RNAi and microRNAs: from animal models to disease therapy

The discovery of the phenomenon of RNA interference (RNAi) and its existence in mammals quickly suggested a great potential for use in disease therapy. Rapid advances have been made in the development of RNAi-based technologies and promising results have been obtained from studies on mammalian cell culture systems and animal in vivo models. However, the progress in our understanding of the RNAi pathway and the related function of microRNAs (miRNAs) have also raised concerns regarding various types of side effects that may restrict the use of this technology in human therapy. At the same time, our new knowledge about the functional roles of miRNAs as regulators of many cellular processes, including proliferation, differentiation, development, and neuronal function, is revolutionizing cell biology and will have a major impact on medical research. In this review, we focus on the discoveries that have been made in animal models and how this insight can be translated to human medicine and disease therapy. In this connection, we will particularly discuss the challenges associated with the efforts to develop RNAi-based therapeutics.     

The therapeutic potential of RNA interference

In recent years, we have witnessed the discovery of a new mechanism of gene regulation called RNA interference (RNAi), which has revitalized interest in the development of nucleic acid-based technologies for therapeutic gene suppression. This review focuses on the potential therapeutic use of RNAi, discussing the theoretical advantages of RNAi-based therapeutics over previous technologies as well as the challenges involved in developing RNAi for clinical use. Also reviewed, are the in vivo proof-of principle experiments that provide the preclinical justification for the continued development of RNAi-based therapeutics.     

Structurally flexible triethanolamine-core poly(amidoamine) dendrimers as effective nanovectors to deliver RNAi-based therapeutics

RNAi-based nucleic acid molecules have attracted considerable attention as compelling therapeutics providing safe and competent delivery systems are available. Dendrimers are emerging as appealing nanocarriers for nucleic acid delivery thanks to their unique well-defined architecture and the resulting cooperativity and multivalency confined within a nanostructure. The present review offers a brief overview of the structurally flexible triethanolamine-core poly(amidoamine) (PAMAM) dendrimers developed in our group as nanovectors for the delivery of RNAi therapeutics. Their excellent activity for delivering different RNAi therapeutics in various disease models in vitro and in vivo will be highlighted here.     

Commercial potential of RNAi

The commercial potential of RNAi is assessed on the basis of successful translation of technology into applications in three areas: (1) drug discovery and research-currently the biggest segment; (2) potential therapeutic applications; and (3) the role of microRNA in molecular diagnostics. RNAi is an important method for analyzing gene function and identifying new drug targets that use dsRNA to knock down or silence specific genes. Sets of siRNAs focused on a specific gene class (siRNA libraries) have the capacity to greatly increase the pace of pathway analysis and functional genomics. RNAi plays an important role in drug discovery by facilitating target validation. The discovery of the role of microRNA (miRNAs) in various pathological processes opens up possible applications in molecular diagnostics, particularly that of cancer. The advantages of RNAi-based therapeutics over traditional pharmaceuticals include the capability for more specific therapies with small molecule siRNA. Drawbacks include the development of resistance in cancer and viral infections as well as the interferon effect. RNAi is closely related to gene therapy and the vectors developed for gene therapy are also being used for delivery of siRNAs. RNAi, along with other related technologies, will contribute to the development of personalised medicine. Although none of the RNAi-based drugs is in the market yet, some are in clinical trials. By the year 2010 the market for RNAi-based drugs is expected to be worth 3.5 billion dollars and is expected to expand to 10.5 billion dollars by the year 2015.     

基于RNAi技术的转基因植物监管现状及其面临的育种领域的挑战

RNA干扰（RNA interference,RNAi）在植物、动物和真菌的生长、发育、病毒防御和转座子失活中起着至关重要的作用。目前已成功利用RNAi技术培育出抗病虫和品质改良等具有优良性状的生物技术产品,为农业绿色发展提供了强有力的支撑。然而,目前RNAi的相关机制尚未完全明确,基于RNAi技术的转基因植物面临着一些亟待解决的问题,同时,对于RNAi转基因植物应用的安全高效监管也需进一步完善。基于此,对RNAi转基因植物的监管现状及其面临的育种上的挑战进行了综述,并对存在的问题提出了解决建议,以期为RNAi技术进一步应用于农业植物改良育种提供新思路,并为其监管评价提供依据。     

RNA interference against viruses: strike and counterstrike

RNA interference (RNAi) is a conserved sequence-specific, gene-silencing mechanism that is induced by double-stranded RNA. RNAi holds great promise as a novel nucleic acid-based therapeutic against a wide variety of diseases, including cancer, infectious diseases and genetic disorders. Antiviral RNAi strategies have received much attention and several compounds are currently being tested in clinical trials. Although induced RNAi is able to trigger profound and specific inhibition of virus replication, it is becoming clear that RNAi therapeutics are not as straightforward as we had initially hoped. Difficulties concerning toxicity and delivery to the right cells that earlier hampered the development of antisense-based therapeutics may also apply to RNAi. In addition, there are indications that viruses have evolved ways to escape from RNAi. Proper consideration of all of these issues will be necessary in the design of RNAi-based therapeutics for successful clinical intervention of human pathogenic viruses.     

Development of new RNAi therapeutics

RNAi-mediated gene inactivation has become a cornerstone of the present day gene function studies that are the foundation of mechanism and target based drug discovery and development, which could potentially shorten the otherwise long process of drug development. In particular, the coming of age of "RNAi drug" could provide new promising therapeutics bypassing traditional approaches. However, there are technological hurdles need to overcome and the biological limitations need to consider for achieving effective therapeutics. Major hurdles include the intrinsic poor pharmacokinetic property of siRNA and major biological restrictions include off-target effects, interferon response and the interference with endogenous miRNA. Recent innovations in nucleic acid chemistry, formulations and delivery methods have gradually rendered it possible to develop effective RNAi-based therapeutics. Careful design based on the newest RNAi/miRNA biology can also help to minimize the potential tissue toxicity. If successful with systemic application, RNAi drug will no doubt revolutionize the whole drug development process. This review attempts to describe the progress in this area, including applications in preclinical models and recent favorable experience in a number of human trials of local diseases, along with the discussion on the potential limitations of RNAi therapeutics.     

病毒siRNA介导的RNA干扰途径与机制

RNA干扰(RNAi)是真核生物体内重要的基因表达调控方式之一.RNAi的一种原始的作用是帮助生物体抵抗病毒,早期的研究表明无脊椎动物可以利用RNAi抵抗病毒,但是哺乳动物是否存在这一机制一直存在争议.最新的研究发现了哺乳动物RNAi抗病毒的强有力的证据,并且研究人员认为,这是一种之前被忽视的、全新的免疫途径.值得注意...     

The silent treatment: RNAi as a defense against virus infection in mammals

RNA interference (RNAi) is a mechanism for sequence-specific gene silencing guided by double-stranded RNA. In plants and insects it is well established that RNAi is instrumental in the response to viral infections; whether RNAi has a similar function in mammals is under intense investigation. Recent studies to address this question have identified some unanticipated interactions between the RNAi machinery and mammalian viruses. Furthermore, introduction of virus-specific small interfering RNAs (siRNAs) into cells, thus programming the RNAi machinery to target viruses, is an effective therapeutic approach to inhibit virus replication in vitro and in animal models. Although several issues remain to be addressed, such as delivery and viral escape, these findings hold tremendous potential for the development of RNAi-based antiviral therapeutics.     

Therapeutic potential of RNAi in metabolic diseases

Over the past years RNA interference (RNAi) has exploded as a new approach to manipulate gene expression in mammalian systems. More recently, RNAi has acquired interest as a potential therapeutic strategy. This review focuses on the potential therapeutic use of RNAi for metabolic diseases, the current understanding of RNAi biology, and how RNAi has been utilized to study the role of different genes in the pathogenesis of diabetes and obesity. Also reviewed are the in vivo proof-of-principle experiments that provide the preclinical justification for the development of RNAi-based therapeutics for diabetes and the key challenges that currently limit its application in the clinical setting.     

Comparison of bovine RNA polymerase III promoters for short hairpin RNA expression

RNA interference (RNAi) mediated by DNA-based expression of short hairpin RNA (shRNA) is a powerful method of sequence-specific gene knockdown. A number of vectors for expression of shRNA have been developed that feature promoters from RNA polymerase III (pol III)-transcribed genes of mouse or human origin. To advance the use of RNAi as a tool for functional genomic research and for future development of specific therapeutics in the bovine species, we have developed shRNA expression vectors that feature novel bovine RNA pol III promoters. We characterized two bovine U6 small nuclear RNA (snRNA) promoters (bU6-2 and bU6-3) and a bovine 7SK snRNA promoter (b7SK). We compared the efficiency of each of these promoters to express shRNA molecules. Promoter activity was measured in the context of RNAi by targeting and suppressing the reporter gene encoding enhanced green fluorescent protein. Results show that the b7SK promoter induced the greatest level of suppression in a range of cell lines. The comparison of these bovine promoters in shRNA expression is an important component for the future development of bovine-specific RNAi-based research.     

Delivery of RNA Interference

Over the last few years, RNA Interference (RNAi), a naturally occurring mechanism of gene regulation conserved in plant and mammalian cells, has opened numerous novel opportunities for basic research across the field of biology. While RNAi has helped accelerate discovery and understanding of gene functions, it also has great potential as a therapeutic and potentially prophylactic modality. Challenging diseases failing conventional therapeutics could become treatable by specific silencing of key pathogenic genes. More specifically, therapeutic targets previously deemed “undruggable” by small molecules, are now coming within reach of RNAi based therapy. For RNAi to be effective and elicit gene silencing response, the double-stranded RNA molecules must be delivered to the target cell. Unfortunately, delivery of these RNA duplexes has been challenging, halting rapid development of RNAi-based therapies. In this review we present current advancements in the field of siRNA delivery methods, including the pros and cons of each method.     

RNAi mechanisms and applications

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.     

Global trends in research and commercialization of exogenous and endogenous RNAi technologies for crops

It has only been about 20 years since the first Nobel Prize-winning work on RNA interference (RNAi) in Caenorhabditis elegans was published in the journal Nature. Fast forward to today, and the use of RNA molecules as gene-silencing elements in crops has helped scientists to unveil possible solutions to the global problems of agricultural losses due to pests, viruses, pathogens, and to other abiotic and biotic stresses. The recent proliferation of publications suggests that the technology has gained significant attention and received ample funding support. In this article, an attempt has been made to visualize recent trends in Research & Development (R&D) investment in this field by analyzing top cited scholarly articles, patent trends, and commercialization activity. The publication and citation analysis identified that the development of RNAi-based crops conferring resistance against viruses, fungi, and pests are at the forefront of RNAi research and that Chinese and US institutions are the leaders in this field. The patent landscape analysis for RNAi technology over all aspects related to RNAi-derived crops provides an overview of patenting activity from a geographical, organizational, and legal perspective. Such an exercise is pivotal to industry players and public institutions aiming at creating intellectual property that is commercially appealing. An upswing in commercial interests in this technology in recent years is reflected by a consistent number of patent filings in US, European, and Chinese patent offices, with multinational giant firms as the most prolific patent filers. The expanding RNAi commercialization landscape is supported by a series of strategic partnerships, licensing agreements, and acquisitions created between agribusinesses, public research institutions, and startup companies. From key observations, we would like to highlight that such investments have very positive impacts on the development of RNAi technology. Nonetheless, the success of this technology is dependent on several factors, such as financial requirements, the complexity, and timeframe of the entire development process, as well as stringent regulations imposed by the relevant authorities. In most countries, RNAi-based transgenic crops are still considered as a genetically modified (GM) product, which necessitates the crops to undergo rigorous evaluation before approval is granted. Recent advancements in exogenous RNAi-derived biopesticides have provided a nontransgenic alternative to GM crops. However, challenges still remain in the form of technical hurdles and regulatory ambiguities surrounding this emerging technology. Its full potential remains to be realized.     

Inhibition of virus replication by RNA interference

RNA interference (RNAi) is a sequence-specific gene-silencing mechanism in eukaryotes, which is believed to function as a defence against viruses and transposons. Since its discovery, RNAi has been developed into a widely used technique for generating genetic knock-outs and for studying gene function by reverse genetics. Additionally, inhibition of virus replication by means of induced RNAi has now been reported for numerous viruses, including several important human pathogens such as human immunodeficiency virus type 1, hepatitis C virus, hepatitis B virus, dengue virus, poliovirus and influenza virus A. In this review, we will summarize the current data on RNAi-mediated inhibition of virus replication and discuss the possibilities for the development of RNAi-based antiviral therapeutics.     

RNA interference against animal viruses: how morbilliviruses generate extended diversity to escape small interfering RNA control

Viruses are serious threats to human and animal health. Vaccines can prevent viral diseases, but few antiviral treatments are available to control evolving infections. Among new antiviral therapies, RNA interference (RNAi) has been the focus of intensive research. However, along with the development of efficient RNAi-based therapeutics comes the risk of emergence of resistant viruses. In this study, we challenged the in vitro propensity of a morbillivirus (peste des petits ruminants virus), a stable RNA virus, to escape the inhibition conferred by single or multiple small interfering RNAs (siRNAs) against conserved regions of the N gene. Except with the combination of three different siRNAs, the virus systematically escaped RNAi after 3 to 20 consecutive passages. The genetic modifications involved consisted of single or multiple point nucleotide mutations and a deletion of a stretch of six nucleotides, illustrating that this virus has an unusual genomic malleability.     

RNAi在基因缺陷模型方面的应用

RNA干扰(RNA interference,RNAi)是指双链RNA(double-stranded RNA,dsRNA)分子导入细胞内后,促进与之同源的mRNA发生特异性的降解,从而高效并特异地阻断或抑制相应基因表达活性的现象。RNAi技术现已成为调控基因的表达,阐明细胞的信号通路和研究功能基因组学的有力工具,并迅速在临床医学上展现出基因药物的诱人前景。目前,人们已开始对RNAi技术在人类疾病预防和治疗中的应用进行研究．这些研究涉及到病毒感染、癌症、代谢性疾患以及遗传病等各个方面。通过综述siRNA分子的作用机制、载体构建以及其在基因缺陷模型的建立等方面的应用,从而展示出RNAi在相关疾病的分子机制研究和基因治疗方面的诱人前景。     

RNA silencing: From discovery and elucidation to application and perspectives

RNA silencing (or RNA interference, RNAi) is a conserved mechanism for regulating gene expression in eukaryotes. The discovery of natural trans-kingdom RNAi indicated that small RNAs act as signaling molecules and enable communication between organisms in different kingdoms. The phenomenon and potential mechanisms of trans-kingdom RNAi are among the most exciting research topics. To better understand trans-kingdom RNAi, we review the history of the discovery and elucidation of RNAi mechanisms. Based on canonical RNAi mechanisms, we summarize the major points of divergence around RNAi pathways in the main eukaryotes’ kingdoms, including plants, animals, and fungi. We review the representative incidents associated with the mechanisms and applications of trans-kingdom RNAi in crop protection, and discuss the critical factors that should be considered to develop successful trans-kingdom RNAi-based crop protection strategies.