**3. Involvement of microRNAs in cancer**

microRNA studies were began in *C. elegans*, as lin-4 and let-7 were identified as noncoding RNAs functioning in larval development. Soon after, the research groups focused on the function of these noncoding RNAs and discovered their homologs in vertebrates as well. The role of miRNAs in tumorigenesis was first reported in chronic lymphocytic leukemia (CLL) by two different groups in 2002. Hemizygous or homozygous loss of 13q14 chromosome was frequently observed among CLL patients [9]. Two different miR-15 and miR-16 expression levels were shown to be downregulated with the deletion of this locus [10]. Both miR-15/16 levels are inversely correlated with antiapoptotic B cell lymphoma-2 (Bcl-2) protein level in the cells. Introduction of miR-15/16 to the leukemic cell lines repressed Bcl-2 expression and induced apoptosis in these cells [11]. It is now very well established that aberrant miRNA expression contributes to cancer [12]. miRNAs are targeting the genes, which involve in cell proliferation, migration, invasion, and metastasis; hence dysregulation of these miRNAs leads to transformation and malignancy of cells [13, 14]. miRNA dysregulation in cancer cells can be result of genomic deletion, mutations, amplification, or epigenetic silencing [14]. A single miRNA can target a variety of mRNAs involved in different cell signaling pathways; interestingly, a single mRNA can be targeted by several miRNAs also [15], such as Let-7, which is one of the initially discovered miRNAs, targets human rat sarcoma (RAS), high-mobility group AT-hook 2 (HMGA2), and MYC mRNAs and downregulates their expression [16]. Phosphotensin homolog (PTEN), which is an important regulator of cell cycle, can be targeted by several different miRNAs including miR-21, miR-22, miR-106b-25, miR-17-92 [17].

In tumorigenesis, miRNAs either act as tumor suppressor or as an oncogene; interestingly, their expression is repressed or induced by transcription factors such as p53 or MYC via their promoter regions. miR-145 is one of the initial examples of tumor suppressor miRNAs. miR-145 was found to be downregulated in a variety of tumors including colon, breast carcinomas [18, 19]. It is interesting that tumor suppressor protein p53 induces miR-145 expression via p53 response element in its promoter. Later, miR-145 targets c-Myc or insulin receptor substrate I (IGF-R1) protooncogenes and silences their expressions, hence preventing tumor cell proliferation [18, 20] . Furthermore miR-145 inhibits invasion and metastasis by targeting Fli-1 or Mucin-1 [20, 21]. miR-145 also targets estrogen receptor-α (ER-α) via its two complementary sites and downregulates ER-α expression [22]. miR-34 family is another target of p53 tumor suppressor protein [23]. Another important tumor suppressor miRNA is miR-34 family. miR-34 family comprises three members: miR-34a, miR-34b, and miR-34c. While miR-34a is ubiquitously expressed in every tissue, expression of miR-34b and miR-34c is restricted to fallopian tubes, lungs, and brain [24, 25]. miR-34a is a very potential tumor suppressor since it is targeting many mRNAs related with proliferation [such as cyclin-dependent kinase-4 (CDK4) and cyclin-dependent kinase-6 (CDK6)], cellular growth [such as Notch2, platelet-derived growth factor receptor A (PDGFRA)], antiapoptosis [Bcl-2, sirtuin 1 (SIRT1), survivin], invasion, and migration [MET, SNAIL, cluster of differentiation (CD44)] [26–28]. Downregulation of miR-34 is observed among many malignancies and associated with poor prognosis [29, 30]. As a result of its role as a tumor suppressor, miR-34 has been applied either alone or in combination with conventional therapies on several tumor cell lines and mouse tumor models and showed promising results [31–34]. miR-34 was first miRNA tested in human Phase I trial (NCT01829971). MRX34, liposomal miR-34 mimic, was tested among patients with solid advanced tumors. While MRX34 treatment showed evidence of antitumor activity in a subset of patients, it exerts some toxicities in patients. Hence, there is need for further studies for improving tolerability among the patients [35, 36].

In addition to tumor suppressor miRNAs, miRNAs behave like oncogenes, called as "oncomiRs." mir-21 is the first miRNA identified as oncogenic; it is significantly upregulated in many tumors including colon cancer, breast cancer, hepatocellular carcinoma, and glioblastoma [37]. miR-21 overexpression contributes to cell proliferation and antiapoptotic responses by targeting important downstream proteins such as phosphotensin homolog (PTEN), programmed cell death protein 4 (PDC4), and tropomyosin I [38–40]. Besides this, miR-21 was shown to be bona fide oncogene by causing pre-B-cell lymphoma in mouse models by overexpression. When mir-21 expression was inactivated, tumors regressed completely in few days [41].

As the importance of miRNAs became evident, miRNA expression profiles for each tumor type have been studied with several methodologies including microarray, QRT-PCR, and next-generation sequencing [15, 42]. miRNA expression profiles can reflect embryonic or development origin of the tissue and able to classify the origin of tissue with high accuracy (>90%), even separate cell subtypes (stem cells vs. progenitor cells) in the same tissue [43–45]. These miRNA profiling studies open the way for biomarker studies. In the biomarker studies, it is aimed to find diagnostic, prognostic, and predictive markers for better characterization of the disease and therapy response as an outcome [46].
