International Mammalian Genome Society

logo18th International Mouse Genome Conference

17-22 October 2004, Seattle, USA



9.45am – 10.00am


Engel NI1, Bartolomei MS2

1 University of Pennsylvania, Philadelphia, United States, 2 Howard Hughes Medical Institute, Philadelphia, United States

Genomic imprinting is an epigenetic mechanism resulting in parental-specific monoallelic expression. H19 and Igf2 are two closely linked oppositely imprinted genes, with H19 expressed maternally and Igf2 expressed paternally. The imprinting regulation at these loci depends on a differentially methylated domain (DMD) upstream of the H19 gene that contains four highly conserved CG-rich repeats. These repeats bind CTCF, a protein involved in establishing chromatin boundaries, and this has led to the proposal that the DMD is a maternal-specific boundary regulating the usage of 3’ enhancers common to H19 and Igf2. The insulator function of the DMD is epigenetically regulated, since the boundary is not established on the paternally hypermethylated allele. We used gene targeting technology to replace the wild-type DMD region with a mutated DMD in which 9 CpGs have been replaced within the repeats. The mice generated from mutant ES cell lines were analyzed. We found that the mutations allow normal establishment of the maternal insulator but disrupt paternal identity at the endogenous locus. Mice with the paternally inherited mutation show hypomethylation of the DMD and biallelic expression of H19. They also have a growth retardation phenotype, consistent with a dramatic reduction of Igf2 expression. CTCF binds ectopically to the paternally inherited mutant repeats in vivo, suggesting that a functional insulator is established on that chromosome and blocks Igf2 expression.  Thus, nine CpG mutations within the CTCF binding sites reveal that the two parental-specific roles of the H19 DMD, maintenance of methylation and insulator assembly, are competitive and antagonistic.

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