Dynamic chromatin organization and regulatory interactions in human endothelial cell differentiation

Vascular endothelial cells are a mesoderm-derived lineage with many essential functions, including angiogenesis and coagulation. The gene-regulatory mechanisms underpinning endothelial specialization are largely unknown, as are the roles of chromatin organization in regulating endothelial cell transcription. To investigate the relationships between chromatin organization and gene expression, we induced endothelial cell differentiation from human pluripotent stem cells and performed Hi-C and RNA-sequencing assays at specific time points. Long-range intrachromosomal contacts increase over the course of differentiation, accompanied by widespread heteroeuchromatic compartment transitions that are tightly associated with transcription. Dynamic topologically associating domain boundaries strengthen and converge on an endothelial cell state, and function to regulate gene expression. Chromatin pairwise point interactions (DNA loops) increase in frequency during differentiation and are linked to the expression of genes essential to vascular biology. Chromatin dynamics guide transcription in endothelial cell development and promote the divergence of endothelial cells from cardiomyocytes

ATF7IP2/MCAF2 directs H3K9 methylation and meiotic gene regulation in the male germline

H3K9 trimethylation (H3K9me3) plays emerging roles in gene regulation, beyond its accumulation on pericentric constitutive heterochromatin. It remains a mystery why and how H3K9me3 undergoes dynamic regulation in male meiosis. Here, we identify a novel, critical regulator of H3K9 methylation and spermatogenic heterochromatin organization: the germline-specific protein ATF7IP2 (MCAF2). We show that in male meiosis, ATF7IP2 amasses on autosomal and X-pericentric heterochromatin, spreads through the entirety of the sex chromosomes, and accumulates on thousands of autosomal promoters and retrotransposon loci. On the sex chromosomes, which undergo meiotic sex chromosome inactivation (MSCI), the DNA damage response pathway recruits ATF7IP2 to X-pericentric heterochromatin, where it facilitates the recruitment of SETDB1, a histone methyltransferase that catalyzes H3K9me3. In the absence of ATF7IP2, male germ cells are arrested in meiotic prophase I. Analyses of ATF7IP2-deficient meiosis reveal the protein's essential roles in the maintenance of MSCI, suppression of retrotransposons, and global up-regulation of autosomal genes. We propose that ATF7IP2 is a downstream effector of the DDR pathway in meiosis that coordinates the organization of heterochromatin and gene regulation through the spatial regulation of SETDB1-mediated H3K9me3 deposition

BRCA1 establishes DNA damage signaling and pericentric heterochromatin of the X chromosome in male meiosis

During meiosis, DNA damage response (DDR) proteins induce transcriptional silencing of unsynapsed chromatin, including the constitutively unsynapsed XY chromosomes in males. DDR proteins are also implicated in double strand break repair during meiotic recombination. Here, we address the function of the breast cancer susceptibility gene Brca1 in meiotic silencing and recombination in mice. Unlike in somatic cells, in which homologous recombination defects of Brca1 mutants are rescued by 53bp1 deletion, the absence of 53BP1 did not rescue the meiotic failure seen in Brca1 mutant males. Further, BRCA1 promotes amplification and spreading of DDR components, including ATR and TOPBP1, along XY chromosome axes and promotes establishment of pericentric heterochromatin on the X chromosome. We propose that BRCA1-dependent establishment of X-pericentric heterochromatin is critical for XY body morphogenesis and subsequent meiotic progression. In contrast, BRCA1 plays a relatively minor role in meiotic recombination, and female Brca1 mutants are fertile. We infer that the major meiotic role of BRCA1 is to promote the dramatic chromatin changes required for formation and function of the XY body

Pioneering meiotic recombination

To induce cell type-specific forms of gene regulation, pioneer factors open tightly packed, inaccessible chromatin sites, enabling the molecular machinery to act on functionally significant information encoded in DNA. While previous studies of pioneer factors have revealed their functions in transcriptional regulation, pioneer factors that open chromatin for other physiological events remain undetermined. In this issue of Genes & Development, Spruce and colleagues (pp. 398-412) report the functional significance of a "pioneer complex" in mouse meiotic recombination. This complex, comprised of the zinc finger DNA-binding protein PRDM9 and the SNF2 family chromatin remodeler HELLS, exposes nucleosomal DNA to designate the sites of DNA double-strand breaks that initiate meiotic recombination. Both HELLS and PRDM9 are required for the determination of these recombination hot spots. Through the identification of a pioneer complex for meiotic recombination, this study broadens the conceptual scope of pioneer factors, indicating their functional significance in biological processes beyond transcriptional regulation