Background: Internucleosomal DNA fragmentation is one of the hallmarks of apoptosis. Because the low molecular weight DNA fragments are extracted during cell staining in aqueous solutions, apoptotic cells can be identified on DNA content frequency histograms as cells with fractional (“sub‐G1”) DNA content. The aim of the present study was to explore whether in situ DNA fragmentation during apoptosis is discontinuous or progresses incessantly and if it is discontinuous, to define the resistant to cleavage fraction of DNA that remains stainable with the fluorochrome. Materials and Methods: The model of activation‐induced apoptosis of human lymphocytes was chosen as it provides uniform cell population with identical DNA content (DI = 1.00) that undergo apoptosis. Their apoptosis was induced by multivalent mitogen phytohemagglutinin (PHA) in the absence and presence of geldanamycin (GA), the benzoquinone ansamycin antibiotic which binds to Hsp90 (Heat Shock Protein 90) and alters its function. The cells were stained with acridine orange, the metachromatic fluorochrome that differentially stains cellular DNA and RNA. Results: A sharp, discrete peak representing the subpopulation of “sub‐G1” cells with highly reproducible DI = 0.42 ± 0.02 (CV = 5.5 ± 1.2) was observed on DNA content histograms of lymphocytes whose apoptosis was induced by PHA alone. Two distinct peaks, one representing cell subpopulations with DI = 0.42 (as above) and another, with DI = 0.79 ± 0.04 (CV = 5.8 ± 0.4), respectively, were seen in apoptotic cells from cultures stimulated with PHA in the presence of GA. The frequency of cells represented by the sub‐G1 peaks varied depending on time of induction of apoptosis and GA concentration. Conclusions: Apoptosis‐induced DNA fragmentation is discontinuous; approximately 42% of DNA is relatively stable and remains within the cell. The data suggest that the stable DNA is associated with nuclear matrix while the degradable fraction represents DNA in loop domains. A transient DNA stabilization is apparent in the presence of GA as evidenced by the presence of cell subpopulations with 79% of DNA retained in the cell. The observed discontinuity of DNA fragmentation appears to reflect sequential involvement of different nucleases and may also be modulated by chromatin structure. © 2007 International Society for Analytical Cytology.

Objectives-We recently postulated that constitutive activation of Ataxia Telangiectasia, Mutated (CAA) and constitutive histone H2AX phosphorylation (CHP) seen in cells not treated with genotoxic agents are the events triggered by DNA damage caused by endogenous reactive oxygen species (ROS), the product of mitochondrial oxidative metabolism. The aim of this study was to seek further evidence in support of this postulate, namely to test whether the levels of CAA and CHP correlate with cells metabolic activity.Materials & Methods-Peripheral blood lymphocytes are non-cycling (G 0 ) cells characterized by minimal rate of oxidative metabolism. A dramatic rise in transcriptional and translational activity, an increase in number of mitochondria, and induction of DNA replication, occur during their mitogenic stimulation. This classic model of cell activation was chosen to study a possible correlation between CAA and CHP versus metabolic activity and generation of ROS.Results-The levels of CAA and CHP in lymphocytes were increased many-fold during their stimulation. This increase was paralleled by the rise in extent of endogenously generated ROS. The growth of stimulated lymphocytes in the presence glucose antimetabolite 2-deoxy-D-glucose led to markedly lowered translational activity, decreased ROS generation and correspondingly attenuated CHA and CAA.Conclusions-The present data are consistent with our postulate that CHP and CAA report DNA damage by endogenous oxidants whose level correlates with metabolic activity. Because cumulative DNA damage by ROS generated via oxidative metabolism is considered the key mechanism responsible for cell ageing and senescence the data imply that these processes are delayed in G 0 quiescent lymphocytes or stem cells as compared with proliferating cells.

Early assessment of cancer response to the treatment is of great importance in clinical oncology. Most antitumor drugs, among them DNA topoisomerase (topo) inhibitors, target nuclear DNA. The aim of the present study was to explore feasibility of the assessment of DNA damage response (DDR) as potential biomarker, eventually related to the clinical response, during treatment of human leukemias. We have measured DDR as reported by activation of ATM through its phosphorylation on Ser 1981 (ATM-S1981P) concurrent with histone H2AX phosphorylation on Ser139 (γH2AX) in leukemic blast cells from the blood of twenty patients, 16 children/adolescents and 4 adults, diagnosed with acute leukemias and treated with topo2 inhibitors doxorubicin, daunomycin, mitoxantrone or idarubicin. Phosphorylation of H2AX and ATM was detected using phospho-specific Abs and measured in individual cells by flow cytometry. The increase in the level of ATM-S1981P and γH2AX, varying in extent between the patients, was observed in blasts from the blood collected one hour after completion of the drug infusion with respect to the pre-treatment level. A modest degree of correlation was observed between the induction of ATM activation and H2AX phosphorylation in blasts of individual patients. The number of the studied patients (20) and the number of the clinically non-responding ones (2) was too low to draw a conclusion whether the assessment of DDR can be clinically prognostic. The present findings, however, demonstrate the feasibility of assessment of DDR during the treatment of leukemias with drugs targeting DNA.

Our previous experiments on leukemic cell lines demonstrated that the intensity of immunofluorescence (IF) that is proportional to the level of the phosphorylated histone H2AX (γH2AX) reveals the frequency of DNA double-strand breaks (DSBs) in chromatin induced by DNA topoisomerase I and II (topo I and II) inhibitors. The aim of the present study was to explore whether ex vivo assessment of the extent of DNA damage in leukemic blasts induced during chemotherapy is feasible. The long term goal is to develop a rapid assay to assess the effectiveness of chemotherapy early in the course of treatment of leukemia. Mononuclear cells were isolated from blood samples of 1 AML and 10 ALL patients with peripheral blasts, 4 of whom were, at diagnosis, high-risk ALL while 4 were in relapse. The median age of the patients was 17 yrs (2–25 yrs) and all were entered into an anthracycline-containing drug regimen. Samples were collected: prior to drug administration; and, one hour after completion of the drug infusion. The extent of DNA damage was assessed by analysis of histone H2AX phosphorylation occurring at sites of DSBs, which could be detected immunocytochemically and measured by multiparameter flow cytometry in conjunction with DNA content. In the only patient not responding to treatment, one of 4 relapsed ALL patients, there was no induction of DSBs and no activation of ATM. In all patients phosphorylation of ATM appeared to be strongly linked to induction of DSBs after treatment targeting topo inhibitors. In a pilot study of 3 adult patients with acute leukemia (two AML and one ALL in relapse) treated with the topo II inhibitor Mitoxantrone (MTX), ATM activation was strongly linked to induction of DSBs 1 h post drug infusion. ATM activation and induction of DSBs was several times higher in two adult responders in comparison to one non responder to therapy. A new approach to detect DNA damage in leukemic cell by flow cytometry using gating strategy which combines CD45 expression on the cell surface with side scatter (SS) is being developed and was applied in 5 cases. This approach will be useful when the proportion of blasts in peripheral blood is low. Although the number of patients is very small in this study, the results suggest that a cytometric assay that reveals the extent of DNA damage based on detection of H2AX phosphorylation during induction therapy targeting topo I and II inhibitors may provide an early prognostic marker.