MicroRNA Expression Signatures Determine Prognosis

Authors: Michael Henriksen Kasper Bendix Johnsen Hjalte Holm Andersen Linda Pilgaard Meg Duroux

Publish Date: 2014/03/12

Volume: 50, Issue: 3, Pages: 896-913

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Abstract

Despite advances in our knowledge about glioblastoma multiforme GBM pathology clinical challenges still lie ahead with respect to treatment in GBM due to high prevalence poor prognosis and frequent tumor relapse The implication of microRNAs miRNAs in GBM is a rapidly expanding field of research with the aim to develop more targeted molecular therapies This review aims to present a comprehensive overview of all the available literature evaluating miRNA signatures as a function of prognosis and survival in GBM The results are presented with a focus on studies derived from clinical data in databases and independent tissue cohorts where smaller samples sizes were investigated Here miRNA associated to longer survival protective and miRNA with shorter survival riskassociated have been identified and their signatures based on different prognostic attributes are described Finally miRNAs associated with disease progression or survival in several studies are identified and functionally described These miRNAs may be valuable for future determination of patient prognosis and could possibly serve as targets for miRNAbased therapies which hold a great potential in the treatment of this severe malignant diseaseGlioblastoma multiforme GBM is a severe type of brain cancer characterized by its large growth potential and very poor clinical outcome It is one of the most aggressive and incurable types of cancer reflected in a median survival of less than 1 year of all GBM cases and a 5year survival rate of less than 5  1 2 GBM affects 2–3 per 100000 persons per year making it a rare type of cancer but still it accounts for 16  of all brain tumors and 54  of all clinically diagnosed gliomas in the USA 2 The clinical presentation of GBM depends on the location of the tumors and generally involves focal neurological deficits headaches and seizures Tumors are most commonly found in the frontal lobes of the supratentorial compartments however they are not restricted to these areas as illustrated by GBM tumors found in other parts of the central nervous system CNS such as the spinal cord and brainstem 3GBM can be subdivided into de novooccurring tumors termed primary GBM or tumors developed from lowergrade astrocytomas termed secondary GBM The most prominent occurring subtype of GBM is the primary tumors These tumors are often characterized by amplification or overexpression of the epidermal growth factor receptor EGFR 40–60  of all primary GBM tumors combined with genetic alterations in the EGFR gene which results in mutated forms of this receptor 1 This is opposed to secondary GBM which is characterized by progressive addition of mutations in p53 plateletderived growth factor receptor and the retinoblastoma gene 4 5 6 Nevertheless this distinctive division of mutations into the different GBM subtypes is not definitive 3Determination of disease prognosis is most often based on histological classification combined with information on patient age and tumor size and location These factors have all been defined as indicators of patient survival and treatment outcome but due to the sustained poor overall survival of GBM patients new arrays of prognostic indicators have been requested to aid in the clinical decision making 1 In recent years several molecular biomarkers have been characterized including chromosomal aberrations methylation status of the methyl guanine methyl transferase MGMT promoter mutations in important genes isocitrate dehydrogenase 1 IDH1 EGFR and p53 and dysregulation of microRNAs 7 Loss of heterozygosity in chromosomes 9p and 10q is associated with decreased survival while codeletion of 1p and 19q correlates with better treatment response and longer survival 7 Hypermethylation of the MGMT promoter leads to lower expression levels of MGMT which sensitizes GBM tumors to chemotherapeutic treatment and thus is associated with a significantly better patient outcome 8 9 Improvement of the disease condition is also observed in patients with mutation in the IDH1 gene which is most often found in secondary GBM Furthermore the expressional profile of specific microRNA signatures also correlates with overall survival time to progression and treatment success 10 11 12MicroRNA miRNA is a class of noncoding singlestranded RNA comprised of approximately 22 nucleotides with the ability to negatively regulate gene expression posttranscriptionally 13 14 miRNAs bind to the 3′ untranslated regions UTRs and sometimes 5′UTRs of their messenger RNA mRNA targets to which they exhibit imperfect complementarity hence enabling one miRNA to inhibit translation of multiple genes 15 16 The first miRNA was discovered in 1993 in Caenorhabditis elegans and denoted lin4 17 Later upon the discovery of let7 found to be conserved in several species miRNA regulation was recognized as an omnipresent phenomenon in eukaryotic organisms 18 19 miRNAs are acknowledged as crucial micromodulators of normal cellular homeostasis and accordingly dysregulation of miRNAs have been associated with a wide range of pathological conditions such as cancer 20 cardiovascular disease 21 22 and autoimmune 23 and neurodegenerative disorders 24 Expression of miRNAs in pathological specimens or biofluids compared to nonpathologic samples is subject to great scientific efforts 25 This poses interesting perspectives in terms of novel diagnostic and prognostic approaches and is inherently the initial step in uncovering the role of individual miRNAs in the context of different diseases eventually paving the way for novel miRNAbased therapiesThe biogenesis of miRNA requires RNA polymerase II/III for the transcription of primiRNA The primiRNA product is then cleaved by the DroshaDGCR8 complex into premiRNA The premiRNA is exported to the cytoplasm by Exportin5 in the presence of RanGTP cofactor Once in the cytoplasm the premiRNA is cleaved by the DicerTRBP complex into a miRNA duplex which is unwound into two products a guide strand bound to Ago2 which is incorporated into the RISC and a passenger strand which is degraded Finally the miRNA binds to its target mRNAs resulting in mRNA target cleavage translational repression or mRNA decay A more novel fate of the miRNAs is the selective secretion via microvesicles or exosomes Ran = Rasrelated nuclear protein GTP = guanosine5′triphosphate TRBP = TAR HIV1 RNA binding protein Ago2 = Argonaute protein 2 RISC = RNAinduced silencing complexThe linear biogenesis of miRNA begins with the transcription of miRNA genes by RNA polymerase II/III giving rise to a primary transcript called primiRNA which is subsequently polyadenylated and capped The transcript then folds into a hairpinloop structure via intrastrand basepairing 26 This structure is cleaved by the Drosha/DGCR8 complex to become premiRNA and transported out of the nucleus by Exportin5 in a RanGTPdependent process 27 In the cell cytoplasm the RNAseIII enzyme known as Dicer cleaves the premiRNA of which only one strand known as guide strand is incorporated into the RNAinduced silencing complex RISC the cytoplasmic effector machine of miRNA The passenger strand is subsequently degraded 28 The RISC is comprised of Dicer doublestranded RNAbinding factor and Argonaut protein 2 Ago2 The posttranscriptional RNA silencing is facilitated via imperfect complementary binding of miRNA attached to RISC to the respective mRNA 3′UTR resulting in translational inhibition 29 Additionally miRNAs are selectively excreted via lipoproteins or microvesicles potentially functioning as a mode of intercellular communication This last notion is important in relation to the nature of sampling material in the sense that plasma miRNA patterns might be a useful diagnostic and/or prognostic marker of ongoing pathological processes 30 31 For a more comprehensive review of miRNA biogenesis please refer to Winter et al 26miRNAs can be regarded as cancer biomarkers when their variation in expression identifies the cancerous state To date almost all tumor tissue analyzed by miRNA profiling has provided distinct miRNA profiles compared to normal tissue 32 These differential profiles can be further associated with prognostic factors and disease progression 33 34 35 In GBM the number of studies pertaining to miRNA expression and functional characterization has grown and miRNA signatures are refining GBM classification differentiating between the different grades and stages providing key regulatory links to disrupted signaling pathways such as those facilitating cell growth This has lead to a more in depth understanding about GBM pathology 36

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