Synchronization of human leukemic cells: Relevance for high-resolution chromosome banding

The cell-cycle kinetics of synchronized K562 human leukemic cells and bone marrow cells from adults with acute leukemia were studied in order to develop more reliable methods for producing increased numbers of mitoses, particularly those with elongated chromosomes suitable for high-resolution banding. Parameters examined included DNA content, mitotic index (MI), and chromosome preparations. K562 cells synchronized with methotrexate (MTX), thymidine (Tdr), or hydroxyurea (HU) showed two-fold increases in peak MI. Optimal harvesting times after release from block were approximately 10.5, 12.5, and 14.5 h for MTX, HU, and Tdr, respectively. MTX was selected for studies with cells from patients. Cells from 7 of the 10 patients studied showed 4.4-fold increases in peak MI. The optimal harvesting time was 9.5 to 11.5 h after release from block, considerably later than the 6 h time previously assumed in studies using stimulated lymphocytes. Cells from the three remaining patients showed no increase in MI after synchronization: and the lack of response may have been related to the high proportion of cells in G₀+G₁ prior to MTX exposure. For both the K562 cell line and most patient specimens, the combination of synchronization with appropriate release times and short Colcemid exposure (10 min) resulted in substantially improved chromosome preparations.