Plasmids bearing a mammalian replication initiation region (IR) and a nuclear matrix attachment region (MAR) are spontaneously amplified in transfected mammalian cells, and such amplification generates chromosomal homogeneously staining regions (HSRs) or extrachromosomal double minutes (DMs). This method provides a novel, efficient, and rapid way to establish cells that stably produce high levels of recombinant proteins. However, because IR/MAR plasmids are amplified as repeats, they are frequently targeted by repeat-induced gene silencing (RIGS), which silences a variety of repeated sequences in transgenes and the genome. To address this problem, we developed a novel screening system using the IR/MAR plasmid to isolate human genome sequences that alleviate RIGS. The screen identified a 3,271 bp sequence (B-3-31) that elevated transgene expression without affecting the amplification process. Neither non-B structure (i.e., the inverted repeats or bending) nor known epigenetic modifier elements such as MARs, insulators, UCOEs, or STARs could explain the anti-silencing activity of B-3-31. Instead, the activity was distributed throughout the entire B-3-31 sequence, which was extremely A/T-rich and CpG-poor. Because B-3-31 effectively and reproducibly alleviated RIGS of repeated genes, it could be used to increase recombinant protein production.
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