National Institute of Genetics
The Igf2 and H19 genes lie within a large imprinted domain on mouse chromosome 7, which is homologous to the Beckwith-Wiedemann region on human chromosome 11p15.5. The two genes are imprinted to the opposite directions: only the paternal allele of Igf2 is active whereas only the maternal allele of H19 is active. By using a comparative sequencing approach, we look for evolutionarily conserved regulatory elements involved in the imprinting processes in the germline and the allele-specific expression in the somatic cells. So far we identified five tissue-specific enhancers in the downstream region of H19 and four CTCF binding motifs in the differentially methylated region (DMR) upstream of H19. To explain the reciprocal imprinting of the two linked genes, a model is proposed in which the DMR exerts two different functions depending on its methylation status. A methylated DMR on the paternal chromosome inactivates the adjacent H19, allowing the downstream enhancers to interact with Igf2, whereas an unmethylated DMR on the maternal chromosome insulates Igf2 from the enhancers, leaving H19 available to the enhancers. This methylation-sensitive insulator activity is probably mediated by CTCF, whose DNA binding is blocked by target site methylation. Others and we have also found that the paternal-specific methylation of the DMR is established in the male germ-line, mainly during the gonocyte or prospermatogonium stage. On the other hand, the allele-specific methylation is completely erased in the female germline by entry into meiosis. The identification of the timings of the methylation and demethylation should help to identify and characterize the biochemical basis of the reprogramming of imprinting. The imprinted Igf2/H19 region provides a unique model to study the roles of DNA methylation, chromatin structure, and long-distance mechanisms in the regulation of genome or chromosome domains.