Preface

Noncoding RNAs (ncRNAs) have made a huge impact on RNA biology. ncRNAs play important roles in regulating gene expression in animals, plants, and various human diseases. This book provides an overview of current knowledge on ncRNA research by dealing with miRNA- and ncRNA-related human diseases, plant miRNA markers, and the roles of lncRNAs in cancer and epigenetics. The book starts with a brief introductory chapter on ncRNAs. The second chapter discusses the pathogenetic role of miR-335-5p in gastrointestinal tumors. The third chapter focuses on miRNA expression in smoking-induced periodontitis. The fourth and fifth chapters describe the role of herpesvirus-encoded miRNAs in avian diseases and the role of three miRNA families in the progression of skin aging. The sixth chapter covers miRNA-based markers in plants and their application in plant genotyping. The seventh chapter elaborates the role of ncRNAs, especially circRNAs, miRNAs, mRNAs, and their interactions, in brain cells. The last two chapters review lncRNAs as hallmarks of cancer with clinical applications and their functions as epigenetic regulators. Overall, the book content provides a unique perspective to scientists in the field.

> **Dr. Lütfi Tutar** Assistant Professor, Kırşehir Ahi Evran University, Kırşehir, Turkey

#### **Dr. Sümer Aras**

Professor, Ankara University, Ankara, Turkey

#### **Dr. Esen Tutar**

Assistant Professor, Kırşehir Ahi Evran University, Kırşehir, Turkey

**1**

**2. miRNAs and siRNAs**

erative diseases [9].

**Chapter 1**

**1. Introduction**

Introductory Chapter: Noncoding

Noncoding RNAs (ncRNAs) are an attractive research field to prompt extensive genome-wide transcriptional efforts by different international initiatives such as the Encyclopedia of DNA Elements (ENCODEs) [1] and the Functional Annotation of the Mammalian Genome (FANTOM) [2]. Existence of ncRNAs is ubiquitous to all three domains of life, but they play different roles according to type of its RNA family [3, 4]. Dysfunction of ncRNAs may lead to various human diseases from tumorigenesis to neurological, cardiovascular, and developmental disorders [5]. Hence, ncRNAs

Findings of Human Genome Project have disclosed that approximately 1.5% of human genome is comprised of protein encoding genes. On the other hand, the majority of the human genome is transcribed and yields ncRNAs. Noncoding RNAs, which are not translated as peptides or proteins, may be categorized as housekeeping noncoding RNAs and regulatory noncoding RNAs. Noncoding RNAs may be grouped into two major classes based on their size: small noncoding RNAs (sncRNAs) are shorter than 200 nucleotides (nts) in length and long noncoding RNAs (lncRNAs) are longer than 200 nts. Albeit these RNAs are named as noncod-

SncRNAs accorporate functional RNAs including r-RNAs, snRNAs, and t-RNAs, which play important roles in transcriptional and translational regulations. Furthermore, sncRNAs also contain regulatory RNAs, which play roles in gene expression such as P-element-induced wimpy testis (PIWI) interacting RNAs (piRNAs), small interfering RNAs (siRNAs), and microRNAs (miRNAs). Per contra, lncRNAs represent a large group of noncoding regulatory RNAs. The lncRNAs are divided according to their mode of action, such as natural antisense transcripts (NATs), intergenic (lincRNAs), intronic lncRNAs, circular RNAs (circRNAs), promoter-associated long RNAs (pRNAs), and enhancer RNAs (eRNAs).

siRNAs and miRNAs are 19–24 nts in size and silent transcription of genes via inducing mRNA degradation or translational repression. Generally, protein-coding genes are negatively regulated by a single miRNA or multiple miRNAs [8]. While miRNAs originated from pri-miRNAs, source of siRNAs is double-stranded RNAs. Moreover, miRNAs potentially play important roles in biological processes in a cell such as cell differentiation, cell proliferation, cell death, and development by inducing mRNA degradation or translational repression. Dysregulation of miRNAs leads to several human diseases including cancer, cardiovascular, and neurodegen-

RNAs—A Brief Overview

have become a hot topic in molecular genetic and epigenetic research.

ing, some lncRNAs code for small bioactive peptides [6, 7].

*Sümer Aras, Esen Tutar and Lütfi Tutar*

### **Chapter 1**
