Going circular: history, present, and future of circRNAs in cancer

Sep 24, 2026

Uploaded by Đào

Review article tracing the discovery and evolving understanding of circular RNAs (circRNAs) in cancer. It summarizes early evidence for RNA circularity, subsequent identification of endogenous spliced circRNAs, and how RNA-seq and bioinformatics revealed widespread abundance and genomic back-splicing mechanisms. The article contrasts canonical splicing with back-splicing, explaining structural features that confer stability and inform biogenesis in normal versus pathological contexts.

LanguageEnglish

CategoryHealthcare

FormatPDF

Pages18

Reading Time45 min

AccessFree to download

Background: the advent of circRNAs

Biogenesis of circRNAs in normal and pathological contexts

General features of circRNAs

Oncogene [www.nature.com/onc](www.nature.com/onc) REVIEW ARTICLE OPEN Going circular: history, present, and future of circRNAs in cancer 1 ✉, David C. Michael 1, Tanvi H. Visal2,3, Radu Pirlog 2,3, Michael Ladomery 4 and George A. Calin 2,3 ✉ Giuseppina Pisignano © The Author(s) 2023 BACKGROUND: THE ADVENT OF CIRCRNAS Despite their recent fame, the discovery of circRNAs dates back over 40 years. CircRNAs were ﬁrst discovered in the murine respirovirus (Sendai virus) [1] and in plant pathogenic viruses termed viroids [2] . The initial physical evidence of the existence of a circular form of RNA was obtained by electron microscope analysis of the cytoplasmic fraction of eukaryotic (HeLa) cells in 1979 [3] and later in 1986 when they were identiﬁed within the hepatitis delta virus (HDV) [4] . Such circular shaped viral genomes possess the distinct property of generating multiple copies of an RNA through the rolling circle replication mechanism that facilitates the spread of infection more efﬁciently. However, evidence of another type of circRNA as intermediate molecules excised from pre-mRNA started emerging [5–7] . In 1991, a group of researchers found the ﬁrst examples of endogenous spliced circRNAs in humans, transcribed from the DCC gene and end-joined in a scrambled order compared to the canonical linear sequence [8] . In parallel, the mouse Sry gene, encoding a crucial molecule responsible for sex determination during embryogenesis, was found to be expressed in adult mice testes exclusively as a 1.23-kb circular RNA [9] . During the late 1990s and early 2000s, several other studies showed that circRNA-producing genes are widespread in eukaryotic cells from ﬂies to mammals including humans [10–18] . Nevertheless, the lack of evidence of their translation into polypeptides left researchers skeptical about the functional signiﬁcance of such RNAs, and for several decades circRNAs were commonly disregarded as mis-splicing artifacts or by-products of pre-mRNA processing [19] . With the advent of RNA sequencing (RNA-seq) technologies and bioinformatics, the true abundance ofcircRNAs was revealed. In 2012, an unexpected number of human genes were reported to express “scrambled exons” resulting in circular RNA isoforms [20] . A subsequent analysis of nonpolyadenylated libraries prepared from ribosomal RNA-depleted RNAs, revealed >25,000 distinct RNA species containing “noncolinear exons” in human ﬁbroblasts [21] and, at the same time, anew genome-wide in-silico approach identiﬁed ~2000 human, 700 nematode and 1900 mouse circRNAs that were more stable than the associated linear mRNAs in vivo [18] . Since their discovery, thousands of circRNAs have been found tobe present in multiple organisms and their expression been associated with developmental stages, physiological conditions, and diseases including cancer. This has opened up a new ﬁeld of study aimed at elucidating their biogenesis and function as an essential part of gene expression programs across eukaryotes (Fig. 1). BIOGENESIS OF CIRCRNAS IN NORMAL AND PATHOLOGICAL CONTEXTS General features of circRNAs Pre-mRNA splicing, facilitated by the spliceosome, is a mechanism that is tightly coupled with transcription [22]. During this process, introns are removed from precursor mRNAs (pre‐mRNAs) and the exons are covalently joined to create linear mRNA molecules that are translated into proteins. By contrast, back-splicing allows a downstream splice donor (5′ splice site) to join backwards to an upstream splice acceptor (3′ splice site), resulting in a closed continuous headto-tail molecule known as a circRNA (Fig. 2). The resulting molecules are devoid of terminal 5’ caps and 3′ polyadenylated tails [23–25], are less accessible to exonucleases and are consequently more stable than linear RNAs [26]. By rearranging the order of genomic information, circRNAs also provide a unique opportunity to further diversify gene expression across eukaryotes. 1Departm

FAQs

How were circRNAs first discovered and what key early evidence supported their circular form?

CircRNAs were first discovered in a murine respirovirus and in plant pathogenic viroids. Electron microscopy later provided physical evidence for circular RNA forms, including identification within hepatitis delta virus.

Why did researchers initially question circRNA biological function?

Lack of evidence for translation of circRNAs into polypeptides led to skepticism, and many circRNAs were treated as mis-splicing artifacts or by-products of pre-mRNA processing.

What is the difference between canonical splicing and back-splicing in circRNA formation?

Canonical splicing joins exons in a linear head-to-tail fashion, while back-splicing links a downstream splice donor to an upstream splice acceptor, producing a closed head-to-tail circRNA lacking terminal 5' caps and 3' polyadenylated tails.

Sep 24, 2026

Uploaded by Đào