Melika Amelimojarad,Mandana AmeliMojarad*

1Department of Molecular Biology,Kharazmi University,Iran.

Abstract Circular RNAs(CircRNA)were recently found as a closed continuous loop subset of competing endogenous RNAs,with an ability to control gene expression by microRNA sponging.There are many studies on circRNAs potential roles in cancer development.lately;studies have described the important roles of CircRNAs in cancers,indicating their oncogene role to promote cancer or as a tumor suppressor to decrease the diseases.Recently discovered CircRNA adam9 has been suggested to be involved in multiple physiological and pathological processes including cancer.With this aspect,aberrant expression of Circ-ADAM9 has been shown in a variety of human malignancies,suggesting an important role for Circ-adam9 in tumor genesis.This review summarizes the recent discoveries done on the role of circ-adam9 in different cancers and discusses its potential prognostic,diagnostic,and therapeutic values as a novel biomarker.

Keywords:Circ-ADAM9,cancer,target gene,oncogene

Introduction

Cancer is one of the main concerns of society both in developed and developing countries,with high risks of morbidity and mortality[1].

Despite researches done in the early treatment of cancer,there is an urgent need for a functional biomarker for early detection and to improve the survival and the cure rate of cancer.Recently long non-coding RNAs are reported to be aberrant in diseases including cancer with the ability to play a regulatory role in modifying the gene expression in multiple levels such as transcriptional and translational levels.Recently,a covalently close long non-coding RNAs Circular RNAs(circRNAs)are widely discovered with their special structure not only in tissues but also in the blood serum,where they are released with high stability and conservation[2],that makes them more stable compare to the linear mRNA.Evidence also proves that CircRNAs can regulate many biological and physiological processes either by function as a competing endogenous RNA(ceRNA)to regulate gene expression and translation of regulatory proteins or by sponging miRNAs,(miRNAs)or RNA-binding proteins[3–5].

Despite,several investigations on circRNAs and their functions in cancer and other diseases,the biological functions of circRNAs are not completely discovered.To date,most of the studies have focused on the ability of circRNAs in targeting miRNA to inhibit the miRNAs affecting their target genes Therefore,miRNAs cannot exert their regulatory function on mRNA expression[6].

Recent findings discovered the relation between CircRNA-ADAM9 and different malignancies especially in cancer such as breast cancer,pancreatic cancer,and melanoma to discover the potential role of circRNA-ADAm9 to be a non-invasive and early diagnosis in this review we summarize the origin and functions of circRNAs and in particular,CircRNA-ADAM9,in cancers to get a better perspective and new direction toward the early detection of cancer.

Origin of circRNAs

The biogenesis of circRNAs is based on back splicing of pre-mRNA which is different from linear RNAs splicing.CircRNAs are plenty found in the cytoplasm of eukaryotic cells and can be divided into three categories including exonic circRNA(EcircRNAs)generated from exons or(EIciRNA)generate from exon-intron,and intronic circRNAs(CiRNAs).Two models are suggested for the origination of circRNAs,namely lariat-driven circularization(exon skipping)and intron-pairing-driven circularization(direct back splicing).These two models differ from each other in the first steps while,the subsequent steps of circRNA formation are largely similar[7],evidence has indicated that intron-pairing-driven circularization might occur more than lariat-driven circularization[8].Another model for circRNA circularization is by the help of RNA-binding protein(RBP)which makes a bridge between the flanking introns,to promote the biogenesis.In addition,there is an alternative circularization pathway in the human genome named intron pairing-driven circularization which is similar to alternative splicing and can promote the complementary pairing of the introns and generate multiple circRNAs[9,10].The schematic illustration of circRNAs biogenesis is demonstrated in Figure 1.

Figure 1.circRNA Formation based on Lariat-driven circulation

Circ-ADAM9

23 different Circ-ADAM9 drives from the ADAM9 gene which is located on the human chromosome 8 base on their splicing site.ADAM9 is a member of the ADAM(a disintegrin and metalloprotease domain)protein family playing important role in so many biological processes involving cell-cell and cell-matrix interactions,fertilization,muscle development,and neurogenesis,homeostasis,and is believed their alternation is contributed to pathologies,such as cancer,ADAM9 is consistently overexpressed in various human cancers,and plays a role in tumorigenesis and angiogenesis in mouse models.ADAM9 can release several molecules with important roles in tumorigenesis and angiogenesis,such as EGF,FGFR2iiib,Tie-2,Flk-1,EphB4,CD40,VCAM-1,and VE-cadherin,and could represent a potential therapeutic target in tumors where it is highly expressed.Circ-ADAM9 has been also reported to be associated with the development of different cancers and disorders including pancreatic cancer,breast cancer,malignant melanoma,and Diabetes mellitus[11–14].

Circ-ADAM9 and cancers

Recently,emerging evidence has declared that circ-ADAM9 has been differentially expressed in distinct cancers.Therefore,we summarized the recent discoveries on the role of circ-ADAM9 in the progression of different malignancies and cancers below.

Breast cancer is the highest morbidity and mortality among women Circ-ADAM9 is found to be remarkably upregulated in breast cancers tissues compared with the paired adjacent samples and its overexpression was correlated with ER receptors[14,15].In another study,circ-ADAM9 is discovered to decrease the inhibitory role of miR-217 and elevate PRSS3 expression by activating the ERK/VEGF pathway to moderate the growth and metastasis of Pancreatic cancer(PC)which is one of a lethal human cancer[16].Diabetes mellitus is a group of heterogeneous metabolic diseases,which occurred base on the significantly increased level of glucose concentrations in blood.Endothelial dysfunction is one of the main causes of vascular complications[17].Endothelial progenitor cells(EPCs)have been found to play important role in vascular complications in diabetes mellitus.Circ-ADAM9 was detected to be significantly increased in EPCs under the high-glucose condition with the ability to target mir-20a-5p to promote autophagy and apoptosis.In addition,downregulation of circ-ADAM9 in EPCs promotes angiogenesis in diabetic mice,therefore Circ-ADAM9represents is a potential new treatment target for diabetic angiopathy[18].

Circ-ADAM9(has-circ-0084043)was found to be significantly up-regulated in melanoma tissue,functioning as an oncogene,to promote melanoma cell proliferation,invasion,and migration by directly binding to miR-153-3p.Therefore, decreasing has-circ-0084043 can be a potential therapeutic target for melanoma patients[19].In another study(has-circ-0084043)was also found to be upregulated in both melanoma tissue and cell lines with the ability to target MiR-31 to promote cell proliferation and glycolysis,and block apoptosis.In addition,knockdown of Circ_0084043 inhibited cell proliferation,induced cell apoptosis,and restrained glycolysis[20].The recent function of circ-ADAM9 in cancers is summarized in Figure 2.

Figure 2.The illustration demonstrating the function of circRNA-ADAM9 and its targets in different cancers

CircRNA and their Therapeutic Strategies

Targeted therapy has been widely used in the clinic due to its excellent efficacy,and it can work on cancerous cells by directly inhibiting cell proliferation,differentiation,and migration[21].Currently,RNAi(siRNAs)is the easiest method to inhibit circRNA expression specifically,without affecting the host gene expression[22].CircRNAs based on their ability to regulate gene expression,emerge as potential therapeutic tools,to modulate the circRNA expression based on their overexpression or knockdown in different cancers[22].

CRISPR/Cas9 genome-editing is a method to suppress circRNA expression,however the complete knockout of circRNA by genome editing is difficult without affecting the linear mRNA transcription since the deletion of circRNA-involved exons that might negatively affect the host gene expression[23].

For example,CRISPR/Cas9 genome-editing tool was performed to remove the CDR1as locus in mice for the first time[25].CRISPR-Cas13 was recently reported as a new tool for RNA modulation with higher knockdown efficiency and improved specificity compared to RNAi.CRISPR-Cas13 is a promising tool for RNA-knockdown studies,including circRNA silencing by targeting the CRISPR-Cas13 guide RNA[24].

However,cas13 protein and guide RNA has not still considered as therapeutic approach since they might be delivered into the organism with unknown side effects[23,24].

Overexpression of circRNA is usually achieved by vector constructs containing the circRNA sequence,the overexpression construct can be delivered by any vector such as plasmid transfection or viral vector systems like adenovirus vectors[24].Overall manipulation of circRNA such as circADAM9 might considered as great tool to discover circRNAs function in different diseases in the early future.

Conclusions

CircRNAs are found to be stably expressed in a tissue-specific manner and dysregulated in different cancers,recently,more than 10,000 CircRNAs have been identified,with the help of high thruputs sequencing.Due to the unique structure of covalently closed CircRNAs,it is generally believed that they take a part in various processes including proliferation,apoptosis,invasion,and migration,and cancer-associated signaling pathway,regulates gene expression by acting as miRNA sponges also some can act as protein translation,and may have potential in diagnosis and treatment.Recently identified circ-ADAM9 found to play a key role in tumorigenesis of several human cancers including,breast cancer(BC),prostate cancer(PCa),melanoma,and diabetes.It is generally thought that circ-ADAM9 is an oncogene and its expression level is frequently measured to be higher in cancerous tissues as compared to normal adjacent tissues with an ability to increase proliferation,migration,and invasion.Although the functions of CircRNAs have not been completely revealed,and there is still a significant gap in the knowledge about CircRNA functions in different cancers.It is our hope that further research in this area may provide a new direction toward the early diagnosis and targeted therapy of cancer.