Spontaneous amplification of interstitial telomeric bands in Chinese hamster ovary cells

Chinese hamster cell lines have been widely used to explore the mechanism of gene amplification under stress conditions. Investigations of gene amplification in Chinese hamster cells under nonstress conditions, however, have been limited due to technical reasons. We initiated studies on the amplification mechanism by examining the frequency of interstitial telomeric bands (ITBs) in continuously growing cultures by fluorescence in situ hybridization (FISH), using a (TTAGGG)n probe. We found that one of the chromosomes uniquely exhibit amplification of ITBs in Chinese hamster ovary (CHO) cells but not in drug-resistant mutant CHO cells or in Chinese hamster V79 cells. Amplification of ITBs could not be induced by DNA damaging agents or DNA synthesis inhibitors. To investigate the early events that mediate ITB amplification, we developed clones and determined that the frequency of acquisition of additional ITBs on the marker chromosome was in the range of 10(-5) to 10(-3) per cell doubling. Using DNA strand-specific FISH, we found that one of the initial events of amplification is isochromosome formation. Our observations support the involvement of sister chromatid fusion in the initiation of spontaneous ITB amplification.