The discovery grew out of a shift in how scientists think about stress responses. Proteotoxic stress responses were traditionally viewed as acute, emergency reactions triggered by noxious stimuli such as heat, toxins, or chemical stressors. But recent evidence has revealed something more rhythmic and more fundamental. Many proteostasis genes do not simply wait for disaster to strike; instead, they oscillate on an approximately 12-hour ultradian cycle under ordinary physiological conditions. This 12-hour oscillator is driven by the transcription factor XBP1s in its spliced, active form, and remarkably, it operates independently of both the canonical circadian clock and the cell cycle. In other words, cells possess a second biological clock, running at twice the frequency of the daily rhythm, dedicated to the maintenance of the protein-folding machinery.

To understand how this oscillator is written into chromatin, the research team, led by Syeda Kubra, Michelle Sun, William Dion, Yu Bian, and senior author Bokai Zhu, mapped the chromatin landscape associated with the murine 12-hour oscillator. Their genome-wide survey pointed decisively toward one factor: RBBP5, an essential subunit of the COMPASS complex, the enzyme complex responsible for depositing trimethyl marks on lysine 4 of histone H3, a modification abbreviated H3K4me3. H3K4me3 is classically associated with active gene promoters, and the new findings place it at the heart of the temporal control of proteostasis gene expression. In contrast, the study found that histone acetyltransferases and the H3K9 acetylation mark were dispensable for the dynamic expression of proteostasis genes, sharpening the conclusion that methylation, rather than acetylation, is the decisive chromatin currency for this program.

The functional experiments were striking in their consistency. RBBP5 was not merely present at the right places at the right times; it was indispensable for the 12-hour oscillator itself. When the researchers depleted RBBP5, the rhythmic transcriptional pulses that normally drive proteostasis gene expression collapsed. Beyond the oscillator, RBBP5 proved essential for the transcriptional regulation of responses to diverse proteotoxic stresses, indicating that the same epigenetic machinery governs both the rhythmic, housekeeping side of proteostasis and the acute, emergency side of the program.

Mechanistically, the study offers a two-pronged model for how RBBP5 exerts its influence. First, RBBP5 acts as a coactivator of XBP1s, the spliced transcription factor that drives the 12-hour oscillator and the broader unfolded protein response. By partnering with XBP1s, RBBP5 helps translate the stress-sensing signal from the endoplasmic reticulum into a productive transcriptional output. Second, RBBP5 promotes the deposition of the H3K4me3 mark at the promoters of proteostasis genes, physically opening the chromatin at precisely the loci that encode chaperones, autophagy components, and other protein-quality-control factors. Together, these two functions make RBBP5 a bridge between the signaling layer of the stress response and the chromatin layer that must execute it.

The consequences of losing this bridge are immediate and measurable. The authors showed that loss of RBBP5 sensitizes cells to proteotoxic stress, in part because autophagy, the cellular recycling system that clears damaged proteins and organelles, becomes impaired. Autophagy is one of the major arms of the proteostasis network, and its dependence on RBBP5-dependent transcription explains why cells lacking this histone methyltransferase subunit cannot mount an effective defense when protein folding goes wrong. The finding suggests that chromatin competence, not just the presence of stress sensors, determines whether a cell survives proteomic insults.

To push the analysis beyond standard genomic assays, the team employed proximity labeling of H3K4me3, a technique that tags and identifies proteins found in the immediate vicinity of this histone mark in living cells. The resulting map uncovered a dynamic, chromatin-associated proteomic architecture that assembles at active proteostasis promoters during the stress response. Among the components identified were members of the COMPASS complex itself, the Integrator complex, a multi-subunit machinery best known for processing small nuclear RNAs and recently implicated in transcriptional pause release, and SWI/SNF chromatin remodelers, the ATP-dependent motors that physically reposition nucleotides to expose regulatory DNA.

This proteomic architecture is significant because it reframes the transcriptional response to proteotoxic stress as a coordinated, multi-complex event rather than the action of a single transcription factor. XBP1s may provide the sequence-specific targeting, but the actual execution depends on a choreographed assembly of methyltransferases, RNA-processing factors, and nucleosome remodelers converging on the same promoters. The proximity-labeling data suggest that this assembly is dynamic, forming and dissolving as the 12-hour oscillator cycles and as stress demands shift, providing a physical substrate for the temporal control of gene expression that the field has been seeking.

The broader implications reach into several of biomedicine’s most pressing territories. Because proteostasis decline is tightly linked to aging, the identification of an epigenetic regulator that sustains the proteostasis program offers a new target for interventions aimed at preserving cellular resilience. In neurodegenerative diseases such as Alzheimer’s and Parkinson’s, where the accumulation of misfolded and aggregated proteins is a defining pathology, strategies that bolster RBBP5-dependent chromatin states could theoretically strengthen neurons’ capacity to cope with proteotoxic burdens. In metabolic disorders, where endoplasmic reticulum stress is a well-documented contributor to tissue dysfunction, the XBP1s-RBBP5 axis provides a mechanistic link between metabolic signaling and long-term chromatin competence.

The study also adds an important chapter to the biology of ultradian rhythms. The 12-hour oscillator, once regarded as a curiosity, is emerging as a physiologically meaningful clock with its own dedicated chromatin machinery. That RBBP5, a factor long appreciated simply as a core component of H3K4 methyltransferase complexes in development and gene regulation, turns out to be the linchpin of this clock illustrates how reexamining well-known molecular players in a rhythmic context can reveal entirely new functions. As the authors conclude, these findings establish RBBP5 as a central regulator of proteostasis dynamics, essential for maintaining cellular resilience, and they open a clear path for future work exploring how chromatin methylation could be therapeutically tuned to protect cells against the protein-folding failures that underlie aging and disease.

Subject of Research: Epigenetic regulation of proteostasis dynamics by the histone methyltransferase complex subunit RBBP5

Article Title: The histone methyltransferase complex subunit RBBP5 serves as a central epigenetic regulator of proteostasis dynamics

Article References: Kubra, S., Sun, M., Dion, W., Bian, Y., Catak, A., Luong, H., Wang, H., Pan, Y., Liu, J.-J., Ponna, A., Liu, L., Sipula, I., Luo, J.-H., Jurczak, M. J., Liu, S., & Zhu, B. (2026). The histone methyltransferase complex subunit RBBP5 serves as a central epigenetic regulator of proteostasis dynamics. PLOS Biology, 24(10), e3004048. https://doi.org/10.1371/journal.pbio.3004048

Image Credits: AI Generated

DOI: 10.1371/journal.pbio.3004048

Keywords: RBBP5, COMPASS complex, H3K4me3, proteostasis, XBP1s, 12-hour oscillator, ultradian rhythms, autophagy, proteotoxic stress, chromatin, SWI/SNF, Integrator complex

The global burden of gallbladder cancer is strikingly uneven. According to GLOBOCAN 2022 data cited in the review, the disease accounted for 122,491 new cases and 89,055 deaths worldwide, with Asia shouldering roughly 71 percent of incidence and 75 percent of mortality. China and India alone contribute more than a quarter and nearly a fifth of global cases, respectively. Within India, incidence in northern and northeastern states such as Bihar, Uttar Pradesh, Assam, Odisha, and West Bengal is nearly seven times higher than in the south, matching rates seen in the world’s highest-prevalence regions of South America and North Africa. Women are two to six times more susceptible than men, and risk factors including gallstones, chronic infections with Salmonella typhi and Helicobacter pylori, obesity, and exposures to aflatoxins and arsenic compound a genetic predisposition to the disease.

A central and increasingly appreciated driver of this carcinogenesis is microbial dysbiosis within the biliary ecosystem. Enrichment of bacterial taxa such as Enterobacteriaceae, Streptococcus, and Helicobacter species shifts bile acid metabolism toward secondary bile acids like deoxycholic acid, which inflict detergent-like DNA damage and activate pro-survival signaling through EGFR and Wnt/beta-catenin pathways. Bacterial genotoxins, including cytolethal distending toxin and colibactin, further destabilize the genome, and chronic inflammation driven by reactive oxygen species contributes directly to TP53 mutations, the most frequent alteration in gallbladder cancer. The review’s authors emphasize that dysbiosis is not a passive bystander but an active sculptor of the tumor microenvironment, accelerating both the generation and selection of oncogenic mutations as chronic cholecystitis progresses through dysplasia to invasive carcinoma.

To organize this complexity, the researchers mined The Cancer Genome Atlas and the COSMIC database to identify twelve recurrently mutated genes: TP53, SMAD4, PIK3CA, CDKN2A, ARID1A, ARID2, KRAS, ELF3, ERBB3, ERBB2, STK11, and CTNNB1. TP53 dominates the landscape, mutated in up to 70 percent of cases, with TCGA reporting 58.6 percent and COSMIC 44 percent. Most are missense mutations in the DNA-binding domain spanning exons 5 to 8, which not only abolish the protein’s tumor-suppressive transcriptional activity but also confer gain-of-function properties. Mutant p53 accumulates to high levels in invasive tumors while being virtually absent in normal gallbladder epithelium, marking it as an early event in malignant transformation and a potential diagnostic marker.

The remaining genes map onto a web of interconnected pathways. SMAD4, altered in roughly 12 to 21 percent of tumors, cripples TGF-beta signaling, with mutations concentrated in the MH2 domain that mediates complex formation with receptor-regulated SMADs. PIK3CA, mutated in about 7 to 10 percent of cases, encodes the p110-alpha catalytic subunit of PI3K; hotspot mutations E542K, E545K, and H1047R constitutively activate the PI3K/AKT survival axis. CDKN2A, altered in roughly 9 to 10 percent of tumors, disables two cell-cycle sentinels: p16, which blocks cyclin D-CDK4/6-driven phosphorylation of the retinoblastoma protein, and p14, which stabilizes p53 by sequestering MDM2. Chromatin remodelers ARID1A and ARID2, mutated in about 20 percent and 8 percent of tumors respectively, destabilize the SWI/SNF complex, impairing DNA damage repair and gene regulation. Notably, ARID1A deficiency in gallbladder cancer drives overexpression of PD-L1 and impairs tumor-infiltrating lymphocytes, promoting immune evasion and worse outcomes.

On the oncogene side, KRAS mutations, found in 7 to 12 percent of cases, lock the small GTPase in its active GTP-bound state, most often through codon 12 substitutions such as G12D and G12V, fueling persistent MAPK and PI3K/AKT signaling. The receptor tyrosine kinases ERBB2 and ERBB3, each mutated in 5 to 8 percent of tumors, heterodimerize to hyperactivate PI3K/AKT; experimental work has shown that ERBB2/ERBB3 mutants robustly drive gallbladder cancer progression in vivo. CTNNB1 mutations in exon 3, including S45 and S37 variants, prevent phosphorylation-dependent degradation of beta-catenin, allowing constitutive Wnt signaling. Perhaps most intriguing is ELF3, which acts as a tumor suppressor in gallbladder cancer: its loss upregulates epiregulin, activating EGFR/mTORC1 signaling and driving epithelial-to-mesenchymal transition, invasion, and advanced tumor stage.

What elevates this review beyond a catalog of mutations is its pathway-level synthesis, drawing on how the same alterations behave in better-studied cancers. Mutant p53 in breast and colon cancers suppresses onco-protective microRNAs, hyperactivates chromatin modifiers, and confers cisplatin resistance, suggesting similar mechanisms may operate in gallbladder tumors. SMAD4’s R361C mutation in colorectal cancer disrupts oligomerization and aberrantly activates Wnt/beta-catenin signaling instead of apoptosis. A protein-protein interaction network analysis using STRING and CytoHubba identified ERBB2 and KRAS as central hub genes in gallbladder cancer, and the authors point to pancreatic cancer evidence that pharmacological co-targeting of these two drivers with the ERBB2 inhibitor neratinib and the KRAS G12C inhibitor ARS-1620 produced profound growth suppression in xenograft models, a strategy they argue could be adapted for gallbladder tumors harboring co-occurring mutations.

The therapeutic implications are concrete. Current first-line treatment combines gemcitabine and cisplatin with immunotherapy: the phase 3 TOPAZ-1 and KEYNOTE-966 trials established that adding durvalumab or pembrolizumab to chemotherapy improves overall survival in biliary tract cancers. But low PD-1 or CTLA-4 expression excludes many patients from benefit, underscoring the need for molecularly matched options. The review catalogs drugs already approved or in trials for other cancers that target the same mutated proteins: p53 reactivators such as APR-246 and arsenic trioxide; PI3K-alpha inhibitors alpelisib and the mutant-selective RLY-2608; KRAS G12C inhibitors sotorasib and adagrasib alongside emerging G12D agents; CDK4/6 inhibitors for CDKN2A-deficient tumors; the EZH2 inhibitor tazemetostat for ARID1A-mutant cancers; and HER2-directed antibodies, antibody-drug conjugates, and kinase inhibitors. The ongoing SAFIR ABC-10 phase 3 trial is testing matched targeted maintenance therapy in advanced biliary tract cancers, with results pending.

The authors also envision liquid biopsy as a practical bridge to precision care. Because tissue sampling in metastatic gallbladder cancer is invasive and often limited, circulating tumor DNA and bile-derived cell-free DNA profiling could capture spatial and temporal tumor heterogeneity, detecting recurrent TP53 and KRAS mutations and actionable alterations to guide treatment selection. Real-world analyses suggest patients receiving matched targeted therapies, including PARP inhibitors for homologous recombination deficiency and ERBB2-directed agents, survive longer than those on conventional chemotherapy. Even the microbiome may become a therapeutic lever, with precision antibiotics, engineered probiotics, or phage-based antimicrobials potentially reducing genotoxic secondary bile acids and the DNA damage that fuels driver mutations.

Challenges remain substantial. Evidence on STK11 and ERBB3 mutations in gallbladder carcinoma is sparse, and the functional consequences of many alterations have been characterized only in other cancer types. The review’s authors call for multi-omics studies integrating genomics, transcriptomics, and proteomics to build comprehensive mutation-to-phenotype maps, and for rigorous preclinical validation of rational drug combinations in gallbladder cancer organoids and patient-derived xenografts. Still, the message is clear: gallbladder cancer is not a single disease but a collection of molecularly defined subtypes, and decoding the cooperative effects of co-occurring mutations may finally transform treatment from empiric cytotoxic regimens into biomarker-driven therapy tailored to each patient’s tumor, offering hope in a cancer where survival has barely improved for decades.

Subject of Research: Mutational landscape and signaling network crosstalk in gallbladder cancer for precision therapy

Article Title: Mapping Mutations and Signaling Network Interactions to Guide Precision Therapy in Gallbladder Cancer

Article References: Manav, N., Kashyap, A., Malhotra, L., Rajegowda, C., & Prasad, C. P. (2026). Mapping Mutations and Signaling Network Interactions to Guide Precision Therapy in Gallbladder Cancer. Cancer Reports, 9(10), Article e70699. https://doi.org/10.1002/cnr2.70699

Image Credits: AI Generated

DOI: 10.1002/cnr2.70699

Keywords: gallbladder cancer, TP53, KRAS, ERBB2, PIK3CA, precision oncology, mutational landscape, signaling pathways, immunotherapy, liquid biopsy, SWI/SNF, targeted therapy