ABSTRACT
Methylation of CpGs is generally thought to repress transcription without significant influence from the sequence surrounding the methylated dinucleotides. Using the mouse Igf2/H19 imprinting control region (ICR), Igf2r differentially methylated region 2 (DMR2) and bacterial sequences, we addressed how methylation-dependent repression (MDR) from a distance varies with CpG number, density and surrounding sequence. In stably transfected F9 cells, the methylated ICR repressed expression from a CpG-free reporter plasmid more than 1000-fold compared with its unmethylated control. A segment of pBluescript, with a CpG number equal to the ICR's but with a higher density, repressed expression only 70-fold when methylated. A bacteriophage lambda fragment and the Igf2r DMR2 showed minimal MDR activity, despite having CpG numbers and densities similar to or greater than the ICR. By rearranging or deleting CpGs, we identified CpGs associated with three CTCF sites in the ICR that are necessary and sufficient for sequence-specific MDR. In contrast to F9 cells, the methylated ICR and pBS fragments exhibited only 3-fold reporter repression in Hela cells and none in Cos7. Our results show that the strength of MDR from a distance can vary a 1000-fold between different cell types and depends on the sequence surrounding the methylated CpGs, but does not necessarily increase with CpG number or density.
