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miRNA / microRNA Introduction
MiRNAs (microRNAs) are small non-coding RNAs, 18-23 nucleotides long, which act as post-transcriptional regulators of gene expression. These small non-coding RNAs regulate gene expression by sequence-specific targeting of mRNAs. Typically, miRNAs target the 3' untranslated region (3'UTR) of mRNA transcripts leading to mRNA degradation or translational repression. To date, more than 1200 validated human miRNAs have been identified in various organisms through random cloning and sequencing or computational prediction. The miRBase, managed by the Griffiths-Jones lab provides miRNA sequence data, annotation and target prediction information.
MicroRNAs (miRNA) are endogenous, short, non-coding RNA that undergo a multistep biogenesis before generating the functional, mature sequence. MiRNA are processed in multiple steps:
DNA→ Pri-miRNA → Pre-miRNA→ Mature miRNA
The core components of the microprocessor complex, consisting of Drosha and DGCR8, are both necessary and sufficient for this process, although accessory proteins have been found that modulate the biogenesis of a subset of miRNA. Curiously, many of the proteins involved in miRNA biogenesis are also needed for ribosomal RNA processing. MicroRNAs (miRNAs) are generated from large RNA precursors (termed pri-miRNAs) that are processed in the nucleus by the RNase III enzyme Drosha and the protein Pasha/DGCR8 into ~70 nucleotides pre-miRNAs, Once in the cytoplasm, the pre-miRNAs undergo an additional processing step by the RNAse III enzyme Dicer generating the miRNA, a double-stranded RNA aproximately 22 nucleotides in length. Dicer also initiates the formation of the RNA-induced silencing complex (RISC). The mature miRNA is part of an active RNA-induced silencing complex (RISC) containing Dicer and many associated proteins. RISC with incorporated miRNA is sometimes referred to as "miRISC."
The function of microRNAs appears to be in gene regulation. For that purpose, a microRNA is complementary to a part of one or more messenger RNAs (mRNAs). Plant miRNAs are usually complementary to coding regions of mRNAs, whereas animal miRNAs are usually complementary to bind to the 3' UTR of their target transcripts to repress translation. A class of eukaryotic non-coding RNAs termed microRNAs (miRNAs) interact with target mRNAs by sequence complementarity to regulate their expression.
A known function of miRNAs is to downregulate the translation of target mRNAs through base-pairing to the target mRNA. The pairing between miRNAs and their target mRNAs usually includes short bulges and/or mismatches. In contrast, in all known cases, plant miRNAs bind to the protein-coding region of their target mRNAs with three or fewer mismatches and induce target mRNA degradation or repress mRNA translation.
miRNAs occasionally also cause histone modification and DNA methylation of promoter sites, which affects the expression of target genes.
It has been estimated that miRNAs can target more than 30% of the human genome. With the great progress of miRNA-related biological studies, miRNAs are being considered as novel biomarkers and potential therapeutic targets for various diseases. As a consequence, sensitive and selective miRNA detection is very significant for miRNA discovery, study and clinical diagnosis. However, miRNA detection is challenged by the characteristics of low cellular abundance and short length. Currently, many methods have been developed for the detection of miRNA, including northernblotting using LNA probes, nanoparticle amplification methods, quantitative RT-PCR, and so on.
OriGene’s unique primer-based, SYBR Green qPCR microRNA detection system not only offers researchers a fast and simple method for profiling miRNA expression levels, but also provides means to quantify the results down to absolute copy number of miRNA.
The revealing role of miRNAs functioning as potential oncogenes and tumor suppressors in tumorigenesis has generated great interest in using them as targets for cancer therapies. General therapeutic strategies involving antisense-mediated inhibition of oncogenic miRNAs and miRNA replacements with miRNA mimetics or viral vector-encoded miRNAs will be discussed. Synthetic anti-miRNA oligonucleotides (AMOs) with 2′-O-methyl modification have been shown to be effective inhibitors of endogenous miRNAs in cell culture and xenograft mice models. Application of 2′-O-methyl AMOs targeting the onco-miR-21 potently inhibited glioblastoma and breast cancer cell growth in vitro and tumor growth in an MCF-7 breast cancer xenograft mice model
With anti-miRNA strategies showing great therapeutic promise, the reverse approach of miRNA replacement may be equally attractive for anticancer therapy. The reasoning follows the observation that miRNA expression profiling studies found that although some specific miRNAs are upregulated in cancer, most miRNAs have reduced expression in tumor tissues compared with normal tissues.
The roles of miRNAs in cancer have been very well established over the last few years. Although there is still much to be learned concerning the mechanism of miRNAs in tumorigenesis, scientists have been able to apply their knowledge to use miRNAs for cancer diagnosis and prognosis and identification of cancer risks. Within the last few years, many studies involving either anti-miR knockdown or miRNA replacement therapy have moved into animal models with highly encouraging results for cancer therapeutics.
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