Condensin-driven remodelling of X chromosome topology during dosage compensation.

The three-dimensional organization of a genome plays a critical role in regulating gene expression, yet little is known about the machinery and mechanisms that determine higher-order chromosome structure. Here we perform genome-wide chromosome conformation capture analysis, fluorescent in situ hybridization (FISH), and RNA-seq to obtain comprehensive three-dimensional (3D) maps of the Caenorhabditis elegans genome and to dissect X chromosome dosage compensation, which balances gene expression between XX hermaphrodites and XO males. The dosage compensation complex (DCC), a condensin complex, binds to both hermaphrodite X chromosomes via sequence-specific recruitment elements on X (rex sites) to reduce chromosome-wide gene expression by half. Most DCC condensin subunits also act in other condensin complexes to control the compaction and resolution of all mitotic and meiotic chromosomes. By comparing chromosome structure in wild-type and DCC-defective embryos, we show that the DCC remodels hermaphrodite X chromosomes into a sex-specific spatial conformation distinct from autosomes. Dosage-compensated X chromosomes consist of self-interacting domains (∼1 Mb) resembling mammalian topologically associating domains (TADs). TADs on X chromosomes have stronger boundaries and more regular spacing than on autosomes. Many TAD boundaries on X chromosomes coincide with the highest-affinity rex sites and become diminished or lost in DCC-defective mutants, thereby converting the topology of X to a conformation resembling autosomes. rex sites engage in DCC-dependent long-range interactions, with the most frequent interactions occurring between rex sites at DCC-dependent TAD boundaries. These results imply that the DCC reshapes the topology of X chromosomes by forming new TAD boundaries and reinforcing weak boundaries through interactions between its highest-affinity binding sites. As this model predicts, deletion of an endogenous rex site at a DCC-dependent TAD boundary using CRISPR/Cas9 greatly diminished the boundary. Thus, the DCC imposes a distinct higher-order structure onto X chromosomes while regulating gene expression chromosome-wide.

The fission yeast homolog of the human transcription factor EAP30 blocks meiotic spindle pole body amplification.

Centrosome aberrations caused by misregulated centrosome maturation result in defective spindle and genomic instability. Here we report that the fission yeast homolog of the human transcription factor EAP30, Dot2, negatively regulates meiotic spindle pole body (SPB, the yeast equivalent of centrosome) maturation. dot2 mutants show excess electron-dense material accumulating near SPBs, which we refer to as aberrant microtubule organization centers (AMtOCs). These AMtOCs assemble multipolar spindles, leading to chromosome missegregation. SPB aberrations were associated with elevated levels of Pcp1, the fission yeast ortholog of pericentrin/kentrin, and reducing pcp1(+) expression significantly suppressed AMtOCs in dot2-439 cells. Our findings, therefore, uncover meiosis-specific regulation of SPB maturation and provide evidence that a member of the conserved EAP30 family is required for maintenance of genome stability through regulation of SPB maturation. EAP30 is part of a transcription factor complex associated with acute myeloid leukemia, so these results may have relevance to human cancer.