Cell cycle perturbations in acute myeloid leukemia samples following in vitro exposures to therapeutic agents

Cell cycle checkpoints establish the timing and strength of arrest, repair and apoptosis responses to damaging treatments. We designed flow cytometric assays to measure cell cycle arrest and apoptosis in acute myeloid leukemia (AML) samples treated in vitro with relevant therapeutic agents so as to functionally characterize checkpoints in these samples and to ask if checkpoint abnormalities are common in AML and contribute to therapeutic failures. We show here that cell cycle responses to daunomycin (DNR), cytosine arabinoside (ARA-C) and gamma irradiation (RAD) were reproducibly treatment agent- and dose-dependent and distinct in different myeloid cell lines. DNR treatments differentially induced cell accumulations in the gap 2 and mitosis (G2/M) phases of the cell cycle and/or in the gap 1 (G1) phase, as did RAD, while ARA-C induced accumulations in the DNA synthesis (S) phase or in the G1 phase. Flow cytometric gates were devised to exclude lymphocytes and mature neutrophils in analyses of primary myeloid cell samples. Cell subsets in bone marrow samples from normal donors were thus enriched for myeloid constituents and used as normal myeloid cell controls. Proliferating cell nuclear antigen (PCNA) immunostaining was used to further identify actively dividing cell subpopulations in primary cell samples. AML samples were similarly analyzed and the majority showed lower DNA synthesis cell cycle phase (S) fractions and lower PCNA-positive fractions than normal myeloid cells, suggesting that AMLs are generally less proliferative in these culture conditions. Exceptional AML samples with high S phase fractions had cytogenetic abnormalities associated with poor prognosis. Most AML samples mounted weak cell cycle responses relative to normal myeloid cells, while a minority showed robust, agent-specific cell cycle arrests. This non-responsiveness was not simply associated with lower cycling indices-neither the response patterns nor the degrees of response were correlated with untreated S phase fractions or with PCNA-positive fractions. Cell cycle responses were also not associated with clinical parameters including patient age, FAB class, or white blood cell count, nor with immunophenotypic features including CD34 status, nor with specific cytogenetic markers. This suggests that functional cell cycle response assays could provide unique diagnostic information in AML. These assays might also have prognostic value as ARA-C induced G1 arrests and DNR-specific G2/M arrests tended to be associated with failure to achieve clinical remission. In addition, G1 arrests after ARA-C and G2/M accumulations after DNR treatments tended to be more robust in samples that had previously been shown to be more highly immunopositive for bcl-2 expression. This data suggests that the association of bcl-2 expression with particular cell cycle responses to therapeutic agents may contribute to the association of bcl-2 with poor clinical responses in AML. These data provide the basis for further laboratory studies aimed at examining specific cell cycle arrests as mechanisms of therapeutic resistance and prospective studies aimed at rigorously assessing the prognostic utility of in vitro assays of checkpoint function.