Identification of Genes That Promote or Antagonize Somatic Homolog Pairing Using a High-Throughput FISH-Based Screen

Department of Genetics, Harvard Medical School, Boston, Massachusetts, United States of America.
PLoS Genetics (Impact Factor: 7.53). 05/2012; 8(5):e1002667. DOI: 10.1371/journal.pgen.1002667
Source: PubMed


The pairing of homologous chromosomes is a fundamental feature of the meiotic cell. In addition, a number of species exhibit homolog pairing in nonmeiotic, somatic cells as well, with evidence for its impact on both gene regulation and double-strand break (DSB) repair. An extreme example of somatic pairing can be observed in Drosophila melanogaster, where homologous chromosomes remain aligned throughout most of development. However, our understanding of the mechanism of somatic homolog pairing remains unclear, as only a few genes have been implicated in this process. In this study, we introduce a novel high-throughput fluorescent in situ hybridization (FISH) technology that enabled us to conduct a genome-wide RNAi screen for factors involved in the robust somatic pairing observed in Drosophila. We identified both candidate "pairing promoting genes" and candidate "anti-pairing genes," providing evidence that pairing is a dynamic process that can be both enhanced and antagonized. Many of the genes found to be important for promoting pairing are highly enriched for functions associated with mitotic cell division, suggesting a genetic framework for a long-standing link between chromosome dynamics during mitosis and nuclear organization during interphase. In contrast, several of the candidate anti-pairing genes have known interphase functions associated with S-phase progression, DNA replication, and chromatin compaction, including several components of the condensin II complex. In combination with a variety of secondary assays, these results provide insights into the mechanism and dynamics of somatic pairing.

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    • "Later on, both condensin complexes play fundamental roles in the segregation of sister chromatids. The position that chromosomes occupy in the next G1 nucleus seems to be dependent on condensin II (Bauer et al. 2012; Joyce et al. 2012). The predictive positions that each chromosome will occupy in the nucleus appear related to its position on the metaphase plate, when chromosomes attach the mitotic spindle and line up to prepare for moving to the opposite poles. "
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    • "Mrg-binding motif is required for Cap-H2-mediated homolog unpairing and axial compaction in cultured cells Condensin II promotes unpairing of homologous chromosomes, a process that has been proposed to result from condensin II–mediated axial compaction (Bauer et al. 2012; Joyce et al. 2012). Overexpression of Cap-H2 is sufficient to drive compaction and unpairing in interphase chromosomes, and does so in an Mrg15-dependent manner (Bauer et al. 2012; Smith et al. 2013). "
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    ABSTRACT: The spatial organization of the genome within the eukaryotic nucleus is a dynamic process that plays a central role in cellular processes such as gene expression, DNA replication, and chromosome segregation. Condensins are conserved multi-subunit protein complexes that contribute to chromosome organization by regulating chromosome compaction and homolog pairing. Previous work in our lab has shown that the Cap-H2 subunit of condensin II physically and genetically interacts with the Drosophila homolog of human MORF4-related gene on chromosome 15 (MRG15). Like Cap-H2, Mrg15 is required for interphase chromosome compaction and homolog pairing. However, the mechanism by which Mrg15 and Cap-H2 cooperate to maintain interphase chromatin organization remains unclear. Here we show that Cap-H2 localizes to interband regions on polytene chromosomes and co-localizes with Mrg15 at regions of active transcription across the genome. We show that co-localization of Cap-H2 on polytene chromosomes is partially dependent on Mrg15. We have identified a binding motif within Cap-H2 that is essential for its interaction with Mrg15, and have found that mutation of this motif results in loss of localization of Cap-H2 on polytene chromosomes and results in partial suppression of Cap-H2-mediated compaction and homolog unpairing. Our data are consistent with a model in which Mrg15 acts as a loading factor to facilitate Cap-H2 binding to chromatin and mediate changes in chromatin organization. Copyright © 2015 Author et al.
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    • "In this organism, homologous chromosomes are paired in diploid somatic cells, leading to a specialized gene regulatory event, known as transvection, in which a gene is transcriptionally activated or repressed in trans by regulatory elements located on the homologous chromosomes. Remarkably, condensin II subunits have been shown to play a role in antagonizing transvection (Hartl et al. 2008), as well as somatic homolog pairing (Joyce et al. 2012). Moreover, condensin II subunits apparently contribute to chromosome territory formation in polyploid nurse cells (Bauer et al. 2012). "
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    ABSTRACT: The primary goal of mitosis is to partition duplicated chromosomes into daughter cells. Eukaryotic chromosomes are equipped with two distinct classes of intrinsic machineries, cohesin and condensins, that ensure their faithful segregation during mitosis. Cohesin holds sister chromatids together immediately after their synthesis during S phase until the establishment of bipolar attachments to the mitotic spindle in metaphase. Condensins, on the other hand, attempt to "resolve" sister chromatids by counteracting cohesin. The products of the balancing acts of cohesin and condensins are metaphase chromosomes, in which two rod-shaped chromatids are connected primarily at the centromere. In anaphase, this connection is released by the action of separase that proteolytically cleaves the remaining population of cohesin. Recent studies uncover how this series of events might be mechanistically coupled with each other and intricately regulated by a number of regulatory factors. Copyright © 2015 Cold Spring Harbor Laboratory Press; all rights reserved.
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