as necrotic cell death induced by exposure to toxic compounds and
other insults (7), and TUNEL staining has also been reported to stain cells
undergoing active DNA repair (8). Therefore TUNEL staining may be considered
generally as a method for the detection of DNA damage (DNA fragmentation),
and under the appropriate circumstances, more specifically as a method
for identifying apoptotic cells