Department of Animal Development and Genetics; Uppsala University
Regulation of the parent of origin-specific silencing of the neighboring Igf2 and H19 loci involves a short stretch of differentially methylated sequence that lies upstream of the H19 locus (1-3). Here we show that this imprinting control region display unique allele-specific chromatin conformations: whereas both parental alleles display phased nucleosomes, only the maternally-derived allele harbors multiple nuclease hypersensitive sites which map at conserved repetitive elements located within linker regions. The nucleosome positioning pattern is intrinsic to the ICR and the methylation status specifically determines nuclease hypersensitivity at the linker regions. By transfecting an episomal-based H19 minigene into human somatic cells, we document that the ICR does not display any silencer activity by default. This observation is in contrast to a previous study using a Drosophila transgenic approach (4). On the other hand, we cannot rule out the possibility that an ICR-specific silencing represents the constitutive ground state for the unmethylated allele and that the maternal allele is protected against the silencing effect. We favor, however, the suggestion that the ICR insulates the Igf2 promoters from the H19 enhancers when maternally inherited (5), since we were able to show that the ICR prevents the communication between a strong enhancer and the H19 promoter in a position-dependent manner.