Genomic imprinting in plants-revisiting existing models.

Genomic imprinting is an epigenetic phenomenon leading to parentally biased gene expression. Throughout the years, extensive efforts have been made to characterize the epigenetic marks underlying imprinting in animals and plants. As a result, DNA methylation asymmetries between parental genomes emerged as the primary factor controlling the imprinting status of many genes. Nevertheless, the data accumulated so far suggest that this process cannot solely explain the imprinting of all genes. In this review, we revisit the current models explaining imprinting regulation in plants, and discuss novel regulatory mechanisms that could function independently of parental DNA methylation asymmetries in the establishment of imprinting.

The role of O-GlcNAcylation in development.

O-GlcNAcylation is a dynamic post-translational modification performed by two opposing enzymes: O-GlcNAc transferase and O-GlcNAcase. O-GlcNAcylation is generally believed to act as a metabolic integrator in numerous signalling pathways. The stoichiometry of this modification is tightly controlled throughout all stages of development, with both hypo/hyper O-GlcNAcylation resulting in broad defects. In this Primer, we discuss the role of O-GlcNAcylation in developmental processes from stem cell maintenance and differentiation to cell and tissue morphogenesis.

Auxin: a molecular trigger of seed development.

The evolution of seeds defines a remarkable landmark in the history of land plants. A developing seed contains three genetically distinct structures: the embryo, the nourishing tissue, and the seed coat. While fertilization is necessary to initiate seed development in most plant species, apomicts have evolved mechanisms allowing seed formation independently of fertilization. Despite their socio-economical relevance, the molecular mechanisms driving seed development have only recently begun to be understood. Here we review the current knowledge on the role of the hormone auxin for the initial development of the three seed structures and as a trigger of fertilization-independent seed development.

Polycomb group (PcG) proteins prevent the assembly of abnormal synaptonemal complex structures during meiosis.

The synaptonemal complex (SC) is a proteinaceous scaffold that is assembled between paired homologous chromosomes during the onset of meiosis. Timely expression of SC coding genes is essential for SC assembly and successful meiosis. However, SC components have an intrinsic tendency to self-organize into abnormal repetitive structures, which are not assembled between the paired homologs and whose formation is potentially deleterious for meiosis and gametogenesis. This creates an interesting conundrum, where SC genes need to be robustly expressed during meiosis, but their expression must be carefully regulated to prevent the formation of anomalous SC structures. In this manuscript, we show that the Polycomb group protein Sfmbt, the Drosophila ortholog of human MBTD1 and L3MBTL2, is required to avoid excessive expression of SC genes during prophase I. Although SC assembly is normal after Sfmbt depletion, SC disassembly is abnormal with the formation of multiple synaptonemal complexes (polycomplexes) within the oocyte. Overexpression of the SC gene corona and depletion of other Polycomb group proteins are similarly associated with polycomplex formation during SC disassembly. These polycomplexes are highly dynamic and have a well-defined periodic structure. Further confirming the importance of Sfmbt, germ line depletion of this protein is associated with significant metaphase I defects and a reduction in female fertility. Since transcription of SC genes mostly occurs during early prophase I, our results suggest a role of Sfmbt and other Polycomb group proteins in downregulating the expression of these and other early prophase I genes during later stages of meiosis.

Polycomb-mediated genome architecture enables long-range spreading of H3K27 methylation.

Polycomb-group proteins play critical roles in gene silencing through the deposition of histone H3 lysine 27 trimethylation (H3K27me3) and chromatin compaction. This process is essential for embryonic stem cell (ESC) pluripotency, differentiation, and development. Polycomb repressive complex 2 (PRC2) can both read and write H3K27me3, enabling progressive spreading of H3K27me3 on the linear genome. Long-range Polycomb-associated DNA contacts have also been described, but their regulation and role in gene silencing remain unclear. Here, we apply H3K27me3 HiChIP, a protein-directed chromosome conformation method, and optical reconstruction of chromatin architecture to profile long-range Polycomb-associated DNA loops that span tens to hundreds of megabases across multiple topological associated domains in mouse ESCs and human induced pluripotent stem cells. We find that H3K27me3 loop anchors are enriched for Polycomb nucleation points and coincide with key developmental genes. Genetic deletion of H3K27me3 loop anchors results in disruption of spatial contact between distant loci and altered H3K27me3 in cis, both locally and megabases away on the same chromosome. In mouse embryos, loop anchor deletion leads to ectopic activation of the partner gene, suggesting that Polycomb-associated loops control gene silencing during development. Further, we find that alterations in PRC2 occupancy resulting from an RNA binding­deficient EZH2 mutant are accompanied by loss of Polycomb-associated DNA looping. Together, these results suggest PRC2 uses RNA binding to enhance long-range chromosome folding and H3K27me3 spreading. Developmental gene loci have unique roles in Polycomb spreading, emerging as important architectural elements of the epigenome.

Polycomb Group Protein CBX7 Represses Cardiomyocyte Proliferation Through Modulation of the TARDBP/RBM38 Axis.

BACKGROUND: Shortly after birth, cardiomyocytes exit the cell cycle and cease proliferation. At present, the regulatory mechanisms for this loss of proliferative capacity are poorly understood. CBX7 (chromobox 7), a polycomb group (PcG) protein, regulates the cell cycle, but its role in cardiomyocyte proliferation is unknown. METHODS: We profiled CBX7 expression in the mouse hearts through quantitative real-time polymerase chain reaction, Western blotting, and immunohistochemistry. We overexpressed CBX7 in neonatal mouse cardiomyocytes through adenoviral transduction. We knocked down CBX7 by using constitutive and inducible conditional knockout mice (Tnnt2-Cre;Cbx7fl/+ and Myh6-MCM;Cbx7fl/fl, respectively). We measured cardiomyocyte proliferation by immunostaining of proliferation markers such as Ki67, phospho-histone 3, and cyclin B1. To examine the role of CBX7 in cardiac regeneration, we used neonatal cardiac apical resection and adult myocardial infarction models. We examined the mechanism of CBX7-mediated repression of cardiomyocyte proliferation through coimmunoprecipitation, mass spectrometry, and other molecular techniques. RESULTS: We explored Cbx7 expression in the heart and found that mRNA expression abruptly increased after birth and was sustained throughout adulthood. Overexpression of CBX7 through adenoviral transduction reduced proliferation of neonatal cardiomyocytes and promoted their multinucleation. On the other hand, genetic inactivation of Cbx7 increased proliferation of cardiomyocytes and impeded cardiac maturation during postnatal heart growth. Genetic ablation of Cbx7 promoted regeneration of neonatal and adult injured hearts. Mechanistically, CBX7 interacted with TARDBP (TAR DNA-binding protein 43) and positively regulated its downstream target, RBM38 (RNA Binding Motif Protein 38), in a TARDBP-dependent manner. Overexpression of RBM38 inhibited the proliferation of CBX7-depleted neonatal cardiomyocytes. CONCLUSIONS: Our results demonstrate that CBX7 directs the cell cycle exit of cardiomyocytes during the postnatal period by regulating its downstream targets TARDBP and RBM38. This is the first study to demonstrate the role of CBX7 in regulation of cardiomyocyte proliferation, and CBX7 could be an important target for cardiac regeneration.

Polycomb group proteins confer robustness to aposematic coloration in the milkweed bug, Oncopeltus fasciatus.

Aposematic coloration offers an opportunity to explore the molecular mechanisms underlying canalization. In this study, the role of epigenetic regulation underlying robustness was explored in the aposematic coloration of the milkweed bug, Oncopeltus fasciatus. Polycomb (Pc) and Enhancer of zeste (E(z)), which encode components of the Polycomb repressive complex 1 (PRC1) and PRC2, respectively, and jing, which encodes a component of the PRC2.2 subcomplex, were knocked down in the fourth instar of O. fasciatus. Knockdown of these genes led to alterations in scutellar morphology and melanization. In particular, when Pc was knocked down, the adults developed a highly melanized abdomen, head and forewings at all temperatures examined. In contrast, the E(z) and jing knockdown led to increased plasticity of the dorsal forewing melanization across different temperatures. Moreover, jing knockdown adults exhibited increased plasticity in the dorsal melanization of the head and the thorax. These observations demonstrate that histone modifiers may play a key role during the process of canalization to confer robustness in the aposematic coloration.

Decoding plant adaptation: deubiquitinating enzymes UBP12 and UBP13 in hormone signaling, light response, and developmental processes.

Ubiquitination, a vital post-translational modification in plants, plays a significant role in regulating protein activity, localization, and stability. This process occurs through a complex enzyme cascade that involves E1, E2, and E3 enzymes, leading to the covalent attachment of ubiquitin molecules to substrate proteins. Conversely, deubiquitinating enzymes (DUBs) work in opposition to this process by removing ubiquitin moieties. Despite extensive research on ubiquitination in plants, our understanding of the function of DUBs is still emerging. UBP12 and UBP13, two plant DUBs, have received much attention recently and are shown to play pivotal roles in hormone signaling, light perception, photoperiod responses, leaf development, senescence, and epigenetic transcriptional regulation. This review summarizes current knowledge of these two enzymes, highlighting the central role of deubiquitination in regulating the abundance and activity of critical regulators such as receptor kinases and transcription factors during phytohormone and developmental signaling.

Polycomb group genes are required for neuronal pruning in Drosophila.

BACKGROUND: Pruning that selectively eliminates unnecessary or incorrect neurites is required for proper wiring of the mature nervous system. During Drosophila metamorphosis, dendritic arbourization sensory neurons (ddaCs) and mushroom body (MB) γ neurons can selectively prune their larval dendrites and/or axons in response to the steroid hormone ecdysone. An ecdysone-induced transcriptional cascade plays a key role in initiating neuronal pruning. However, how downstream components of ecdysone signalling are induced remains not entirely understood. RESULTS: Here, we identify that Scm, a component of Polycomb group (PcG) complexes, is required for dendrite pruning of ddaC neurons. We show that two PcG complexes, PRC1 and PRC2, are important for dendrite pruning. Interestingly, depletion of PRC1 strongly enhances ectopic expression of Abdominal B (Abd-B) and Sex combs reduced, whereas loss of PRC2 causes mild upregulation of Ultrabithorax and Abdominal A in ddaC neurons. Among these Hox genes, overexpression of Abd-B causes the most severe pruning defects, suggesting its dominant effect. Knockdown of the core PRC1 component Polyhomeotic (Ph) or Abd-B overexpression selectively downregulates Mical expression, thereby inhibiting ecdysone signalling. Finally, Ph is also required for axon pruning and Abd-B silencing in MB γ neurons, indicating a conserved function of PRC1 in two types of pruning. CONCLUSIONS: This study demonstrates important roles of PcG and Hox genes in regulating ecdysone signalling and neuronal pruning in Drosophila. Moreover, our findings suggest a non-canonical and PRC2-independent role of PRC1 in Hox gene silencing during neuronal pruning.

Analysis of polycomb repressive complex 2 (PRC2) subunits in Picea abies with a focus on embryo development.

BACKGROUND: Conserved polycomb repressive complex 2 (PRC2) mediates H3K27me3 to direct transcriptional repression and has a key role in cell fate determination and cell differentiation in both animals and plants. PRC2 subunits have undergone independent multiplication and functional divergence in higher plants. However, relevant information is still absent in gymnosperms. RESULTS: To launch gymnosperm PRC2 research, we identified and cloned the PRC2 core component genes in the conifer model species Picea abies, including one Esc/FIE homolog PaFIE, two p55/MSI homologs PaMSI1a and PaMSI1b, two E(z) homologs PaKMT6A2 and PaKMT6A4, a Su(z)12 homolog PaEMF2 and a PaEMF2-like fragment. Phylogenetic and protein domain analyses were conducted. The Esc/FIE homologs were highly conserved in the land plant, except the monocots. The other gymnospermous PRC2 subunits underwent independent evolution with angiospermous species to different extents. The relative transcript levels of these genes were measured in endosperm and zygotic and somatic embryos at different developmental stages. The obtained results proposed the involvement of PaMSI1b and PaKMT6A4 in embryogenesis and PaKMT6A2 and PaEMF2 in the transition from embryos to seedlings. The PaEMF2-like fragment was predominantly expressed in the endosperm but not in the embryo. In addition, immunohistochemistry assay showed that H3K27me3 deposits were generally enriched at meristem regions during seed development in P. abies. CONCLUSIONS: This study reports the first characterization of the PRC2 core component genes in the coniferous species P. abies. Our work may enable a deeper understanding of the cell reprogramming process during seed and embryo development and may guide further research on embryonic potential and development in conifers.

Modulation of Chromatin by Noncoding RNA.

Noncoding RNAs (ncRNAs) are remarkably powerful, flexible, and pervasive cellular regulators. The involvement of these molecules in virtually all aspects of eukaryotic chromatin function is notable. Long and short ncRNAs play broadly complementary roles in these processes. Short ncRNAs underlie a programmable system of chromatin modification that silences mobile elements, identifies boundaries, and initiates the formation of constitutive heterochromatin in yeast. In contrast, long noncoding RNAs (lncRNAs) enforce developmentally appropriate expression and switch gene expression programs. lncRNAs accomplish this through diverse mechanisms, but often by modulating the activity or localization of chromatin regulatory complexes. Both long and short ncRNAs play key roles in organization of complex genomes of higher eukaryotes, and their coordinated actions appear to underlie some of the more dramatic examples of epigenetic regulation. This review contrasts well-studied examples of chromatin regulation by RNA and introduces examples of coordination between these systems.

Brain regionalization by Polycomb-group proteins and chromatin accessibility.

During brain development, neural precursor cells (NPCs) in different brain regions produce different types of neurons, and each of these regions plays a different role in the adult brain. Therefore, precise regionalization is essential in the early stages of brain development, and irregular regionalization has been proposed as the cause of neurodevelopmental disorders. The mechanisms underlying brain regionalization have been well studied in terms of morphogen-induced expression of critical transcription factors for regionalization. NPC potential in different brain regions is defined by chromatin structures that regulate the plasticity of gene expression. Herein, we present recent findings on the importance of chromatin structure in brain regionalization, particularly with respect to its regulation by Polycomb-group proteins and chromatin accessibility.

Single-molecule and in silico dissection of the interaction between Polycomb repressive complex 2 and chromatin.

Polycomb repressive complex 2 (PRC2) installs and spreads repressive histone methylation marks on eukaryotic chromosomes. Because of the key roles that PRC2 plays in development and disease, how this epigenetic machinery interacts with DNA and nucleosomes is of major interest. Nonetheless, the mechanism by which PRC2 engages with native-like chromatin remains incompletely understood. In this work, we employ single-molecule force spectroscopy and molecular dynamics simulations to dissect the behavior of PRC2 on polynucleosome arrays. Our results reveal an unexpectedly diverse repertoire of PRC2 binding configurations on chromatin. Besides reproducing known binding modes in which PRC2 interacts with bare DNA, mononucleosomes, and adjacent nucleosome pairs, our data also provide direct evidence that PRC2 can bridge pairs of distal nucleosomes. In particular, the "1-3" bridging mode, in which PRC2 engages two nucleosomes separated by one spacer nucleosome, is a preferred low-energy configuration. Moreover, we show that the distribution and stability of different PRC2-chromatin interaction modes are modulated by accessory subunits, oncogenic histone mutations, and the methylation state of chromatin. Overall, these findings have implications for the mechanism by which PRC2 spreads histone modifications and compacts chromatin. The experimental and computational platforms developed here provide a framework for understanding the molecular basis of epigenetic maintenance mediated by Polycomb-group proteins.

Sex comb on midleg (Scm) is a functional link between PcG-repressive complexes in Drosophila.

The Polycomb group (PcG) proteins are key regulators of development in Drosophila and are strongly implicated in human health and disease. How PcG complexes form repressive chromatin domains remains unclear. Using cross-linked affinity purifications of BioTAP-Polycomb (Pc) or BioTAP-Enhancer of zeste [E(z)], we captured all PcG-repressive complex 1 (PRC1) or PRC2 core components and Sex comb on midleg (Scm) as the only protein strongly enriched with both complexes. Although previously not linked to PRC2, we confirmed direct binding of Scm and PRC2 using recombinant protein expression and colocalization of Scm with PRC1, PRC2, and H3K27me3 in embryos and cultured cells using ChIP-seq (chromatin immunoprecipitation [ChIP] combined with deep sequencing). Furthermore, we found that RNAi knockdown of Scm and overexpression of the dominant-negative Scm-SAM (sterile α motif) domain both affected the binding pattern of E(z) on polytene chromosomes. Aberrant localization of the Scm-SAM domain in long contiguous regions on polytene chromosomes revealed its independent ability to spread on chromatin, consistent with its previously described ability to oligomerize in vitro. Pull-downs of BioTAP-Scm captured PRC1 and PRC2 and additional repressive complexes, including PhoRC, LINT, and CtBP. We propose that Scm is a key mediator connecting PRC1, PRC2, and transcriptional silencing. Combined with previous structural and genetic analyses, our results strongly suggest that Scm coordinates PcG complexes and polymerizes to produce broad domains of PcG silencing.

Genome-wide occupancy of Arabidopsis SWI/SNF chromatin remodeler SPLAYED provides insights into its interplay with its close homolog BRAHMA and Polycomb proteins.

SPLAYED (SYD) is a SWItch/Sucrose Non-Fermentable (SWI/SNF)-type chromatin remodeler identified in Arabidopsis thaliana (Arabidopsis). It is believed to play both redundant and differential roles with its closest homolog BRAHMA (BRM) in diverse plant growth and development processes. To better understand how SYD functions, we profiled the genome-wide occupancy of SYD and its impact on the global transcriptome and trimethylation of histone H3 on lysine 27 (H3K27me3). To map the global occupancy of SYD, we generated a GFP-tagged transgenic line and used it for chromatin immunoprecipitation experiments followed by next-generation sequencing, by which more than 6000 SYD target genes were identified. Through integrating SYD occupancy and transcriptome profiles, we found that SYD preferentially targets to nucleosome-free regions of expressed genes. Further analysis revealed that SYD occupancy peaks exhibit five distinct patterns, which were also shared by BRM and BAF60, a conserved SWI/SNF complex component, indicating the common target sites of these SWI/SNF chromatin remodelers and the functional relevance of such distinct patterns. To investigate the interplay between SYD and Polycomb-group (PcG) proteins, we performed a genome-wide analysis of H3K27me3 in syd-5. We observed both increases and decreases in H3K27me3 levels at a few hundred genes in syd-5 compared to wild type. Our results imply that SYD can act antagonistically or synergistically with PcG at specific genes. Together, our SYD genome-wide occupancy data and the transcriptome and H3K27me3 profiles provide a much-needed resource for dissecting SYD's crucial roles in the regulation of plant growth and development.

Sonic hedgehog signals hinder the transcriptional network necessary for pancreatic endoderm formation from human embryonic stem cells.

Hedgehog morphogens govern multiple aspects of pancreas organogenesis and functioning with diverse outcomes across species. Although most current differentiation protocols repress Sonic hedgehog (SHH) signals during in vitro endocrine specification, the role and mechanisms through which the SHH pathway antagonizes pancreas development during in vitro human embryonic stem (hES) cell differentiation remain unclear. We modulated SHH signaling at transitory stages of hES cell-derived pancreatic progenitors and analyzed the effect on cellular fate decisions. We identify the Hedgehog pathway as a negative regulator of pancreatic endoderm formation through up-regulation of a set of pancreatobiliary markers required for ductal specification, including SOX17, FOXA2, HNF1ß, HNF6, PDX1, and SOX9. Surprisingly, active Hedgehog signals impeded a group of pancreatic epithelium markers, including HNF4α, HHEX, PAX6, and PTF1α. To understand how SHH signals repress the transcription of these specific markers, we analyzed Polycomb group proteins. We found differential expression of Polycomb Repressive Complex 1 subunit, BMI1 upon Shh pathway modulation in the pancreatic progenitors. Ectopic activation of Sonic hedgehog results in over-expression of BMI1 and its associated repressive histone mark, H2AK119Ub1, in the multipotent progenitors. Our data suggest that Sonic hedgehog restricts the pancreatic differentiation program by limiting progenitor cells acquiring pancreatic epithelial fates and instead promotes pancreatobiliary differentiation. We further provide mechanistic cues of an association between Hedgehog signaling and epigenetic silencers during pancreatic lineage decisions.

Repression of the soma-specific transcriptome by Polycomb-repressive complex 2 promotes male germ cell development.

Polycomb-repressive complex 2 (PRC2) catalyzes the methylation of histone H3 Lys27 (H3K27) and functions as a critical epigenetic regulator of both stem cell pluripotency and somatic differentiation, but its role in male germ cell development is unknown. Using conditional mutagenesis to remove the core PRC2 subunits EED and SUZ12 during male germ cell development, we identified a requirement for PRC2 in both mitotic and meiotic germ cells. We observed a paucity of mutant spermatogonial stem cells (SSCs), which appears independent of repression of the known cell cycle inhibitors Ink4a/Ink4b/Arf. Moreover, mutant spermatocytes exhibited ectopic expression of somatic lamins and an abnormal distribution of SUN1 proteins on the nuclear envelope. These defects were coincident with abnormal chromosome dynamics, affecting homologous chromosome pairing and synapsis. We observed acquisition of H3K27me3 on stage-specific genes during meiotic progression, indicating a requirement for PRC2 in regulating the meiotic transcriptional program. Together, these data demonstrate that transcriptional repression of soma-specific genes by PRC2 facilitates homeostasis and differentiation during mammalian spermatogenesis.

The polycomb group proteins functions in epithelial to mesenchymal transition in lung cancer.

Polycomb group proteins (PcG) play important roles in the maintenance of DNA sequencing and multi-dimensional organization of genome. The main PcG complexes are consisted of Polycomb repressive complex 1 and 2, of which the diversity is dependent upon target gene sequences and functions. The present review initially explores the mechanism-based relationship and functional roles of PcG proteins in the interplay between epithelial mesenchymal transition (EMT) and chromatin dynamics in lung cancer. PcG proteins regulate the target genes by modifying histone and chromosome conformation and influencing chromatin looping and long-range interactions between topologically associating domains (TADs). PcG proteins regulate target genes expression and long-distance interactions between TADs in nucleus in the development of EMT and lung cancer. PcG plays decisive regulatory roles in epithelial differentiation and transition or signaling and activation of oncogenes, by promoting the isoforms at the transcriptional levels, to drive EMT to greater invasive ability and carcinogenesis. With the development of single cell systems biology and gene editing, PcG roles in 3D genome organization, heterogeneity, and EMT will be furthermore understood at single cell levels.

The polycomb group protein PCGF6 mediates germline gene silencing by recruiting histone-modifying proteins to target gene promoters.

Polycomb group (PcG) proteins are essential for maintenance of lineage fidelity by coordinating developmental gene expression programs. Polycomb group ring finger 6 (PCGF6) has been previously reported to repress expression of lineage-specific genes, especially germ cell-related genes in mouse embryonic stem cells (ESCs) via the noncanonical polycomb repressive complex PRC1.6. However, the molecular mechanism of this repression remains largely unknown. Here, using RNA-Seq, real-time RT-PCR, immunohistochemistry, immunoprecipitation, and ChIP analyses, we demonstrate that PCGF6 plays an essential role in embryonic development, indicated by the partially penetrant embryonic lethality in homozygous PCGF6 (Pcgf6-/-)-deficient mice. We also found that surviving Pcgf6-deficient mice exhibit reduced fertility. Using the Pcgf6-deficient mice, we observed that ablation of Pcgf6 in somatic tissues robustly derepresses germ cell-related genes. We further provide evidence that these genes are direct targets of PCGF6 in ESCs and that endogenous PCGF6 co-localizes with the histone-modifying proteins G9A histone methyltransferase (G9A)/G9a-like protein (GLP) and histone deacetylase 1/2 (HDAC1/2) on the promoters of the germ cell-related genes. Moreover, the binding of these proteins to their target genes correlated with methylation of Lys-9 of histone 3 and with the status of histone acetylation at these genes. Moreover, the recruitment of G9A/GLP and HDAC1/2 to target promoters depended on the binding of PCGF6. Our findings indicate that PCGF6 has a critical role in safeguarding lineage decisions and in preventing aberrant expression of germ cell-related genes.

Missense Mutations of the Pro65 Residue of PCGF2 Cause a Recognizable Syndrome Associated with Craniofacial, Neurological, Cardiovascular, and Skeletal Features.

PCGF2 encodes the polycomb group ring finger 2 protein, a transcriptional repressor involved in cell proliferation, differentiation, and embryogenesis. PCGF2 is a component of the polycomb repressive complex 1 (PRC1), a multiprotein complex which controls gene silencing through histone modification and chromatin remodelling. We report the phenotypic characterization of 13 patients (11 unrelated individuals and a pair of monozygotic twins) with missense mutations in PCGF2. All the mutations affected the same highly conserved proline in PCGF2 and were de novo, excepting maternal mosaicism in one. The patients demonstrated a recognizable facial gestalt, intellectual disability, feeding problems, impaired growth, and a range of brain, cardiovascular, and skeletal abnormalities. Computer structural modeling suggests the substitutions alter an N-terminal loop of PCGF2 critical for histone biding. Mutant PCGF2 may have dominant-negative effects, sequestering PRC1 components into complexes that lack the ability to interact efficiently with histones. These findings demonstrate the important role of PCGF2 in human development and confirm that heterozygous substitutions of the Pro65 residue of PCGF2 cause a recognizable syndrome characterized by distinctive craniofacial, neurological, cardiovascular, and skeletal features.