Imprinting along the Kcnq1 domain on mouse chromosome 7 involves repressive histone methylation and recruitment of Polycomb group complexes

Imprinted genes are clustered in domains, and their allelic repression is mediated by imprinting control regions. These imprinting control regions are marked by DNA methylation, which is essential to maintain imprinting in the embryo. To explore how imprinting is regulated in placenta, we studied the Kcnq1 domain on mouse distal chromosome 7. This large domain is controlled by an intronic imprinting control region and comprises multiple genes that are imprinted in placenta, without the involvement of promoter DNA methylation. We found that the paternal repression along the domain involves acquisition of trimethylation at Lys27 and dimethylation at Lys9 of histone H3. Eed-Ezh2 Polycomb complexes are recruited to the paternal chromosome and potentially regulate its repressive histone methylation. Studies on embryonic stem cells and early embryos support our proposal that chromatin repression is established early in development and is maintained in the placenta. In the embryo, however, imprinting is stably maintained only at genes that have promoter DNA methylation. These data underscore the importance of histone methylation in placental imprinting and identify mechanistic similarities with X-chromosome inactivation in extraembryonic tissues, suggesting that the two epigenetic mechanisms are evolutionarily linked.     

SUMO modification is required for in vivo Hox gene regulation by the Caenorhabditis elegans Polycomb group protein SOP-2

Post-translational modification of proteins by the ubiquitin-like molecule SUMO (sumoylation) regulates their subcellular localization and affects their functional properties in vitro, but the physiological function of sumoylation in multicellular organisms is largely unknown. Here, we show that the C. elegans Polycomb group (PcG) protein SOP-2 interacts with the SUMO-conjugating enzyme UBC-9 through its evolutionarily conserved SAM domain. Sumoylation of SOP-2 is required for its localization to nuclear bodies in vivo and for its physiological repression of Hox genes. Global disruption of sumoylation phenocopies a sop-2 mutation by causing ectopic Hox gene expression and homeotic transformations. Chimeric constructs in which the SOP-2 SAM domain is replaced with that derived from fruit fly or mammalian PcG proteins, but not those in which the SOP-2 SAM domain is replaced with the SAM domains of non-PcG proteins, confer appropriate in vivo nuclear localization and Hox gene repression. These observations indicate that sumoylation of PcG proteins, modulated by their evolutionarily conserved SAM domain, is essential to their physiological repression of Hox genes.     

Interaction between differentially methylated regions partitions the imprinted genes Igf2 and H19 into parent-specific chromatin loops

Imprinted genes are expressed from only one of the parental alleles and are marked epigenetically by DNA methylation and histone modifications. The paternally expressed gene insulin-like growth-factor 2 (Igf2) is separated by approximately 100 kb from the maternally expressed noncoding gene H19 on mouse distal chromosome 7. Differentially methylated regions in Igf2 and H19 contain chromatin boundaries, silencers and activators and regulate the reciprocal expression of the two genes in a methylation-sensitive manner by allowing them exclusive access to a shared set of enhancers. Various chromatin models have been proposed that separate Igf2 and H19 into active and silent domains. Here we used a GAL4 knock-in approach as well as the chromosome conformation capture technique to show that the differentially methylated regions in the imprinted genes Igf2 and H19 interact in mice. These interactions are epigenetically regulated and partition maternal and paternal chromatin into distinct loops. This generates a simple epigenetic switch for Igf2 through which it moves between an active and a silent chromatin domain.     

A mutation in OTOF, encoding otoferlin, a FER-1-like protein, causes DFNB9, a nonsyndromic form of deafness

Using a candidate gene approach, we identified a novel human gene, OTOF, underlying an autosomal recessive, nonsyndromic prelingual deafness, DFNB9. The same nonsense mutation was detected in four unrelated affected families of Lebanese origin. OTOF is the second member of a mammalian gene family related to Caenorhabditis elegans fer-1. It encodes a predicted cytosolic protein (of 1,230 aa) with three C2 domains and a single carboxy-terminal transmembrane domain. The sequence homologies and predicted structure of otoferlin, the protein encoded by OTOF, suggest its involvement in vesicle membrane fusion. In the inner ear, the expression of the orthologous mouse gene, mainly in the sensory hair cells, indicates that such a role could apply to synaptic vesicles.