Regularities of karyotypic evolution during stepwise amplification of genes determining drug resistance

Analysis of chromosomal alterations during stepwise development of mdr1, dhfr, or CAD gene amplifications in a large number of independently selected Djungarian hamster DM-15 and murine P388 sublines revealed typical patterns of karyotypic evolution, specific for multiplication of each of these genes in each cell type. Some principal similarities of karyotypic evolution were noted in at least two different systems. They include: (i) appearance at the first selection step of a new chromosomal arm bearing the resident gene copy followed at the next selection steps by the formation in these specific chromosomal arms of amplified DNA tandem arrays; (ii) translocations of amplified DNA from its initial site to other, also non-random, chromosomal sites; and (iii) emergence in the cell variants with high degrees of gene amplification of multiple extra-chromosomal elements. The most prominent distinctions among the systems were as follows: (i) different structures, evidently containing amplified DNAs, appeared at the initial steps of amplification of different genes--additional heterogeneously staining regions in specific chromosomal segments in the case of amplification of dhfr or CAD genes in DM-15 cells, and mini-chromosomes in the case of mdr1 gene amplification in both DM-15 and P380 cells; (ii) distinct patterns of location of the amplified mdr1 gene copies are characteristic of Djungarian hamster DM-15 and murine P388 cell derivatives after subsequent steps of selection--at the site of resident gene localization or in some other, also non-random, chromosomal sites in DM-15 sublines, and predominantly extra-chromosomal in P388 sublines. We propose that different mechanisms are responsible for the initial steps of amplification of dhfr and CAD genes on the one hand and the mdr1 gene on the other: non-equal sister-chromatid exchanges and autonomous replication of the extra-chromosomal elements. It seems, however, that both mechanisms may be involved in further rounds of amplification of each of these three genes.