Gene amplification plays a crucial role in the development of many human malignancies. Amplified genes are frequently localized on double minutes (DMs). We show here that plasmids bearing both a mammalian replication origin and a nuclear matrix attachment region were able to integrate into DMs if transfected to cells having DMs (COLO 320DM). Furthermore, these plasmids triggered the events leading to the de novo formation of the structure similar to DMs if transfected to the cells without DMs (COLO 320HSR or HeLa). Autonomous replication of these plasmids was suggested to be a prerequisite for these events to occur, because the presence of the origin sequences in the plasmids was required. The presence of matrix attachment region in the plasmids is also required for these events to occur, suggesting that matrix attachment plays an indispensable role in extrachromosomal replication. This model system will allow us to investigate the mechanism of gene amplification as well as to analyze the autonomous replication of the plasmid with mammalian replication origins.
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