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1.  Retention and Loss of RNA Interference Pathways in Trypanosomatid Protozoans 
PLoS Pathogens  2010;6(10):e1001161.
RNA interference (RNAi) pathways are widespread in metaozoans but the genes required show variable occurrence or activity in eukaryotic microbes, including many pathogens. While some Leishmania lack RNAi activity and Argonaute or Dicer genes, we show that Leishmania braziliensis and other species within the Leishmania subgenus Viannia elaborate active RNAi machinery. Strong attenuation of expression from a variety of reporter and endogenous genes was seen. As expected, RNAi knockdowns of the sole Argonaute gene implicated this protein in RNAi. The potential for functional genetics was established by testing RNAi knockdown lines lacking the paraflagellar rod, a key component of the parasite flagellum. This sets the stage for the systematic manipulation of gene expression through RNAi in these predominantly diploid asexual organisms, and may also allow selective RNAi-based chemotherapy. Functional evolutionary surveys of RNAi genes established that RNAi activity was lost after the separation of the Leishmania subgenus Viannia from the remaining Leishmania species, a divergence associated with profound changes in the parasite infectious cycle and virulence. The genus Leishmania therefore offers an accessible system for testing hypothesis about forces that may select for the loss of RNAi during evolution, such as invasion by viruses, changes in genome plasticity mediated by transposable elements and gene amplification (including those mediating drug resistance), and/or alterations in parasite virulence.
Author Summary
RNAi interference pathways play fundamental roles in eukaryotes and provide important methods for the analysis of gene function. Occasionally RNAi has been lost, precluding its use as a tool, as well as raising the question of what forces could lead to loss of such a key pathway. Genomic and functional studies previously showed that within trypanosomatids protozoans RNAi was absent in both Leishmania major and Trypanosoma cruzi. The genome of L. braziliensis, a member of the early diverging Leishmania subgenus Viannia, retained key genes required for RNAi such as an Argonaute. We demonstrated that in fact L. braziliensis shows strong RNAi activity with reporter and endogenous genes affecting flagellar function. These data suggest that RNAi may be productively applied for functional genomic studies in L. braziliensis. We mapped the evolutionary point at which RNAi was lost in lineage leading to Leishmania and Crithidia, and establish that RNAi must have been lost at least twice in the trypanosomatids, once on the lineage leading to T. cruzi and independently following the divergence of the Viannia subgenus from other Leishmania species. Lastly, we discuss hypotheses concerning the forces leading to the loss of RNAi in Leishmania evolution, including viral invasion, increased genome plasticity, and altered virulence.
doi:10.1371/journal.ppat.1001161
PMCID: PMC2965760  PMID: 21060810
2.  RNA Interference in Schistosoma mansoni Schistosomula: Selectivity, Sensitivity and Operation for Larger-Scale Screening 
Background
The possible emergence of resistance to the only available drug for schistosomiasis spurs drug discovery that has been recently incentivized by the availability of improved transcriptome and genome sequence information. Transient RNAi has emerged as a straightforward and important technique to interrogate that information through decreased or loss of gene function and identify potential drug targets. To date, RNAi studies in schistosome stages infecting humans have focused on single (or up to 3) genes of interest. Therefore, in the context of standardizing larger RNAi screens, data are limited on the extent of possible off-targeting effects, gene-to-gene variability in RNAi efficiency and the operational capabilities and limits of RNAi.
Methodology/Principal Findings
We investigated in vitro the sensitivity and selectivity of RNAi using double-stranded (ds)RNA (approximately 500 bp) designed to target 11 Schistosoma mansoni genes that are expressed in different tissues; the gut, tegument and otherwise. Among the genes investigated were 5 that had been previously predicted to be essential for parasite survival. We employed mechanically transformed schistosomula that are relevant to parasitism in humans, amenable to screen automation and easier to obtain in greater numbers than adult parasites. The operational parameters investigated included defined culture media for optimal parasite maintenance, transfection strategy, time- and dose- dependency of RNAi, and dosing limits. Of 7 defined culture media tested, Basch Medium 169 was optimal for parasite maintenance. RNAi was best achieved by co-incubating parasites and dsRNA (standardized to 30 µg/ml for 6 days); electroporation provided no added benefit. RNAi, including interference of more than one transcript, was selective to the gene target(s) within the pools of transcripts representative of each tissue. Concentrations of dsRNA above 90 µg/ml were directly toxic. RNAi efficiency was transcript-dependent (from 40 to >75% knockdown relative to controls) and this may have contributed to the lack of obvious phenotypes observed, even after prolonged incubations of 3 weeks. Within minutes of their mechanical preparation from cercariae, schistosomula accumulated fluorescent macromolecules in the gut indicating that the gut is an important route through which RNAi is expedited in the developing parasite.
Conclusions
Transient RNAi operates gene-selectively in S. mansoni newly transformed schistosomula yet the sensitivity of individual gene targets varies. These findings and the operational parameters defined will facilitate larger RNAi screens.
Author Summary
RNA interference (RNAi) is a technique to selectively suppress mRNA of individual genes and, consequently, their cognate proteins. RNAi using double-stranded (ds) RNA has been used to interrogate the function of mainly single genes in the flatworm, Schistosoma mansoni, one of a number of schistosome species causing schistosomiasis. In consideration of large-scale screens to identify candidate drug targets, we examined the selectivity and sensitivity (the degree of suppression) of RNAi for 11 genes produced in different tissues of the parasite: the gut, tegument (surface) and otherwise. We used the schistosomulum stage prepared from infective cercariae larvae which are accessible in large numbers and adaptable to automated screening platforms. We found that RNAi suppresses transcripts selectively, however, the sensitivity of suppression varies (40%–>75%). No obvious changes in the parasite occurred post-RNAi, including after targeting the mRNA of genes that had been computationally predicted to be essential for survival. Additionally, we defined operational parameters to facilitate large-scale RNAi, including choice of culture medium, transfection strategy to deliver dsRNA, dose- and time-dependency, and dosing limits. Finally, using fluorescent probes, we show that the developing gut allows rapid entrance of dsRNA into the parasite to initiate RNAi.
doi:10.1371/journal.pntd.0000850
PMCID: PMC2957409  PMID: 20976050
3.  RNAi mechanisms and applications 
BioTechniques  2008;44(5):613-616.
Within the past two decades we have become increasingly aware of the roles that RNAs play in regulation of gene expression. The RNA world was given a booster shot with the discovery of RNA interference (RNAi), a compendium of mechanisms involving small RNAs (less than 30 bases long) that regulate the expression of genes in a variety of eukaryotic organisms. Rapid progress in our understanding of RNAi-based mechanisms has led to applications of this powerful process in studies of gene function as well as in therapeutic applications for the treatment of disease. RNAi-based therapies involve two-dimensional drug designs using only identification of good Watson-Crick base pairing between the RNAi guide strand and the target, thereby resulting in rapid design and testing of RNAi triggers. To date there are several clinical trials using RNAi, and we should expect the list of new applications to grow at a phenomenal rate. This article summarizes our current knowledge about the mechanisms and applications of RNAi.
doi:10.2144/000112792
PMCID: PMC2727154  PMID: 18474035
4.  The miR-35-41 Family of MicroRNAs Regulates RNAi Sensitivity in Caenorhabditis elegans 
PLoS Genetics  2012;8(3):e1002536.
RNA interference (RNAi) utilizes small interfering RNAs (siRNAs) to direct silencing of specific genes through transcriptional and post-transcriptional mechanisms. The siRNA guides can originate from exogenous (exo–RNAi) or natural endogenous (endo–RNAi) sources of double-stranded RNA (dsRNA). In Caenorhabditis elegans, inactivation of genes that function in the endo–RNAi pathway can result in enhanced silencing of genes targeted by siRNAs from exogenous sources, indicating cross-regulation between the pathways. Here we show that members of another small RNA pathway, the mir-35-41 cluster of microRNAs (miRNAs) can regulate RNAi. In worms lacking miR-35-41, there is reduced expression of lin-35/Rb, the C. elegans homolog of the tumor suppressor Retinoblastoma gene, previously shown to regulate RNAi responsiveness. Genome-wide microarray analyses show that targets of endo–siRNAs are up-regulated in mir-35-41 mutants, a phenotype also displayed by lin-35/Rb mutants. Furthermore, overexpression of lin-35/Rb specifically rescues the RNAi hypersensitivity of mir-35-41 mutants. Although the mir-35-41 miRNAs appear to be exclusively expressed in germline and embryos, their effect on RNAi sensitivity is transmitted to multiple tissues and stages of development. Additionally, we demonstrate that maternal contribution of miR-35-41 or lin-35/Rb is sufficient to reduce RNAi effectiveness in progeny worms. Our results reveal that miRNAs can broadly regulate other small RNA pathways and, thus, have far reaching effects on gene expression beyond directly targeting specific mRNAs.
Author Summary
RNA interference (RNAi) has become a widely used approach for silencing genes of interest. This tool is possible because endogenous RNA silencing pathways exist broadly across organisms, including humans, worms, and plants. The general RNAi pathway utilizes small ∼21-nucleotide RNAs to target specific protein-coding genes through base-pairing interactions. Since RNAs from exogenous sources require some of the same factors as endogenous small RNAs to silence gene expression, there can be competition between the pathways. Thus, perturbations in the endogenous RNAi pathway can result in enhanced silencing efficiency by exogenous small RNAs. MicroRNAs (miRNAs) comprise another endogenous small RNA pathway, but their biogenesis and mechanism of gene silencing are distinct in many ways from RNAi pathways. Here we show that a family of miRNAs regulates the effectiveness of RNAi in Caenorhabditis elegans. Loss of mir-35-41 results in enhanced RNAi by exogenous RNAs and reduced silencing of endogenous RNAi targets. The embryonic miR-35-41 miRNAs regulate the sensitivity to RNAi through lin-35/Rb, a homolog of the human Retinoblastoma tumor suppressor gene previously shown to regulate RNAi effectiveness in C. elegans. Additionally, we show that this sensitivity can be passed on to the next generation of worms, demonstrating a far-reaching effect of the miR-35-41 miRNAs on gene regulation by other small RNA pathways.
doi:10.1371/journal.pgen.1002536
PMCID: PMC3297572  PMID: 22412382
5.  Selective gene silencing by viral delivery of short hairpin RNA 
Virology Journal  2010;7:248.
RNA interference (RNAi) technology has not only become a powerful tool for functional genomics, but also allows rapid drug target discovery and in vitro validation of these targets in cell culture. Furthermore, RNAi represents a promising novel therapeutic option for treating human diseases, in particular cancer. Selective gene silencing by RNAi can be achieved essentially by two nucleic acid based methods: i) cytoplasmic delivery of short double-stranded (ds) interfering RNA oligonucleotides (siRNA), where the gene silencing effect is only transient in nature, and possibly not suitable for all applications; or ii) nuclear delivery of gene expression cassettes that express short hairpin RNA (shRNA), which are processed like endogenous interfering RNA and lead to stable gene down-regulation. Both processes involve the use of nucleic acid based drugs, which are highly charged and do not cross cell membranes by free diffusion. Therefore, in vivo delivery of RNAi therapeutics must use technology that enables the RNAi therapeutic to traverse biological membrane barriers in vivo. Viruses and the vectors derived from them carry out precisely this task and have become a major delivery system for shRNA. Here, we summarize and compare different currently used viral delivery systems, give examples of in vivo applications, and indicate trends for new developments, such as replicating viruses for shRNA delivery to cancer cells.
doi:10.1186/1743-422X-7-248
PMCID: PMC2949849  PMID: 20858246
6.  Nanoparticle-Based Delivery of RNAi Therapeutics: Progress and Challenges 
RNA interference (RNAi) is an evolutionarily conserved, endogenous process for post-transcriptional regulation of gene expression. Although RNAi therapeutics have recently progressed through the pipeline toward clinical trials, the application of these as ideal, clinical therapeutics requires the development of safe and effective delivery systems. Inspired by the immense progress with nanotechnology in drug delivery, efforts have been dedicated to the development of nanoparticle-based RNAi delivery systems. For example, a precisely engineered, multifunctional nanocarrier with combined passive and active targeting capabilities may address the delivery challenges for the widespread use of RNAi as a therapy. Therefore, in this review, we introduce the major hurdles in achieving efficient RNAi delivery and discuss the current advances in applying nanotechnology-based delivery systems to overcome the delivery hurdles of RNAi therapeutics. In particular, some representative examples of nanoparticle-based delivery formulations for targeted RNAi therapeutics are highlighted.
doi:10.3390/ph6010085
PMCID: PMC3649006  PMID: 23667320
small interfering (si)RNA; non-viral vector; multifunctional nanoparticle; targeting delivery; passive and active targeting
7.  Nanoparticle-Based Delivery of RNAi Therapeutics: Progress and Challenges 
Pharmaceuticals  2013;6(1):85-107.
RNA interference (RNAi) is an evolutionarily conserved, endogenous process for post-transcriptional regulation of gene expression. Although RNAi therapeutics have recently progressed through the pipeline toward clinical trials, the application of these as ideal, clinical therapeutics requires the development of safe and effective delivery systems. Inspired by the immense progress with nanotechnology in drug delivery, efforts have been dedicated to the development of nanoparticle-based RNAi delivery systems. For example, a precisely engineered, multifunctional nanocarrier with combined passive and active targeting capabilities may address the delivery challenges for the widespread use of RNAi as a therapy. Therefore, in this review, we introduce the major hurdles in achieving efficient RNAi delivery and discuss the current advances in applying nanotechnology-based delivery systems to overcome the delivery hurdles of RNAi therapeutics. In particular, some representative examples of nanoparticle-based delivery formulations for targeted RNAi therapeutics are highlighted.
doi:10.3390/ph6010085
PMCID: PMC3649006  PMID: 23667320
small interfering (si)RNA; non-viral vector; multifunctional nanoparticle; targeting delivery; passive and active targeting
8.  Multimodality Imaging of RNA Interference 
Current medicinal chemistry  2013;20(29):3664-3675.
The discovery of small interfering RNAs (siRNAs) and their potential to knock down virtually any gene of interest has ushered in a new era of RNA interference (RNAi). Clinical use of RNAi faces severe limitations due to inefficiency delivery of siRNA or short hairpin RNA (shRNA). Many molecular imaging techniques have been adopted in RNAi-related research for evaluation of siRNA/shRNA delivery, biodistribution, pharmacokinetics, and the therapeutic effect. In this review article, we summarize the current status of in vivo imaging of RNAi. The molecular imaging techniques that have been employed include bioluminescence/fluorescence imaging, magnetic resonance imaging/spectroscopy, positron emission tomography, single-photon emission computed tomography, and various combinations of these techniques. Further development of non-invasive imaging strategies for RNAi, not only focusing on the delivery of siRNA/shRNA but also the therapeutic efficacy, is critical for future clinical translation. Rigorous validation will be needed to confirm that biodistribution of the carrier is correlated with that of siRNA/shRNA, since imaging only detects the label (e.g. radioisotopes) but not the gene or carrier themselves. It is also essential to develop multimodality imaging approaches for realizing the full potential of therapeutic RNAi, as no single imaging modality may be sufficient to simultaneously monitor both the gene delivery and silencing effect of RNAi.
PMCID: PMC3749287  PMID: 23745567
RNA interference (RNAi); small interfering RNA (siRNA); gene therapy; molecular imaging; magnetic resonance imaging (MRI); positron emission tomography (PET); near-infrared fluorescence (NIRF); bioluminescence imaging (BLI)
9.  Nanoparticle-based delivery of small interfering RNA: challenges for cancer therapy 
During recent decades there have been remarkable advances and profound changes in cancer therapy. Many therapeutic strategies learned at the bench, including monoclonal antibodies and small molecule inhibitors, have been used at the bedside, leading to important successes. One of the most important advances in biology has been the discovery that small interfering RNA (siRNA) is able to regulate the expression of genes, by a phenomenon known as RNA interference (RNAi). RNAi is one of the most rapidly growing fields of research in biology and therapeutics. Much research effort has gone into the application of this new discovery in the treatment of various diseases, including cancer. However, even though these molecules may have potential and strong utility, some limitations make their clinical application difficult, including delivery problems, side effects due to off-target actions, disturbance of physiological functions of the cellular machinery involved in gene silencing, and induction of the innate immune response. Many researchers have attempted to overcome these limitations and to improve the safety of potential RNAi-based therapeutics. Nanoparticles, which are nanostructured entities with tunable size, shape, and surface, as well as biological behavior, provide an ideal opportunity to modify current treatment regimens in a substantial way. These nanoparticles could be designed to surmount one or more of the barriers encountered by siRNA. Nanoparticle drug formulations afford the chance to improve drug bioavailability, exploiting superior tissue permeability, payload protection, and the “stealth” features of these entities. The main aims of this review are: to explain the siRNA mechanism with regard to potential applications in siRNA-based cancer therapy; to discuss the possible usefulness of nanoparticle-based delivery of certain molecules for overcoming present therapeutic limitations; to review the ongoing relevant clinical research with its pitfalls and promises; and to evaluate critically future perspectives and challenges in siRNA-based cancer therapy.
doi:10.2147/IJN.S23696
PMCID: PMC3418108  PMID: 22915840
small interfering RNA; nanoparticles; cancer therapy; delivery strategies; biological barriers; clinical trials
10.  Delivery of small-interfering RNA (siRNA) to the brain 
Background
Two fundamental difficulties in the delivery of drugs to treat central nervous system (CNS) diseases are the systemic delivery of therapeutics across the bloodbrain-barrier (BBB), and the targeting of drugs to specific tissues or cells within the brain. With the advent and promise of RNA-based therapeutics that utilize RNA interference (RNAi) to trigger specific silencing of genes within diseased tissues, the necessity to surmount such obstacles has become even more urgent.
Objective
Most pre-clinical and clinical studies on delivery of RNAi to the CNS have utilized invasive, intra-cerebral delivery of RNA to the targeted tissue. Thus, methods need to be developed to facilitate delivery of therapeutically significant quantities of RNA to the CNS via the systemic route, and to elicit clinically significant RNAi effects within the CNS tissues.
Methods
Cell-penetrating-peptides (CPPs) are ‘molecular delivery vehicles’ that can traverse cell membranes and co-transport peptides or polynucleotides. The present invention examines 1) the utility of CPP-RNA duplexes for delivery of RNA to CNS tissues and, 2) cell-mediated release of the RNA payload once the CPP-RNA duplex is internalized by the CNS cells.
Conclusions
The invention and embodiments listed therein outline molecular tools that can be adapted for non-invasive, systemic delivery of therapeutic RNA to the CNS in a future clinical setting.
doi:10.1517/13543770802680195
PMCID: PMC2698461  PMID: 19441914
messenger RNA; shRNA, microRNA; mRNA, miRNA, short-hairpin RNA; siRNA, short-interfering RNA
11.  Bacterial Delivery of RNAi Effectors: Transkingdom RNAi 
RNA interference (RNAi) represents a high effective mechanism for specific inhibition of mRNA expression. Besides its potential as a powerful laboratory tool, the RNAi pathway appears to be promising for therapeutic utilization. For development of RNA interference (RNAi)-based therapies, delivery of RNAi-mediating agents to target cells is one of the major obstacles. A novel strategy to overcome this hurdle is transkingdom RNAi (tkRNAi). This technology uses non-pathogenic bacteria, e.g. Escherichia coli, to produce and deliver therapeutic short hairpin RNA (shRNA) into target cells to induce RNAi. A first-generation tkRNAi-mediating vector, TRIP, contains the bacteriophage T7 promoter for expression regulation of a therapeutic shRNA of interest. Furthermore, TRIP has the Inv locus from Yersinia pseudotuberculosis that encodes invasin, which permits natural noninvasive bacteria to enter β1-integrin-positive mammalian cells and the HlyA gene from Listeria monocytogenes, which produces listeriolysin O. This enzyme allows the therapeutic shRNA to escape from entry vesicles within the cytoplasm of the target cell. TRIP constructs are introduced into a competent non-pathogenic Escherichia coli strain, which encodes T7 RNA polymerase necessary for the T7 promoter-driven synthesis of shRNAs. A well-characterized cancer-associated target molecule for different RNAi strategies is ABCB1 (MDR1/P-glycoprotein, MDR1/P-gp). This ABC-transporter acts as a drug extrusion pump and mediates the "classical" ABCB1-mediated multidrug resistance (MDR) phenotype of human cancer cells which is characterized by a specific cross resistance pattern. Different ABCB1-expressing MDR cancer cells were treated with anti-ABCB1 shRNA expression vector bearing E. coli. This procedure resulted in activation of the RNAi pathways within the cancer cells and a considerable down regulation of the ABCB1 encoding mRNA as well as the corresponding drug extrusion pump. Accordingly, drug accumulation was enhanced in the pristine drug-resistant cancer cells and the MDR phenotype was reversed. By means of this model the data provide the proof-of-concept that tkRNAi is suitable for modulation of cancer-associated factors, e.g. ABCB1, in human cancer cells.
doi:10.3791/2099
PMCID: PMC3156018  PMID: 20811323
12.  Comparative Genomics Reveals Two Novel RNAi Factors in Trypanosoma brucei and Provides Insight into the Core Machinery 
PLoS Pathogens  2012;8(5):e1002678.
The introduction ten years ago of RNA interference (RNAi) as a tool for molecular exploration in Trypanosoma brucei has led to a surge in our understanding of the pathogenesis and biology of this human parasite. In particular, a genome-wide RNAi screen has recently been combined with next-generation Illumina sequencing to expose catalogues of genes associated with loss of fitness in distinct developmental stages. At present, this technology is restricted to RNAi-positive protozoan parasites, which excludes T. cruzi, Leishmania major, and Plasmodium falciparum. Therefore, elucidating the mechanism of RNAi and identifying the essential components of the pathway is fundamental for improving RNAi efficiency in T. brucei and for transferring the RNAi tool to RNAi-deficient pathogens. Here we used comparative genomics of RNAi-positive and -negative trypanosomatid protozoans to identify the repertoire of factors in T. brucei. In addition to the previously characterized Argonaute 1 (AGO1) protein and the cytoplasmic and nuclear Dicers, TbDCL1 and TbDCL2, respectively, we identified the RNA Interference Factors 4 and 5 (TbRIF4 and TbRIF5). TbRIF4 is a 3′-5′ exonuclease of the DnaQ superfamily and plays a critical role in the conversion of duplex siRNAs to the single-stranded form, thus generating a TbAGO1-siRNA complex required for target-specific cleavage. TbRIF5 is essential for cytoplasmic RNAi and appears to act as a TbDCL1 cofactor. The availability of the core RNAi machinery in T. brucei provides a platform to gain mechanistic insights in this ancient eukaryote and to identify the minimal set of components required to reconstitute RNAi in RNAi-deficient parasites.
Author Summary
RNA interference (RNAi), a naturally-occurring pathway whereby the presence of double-stranded RNA in a cell triggers the degradation of homologous mRNA, has been harnessed in many organisms as an invaluable molecular biology tool to interrogate gene function. Although this technology is widely used in the protozoan parasite Trypanosoma brucei, other parasites of considerable public health significance, such as Trypanosoma cruzi, Leishmania major, and Plasmodium falciparum do not perform RNAi. Since RNAi has recently been introduced into budding yeast, this opens up the possibility that RNAi can be reconstituted in these pathogens. The key to this is getting a handle on the essential RNAi factors in T. brucei. By applying comparative genomics we identified five genes that are present in the RNAi-proficient species, but not in RNAi-deficient species: three previously identified RNAi factors, and two novel ones, which are described here. This insight into the core T. brucei RNAi machinery represents a major step towards transferring this pathway to RNAi-deficient parasites.
doi:10.1371/journal.ppat.1002678
PMCID: PMC3359990  PMID: 22654659
13.  In Vivo Imaging of RNA Interference 
RNA interference (RNAi), an effective technique for regulating/silencing specific genes, can be applied to treat various diseases. Multiple clinical trials using RNAi are ongoing and molecular imaging can serve as a powerful tool in RNAi-based therapies. This brief review will highlight the current progress on in vivo imaging of RNAi delivery and silencing effects. Incorporation of suitable molecular imaging techniques into future RNAi-based clinical trials will provide more pieces of the puzzle, thus facilitating the transformation of RNAi into a powerful therapeutic modality in the clinic.
doi:10.2967/jnumed.109.066878
PMCID: PMC2832477  PMID: 20080892
RNA interference (RNAi); molecular imaging; small interfering RNA (siRNA); small hairpin RNA (shRNA); positron emission tomography (PET)
14.  Establishing an In Vivo Assay System to Identify Components Involved in Environmental RNA Interference in the Western Corn Rootworm 
PLoS ONE  2014;9(7):e101661.
The discovery of environmental RNA interference (RNAi), in which gene expression is suppressed via feeding with double-stranded RNA (dsRNA) molecules, opened the door to the practical application of RNAi-based techniques in crop pest management. The western corn rootworm (WCR, Diabrotica virgifera virgifera) is one of the most devastating corn pests in North America. Interestingly, WCR displays a robust environmental RNAi response, raising the possibility of applying an RNAi-based pest management strategy to this pest. Understanding the molecular mechanisms involved in the WCR environmental RNAi process will allow for determining the rate limiting steps involved with dsRNA toxicity and potential dsRNA resistance mechanisms in WCR. In this study, we have established a two-step in vivo assay system, which allows us to evaluate the involvement of genes in environmental RNAi in WCR. We show that laccase 2 and ebony, critical cuticle pigmentation/tanning genes, can be used as marker genes in our assay system, with ebony being a more stable marker to monitor RNAi activity. In addition, we optimized the dsRNA dose and length for the assay, and confirmed that this assay system is sensitive to detect well-known RNAi components such as Dicer-2 and Argonaute-2. We also evaluated two WCR sid1- like (sil) genes with this assay system. This system will be useful to quickly survey candidate systemic RNAi genes in WCR, and also will be adaptable for a genome-wide RNAi screening to give us an unbiased view of the environmental/systemic RNAi pathway in WCR.
doi:10.1371/journal.pone.0101661
PMCID: PMC4086966  PMID: 25003334
15.  Expression Strategies for Short Hairpin RNA Interference Triggers 
Human gene therapy  2008;19(4):313-317.
Since the discovery that the triggers for RNA interference (RNAi), small interfering RNAs, could mediate silencing in mammalian cells without triggering a toxic response, RNAi has become the standard tool for sequence-specific knockdown of gene expression in molecular biology. This is due in part to the development of methods for promoter-based expression of RNAi triggers that can mediate stable silencing in mammalian cells. Numerous systems with slightly different characteristics exist, but despite incredible progress in a field that moves very rapidly, challenges still remain. The biggest challenge is to successfully and safely apply RNAi in vivo. Aside from potential issues of delivery, which is one of the most important considerations, successful application of short hairpin RNAs (shRNAs) in vivo requires expression systems that yield potent and specific knockdown of the target in the absence of toxicity. With a couple of exceptions, the current systems available for shRNA expression have not generally resulted in unexpected toxicities, while still providing strong knockdown of the intended targets; however, we do not know enough about how sequence-specific off-target effects will affect various cell and tissue types, or to what extent ectopic expression of RNAi triggers will perturb the endogenous RNAi mechanisms.
doi:10.1089/hum.2008.026
PMCID: PMC2702704  PMID: 18363506
16.  Systemic RNAi mediated gene silencing in the anhydrobiotic nematode Panagrolaimus superbus 
Background
Gene silencing by RNA interference (RNAi) is a powerful tool for functional genomics. Although RNAi was first described in Caenorhabditis elegans, several nematode species are unable to mount an RNAi response when exposed to exogenous double stranded RNA (dsRNA). These include the satellite model organisms Pristionchus pacificus and Oscheius tipulae. Available data also suggest that the RNAi pathway targeting exogenous dsRNA may not be fully functional in some animal parasitic nematodes. The genus Panagrolaimus contains bacterial feeding nematodes which occupy a diversity of niches ranging from polar, temperate and semi-arid soils to terrestrial mosses. Thus many Panagrolaimus species are adapted to tolerate freezing and desiccation and are excellent systems to study the molecular basis of environmental stress tolerance. We investigated whether Panagrolaimus is susceptible to RNAi to determine whether this nematode could be used in large scale RNAi studies in functional genomics.
Results
We studied two species: Panagrolaimus sp. PS1159 and Panagrolaimus superbus. Both nematode species displayed embryonic lethal RNAi phenotypes following ingestion of Escherichia coli expressing dsRNA for the C. elegans embryonic lethal genes Ce-lmn-1 and Ce-ran-4. Embryonic lethal RNAi phenotypes were also obtained in both species upon ingestion of dsRNA for the Panagrolaimus genes ef1b and rps-2. Single nematode RT-PCR showed that a significant reduction in mRNA transcript levels occurred for the target ef1b and rps-2 genes in RNAi treated Panagrolaimus sp. 1159 nematodes. Visible RNAi phenotypes were also observed when P. superbus was exposed to dsRNA for structural genes encoding contractile proteins. All RNAi phenotypes were highly penetrant, particularly in P. superbus.
Conclusion
This demonstration that Panagrolaimus is amenable to RNAi by feeding will allow the development of high throughput methods of RNAi screening for P. superbus. This greatly enhances the utility of this nematode as a model system for the study of the molecular biology of anhydrobiosis and cryobiosis and as a possible satellite model nematode for comparative and functional genomics. Our data also identify another nematode infraorder which is amenable to RNAi and provide additional information on the diversity of RNAi phenotypes in nematodes.
doi:10.1186/1471-2199-9-58
PMCID: PMC2453295  PMID: 18565215
17.  Computational Design of Antiviral RNA Interference Strategies That Resist Human Immunodeficiency Virus Escape†  
Journal of Virology  2005;79(3):1645-1654.
Recently developed antiviral strategies based upon RNA interference (RNAi), which harnesses an innate cellular system for the targeted down-regulation of gene expression, appear highly promising and offer alternative approaches to conventional highly active antiretroviral therapy or efforts to develop an AIDS vaccine. However, RNAi is faced with several challenges that must be overcome to fully realize its promise. Specifically, it degrades target RNA in a highly sequence-specific manner and is thus susceptible to viral mutational escape, and there are also challenges in delivery systems to induce RNAi. To aid in the development of anti-human immunodeficiency virus (anti-HIV) RNAi therapies, we have developed a novel stochastic computational model that simulates in molecular-level detail the propagation of an HIV infection in cells expressing RNAi. The model provides quantitative predictions on how targeting multiple locations in the HIV genome, while keeping the overall RNAi strength constant, significantly improves efficacy. Furthermore, it demonstrates that delivery systems must be highly efficient to preclude leaving reservoirs of unprotected cells where the virus can propagate, mutate, and eventually overwhelm the entire system. It also predicts how therapeutic success depends upon a relationship between RNAi strength and delivery efficiency and uniformity. Finally, targeting an essential viral element, in this case the HIV TAR region, can be highly successful if the RNAi target sequence is correctly selected. In addition to providing specific predictions for how to optimize a clinical therapy, this system may also serve as a future tool for investigating more fundamental questions of viral evolution.
doi:10.1128/JVI.79.3.1645-1654.2005
PMCID: PMC544124  PMID: 15650190
18.  Programmed fluctuations in sense/antisense transcript ratios drive sexual differentiation in S. pombe 
Strand-specific RNA sequencing of S. pombe reveals a highly structured programme of ncRNA expression at over 600 loci. Functional investigations show that this extensive ncRNA landscape controls the complex programme of sexual differentiation in S. pombe.
The model eukaryote S. pombe features substantial numbers of ncRNAs many of which are antisense regulatory transcripts (ARTs), ncRNAs expressed on the opposing strand to coding sequences.Individual ARTs are generated during the mitotic cycle, or at discrete stages of sexual differentiation to downregulate the levels of proteins that drive and coordinate sexual differentiation.Antisense transcription occurring from events such as bidirectional transcription is not simply artefactual ‘chatter', it performs a critical role in regulating gene expression.
Regulation of the RNA profile is a principal control driving sexual differentiation in the fission yeast Schizosaccharomyces pombe. Before transcription, RNAi-mediated formation of heterochromatin is used to suppress expression, while post-transcription, regulation is achieved via the active stabilisation or destruction of transcripts, and through at least two distinct types of splicing control (Mata et al, 2002; Shimoseki and Shimoda, 2001; Averbeck et al, 2005; Mata and Bähler, 2006; Xue-Franzen et al, 2006; Moldon et al, 2008; Djupedal et al, 2009; Amorim et al, 2010; Grewal, 2010; Cremona et al, 2011).
Around 94% of the S. pombe genome is transcribed (Wilhelm et al, 2008). While many of these transcripts encode proteins (Wood et al, 2002; Bitton et al, 2011), the majority have no known function. We used a strand-specific protocol to sequence total RNA extracts taken from vegetatively growing cells, and at different points during a time course of sexual differentiation. The resulting data redefined existing gene coordinates and identified additional transcribed loci. The frequency of reads at each of these was used to monitor transcript abundance.
Transcript levels at 6599 loci changed in at least one sample (G-statistic; False Discovery Rate <5%). 4231 (72.3%), of which 4011 map to protein-coding genes, while 809 loci were antisense to a known gene. Comparisons between haploid and diploid strains identified changes in transcript levels at over 1000 loci.
At 354 loci, greater antisense abundance was observed relative to sense, in at least one sample (putative antisense regulatory transcripts—ARTs). Since antisense mechanisms are known to modulate sense transcript expression through a variety of inhibitory mechanisms (Faghihi and Wahlestedt, 2009), we postulated that the waves of antisense expression activated at different stages during meiosis might be regulating protein expression.
To ask whether transcription factors that drive sense-transcript levels influenced ART production, we performed RNA-seq of a pat1.114 diploid meiosis in the absence of the transcription factors Atf21 and Atf31 (responsible for late meiotic transcription; Mata et al, 2002). Transcript levels at 185 ncRNA loci showed significant changes in the knockout backgrounds. Although meiotic progression is largely unaffected by removal of Atf21 and Atf31, viability of the resulting spores was significantly diminished, indicating that Atf21- and Atf31-mediated events are critical to efficient sexual differentiation.
If changes to relative antisense/sense transcript levels during a particular phase of sexual differentiation were to regulate protein expression, then the continued presence of the antisense at points in the differentiation programme where it would normally be absent should abolish protein function during this phase. We tested this hypothesis at four loci representing the three means of antisense production: convergent gene expression, improper termination and nascent transcription from an independent locus. Induction of the natural antisense transcripts that opposed spo4+, spo6+ and dis1+ (Figures 3 and 7) in trans from a heterologous locus phenocopied a loss of function of the target protein. ART overexpression decreased Dis1 protein levels. Antisense transcription opposing spk1+ originated from improper termination of the sense ups1+ transcript on the opposite strand (Figure 3B, left locus). Expression of either the natural full-length ups1+ transcript or a truncated version, restricted to the portion of ups1+ overlapping spk1+ (Figure 3, orange transcripts) in trans from a heterologous locus phenocopied the spk1.Δ differentiation deficiency. Convergent transcription from a neighbouring gene on the opposing strand is, therefore, an effective mechanism to generate RNAi-mediated (below) silencing in fission yeast. Further analysis of the data revealed, for many loci, substantial changes in UTR length over the course of meiosis, suggesting that UTR dynamics may have an active role in regulating gene expression by controlling the transcriptional overlap between convergent adjacent gene pairs.
The RNAi machinery (Grewal, 2010) was required for antisense suppression at each of the dis1, spk1, spo4 and spo6 loci, as antisense to each locus had no impact in ago1.Δ, dcr1.Δ and rdp1.Δ backgrounds. We conclude that RNAi control has a key role in maintaining the fidelity of sexual differentiation in fission yeast. The histone H3 methyl transferase Clr4 was required for antisense control from a heterologous locus.
Thus, a significant portion of the impact of ncRNA upon sexual differentiation arises from antisense gene silencing. Importantly, in contrast to the extensively characterised ability of the RNAi machinery to operate in cis at a target locus in S. pombe (Grewal, 2010), each case of gene silencing generated here could be achieved in trans by expression of the antisense transcript from a single heterologous locus elsewhere in the genome.
Integration of an antibiotic marker gene immediately downstream of the dis1+ locus instigated antisense control in an orientation-dependent manner. PCR-based gene tagging approaches are widely used to fuse the coding sequences of epitope or protein tags to a gene of interest. Not only do these tagging approaches disrupt normal 3′UTR controls, but the insertion of a heterologous marker gene immediately downstream of an ORF can clearly have a significant impact upon transcriptional control of the resulting fusion protein. Thus, PCR tagging approaches can no longer be viewed as benign manipulations of a locus that only result in the production of a tagged protein product.
Repression of Dis1 function by gene deletion or antisense control revealed a key role this conserved microtubule regulator in driving the horsetail nuclear migrations that promote recombination during meiotic prophase.
Non-coding transcripts have often been viewed as simple ‘chatter', maintained solely because evolutionary pressures have not been strong enough to force their elimination from the system. Our data show that phenomena such as improper termination and bidirectional transcription are not simply interesting artifacts arising from the complexities of transcription or genome history, but have a critical role in regulating gene expression in the current genome. Given the widespread use of RNAi, it is reasonable to anticipate that future analyses will establish ARTs to have equal importance in other organisms, including vertebrates.
These data highlight the need to modify our concept of a gene from that of a spatially distinct locus. This view is becoming increasingly untenable. Not only are the 5′ and 3′ ends of many genes indistinct, but that this lack of a hard and fast boundary is actively used by cells to control the transcription of adjacent and overlapping loci, and thus to regulate critical events in the life of a cell.
Strand-specific RNA sequencing of S. pombe revealed a highly structured programme of ncRNA expression at over 600 loci. Waves of antisense transcription accompanied sexual differentiation. A substantial proportion of ncRNA arose from mechanisms previously considered to be largely artefactual, including improper 3′ termination and bidirectional transcription. Constitutive induction of the entire spk1+, spo4+, dis1+ and spo6+ antisense transcripts from an integrated, ectopic, locus disrupted their respective meiotic functions. This ability of antisense transcripts to disrupt gene function when expressed in trans suggests that cis production at native loci during sexual differentiation may also control gene function. Consistently, insertion of a marker gene adjacent to the dis1+ antisense start site mimicked ectopic antisense expression in reducing the levels of this microtubule regulator and abolishing the microtubule-dependent ‘horsetail' stage of meiosis. Antisense production had no impact at any of these loci when the RNA interference (RNAi) machinery was removed. Thus, far from being simply ‘genome chatter', this extensive ncRNA landscape constitutes a fundamental component in the controls that drive the complex programme of sexual differentiation in S. pombe.
doi:10.1038/msb.2011.90
PMCID: PMC3738847  PMID: 22186733
antisense; meiosis; ncRNA; S. pombe; siRNA
19.  Evidence of RNAi in humans from systemically administered siRNA via targeted nanoparticles 
Nature  2010;464(7291):1067-1070.
Therapeutics that are designed to engage RNA interference (RNAi) pathways have the potential to provide new, major ways of imparting therapy to patients.1,2 Fire et al. first demonstrated that long, double stranded RNAs mediate RNAi in Caenorhabditis elegans,3 and Elbashir et al. opened the pathway to the use of RNAi for human therapy by showing that small interfering RNAs (siRNAs: ca. 21 base pair double stranded RNA) can elicit RNAi in mammalian cells without producing an interferon response.4 We are currently conducting the first-in-human Phase I clinical trial involving the systemic administration of siRNA to patients with solid cancers using a targeted, nanoparticle delivery system. Here we provide evidence of inducing an RNAi mechanism of action in a human from the delivered siRNA. Tumor biopsies from melanoma patients obtained after treatment reveal: (i) the presence of intracellularly-localized nanoparticles in amounts that correlate with dose levels of the nanoparticles administered (this is a first for systemically delivered nanoparticles of any kind), and (ii) reduction in both the specific mRNA (M2 subunit of ribonucleotide reductase (RRM2)) and the protein (RRM2) when compared to pre-dosing tissue. Most importantly, we detect the presence of an mRNA fragment that demonstrates siRNA mediated mRNA cleavage occurs specifically at the site predicted for an RNAi mechanism from a patient who received the highest dose of the nanoparticles. These data when taken in total demonstrate that siRNA administered systemically to a human can produce a specific gene inhibition (reduction in mRNA and protein) by an RNAi mechanism of action.
doi:10.1038/nature08956
PMCID: PMC2855406  PMID: 20305636
20.  Current Progress of siRNA/shRNA Therapeutics in Clinical Trials 
Biotechnology Journal  2011;6(9):1130-1146.
Through a mechanism known as RNA interference (RNAi), small interfering RNA (siRNA) molecules can target complementary mRNA strands for degradation, thus specifically inhibiting gene expression. The ability of siRNAs to inhibit gene expression offers a mechanism that can be exploited for novel therapeutics. Indeed, over the past decade, at least 21 siRNA therapeutics have been developed for more than a dozen diseases, including various cancers, viruses, and genetic disorders. Like other biological drugs, RNAi-based therapeutics often require a delivery vehicle to transport them to the targeted cells. Thus, the clinical advancement of numerous siRNA drugs has relied on the development of siRNA carriers including biodegradable nanoparticles, lipids, bacteria, and attenuated viruses. Most therapies permit systemic delivery of the siRNA drug, while others use ex vivo delivery by autologous cell therapy. For some of the drugs, advancements in bioengineering and nanotechnology have led to improved control of delivery and release of the siRNA. Likewise, progress in molecular biology has allowed for improved design of the siRNA molecules. Here, we provide an overview of siRNA therapeutics in clinical trials, including their clinical progress, the challenges they have encountered, and the future they hold in the treatment of human diseases.
doi:10.1002/biot.201100054
PMCID: PMC3388104  PMID: 21744502
RNAi; siRNA; shRNA; clinical trial; delivery
21.  Molecular Mechanisms of RNA-Triggered Gene Silencing Machineries 
Accounts of Chemical Research  2012;45(7):1122-1131.
Gene silencing by RNA triggers is an ancient, evolutionarily conserved, and widespread phenomenon. This process, known as RNA interference (RNAi), occurs when double-stranded RNA helices induce cleavage of their complementary mRNAs. Because these RNA molecules can be introduced exogenously as small interfering RNAs (siRNAs), RNAi has become an everyday experimental tool in laboratory research. In addition, the number of RNA-based therapeutics that are currently in clinical trials for a variety of human diseases demonstrate the therapeutic potential of RNAi.
In this Account, we focus on our current understanding of the structure and function of various classes of RNAi triggers and how this knowledge has contributed to our understanding of the biogenesis and catalytic functions of siRNA and microRNA in mammalian cells. Mechanistic studies to understand the structure and function of small RNAs that induce RNAi have illuminated broad functions of the ancient RNAi machinery in animals and plants. In addition, such studies have provided insight to identify endogenous physiological gene silencing RNA triggers that engage functional machineries similar to siRNAs. Several endogenous small RNA species have been identified: small noncoding RNAs (microRNAs), piwi-interacting RNAs (piRNAs), and endogenous siRNAs (endo-siRNAs). microRNAs are the most widespread class of small RNAs in mammalian cells. Despite their importance in biology and medicine, the molecular and cellular mechanisms of microRNA biogenesis and function are not fully understood. We provide an overview of the current understanding of how these molecules are synthesized within cells and how they act on gene targets. Interesting questions remain both for understanding the effects of modifications and editing on microRNAs and the interactions between microRNAs and other cellular RNAs such as long noncoding RNAs.
doi:10.1021/ar200253u
PMCID: PMC3398549  PMID: 22304792
22.  Exploring systemic RNA interference in insects: a genome-wide survey for RNAi genes in Tribolium 
Genome Biology  2008;9(1):R10.
Tribolium resembles C. elegans in showing a robust systemic RNAi response, but does not have C. elegans-type RNAi mechanisms; insect systemic RNAi probably uses a different mechanism.
Background
RNA interference (RNAi) is a highly conserved cellular mechanism. In some organisms, such as Caenorhabditis elegans, the RNAi response can be transmitted systemically. Some insects also exhibit a systemic RNAi response. However, Drosophila, the leading insect model organism, does not show a robust systemic RNAi response, necessitating another model system to study the molecular mechanism of systemic RNAi in insects.
Results
We used Tribolium, which exhibits robust systemic RNAi, as an alternative model system. We have identified the core RNAi genes, as well as genes potentially involved in systemic RNAi, from the Tribolium genome. Both phylogenetic and functional analyses suggest that Tribolium has a somewhat larger inventory of core component genes than Drosophila, perhaps allowing a more sensitive response to double-stranded RNA (dsRNA). We also identified three Tribolium homologs of C. elegans sid-1, which encodes a possible dsRNA channel. However, detailed sequence analysis has revealed that these Tribolium homologs share more identity with another C. elegans gene, tag-130. We analyzed tag-130 mutants, and found that this gene does not have a function in systemic RNAi in C. elegans. Likewise, the Tribolium sid-like genes do not seem to be required for systemic RNAi. These results suggest that insect sid-1-like genes have a different function than dsRNA uptake. Moreover, Tribolium lacks homologs of several genes important for RNAi in C. elegans.
Conclusion
Although both Tribolium and C. elegans show a robust systemic RNAi response, our genome-wide survey reveals significant differences between the RNAi mechanisms of these organisms. Thus, insects may use an alternative mechanism for the systemic RNAi response. Understanding this process would assist with rendering other insects amenable to systemic RNAi, and may influence pest control approaches.
doi:10.1186/gb-2008-9-1-r10
PMCID: PMC2395250  PMID: 18201385
23.  Genome wide screening of RNAi factors of Sf21 cells reveal several novel pathway associated proteins 
BMC Genomics  2014;15(1):775.
Background
RNA interference (RNAi) leads to sequence specific knock-down of gene expression and has emerged as an important tool to analyse gene functions, pathway analysis and gene therapy. Although RNAi is a conserved cellular process involving common elements and factors, species-specific differences have been observed among different eukaryotes. Identification of components for RNAi pathway is pursued intensively and successful genome-wide screens have been performed for components of RNAi pathways in various organisms. Functional comparative genomics analysis offers evolutionary insight that forms basis of discoveries of novel RNAi-factors within related organisms. Keeping in view the academic and commercial utility of insect derived cell-line from Spodoptera frugiperda, we pursued the identification and functional analysis of components of RNAi-machinery of Sf21 cell-line using genome-wide application.
Results
The genome and transcriptome of Sf21 was assembled and annotated. In silico application of comparative genome analysis among insects allowed us to identify several RNAi factors in Sf21 line. The candidate RNAi factors from assembled genome were validated by knockdown analysis of candidate factors using the siRNA screens on the Sf21-gfp reporter cell-line. Forty two (42) potential factors were identified using the cell based assay. These include core RNAi elements including Dicer-2, Argonaute-1, Drosha, Aubergine and auxiliary modules like chromatin factors, RNA helicases, RNA processing module, signalling allied proteins and others. Phylogenetic analyses and domain architecture revealed that Spodoptera frugiperda homologs retained identity with Lepidoptera (Bombyx mori) or Coleoptera (Tribolium castaneum) sustaining an evolutionary conserved scaffold in post-transcriptional gene silencing paradigm within insects.
Conclusion
The database of RNAi-factors generated by whole genome association survey offers comprehensive outlook about conservation as well as specific differences of the proteins of RNAi machinery. Understanding the interior involved in different phases of gene silencing also offers impending tool for RNAi-based applications.
Electronic supplementary material
The online version of this article (doi:10.1186/1471-2164-15-775) contains supplementary material, which is available to authorized users.
doi:10.1186/1471-2164-15-775
PMCID: PMC4247154  PMID: 25199785
RNA interference; siRNA screening; Sf21 cells; Genome-wide screening; Insect RNAi; Spodoptera frugiperda
24.  Dissecting Systemic RNA Interference in the Red Flour Beetle Tribolium castaneum: Parameters Affecting the Efficiency of RNAi 
PLoS ONE  2012;7(10):e47431.
The phenomenon of RNAi, in which the introduction of dsRNA into a cell triggers the destruction of the corresponding mRNA resulting in a gene silencing effect, is conserved across a wide array of plant and animal phyla. However, the mechanism by which the dsRNA enters a cell, allowing the RNAi effect to occur throughout a multicellular organism (systemic RNAi), has only been studied extensively in certain plants and the nematode Caenorhabditis elegans. In recent years, RNAi has become a popular reverse genetic technique for gene silencing in many organisms. Although many RNAi techniques in non-traditional model organisms rely on the systemic nature of RNAi, little has been done to analyze the parameters required to obtain a robust systemic RNAi response. The data provided here show that the concentration and length of dsRNA have profound effects on the efficacy of the RNAi response both in regard to initial efficiency and duration of the effect in Tribolium castaneum. In addition, our analyses using a series of short dsRNAs and chimeric dsRNA provide evidence that dsRNA cellular uptake (and not the RNAi response itself) is the major step affected by dsRNA size in Tribolium. We also demonstrate that competitive inhibition of dsRNA can occur when multiple dsRNAs are injected together, influencing the effectiveness of RNAi. These data provide specific information essential to the design and implementation of RNAi based studies, and may provide insight into the molecular basis of the systemic RNAi response in insects.
doi:10.1371/journal.pone.0047431
PMCID: PMC3484993  PMID: 23133513
25.  A status report on RNAi therapeutics 
Silence  2010;1:14.
Fire and Mello initiated the current explosion of interest in RNA interference (RNAi) biology with their seminal work in Caenorhabditis elegans. These observations were closely followed by the demonstration of RNAi in Drosophila melanogaster. However, the full potential of these new discoveries only became clear when Tuschl and colleagues showed that 21-22 bp RNA duplexes with 3" overhangs, termed small interfering (si)RNAs, could reliably execute RNAi in a range of mammalian cells. Soon afterwards, it became clear that many different human cell types had endogenous machinery, the RNA-induced silencing complex (RISC), which could be harnessed to silence any gene in the genome. Beyond the availability of a novel way to dissect biology, an important target validation tool was now available. More importantly, two key properties of the RNAi pathway - sequence-mediated specificity and potency - suggested that RNAi might be the most important pharmacological advance since the advent of protein therapeutics. The implications were profound. One could now envisage selecting disease-associated targets at will and expect to suppress proteins that had remained intractable to inhibition by conventional methods, such as small molecules. This review attempts to summarize the current understanding on siRNA lead discovery, the delivery of RNAi therapeutics, typical in vivo pharmacological profiles, preclinical safety evaluation and an overview of the 14 programs that have already entered clinical practice.
doi:10.1186/1758-907X-1-14
PMCID: PMC2908561  PMID: 20615220

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