RNA polymerase II senses obstruction in the DNA minor groove via a conserved sensor motif

Redox-Regulated and Guest-Driven Transformations of Aromatic Oligoamide Foldamers in Advanced Structures

In this study, we demonstrate that an aromatic oligoamide sequence assembles into a trimeric helix-turn-helix architecture with a disulfide linkage, and upon cleavage of this linkage, it reconstructs into an antiparallel double helix. The antiparallel double helix is accessible to encapsulate a diacid guest within its cavity, forming a 2:1 host-guest complex. In contrast, hydrogen-bonding interactions between the trimeric-assembled structure and guests induce a conformational shift in the trimeric helix, resulting in a cross-shaped double-helix complex at a 2:2 host-guest ratio. Interconversions between the trimeric helix and the antiparallel double helix, along with their respective host-guest complexes, can be initiated through thiol/disulfide redox-mediated regulation.

A cyclic pyrrole-imidazole polyamide reduces pathogenic RNA in CAG/CTG triplet repeat neurological disease models

Expansion of CAG and CTG (CWG) triplet repeats causes several inherited neurological diseases. The CWG repeat diseases are thought to involve complex pathogenic mechanisms through expanded CWG repeat-derived RNAs in a noncoding region and polypeptides in a coding region, respectively. However, an effective therapeutic approach has not been established for the CWG repeat diseases. Here, we show that a CWG repeat DNA-targeting compound, cyclic pyrrole-imidazole polyamide (CWG-cPIP), suppressed the pathogenesis of coding and noncoding CWG repeat diseases. CWG-cPIP bound to the hairpin form of mismatched CWG DNA, interfering with transcription elongation by RNA polymerase through a preferential activity toward repeat-expanded DNA. We found that CWG-cPIP selectively inhibited pathogenic mRNA transcripts from expanded CWG repeats, reducing CUG RNA foci and polyglutamine accumulation in cells from patients with myotonic dystrophy type 1 (DM1) and Huntington's disease (HD). Treatment with CWG-cPIP ameliorated behavioral deficits in adeno-associated virus-mediated CWG repeat-expressing mice and in a genetic mouse model of HD, without cytotoxicity or off-target effects. Together, we present a candidate compound that targets expanded CWG repeat DNA independently of its genomic location and reduces both pathogenic RNA and protein levels. CWG-cPIP may be used for the treatment of CWG repeat diseases and improvement of clinical outcomes.

Design and Synthesis of Artificial Nucleobases for Sequence-Selective DNA Recognition within the Major Groove

We present the design and synthesis of artificial specific nucleobases, each one recognizing a single base pair within the major groove of duplex DNA. Computational calculations indicate that PNAs modified with these nucleobases enable the formation of highly stable triple helices with no sequence restrictions through multiple hydrogen bonding and π⋅⋅⋅π stacking interactions, without significantly widening the DNA double helix. New synthetic routes were developed to the structures of these fused heterocycles which have rarely been described in the literature. NMR titration experiments indicate specific hydrogen bonding at the Hoogsteen sites. The new building blocks allow the construction of four PNA monomers for each canonic base pair and their covalent connection to PNA oligomers. These can be designed complementary to any given DNA sequence. With high efficiency and relative simplicity of operation, the described methodologies and strategies hence form the basis for a new supramolecular ligand system targeting double-stranded DNA without strand invasion.

Polymerases and DNA Repair in Neurons: Implications in Neuronal Survival and Neurodegenerative Diseases

Most of the neurodegenerative diseases and aging are associated with reactive oxygen species (ROS) or other intracellular damaging agents that challenge the genome integrity of the neurons. As most of the mature neurons stay in G0/G1 phase, replication-uncoupled DNA repair pathways including BER, NER, SSBR, and NHEJ, are pivotal, efficient, and economic mechanisms to maintain genomic stability without reactivating cell cycle. In these progresses, polymerases are prominent, not only because they are responsible for both sensing and repairing damages, but also for their more diversified roles depending on the cell cycle phase and damage types. In this review, we summarized recent knowledge on the structural and biochemical properties of distinct polymerases, including DNA and RNA polymerases, which are known to be expressed and active in nervous system; the biological relevance of these polymerases and their interactors with neuronal degeneration would be most graphically illustrated by the neurological abnormalities observed in patients with hereditary diseases associated with defects in DNA repair; furthermore, the vicious cycle of the trinucleotide repeat (TNR) and impaired DNA repair pathway is also discussed. Unraveling the mechanisms and contextual basis of the role of the polymerases in DNA damage response and repair will promote our understanding about how long-lived postmitotic cells cope with DNA lesions, and why disrupted DNA repair contributes to disease origin, despite the diversity of mutations in genes. This knowledge may lead to new insight into the development of targeted intervention for neurodegenerative diseases.

RNA polymerase II trapped on a molecular treadmill: Structural basis of persistent transcriptional arrest by a minor groove DNA binder

Elongating RNA polymerase II (Pol II) can be paused or arrested by a variety of obstacles. These obstacles include DNA lesions, DNA-binding proteins, and small molecules. Hairpin pyrrole-imidazole (Py-Im) polyamides bind to the minor groove of DNA in a sequence-specific manner and induce strong transcriptional arrest. Remarkably, this Py-Im-induced Pol II transcriptional arrest is persistent and cannot be rescued by transcription factor TFIIS. In contrast, TFIIS can effectively rescue the transcriptional arrest induced by a nucleosome barrier. The structural basis of Py-Im-induced transcriptional arrest and why TFIIS cannot rescue this arrest remain elusive. Here we determined the X-ray crystal structures of four distinct Pol II elongation complexes (Pol II ECs) in complex with hairpin Py-Im polyamides as well as of the hairpin Py-Im polyamides-dsDNA complex. We observed that the Py-Im oligomer directly interacts with RNA Pol II residues, introduces compression of the downstream DNA duplex, prevents Pol II forward translocation, and induces Pol II backtracking. These results, together with biochemical studies, provide structural insight into the molecular mechanism by which Py-Im blocks transcription. Our structural study reveals why TFIIS fails to promote Pol II bypass of Py-Im-induced transcriptional arrest.

Opposite roles of transcription elongation factors Spt4/5 and Elf1 in RNA polymerase II transcription through B-form versus non-B DNA structures

Transcription elongation can be affected by numerous types of obstacles, such as nucleosome, pausing sequences, DNA lesions and non-B-form DNA structures. Spt4/5 and Elf1 are conserved transcription elongation factors that promote RNA polymerase II (Pol II) bypass of nucleosome and pausing sequences. Importantly, genetic studies have shown that Spt4/5 plays essential roles in the transcription of expanded nucleotide repeat genes associated with inherited neurological diseases. Here, we investigate the function of Spt4/5 and Elf1 in the transcription elongation of CTG•CAG repeat using an in vitro reconstituted yeast transcription system. We found that Spt4/5 helps Pol II transcribe through the CTG•CAG tract duplex DNA, which is in good agreement with its canonical roles in stimulating transcription elongation. In sharp contrast, surprisingly, we revealed that Spt4/5 greatly inhibits Pol II transcriptional bypass of CTG and CAG slip-out structures. Furthermore, we demonstrated that transcription elongation factor Elf1 individually and cooperatively with Spt4/5 inhibits Pol II bypass of the slip-out structures. This study uncovers the important functional interplays between template DNA structures and the function of transcription elongation factors. This study also expands our understanding of the functions of Spt4/5 and Elf1 in transcriptional processing of trinucleotide repeat DNA.

Natural and Synthetic Oligoarylamides: Privileged Structures for Medical Applications

The term "privileged structure" refers to a single molecular substructure or scaffold that can serve as a starting point for high-affinity ligands for more than one receptor type. In this report, a hitherto overlooked group of privileged substructures is addressed, namely aromatic oligoamides, for which there are natural models in the form of cystobactamids, albicidin, distamycin A, netropsin, and others. The aromatic and heteroaromatic core, together with a flexible selection of substituents, form conformationally well-defined scaffolds capable of specifically binding to conformationally well-defined regions of biomacromolecules such as helices in proteins or DNA often by acting as helices mimics themselves. As such, these aromatic oligoamides have already been employed to inhibit protein-protein and nucleic acid-protein interactions. This article is the first to bring together the scattered knowledge about aromatic oligoamides in connection with biomedical applications.

Single position substitution of hairpin pyrrole-imidazole polyamides imparts distinct DNA-binding profiles across the human genome

Pyrrole-imidazole (Py-Im) polyamides are synthetic molecules that can be rationally designed to target specific DNA sequences to both disrupt and recruit transcriptional machinery. While in vitro binding has been extensively studied, in vivo effects are often difficult to predict using current models of DNA binding. Determining the impact of genomic architecture and the local chromatin landscape on polyamide-DNA sequence specificity remains an unresolved question that impedes their effective deployment in vivo. In this report we identified polyamide-DNA interaction sites across the entire genome, by covalently crosslinking and capturing these events in the nuclei of human LNCaP cells. This technique confirms the ability of two eight ring hairpin-polyamides, with similar architectures but differing at a single ring position (Py to Im), to retain in vitro specificities and display distinct genome-wide binding profiles.

Molecular basis of transcriptional pausing, stalling, and transcription-coupled repair initiation

Transcription elongation by RNA polymerase II (Pol II) is constantly challenged by numerous types of obstacles that lead to transcriptional pausing or stalling. These obstacles include DNA lesions, DNA epigenetic modifications, DNA binding proteins, and non-B form DNA structures. In particular, lesion-induced prolonged transcriptional blockage or stalling leads to genome instability, cellular dysfunction, and cell death. Transcription-coupled nucleotide excision repair (TC-NER) pathway is the first line of defense that detects and repairs these transcription-blocking DNA lesions. In this review, we will first summarize the recent research progress toward understanding the molecular basis of transcriptional pausing and stalling by different kinds of obstacles. We will then discuss new insights into Pol II-mediated lesion recognition and the roles of CSB in TC-NER.

Formation and Recognition of UV-Induced DNA Damage within Genome Complexity

Ultraviolet (UV) light is a natural genotoxic agent leading to the formation of photolesions endangering the genomic integrity and thereby the survival of living organisms. To prevent the mutagenetic effect of UV, several specific DNA repair mechanisms are mobilized to accurately maintain genome integrity at photodamaged sites within the complexity of genome structures. However, a fundamental gap remains to be filled in the identification and characterization of factors at the nexus of UV-induced DNA damage, DNA repair, and epigenetics. This review brings together the impact of the epigenomic context on the susceptibility of genomic regions to form photodamage and focuses on the mechanisms of photolesions recognition through the different DNA repair pathways.

Role of the trigger loop in translesion RNA synthesis by bacterial RNA polymerase

DNA lesions can severely compromise transcription and block RNA synthesis by RNA polymerase (RNAP), leading to subsequent recruitment of DNA repair factors to the stalled transcription complex. Recent structural studies have uncovered molecular interactions of several DNA lesions within the transcription elongation complex. However, little is known about the role of key elements of the RNAP active site in translesion transcription. Here, using recombinantly expressed proteins, in vitro transcription, kinetic analyses, and in vivo cell viability assays, we report that point amino acid substitutions in the trigger loop, a flexible element of the active site involved in nucleotide addition, can stimulate translesion RNA synthesis by Escherichia coli RNAP without altering the fidelity of nucleotide incorporation. We show that these substitutions also decrease transcriptional pausing and strongly affect the nucleotide addition cycle of RNAP by increasing the rate of nucleotide addition but also decreasing the rate of translocation. The secondary channel factors DksA and GreA modulated translesion transcription by RNAP, depending on changes in the trigger loop structure. We observed that although the mutant RNAPs stimulate translesion synthesis, their expression is toxic in vivo, especially under stress conditions. We conclude that the efficiency of translesion transcription can be significantly modulated by mutations affecting the conformational dynamics of the active site of RNAP, with potential effects on cellular stress responses and survival.

The Road Not Taken with Pyrrole-Imidazole Polyamides: Off-Target Effects and Genomic Binding

The high sequence specificity of minor groove-binding N-methylpyrrole-N-methylimidazole polyamides have made significant advances in cancer and disease biology, yet there have been few comprehensive reports on their off-target effects, most likely as a consequence of the lack of available tools in evaluating genomic binding, an essential aspect that has gone seriously underexplored. Compared to other N-heterocycles, the off-target effects of these polyamides and their specificity for the DNA minor groove and primary base pair recognition require the development of new analytical methods, which are missing in the field today. This review aims to highlight the current progress in deciphering the off-target effects of these N-heterocyclic molecules and suggests new ways that next-generating sequencing can be used in addressing off-target effects.

Structural basis of DNA duplex distortion induced by thiazole-containing hairpin polyamides

This manuscript reports the molecular basis for double-stranded DNA (dsDNA) binding of hairpin polyamides incorporating a 5-alkyl thiazole (Nt) unit. Hairpin polyamides containing an N-terminal Nt unit induce higher melting stabilisation of target dsDNA sequences relative to an archetypical hairpin polyamide incorporating an N-terminal imidazole (Im) unit. However, modification of the N-terminus from Im to Nt-building blocks results in an increase in dsDNA binding affinity but lower G-selectivity. A general G-selectivity trend is observed for Nt-containing polyamide analogues. G-selectivity increases as the steric bulk in the Nt 5-position increases. Solution-based NMR structural studies reveal differences in the modulation of the target DNA duplex of Nt-containing hairpin polyamides relative to the Im-containing archetype. A structural hallmark of an Nt polyamide•dsDNA complex is a more significant degree of major groove compression of the target dsDNA sequence relative to the Im-containing hairpin polyamide.

Structural and biochemical analysis of DNA lesion-induced RNA polymerase II arrest

Transcription, catalyzed by RNA polymerase II (Pol II) in eukaryotes, is the first step in gene expression. RNA Pol II is a 12-subunit enzyme complex regulated by many different transcription factors during transcription initiation, elongation, and termination. During elongation, Pol II encounters various types of obstacles that can cause transcriptional pausing and arrest. Through decades of research on transcriptional pausing, it is widely known that Pol II can distinguish between different types of obstacles by its active site. A major class of obstacles is DNA lesions. While some DNA lesions can cause transient transcriptional pausing, which can be bypassed by Pol II itself or with the help from other elongation factors, bulky DNA damage can cause prolonged transcriptional pausing and arrest, which signals for transcription coupled repair. Using biochemical and structural biology approaches, the outcomes of many different types of DNA lesions, DNA modifications, and DNA binding molecules to transcription were studied. In this mini review, we will describe the in vitro transcription assays with Pol II to investigate the impacts of various DNA lesions on transcriptional outcomes and the crystallization method of lesion-arrested Pol II complex. These methods can provide a general platform for the structural and biochemical analysis of Pol II transcriptional pausing and bypass mechanisms.

Structural basis of DNA lesion recognition for eukaryotic transcription-coupled nucleotide excision repair

Eukaryotic transcription-coupled nucleotide excision repair (TC-NER) is a pathway that removes DNA lesions capable of blocking RNA polymerase II (Pol II) transcription from the template strand. This process is initiated by lesion-arrested Pol II and the recruitment of Cockayne Syndrome B protein (CSB). In this review, we will focus on the lesion recognition steps of eukaryotic TC-NER and summarize the recent research progress toward understanding the structural basis of Pol II-mediated lesion recognition and Pol II-CSB interactions. We will discuss the roles of CSB in both TC-NER initiation and transcription elongation. Finally, we propose an updated model of tripartite lesion recognition and verification for TC-NER in which CSB ensures Pol II-mediated recognition of DNA lesions for TC-NER.

Interference with DNA repair after ionizing radiation by a pyrrole-imidazole polyamide

Pyrrole-imidazole (Py–Im) polyamides are synthetic non-genotoxic minor groove-binding small molecules. We hypothesized that Py–Im polyamides can modulate the cellular response to ionizing radiation. Pre-treatment of cells with a Py-Im polyamide prior to exposure to ionizing radiation resulted in a delay in resolution of phosphorylated γ-H2AX foci, increase in XRCC1 foci, and reduced cellular replication potential. RNA-sequencing of cell lines exposed to the polyamide showed induction of genes related to the ultraviolet radiation response. We observed that the polyamide is almost 10-fold more toxic to a cell line deficient in DNA ligase 3 as compared to the parental cell line. Alkaline single cell gel electrophoresis reveals that the polyamide induces genomic fragmentation in the ligase 3 deficient cell line but not the corresponding parental line. The polyamide interferes directly with DNA ligation in vitro. We conclude that Py-Im polyamides may be further explored as sensitizers to genotoxic therapies.

G-Quadruplex Induction by the Hairpin Pyrrole-Imidazole Polyamide Dimer

The G-quadruplex (G4) is one type of higher-order structure of nucleic acids and is thought to play important roles in various biological events such as regulation of transcription and inhibition of DNA replication. Pyrrole-imidazole polyamides (PIPs) are programmable small molecules that can sequence-specifically bind with high affinity to the minor groove of double-stranded DNA (dsDNA). Herein, we designed head-to-head hairpin PIP dimers and their target dsDNA in a model G4-forming sequence. Using an electrophoresis mobility shift assay and transcription arrest assay, we found that PIP dimers could induce the structural change to G4 DNA from dsDNA through the recognition by one PIP dimer molecule of two duplex-binding sites flanking both ends of the G4-forming sequence. This induction ability was dependent on linker length. This is the first study to induce G4 formation using PIPs, which are known to be dsDNA binders. The results reported here suggest that selective G4 induction in native sequences may be achieved with PIP dimers by applying the same design strategy.

Structural basis for the initiation of eukaryotic transcription-coupled DNA repair

Eukaryotic transcription-coupled repair (TCR), or transcription-coupled nucleotide excision repair (TC-NER), is an important and well-conserved sub-pathway of nucleotide excision repair (NER) that preferentially removes DNA lesions from the template strand blocking RNA polymerase II (Pol II) translocation^1,2. Cockayne syndrome group B protein in humans (CSB, or ERCC6), or its yeast orthologs (Rad26 in Saccharomyces cerevisiae and Rhp26 in Schizosaccharomyces pombe), is among the first proteins to be recruited to the lesion-arrested Pol II during initiation of eukaryotic TCR^1,3–10. Mutations in CSB are associated with Cockayne syndrome, an autosomal-recessive neurologic disorder characterized by progeriod features, growth failure, and photosensitivity¹. The molecular mechanism of eukaryotic TCR initiation remains elusive, with several long-standing questions unanswered: How do cells distinguish DNA lesion-arrested Pol II from other forms of arrested Pol II? How does CSB interact with the arrested Pol II complex? What is the role of CSB in TCR initiation? The lack of structures of CSB or the Pol II-CSB complex have hindered our ability to answer those questions. Here we report the first structure of S. cerevisiae Pol II-Rad26 complex solved by cryo-electron microscopy (cryo-EM). The structure reveals that Rad26 binds to the DNA upstream of Pol II where it dramatically alters its path. Our structural and functional data suggest that the conserved Swi2/Snf2-family core ATPase domain promotes forward movement of Pol II and elucidate key roles for Rad26/CSB in both TCR and transcription elongation.

Mechanism of RNA polymerase II stalling by DNA alkylation

RNA polymerase II (Pol II) catalyzes the transcription of DNA to RNA in the nucleus. DNA alkylating cancer drugs can stall transcription; however, the basis for Pol II stalling when encountering a DNA template with minor-groove alkylation adducts has remained elusive due to its inherent chemical instability. We characterized the behavior of Pol II in transcription over minor-groove alkylation adducts and uncovered a previously unobserved mode of Pol II stalling wherein clashes between DNA adducts and the mobile trigger loop of RNA Pol II prevent translocation of the enzyme after nucleotide insertion. These results provide a molecular basis for how DNA damage in transcribed portions of the genome initiates DNA repair contributing to drug resistance.

Several anticancer agents that form DNA adducts in the minor groove interfere with DNA replication and transcription to induce apoptosis. Therapeutic resistance can occur, however, when cells are proficient in the removal of drug-induced damage. Acylfulvenes are a class of experimental anticancer agents with a unique repair profile suggesting their capacity to stall RNA polymerase (Pol) II and trigger transcription-coupled nucleotide excision repair. Here we show how different forms of DNA alkylation impair transcription by RNA Pol II in cells and with the isolated enzyme and unravel a mode of RNA Pol II stalling that is due to alkylation of DNA in the minor groove. We incorporated a model for acylfulvene adducts, the stable 3-deaza-3-methoxynaphtylethyl-adenosine analog (3d-Napht-A), and smaller 3-deaza-adenosine analogs, into DNA oligonucleotides to assess RNA Pol II transcription elongation in vitro. RNA Pol II was strongly blocked by a 3d-Napht-A analog but bypassed smaller analogs. Crystal structure analysis revealed that a DNA base containing 3d-Napht-A can occupy the +1 templating position and impair closing of the trigger loop in the Pol II active center and polymerase translocation into the next template position. These results show how RNA Pol II copes with minor-groove DNA alkylation and establishes a mechanism for drug resistance.

Mechanism of DNA alkylation-induced transcriptional stalling, lesion bypass, and mutagenesis

Alkylated DNA lesions, induced by both exogenous chemical agents and endogenous metabolites, interfere with the efficiency and accuracy of DNA replication and transcription. However, the molecular mechanisms of DNA alkylation-induced transcriptional stalling and mutagenesis remain unknown. In this study, we systematically investigated how RNA polymerase II (pol II) recognizes and bypasses regioisomeric O²-, N3-, and O⁴-ethylthymidine (O²-, N3-, and O⁴-EtdT) lesions. We observed distinct pol II stalling profiles for the three regioisomeric EtdT lesions. Intriguingly, pol II stalling at O²-EtdT and N3-EtdT sites is exacerbated by TFIIS-stimulated proofreading activity. Assessment for the impact of the EtdT lesions on individual fidelity checkpoints provided further mechanistic insights, where the transcriptional lesion bypass routes for the three EtdT lesions are controlled by distinct fidelity checkpoints. The error-free transcriptional lesion bypass route is strongly favored for the minor-groove O²-EtdT lesion. In contrast, a dominant error-prone route stemming from GMP misincorporation was observed for the major-groove O⁴-EtdT lesion. For the N3-EtdT lesion that disrupts base pairing, multiple transcriptional lesion bypass routes were found. Importantly, the results from the present in vitro transcriptional studies are well correlated with in vivo transcriptional mutagenesis analysis. Finally, we identified a minor-groove-sensing motif from pol II (termed Pro-Gate loop). The Pro-Gate loop faces toward the minor groove of RNA:DNA hybrid and is involved in modulating the translocation of minor-groove alkylated DNA template after nucleotide incorporation opposite the lesion. Taken together, this work provides important mechanistic insights into transcriptional stalling, lesion bypass, and mutagenesis of alkylated DNA lesions.

A Pyrrole-Imidazole Polyamide Is Active against Enzalutamide-Resistant Prostate Cancer

The LREX' prostate cancer model is resistant to the antiandrogen enzalutamide via activation of an alternative nuclear hormone receptor, glucocorticoid receptor (GR), which has similar DNA-binding specificity to the androgen receptor (AR). Small molecules that target DNA to interfere with protein-DNA interactions may retain activity against enzalutamide-resistant prostate cancers where ligand-binding domain antagonists are ineffective. We reported previously that a pyrrole-imidazole (Py-Im) polyamide designed to bind the consensus androgen response element half-site has antitumor activity against hormone-sensitive prostate cancer. In enzalutamide-resistant LREX' cells, Py-Im polyamide interfered with both AR- and GR-driven gene expression, whereas enzalutamide interfered with only that of AR. Genomic analyses indicated immediate interference with the AR transcriptional pathway. Long-term treatment with Py-Im polyamide demonstrated a global decrease in RNA levels consistent with inhibition of transcription. The polyamide was active against two enzalutamide-resistant xenografts with minimal toxicity. Overall, our results identify Py-Im polyamide as a promising therapeutic strategy in enzalutamide-resistant prostate cancer. Cancer Res; 77(9); 2207-12. ©2017 AACR.