Enhancers are regulatory DNA elements that can be located thousands or even hundreds of thousands of bases away from the promoters of the genes they regulate. Their activity depends in part on the formation of three-dimensional chromatin loops that allow enhancer-bound regulatory factors to contact promoter-bound transcription machinery. Although researchers have known that many enhancers produce short-lived transcripts called enhancer RNAs, or eRNAs, the precise role of these molecules has remained difficult to define. Promoter regions also generate nascent RNAs as transcription begins, creating an opportunity for RNA molecules from the two regulatory regions to interact. The new study proposes that hnRNPK recognizes and organizes these emerging transcripts, converting RNA–RNA contacts into a stable molecular bridge between enhancer and promoter.

Rather than functioning only as a conventional RNA-binding factor, hnRNPK appears to provide an architectural framework for transcriptional regulation. The researchers found that it binds preferentially to nascent RNAs produced from active enhancers and promoters. These RNAs are generated locally while the relevant chromatin regions are being transcribed, placing them close to the DNA sites that need to communicate. By associating with both classes of transcript, hnRNPK may concentrate enhancer- and promoter-derived RNAs in the same molecular environment. Such proximity could stabilize interactions between the RNAs and help fold the intervening chromatin into a loop. The model offers a mechanistic explanation for how transient RNA molecules can influence a much larger and more persistent structure in the genome.

The study further indicates that hnRNPK can assemble into phase-separated condensates. These are dynamic, concentrated compartments formed when proteins and nucleic acids cluster through numerous weak interactions rather than through a single permanent molecular bond. Phase separation is increasingly recognized as a way for cells to organize biochemical reactions without enclosing them in a membrane. In the case of hnRNPK, the condensates were described as containing cavities that encapsulate RNA polymerase II. This arrangement could create a specialized transcriptional environment in which regulatory RNAs, chromatin-associated factors and the polymerase are brought together at high local concentrations, making productive transcription more likely.

A particularly important feature of the proposed mechanism involves the RPB3 subunit of RNA polymerase II. The investigators report that hnRNPK interacts with Pol II through RPB3, providing a direct protein-based route for incorporating the transcriptional enzyme into hnRNPK condensates. Once Pol II is concentrated within these assemblies, enhancer-associated complexes may be able to influence its delivery to nearby or physically connected promoters. The researchers suggest that hnRNPK dimerization could contribute to this process: one portion of an hnRNPK assembly could engage enhancer-derived RNA and regulatory factors, while another connects with promoter-associated RNA and Pol II. In this framework, enhancer–promoter looping is not merely a passive consequence of chromatin folding but an active process coordinated by RNA-binding proteins.

This mechanism also helps explain why enhancer transcription may be functionally important even when eRNAs are rapidly degraded and do not encode proteins. Their role may depend less on their final abundance than on their production at the correct genomic location and time. As these RNAs emerge from the DNA, they can serve as temporary molecular signals or scaffolds for proteins such as hnRNPK. The resulting assemblies could integrate several signals at once, including the identity of the enhancer, the activity state of the promoter and the availability of Pol II. By linking these signals, hnRNPK may help ensure that developmental genes are activated only when the appropriate regulatory elements are engaged.

The biological significance of the findings was tested using a mutation in hnRNPK associated with Au–Kline syndrome, a rare developmental disorder. The mutation, designated c.953+1dupG, altered the physical properties of hnRNPK condensates. Instead of remaining liquid-like and dynamic, the mutant condensates became more gel-like. Liquid-like condensates can continuously exchange their molecular components with the surrounding nucleus, allowing them to assemble, reorganize and dissolve as transcriptional requirements change. A gel-like state may restrict this exchange, trapping components or preventing the rapid rearrangements required for enhancer–promoter communication. The results suggest that the material state of a condensate is not a cosmetic property but a critical determinant of gene regulation.

Knock-in mice carrying the disease-associated mutation developed developmental defects, providing evidence that the molecular changes have consequences at the level of the organism. Fibroblasts derived from these animals showed reduced enhancer–promoter looping and diminished recruitment of Pol II to the promoters of key developmental genes. These observations connect the mutation’s effects across several scales: it changes the physical behavior of hnRNPK condensates, weakens three-dimensional contacts between regulatory DNA elements, reduces transcriptional machinery at gene promoters and ultimately contributes to developmental abnormalities. The findings are consistent with the idea that many congenital disorders may result not only from loss of a protein’s biochemical activity but also from changes in the dynamics and material properties of nuclear assemblies.

The work presents hnRNPK as a general structural regulator of gene expression rather than a factor restricted to a small set of specialized genes. By mediating interactions between enhancer and promoter RNAs, organizing phase-separated condensates and engaging Pol II through RPB3, hnRNPK may provide a common platform for communication across the genome. The proposed model also broadens the view of noncoding transcription: RNAs produced from regulatory DNA may act as active architectural components, even when they do not persist as stable cellular molecules. Further research will be needed to determine how hnRNPK selects particular RNA sequences, how its condensates are regulated by signaling pathways and whether similar defects in condensate dynamics contribute to other human diseases. For now, the study identifies a molecular link between nascent RNA, chromatin looping and transcriptional activation, revealing how the genome’s distant control elements can work together with remarkable precision.

Subject of Research: hnRNPK-mediated enhancer–promoter looping, RNA–RNA interactions, phase-separated condensates and RNA polymerase II recruitment

Article Title: hnRNPK condensates facilitate enhancer–promoter looping and RNA polymerase II recruitment

Article References: Ye, R., Zhao, H., Chen, J. et al. hnRNPK condensates facilitate enhancer–promoter looping and RNA polymerase II recruitment. Nat Genet (2026). https://doi.org/10.1038/s41588-026-02710-y

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41588-026-02710-y

Keywords: hnRNPK, enhancer RNA, promoter RNA, enhancer–promoter looping, RNA polymerase II, phase separation, condensates, chromatin architecture, Au–Kline syndrome, gene regulation

Every cell in the human body harbors an identical DNA blueprint, yet the diversity in cell types arises from the distinct ways DNA is folded, packaged, and regulated. Within the nucleus, approximately two meters of DNA strand must be intricately folded to fit into a microscopic space, a process meticulously organized into three-dimensional domains delineated by boundary regions known as insulation. These boundaries prevent cross-talk between different genetic elements, ensuring precise gene expression patterns. Cohesin complexes, multi-protein rings known for holding sister chromatids together during cell division, have recently gained recognition as vital architects of these boundaries, mediating chromatin looping and genome organization.

Before this study, two primary cohesin complexes were characterized: mitotic cohesins comprising STAG1 or STAG2 paired with RAD21, active during the cell cycles that produce identical daughter cells; and meiotic cohesins containing STAG3 partnered with REC8 or RAD21L, operative during gamete formation. However, SSCs, the stem cells responsible for sustaining spermatogenesis, seemed to challenge these categorizations. These cells feature uniquely weak DNA boundary structures, an unusual characteristic that hinted at the possibility of unexplored regulatory mechanisms.

In pursuit of elucidating the molecular underpinnings of SSC-specific DNA organization, the Kyoto University team employed state-of-the-art immunoprecipitation followed by mass spectrometry to map cohesin components within in vitro cultured SSCs. Their results were startling: instead of associating with STAG1 or STAG2 as conventional mitotic cells do, RAD21 partnered predominantly with STAG3. This unexpected pairing revealed a heretofore unknown mitotic cohesin complex, now named STAG3-cohesin, shattering the dogma that positioned STAG3’s function solely within meiotic contexts.

To interrogate the functional consequences of this novel complex, the scientists engineered genetically modified SSC lines. One variant was deficient in STAG3, while another expressed STAG3 exclusively, lacking STAG1 and STAG2. Through meticulous chromatin conformation analyses, the team demonstrated that STAG3-cohesin is responsible for the distinctive weak boundary formation in SSC chromatin architecture. Crucially, SSCs devoid of STAG3 failed to transition efficiently from stem cell status to differentiated germ cells, indicating that STAG3 not only sculpts the nucleome but is indispensable for proper sperm development.

Extending beyond basic biology, the implications of STAG3’s mitotic role ripple into human health and disease. Leveraging extensive transcriptomic datasets, the researchers established that STAG3 exhibits high expression levels in human immune B cells and, notably, in B-cell lymphomas — cancers derived from malignant B lymphocytes. Functional assays revealed that targeted inhibition of STAG3 in these cancer cells significantly impeded their proliferative capacity in vitro, indicating a promising therapeutic target in oncology.

This discovery of STAG3-cohesin as a distinct mitotic complex with dual roles in germline development and cancer pathophysiology invites a reevaluation of cohesin biology. Unlike classical cohesins, which form robust DNA boundaries, STAG3-cohesin creates weaker, more flexible insulation zones, potentially facilitating the dynamic chromatin remodeling essential for SSC differentiation. This finding advances the understanding of how subtle modulation of chromatin topology can influence cell fate decisions, particularly at the critical juncture between mitosis and meiosis.

The stakes of this research are profound. Male infertility remains a global challenge with complex underlying causes, many of which relate to defects in germ cell development. Illuminating the role of STAG3-cohesin offers a tangible molecular mechanism that could underpin certain infertility cases linked to SSC dysfunction. Furthermore, the connection of STAG3 to B-cell lymphoma progression opens novel avenues for cancer research targeting cohesin complexes, a class of proteins not traditionally exploited in cancer therapeutics.

Beyond immediate clinical applications, the study catalyzes fundamental questions about genome regulation. How does the presence of STAG3 in mitotic cells alter the cohesin-mediated chromatin loop formation? What regulatory pathways control the switch between the usage of STAG3-cohesin and canonical cohesins during stem cell differentiation? Addressing these will be pivotal in decoding the complex choreography of the male germline nucleome.

The Kyoto University team, led by Professor Mitinori Saitou, combined sophisticated biochemical techniques, genetic engineering, and computational biology to dissect the chromatin landscape of SSCs. Their integrative approach exemplifies how interdisciplinary methods can unravel previously inaccessible layers of cellular regulation. Published in the prestigious journal Nature Structural & Molecular Biology, this study sets a new standard for investigations into the molecular architecture of stem cells.

Looking forward, the scientific community anticipates that research into STAG3’s functions will expand into other cell types, potentially revealing broader roles for this cohesin variant in human development and disease. Its unexpected presence and function in immune cells hint at an intricate network of genome regulation that transcends traditional boundaries of cell identity and division modes.

In sum, the revelation of STAG3-cohesin reshapes our conception of the mitotic machinery within spermatogonial stem cells, linking genome structure remodeling directly to stem cell fate and fertility. At the intersection of structural biology, stem cell research, and oncology, this discovery heralds a new chapter in understanding the dynamic interplay between chromatin architecture and cellular function, with promising translational potential in medicine.

Subject of Research: Cells

Article Title: The mitotic STAG3–cohesin complex shapes male germline nucleome

News Publication Date: August 25, 2025

Web References:

• Institute for the Advanced Study of Human Biology (ASHBi), Kyoto University
• Journal article DOI

References:

• Saitou, M., Nagano, M., Hu, B. et al. The mitotic STAG3–cohesin complex shapes male germline nucleome. Nature Structural & Molecular Biology (2025).

Keywords: Gametogenesis, Chromatin