Developmentally regulated functions of the H19 differentially methylated domain.

Igf2 and H19 are physically linked imprinted genes. In embryonic liver, their reciprocal expression (paternal for Igf2 and maternal for H19) is controlled by a paternally methylated region (H19 DMD) located 5' of H19. This region contains a methylation-sensitive insulator that prevents the Igf2 promoters being activated by downstream enhancers on the maternal chromosome. In adult liver, Igf2 is normally not expressed but is reactivated upon tumour formation. By analysing three deletions of the H19 locus, we investigated the mechanism regulating the imprinted expression of the Igf2 gene in the course of liver tumourigenesis. We observed that the role of the H19 DMD in the control of Igf2 expression changes during tumourigenesis. The H19 DMD is required on the paternal chromosome for Igf2 activation in the early stages while its maternal allele is necessary for maintaining Igf2 imprinting only in the late stages. A positive regulatory function of the paternal H19 DMD is also evident in normal neonatal liver, but its relevance for Igf2 expression becomes higher in the second post-natal week. Our results support a model in which both methylated and non-methylated parental copies of the H19 DMD have active roles in the regulation of Igf2 expression in the liver and these activities are under developmental control.