The research team, led by Jing-bin Yan and Fanyi Zeng, focused on a question that has received surprisingly little attention: what happens to the proteins that cluster around centromeres in fetal cells carrying three copies of chromosome 21? Centromeres are not merely passive anchors for spindle fibers during cell division. They coordinate kinetochore assembly, microtubule attachment, and checkpoint signaling, and their dysfunction is known to destabilize the genome. Because chromosome 21 non-disjunction—the meiotic error that produces trisomy 21—depends entirely on intact centromere-kinetochore function, the investigators reasoned that the centromere-proximal proteome might hold clues to how the extra chromosome wreaks havoc on cellular physiology.

To map this proteomic landscape, the team employed a sophisticated technique called APEX2 proximity labeling. They engineered a fusion protein combining CENPA, the defining histone variant of centromeres, with GFP and the engineered peroxidase APEX2. When expressed in living cells and activated with hydrogen peroxide, APEX2 covalently tags every protein within roughly 20 nanometers of the centromere with biotin. These tagged proteins can then be pulled down with streptavidin beads and identified by quantitative mass spectrometry. Unlike conventional co-immunoprecipitation, which can lose weak or transient interactions and disrupt subcellular compartments, proximity labeling preserves the native molecular environment of the centromere in living cells.

The researchers applied this system to primary human amniocytes—cells shed by the fetus into amniotic fluid—obtained from pregnant women undergoing prenatal diagnosis at the Shanghai Ji Ai Genetics and IVF Institute. They compared trisomy 21-derived amniocytes with euploid normal amniocytes matched for gestational age, fetal sex, and passage number. Karyotyping confirmed the chromosomal constitution of each sample, and all experiments used cells at passage five or earlier to avoid senescence-related artifacts. After validating that the CENPA-GFP-APEX2 fusion protein localized exclusively to centromeres and exhibited minimal background biotinylation, the team proceeded to quantitative proteomic profiling.

Using label-free liquid chromatography-tandem mass spectrometry, the investigators quantified 999 CENPA-proximal proteins across three biological replicates per group. Among these, 32 proteins showed significantly altered abundance in trisomy 21 amniocytes: 19 were upregulated and 13 were downregulated. Principal component analysis revealed tight clustering within each group and clear separation between the two conditions, indicating widespread proteome remodeling driven by the extra chromosome. Gene Ontology and Gene Set Enrichment analyses pointed to dysregulation of interferon response pathways and ribosomal protein networks, while Kyoto Encyclopedia of Genes and Genomes pathway analysis implicated thyroid hormone signaling, platelet activation, and RAP1 signaling among the perturbed pathways.

Standing out among all the differentially abundant proteins was IFIT3, or Interferon-induced protein with tetratricopeptide repeats 3, which showed the most significant upregulation in trisomy 21 amniocytes. The team validated this finding by Western blot using independent donor-matched samples, confirming that IFIT3 protein levels were indeed elevated in the trisomy 21 cells. IFIT3 is a canonical interferon-stimulated gene, part of a family of antiviral effector proteins that are chronically hyperactivated across all trisomy 21 cell types—a phenomenon sometimes referred to as the interferonopathy of Down syndrome. What made the detection of IFIT3 within the centromere-proximal proteome particularly intriguing is that this protein is predominantly cytoplasmic at steady state, suggesting it may be transiently recruited to pericentromeric chromatin during mitosis.

To determine whether elevated IFIT3 is merely a bystander or an active driver of cellular dysfunction, the researchers generated human induced pluripotent stem cells overexpressing IFIT3 via lentiviral transduction. Before conducting any functional assays, they confirmed that the engineered cells retained intact pluripotency, with normal expression of OCT4, NANOG, SOX2, and SSEA4. The results were striking. Forced IFIT3 overexpression triggered G1-phase cell cycle arrest, suppressed cell proliferation, and elevated apoptotic rates in the pluripotent stem cells. Western blot analysis revealed that P21 and P27, two master negative regulators of cell cycle progression, were substantially downregulated in primary trisomy 21 amniocytes, providing a molecular link between IFIT3 dysregulation and cell cycle perturbation.

The most consequential findings, however, concerned mitochondrial function. Transcriptome-wide RNA sequencing of IFIT3-overexpressing stem cells identified 200 differentially expressed genes, with strong transcriptional repression of oxidative phosphorylation and mitochondrial ATP biogenesis gene sets. Gene Set Enrichment Analysis independently validated suppressed mitochondrial respiration signatures alongside enrichment of axonal guidance and neurogenesis gene sets. The team then performed direct functional measurements using JC-1 staining to assess mitochondrial membrane potential and luminescence-based assays to quantify intracellular ATP. Compared with control cells, IFIT3-overexpressing stem cells displayed a significantly reduced red-to-green fluorescence ratio, indicating depolarized mitochondria, and approximately 20 percent lower intracellular ATP levels. These results confirmed that IFIT3 overexpression alone is sufficient to impair mitochondrial bioenergetics.

The study’s authors note an intriguing discrepancy: forced IFIT3 overexpression in pluripotent stem cells induced G1-phase arrest, whereas endogenous IFIT3 upregulation in primary trisomy 21 amniocytes promoted increased S-phase entry. They attribute this to cell-type-specific divergence in cell cycle checkpoint wiring between differentiated primary amniocytes and pluripotent stem cells, highlighting the context-dependent nature of IFIT3’s mitotic regulatory functions. This nuance underscores the complexity of translating findings across cellular models and suggests that IFIT3’s effects on cell cycle progression may depend on the developmental stage and differentiation status of the cell.

Looking forward, the researchers propose that pharmacological suppression of IFIT3 activity may restore mitochondrial bioenergetics and reverse core cellular pathologies in Down syndrome. This builds on prior preclinical evidence that 7,8-dihydroxyflavone, a small molecule that ameliorates mitochondrial dysfunction, can rescue neurodevelopmental defects in the Ts65Dn mouse model of trisomy 21. The current work distinguishes itself from a previous study by the same group, which showed that the chromosome 21-encoded transcription factor RUNX1 disrupts mitochondrial function through PI3K-Akt signaling. The present findings identify a genetically independent, interferon-dependent IFIT3 signaling axis that acts upstream of mitochondrial impairment, providing a novel proteomic framework that links centromere-proximal immune signaling to bioenergetic failure in trisomy 21 and nominating IFIT3 as a candidate therapeutic target for future intervention studies.

Subject of Research: The role of centromere-proximal IFIT3 overexpression in mitochondrial dysfunction and cell cycle disruption in Down syndrome

Article Title: Overexpression of centromere-localized IFIT3 impairs mitochondrial function and disrupts cellular proliferation and cell cycle progression in down syndrome

Article References: Overexpression of centromere-localized IFIT3 impairs mitochondrial function and disrupts cellular proliferation and cell cycle progression in down syndrome. (n.d.). https://doi.org/10.3389/fgene.2026.1904151

Image Credits: AI Generated

DOI: 10.3389/fgene.2026.1904151

Keywords: Down syndrome, trisomy 21, IFIT3, centromere, CENPA, mitochondrial dysfunction, APEX2 proximity labeling, proteomics, oxidative phosphorylation, cell cycle, induced pluripotent stem cells, interferon signaling

The technique, developed by Iwona Nowak, Ahsan H. Polash, Hang T. Huynh, Mahekdeep Kaur and colleagues in the laboratories of Aishe A. Sarshad, Markus Hafner, and Daniel Benhalevy, fuses two powerful approaches that had never been married at this scale. The first is APEX2-based proximity labeling, an enzymatic tagging system that marks proteins and nucleic acids within a narrow radius of a genetically targeted protein. The second is 4-thiouridine-enhanced ultraviolet crosslinking, which covalently locks RNA-binding proteins onto their RNA targets and allows precise identification of crosslinked nucleotides. By combining these with sequential affinity purifications, first for the RNA-binding protein of interest and then for the biotin tag, the researchers built a pipeline that isolates only those protein-RNA contacts that occurred in the marked compartment.

The logic of the method is elegant in its layering. A protein of interest, such as the RNA-binding factor being studied, is expressed as a fusion with APEX2, a peroxidase that in the presence of biotin-phenol and hydrogen peroxide converts nearby molecules into biotin-tagged derivatives. Cells are first treated with 4-thiouridine, which is incorporated into newly synthesized RNA and dramatically strengthens ultraviolet-induced crosslinking between RNA and any protein touching it, a strategy borrowed from fPAR-CLIP protocols. When ultraviolet light fires, RNA-binding proteins are frozen onto their targets with nucleotide precision. APEX2 then labels everything in its immediate vicinity with biotin, creating a spatial signature.

What follows is a double selection. The researchers purify the RNA-binding protein and its crosslinked RNA fragments through an initial affinity step, recovering crosslinked RNA with mutations that reveal the exact binding sites, the hallmark of enhanced crosslinking-and-immunoprecipitation approaches. They then recover only those complexes that also carry the APEX2-deposited biotin tag in a second purification. A transcript must pass both filters to be counted: it must have been physically touched by the protein and must have been within the proximity-labeling radius, effectively bracketing the interaction in space and time. The result is a map of protein-RNA contacts that is simultaneously compartment-specific and nucleotide-resolved.

Control experiments were central to validating the approach. The team confirmed that APEX2 fusion proteins localized correctly to their intended compartments and that proximity-dependent biotinylation was restricted to the expected neighborhoods, presenting this evidence across multiple supplementary figures alongside imaging and immunoblot analyses. Without accurate spatial confinement, any apparent compartment specificity could simply reflect leakage or overexpression artifacts, so establishing the fidelity of the labeling step was essential before biological conclusions could be drawn.

With the method proven, the researchers turned it loose on three of the most intensively studied RNA-binding proteins: AGO2, the central effector of the microRNA silencing machinery; YBX1, a multifunctional nucleic acid-binding protein involved in mRNA stability and translation; and ELAVL1, better known in some circles as HuR, a regulator of mRNA stability and stress responses. All three proteins are known to operate in multiple cellular compartments, making them ideal test cases for a technology designed to disentangle location-dependent behavior.

The findings were striking. Each of the three proteins displayed distinct, compartment-specific RNA-binding patterns, targeting different sets of transcripts and even different sequence contexts depending on where in the cell it was acting. Importantly, the researchers found that these spatial differences arose through canonical motif recognition: the same proteins continued to recognize their characteristic sequence motifs in each compartment, but the repertoire of available targets, and thus the functional consequences of binding, shifted dramatically with location. In other words, the proteins did not change their binding preferences so much as their access to substrates changed, partitioning the accessible RNA targets and reshaping each protein’s regulome across the cell.

This partitioning concept has broad implications. For AGO2, compartment-specific mapping opens a window onto how microRNA targeting might differ between the cytoplasm, where canonical silencing occurs, and other locales where AGO2 has been implicated in less conventional roles. For YBX1 and ELAVL1, both implicated in cancer biology and stress physiology, knowing which transcripts are bound in which compartments could clarify how these proteins execute distinct functions from seemingly identical biochemical activities. The method essentially converts a single flat binding profile into a set of spatially annotated maps, revealing that what looked like one interactome is actually several overlapping ones.

Technically, RBProximity-CLIP also demonstrates the value of the 4-thiouridine enhancement inherited from fPAR-CLIP. Enhanced crosslinking increases crosslinking efficiency and yields the characteristic signature of crosslink-induced mutations, allowing binding sites to be pinpointed to individual nucleotides. Combined with proximity tagging, this delivers a rare combination of spatial and sequence resolution in a single experiment, something neither classical CLIP nor proximity labeling alone can provide. The authors provide detailed reagent lists, including antibodies, detection reagents for imaging and immunoblotting, and oligonucleotide sequences for library preparation, alongside full-length uncropped gel and blot images, underscoring the reproducibility infrastructure that accompanies methodological advances of this kind.

The work was a genuinely international effort, jointly led from the University of Gothenburg’s Wallenberg Centre for Molecular and Translational Medicine, the RNA Molecular Biology Laboratory at the National Institute of Arthritis and Musculoskeletal and Skin Diseases in Bethesda, and Tel Aviv University, with support from the Swedish Research Council, the Knut and Alice Wallenberg Foundation, the Swedish Society for Medical Research, the National Institutes of Health Intramural Research Program, the Israel Science Foundation, and the Hellenic Foundation for Research and Innovation. Published as an open-access article in Genome Biology, the study arrives at a moment when the field is increasingly aware that spatial context is not a luxury but a fundamental dimension of gene regulation. As RBProximity-CLIP is adopted and extended, it promises to turn the cell from a featureless bag of molecules into a mapped territory, where every protein-RNA encounter has an address, and where the grammar of gene expression can finally be read with both sequence and location in view.

Subject of Research: A subcellular-resolution RNA-binding protein mapping method combining proximity labeling and enhanced crosslinking

Article Title: RBProximity-CLIP enables subcellular mapping of RNA-binding protein interactions at nucleotide resolution

Article References: Nowak, I., Polash, A. H., Huynh, H. T., Kaur, M., Lobo, V., Scutenaire, J., Fong, M., Alluhaibi, G., Anastasakis, D. G., Hafner, M., Benhalevy, D., & Sarshad, A. A. (2026). RBProximity-CLIP enables subcellular mapping of RNA-binding protein interactions at nucleotide resolution. Genome Biology. https://doi.org/10.1186/s13059-026-04278-6

Image Credits: AI Generated

DOI: 10.1186/s13059-026-04278-6

Keywords: RNA-binding proteins, RBProximity-CLIP, proximity labeling, APEX2, CLIP-seq, AGO2, YBX1, ELAVL1, 4-thiouridine, subcellular resolution, protein-RNA interactions, Genome Biology