Early dihydrofolate reductase gene amplification events in CHO cells usually occur on the same chromosome arm as the original locus.

We used fluorescence in situ hybridization to examine the products of early DNA sequence amplification events in CHO cells. Nine independent populations of cells were selected for resistance to 0.4 microM methotrexate (MTX), and mitotic chromosome spreads were hybridized to a mixture of cloned cosmids representing approximately 273 kb of contiguous DNA sequence from the dihydrofolate reductase (DHFR) locus. Of the nine populations, eight contain cells that have amplified the DHFR domain. Cells in the remaining population displayed only the two single-copy loci on chromosomes 2 and Z2. Of the eight amplificants, one carries amplified DHFR genes on chromosome 2, six on chromosome Z2, and one on an unidentified chromosome. Some cultures carry additional amplified genes on other chromosomes, probably resulting from bridge/breakage/fusion cycles or translocations. In six of the eight amplificants, both single-copy parental loci are detected at their original positions, and amplicon clusters are situated at least 50 megabases (Mb) away on the same chromosome arm, often at the termini. Amplification occurred at or close to the original site of the DHFR gene in only one population. Our results are not consistent with models in which initial amplification events occur by over-replication of the parental locus followed by recombination in loco. Because amplified DHFR sequences occur most often on the same chromosome arm as the parental DHFR gene but at a considerable distance from it, our results are most compatible with either sister chromatid exchange between widely separated sites or with a form of conservative intrachromosomal duplication analogous to transposition in bacteria.