Cytogenetical changes during early stages of development of methotrexate resistance in HeLa cells

HeLa cells cultured in ever-increasing doses of methotrexate (MTX) acquired resistance to this drug. Chromosomal changes occurring at early stages during the development of resistance to various doses (0.1, 0.3, 0.5, 1.0, 2.1, 3.0, 10.0 and 100.0 μg/ml) of MTX included: (a) an increase in the percentage of hypertetraploid cells (>92 chromosomes) at all doses, and most profoundly at 1.0, 2.1 and 3.0 μg/ml; (b) an increase in the percentage of cells with structural abnormalities; (c) a remarkable increase in the percentage of hypertetraploid cells containing dicentrics, particularly at MTX dose levels 0.5–3.0 μg/ml and (d) emergence of increasing numbers of double minutes per cell with increasing MTX doses. At dose levels 2.1 and 3.0 μg/ml the modal chromosome number increased to 82, while at 0.1–1.0 μg/ml it was similar to the mode of sensitive HeLa (64, 66) and at 10.0 and 100.0 μg/ml dropped to 62, 63. The results obtained suggest that polyploidization and formation of dicentrics are associated with the earlier stages of development of resistance to methotrexate.