KARYOTYPIC CHANGE FROM HETEROPLOIDY TO NEAR DIPLOIDY ASSOCIATED WITH DEVELOPMENT OF CISPLATIN RESISTANCE IN A RAT OVARIAN TUMOR-CELL LINE

In a rat ovarian tumour cell line a 33-fold resistance to cisplatin (O-342/DDP) was developed in vitro by continuous exposure of the parental cell line (O-342) to stepwise increase cisplatin concentration in the culture medium. Both cell lines had a similar growth rate in vitro. Development of resistance was accompanied by a change of the karyotype from heteroploidy in chemosensitive O-342 cells to near diploidy in resistant O-342/DDP cells as shown by chromosome number distribution. This finding was confirmed by measuring cellular DNA content using flow-cytometry analysis. Flow karyotyping showed significant differences in chromosomal DNA contents between both cell lines. Our results suggest that the parent line O-342 consists of at least two subpopulations, a cisplatin-sensitive and a cisplatin-resistant one, corresponding to hyperploidy and near diploidy, respectively. Continuous cisplatin exposure of O-342 cells selectively killed the sensitive fraction, resulting in the karyotypic change observed.