Long non-coding RNAs (lncRNAs) are a diverse class of RNAs that engage in numerous biological processes across every branch of life. Although initially discovered as mRNA-like transcripts that do not encode proteins, recent studies have revealed features of lncRNAs that further distinguish them from mRNAs. In this Review, we describe special events in the lifetimes of lncRNAs - before, during and after transcription - and discuss how these events ultimately shape the unique characteristics and functional roles of lncRNAs.

Unique features of long non-coding RNA biogenesis and function

Circular RNAs (circRNAs) are covalently closed, endogenous biomolecules in eukaryotes with tissue-specific and cell-specific expression patterns, whose biogenesis is regulated by specific cis-acting elements and trans-acting factors. Some circRNAs are abundant and evolutionarily conserved, and many circRNAs exert important biological functions by acting as microRNA or protein inhibitors ('sponges'), by regulating protein function or by being translated themselves. Furthermore, circRNAs have been implicated in diseases such as diabetes mellitus, neurological disorders, cardiovascular diseases and cancer. Although the circular nature of these transcripts makes their detection, quantification and functional characterization challenging, recent advances in high-throughput RNA sequencing and circRNA-specific computational tools have driven the development of state-of-the-art approaches for their identification, and novel approaches to functional characterization are emerging.

The biogenesis, biology and characterization of circular RNAs.

Unlike linear RNAs terminated with 5′ caps and 3′ tails, circular RNAs are characterized by covalently closed loop structures with neither 5′ to 3′ polarity nor polyadenylated tail. This intrinsic characteristic has led to the general under-estimation of the existence of circular RNAs in previous polyadenylated transcriptome analyses. With the advent of specific biochemical and computational approaches, a large number of circular RNAs from back-spliced exons (circRNAs) have been identified in various cell lines and across different species. Recent studies have uncovered that back-splicing requires canonical spliceosomal machinery and can be facilitated by both complementary sequences and specific protein factors. In this review, we highlight our current understanding of the regulation of circRNA biogenesis, including both the competition between splicing and back-splicing and the previously under-appreciated alternative circularization.

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Regulation of circRNA biogenesis

Circular RNA: A new star of noncoding RNAs

The Biogenesis, Functions, and Challenges of Circular RNAs.

Noncoding RNAs (ncRNAs) have numerous roles in development and disease, and one of the prominent roles is to regulate gene expression A vast number of circular RNAs (circRNAs) have been identified, and some have been shown to function as microRNA sponges in animal cells Here, we report a class of circRNAs associated with RNA polymerase II in human cells In these circRNAs, exons are circularized with introns 'retained' between exons; we term them exon-intron circRNAs or EIciRNAs EIciRNAs predominantly localize in the nucleus, interact with U1 snRNP and promote transcription of their parental genes Our findings reveal a new role for circRNAs in regulating gene expression in the nucleus, in which EIciRNAs enhance the expression of their parental genes in cis, and highlight a regulatory strategy for transcriptional control via specific RNA-RNA interaction between U1 snRNA and EIciRNAs

Exon-intron circular RNAs regulate transcription in the nucleus

Nat. Struct. Mol. Biol. 22, 256–264 (2015); published online 9 February 2015; corrected after print 19 August 2016 In the version of this article initially published, the convergent primers depicted in the schematic in Figure 1b were incorrectly placed. The error has been corrected in the HTML and PDF versions of the article.

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Corrigendum: Exon-intron circular RNAs regulate transcription in the nucleus.

A major portion of the mammalian genome is transcribed to produce large numbers of noncoding RNAs (ncRNAs). During the past decade, the discovery of small RNAs, including the microRNAs (miRNA) and small interfering RNAs (siRNA), has led to important advances in biology. The breadth of the ncRNA field of study has substantially expanded and many recent results have revealed a range of functions that can be attributed to the miRNAs and other ncRNAs. For example, H19 RNA, HOTAIR RNA, transcribed ultraconserved regions (T-UCRs), natural antisense RNA, transfer RNA and mitochondrial noncoding RNA have been suggested to play important roles in cancers and other diseases as well as in diverse cellular processes. In this review, we focus on the current status of several classes of ncRNAs associated with cancer with the emphasis on those that are not microRNAs.

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Mei Lin

Papers

Exon-intron circular RNAs regulate transcription in the nucleus

Corrigendum: Exon-intron circular RNAs regulate transcription in the nucleus.

Noncoding RNAs: Different roles in tumorigenesis