Nucleic Acids Research

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Accurate detection for a wide range of mutation and editing sites of microRNAs from small RNA high-throughput sequencing profiles

Fri, 2016-08-19 06:31

Various types of mutation and editing (M/E) events in microRNAs (miRNAs) can change the stabilities of pre-miRNAs and/or complementarities between miRNAs and their targets. Small RNA (sRNA) high-throughput sequencing (HTS) profiles can contain many mutated and edited miRNAs. Systematic detection of miRNA mutation and editing sites from the huge volume of sRNA HTS profiles is computationally difficult, as high sensitivity and low false positive rate (FPR) are both required. We propose a novel method (named MiRME) for an accurate and fast detection of miRNA M/E sites using a progressive sequence alignment approach which refines sensitivity and improves FPR step-by-step. From 70 sRNA HTS profiles with over 1.3 billion reads, MiRME has detected thousands of statistically significant M/E sites, including 3'-editing sites, 57 A-to-I editing sites (of which 32 are novel), as well as some putative non-canonical editing sites. We demonstrated that a few non-canonical editing sites were not resulted from mutations in genome by integrating the analysis of genome HTS profiles of two human cell lines, suggesting the existence of new editing types to further diversify the functions of miRNAs. Compared with six existing studies or methods, MiRME has shown much superior performance for the identification and visualization of the M/E sites of miRNAs from the ever-increasing sRNA HTS profiles.

CasHRA (Cas9-facilitated Homologous Recombination Assembly) method of constructing megabase-sized DNA

Fri, 2016-08-19 06:31

Current DNA assembly methods for preparing highly purified linear subassemblies require complex and time-consuming in vitro manipulations that hinder their ability to construct megabase-sized DNAs (e.g. synthetic genomes). We have developed a new method designated ‘CasHRA (Cas9-facilitated Homologous Recombination Assembly)’ that directly uses large circular DNAs in a one-step in vivo assembly process. The large circular DNAs are co-introduced into Saccharomyces cerevisiae by protoplast fusion, and they are cleaved by RNA-guided Cas9 nuclease to release the linear DNA segments for subsequent assembly by the endogenous homologous recombination system. The CasHRA method allows efficient assembly of multiple large DNA segments in vivo; thus, this approach should be useful in the last stage of genome construction. As a proof of concept, we combined CasHRA with an upstream assembly method (Gibson procedure of genome assembly) and successfully constructed a 1.03 Mb MGE-syn1.0 (Minimal Genome of Escherichia coli) that contained 449 essential genes and 267 important growth genes. We expect that CasHRA will be widely used in megabase-sized genome constructions.

Transcriptional gene silencing in humans

Fri, 2016-08-19 06:31

It has been over a decade since the first observation that small non-coding RNAs can functionally modulate epigenetic states in human cells to achieve functional transcriptional gene silencing (TGS). TGS is mechanistically distinct from the RNA interference (RNAi) gene-silencing pathway. TGS can result in long-term stable epigenetic modifications to gene expression that can be passed on to daughter cells during cell division, whereas RNAi does not. Early studies of TGS have been largely overlooked, overshadowed by subsequent discoveries of small RNA-directed post-TGS and RNAi. A reappraisal of early work has been brought about by recent findings in human cells where endogenous long non-coding RNAs function to regulate the epigenome. There are distinct and common overlaps between the proteins involved in small and long non-coding RNA transcriptional regulatory mechanisms, suggesting that the early studies using small non-coding RNAs to modulate transcription were making use of a previously unrecognized endogenous mechanism of RNA-directed gene regulation. Here we review how non-coding RNA plays a role in regulation of transcription and epigenetic gene silencing in human cells by revisiting these earlier studies and the mechanistic insights gained to date. We also provide a list of mammalian genes that have been shown to be transcriptionally regulated by non-coding RNAs. Lastly, we explore how TGS may serve as the basis for development of future therapeutic agents.

The delivery of therapeutic oligonucleotides

Fri, 2016-08-19 06:31

The oligonucleotide therapeutics field has seen remarkable progress over the last few years with the approval of the first antisense drug and with promising developments in late stage clinical trials using siRNA or splice switching oligonucleotides. However, effective delivery of oligonucleotides to their intracellular sites of action remains a major issue. This review will describe the biological basis of oligonucleotide delivery including the nature of various tissue barriers and the mechanisms of cellular uptake and intracellular trafficking of oligonucleotides. It will then examine a variety of current approaches for enhancing the delivery of oligonucleotides. This includes molecular scale targeted ligand-oligonucleotide conjugates, lipid- and polymer-based nanoparticles, antibody conjugates and small molecules that improve oligonucleotide delivery. The merits and liabilities of these approaches will be discussed in the context of the underlying basic biology.

Splice-switching antisense oligonucleotides as therapeutic drugs

Fri, 2016-08-19 06:31

Splice-switching oligonucleotides (SSOs) are short, synthetic, antisense, modified nucleic acids that base-pair with a pre-mRNA and disrupt the normal splicing repertoire of the transcript by blocking the RNA–RNA base-pairing or protein–RNA binding interactions that occur between components of the splicing machinery and the pre-mRNA. Splicing of pre-mRNA is required for the proper expression of the vast majority of protein-coding genes, and thus, targeting the process offers a means to manipulate protein production from a gene. Splicing modulation is particularly valuable in cases of disease caused by mutations that lead to disruption of normal splicing or when interfering with the normal splicing process of a gene transcript may be therapeutic. SSOs offer an effective and specific way to target and alter splicing in a therapeutic manner. Here, we discuss the different approaches used to target and alter pre-mRNA splicing with SSOs. We detail the modifications to the nucleic acids that make them promising therapeutics and discuss the challenges to creating effective SSO drugs. We highlight the development of SSOs designed to treat Duchenne muscular dystrophy and spinal muscular atrophy, which are currently being tested in clinical trials.

The use of an artificial nucleotide for polymerase-based recognition of carcinogenic O6-alkylguanine DNA adducts

Fri, 2016-08-19 06:31

Enzymatic approaches for locating alkylation adducts at single-base resolution in DNA could enable new technologies for understanding carcinogenesis and supporting personalized chemotherapy. Artificial nucleotides that specifically pair with alkylated bases offer a possible strategy for recognition and amplification of adducted DNA, and adduct-templated incorporation of an artificial nucleotide has been demonstrated for a model DNA adduct O6-benzylguanine by a DNA polymerase. In this study, DNA adducts of biological relevance, O6-methylguanine (O6-MeG) and O6-carboxymethylguanine (O6-CMG), were characterized to be effective templates for the incorporation of benzimidazole-derived 2'-deoxynucleoside-5'-O-triphosphates (BenziTP and BIMTP) by an engineered KlenTaq DNA polymerase. The enzyme catalyzed specific incorporation of the artificial nucleotide Benzi opposite adducts, with up to 150-fold higher catalytic efficiency for O6-MeG over guanine in the template. Furthermore, addition of artificial nucleotide Benzi was required for full-length DNA synthesis during bypass of O6-CMG. Selective incorporation of the artificial nucleotide opposite an O6-alkylguanine DNA adduct was verified using a novel 2',3'-dideoxy derivative of BenziTP. The strategy was used to recognize adducts in the presence of excess unmodified DNA. The specific processing of BenziTP opposite biologically relevant O6-alkylguanine adducts is characterized herein as a basis for potential future DNA adduct sequencing technologies.

Mechanical properties of DNA origami nanoassemblies are determined by Holliday junction mechanophores

Fri, 2016-08-19 06:31

DNA nanoassemblies have demonstrated wide applications in various fields including nanomaterials, drug delivery and biosensing. In DNA origami, single-stranded DNA template is shaped into desired nanostructure by DNA staples that form Holliday junctions with the template. Limited by current methodologies, however, mechanical properties of DNA origami structures have not been adequately characterized, which hinders further applications of these materials. Using laser tweezers, here, we have described two mechanical properties of DNA nanoassemblies represented by DNA nanotubes, DNA nanopyramids and DNA nanotiles. First, mechanical stability of DNA origami structures is determined by the effective density of Holliday junctions along a particular stress direction. Second, mechanical isomerization observed between two conformations of DNA nanotubes at 10–35 pN has been ascribed to the collective actions of individual Holliday junctions, which are only possible in DNA origami with rotational symmetric arrangements of Holliday junctions, such as those in DNA nanotubes. Our results indicate that Holliday junctions control mechanical behaviors of DNA nanoassemblies. Therefore, they can be considered as ‘mechanophores’ that sustain mechanical properties of origami nanoassemblies. The mechanical properties observed here provide insights for designing better DNA nanostructures. In addition, the unprecedented mechanical isomerization process brings new strategies for the development of nano-sensors and actuators.

Novel small molecules potentiate premature termination codon readthrough by aminoglycosides

Fri, 2016-08-19 06:31

Nonsense mutations introduce premature termination codons and underlie 11% of genetic disease cases. High concentrations of aminoglycosides can restore gene function by eliciting premature termination codon readthrough but with low efficiency. Using a high-throughput screen, we identified compounds that potentiate readthrough by aminoglycosides at multiple nonsense alleles in yeast. Chemical optimization generated phthalimide derivative CDX5-1 with activity in human cells. Alone, CDX5-1 did not induce readthrough or increase TP53 mRNA levels in HDQ-P1 cancer cells with a homozygous TP53 nonsense mutation. However, in combination with aminoglycoside G418, it enhanced readthrough up to 180-fold over G418 alone. The combination also increased readthrough at all three nonsense codons in cancer cells with other TP53 nonsense mutations, as well as in cells from rare genetic disease patients with nonsense mutations in the CLN2, SMARCAL1 and DMD genes. These findings open up the possibility of treating patients across a spectrum of genetic diseases caused by nonsense mutations.

Conformational selection and dynamic adaptation upon linker histone binding to the nucleosome

Fri, 2016-08-19 06:31

Linker histones are essential for DNA compaction in chromatin. They bind to nucleosomes in a 1:1 ratio forming chromatosomes. Alternative configurations have been proposed in which the globular domain of the linker histone H5 (gH5) is positioned either on- or off-dyad between the nucleosomal and linker DNAs. However, the dynamic pathways of chromatosome assembly remain elusive. Here, we studied the conformational plasticity of gH5 in unbound and off-dyad nucleosome-bound forms with classical and accelerated molecular dynamics simulations. We find that the unbound gH5 converts between open and closed conformations, preferring the closed form. However, the open gH5 contributes to a more rigid chromatosome and restricts the motion of the nearby linker DNA through hydrophobic interactions with thymidines. Moreover, the closed gH5 opens and reorients in accelerated simulations of the chromatosome. Brownian dynamics simulations of chromatosome assembly, accounting for a range of amplitudes of nucleosome opening and different nucleosome DNA sequences, support the existence of both on- and off-dyad binding modes of gH5 and reveal alternative, sequence and conformation-dependent chromatosome configurations. Taken together, these findings suggest that the conformational dynamics of linker histones and nucleosomes facilitate alternative chromatosome configurations through an interplay between induced fit and conformational selection.

NCBI prokaryotic genome annotation pipeline

Fri, 2016-08-19 06:31

Recent technological advances have opened unprecedented opportunities for large-scale sequencing and analysis of populations of pathogenic species in disease outbreaks, as well as for large-scale diversity studies aimed at expanding our knowledge across the whole domain of prokaryotes. To meet the challenge of timely interpretation of structure, function and meaning of this vast genetic information, a comprehensive approach to automatic genome annotation is critically needed. In collaboration with Georgia Tech, NCBI has developed a new approach to genome annotation that combines alignment based methods with methods of predicting protein-coding and RNA genes and other functional elements directly from sequence. A new gene finding tool, GeneMarkS+, uses the combined evidence of protein and RNA placement by homology as an initial map of annotation to generate and modify ab initio gene predictions across the whole genome. Thus, the new NCBI's Prokaryotic Genome Annotation Pipeline (PGAP) relies more on sequence similarity when confident comparative data are available, while it relies more on statistical predictions in the absence of external evidence. The pipeline provides a framework for generation and analysis of annotation on the full breadth of prokaryotic taxonomy. For additional information on PGAP see https://www.ncbi.nlm.nih.gov/genome/annotation_prok/ and the NCBI Handbook, https://www.ncbi.nlm.nih.gov/books/NBK174280/.

The role of repressor kinetics in relief of transcriptional interference between convergent promoters

Fri, 2016-08-19 06:31

Transcriptional interference (TI), where transcription from a promoter is inhibited by the activity of other promoters in its vicinity on the same DNA, enables transcription factors to regulate a target promoter indirectly, inducing or relieving TI by controlling the interfering promoter. For convergent promoters, stochastic simulations indicate that relief of TI can be inhibited if the repressor at the interfering promoter has slow binding kinetics, making it either sensitive to frequent dislodgement by elongating RNA polymerases (RNAPs) from the target promoter, or able to be a strong roadblock to these RNAPs. In vivo measurements of relief of TI by CI or Cro repressors in the bacteriophage PR–PRE system show strong relief of TI and a lack of dislodgement and roadblocking effects, indicative of rapid CI and Cro binding kinetics. However, repression of the same promoter by a catalytically dead CRISPR Cas9 protein gave either compromised or no relief of TI depending on the orientation at which it binds DNA, consistent with dCas9 being a slow kinetics repressor. This analysis shows how the intrinsic properties of a repressor can be evolutionarily tuned to set the magnitude of relief of TI.

Comprehensive analysis of high-throughput screens with HiTSeekR

Fri, 2016-08-19 06:31

High-throughput screening (HTS) is an indispensable tool for drug (target) discovery that currently lacks user-friendly software tools for the robust identification of putative hits from HTS experiments and for the interpretation of these findings in the context of systems biology. We developed HiTSeekR as a one-stop solution for chemical compound screens, siRNA knock-down and CRISPR/Cas9 knock-out screens, as well as microRNA inhibitor and -mimics screens. We chose three use cases that demonstrate the potential of HiTSeekR to fully exploit HTS screening data in quite heterogeneous contexts to generate novel hypotheses for follow-up experiments: (i) a genome-wide RNAi screen to uncover modulators of TNFα, (ii) a combined siRNA and miRNA mimics screen on vorinostat resistance and (iii) a small compound screen on KRAS synthetic lethality. HiTSeekR is publicly available at http://hitseekr.compbio.sdu.dk. It is the first approach to close the gap between raw data processing, network enrichment and wet lab target generation for various HTS screen types.

Identification of consensus binding sites clarifies FMRP binding determinants

Fri, 2016-08-19 06:31

Fragile X mental retardation protein (FMRP) is a multifunctional RNA-binding protein with crucial roles in neuronal development and function. Efforts aimed at elucidating how FMRP target mRNAs are selected have produced divergent sets of target mRNA and putative FMRP-bound motifs, and a clear understanding of FMRP's binding determinants has been lacking. To clarify FMRP's binding to its target mRNAs, we produced a shared dataset of FMRP consensus binding sequences (FCBS), which were reproducibly identified in two published FMRP CLIP sequencing datasets. This comparative dataset revealed that of the various sequence and structural motifs that have been proposed to specify FMRP binding, the short sequence motifs TGGA and GAC were corroborated, and a novel TAY motif was identified. In addition, the distribution of the FCBS set demonstrates that FMRP preferentially binds to the coding region of its targets but also revealed binding along 3' UTRs in a subset of target mRNAs. Beyond probing these putative motifs, the FCBS dataset of reproducibly identified FMRP binding sites is a valuable tool for investigating FMRP targets and function.

The MazF-regulon: a toolbox for the post-transcriptional stress response in Escherichia coli

Fri, 2016-08-19 06:31

Flexible adaptation to environmental stress is vital for bacteria. An energy-efficient post-transcriptional stress response mechanism in Escherichia coli is governed by the toxin MazF. After stress-induced activation the endoribonuclease MazF processes a distinct subset of transcripts as well as the 16S ribosomal RNA in the context of mature ribosomes. As these ‘stress-ribosomes’ are specific for the MazF-processed mRNAs, the translational program is changed. To identify this ‘MazF-regulon’ we employed Poly-seq (polysome fractionation coupled with RNA-seq analysis) and analyzed alterations introduced into the transcriptome and translatome after mazF overexpression. Unexpectedly, our results reveal that the corresponding protein products are involved in all cellular processes and do not particularly contribute to the general stress response. Moreover, our findings suggest that translational reprogramming serves as a fast-track reaction to harsh stress and highlight the so far underestimated significance of selective translation as a global regulatory mechanism in gene expression. Considering the reported implication of toxin-antitoxin (TA) systems in persistence, our results indicate that MazF acts as a prime effector during harsh stress that potentially introduces translational heterogeneity within a bacterial population thereby stimulating persister cell formation.

Transcription facilitates sister chromatid cohesion on chromosomal arms

Fri, 2016-08-19 06:31

Cohesin is a multi-subunit protein complex essential for sister chromatid cohesion, gene expression and DNA damage repair. Although structurally well studied, the underlying determinant of cohesion establishment on chromosomal arms remains enigmatic. Here, we show two populations of functionally distinct cohesin on chromosomal arms using a combination of genomics and single-locus specific DNA-FISH analysis. Chromatin bound cohesin at the loading sites co-localizes with Pds5 and Eso1 resulting in stable cohesion. In contrast, cohesin independent of its loader is unable to maintain cohesion and associates with chromatin in a dynamic manner. Cohesive sites coincide with highly expressed genes and transcription inhibition leads to destabilization of cohesin on chromatin. Furthermore, induction of transcription results in de novo recruitment of cohesive cohesin. Our data suggest that transcription facilitates cohesin loading onto chromosomal arms and is a key determinant of cohesive sites in fission yeast.

Protection of CpG islands from DNA methylation is DNA-encoded and evolutionarily conserved

Fri, 2016-08-19 06:31

DNA methylation is a repressive epigenetic modification that covers vertebrate genomes. Regions known as CpG islands (CGIs), which are refractory to DNA methylation, are often associated with gene promoters and play central roles in gene regulation. Yet how CGIs in their normal genomic context evade the DNA methylation machinery and whether these mechanisms are evolutionarily conserved remains enigmatic. To address these fundamental questions we exploited a transchromosomic animal model and genomic approaches to understand how the hypomethylated state is formed in vivo and to discover whether mechanisms governing CGI formation are evolutionarily conserved. Strikingly, insertion of a human chromosome into mouse revealed that promoter-associated CGIs are refractory to DNA methylation regardless of host species, demonstrating that DNA sequence plays a central role in specifying the hypomethylated state through evolutionarily conserved mechanisms. In contrast, elements distal to gene promoters exhibited more variable methylation between host species, uncovering a widespread dependence on nucleotide frequency and occupancy of DNA-binding transcription factors in shaping the DNA methylation landscape away from gene promoters. This was exemplified by young CpG rich lineage-restricted repeat sequences that evaded DNA methylation in the absence of co-evolved mechanisms targeting methylation to these sequences, and species specific DNA binding events that protected against DNA methylation in CpG poor regions. Finally, transplantation of mouse chromosomal fragments into the evolutionarily distant zebrafish uncovered the existence of a mechanistically conserved and DNA-encoded logic which shapes CGI formation across vertebrate species.

Transcript degradation and noise of small RNA-controlled genes in a switch activated network in Escherichia coli

Fri, 2016-08-19 06:31

Post-transcriptional regulatory processes may change transcript levels and affect cell-to-cell variability or noise. We study small-RNA downregulation to elucidate its effects on noise in the iron homeostasis network of Escherichia coli. In this network, the small-RNA RyhB undergoes stoichiometric degradation with the transcripts of target genes in response to iron stress. Using single-molecule fluorescence in situ hybridization, we measured transcript numbers of the RyhB-regulated genes sodB and fumA in individual cells as a function of iron deprivation. We observed a monotonic increase of noise with iron stress but no evidence of theoretically predicted, enhanced stoichiometric fluctuations in transcript numbers, nor of bistable behavior in transcript distributions. Direct detection of RyhB in individual cells shows that its noise is much smaller than that of these two targets, when RyhB production is significant. A generalized two-state model of bursty transcription that neglects RyhB fluctuations describes quantitatively the dependence of noise and transcript distributions on iron deprivation, enabling extraction of in vivo RyhB-mediated transcript degradation rates. The transcripts’ threshold-linear behavior indicates that the effective in vivo interaction strength between RyhB and its two target transcripts is comparable. Strikingly, the bacterial cell response exhibits Fur-dependent, switch-like activation instead of a graded response to iron deprivation.

Jointly characterizing epigenetic dynamics across multiple human cell types

Fri, 2016-08-19 06:31

Advanced sequencing technologies have generated a plethora of data for many chromatin marks in multiple tissues and cell types, yet there is lack of a generalized tool for optimal utility of those data. A major challenge is to quantitatively model the epigenetic dynamics across both the genome and many cell types for understanding their impacts on differential gene regulation and disease. We introduce IDEAS, an integrative and discriminative epigenome annotation system, for jointly characterizing epigenetic landscapes in many cell types and detecting differential regulatory regions. A key distinction between our method and existing state-of-the-art algorithms is that IDEAS integrates epigenomes of many cell types simultaneously in a way that preserves the position-dependent and cell type-specific information at fine scales, thereby greatly improving segmentation accuracy and producing comparable annotations across cell types.

A universal transcription pause sequence is an element of initiation factor {sigma}70-dependent pausing

Fri, 2016-08-19 06:31

The Escherichia coli 70 initiation factor is required for a post-initiation, promoter-proximal pause essential for regulation of lambdoid phage late gene expression; potentially, 70 acts at other sites during transcription elongation as well. The pause is induced by 70 binding to a repeat of the promoter –10 sequence. After 70 binding, further RNA synthesis occurs as DNA is drawn (or ‘scrunched’) into the enzyme complex, presumably exactly as occurs during initial synthesis from the promoter; this synthesis then pauses at a defined site several nucleotides downstream from the active center position when 70 first engages the –10 sequence repeat. We show that the actual pause site in the stabilized, scrunched complex is the ‘elemental pause sequence’ recognized from its frequent occurrence in the E. coli genome. 70 binding and the elemental pause sequence together, but neither alone, produce a substantial transcription pause.

ISL1 and JMJD3 synergistically control cardiac differentiation of embryonic stem cells

Fri, 2016-08-19 06:31

ISL1 is expressed in cardiac progenitor cells and plays critical roles in cardiac lineage differentiation and heart development. Cardiac progenitor cells hold great potential for clinical and translational applications. However, the mechanisms underlying ISL1 function in cardiac progenitor cells have not been fully elucidated. Here we uncover a hierarchical role of ISL1 in cardiac progenitor cells, showing that ISL1 directly regulates hundreds of potential downstream target genes that are implicated in cardiac differentiation, through an epigenetic mechanism. Specifically, ISL1 promotes the demethylation of tri-methylation of histone H3K27 (H3K27me3) at the enhancers of key downstream target genes, including Myocd and Mef2c, which are core cardiac transcription factors. ISL1 physically interacts with JMJD3, a H3K27me3 demethylase, and conditional depletion of JMJD3 leads to impaired cardiac progenitor cell differentiation, phenocopying that of ISL1 depletion. Interestingly, ISL1 is not only responsible for the recruitment of JMJD3 to specific target loci during cardiac progenitor differentiation, but also modulates its demethylase activity. In conclusion, ISL1 and JMJD3 partner to alter the cardiac epigenome, instructing gene expression changes that drive cardiac differentiation.