**2. Biogenesis of miRNA**

The biogenesis of miRNA is a complicated process, starting in the nucleus, following with posttranslational modifications, and finalized in the cytoplasm. Similar to gene encoding, biogenesis of primary miRNAs (pri-miRNAs) is starting with the transcription by RNA polymerase II or RNA polymerase III enzyme. In the nucleus, pri-miRNA is recognized and cleaved by Drosha enzyme to form precursor miRNA (pre-miRNA). The pre-miRNA is exported to cytoplasm by exportin-5. In the cytoplasm, pre-miRNA is bound to cytoplasmic RNase Dicer and RNA-induced silencing complex (RISC), which is composed of argonaute 2 (AGO2) and transactivation response (TAR) RNA-binding protein (TRBP). Firstly, AGO2 cleaves the pre-miRNA from its 3′ end, and the cleaved pre-miRNA is further cleaved by Dicer into mature miRNA duplex. Mature miRNA duplex is then unwounded; while one strand of the miRNA remains on AGO2 protein, and the other strand (passenger strand) is degraded. Mostly, miRNAs are recognizing the complementary sequence of 3′ UTR of mRNAs, hence directing RISC to cleave mRNAs and translational repression of mRNAs [5, 6] (**Figure 1**).

### **Figure 1.**

*miRNA biogenesis. The pathway starts miRNA transcription by RNA polymerase II or III to generate the primary transcripts (pri-miRNAs). Pri-miRNA is processed by the Drosha-DiGeorge syndrome critical region gene 8 (DGCR8, Pasha Pasha in Drosophila melanogaster and Caenorhabditis elegans) complex (also known as the microprocessor complex) that generates ~70 nucleotide (nt) pre-miRNAs. Pre-miRNA, which is recognized by the nuclear export factor exportin-5, is transferred to the cytoplasm. In the cytoplasm, the cytoplasmic RNase Dicer cleaves the pre-miRNA hairpin to its mature length. Dicer in complex with the transactivation response (TAR) RNA-binding protein (also known as TRBP and TARBP2) and argonaute (AGO) 1–4 mediate the processing of pre-miRNA and the assembly of the RISC (RNA-induced silencing complex). With the formation of this complex structure, one strand of the miRNA duplex is removed and single-stranded miRNA is generated. Interaction between microRNA complex and target mRNA induces posttranscriptional silencing by destabilization of mRNA and suppression of translation [7, 8].*

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*MicroRNAs (miRNAs) in Colorectal Cancer DOI: http://dx.doi.org/10.5772/intechopen.80828*

miR-22, miR-106b-25, miR-17-92 [17].

**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,

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 *Oncogenes and Carcinogenesis*

**2. Biogenesis of miRNA**

encoded miRNAs have been determined [4].

repression of mRNAs [5, 6] (**Figure 1**).

lin-28, lin-41, lin-42, and daf-12. After the discovery of lin-4 and let-7, miRNAs were shown in other organisms including plants and animals [2, 3], and over 10,000 miRNAs have been identified in various organisms. In humans, over 2500 types of

The biogenesis of miRNA is a complicated process, starting in the nucleus, following with posttranslational modifications, and finalized in the cytoplasm. Similar to gene encoding, biogenesis of primary miRNAs (pri-miRNAs) is starting with the transcription by RNA polymerase II or RNA polymerase III enzyme. In the nucleus, pri-miRNA is recognized and cleaved by Drosha enzyme to form precursor miRNA (pre-miRNA). The pre-miRNA is exported to cytoplasm by exportin-5. In the cytoplasm, pre-miRNA is bound to cytoplasmic RNase Dicer and RNA-induced silencing complex (RISC), which is composed of argonaute 2 (AGO2) and transactivation response (TAR) RNA-binding protein (TRBP). Firstly, AGO2 cleaves the pre-miRNA from its 3′ end, and the cleaved pre-miRNA is further cleaved by Dicer into mature miRNA duplex. Mature miRNA duplex is then unwounded; while one strand of the miRNA remains on AGO2 protein, and the other strand (passenger strand) is degraded. Mostly, miRNAs are recognizing the complementary sequence of 3′ UTR of mRNAs, hence directing RISC to cleave mRNAs and translational

*miRNA biogenesis. The pathway starts miRNA transcription by RNA polymerase II or III to generate the primary transcripts (pri-miRNAs). Pri-miRNA is processed by the Drosha-DiGeorge syndrome critical region gene 8 (DGCR8, Pasha Pasha in Drosophila melanogaster and Caenorhabditis elegans) complex (also known as the microprocessor complex) that generates ~70 nucleotide (nt) pre-miRNAs. Pre-miRNA, which is recognized by the nuclear export factor exportin-5, is transferred to the cytoplasm. In the cytoplasm, the cytoplasmic RNase Dicer cleaves the pre-miRNA hairpin to its mature length. Dicer in complex with the transactivation response (TAR) RNA-binding protein (also known as TRBP and TARBP2) and argonaute (AGO) 1–4 mediate the processing of pre-miRNA and the assembly of the RISC (RNA-induced silencing complex). With the formation of this complex structure, one strand of the miRNA duplex is removed and single-stranded miRNA is generated. Interaction between microRNA complex and target mRNA induces post-*

*transcriptional silencing by destabilization of mRNA and suppression of translation [7, 8].*

**32**

**Figure 1.**
