Genomic imprinting describes the differential expression of the maternally and paternally inherited alleles of an autosomal gene. Many important growth regulatory molecules are subject to genomic imprinting, and loss of imprinting results in human growth disorders and cancer. Previous work by the PI has characterized an imprinted region located on mouse chromosome 12/human 14q32. This region contains the paternally expressed Dlk1 gene, encoding a Notch-family signaling molecule involved in cellular proliferation, and the maternally expressed Gtl2 and Meg8 genes, both producing untranslated RNAs. Mutation of a region upstream of Gtl2 in the mouse results in altered growth, with dwarfism upon paternal inheritance and overgrowth upon maternal inheritance. The Pl's laboratory has now characterized the molecular defect in these animals, and shown that the growth phenotypes are the result of loss of imprinting of at least three linked genes in the region, with expression of the normally silent alleles from the chromosome carrying the mutation. This loss of imprinting causes altered expression levels of the genes involved, with decreased Dlk1 and increased Gtl2 and Meg8 upon paternal inheritance, and increased Dlk1 and decreased Gtl2 and Meg8 upon maternal inheritance. As the regulated Gtl2 and Meg8 genes are noncoding, it is proposed that the altered levels of Dlk1 are causative for the growth effects. This mutation therefore localizes an imprinting control region (ICR) that is required on both the maternal and paternal chromosomes for proper imprinting. A comparison of this element, termed the Gtl2 ICR, with the well characterized H19 ICR has identified both similarities and differences in their regulatory mechanisms. Both ICRs appear to function as paternal silencers, but while the H19 ICR utilizes a chromatin boundary element for its function on the maternal allele, the Gtl2 ICR does not display the properties of a boundary. These data suggest that the Gtl2 ICR contains a silencer that functions on the paternal chromosome, and a novel maternal imprinting regulatory element that functions by a currently unknown mechanism. Further investigation of this region promises to provide essential information about the range of imprinting regulatory mechanisms. This proposal details a plan to characterize the role of the Gtl2 ICR on both the maternal and paternal chromosome, isolate the trans-acting factors that recognize this ICR and localize additional regulatory elements in the Dlk1-Gtl2 region with which it interacts to regulate genomic imprinting.
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