The heart of the discovery concerns a chemical tag on histones, the protein spools around which DNA is wound. Specifically, the team focused on trimethylation of lysine 27 on histone H3, universally abbreviated H3K27me3. This mark is the calling card of Polycomb repressive complexes, evolutionarily ancient machines that silence genes by altering chromatin structure without changing the underlying DNA sequence. In animals, H3K27me3 is central to maintaining cell identity and to the epigenetic crosstalk between mother and offspring during pregnancy. In flowering plants, Polycomb proteins were already known to coordinate seed coat growth with fertilization, but whether the maternal tissues’ epigenetic state could reach into the embryo and steer its patterning had never been demonstrated.

Using CUT&Tag, a sensitive chromatin-profiling technique that maps histone modifications in tiny amounts of tissue, the researchers charted the H3K27me3 landscape in the integuments, the maternal cell layers that develop into the seed coat, of the reference plant Arabidopsis thaliana. They compared the landscape before fertilization and at 24 hours after pollination, a window during which the fertilized egg, the zygote, elongates and divides into the first embryonic lineages. What emerged was a strikingly dynamic picture: thousands of genomic regions gained or lost the repressive mark after fertilization, and the genes associated with those regions shifted in expression accordingly. The maternal seed coat, in other words, undergoes a coordinated epigenetic reprogramming precisely when the embryo is making its earliest patterning decisions.

To test whether this reprogramming matters functionally, the team turned to mutants lacking the three Arabidopsis H3K27me3 demethylases, the enzymes ELF6, REF6 and JMJ13, which erase the repressive mark. Plants carrying mutations in all three genes, a triple mutant the authors call erj-1, produced embryos with pronounced defects in their apical region, the part that gives rise to the shoot and the cotyledons. Crucially, the defect was not a failure of the embryo’s basic cell fate decisions. Single-cell transcriptome analysis of the apical and basal cell lineages showed that the early lineage-specification program ran normally in erj-1 embryos. Instead, the embryos failed at a later step: organizing the apical structures that define a mature body plan. This distinction, between specifying cell identities and arranging them into a pattern, proved to be the key to the whole mechanism.

Genetic detective work then localized the action to the mother. In reciprocal crosses between wild-type and mutant plants, the embryonic defects followed the maternal genotype, not the embryo’s own. When the demethylases were expressed only in maternal tissues, driven by the integument-specific SEEDSTICK promoter, the mutant phenotype was rescued. Allele-specific expression analysis confirmed that ELF6, REF6 and JMJ13 transcripts in early embryos are overwhelmingly of maternal origin, and fluorescently tagged versions of the proteins were detected in the maternal integuments rather than in the embryo itself. The epigenetic erasers, in short, work in the seed coat, and their influence travels across the maternal-embryonic boundary.

How does a chemical change in the seed coat reach the embryo? The answer is the plant hormone auxin, the great morphogen of plant development, which forms gradients that pattern organs from roots to leaves. In the erj-1 integuments, where H3K27me3 accumulates abnormally because it can no longer be erased, the researchers found that auxin levels dropped specifically in the micropylar integuments, the region where the embryo attaches to maternal tissue. Auxin signaling reporters such as R2D2 and DR5::GFP lit up weakly in these tissues, and the embryos themselves showed reduced auxin signaling. The maternal tissue, it seems, normally builds an auxin reservoir at the embryo’s doorstep and hands the hormone over to the developing offspring.

The molecular link between the histone mark and the hormone turned out to be two auxin transport genes, AUX1 and LAX1, which encode influx carriers that move auxin across cell membranes. In the erj-1 mutant, both genes carry elevated H3K27me3 and are transcriptionally downregulated in the integuments. The team showed that REF6, one of the demethylases, binds directly to specific sequence motifs in the promoters of AUX1 and LAX1, using yeast one-hybrid assays and published chromatin immunoprecipitation data as evidence. When AUX1 was expressed specifically in the integuments of erj-1 or aux1 lax1 mutant plants, using the TT10 seed coat promoter, auxin accumulated again in the micropylar region and embryonic patterning was substantially restored. The causal chain is therefore remarkably clean: maternal demethylases remove H3K27me3 from auxin transporter genes, the transporters load auxin into the micropylar integuments, and that auxin flows into the embryo where it instructs apical patterning.

Notably, the study found that the opposite side of the Polycomb system, the methyltransferases that write H3K27me3, played no detectable role in early embryonic patterning. Mutants in components of Polycomb repressive complex 2, including CURLY LEAF and SWINGER, produced embryos at normal frequencies. This asymmetry suggests that the dynamic removal of the mark, rather than its deposition, is the regulatory lever that maternal tissues pull during the fertilization window. It also echoes a theme familiar from animal biology: in mammals, oocytes actively remodel the epigenetic state of the genome they contribute, and placental epigenetics shapes pregnancy outcomes. The plant seed coat, functionally analogous to a placenta, now appears to run an epigenetic script of its own with direct consequences for the offspring’s form.

The implications extend well beyond basic developmental biology. Seed formation is the foundation of agriculture, and embryonic patterning defects translate directly into poor seed set and abnormal seedlings. Understanding that the maternal seed coat exerts epigenetic control over embryo architecture opens a potential avenue for crop improvement: modulating the expression of histone demethylases or auxin transporters in maternal tissues could strengthen embryo development under stress conditions or in hybrid breeding programs, where maternal effects are often decisive. The datasets generated in the study, including RNA sequencing of eight-cell embryos and seed coats and CUT&Tag maps deposited in the National Genomics Data Center, provide a public resource for exploring these possibilities.

Conceptually, the work reframes the maternal microenvironment from a passive incubator into an active epigenetic instructor. A mother plant, through the controlled erasure of a repressive histone mark in her seed coats, decides how much auxin her embryo will receive and, by extension, how that embryo will be shaped. The finding that this instruction crosses the generational boundary via a hormone rather than via inherited chromatin states adds a new mechanism to the growing catalog of maternal effects in biology. As Li, Zhao and their colleagues continue to dissect the pathway, one thing is already clear: in the quiet world of seeds, the mother’s epigenome speaks, and the embryo listens.

Subject of Research: Maternal epigenetic regulation of embryonic patterning via H3K27me3 demethylation and auxin transport in plant seed coats

Article Title: Maternal epigenetic regulation of embryonic patterning in plants

Article References: Maternal epigenetic regulation of embryonic patterning in plants. (n.d.). https://doi.org/10.1038/s41477-026-02421-6

Image Credits: AI Generated

DOI: 10.1038/s41477-026-02421-6

Keywords: plant embryogenesis, epigenetics, H3K27me3, histone demethylases, auxin, seed coat, Arabidopsis, maternal effects, Polycomb, AUX1, LAX1, seed development

Polycomb repressive complexes were first discovered in fruit flies through mutations that transformed body segments, a striking phenotype that hinted at their role in maintaining the expression patterns of developmental genes. In mammals, the two major Polycomb systems, PRC1 and PRC2, work in concert to silence lineages-inappropriate genes. PRC2 deposits the histone mark H3K27me3, a chemical flag on lysine 27 of histone H3, while PRC1 deposits a complementary mark, monoubiquitinated lysine 119 on histone H2A, abbreviated H2AK119ub. Both marks are hallmarks of facultative heterochromatin, the compacted but still potentially reversible form of chromatin that keeps developmental regulators quiet without permanently altering the underlying DNA sequence.

Canonical PRC1 is not a single protein but a holocomplex built from four families of subunits. At its catalytic heart sits a heterodimer of a RING-family E3 ubiquitin ligase component, RING1A or RING1B, paired with one of six PCGF proteins, most commonly PCGF2 or PCGF4 in the canonical form. Surrounding this catalytic core are the Polyhomeotic proteins PHC1, PHC2 or PHC3, which mediate higher-order chromatin interactions, and the chromodomain proteins of the CBX family, CBX2, CBX4, CBX6, CBX7 or CBX8, which read the H3K27me3 mark laid down by PRC2 and help recruit PRC1 to its genomic destinations. This modular organisation has made PRC1 notoriously difficult to study as an intact entity: the complex flexes, adopts multiple compositions, and interacts dynamically with its nucleosome substrate.

Cryo-electron microscopy is uniquely suited to this challenge. The technique flash-freezes purified protein complexes in a thin film of vitreous ice and images thousands of individual particles, computationally sorting the views into a three-dimensional map at near-atomic resolution. For PRC1, the technical hurdles were formidable. The complex must be purified in a homogeneous state, yet its different subunit paralogs generate compositional heterogeneity that can blur reconstructions. By isolating a defined canonical holocomplex and pushing the limits of particle classification and map refinement, Ciapponi and colleagues were able to visualise the architecture of the entire assembly, resolving how each subunit docks onto its neighbours and where the catalytic machinery sits relative to the rest of the complex.

The resulting structure reveals a striking spatial economy. The RING1 catalytic subunit and its PCGF partner form a central scaffold onto which the Polyhomeotic and CBX subunits assemble in a defined arrangement. Critically, the positions of these accessory modules explain why they are not passive passengers. The chromodomain subunit projects away from the catalytic core, positioned to reach out and engage neighbouring nucleosomes bearing the H3K27me3 mark, thereby coupling recognition of the PRC2 histone code to the placement of the PRC1 ligase. The Polyhomeotic subunits, meanwhile, occupy an architecture consistent with their established role in bridging distant chromatin regions, the process of chromatin compaction that physically reinforces gene silencing.

Beyond the static snapshot, the study directly interrogated enzymatic function. The catalytic RING1B-containing core acts as an E3 ubiquitin ligase, transferring ubiquitin from an E2 conjugating enzyme onto lysine 119 of histone H2A within the nucleosome. In experiments with reconstituted nucleosomes and purified complex variants, the authors examined how mutations or perturbations that disrupt the interfaces seen in the structure affect H2A monoubiquitination activity. The results support a model in which the holocomplex architecture coordinates substrate engagement: the catalytic module must be correctly positioned relative to the nucleosome acidic patch, the docking surface on the histone disc where RING1B binds, while the peripheral subunits stabilise the active conformation and mediate the multivalent contacts that make targeting efficient.

Perhaps the most consequential aspect of the work concerns genome targeting. How does PRC1 find the hundreds of genomic loci it represses, including the CpG islands that overlap promoters of developmental regulators? The prevailing model holds that recruitment depends on combinatorial recognition: CBX chromodomains bind H3K27me3 deposited by PRC2, while additional factors recognise unmethylated CpG DNA. The new structure shows how the CBX subunit is oriented within the holocomplex such that its chromodomain can sample adjacent nucleosomes, and how the overall geometry of the complex allows multiple weak interactions to cooperate into a stable, specific engagement with its chromatin substrate. This multivalent design explains how PRC1 achieves both specificity, favouring the correct genomic loci, and stability, maintaining silencing through cell divisions.

The biological stakes of this work are considerable. Aberrant Polycomb function is implicated in a long list of human diseases. Overexpression of BMI1, the mammalian homolog of PCGF4, drives proliferation in numerous cancers and is a recognised marker of tumour-initiating stem-like cells. Loss-of-function mutations in Polycomb components cause developmental syndromes, including Weaver syndrome and other overgrowth disorders linked to EZH2 and EED of PRC2, and Shashi-Penman syndrome linked to EED and EZHIP, while somatic alterations in PRC1 genes recur across lymphoma, leukaemia and solid tumours. Understanding the assembly pathway and enzymatic mechanism of canonical PRC1 at structural resolution therefore provides a rational foundation for therapeutic intervention, informing the design of molecules that destabilise pathological complexes or block their catalytic activity.

The study also resolves longstanding debates about the relationship between structure and function in Polycomb biology. Earlier biochemical work established the individual interactions, RING1B with PCGF subunits, CBX chromodomains with H3K27me3, Polyhomeotic with chromatin, but it remained unclear how these pieces fit together in the intact holocomplex and whether the intact assembly behaves differently from its parts. By presenting the architecture of the canonical PRC1 holocomplex alongside enzymatic assays and genome-targeting data within a single integrated analysis, Ciapponi and colleagues provide the kind of mechanistic completeness that individual fragment studies could not. Their structure serves as a framework onto which decades of genetic, biochemical and genomic observations can now be mapped.

Future work will build on this foundation in several directions. Cryo-EM structures of PRC1 variants containing different PCGF, CBX and Polyhomeotic paralogs will reveal how compositional diversity translates into functional specialisation. Higher-resolution reconstructions of the complex bound to nucleosome substrates will clarify the exact geometry of ubiquitin transfer onto H2A lysine 119. And comparative studies with the non-canonical PRC1 complexes, which lack CBX subunits and rely instead on KDM2B and other recruitment factors, will illuminate how evolution has tuned the same catalytic core to serve distinct targeting strategies. For now, the structure determined by Ciapponi and colleagues stands as a landmark: the first comprehensive view of one of the cell’s most important gene-silencing machines, and a vivid demonstration of how cryo-electron microscopy is transforming our understanding of the molecular logic of chromatin regulation.

Subject of Research: Cryo-EM structure, enzymatic activity and genome targeting of canonical Polycomb repressive complex 1

Article Title: Cryo-EM structure, enzymatic activity and genome targeting of canonical PRC1

Article References: Cryo-EM structure, enzymatic activity and genome targeting of canonical PRC1. (n.d.). https://doi.org/10.1038/s41594-026-01885-6

Image Credits: AI Generated

DOI: 10.1038/s41594-026-01885-6

Keywords: PRC1, cryo-EM, Polycomb, chromatin, epigenetics, gene silencing, H2A ubiquitination, RING1B, structural biology, histone modification, genome targeting, structure

The review centers on CBX7, a member of the Chromobox protein family and a core subunit of Polycomb Repressive Complex 1, one of the two major Polycomb complexes that shape the epigenetic landscape of mammalian cells. Epigenetics, a field now seventy years old, concerns heritable changes in gene expression that occur without altering the underlying DNA sequence, chiefly through DNA methylation and histone modifications. Polycomb group proteins sit at the heart of this regulatory architecture, controlling cell fate decisions by modulating the expression of genes responsible for development, differentiation and, when things go wrong, cancer. Genome-wide studies have established that Polycomb Repressive Complex 1 and Polycomb Repressive Complex 2 function as crucial transcriptional regulators targeting a wide range of genes involved in tumor development, and the chromodomain-containing CBX proteins are the molecular anchors that tether Polycomb activity to specific chromatin regions.

Structurally, CBX7 is a study in molecular precision. Its chromodomain, spanning amino acids 9 to 48, descends from a conserved 37-amino acid sequence first identified in the Polycomb and heterochromatin protein 1 proteins of the fruit fly Drosophila melanogaster. Phylogenetic analyses show that the CBX7 chromodomain is more closely related to other Polycomb group proteins such as CBX2, CBX4, CBX6 and CBX8 than to the HP1 family members CBX1, CBX3 and CBX5, and unlike HP1 proteins, CBX7 lacks a chromoshadow domain. Instead, its chromodomain carries unique conserved residues exclusive to Polycomb proteins that are necessary for Polycomb dimer formation and for recognizing the tri-methylated lysine 27 mark on histone H3, known as H3K27me3. Thanks to the stable geometry of this domain, CBX7 preferentially binds H3K27me3, a mark laid down by the PRC2 complex through its core subunits EZH2, SUZ12 and EED. Once anchored, CBX7 recruits the canonical PRC1 complex, whose catalytic core ubiquitinates lysine 119 on histone H2A, compacting chromatin and silencing target genes. In this way, CBX7 acts as a functional bridge between PRC2 activity and stable transcriptional repression.

The protein also exists in two isoforms with strikingly divergent behavior. The 36-kilodalton nuclear form, p36CBX7, is expressed in proliferating cells and drives gene regulation, while the 22-kilodalton cytoplasmic form, p22CBX7, is induced by serum deprivation and restrains cell proliferation. This context- and tissue-specific duality foreshadows the protein’s complex role in cancer. Across human tumors, CBX7 behaves as either an oncogene or a tumor suppressor depending on the dominant signaling pathways and the genetic and epigenetic makeup of each tumor type. In lymphomas and gastric cancer, overexpressed CBX7 represses tumor-suppressive genes such as Ink4a/Arf and Trail, allowing malignant cells to evade senescence and apoptosis. In contrast, cancers of the breast, pancreas, lung, thyroid, colon, bladder and brain, including glioblastoma, consistently show reduced CBX7 expression compared with normal tissue, pointing to a tumor-suppressive function in these settings.

In glioblastoma specifically, the evidence for CBX7 as a tumor suppressor is compelling. High-grade gliomas exhibit significantly lower CBX7 levels than low-grade gliomas, and this downregulation correlates with aggressive disease and poor prognosis. When lost CBX7 is restored in glioma cells, tumor growth is inhibited and the cell cycle arrests at the G0/G1 phase. One well-characterized mechanism involves competition with the architectural transcription factor HMGA1 for control of the CCNE1 promoter, which encodes cyclin E1, a key driver of the G1/S transition. Under normal conditions, CBX7 partners with histone deacetylase 2 to form a repressive complex on the CCNE1 promoter. When CBX7 is lost, HMGA1 evicts this complex, cyclin E1 expression surges, and glioblastoma cells proliferate uncontrollably. HMGA1 also directly binds the CBX7 promoter itself, adding a further layer of antagonism.

The tumor-suppressive reach of CBX7 extends well beyond cell-cycle control into the machinery of invasion and stemness. CBX7 directly binds the promoter of CDH1, the gene encoding the adhesion molecule E-cadherin, where it restrains HDAC2 activity and promotes activating acetylation and methylation marks on histones H3 and H4, stabilizing the epithelial phenotype. Loss of CBX7 silences CDH1, promotes epithelial-to-mesenchymal transition and drives the invasiveness that typifies glioblastoma progression. CBX7 also prevents HMGA1 from transcriptionally activating SPP1, the gene encoding osteopontin, another potent EMT stimulator. Perhaps most strikingly, recent work shows that CBX7 engages PRC1 to facilitate ubiquitin-proteasome degradation of myosin heavy chain 9, thereby suppressing NF-kappaB signaling and stripping glioblastoma cells of their stem-like characteristics. Restoring MYH9 reverses this effect, re-establishing stemness, proliferation and invasion, and defining a CBX7-MYH9-NF-kappaB regulatory axis that researchers regard as a desirable therapeutic target.

Two further oncogenic pathways fall under CBX7’s control. The Wnt/beta-catenin pathway, aberrantly activated in many glioblastomas, is dampened when CBX7 binds the promoter of DKK1, a Wnt inhibitor, upregulating its expression and simultaneously reducing ZEB1, a transcription factor that promotes invasion, metastasis and epithelial-to-mesenchymal transition. Restoring CBX7 in glioblastoma cells suppresses both Wnt signaling and ZEB1, curtailing tumor growth and metastatic potential. CBX7 likewise modulates the Hippo pathway through connective tissue growth factor, encoded by CTGF, a key component of the YAP/TAZ signaling axis. When CBX7 is lost, PRC1-mediated gene suppression fails, YAP/TAZ signaling becomes hyperactive, TEAD-dependent transcription rises and CTGF is overexpressed, activating kinases such as SAPK/JNK and fueling glioma cell invasion and migration. Ectopic CBX7 expression additionally reduces the matrix metalloproteinases MMP2 and MMP9, major markers of glioblastoma invasiveness, angiogenesis and immune modulation.

How does such a critical tumor suppressor get silenced in the first place? The review details two principal mechanisms. The first is epigenetic: hypermethylation of the CBX7 promoter, confirmed by bisulfite sequencing in glioblastoma tissues, glioma-derived cell lines and normal brain, with clear enrichment of methylation in tumor samples. Knockdown studies identified the DNA methyltransferases DNMT1 and DNMT3A, but not DNMT3B, as the enzymes responsible for this silencing, and treatment with a methylation inhibitor restored CBX7 mRNA expression in glioma cell lines. The second mechanism is post-transcriptional, mediated by microRNAs. The MYC-responsive microRNA miR-9 lowers CBX7 protein levels, while CBX7 in turn binds the miR-9 promoter and represses its expression, creating an autoregulatory loop that also feeds into the control of p16-INK4a, a key effector of replicative senescence. Separately, the oncogenic miR-18a, highly expressed in glioblastoma, directly targets CBX7 mRNA, and silencing miR-18a in animal models markedly slows tumor growth and prolongs survival.

The translational implications are substantial, though tempered by caution. No CBX7-targeted therapy is yet in clinical use for glioblastoma, and the review’s authors stress that the protein’s role in normal cells must be fully understood before therapeutic modulation is attempted. Nevertheless, pharmacological proof of concept already exists in other malignancies: chromodomain-targeting inhibitors such as UNC3866, UNC4976, MS452, EC-134 and BDA-41 have dislodged overexpressed CBX7 from chromatin in lymphoid leukemia models, inducing differentiation and growth arrest. For glioblastoma, the therapeutic logic runs in the opposite direction, toward restoring rather than inhibiting CBX7 function, using epigenetic drugs, miRNA-based approaches or targeted modulation of CBX7-associated complexes and downstream pathways. Beyond therapy, CBX7’s consistent downregulation and correlation with tumor aggressiveness make it a promising prognostic biomarker, measurable through immunohistochemistry, methylation profiling or miRNA expression analysis to classify patients and guide treatment decisions. As the authors conclude, CBX7 represents a particularly promising starting point for developing more accurate and potent targeted treatments, and for deepening our understanding of glioblastoma biology in a disease that urgently needs both.

Subject of Research: The role of the Polycomb protein CBX7 as an epigenetic tumor suppressor in glioblastoma and its therapeutic potential.

Article Title: Epigenetic regulation by CBX7 in glioblastoma: molecular mechanisms and translational perspectives

Article References: Fayyaz, F., Favale, G., Capasso, L., Casalino, R., Mele, D., Verrilli, G., Conte, M., Carafa, V., Nebbioso, A., & Altucci, L. (2025). Epigenetic regulation by CBX7 in glioblastoma: molecular mechanisms and translational perspectives. Epigenetics Communications, 6(1), Article 2. https://doi.org/10.1186/s43682-025-00043-1

Image Credits: AI Generated

DOI: 10.1186/s43682-025-00043-1

Keywords: CBX7, glioblastoma, epigenetics, Polycomb, PRC1, PRC2, chromodomain, DNA methylation, microRNA, Wnt signaling, Hippo pathway, tumor suppressor