Next-Generation Sequencing Studies Guide the Design of Pyrrole-Imidazole Polyamides with Improved Binding Specificity by the Addition of β-Alanine

Chem-map profiles drug binding to chromatin in cells

Characterizing drug-target engagement is essential to understand how small molecules influence cellular functions. Here we present Chem-map for in situ mapping of small molecules that interact with DNA or chromatin-associated proteins, utilizing small-molecule-directed transposase Tn5 tagmentation. We demonstrate Chem-map for three distinct drug-binding modalities as follows: molecules that target a chromatin protein, a DNA secondary structure or that intercalate in DNA. We map the BET bromodomain protein-binding inhibitor JQ1 and provide interaction maps for DNA G-quadruplex structure-binding molecules PDS and PhenDC3. Moreover, we determine the binding sites of the widely used anticancer drug doxorubicin in human leukemia cells; using the Chem-map of doxorubicin in cells exposed to the histone deacetylase inhibitor tucidinostat reveals the potential clinical advantages of this combination therapy. In situ mapping with Chem-map of small-molecule interactions with DNA and chromatin proteins provides insights that will enhance understanding of genome and chromatin function and therapeutic interventions.

Strong and Specific Recognition of CAG/CTG Repeat DNA (5'-dWGCWGCW-3') by a Cyclic Pyrrole-Imidazole Polyamide

Abnormally expanded CAG/CTG repeat DNA sequences lead to a variety of neurological diseases, such as Huntington's disease. Here, we synthesized a cyclic pyrrole‐imidazole polyamide (cPIP), which can bind to the minor groove of the CAG/CTG DNA sequence. The double‐stranded DNA melting temperature (T_m) and surface plasmon resonance assays revealed the high binding affinity of the cPIP. In addition, next‐generation sequencing showed that the cPIP had high specificity for its target DNA sequence.

Targeted epigenetic modulation using a DNA-based histone deacetylase inhibitor enhances cardiomyogenesis in mouse embryonic stem cells

The epigenome has an essential role in orchestrating transcriptional activation and modulating key developmental processes. Previously, we developed a library of pyrrole-imidazole polyamides (PIPs) conjugated with suberoylanilide hydroxamic acid (SAHA), a histone deacetylase (HDAC) inhibitor, for the purpose of sequence-specific modification of epigenetics. Based on the gene expression profile of SAHA-PIPs and screening studies using the α-myosin heavy chain promoter-driven reporter and SAHA-PIP library, we identified that SAHA-PIP G activates cardiac-related genes. Studies in mouse ES cells showed that SAHA-PIP G could enhance the generation of spontaneous beating cells, which is consistent with upregulation of several cardiac-related genes. Moreover, ChIP-seq results confirmed that the upregulation of cardiac-related genes is highly correlated with epigenetic activation, relevant to the sequence-specific binding of SAHA-PIP G. This proof-of-concept study demonstrating the applicability of SAHA-PIP not only improves our understanding of epigenetic alterations involved in cardiomyogenesis but also provides a novel chemical-based strategy for stem cell differentiation.

Therapeutic gene regulation using pyrrole-imidazole polyamides

Recent innovations in cutting-edge sequencing platforms have allowed the rapid identification of genes associated with communicable, noncommunicable and rare diseases. Exploitation of this collected biological information has facilitated the development of nonviral gene therapy strategies and the design of several proteins capable of editing specific DNA sequences for disease control. Small molecule-based targeted therapeutic approaches have gained increasing attention because of their suggested clinical benefits, ease of control and lower costs. Pyrrole-imidazole polyamides (PIPs) are a major class of DNA minor groove-binding small molecules that can be predesigned to recognize specific DNA sequences. This programmability of PIPs allows the on-demand design of artificial genetic switches and fluorescent probes. In this review, we detail the progress in the development of PIP-based designer ligands and their prospects as advanced DNA-based small-molecule drugs for therapeutic gene modulation.

Estimating genome-wide off-target effects for pyrrole-imidazole polyamide binding by a pathway-based expression profiling approach

In the search for new pharmaceutical leads, especially with DNA-binding molecules or genome editing methods, the issue of side and off-target effects have always been thorny in nature. A particular case is the investigation into the off-target effects of N-methylpyrrole-N-methylimidazole polyamides, a naturally inspired class of DNA binders with strong affinity to the minor-groove and sequence specificity, but at < 20 bases, their relatively short motifs also insinuate the possibility of non-unique genomic binding. Binding at non-intended loci potentially lead to the rise of off-target effects, issues that very few approaches are able to address to-date. We here report an analytical method to infer off-target binding, via expression profiling, based on probing the relative impact to various biochemical pathways; we also proposed an accompanying side effect prediction engine for the systematic screening of candidate polyamides. This method marks the first attempt in PI polyamide research to identify elements in biochemical pathways that are sensitive to the treatment of a candidate polyamide as an approach to infer possible off-target effects. Expression changes were then considered to assess possible outward phenotypic changes, manifested as side effects, should the same PI polyamide candidate be administered clinically. We validated some of these effects with a series of animal experiments, and found agreeable corroboration in certain side effects, such as changes in aspartate transaminase levels in ICR and nude mice post-administration.

Sequence-specific DNA binding Pyrrole-imidazole polyamides and their applications

Pyrrole-imidazole polyamides (Py-Im polyamides) are cell-permeable compounds that bind to the minor groove of double-stranded DNA in a sequence-specific manner without causing denaturation of the DNA. These compounds can be used to control gene expression and to stain specific sequences in cells. Here, we review the history, structural variations, and functional investigations of Py-Im polyamides.

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.

A synthetic DNA-binding inhibitor of SOX2 guides human induced pluripotent stem cells to differentiate into mesoderm

Targeted differentiation of human induced pluripotent stem cells (hiPSCs) using only chemicals would have value-added clinical potential in the regeneration of complex cell types including cardiomyocytes. Despite the availability of several chemical inhibitors targeting proteins involved in signaling pathways, no bioactive synthetic DNA-binding inhibitors, targeting key cell fate-controlling genes such as SOX2, are yet available. Here, we demonstrate a novel DNA-based chemical approach to guide the differentiation of hiPSCs using pyrrole-imidazole polyamides (PIPs), which are sequence-selective DNA-binding synthetic molecules. Harnessing knowledge about key transcriptional changes during the induction of cardiomyocyte, we developed a DNA-binding inhibitor termed PIP-S2, targeting the 5'-CTTTGTT-3' and demonstrated that inhibition of SOX2-DNA interaction by PIP-S2 triggers the mesoderm induction in hiPSCs. Genome-wide gene expression analyses revealed that PIP-S2 induced mesoderm by targeted alterations in SOX2-associated gene regulatory networks. Also, employment of PIP-S2 along with a Wnt/β-catenin inhibitor successfully generated spontaneously contracting cardiomyocytes, validating our concept that DNA-binding inhibitors could drive the directed differentiation of hiPSCs. Because PIPs can be fine-tuned to target specific DNA sequences, our DNA-based approach could be expanded to target and regulate key transcription factors specifically associated with desired cell types.

Synthetic genome readers target clustered binding sites across diverse chromatin states

Targeting the genome with sequence-specific DNA-binding molecules is a major goal at the interface of chemistry, biology, and precision medicine. Polyamides, composed of N-methylpyrrole and N-methylimidazole monomers, are a class of synthetic molecules that can be rationally designed to "read" specific DNA sequences. However, the impact of different chromatin states on polyamide binding in live cells remains an unresolved question that impedes their deployment in vivo. Here, we use cross-linking of small molecules to isolate chromatin coupled to sequencing to map the binding of two bioactive and structurally distinct polyamides to genomes directly within live H1 human embryonic stem cells. This genome-wide view from live cells reveals that polyamide-based synthetic genome readers bind cognate sites that span a range of binding affinities. Polyamides can access cognate sites within repressive heterochromatin. The occupancy patterns suggest that polyamides could be harnessed to target loci within regions of the genome that are inaccessible to other DNA-targeting molecules.

Identification of Binding Targets of a Pyrrole-Imidazole Polyamide KR12 in the LS180 Colorectal Cancer Genome

Pyrrole-imidazole polyamides are versatile DNA minor groove binders and attractive therapeutic options against oncological targets, especially upon functionalization with an alkylating agent such as seco-CBI. These molecules also provide an alternative for oncogenes deemed "undruggable" at the protein level, where the absence of solvent-accessible pockets or structural crevices prevent the formation of protein-inhibitor ligands; nevertheless, the genome-wide effect of pyrrole-imidazole polyamide binding remain largely unclear to-date. Here we propose a next-generation sequencing-based workflow combined with whole genome expression arrays to address such issue using a candidate anti-cancer alkylating agent, KR12, against codon 12 mutant KRAS. Biotinylating KR12 enables the means to identify its genome-wide effects in living cells and possible biological implications via a coupled workflow of enrichment-based sequencing and expression microarrays. The subsequent computational pathway and expression analyses allow the identification of its genomic binding sites, as well as a route to explore a polyamide's possible genome-wide effects. Among the 3,343 KR12 binding sites identified in the human LS180 colorectal cancer genome, the reduction of KR12-bound gene expressions was also observed. Additionally, the coupled microarray-sequencing analysis also revealed some insights about the effect of local chromatin structure on pyrrole-imidazole polyamide, which had not been fully understood to-date. A comparative analysis with KR12 in a different human colorectal cancer genome SW480 also showed agreeable agreements of KR12 binding affecting gene expressions. Combination of these analyses thus suggested the possibility of applying this approach to other pyrrole-imidazole polyamides to reveal further biological details about the effect of polyamide binding in a genome.

Sequence-specific DNA binding by long hairpin pyrrole-imidazole polyamides containing an 8-amino-3,6-dioxaoctanoic acid unit

With the aim of improving aqueous solubility, we designed and synthesized five N-methylpyrrole (Py)-N-methylimidazole (Im) polyamides capable of recognizing 9-bp sequences. Their DNA-binding affinities and sequence specificities were evaluated by SPR and Bind-n-Seq analyses. The design of polyamide 1 was based on a conventional model, with three consecutive Py or Im rings separated by a β-alanine to match the curvature and twist of long DNA helices. Polyamides 2 and 3 contained an 8-amino-3,6-dioxaoctanoic acid (AO) unit, which has previously only been used as a linker within linear Py-Im polyamides or between Py-Im hairpin motifs for tandem hairpin. It is demonstrated herein that AO also functions as a linker element that can extend to 2-bp in hairpin motifs. Notably, although the AO-containing unit can fail to bind the expected sequence, polyamide 4, which has two AO units facing each other in a hairpin form, successfully showed the expected motif and a KD value of 16nM was recorded. Polyamide 5, containing a β-alanine-β-alanine unit instead of the AO of polyamide 2, was synthesized for comparison. The aqueous solubilities and nuclear localization of three of the polyamides were also examined. The results suggest the possibility of applying the AO unit in the core of Py-Im polyamide compounds.

Deciphering the genomic targets of alkylating polyamide conjugates using high-throughput sequencing

Chemically engineered small molecules targeting specific genomic sequences play an important role in drug development research. Pyrrole-imidazole polyamides (PIPs) are a group of molecules that can bind to the DNA minor-groove and can be engineered to target specific sequences. Their biological effects rely primarily on their selective DNA binding. However, the binding mechanism of PIPs at the chromatinized genome level is poorly understood. Herein, we report a method using high-throughput sequencing to identify the DNA-alkylating sites of PIP-indole-seco-CBI conjugates. High-throughput sequencing analysis of conjugate 2: showed highly similar DNA-alkylating sites on synthetic oligos (histone-free DNA) and on human genomes (chromatinized DNA context). To our knowledge, this is the first report identifying alkylation sites across genomic DNA by alkylating PIP conjugates using high-throughput sequencing.

A Synthetic DNA-Binding Domain Guides Distinct Chromatin-Modifying Small Molecules to Activate an Identical Gene Network

Synthetic dual-function ligands targeting specific DNA sequences and histone-modifying enzymes were applied to achieve regulatory control over multi-gene networks in living cells. Unlike the broad array of targeting small molecules for histone deacetylases (HDACs), few modulators are known for histone acetyltransferases (HATs), which play a central role in transcriptional control. As a novel chemical approach to induce selective HAT-regulated genes, we conjugated a DNA-binding domain (DBD) "I" to N-(4-chloro-3-trifluoromethyl-phenyl)-2-ethoxy-benzamide (CTB), an artificial HAT activator. In vitro enzyme activity assays and microarray studies were used to demonstrate that distinct functional small molecules could be transformed to have identical bioactivity when conjugated with a targeting DBD. This proof-of-concept synthetic strategy validates the switchable functions of HDACs and HATs in gene regulation and provides a molecular basis for developing versatile bioactive ligands.