MicroRNA’s in cancer as biomarkers and therapeutic keys

MicroRNA (miRNA), the noncoding RNAs, are short length with 22 nucleotides. It involved in various biological process. Its expression is found varied in cancer and hence used as a marker. miRNAs are become important entity that changes the expression of genes in disease particularly cancer. In this review, different types of miRNAs were addressed with its relationship in different types of cancer. The level of expression miRNA is depends upon the different stages and could be used as a marker for early diagnosis. The circulating as well as exosome miRNA in cancer was also discussed. This review could facilitates us to study the miRNA as biomarker and it additionally paves way for therapeutic approaches. Background A couple of decades ago, provenances of microRNA (miRNA) discovery lead to the new arena in molecular biology. In humans, more than 2000 miRNAs were discovered and it regulate more than 25% of the genes. It’s often well connected with various diseases and hence they are useful for diagnosis [1]. miRNAs, the noncoding RNAs, are short length with nearly twenty two nucleotides. It is found in eukaryotes and they are part of all pathways in our biological system [2]. It controls the cellular process such as cell cycle, inflammation, cell differentiation and cell death, through inhibition of mRNAs stability and translation. Hence, this miRNAs are inevitable in all biological process and signaling in a cell. Its dysregulation is often leads to genesis of cancer [3]. The first miRNA was discovered in the year 1993 and was outcome of the two different studies reported Lin-4 as small non coding RNA from Ceanorhabditis elegans heterochronic gene lin-4 [4]. At that time, this small non coding RNA was considered as a specific tool used by the worms to manage their heterochronic gene expressions. After 7 years, Reinhart et al. [5] other small ncRNAs in C. elegance represented Let-7, the heterochronic gene. They together with lin-4 RNA were initiating the cascades of heterochronic genes regulation via RNA-RNA interaction at 3′ untrasnlated region of the gene target [5]. It lead to trace the other small ncRNA and it unveiled the existence of ncRNAs in different organisms in which it plays as potential regulatory control and then named as microRNAs [6–8]. Later it was identified that they are all inhabitant of plants, animals and now miRNA database revealed a total of 2042 and 1281 mature RNAs in human and mouse respectively [9]. In mammals, miRNAs genes have paralogue, i.e., different isoforms while it is well conserved in animals. There were 8 isoforms were existing in 11 genomic loci. Interestingly, in C. elegans almost 55% of the miRNAs were found similar to humans. The majority of miRNA gene occupy regions away from annotated gene often due to one transcriptional unit. Over 50% of the miRNA genes are clustered and normally transcribed as multcistronic RNA transcript. In animals, it constitute a gene regulatory molecule and had impact on gene expression and thus in cancer it exert unique role in phenotype of the disease [3]. In cancer, miRNA play a role in disease initiation, movement of cell from site of origin, disease prognosis and response during treatment [10].


Background
A couple of decades ago, provenances of microRNA (miRNA) discovery lead to the new arena in molecular biology. In humans, more than 2000 miRNAs were discovered and it regulate more than 25% of the genes. It's often well connected with various diseases and hence they are useful for diagnosis [1]. miRNAs, the noncoding RNAs, are short length with nearly twenty two nucleotides. It is found in eukaryotes and they are part of all pathways in our biological system [2]. It controls the cellular process such as cell cycle, inflammation, cell differentiation and cell death, through inhibition of mRNAs stability and translation. Hence, this miRNAs are inevitable in all biological process and signaling in a cell. Its dysregulation is often leads to genesis of cancer [3].
The first miRNA was discovered in the year 1993 and was outcome of the two different studies reported Lin-4 as small non coding RNA from Ceanorhabditis elegans heterochronic gene lin-4 [4]. At that time, this small non coding RNA was considered as a specific tool used by the worms to manage their heterochronic gene expressions. After 7 years, Reinhart et al. [5] other small ncRNAs in C. elegance represented Let-7, the heterochronic gene. They together with lin-4 RNA were initiating the cascades of heterochronic genes regulation via RNA-RNA interaction at 3′ untrasnlated region of the gene target [5]. It lead to trace the other small ncRNA and it unveiled the existence of ncRNAs in different organisms in which it plays as potential regulatory control and then named as microRNAs [6][7][8]. Later it was identified that they are all inhabitant of plants, animals and now miRNA database revealed a total of 2042 and 1281 mature RNAs in human and mouse respectively [9]. In mammals, miRNAs genes have paralogue, i.e., different isoforms while it is well conserved in animals. There were 8 isoforms were existing in 11 genomic loci. Interestingly, in C. elegans almost 55% of the miRNAs were found similar to humans. The majority of miRNA gene occupy regions away from annotated gene often due to one transcriptional unit. Over 50% of the miRNA genes are clustered and normally transcribed as multcistronic RNA transcript. In animals, it constitute a gene regulatory molecule and had impact on gene expression and thus in cancer it exert unique role in phenotype of the disease [3]. In cancer, miRNA play a role in disease initiation, movement of cell from site of origin, disease prognosis and response during treatment [10].

Biological roles Of miRNAs
The Dicer and DGCR8 deficient mouse model are the first initiative to explore the significance of miRNAs in developmental stages of mammals since biogenesis of miRNA associated with DGCR8. Break in any of the step in biogenesis of miRNA is found leathal to embryo [19,20]. Its, any of the genes, loss of function in tissues also causes developmental dysfunction in associated tissues [21]. Mouse models with miRNA knockout already demonstrated its, the genes, role in all tissue type and its associated developmental defects [21,22].

Dysregulation of miRNAs
Literatures portray the existence of miRNA role either in up-regulation or down regulation in diseases experienced by human. MiRNA dysregulation often thought to be linked with one of the factor in progression of disease. In cancers, its altered expressions were highly reported [1]. The list of miRNA's expressed in breast, esophageal and gastric caner are listed in Table 1.

Breast cancer
Oncogenic miRNA In cancer, upregulation of oncogenic miRNA was frequently noticed and they deliver its action through suppressing the tumor suppressor gene that regulates the normal cell regulatory and cleaning process such as apoptosis etc. [42]. The following are the some of the miRNAs recorded in breast cancers. miR-10 family miR-10a, 10b are the members of miR-10 family resides in Hox cluster. In murine xenograft breast cancers model, miR-10b is, found over expressed, inducing metastasis and invasion through HOXD10 gene [23]. Its expression level was found positively associated with all clinical features such as size of the tumor, proliferation, stages, metastasis while it is negatively associated with PR + , ER + and level of E-cadherin. Its, miR-10b, transcription factor enhances invasion and metastatic ability through HOXD10 in cell lines and animal models [23,43,44].

miR -21
It is also involved in breast cancer cell migration and invasion [45,46]. Chan et al. recorded higher miR-21 level in tumor tissue of human glioblastoma and generated cell line out of it. It helped them to compare it with brain tissues of fetal and adult non neoplastic tissues [24]. Besides miR-21, miR-125b, 145, 155 expressions were also noticed aberrantly in breast cancer [47]. Its, miR-21, upregulation is found correlated with increasing in grade of the tumor, status of the receptors of hormones and ductal carcinoma.
Tumor suppressor In C. elegans embryogenesis importance of let-7 family in determination of cell type was noticed by Reinhart et al. [49]. It consists of let-7a-g, miR-98 and miR-202. It involved in various physiological such as development, cell adhesion, muscle formation and gene expression regulation. Let-7 family is found lost during early disease progression stage in breast cancer [28].  [50][51][52]. They were found lost in mesenchymal phenotypic cell lines of invasive breast cancer. The drug resistances found in human breast cancers were found linked to miR-200 down regulation [27].
miR-205 It is found down regulated in triple negative breast cancer cell [26,52]. Its expression prevents the growth and development of the breast cancer cell. Triple negative breast cancer cell can be protected by miR-205 as tumor suppressor. Its expression supports the inhibition of different physiological mechanisms of the cell in in vitro and in vivo.
miR-145 It is found down regulated in breast cancer tissues compared to normal breast tissue in a study by Iorio et al. [47]. It is often used as early diagnostic biomarker due to its expression pattern in breast cancer [28,53].

Esophageal cancer
Oncogenic miRNA miR- 21 It, used for the ESCC prognosis, has strong relationship with development of EAC and ESCC. In many human cancers, it is highly expressed. It has strong control over PTEN, tropomysin-1, programmed cell death 4 and maspin that are regulators of survival, invasive and apoptosis [29].
miR-106-25 It is a polycistronic and found in chromosome 7q22.1. It encodes miR-25, 93 and miR-106b. It is found upregulaed in NSE and from which to BE and EAC. It is also shows potential cell development, anti-apoptotic as well as enhance the cell cycle in in vitro and in vivo tumorigenic activity [54]. miR-25 targets the E-cadherin gene at 3'UTR and enhances migration and cell invasion in ESCC. This miR-25 showed the oncogenic activity by inhibiting CDH1.
miR-17-92 miR-17-92, the oncomir-1, is a oncogenic miRNA [55,56] found located in 13q31.3 and codes for miR-17, 18a, 19a, 20a, 19b-1 and miR-92-1. Its cluster found expressed about 75% in tumors of ESCC [31] and its expression enhances the cell growth in both in vitro and in vivo. miR-10b Its over expression was noticed in different cancer types [57][58][59][60] such as ESCC [32]. Its role in ESCC as inducing factor of invasiveness and motility of cell was noticed by Tian et al. [32]. A gene that suppress the migration of cell and invasiveness of the esophageal cancer was identified as KLF4, which is an miR-10b target.

Tumor suppressor miR-375
In EC, it function as PDK1 regulator (negative) and its promoter regions is hypermethylated adequately [68]. In mice, its role as inhibitors of motility of cell, proliferation of cell, formation of tumor, clonogenicity and metastasis was noticed. Its interaction with IGFIR 3′ UTR often down regulates it and both are negatively correlated [35].

miR-145
It is induced by p53, the tumor suppressor and it targets the c-Myc [69]. Another gene called FSCN1, which was expressed highly in ESCC (tumors) while not in normal epithelium, was also found recorded as target gene for this miRNA. The reduction in the prognosis and metastasis in lymphnode was found correlated with FSCN1 expression [34].

miR-106a
In GC, compared to normal tissues, it is found upregulated in various human tumors [74]. It reflect as G1-S transition positive regulator [74] and its binding with 3'UTR down regulates the cytokine such as IL10 in hematopoietic stem cells [40]. It was also regulated by SP1 and EGR1 for down regulating IL10 [40].

Tumor suppressor miR-101
In most of the cancers, the progression of cancer was due to epigenetic pathway dysregulation by inhibiting miR-101 and inducing EZHR over expression [75,76]. Since, in GC, this miRNA targeting the COX2, its down regulation causes expression of COX2 [41] which controls the activation of prostaglandin E2 and arachidonic acid pathways.
Let-7 Its expression controls (reduce) the expression of RAS genes such as H, K and NRAS. Its expression, compared to normal lung tissue, was found noticed low in lung tumor while inversely higher expression of RAS protein was noticed in lung tumor [77]. In gastric tumorigenesis RAB40C, which is a let-7a target, role was well recorded [78].

miR-148
It is found inactivated in GC by hypermethylating its promoter region [79] and this causes upregulation of DNA methyltransferase [79]. It act on the ROCK1 to down regulate it and tumor cell invasion [80].

Oncogenic miRNA miR-21
It support the cell growth, invasiveness of tumor and metastasis [81] by suppressing the tumor suppressor genes. Its expression was found noticed high in mouse lung cancer (K-ras) dependent and controls the Spry1, Spry2, Btg2 and Pdcd4 by targeting the regulators of ERK/MEK/Ras [81][82][83]. It also act on the Apaf1, Fas1g, Pdcd4 and RhoB, the pro-apoptotic gene products, to apoptosis inhibition while PDCD4 directly associated with metastasis and invasion [83].
miR-197 Its knockdown results in induction of apoptosis and involved in p53 pathway which causes uncontrolled cell proliferation [84].
miR-212 AChe level was found associated with rapid development of tumor and lower survival rate [85] since it was altered by this miRNA by acting at 3′ UTR.

Tumor suppressor B-Cell Lymphocyte 2 (BCL2)
It is known for its apoptosis regulation. Its over expression was found associated with cancer development and it also act as resistance exerting member against anticancer drugs or agents [88][89][90].
miR-608 Its upregulation was found recorded in mostly tissues [91] and its higher expression cause apoptosis [92]. It was proposed to have higher interaction with signaling pathways [93].
Mechanism for miRNA dysregulation The mechanisms by which over expression of miRNA were recorded in different cancer through following types, Genomic abnormality Chromosomal aberration is associated with tumorigenesis. In humans [94] and mouse [95] occurrence of miRNA was recorded in regions or fragile sites associated with cancer. Different analytical tools revealed the existence of association between miRNA expression level and copy number variations [96][97][98][99][100][101][102].
Epigenetic factors In different cancer types, silence of tumor suppressor gene was due to CpG hypermethylation which also includes histone modifications [103].
Transcriptional regulation miRNA expression often associated with transcription factors also. In many cases, during differentiation, switching on of tissue specific miRNAs were activated by transcription factors which includes oncogenes and tumor suppressor gene. Its association was well documented in cancer.
Circulating miRNA in Cancer Circulating miRNA play an important role in different cancer as prognostic as well as therapeutic markers ( Table 2). Low level mir-375 and mir-125b expression were noticed in OSCC patients that are involved in radiotherapy and proliferation [112,113]. Similarly, mir-196 was found highly expressed in Head and Neck sqamous cell carcinoma and caused resistance to radiotherapy [114]. In, Eosphoageal squamous cell carcinoma (SCC), miR-150 was found in low level which are involved in tumor malignancy such as metastasis and lymph node invasion [115]. Similarly, Tongue squamous cell carcinoma, mir-21 was found in high expression and was used as marker for late stage as well as metastasis in lymph node [116]. Mir-134 found involved in metastasis in HNSCC [117]. Higher expression level of mir-146a was noticed in OSCC that caused increased metastasis and tumorogenisis [118].
Exosomal miRNA in Cancer Exosomal miRNA are often involved in exchange of RNA (Table 3). miRNA-200b was involved in increased proliferation in colorectal cancer [119]. Similarly, in Papillary thyroid cancer, negative proliferation was exerted by miRNA-146b and miRNA222 [120]. Chemotherapeutic drug, gemcitabine resistance to pancreatic cancer was induced by miRNA-106b [121]. Thus, the exosomal miRNA involved in regulating cancer proliferation and conferring resistance to the cancer cells.
Conclusion miRNA are the key biomarker in the field of cancer research. Its regulation describes the exact status like nature, development and its metastatic condition of the cancer. This review have discussed many such miRNA and emphasized the importance in the developmental cancer research. It could facilitate the early diagnosis and the therapeutic targets simultaneously for the greater reduction in cancer mortality. miRNA controlled signalling pathways portrays the precise key paths need to be focused for the treatment of cancer in efficient way. Authors' contributions Authors Sindhuja SS and P.V. Sona prepared this manuscript; Dr.PVM corrected, aligned and checking the proof reading of this article. The author (s) read and approved the final manuscript.

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