Genomic imprinting is the process in mammals by which gametespecific epigenetic modifications establish the differential expression of the two alleles of a gene. The tightly linked H19 and Igf2 genes are expressed in tissues of endodermal and mesoderm origin, with H19 express from the maternal chromosome and Igf2 expressed from the paternal chromosome. A model has been proposed to explain the reciprocal imprinting of these genes; in this model, expression of the genes is governed by competition between their promoters for a common set of enhancers. An extra set of enhancers might be predicted to relieve the competition, thereby eliminating imprinting. Here we tested this prediction by generating mice with a duplication of the endoderm- specific enhancers. The normally silent Igf2 gene on the maternal chromosome was expressed in liver, consistent with relief from competition. We then generated a maternal chromosome containing a single set of enhancers located equidistant from Igf2 and H19; the direction of the imprint was reversed. Thus, the location of the enhancers determines the outcome of competition in liver, and the strength of the H19 promoter is not sufficient to silence Igf2.
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