Induced sister chromatid exchange frequency is not increased in homogeneously staining regions that contain amplified genes.

Gene amplification is a process by which cells become resistant to selective agents by increasing gene copy number and overproducing specific enzymes. The molecular mechanism by which gene amplification occurs is unknown, but unequal sister chromatid exchange (SCE) has been suggested as one possibility. Unequal SCE results in one chromatid containing an extra copy of a selected gene that is deleted in the sister chromatid. Two predictions of the unequal SCE model are that agents that increase SCE frequency would increase gene amplification, and that SCE would be more prevalent in arrays of amplified units. We examined SCE frequency in the Chinese hamster ovary cell line MK42, which contains amplified dihydrofolate reductase genes that are stably integrated as a homogeneously staining region in chromosome #2. Under treatment conditions known to increase amplification of the dihydrofolate reductase gene, a number of agents did increase SCE frequency in MK42 cells. The frequency of SCE in the amplified region of chromosome #2, however, was no different from that expected if SCE were induced randomly as a function of chromosome length. These data, therefore, do not support the prediction of unequal SCE as a model for gene amplification. Our experiments, however, do not test the idea that unequal SCE at normal frequencies may lead to gene amplification, nor do they rule out the possibility of transient periods early in amplification, during which the homogeneously staining region may be a "hot spot" for SCE.