MULTIPLE MECHANISMS OF N-(PHOSPHONOACETYL)-L-ASPARTATE DRUG-RESISTANCE IN SV40-INFECTED PRECRISIS HUMAN FIBROBLASTS

Normal and SV40-infected human fibroblasts were grown in the presence of the drug N-(phosphonoacetyl)-L-aspartate (PALA) and examined for ev idence of genetic instability. Both cell populations were precrisis an d showed a normal, diploid karyotype at early passage. In contrast to the normal IMR-90 cells, which showed growth arrest and did not form c olonies in PALA, the SV40-infected IMR-90 cells formed colonies at a v ery high frequency and continued to cycle in the drug. The drug-resist ant colonies senesced after continued growth in culture, indicating th at this change in ability to amplify preceded immortalization. This is the first observation of mortal human cells overcoming the drug-induc ed growth arrest. Although all previously isolated PALA-resistant colo nies demonstrated CAD gene amplification as the mechanism of the drug- resistant phenotype, these SV40-infected human cells also showed alter native mechanisms, including increases in gene copy number by aneuploi dy and formation of an isochromosome 2p.