Newly formed chromosome-like structures in independent mouse P388 sublines with developed in vivomdr1 gene amplification

Mouse leukemia P388 sublines that aquired the resistance to multiple drugs as a result of treatment in vivo with anthracyclines (rubomycin, ruboxyl) and/or vincristine were studied. Themdr gene amplification was found in all tested cell lines: in four of five sublines all three members of themdr gene family showed increased copy numbers, and in one cell line, developed after treatment with ruboxyl,mdr1a andmdr1b genes were amplified to the same degree, whereas themdr2 gene was not amplified at all. The levels of amplification ofmdr genes varied in different cell lines from 30-fold to 50-fold. Unusual cytological manifestations—relatively large newly formed chromosomelike structures, were revealed in four of five long-term independent sublines. Some of these structures did not contained C blocks; the others, in contrast, were enriched by C-heterochromatin. In situ hybridization showed the presence ofmdr genes in newly formed bodies. In the majority of cases, the formation of chromosomelike structures was preceded by the appearance of other, smaller size, structures: the so-called “small chromatin bodies” (minichromosomes) and/or homogeneously G-positive small ring chromosomes.