FLUORESCENCE-LABELING OF NICKS IN DNA FROM LEUKEMIC BLAST CELLS AS A MEASURE OF DAMAGE FOLLOWING CYTOSINE-ARABINOSIDE - APPLICATION TO THE STUDY OF REGULATED DRUG-SENSITIVITY

Damage to DNA can be assessed using a technique for labeling nicks in DNA by incubating paraformaldehyde-fixed cells in a mixture of biotin- laheled dUTP, dATP with dNTP and DNA polymerase I. The addition of lab eled nucleotides can then be identified by fluorescence by their react ion with streptavidin. We have used this method to examine damage to t he DNA of OCI/AML-2 cells caused by cytosine arabinoside (ara-C) and t he effects of hydrocortisone and retinoic acid on this damage (regulat ed drug sensitivity). Concurrent measurements of clonogenic cells were used to allow a comparison of damage as shown by labeled nicks in DNA with loss of colony-forming capacity. Both methods gave comparable ar a-C dose-response curves, for cells incubated with the drug for 24 h. Both methods showed that exposure of OCI/AML-2 cells to hydrocortisone before ara-C greatly reduced the toxicity of the drug; and that retin oic acid given after ara-C increased both its lethal effects on colony formation and the extent of DNA damage as assessed by labeled nicks. Clonogenic assays required for colony formation are not readily adapte d to the study of development and repair of damage. The labeled nick a ssay is suitable for such kinetic studies. OCI/AML-2 cells were expose d in suspension to either hydrocortisone before ara-C or retinoic acid after ara-C. At 24 h intervals thereafter, cells were harvested, assa yed by both methods, and recultured after dilution to the original cel l concentration. In cultures exposed only to ara-C (controls), the num ber of cells with labeled nicks increased during the first 24 h and ce lls with damaged DNA could be detected for 48-72 h, depending on the a ra-C dose in spite of the dilution at each passage. OCI/AML-2 cells ex posed to hydrocortisone before drug showed fewer nick-labeled cells th an controls at the first observation and damaged cells rapidly disappe ared from the population with increasing time. For cells treated with retinoic acid after ara-C, the nick-labeled cell population was greate r than controls and remained greater throughout subsequent observation s. We propose that in the control cultures, sublethal damage either be came lethal with time and was seen as increased numbers of cells with damaged DNA, or alternatively, sublethal damage was repaired. From thi s point of view we consider that hydrocortisone promotes repair of sub lethal damage while retinoic acid inhibits repair.