**2. Short overview about biogenesis and function of miRNAs**

If deciphering the whole human genome has represented a milestone of modern biology, the identification of its precise functionality is still a great challenge. However, by completing the ENCODE project, many data about how the human genome is functioning were revealed. Such as, it is estimated that about 1.5% of human genome includes coding DNA exons from protein-coding genes (PCGs), while the rest of 98% represents noncoding DNAs including regulatory sequences such as the ones defining noncoding RNAs (ncRNAs), as well as introns, and other DNA sequences with unknown functions [7].

About 80% of human genome is activated in cell physiology, and an important part of noncoding regulatory elements involved in the regulation of PCGs includes noncoding RNAs. Since their recognition as a distinct class of biological regulators [8], micro-RNAs (miRNAs) have become the most studied species of noncoding RNAs. miRNAs are coded by genes located in almost all regions of the genome, including both PCGs and noncoding transcripts. About a half of miRNA genes are located in both intronic (40%) and exonic (10%) regions of noncoding genes, while the majority of the other miRNA loci are located in intragenic regions of PCGs [9]. The first step of miRNA biogenesis includes the transcription of pri-miRNA, a primary long hairpin transcript with a length of hundreds or thousands of nucleotides (**Figure 1**). Furthermore, after its processing to a shorter

**33**

**Figure 1.**

exonucleolytic decay [9].

*degradation or repression.*

*MiRNA-Based Therapeutics in Oncology, Realities, and Challenges*

hairpin structure of about 70 nucleotides, the pre-miRNA is exported into the cytoplasm, where under enzymatic processing, it is reduced to a single-stranded RNA (mature miRNA) of about 21–23 nucleotides in length. Afterward, by its incorporation into Argonaute 2 protein and then in the RNA-induced silencing complex (RISC), the mature miRNA will function as a guide molecule for silencing complex, targeting specific mRNA transcripts, usually by base-pairing specific mRNA transcripts, in the 3′ untranslated region (3′ UTR). Targeting the mRNA by a specific miRNA leads to the translational repression of mRNA and its

*miRNA biogenesis. The miRNA biogenesis starts in the nucleus, with a pri-miRNA transcription. Afterward, the pri-miRNA is processed to a shorter hairpin structure of about 70 nucleotides, by Drosha and DGCR8, and it is exported in the cytoplasm by Exportin 5. After enzymatic processing by DICER, double-stranded miRNAs are reduced to a single-stranded mature RNA (21–23 nucleotides in length). Furthermore, by incorporating into Argonaute 2 and RISC, mature miRNA will target mRNA transcripts, usually in the 3*′ *UTR, leading to mRNA* 

Because of their capacity to modulate up to 60% of PCGs, miRNAs are defined

as "master modulators" of the human genome [10]. An important feature of miRNAs is that a single miRNA can target up to 200 mRNAs, while a single mRNA can be modulated by different miRNAs [11]. Nevertheless, to increase the accuracy of miRNA-mRNA binding, several combinatorial prediction tools based on thermodynamic modeling and machine learning techniques have been developed lately [12, 13], bringing new understanding about how miRNAs can exert their regulatory

function through a combinatorial-cooperative activity.

*DOI: http://dx.doi.org/10.5772/intechopen.81847*

#### **Figure 1.**

*Antisense Therapy*

involved in invasion and metastasis.

tion to distant organs [6].

none of the currently available antitumor therapies target the molecular pathways

Tumor invasion and metastasis, as they were pointed out by Hanahan and Weinberg [4], represent one of the most important hallmarks of cancer, and therefore, exploiting these features of tumor cells could bring new data to develop more powerful anticancer therapies. Tumor invasion and metastasis are very complex processes that involve a series of sequential and interrelated steps. In this line, epithelial-to-mesenchymal transition (EMT) represents the most important event underlying the tumor invasion [5]. During EMT, tumor cells lose their epithelial characteristics and adhesion and acquire increased motility by shifting toward a mesenchymal phenotype while also diminishing apoptosis and senescence and gaining stem cell properties. The EMT regulation includes a network of many regulators, inducers, and effector molecules, which sustains tumor cell dissemina-

The "omics" revolution has brought us new data about the complexity of signaling pathways in cancer, the type of molecules that are involved in them, and which alterations are associated with cancer. Moreover, noncoding RNAs, including miR-NAs, have proved their crucial role in the regulation of mRNA translation in both physiological and pathological status. Because of their high capacity to modulate mRNA expression, miRNAs are defined as master modulators of the human genome. Therefore, miRNAs are involved in all cancer hallmarks, disrupting the normal function of their targets. By gaining or losing the function, miRNAs lead to the validation of tumor phenotype, its progression, and metastasis as well as to drug resistance. Increasing the evidence suggests that the modulation of miRNA expression in cancer cells, through the inhibition of oncogenic miRNAs (oncomiRs) and the substitution of deficient tumor suppressive miRNAs (TS-miRNAs), could represent a reliable tool for improving the cancer therapy. In this chapter, we will present an up-to-date overview about the role of miRNA-based therapeutics in oncology, highlighting their role in cancer management, how these therapies can be used, and

which would be the future challenges related to miRNA-based therapies.

**2. Short overview about biogenesis and function of miRNAs**

DNA sequences with unknown functions [7].

If deciphering the whole human genome has represented a milestone of modern biology, the identification of its precise functionality is still a great challenge. However, by completing the ENCODE project, many data about how the human genome is functioning were revealed. Such as, it is estimated that about 1.5% of human genome includes coding DNA exons from protein-coding genes (PCGs), while the rest of 98% represents noncoding DNAs including regulatory sequences such as the ones defining noncoding RNAs (ncRNAs), as well as introns, and other

About 80% of human genome is activated in cell physiology, and an important part of noncoding regulatory elements involved in the regulation of PCGs includes noncoding RNAs. Since their recognition as a distinct class of biological regulators [8], micro-RNAs (miRNAs) have become the most studied species of noncoding RNAs. miRNAs are coded by genes located in almost all regions of the genome, including both PCGs and noncoding transcripts. About a half of miRNA genes are located in both intronic (40%) and exonic (10%) regions of noncoding genes, while the majority of the other miRNA loci are located in intragenic regions of PCGs [9]. The first step of miRNA biogenesis includes the transcription of pri-miRNA, a primary long hairpin transcript with a length of hundreds or thousands of nucleotides (**Figure 1**). Furthermore, after its processing to a shorter

**32**

*miRNA biogenesis. The miRNA biogenesis starts in the nucleus, with a pri-miRNA transcription. Afterward, the pri-miRNA is processed to a shorter hairpin structure of about 70 nucleotides, by Drosha and DGCR8, and it is exported in the cytoplasm by Exportin 5. After enzymatic processing by DICER, double-stranded miRNAs are reduced to a single-stranded mature RNA (21–23 nucleotides in length). Furthermore, by incorporating into Argonaute 2 and RISC, mature miRNA will target mRNA transcripts, usually in the 3*′ *UTR, leading to mRNA degradation or repression.*

hairpin structure of about 70 nucleotides, the pre-miRNA is exported into the cytoplasm, where under enzymatic processing, it is reduced to a single-stranded RNA (mature miRNA) of about 21–23 nucleotides in length. Afterward, by its incorporation into Argonaute 2 protein and then in the RNA-induced silencing complex (RISC), the mature miRNA will function as a guide molecule for silencing complex, targeting specific mRNA transcripts, usually by base-pairing specific mRNA transcripts, in the 3′ untranslated region (3′ UTR). Targeting the mRNA by a specific miRNA leads to the translational repression of mRNA and its exonucleolytic decay [9].

Because of their capacity to modulate up to 60% of PCGs, miRNAs are defined as "master modulators" of the human genome [10]. An important feature of miRNAs is that a single miRNA can target up to 200 mRNAs, while a single mRNA can be modulated by different miRNAs [11]. Nevertheless, to increase the accuracy of miRNA-mRNA binding, several combinatorial prediction tools based on thermodynamic modeling and machine learning techniques have been developed lately [12, 13], bringing new understanding about how miRNAs can exert their regulatory function through a combinatorial-cooperative activity.

#### *Antisense Therapy*

At the moment, 48,885 mature miRNA products from 271 species, including 2654 mature human miRNAs, have been reported in the latest available miRNA database (miRBase release 22; http://www.mirbase.org/) [14]. In normal phenotype, by their modulatory effects, miRNAs maintain the cell physiology, while by their aberrant expression, miRNAs lead to the validation of many diseases including cancer.
