Vincristine-resistant (VCR) Chinese hamster ovary (CHO) cells have been established by stepwise selection in increasing concentrations of vincristine. These cells exhibit multidrug cross-resistance to a number of drugs that have no structural or functional similarities. Cytogenetic analyses of resistant cells revealed the presence of double minutes and expanded chromosomal segments, thus implicating gene amplification as a possible mechanism of resistance. An amplified DNA segment isolated from other multidrug cross-resistant CHO cell lines (Roninson, I. B., H. T. Abelson, D. E. Housman, N. Howell, and A. Varshavsky, 1984, Nature (Lond.), 309:626-628) is also amplified in our VCR lines. This DNA segment was used as a probe to screen a cosmid library of VCR genomic DNA, and overlapping clones were retrieved. All of these segments, totaling approximately 45 kilobases (kb), were amplified in VCR cells. Using in situ hybridization, we localized the amplification domain to the long arm of CHO chromosome 1 or Z1. Northern hybridization analysis revealed that a 4.3-kb mRNA was encoded by this amplified DNA domain and was over-produced in the VCR cells. Suggestions for the involvement of these amplified DNA segments in the acquisition of multidrug cross-resistance in animal cells are also presented.

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