RNA utilizes three-dimensional structure in addition to sequence to carry out diverse functions in gene expression and disease. Much like well-folded proteins, RNAs adopt specific three-dimensional structures to carry out their function. Yet comparatively few RNA structures have been solved by atomic resolution structural techniques, in part because unlike structured proteins, RNAs fold into heterogeneous ensembles of interconverting structures that pose a challenge for high-resolution structure probing methods. In this work, we introduce RNAnneal as a method for RNA structural ensemble prediction that seamlessly integrates generative deep learning with statistical physics and molecular dynamics modeling. Given the primary sequence, RNAnneal uses ab inito (i.e., first principles) modeling to sample an ensemble of 3D structures, which, in turn, are used to train an ensemble of unsupervised deep learning models. The RNAnneal score, representing the consensus of the deep learning models, is then used to evaluate the 3D structures. We evaluated RNAnneal structures against 16 experimentally-resolved conformations (ERCs) of riboswitch RNAs and found that pseudoknot-free (PK-free) ERCs were well-reproduced by RNAnneal, with clear avenues for improving performance even on PK-comprising structures. Furthermore, we found that the RNAnneal score outperforms the Rosetta score and a state-of-the-art RNA forcefield on the task of classifying ERCs from decoys. We then introduce the interaction entropy as a measure of conformational heterogeneity within an ensemble and use it to assess our predictions. RNAnneal thus provides a generalizable framework for predicting RNA structural ensembles that will accelerate RNA-targeted drug discovery and the design of functional RNA molecules.

The targeted degradation of RNA, particularly long noncoding RNAs (lncRNAs), holds immense potential for therapeutic interventions in diseases associated with aberrant RNA regulation. Here, we introduce a novel Proximity-Induced Nucleic Acid Degrader (PINAD-1), a first-in-class small molecule that selectively induces the degradation of MALAT1, a lncRNA implicated in the regulation of metastatic processes. PINAD-1 is designed by conjugating a binder specific for the triple helix structure of MALAT1 to an imidazole-based RNA-degrading warhead, enabling specific cleavage of the MALAT1 transcript in vitro and in cellulo, with minimal off-target effects on the structurally similar NEAT1 lncRNA. Through mechanistic studies, we show that effective RNA degradation is not solely dependent on proximity but requires a precise structural context, as demonstrated by the differential activity of PINAD-1 compared to the structurally analogous but functionally inert conjugate PINAD-2. Our findings underscore the importance of binder-induced destabilization and RNA geometry in facilitating RNA degradation. This work lays the foundation for the design of bifunctional small-molecule RNA degraders as powerful tools for the modulation of structured noncoding RNAs, offering potential applications in RNA-based therapeutics.

Targeting RNA with small molecules offers a strategy to modulate gene expression at undruggable targets. Traditional screens favor thermodynamically stable, low-entropy RNA motifs with defined conformations, yet these provide limited energetic leverage for functional modulation. Many RNAs instead sample dynamic structural ensembles that small molecules can repartition. Using group I self-splicing introns as a model, we identified the antineoplastic drug Mitoxantrone as a competitive inhibitor of RNA self-splicing (IC50 = 4.3 μM) that stabilizes the native conformation of the T4 td intron. Structure-activity analysis showed that the anthraquinone scaffold alone is insufficient, and basic amine-containing side chains are required for RNA structural modulation. Transcriptome-wide chemical probing in human cells revealed preferential binding to GC-rich structured regions, although only a subset showed structural change. Furthermore, global analysis of 5' UTR ensembles showed altered structural heterogeneity and translation, demonstrating functional repartitioning of RNA conformational landscapes.

Abstract The chromosome 5p15.33 region, which encodes telomerase reverse transcriptase (TERT), harbors multiple germline variants identified by genome-wide association studies (GWAS) as risk for some cancers but protective for others. Here, we characterize a variable number tandem repeat within TERT intron 6, VNTR6-1 (38-bp repeat unit), and detect a strong link between VNTR6-1 alleles (Short: 24-27 repeats, Long: 40.5-66.5 repeats) and GWAS signals rs2242652 and rs10069690 within TERT intron 4. Bioinformatics analyses reveal that rs10069690-T allele increases intron 4 retention while VNTR6-1-Long allele expands a polymorphic G-quadruplex (G4, 35-113 copies) within intron 6, with both variants contributing to variable TERT expression through alternative splicing and nonsense-mediated decay. In two cell lines, CRISPR/Cas9 deletion of VNTR6-1 increases the ratio of TERT-full-length (FL) to the alternative TERT-β isoform, promoting apoptosis and reducing cell proliferation. In contrast, treatment with G4-stabilizing ligands shifts splicing from TERT-FL to TERT-β isoform, implicating VNTR6-1 as a splicing switch. We associate the functional variants VNTR6-1, rs10069690, and their haplotypes with multi-cancer risk and age-related telomere shortening. By regulating TERT splicing, these variants may contribute to fine-tuning cellular longevity and replicative potential in the context of stress due to tissue-specific endogenous and exogenous exposures, thereby influencing the cancer risk conferred by this locus.

Topoisomerase poisons are clinically used anticancer drugs that can induce DNA cleavage complexes to block replication. TOP3B is the only topoisomerase that can catalyze topological changes on either DNA or RNA and induce cleavage complexes on both nucleic acids. We proposed that TOP3B poisons may inhibit coronavirus RNA genome replication and tested this hypothesis by using mouse hepatitis coronavirus (MHV). We found that one of the two types of reported TOP3B poisons, thiacyanine dyes, possess potent inhibitory activities for MHV. Interestingly, the antiviral activity of the thiacyanine dyes is unaltered in Top3b-KO cells, suggesting that these dyes inhibit viral replication independent of TOP3B. Subsequent screening revealed that multiple members of the thiacyanine dye family have antiviral activity comparable to or stronger than remdesivir, the U.S. Food and Drug Administration (FDA)-approved drug for coronavirus, in an MHV-infected cell line model. One thiacyanine dye (NSC93472) significantly inhibits MHV replication in mouse lungs, showing its potential as an anticoronavirus drug. Mechanistic studies showed that NSC93472 preferentially binds two RNA fragments derived from SARS-CoV-2 genome over random ssRNA, interferes with assembly of an elongation-competent complex between the viral RNA-dependent RNA polymerase and the RNA template, and inhibits the RNA synthesis mediated by the RNA-dependent RNA polymerase. Moreover, NSC93472 can inhibit RNA synthesis by the reverse transcriptase of Moloney murine leukemia virus. Our studies demonstrate that thiacyanine dyes represent a new family of coronavirus inhibitors and suggest that TOP3B poisons and anti-RNA virus drugs share common characteristics in RNA binding and inhibition of enzymatic reactions on RNA.

The challenge of targeting RNA with small molecules necessitates a better understanding of RNA-ligand interaction mechanisms. However, the dynamic nature of nucleic acids, their ligand-induced stabilization, and how conformational changes influence gene expression pose significant difficulties for experimental investigation. This work employs a combination of computational and experimental methods to address these challenges. By integrating structure-informed design, crystallography, and machine learning-augmented all-atom molecular dynamics simulations (MD) we synthesized, biophysically and biochemically characterized, and studied the dissociation of a library of small molecule activators of the ZTP riboswitch, a ligand-binding RNA motif that regulates bacterial gene expression. We uncovered key interaction mechanisms, revealing valuable insights into the role of ligand binding kinetics on riboswitch activation. Further, we established that ligand on-rates determine activation potency as opposed to binding affinity and elucidated RNA structural differences, which provide mechanistic insights into the interplay of RNA structure on riboswitch activation.

Chemical probes that control the function of complex RNA molecules offer unique opportunities to interrogate biological systems. In this study, we demonstrate that a small molecule ligand selectively recognizes and undergoes traceless, reversible photocrosslinking to PreQ1 RNA aptamers. This effect is selective and dependent on both the chemical structure and RNA sequence/structure. A homogeneously modified, caged mRNA construct containing a PreQ1 aptamer and an eGFP or wild type p53 coding sequence displayed repressed translation in vitro or in cells until irradiated with 302 nm light, resulting in cleavage of the photocage and restoration of translation. This method demonstrates for the first time that aptamer‐based molecular recognition of a small molecule ligand can be used to precisely and photochemically activate the translation of a complex mRNA in cells.

Crosslinking mass spectrometry (MS) is a powerful approach for probing protein structures. However, most widely used crosslinkers rely on N-hydroxysuccinimide (NHS) esters, restricting reactivity primarily to lysine residues and protein N-termini, and rendering them incompatible with many amine-containing buffers (e.g., Tris) and key biochemical cofactors (e.g., ATP). To address these limitations, we introduce two novel vinyl-sulfone-based crosslinkers. Alkyne-BVSC is an enrichable, homobifunctional crosslinker featuring an acid-cleavable alkyne handle for downstream peptide enrichment. VSD is a heterobifunctional crosslinker combining a vinyl sulfone with a diazirine moiety for UV-activated photo-crosslinking. Both reagents are synthetically accessible from inexpensive precursors and retain reactivity in amine-rich biochemical environments. We show that vinyl sulfones react with cysteine, histidine, and lysine residues, thereby expanding crosslinkable residues beyond those accessible to NHS-esters. Moreover, we develop a stub-based post-search filtering strategy that leverages the MS-cleavable nature of vinyl sulfone linkages to improve crosslink identification sensitivity. Together, these advances establish vinyl-sulfone-based crosslinkers as versatile and complementary tools for structural proteomics.

The spliceosome is a large ribonucleoprotein complex that regulates pre-mRNA splicing and has been an intriguing target for drug discovery. Essential to the assembly of the spliceosome are the small nuclear RNAs (snRNAs), which form RNA-RNA and RNA-protein interactions in the intact spliceosome and during its assembly. Here, we study the yeast U4/U6 snRNA assembly and report the rapid discovery of small molecule K-turn ligands via parallel small molecule microarray (SMM) screening of multiple related RNA constructs. For hit validation, biophysical analyses were conducted to confirm the binding and effects on thermodynamic stability and of RNA structure. One analog of the hit molecule (22) exhibited improved affinity towards the yeast U4/U6 snRNA (KD = 3.9 ± 2.2 μM). The specific interaction between 22 and the K-turn region was studied experimentally using deltaSHAPE and in silico with MD simulations. Moreover, this molecule was found to inhibit the binding of the U4 to Snu13 in biochemical assays (IC50 = 3.2 ± 0.4 μM). This work reports new ligands for the U4 snRNA and reveals that a multiplexed, structure-based approach can be used to identify small molecules that bind to specific regions of complex RNAs.

Huntington disease (HD) is caused by inherited CAG expansions, which continue expanding somatically in affected brain regions to hasten disease onset and progression. Therapeutically diminishing somatic expansions is expected to be clinically beneficial. However, it is not known if interventionally modifying somatic CAG expansions will actually modify in vivo clinically-relevant phenotypes, what the therapeutic window is, or which phenotypes will be altered. Here we show that acute (6-week) delivery of the contraction-inducing slipped-CAG DNA ligand naphthyridine-azaquinolone to young (4-week-old) (CAG)120 HD mice, induces contractions throughout brain regions, improves motor function (locomotion, balance, coordination, muscle strength), molecular disease landmarks (mHTT aggregates, nuclear envelope morphology, nucleocytoplasmic mRNA transport, transcriptomic dysregulation, neuroinflammation), and neurodegeneration. Beneficial effects of modifying somatic expansions were also evident in muscle and blood, where blood CAG instability correlated with brain instability and blood serum had diminished levels of neurofilament light (a biomarker for neurodegeneration) - offering blood as having elements of target engagement and efficacy. These data support that targeting somatic repeat expansions can be a rapid disease-modifying therapeutic avenue for HD and possibly other repeat expansion diseases. Our findings support an etiologic pathway interconnected to somatic CAG expansions that will inform the design of clinical trials expecting clinical benefit by modulating somatic expansions.

Despite interest in developing therapeutics that leverage binding pockets in structured RNAs-whose dysregulation leads to diseases-such drug discovery efforts are limited. Here, we have used a small molecule microarray (SMM) screen to find inhibitors of a large ribozyme: the Methanobrevibacter smithii RNase P RNA (Msm RPR, ∼300 nt). The ribonucleoprotein form of RNase P, which catalyzes the 5'-maturation of precursor tRNAs, is a suitable drug target as it is essential, structurally diverse across life domains, and present in low copy. From an SMM screen of 7,300 compounds followed by selectivity profiling, we identified 48 hits that bound specifically to the Msm RPR-the catalytic subunit in Msm (archaeal) RNase P. When we tested these hits in precursor-tRNA cleavage assays, we discovered that the drug-like M1, a diaryl-piperidine, inhibits Msm RPR (KI, 17 ± 1 μM) but not a structurally related archaeal RPR, and binds to Msm RPR with a KD(app) of 8 ± 3 μM. Structure-activity relationship analyses performed with synthesized analogs pinpointed groups in M1 that are important for its ability to inhibit Msm RPR. Overall, the SMM method offers prospects for advancing RNA druggability by identifying new privileged scaffolds/chemotypes that bind large, structured RNAs.

The development of selective ligands to target DNA G-quadruplexes (G4s) and i-motifs (iMs) has revealed their relevance in transcriptional regulation. However, most of these ligands are unable to target individual G4s or iMs in the genome, limiting their scope. Herein, we describe an Approach to Target Exact Nucleic Acid alternative structures (ATENA) that relies on the chemical conjugation of established G4 and iM ligands to a catalytically inactive Cas9 protein (dCas9), enabling their individual targeting in living cells. ATENA demonstrates that the selective targeting of the G4 present in the oncogene c-MYC leads to the suppression of transcripts regulated exclusively by one of its promoters (P1). Conversely, targeting the c-MYC iMs on the opposite strand leads to the selective increase of P1-driven transcripts. ATENA reveals that G4-mediated transcriptional responses are highly ligand-specific, with different ligands eliciting markedly different effects at the same G4 site. We further demonstrate that the basal expression levels of the gene targeted can be used to predict the transcriptional impact associated with G4-stabilization. Our study provides a platform for investigating G4- and iM-biology with high precision, unveiling the therapeutic relevance of individual DNA structures with selectivity. Targeting individual DNA secondary structures in the genome with small molecules is challenging. Here, the authors develop ATENA, a CRISPR-based platform for targeting specific DNA structures in cells with high precision, helping to elucidate their biological roles and guide therapeutic design.

Chimeric kinase J-PKAcα represents a potential therapeutic target for fibrolamellar hepatocellular carcinoma (FLHCC). Structure-based design and screening were applied to improve the potency of the marine-derived kinase inhibitor aplithianine A targeting J-PKAcα. Three classes of aplithianines (I, II, and III) including >150 analogs were synthesized, significantly improving biochemical IC50 values to the low nanomolar range. X-ray diffraction experiments confirmed that the class II aplithianines adopted a novel binding mode to J-PKAcα by interacting with the DFG residue Asp239. The kinase selectivity profiles were assessed by kinome profiling. In vitro profiles of selected class II analogs were evaluated to determine compound solubility, protein binding, permeability, metabolism, and hERG binding. Selected aplithianine analogs inhibited intracellular phosphorylation of the peptide substrate CREB following stimulation of the J-PKAcα fusion kinase in NIH/3T3 cells and exhibited antiproliferative/cytotoxic activities against select cancer cell lines from the NCI-60 cell panel at nanomolar concentrations.

RNA-based fluorogenic aptamers, such as Mango, are uniquely powerful tools for imaging RNA that activate the fluorescence of a weakly or non-fluorescent small molecule when bound. A central challenge has been to develop brighter, more specific and high-affinity aptamer–ligand systems for cellular imaging. Here we report an ultrabright fluorophore for the Mango II system discovered using a structure-informed, fragment-based small-molecule microarray approach. This dye—termed SALAD1 (structure-informed, array-enabled LigAnD 1)—exhibits subnanomolar aptamer affinity and 3.5-fold brighter fluorescence than Mango II-TO1–biotin pair, a widely used fluorogenic system. Performance was improved by modulating RNA-dye molecular recognition without altering the fluorophore’s π-system. High-resolution X-ray structures reveal the binding mode for SALAD1, which exhibits improved pocket occupancy, a more defined binding pose and a unique bonding interaction with potassium. SALAD1 is cell-permeable and facilitates improved in-cell confocal RNA imaging. This work introduces an additional RNA-activated fluorophore demonstrating how fragment-based ligand discovery can be used to create high-performance ligands for RNA targets. Fluorogenic RNA aptamers such as Mango are powerful tools for RNA imaging. An ultrabright RNA-activated fluorophore with improved photophysical performance has now been developed using a structure-informed, fragment-based screening approach. This system was studied using high-resolution crystallography and applied to image RNA in cells.

RNA sequences encode structures that impact protein production and other cellular processes. Misfolded RNAs can also potentiate disease, but a complete picture is lacking. To establish more comprehensive and accurate RNA structure-function relationships, new methods are needed to interrogate RNA in native environments. Existing tools rely primarily on electrophiles that are constitutively "on" or triggered by UV light, often resulting in high background. Here we describe an alternative, chemically triggered approach to cross-link RNAs using bioorthogonal cyclopropenones (CpOs). These reagents selectively react with phosphines to provide ketenes-electrophiles that can trap neighboring nucleophiles to forge covalent cross-links. As a proof-of-concept, we conjugated a CpO motif to thiazole orange (TO-1). TO-1-CpO bound selectively to a model RNA aptamer (Mango) with nanomolar affinity, as confirmed by fluorescence turn-on. After phosphine administration, covalent cross-links were formed between the CpO and RNA. Cross-linking was both time and dose dependent. We further applied the chemically triggered tools to model RNAs under biologically relevant conditions. Collectively, this work expands the toolkit of probes for studying RNA and its native conformations.

The development of selective ligands to target DNA G-quadruplexes (G4s) has been pivotal in revealing their role in transcriptional regulation. However, most of the ligands described to date lack intra-G4 selectivity, severely limiting their potential for uncovering the biological function of individual G4s across the genome. To overcome these limitations, we developed ATENA (Approach to Target Exact Nucleic Acid alternative structures). ATENA relies on the chemical modification of established G4-ligands to enable their conjugation onto a catalytically inactive Cas9 protein (dCas9) using HaloTag, allowing for the targeting of individual G4s in living cells. We have systematically screened the length of the PEG-linkers connecting the G4-ligands to the HaloTag and sgRNA sequences to attain optimal G4 engagement both in vitro and in cells. Using optimized conditions, we leveraged ATENA to demonstrate how the selective targeting of the well-studied G4 in the promoter of the oncogene c-MYC suppresses its transcription exclusively from the P1 promoter. We also show that positioning ligands in the proximity of regulatory elements suppresses c-MYC transcription in a G4-independent manner, highlighting the importance of appropriate design to measure genuine G4-mediated transcriptional changes. We also demonstrate that selective targeting of a G4 in the PVT1 promoter can either stimulate or repress its transcription depending on the type of G4-ligand used, indicating that functional responses associated with G4-stabilization can highly depend on the type of ligand used. We further harnessed ATENA to study transcriptional perturbation associated with cell-specific G4s, revealing that the functional responses associated with these structures are tightly linked with the expression levels of the targeted gene. Our study provides critical insights into G4-based therapeutic design, offering an innovative platform to investigate G4 biology with high precision. ### Competing Interest Statement Marco Di Antonio is a co-funder of Project Maestro.

Riboswitches are structured RNA elements that regulate gene expression upon binding to small molecule ligands. Understanding the mechanisms by which small molecules impact riboswitch activity is key to developing potent, selective ligands for these and other RNA targets. We report the structure-informed design of chemically diverse synthetic ligands for PreQ 1 riboswitches. Multiple X-ray co-crystal structures of synthetic ligands with the Thermoanaerobacter tengcongensis ( Tte )-PreQ 1 riboswitch confirm a common binding site with the cognate ligand, despite considerable chemical differences among the ligands. Structure probing assays demonstrate that one ligand causes conformational changes similar to PreQ 1 in six structurally and mechanistically diverse PreQ 1 riboswitch aptamers. Single-molecule force spectroscopy is used to demonstrate differential modes of riboswitch stabilization by the ligands. Binding of the natural ligand brings about the formation of a persistent, folded pseudoknot structure, whereas a synthetic ligand decreases the rate of unfolding through a kinetic mechanism. Single round transcription termination assays show the biochemical activity of the ligands, while a GFP reporter system reveals compound activity in regulating gene expression in live cells without toxicity. Taken together, this study reveals that diverse small molecules can impact gene expression in live cells by altering conformational changes in RNA structures through distinct mechanisms.

Fusion-positive rhabdomyosarcoma (FP-RMS) is an aggressive pediatric sarcoma driven primarily by the PAX3-FOXO1 fusion oncogene, for which therapies targeting PAX3-FOXO1 are lacking. Here, we screen 62,643 compounds using an engineered cell line that monitors PAX3-FOXO1 transcriptional activity identifying a hitherto uncharacterized compound, P3FI-63. RNA-seq, ATAC-seq, and docking analyses implicate histone lysine demethylases (KDMs) as its targets. Enzymatic assays confirm the inhibition of multiple KDMs with the highest selectivity for KDM3B. Structural similarity search of P3FI-63 identifies P3FI-90 with improved solubility and potency. Biophysical binding of P3FI-90 to KDM3B is demonstrated using NMR and SPR. P3FI-90 suppresses the growth of FP-RMS in vitro and in vivo through downregulating PAX3-FOXO1 activity, and combined knockdown of KDM3B and KDM1A phenocopies P3FI-90 effects. Thus, we report KDM inhibitors P3FI-63 and P3FI-90 with the highest specificity for KDM3B. Their potent suppression of PAX3-FOXO1 activity indicates a possible therapeutic approach for FP-RMS and other transcriptionally addicted cancers.

Phase transitions are ubiquitous across life, yet hard to quantify and describe accurately. In this work, we develop an approach for characterizing generic attributes of phase transitions from very limited observations made deep within different phases' domains of stability. Our approach is called Thermodynamic Maps, which combines statistical mechanics and molecular simulations with score-based generative models. Thermodynamic Maps enable learning the temperature dependence of arbitrary thermodynamic observables across a wide range of temperatures. We show its usefulness by calculating phase transition attributes such as melting temperature, temperature-dependent heat capacities, and critical exponents. For instance, we demonstrate the ability of thermodynamic maps to infer the ferromagnetic phase transition of the Ising model, including temperature-dependent heat capacity and critical exponents, despite never having seen samples from the transition region. In addition, we efficiently characterize the temperature-dependent conformational ensemble and compute melting curves of the two RNA systems GCAA tetraloop and HIV-TAR, which are notoriously hard to sample due to glassy-like landscapes.