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Systematic interrogation of gene function requires the ability to perturb gene expression in a robust and generalizable manner. Here we describe structure-guided engineering of a CRISPR-Cas9 complex to mediate efficient transcriptional activation at endogenous genomic loci. We used these engineered Cas9 activation complexes to investigate single-guide RNA (sgRNA) targeting rules for effective transcriptional activation, to demonstrate multiplexed activation of ten genes simultaneously, and to upregulate long intergenic non-coding RNA (lincRNA) transcripts. We also synthesized a library consisting of 70,290 guides targeting all human RefSeq coding isoforms to screen for genes that, upon activation, confer resistance to a BRAF inhibitor. The top hits included genes previously shown to be able to confer resistance, and novel candidates were validated using individual sgRNA and complementary DNA overexpression. A gene expression signature based on the top screening hits correlated with markers of BRAF inhibitor resistance in cell lines and patient-derived samples. These results collectively demonstrate the potential of Cas9-based activators as a powerful genetic perturbation technology. The CRISPR-Cas9 system, a powerful tool for genome editing, has been engineered to activate endogenous gene transcription specifically and potently on a genome-wide scale and applied to a large-scale gain-of-function screen for studying melanoma drug resistance. CRISPR-Cas9 used for gene-expression regulation The CRISPR-Cas9 system has emerged as a powerful tool for genome editing and transcriptional regulation of specific genes. Feng Zhang and colleagues have successfully modified the system to specifically and potently activate endogenous gene transcription on a genome-wide scale, such that it can be used for large-scale functional genomics screens. Application to a genome-wide screen of melanoma cells for genes which when overexpressed can confer resistance to a BRAF inhibitor demonstrates the feasibility of such screens, and also led to the discovery of potential new resistance mechanisms.

Insertion sequence (IS) elements are the simplest autonomous transposable elements found in prokaryotic genomes 1 . We recently discovered that IS110 family elements encode a recombinase and a non-coding bridge RNA (bRNA) that confers modular specificity for target DNA and donor DNA through two programmable loops 2 . Here we report the cryo-electron microscopy structures of the IS110 recombinase in complex with its bRNA, target DNA and donor DNA in three different stages of the recombination reaction cycle. The IS110 synaptic complex comprises two recombinase dimers, one of which houses the target-binding loop of the bRNA and binds to target DNA, whereas the other coordinates the bRNA donor-binding loop and donor DNA. We uncovered the formation of a composite RuvC–Tnp active site that spans the two dimers, positioning the catalytic serine residues adjacent to the recombination sites in both target and donor DNA. A comparison of the three structures revealed that (1) the top strands of target and donor DNA are cleaved at the composite active sites to form covalent 5′-phosphoserine intermediates, (2) the cleaved DNA strands are exchanged and religated to create a Holliday junction intermediate, and (3) this intermediate is subsequently resolved by cleavage of the bottom strands. Overall, this study reveals the mechanism by which a bispecific RNA confers target and donor DNA specificity to IS110 recombinases for programmable DNA recombination. Using cryo-electron microscopy, the structural mechanism by which non-coding bridge RNA confers target and donor DNA specificity to IS110 recombinases for programmable DNA recombination is explored.

The finding that sequence-specific gene silencing occurs in response to the presence of double-stranded RNAs has had an enormous impact on biology, uncovering an unsuspected level of regulation of gene expression. This process, known as RNA interference (RNAi) or RNA silencing, involves small non-coding RNAs, which associate with nuclease-containing regulatory complexes and then pair with complementary messenger RNA targets, thereby preventing the expression of these mRNAs. Remarkable progress has been made towards understanding the underlying mechanisms of RNAi, raising the prospect of deciphering the 'RNAi code' that, like transcription factors, allows the fine-tuning and networking of complex suites of gene activity, thereby specifying cellular physiology and development.

Circular RNAs (circRNAs) are novel clusters of endogenous noncoding RNAs (ncRNAs) that are widely expressed in eukaryotic cells. In contrast to the generation of linear RNA transcripts, circRNAs undergo a \"back-splicing\" process to form a continuous, covalently closed, stable loop structure without 5' or 3' polarities and poly (A) tails during posttranscriptional modification. Due to the widespread availability of several technologies, especially high-throughput RNA sequencing, numerous circRNAs have been discovered not only in mammals but also in plants and insects. Notably, due to their abilities to serve as microRNA (miRNA) \"sponges\", miRNA \"reservoirs\", regulate gene expression and encode proteins, circRNAs participate in the development and progression of different immune responses and immune diseases by enriching various forms of epigenetic modification. CircRNAs have been demonstrated to be expressed in a tissue-specific and pathogenesis-related manner during the occurrence of multiple immune diseases. Additionally, because of their circular configurations, expression in blood and peripheral tissues and coexistence with exosomes, circRNAs show inherent conservation along with environmental resistance stability and may be regarded as potential biomarkers or therapeutic targets for some immune diseases. In this review, we summarize the characteristics, functions and mechanisms of circRNAs and their involvement in immune responses and diseases. Although our knowledge of circRNAs remains preliminary, this field is worthy of deeper exploration and greater research efforts.

Rhizobial infection and root nodule formation in legumes require recognition of signal molecules produced by the bacteria and their hosts. Here, we show that rhizobial transfer RNA (tRNA)-derived small RNA fragments (tRFs) are signal molecules that modulate host nodulation. Three families of rhizobial tRFs were confirmed to regulate host genes associated with nodule initiation and development through hijacking the host RNA-interference machinery that involves ARGONAUTE 1. Silencing individual tRFs with the use of short tandem target mimics or by overexpressing their targets represses root hair curling and nodule formation, whereas repressing these targets with artificial microRNAs identical to the respective tRFs or mutating these targets with CRISPR-Cas9 promotes nodulation. Our findings thus uncover a bacterial small RNA–mediated mechanism for prokaryote-eukaryote interaction and may pave the way for enhancing nodulation efficiency in legumes.

Role for long non-coding RNAs The RNA-binding protein Staufen1 (STAU1) promotes the degradation of double-stranded messenger RNA in the process known as Staufen-mediated decay (SMD). STAU1 binds to transcripts in the 3′ untranslated region (UTR), and although a specific stem-loop binding site had been defined for one SMD target, it was unclear how STAU1 was directed to other SMD targets that lack this structure. Chenguang Gong and Lynne Maquat report that pairing of Alu element sequences in long non-coding RNAs (lncRNAs) and in the 3′ UTR of the SMD target generates a double-stranded RNA structure that STAU1 recognizes. This result highlights a new function for lncRNAs. Staufen 1 (STAU1) protein binds regions of dsRNA in the 3′ UTR of mRNAs and promotes their degradation, a process known as SMD (Staufen-mediated mRNA decay). Although a specific stem-loop binding site had been defined for one SMD target, it was unclear how STAU1 was directed to other SMD targets that lack this structure. This paper reports that pairing of Alu element sequences in long non-coding RNAs (lncRNAs) and in the 3′ UTR of the SMD target generates a dsRNA structure that STAU1 recognizes. This result highlights a new function for lncRNAs. Staufen 1 (STAU1)-mediated messenger RNA decay (SMD) involves the degradation of translationally active mRNAs whose 3′-untranslated regions (3′ UTRs) bind to STAU1, a protein that binds to double-stranded RNA 1 , 2 . Earlier studies defined the STAU1-binding site within ADP-ribosylation factor 1 ( ARF1 ) mRNA as a 19-base-pair stem with a 100-nucleotide apex 2 . However, we were unable to identify comparable structures in the 3′ UTRs of other targets of SMD. Here we show that STAU1-binding sites can be formed by imperfect base-pairing between an Alu element in the 3′ UTR of an SMD target and another Alu element in a cytoplasmic, polyadenylated long non-coding RNA (lncRNA). An individual lncRNA can downregulate a subset of SMD targets, and distinct lncRNAs can downregulate the same SMD target. These are previously unappreciated functions of non-coding RNAs and Alu elements 3 , 4 , 5 . Not all mRNAs that contain an Alu element in the 3′ UTR are targeted for SMD even in the presence of a complementary lncRNA that targets other mRNAs for SMD. Most known trans -acting RNA effectors consist of fewer than 200 nucleotides, and these include small nucleolar RNAs and microRNAs. Our finding that the binding of STAU1 to mRNAs can be transactivated by lncRNAs uncovers an unexpected strategy that cells use to recruit proteins to mRNAs and mediate the decay of these mRNAs. We name these lncRNAs half-STAU1-binding site RNAs (1/2-sbsRNAs).

Noncoding RNAs (ncRNAs) are a large segment of the transcriptome that do not have apparent protein-coding roles, but they have been verified to play important roles in diverse biological processes, including disease pathogenesis. With the development of innovative technologies, an increasing number of novel ncRNAs have been uncovered; information about their prominent tissue-specific expression patterns, various interaction networks, and subcellular locations will undoubtedly enhance our understanding of their potential functions. Here, we summarized the principles and innovative methods for identifications of novel ncRNAs that have potential functional roles in cancer biology. Moreover, this review also provides alternative ncRNA databases based on high-throughput sequencing or experimental validation, and it briefly describes the current strategy for the clinical translation of cancer-associated ncRNAs to be used in diagnosis.

Non-coding RNAs have distinct regulatory roles in the pathogenesis of joint diseases including osteoarthritis (OA) and rheumatoid arthritis (RA). As the amount of high-throughput profiling studies and mechanistic investigations of microRNAs, long non-coding RNAs and circular RNAs in joint tissues and biofluids has increased, data have emerged that suggest complex interactions among non-coding RNAs that are often overlooked as critical regulators of gene expression. Identifying these non-coding RNAs and their interactions is useful for understanding both joint health and disease. Non-coding RNAs regulate signalling pathways and biological processes that are important for normal joint development but, when dysregulated, can contribute to disease. The specific expression profiles of non-coding RNAs in various disease states support their roles as promising candidate biomarkers, mediators of pathogenic mechanisms and potential therapeutic targets. This Review synthesizes literature published in the past 2 years on the role of non-coding RNAs in OA and RA with a focus on inflammation, cell death, cell proliferation and extracellular matrix dysregulation. Research to date makes it apparent that ‘non-coding’ does not mean ‘non-essential’ and that non-coding RNAs are important parts of a complex interactome that underlies OA and RA.Non-coding RNAs such as microRNAs, long non-coding RNAs and circular RNAs regulate signalling pathways that are important in joint development, homeostasis and disease. A better understanding of the non-coding RNA interactome could lead to new therapies for joint diseases.

Small noncoding RNAs regulate processes essential for cell growth and development, including mRNA degradation, translational repression, and transcriptional gene silencing (TGS). During a search for candidate mammalian factors for TGS, we purified a complex that contains small RNAs and Riwi, the rat homolog to human Piwi. The RNAs, frequently 29 to 30 nucleotides in length, are called Piwi-interacting RNAs (piRNAs), 94% of which map to 100 defined (<=101 kb) genomic regions. Within these regions, the piRNAs generally distribute across only one genomic strand or distribute on two strands but in a divergent, nonoverlapping manner. Preparations of piRNA complex (piRC) contain rRecQ1, which is homologous to qde-3 from Neurospora, a gene implicated in silencing pathways. Piwi has been genetically linked to TGS in flies, and slicer activity cofractionates with the purified complex. These results are consistent with a gene-silencing role for piRC in mammals.

The 11-subunit RNA exosome is thought to regulate the mammalian noncoding transcriptome; here, a mouse model is generated in which the essential Exosc3 subunit of the RNA exosome in B cells is conditionally deleted, revealing a link between sites of genomic RNA exosome function and AID-mediated chromosomal translocations. Noncoding RNAs pinpoint AID in B cells It is difficult to identify rare non-coding RNA (ncRNA) species because of their low abundance in cells and the fact that they are rapidly degraded, mainly through the action of the cellular non-coding RNA 3′–5′ degradation complex, RNA exosome. Uttiya Basu and colleagues have generated a mouse model in which an essential subunit (exosome component 3, Exosc3 ) of the RNA exosome can be conditionally inactivated in B cells. Exosc3-deficient B cells lack the recombination and mutagenesis activities that are necessary for generating antibodies. Many non-coding RNAs normally degraded are found in these cells, including xTSS-RNAs, a type of antisense RNA encoded at transcription start sites. Surprisingly, the locations of the xTSS-RNAs correlate with sites of translocation breakages. The model suggested is that antisense transcription of the ncRNAs recruits activation-induced cytidine deaminase (AID) and results in formation of single-strand DNA; pairing with the RNAs makes R-loops that can lead to genomic instability. The vast majority of the mammalian genome has the potential to express noncoding RNA (ncRNA). The 11-subunit RNA exosome complex is the main source of cellular 3′–5′ exoribonucleolytic activity and potentially regulates the mammalian noncoding transcriptome 1 . Here we generated a mouse model in which the essential subunit Exosc3 of the RNA exosome complex can be conditionally deleted. Exosc3 -deficient B cells lack the ability to undergo normal levels of class switch recombination and somatic hypermutation, two mutagenic DNA processes used to generate antibody diversity via the B-cell mutator protein activation-induced cytidine deaminase (AID) 2 , 3 . The transcriptome of Exosc3 -deficient B cells has revealed the presence of many novel RNA exosome substrate ncRNAs. RNA exosome substrate RNAs include xTSS-RNAs, transcription start site (TSS)-associated antisense transcripts that can exceed 500 base pairs in length and are transcribed divergently from cognate coding gene transcripts. xTSS-RNAs are most strongly expressed at genes that accumulate AID-mediated somatic mutations and/or are frequent translocation partners of DNA double-strand breaks generated at Igh in B cells 4 , 5 . Strikingly, translocations near TSSs or within gene bodies occur over regions of RNA exosome substrate ncRNA expression. These RNA exosome-regulated, antisense-transcribed regions of the B-cell genome recruit AID and accumulate single-strand DNA structures containing RNA–DNA hybrids. We propose that RNA exosome regulation of ncRNA recruits AID to single-strand DNA-forming sites of antisense and divergent transcription in the B-cell genome, thereby creating a link between ncRNA transcription and overall maintenance of B-cell genomic integrity.