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WIREs Nanomed Nanobiotechnol
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Toward biosensors for the detection of circulating microRNA as a cancer biomarker: an overview of the challenges and successes

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Considerable attention has been dedicated to developing feasible point‐of‐care tests for cancer diagnosis and prognosis. An ideal biomarker for clinical use should be easily assayed with minimally invasive medical procedures but possess high sensitivity and specificity. The role of microRNAs (miRNAs) in the regulation of different cellular processes, the unique altered patterns in cancer patients and presence in body fluids in the stable form, points to their clinical utility as blood‐based biomarkers for diagnosis, prognosis, and treatment of cancer. Although a variety of selective and sensitive laboratory‐based methods are already exist for the detection of circulating miRNA, having a simple, low‐cost and rapid assay, which could be routinely used in clinical practice, is still required. Among different approaches that have developed for circulating miRNA detection, biosensors, due to the high sensitivity, ease of use, short assay time, non‐toxic experimental steps, and adaptability to point‐of‐care testing, exhibit very attractive properties for developing portable devices. With this view, we present an overview of some of the challenges that still need to be met to be able to use circulating miRNAs in clinical practice, including their clinical significance, sample preparation, and detection. In particular, we highlight the recent advances in the rapidly developing area of biosensors for circulating miRNA detection, along with future prospects and challenges. WIREs Nanomed Nanobiotechnol 2015, 7:580–592. doi: 10.1002/wnan.1324 This article is categorized under: Diagnostic Tools > Biosensing Diagnostic Tools > Diagnostic Nanodevices
Cellular release mechanisms and extracellular transportation systems of micro RNAs (miRNAs): after being transported into the cytoplasm, the pre‐miRNA can be further cleaved into 19‐ to 23‐nucleotide mature miRNA duplexes. One strand of the miRNA duplex can be loaded into the RISC, where it can guide the RISC to specific mRNA targets to prevent translation of the mRNA into protein (1). The other strand may be degraded or released from the cell through export mechanisms described below. In the cytoplasm, pre‐miRNAs can also be incorporated into small vesicles called exosomes, which originate from the endosome and are released from cells when multivesicular bodies (MVB) fuse with the plasma membrane (2). Cytoplasmic miRNAs (pre‐miRNA or mature miRNA) can also be released by microvesicles, which are released from the cell through blebbing of the plasma membrane (3). miRNAs are also found in circulation in microparticle‐free form. These miRNAs can be associated with high‐density lipoproteins (HDLs) or bound to RNA‐binding proteins such as Ago2. It is not known how these miRNA–protein complexes are released from the cell. These miRNAs may be released passively, as by‐products of dead cells, or actively, in an miRNA‐specific manner, through interaction with specific membrane channels or proteins (4). Although pre‐miRNAs have been detected in exosomes and microvesicles, and mature miRNAs have been found in complex with Ago28 and high‐density lipoprotein (HDL), the exact proportion of mature and pre‐miRNAs in the different extracellular compartments is not known.
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The biogenesis of micro RNA (miRNA): in the nucleus, miRNAs are transcribed from DNA. Precursor hairpin primary miRNA (pri‐miRNA) are transcribed by RNA polymerase. Pri‐miRNAs are processed by the microprocessor complex into precursor miRNA (pre‐miRNA) hairpins. These are transported into the cytoplasm, where they are further processed by Dicer into miRNA duplexes. Following strand separation, the mature miRNAs are loaded into RISCs to guide the repression of protein synthesis or mRNA degradation.
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