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Non-coding RNAs (ncRNAs) are a heterogeneous group of transcripts that, by definition, are not translated into proteins. Since their discovery, ncRNAs have emerged as important regulators of multiple biological functions across a range of cell types and tissues, and their dysregulation has been implicated in disease. Notably, much research has focused on the link between microRNAs (miRNAs) and human cancers, although other ncRNAs, such as long non-coding RNAs (lncRNAs) and circular RNAs (circRNAs), are also emerging as relevant contributors to human disease. In this Review, we summarize our current understanding of the roles of miRNAs, lncRNAs and circRNAs in cancer and other major human diseases, notably cardiovascular, neurological and infectious diseases. Further, we discuss the potential use of ncRNAs as biomarkers of disease and as therapeutic targets.In this Review, the authors describe our current knowledge of the role of microRNAs, long non-coding RNAs and circular RNAs in disease, with a focus on cardiovascular, neurological, infectious diseases and cancer. Further, they discuss the potential use of non-coding RNAs as disease biomarkers and as therapeutic targets.

Discoveries over the past decade portend a paradigm shift in molecular biology; evidence suggests that RNA is not only functional as a messenger between DNA and protein but also involved in the regulation of genome organization and gene expression. This Timeline article surveys the emergence of the previously unsuspected world of regulatory RNA from a historical perspective. Discoveries over the past decade portend a paradigm shift in molecular biology. Evidence suggests that RNA is not only functional as a messenger between DNA and protein but also involved in the regulation of genome organization and gene expression, which is increasingly elaborate in complex organisms. Regulatory RNA seems to operate at many levels; in particular, it plays an important part in the epigenetic processes that control differentiation and development. These discoveries suggest a central role for RNA in human evolution and ontogeny. Here, we review the emergence of the previously unsuspected world of regulatory RNA from a historical perspective.

Key Points Functional diversification and expansion of silencing pathways in plants relies on duplication of DICER-LIKE proteins (DCLs) and ARGONAUTE proteins (AGOs). The main small-RNA classes in plants are microRNAs (miRNAs), 21–22-nucleotide secondary siRNAs and 24-nucleotide heterochromatic siRNAs (hetsiRNAs). All small RNAs in plants are modified at their 3′-end by 2′- O -methylation, including miRNAs, which lack this modification in animals. This modification confers stability and protection from degradation. Plant miRNAs are mainly involved in post-transcriptional gene silencing (PTGS) by transcript cleavage or translational repression, and also trigger secondary siRNA production from RNA polymerase II (Pol II) transcripts. Secondary small RNAs of 21 and 22 nucleotides in length are involved in cleavage or translational repression of target transcripts in cis and in trans . They are also able to initiate TGS by establishing DNA methylation at particular loci. The majority of siRNAs in plants are 24-nucleotide hetsiRNAs and are involved in silencing repeats and transposable elements by RNA-directed DNA methylation (RdDM). Small RNAs in plants are involved in reproductive transitions, including meiosis and gametogenesis, and regulate important epigenetic mechanisms such as genomic imprinting and paramutation. Plant genomes encode diverse small RNAs, such as microRNAs, secondary siRNAs, heterochromatic siRNAs and various RNA-dependent RNA polymerases, DICER proteins and ARGONAUTE proteins. Together, these constitute several genetic and epigenetic silencing pathways with diverse cellular and developmental functions, in processes including reproductive transitions, genomic imprinting and paramutation. Plant genomes encode various small RNAs that function in distinct, yet overlapping, genetic and epigenetic silencing pathways. However, the abundance and diversity of small-RNA classes varies among plant species, suggesting coevolution between environmental adaptations and gene-silencing mechanisms. Biogenesis of small RNAs in plants is well understood, but we are just beginning to uncover their intricate regulation and activity. Here, we discuss the biogenesis of plant small RNAs, such as microRNAs, secondary siRNAs and heterochromatic siRNAs, and their diverse cellular and developmental functions, including in reproductive transitions, genomic imprinting and paramutation. We also discuss the diversification of small-RNA-directed silencing pathways through the expansion of RNA-dependent RNA polymerases, DICER proteins and ARGONAUTE proteins.

Spatial transcriptomics reveals the spatial context of gene expression, but current methods are limited to assaying polyadenylated (A-tailed) RNA transcripts. Here we demonstrate that enzymatic in situ polyadenylation of RNA enables detection of the full spectrum of RNAs, expanding the scope of sequencing-based spatial transcriptomics to the total transcriptome. We demonstrate that our spatial total RNA-sequencing (STRS) approach captures coding RNAs, noncoding RNAs and viral RNAs. We apply STRS to study skeletal muscle regeneration and viral-induced myocarditis. Our analyses reveal the spatial patterns of noncoding RNA expression with near-cellular resolution, identify spatially defined expression of noncoding transcripts in skeletal muscle regeneration and highlight host transcriptional responses associated with local viral RNA abundance. STRS requires adding only one step to the widely used Visium spatial total RNA-sequencing protocol from 10x Genomics, and thus could be easily adopted to enable new insights into spatial gene regulation and biology. Spatial RNA sequencing is extended beyond poly-A transcripts to capture the full transcriptome.

MicroRNAs (miRNAs) are short non-coding RNAs that inhibit the expression of target genes by directly binding to their mRNAs. miRNAs are transcribed as precursor molecules, which are subsequently cleaved by the endoribonucleases Drosha and Dicer. Mature miRNAs are bound by a member of the Argonaute (AGO) protein family to form the RNA-induced silencing complex (RISC) in a process termed RISC loading. Advances in structural analyses of Drosha and Dicer complexes enabled elucidation of the mechanisms that drive these molecular machines. Transcription of miRNAs, their processing by Drosha and Dicer and RISC loading are key steps in miRNA biogenesis, and various additional factors facilitate, support or inhibit these processes. Recent work has revealed that regulatory factors not only coordinate individual miRNA processing steps but also connect miRNA biogenesis with other cellular processes. Protein phosphorylation, for example, links miRNA biogenesis to various signalling pathways, and such modifications are often associated with disease. Furthermore, not all miRNAs follow canonical processing routes, and many non-canonical miRNA biogenesis pathways have recently been characterized.

Over the past decade, the amount of research and the number of publications on associations between circulating small and long non-coding RNAs (ncRNAs) and cancer have grown exponentially. Particular focus has been placed on the development of diagnostic and prognostic biomarkers to enable efficient patient management — from early detection of cancer to monitoring for disease recurrence or progression after treatment. Owing to their high abundance and stability, circulating ncRNAs have potential utility as non-invasive, blood-based biomarkers that can provide information on tumour biology and the effects of treatments, such as targeted therapies and immunotherapies. Increasing evidence highlights the roles of ncRNAs in cell-to-cell communication, with a number of ncRNAs having the capacity to regulate gene expression outside of the cell of origin through extracellular vesicle-mediated transfer to recipient cells, with implications for cancer progression and therapy resistance. Moreover, ‘foreign’ microRNAs (miRNAs) encoded by non-human genomes (so-called xeno-miRNAs), such as viral miRNAs, have been shown to be present in human body fluids and can be used as biomarkers. Herein, we review the latest developments in the use of circulating ncRNAs as diagnostic and prognostic biomarkers and discuss their roles in cell-to-cell communication in the context of cancer. We provide a compendium of miRNAs and long ncRNAs that have been reported in the literature to be present in human body fluids and that have the potential to be used as diagnostic and prognostic cancer biomarkers.

Key Points Non-coding RNAs (ncRNAs) — which include microRNAs (miRNAs), repetitive RNAs, intronic RNAs, and long ncRNAs (lncRNAs) — are a diverse group of biomolecules with broad potential to control gene expression. Compounds that target ncRNAs have the potential to control expression of disease-related genes Development of compounds to target ncRNAs can benefit from understanding the lessons learnt from decades of research using antisense oligonucleotides (ASOs) and duplex RNAs to control expression of mRNA. ASOs that affect splicing or that are complementary to miRNAs are already being tested in multiple clinical trials in a variety of diseases, including cancer and muscular dystrophy lncRNAs can affect transcription or splicing and are emerging as a promising class of novel drug targets. Non-coding RNAs (ncRNAs) may affect normal gene expression and disease progression, thereby representing potential drug targets. Here, Matsui and Corey assess the potential and challenges in therapeutically exploiting ncRNA species — including microRNA, intronic RNA, repetitive RNA and long ncRNA — highlighting key lessons learned during the development of technologies targeting mRNA. Most of the human genome encodes RNAs that do not code for proteins. These non-coding RNAs (ncRNAs) may affect normal gene expression and disease progression, making them a new class of targets for drug discovery. Because their mechanisms of action are often novel, developing drugs to target ncRNAs will involve equally novel challenges. However, many potential problems may already have been solved during the development of technologies to target mRNA. Here, we discuss the growing field of ncRNA — including microRNA, intronic RNA, repetitive RNA and long non-coding RNA — and assess the potential and challenges in their therapeutic exploitation.

Key Points MicroRNAs (miRNAs) are small non-coding RNAs that negatively regulate target gene expression through mRNA degradation or translational inhibition. The miRNA biogenesis pathway is a multi-step process that has a crucial role in regulating miRNA maturation. miRNAs can be oncogenes or tumour suppressors and are globally repressed in cancers. Mutations in or dysregulation of components of the miRNA biogenesis pathway are frequently found in cancers and have important functions in oncogenesis. Important oncogenic signalling proteins — such as LIN28A, LIN28B, epidermal growth factor receptor (EGFR) and Hippo — target miRNA biogenesis in cancers. The targeting of abnormal miRNA biogenesis pathways is a novel, promising therapeutic strategy for cancers. The microRNA (miRNA) biogenesis pathway is frequently altered in cancer, leading to global downregulation of miRNA levels in some cancer types. This Review discusses the alterations that affect miRNA biogenesis in cancer. MicroRNAs (miRNAs) are critical regulators of gene expression. Amplification and overexpression of individual 'oncomiRs' or genetic loss of tumour suppressor miRNAs are associated with human cancer and are sufficient to drive tumorigenesis in mouse models. Furthermore, global miRNA depletion caused by genetic and epigenetic alterations in components of the miRNA biogenesis machinery is oncogenic. This, together with the recent identification of novel miRNA regulatory factors and pathways, highlights the importance of miRNA dysregulation in cancer.

Tumor-released RNA may mediate intercellular communication and serve as biomarkers. Here we develop a protocol enabling quantitative, minimally biased analysis of extracellular RNAs (exRNAs) associated with microvesicles, exosomes (collectively called EVs), and ribonucleoproteins (RNPs). The exRNA complexes isolated from patient-derived glioma stem-like cultures exhibit distinct compositions, with microvesicles most closely reflecting cellular transcriptome. exRNA is enriched in small ncRNAs, such as miRNAs in exosomes, and precisely processed tRNA and Y RNA fragments in EVs and exRNPs. EV-enclosed mRNAs are mostly fragmented, and UTRs enriched; nevertheless, some full-length mRNAs are present. Overall, there is less than one copy of non-rRNA per EV. Our results suggest that massive EV/exRNA uptake would be required to ensure functional impact of transferred RNA on brain recipient cells and predict the most impactful miRNAs in such conditions. This study also provides a catalog of diverse exRNAs useful for biomarker discovery and validates its feasibility on cerebrospinal fluid. While circulating DNA has been extensively explored as a potential cancer biomarker, RNA potential has been overlooked so far. Here the authors present a comprehensive analysis of extracellular RNA secreted by glioblastoma cells that could prove a valuable resource for biomarker discovery and a means of intercellular communication.

RNA silencing is a well-established antiviral immunity system in plants, in which small RNAs guide Argonaute proteins to targets in viral RNA or DNA, resulting in virus repression. Virus-encoded suppressors of silencing counteract this defence system. In this Review, we discuss recent findings about antiviral RNA silencing, including the movement of RNA through plasmodesmata and the differentiation between plant self and viral RNAs. We also discuss the emerging role of RNA silencing in plant immunity against non-viral pathogens. This immunity is mediated by transkingdom movement of RNA into and out of the infected plant cells in vesicles or as extracellular nucleoproteins and, like antiviral immunity, is influenced by the silencing suppressors encoded in the pathogens’ genomes. Another effect of RNA silencing on general immunity involves host-encoded small RNAs, including microRNAs, that regulate NOD-like receptors and defence signalling pathways in the innate immunity system of plants. These RNA silencing pathways form a network of processes with both positive and negative effects on the immune systems of plants.RNA silencing through small RNAs is a major antiviral immunity system in plants. Recent findings are uncovering the roles of RNA silencing in immunity against non-viral pathogens, which is mediated by trans-kingdom RNA movements in vesicles or as extracellular nucleoproteins. RNA silencing also enables the crosstalk between other plant immunity systems.