The function and mechanism of circular RNAs in gastrointestinal tumours

Abstract Gastrointestinal tumours are tumours that originate in the digestive tract and have extremely high morbidity and mortality. The main categories include: oesophageal, gastric, and colorectal cancers. Circular RNAs are a new class of non‐coding RNAs with a covalent closed‐loop structure without a 5’ cap or a 3’ poly A tail, which can encode a small amount of polypeptide. Recent studies have shown that circRNAs are involved in multiple biological processes during the development of gastrointestinal tumours including proliferation, invasion and metastasis, radio‐ and chemoresistance, and inflammatory responses. Also, the clinical and pathological characteristics of the patient, such as staging and lymph node metastasis, are closely associated with the expression level of circRNAs. Further investigation of the function and the role of circRNAs in the development of gastrointestinal tumours will provide new directions for its clinical diagnosis and treatment.

many different species. 8 In the beginning, circRNAs were considered as "junk" RNAs without any real function. They were regarded as by-products of incorrect splicing, or by-products of processing of precursor mRNAs at their low-abundance stages. 9 Until 2012, large numbers of circRNAs were discovered and identified owing to the advancements in high-throughput sequencing. 8 As research progressed, it became increasingly clear that circRNAs play an important role in various cellular activities and development. 10 Existing evidence shows that circRNAs are closely associated with several pathological and physiological processes in tumours including growth, differentiation, metastasis and invasion of cancer cells. 11 Liu et al 12 demonstrated that circRNA YAP1 inhibits proliferation and invasion of gastric cancer cells. circITGA7 has been found that it has the ability of promoted the growth and metastasis of colorectal cancer cells by Li et al 13 Xia et al 14 observed that circ_0067934 promotes the differentiation of OC cells. A growing number of studies have characterized circRNAs as early diagnostic and prognostic markers. Beyond that, circRNAs can also serve as potential therapeutic targets. 15,16 Although several studies have focused on the circRNAs and tumours of the digestive system, the precise roles and mechanisms of circRNAs remain unclear. Therefore, further elucidation of the specific roles and mechanisms of circRNAs in the development of digestive system tumours is of great significance for guiding clinical diagnosis and treatment.

| Biological Characteristics of CircRNAs
CircRNAs are a newly discovered class of endogenous ncRNAs.
Unlike conventional linear RNAs, the 3' and 5' ends of circRNAs are ligated to form a covalent closed-loop structure. 3 CircRNAs are mainly composed of exons and/or introns. 17 According to their source of sequence, circRNAs can be classified into four categories In addition to the above three main sources, circRNAs can also be generated by cyclization of viral RNA genomes, tRNA, rRNA and snRNA among others. The formation of circRNAs is relatively complex process involving a series of biological steps

| Function and Method of CircRNAs
As an emerging class of ncRNAs that regulate gene expression, the functional mechanisms of circRNAs have received widespread attention. Several recent studies have shown that circRNAs exert their effects mainly in the following ways: 1) Because they contain miRNAbinding sites, circRNAs can act as miRNA sponges (Table 1). They can indirectly regulate the expression of miRNA downstream target genes by preventing the miRNAs from binding to the 3' untranslated regions of the mRNAs. 2) CircRNAs, along with RNA-binding proteins (RBPs), play an important role in changing the RNA splicing modes and mRNA stability. Du et al found that circ-foxo3 binds with CDK2 and P21 to form an RNA-protein complex to inhibit cell cycle progression from G1 to S phase. 22 In addition, circRNAs also interact with RNA polymerases to affect the process of gene transcription. Furthermore, the interactions between ElcircRNAs, microribonucleoprotein and RNA polymerase have an important effect on gene transcription in vivo. For example, EiciRNAs-u1 SNRNP compounds, which are formed by circ-EIF3J and circ-PIAP2 can interact with RNA polymerase Ⅱ to promote the process of parental gene transcription. 23 3)While it is well known that circRNAs are ncRNAs without 5' caps or 3' poly A tails, some studies have found that many circRNAs have internal ribosome entry sites or open reading frames that participate in the transcription and translation of functional proteins. For example, circ-ZNF609 has two initiation factors, the existence of which makes it possible to encode functional proteins. 24 Zheng et al 25  the highly expressed ciRS-7 has the ability of storing miR-7s and releasing them in specific places and at specific times to suppress miR-671-stimulated expression of miR-7. Although the existing reports on circRNAs have laid the foundation for understanding its important cellular roles, it is still necessary to verify the functions that remain unclear and further explore the hereto unknown mechanisms of action of circRNAs.

| CIRCRNA S AND G A S TROINTE S TINAL TUMOUR S
Tumorigenesis is the process of uncontrolled growth of cells which may be stimulated by internal and external carcinogens. The mechanism of tumorigenesis is complex and involves changes at the tissue, cellular and molecular levels. Hanahan and Weinberg 28 reported that tumour cells possess several unique traits including self-sufficiency of growth signals, insensitivity to antigrowth signals, evasion of apoptosis, limitless replicative potential, sustained angiogenesis, tissue invasion and metastasis, avoiding immune destruction, tumour promotion inflammation, deregulating cellular energetics and genome instability and mutation. Recent studies have shown that circRNAs play a significant role in the development of tumours, and are closely associated with the characteristics processes in tumours including proliferation, invasion and metastasis, chemoradiotherapy resistance and inflammatory response 29 (Figure 2).

| CircRNA and the inflammatory responses of gastrointestinal tumours
In the recent years, the relationship between inflammation and tumour has become the focus of intense research in tumour immunity.
It has been suggested that approximately 25% of human tumours are caused by uncontrolled inflammation. 69

| A SSO CIATI ON B E T WEEN CIRCRNA AND THE CLINI COPATHOLOGY OF G A S TROINTE S TINAL TUMOUR S
In   Tissue  Tissue  Tissue  Tissue  Tissue, Plasma  plasma  Tissue  Tissue, Plasma  Tissue  Tissue  Tissue  Tissue  Tissue, Plasma  Tissue  Tissue  Tissue  Tissue  Tissue  Tissue  Tissue  Tissue 84  80  81  59  45  43  82  85  87  86  137  104  83  138  58  139  139  140  141  113  142  143  144  145  146  47  147 Tissue  Tissue  Tissue  Tissue  Tissue  Tissue  Tissue  Tissue  Tissue  Tissue  Tissue  Tissue  Tissue  Tissue  Tissue  Tissue  Tissue  Tissue  Tissue  Tissue  Among the biological functions, the most important and useful function is its potential in early diagnosis and therapeutics. The stable differential expression of some circRNAs in the serum of tumour patients can be used as early diagnostic markers. In addition, studies on the mechanism of circRNAs can provide more effective clinical solutions for the treatment of gastrointestinal tumours.

| CON CLUS IONS
This review mainly discussed the role of circRNAs in the development and clinicopathological characteristics of digestive tract tumours. We briefly highlighted the connecting links among circRNAs, miRNAs and target genes, and summarized the biological roles of circRNAs in the three major digestive tract tumours. Finally, we discussed relationship between circRNAs expression and clinicopathological characteristics, all of which will lay a solid foundation for further functional studies of various circRNAs.

ACK N OWLED G EM ENTS
This work was supported by the National Natural Science

CO N FLI C T S O F I NTE R E S T
The authors declare no competing financial interest.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available from the corresponding author upon reasonable request.