Common acute lymphoblastic leukemia characterized by four different DNA stemlines with heterogeneity in RNA content, antigen expression and sensitivity to chemotherapy.

Four subpopulations of cells with different DNA content were present in the bone marrow of a pediatric patient with acute lymphoblastic leukemia. Flow cytometry of DNA/RNA and DNA/surface antigen expression enabled the identification and characterization of diploid, hypertriploid, tetraploid and hypertetraploid leukemic cells. This was not appreciated by cytogenetic analysis, which identified only some tetraploid cells (3/20 metaphases). Common acute lymphoblastic leukemia antigen was expressed in all aneuploid and also on 17% of diploid cells. Quantitative CALLA expression was unrelated to ploidy and cell size. Cellular RNA content paralleled ploidy, i.e. the more aneuploid cells had increased RNA content and there was pronounced RNA heterogeneity within each DNA stemline. The different subclones showed almost identical stages of early B-cell differentiation. The early pre B-cell antigens BL1 and BL2 were expressed in approx. 80 and 60%, respectively, of aneuploid leukemic cells. Cytoplasmic immunoglobulin heavy chain was also present. Clonal excess of lambda light chain immunoglobulin on the cell surface was present on less than 10% of hypertriploid, tetraploid and hypertetraploid leukemic cells indicating differentiation of a subpopulation of aneuploid leukemic cells to mature B cells. This was not seen for any diploid cells. The heterogeneity of the different subpopulations was also evident in the differences in response to chemotherapy: the hypertriploid and hypertetraploid subpopulations were most sensitive to initial induction chemotherapy.