Genomics & Molecular Diagnostics: Molecular Techniques 4 - MicroRNAs


Small Non-Coding RNA

There are two primary types of small non-coding RNAs known today:

  1. Micro RNA (miRNA)
    MiRNAs are integral to the regulation of gene expression and are characterized by their short length, typically ranging from 21 to 24 nucleotides.

  2. Short Interfering RNA (siRNA)
    siRNAs are longer double-stranded RNA molecules, generally 20-25 base pairs long, and are primarily involved in RNA interference (RNAi), which can lead to the degradation of target mRNA.

Characteristics of small non-coding RNAs:

  • Typically composed of 21-24 nucleotides in length.

  • Functionally important in silencing mRNA transcripts via base pairing mechanisms.

  • Synthesized by processing longer precursor sequences that eventually give rise to functional small RNAs.

MicroRNAs (miRNAs)

MicroRNAs (miRNAs) are small, single-stranded, non-coding RNA molecules, usually 19-23 nucleotides long. They are encoded within the genomes of both plants and animals and play a crucial regulatory role in gene expression by specifically binding to the 3' UTR of target mRNAs.

Formation and Processing of miRNAs

  1. MiRNAs are transcribed as long RNA molecules, referred to as primary-miRNAs, typically ranging from 500 to 3000 nucleotides in length.

  2. In the nucleus, the enzyme Drosha, along with the DGCR8 protein, processes these primary-miRNAs into shorter precursor miRNAs (pre-miRNAs), which form hairpin structures approximately 70-100 nucleotides long.

  3. Pre-miRNAs are then transported from the nucleus to the cytoplasm via an exportin 5-dependent mechanism.

  4. In the cytoplasm, the Dicer enzyme further processes pre-miRNAs into mature single-stranded miRNAs, measuring 19-23 nucleotides.

  5. The mature miRNA is then incorporated into the RNA-induced silencing complex (RISC), a multi-protein complex essential for miRNA-mediated gene silencing.

  6. Within RISC, the miRNA guides the complex to target mRNAs by binding to the 3' UTR, leading to reduced expression through either translational repression or the degradation of the target mRNA.

Features of miRNAs

  • A majority of miRNAs initiate with a uracil (U) base.

  • The “seed” sequence, which includes the second 7-mer on the 5' end of the miRNA, must align closely with the target mRNA sequence to ensure effective targeting and regulatory action.

  • MiRNAs often display conservation across species, with many seed sequences found in different organisms, including humans and rodents, highlighting their evolutionary significance.

  • The binding interactions can exhibit variations, such as non-complementary loops or bulges, which often arise from the structural stability of the hairpin configuration of miRNAs.

Small Interfering RNA (siRNA)

siRNAs are critical double-stranded RNA molecules, comprising 20-25 base pairs in length, and are instrumental in the RNA interference (RNAi) process. They exert their function by promoting the cleavage or degradation of target mRNA transcripts through precise base-pair matching with their complementary RNA sequences.

Differences between siRNA and miRNA
siRNA:

  • Causes specific cleavage or degradation of mRNA transcripts, leading to a direct silencing effect.

  • Sense strands of siRNAs share the same sequence as their corresponding target strands, allowing for precise matching.

miRNA:

  • Regulates the expression of target mRNAs, often without requiring exact sequence matching.

  • Typically demonstrates imperfect base-pairing with target mRNAs, which can result in the targeting of multiple genes simultaneously.

Processing Mechanisms

Both miRNA and siRNA undergo processing by the Dicer enzyme before being integrated into RISC. While miRNAs are generally considered endogenous, siRNAs are often exogenous, originating from viral infection or other external sources.

miRNA Research Trends

The significant role of miRNAs in the field of oncology was first recognized in 2002. Since then, research has surged, focusing on unraveling miRNA functions and their implications in cancer progression, diagnosis, and treatment. This expanding field has led to discoveries that highlight miRNAs as pivotal regulators in numerous biological pathways.

miRNA Nomenclature

The nomenclature for miRNAs follows standard conventions, which include:

  • A prefix indicating the species of origin (e.g., hsa for humans).

  • Specific designations such as pri-mir (primary transcript) and pre-mir (precursor miRNA).

  • Thesuffixes (e.g., -3p or -5p) that signify the maturity of a specific miRNA product formed from the precursor.

miRNA Profiling Techniques

  1. TaqMan qRT-PCR: Leverages stem-loop primers for specificity in reverse transcription, enabling accurate quantification of miRNAs.

  2. SYBR Green-based qRT-PCR: Utilizes polyadenylation and intercalating dyes to amplify and quantify miRNA levels effectively.

  3. miRNA Microarray: Employs DNA probes that capture fluorescently-tagged miRNAs, allowing for a comprehensive analysis of miRNA expression profiles.

  4. RNA Sequencing: Involves reverse transcription to cDNA, followed by steps including adapter ligation, facilitating deep sequencing and quantitative analysis of miRNAs.

  5. Nanostring nCounter: Utilizes capture and reporter probes for direct quantification of miRNAs without amplification, promoting accuracy in measuring miRNA levels.

Applications of miRNA Profiling

  • Investigating developmental biology by comparing miRNA profiles at various developmental stages to understand their roles.

  • Identifying novel miRNAs and exploring gene regulation in less-characterized organisms through RNA-Seq analysis.

  • Analyzing interactions between miRNAs, mRNAs, and proteins to identify potential target genes and elucidate regulatory networks.

  • Utilizing miRNAs as biomarkers in disease contexts, given their stability and sensitivity across diverse biological samples.

  • Forensic applications exploit distinct miRNA profiles derived from different body fluids, providing a novel tool for tissue identification.

Bioinformatic Target Prediction

Several bioinformatics programs are available for predicting potential miRNA targets, including miRanda, microCosm, TargetScan, and PicTar. Among them, TargetScan emphasizes predicting conserved microRNA targets specifically within mammalian species, further enhancing the understanding of miRNA-target interactions.

In-vitro UTR Analysis

Common approaches for in-vitro UTR analysis often involve cloning the 3' UTR of predicted target genes into luciferase reporter constructs. Monitoring changes in luciferase activity offers insights into the binding dynamics and regulatory influence of miRNAs on target mRNAs.

Experimental Methods for Assessing miRNA Targeting

  1. Wild-type vs. Mutated UTR: Constructs exhibiting changes in luciferase expression serve as indicators of miRNA binding activity.

  2. miRNA Inhibitors: Experimental inhibition of specific miRNAs allows researchers to assess corresponding differences in luciferase expression, revealing targets that are regulated by those miRNAs.

  3. Dose-Dependent Experiments: These experiments measure luciferase response to varying quantities of miRNA, aiding in the confirmation of specific target interactions and regulatory effects.