Role of Silencers in Genomic Imprinting

Wolf Reik
Developmental Genetics; The Babraham Institute

We are interested in the regulation of imprinted genes in the mammalian genome, and in their role in development and disease. We have carried out comparative sequencing of the 1 Mb imprinted gene cluster on distal chromosome 7 in the mouse. Amongst the most consistent organizational features of imprinted genes are differentially methylated CpG islands or CpG rich sequences which are often near direct tandem repeat clusters. The Igf2 gene contains three differentially methylated regions (DMRs), two of which are methylated on the active paternal copy. This has given rise to the proposal that these DMRs contain silencers which can be regulated by DNA methylation. DMR1 has been deleted by knockout and this results in tissue specific derepression of the maternal allele. Using in vitro transfection assays we found that DMR2 also contains silencer sequences located next to a core methylated region with antisilencer function when methylated. This core region of 54 bp was deleted by knockout, resulting in down regulation of the paternal Igf2 allele. This confirms that the methylated core region is needed for expression of Igf2. Our results reveal that epigenetic control of silencers is an important mechanistic principle in imprinting.