Imprinting represents a curious defiance of normal Mendelian genetics. Mammals inherit two complete sets of chromosomes, one from the mother and one from the father, and most autosomal genes will be expressed equally from maternal and paternal alleles. Imprinted genes, however, are expressed from only one chromosome in a parent-of-origin dependent manner. Because silent and active promoters are present in a single nucleus, the differences in activity cannot be explained by transcription factor abundance. Thus the transcriptional of imprinted genes represents a clear situation in which epigenetic mechanisms restrict gene expression. Therefore imprinted genes are a model for understanding the role of DNA modifications and chromatin structure in maintaining appropriate patterns of gene expression. Further, because of parent-of-origin restricted expression, phenotypes determined by imprinted genes are not only susceptible to mutations of the genes themselves but also to disruptions in the epigenetic programs controlling regulation. Thus imprinted genes are frequently associated with human diseases, including disorders affecting cell growth, development, and behavior. Our Unit is investigating a cluster of genes on the distal end of mouse chromosome 7. The syntenic region in humans on chromosome 11p15.5 is conserved in genomic organization and in monoallelic expression patterns. Specifically we are dissecting the molecular basis for the maternal specific expression of the H19 gene and the paternal specific expression of the Igf2 gene. Loss of imprinting mutations in these two genes is associated with Beckwith Wiedemann Syndrome (BWS) and with Wilms' tumor. We have demonstrated that sequences upstream of the H19 promoter are required for imprinted expression of H19 transgenes. These sequences are called the H19DMR (for differentially methylated region) because they are specifically hypermethylated only on the paternal chromosome. We have deleted this region from the endogenous locus and shown that mice inheriting this mutation paternally show biallelic expression of H19 while mice inheriting the mutation through the maternal germline show loss of repression of the normally silent Igf2 allele. Thus the H19DMR is a parent-of-origin specific silencer. By constructing alleles in which we could delete this element in specific cells and at specific developmental time points we were able to demonstrate that the DMR silences H19 and Igf2 by distinct mechanisms. Specifically, we demonstrate that the DMR contains a transcriptional insulator that is inactivated upon maternal inheritance and that this activity is responsible for monoallelic expression of Igf2. In contrast, the H19DMR silences the H19 gene by directing epigenetic modifications of the H19 promoter that themeselves directly interfere with transcriptional activation. A second focus of our research is to uncover the biological function of the KCNQ1 gene, also in this locus. This gene has been identified independently by groups looking for genes important in the etiology of BWS, a disease with parent-of-origin inheritance patterns, and for genes important in Long QT syndromes mapping to 11p15.5, a disease with no parent-of-origin effects. We have elucidated the complex developmental regulation of imprinting of this gene so to resolve this apparent paradox. Recently, we have developed a model for inherited LQTS by generating mice deficient in KCNQ1. ECGs from these mice show abnormal T-wave and P-wave morphologies and prolongation of the QT and JT intervals. These changes are both indicative of cardiac repolarization defects. In addition, these mice show profound bilateral deafness and balance disorders. Histological analyses demonstrate that mutant mice cannot secrete functional endolymph.
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