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