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January 2010 = Volume 28, Number = 1

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The many roles of microRNAs in brain = tumor=20 biology

Jay=20 D. Turner, M.D., = Ph.D.^*,¹,=20 Richard Williamson,=20 M.D.^*,¹, Kaith K. Almefty,=20 M.D.^*,¹, Peter Nakaji, = M.D.¹,=20 Randall Porter,=20 M.D.¹, Victor Tse, M.D.,=20 Ph.D.², and M. = Yashar=20 S. Kalani, M.D., Ph.D.¹

¹Division of Neurosurgery, = Barrow=20 Neurological Institute, Phoenix, Arizona; = and ²Department of = Neurosurgery, The=20 Permanente Medical Group, Redwood City,=20 California

Abbreviations used in this = paper:=20 miRNA =3D microRNA; npBAF =3D = neural-progenitor-specific BAF; REST=20 =3D repressor-element-1-silencing transcription = factor; RISC =3D=20 RNA-induced silencing complex.

* These authors contributed = equally to=20 this work.

Address correspondence = to: M.=20 Yashar S. Kalani, M.D., Ph.D., Department of = Neurosurgery,=20 Kaiser Permanente Medical Center, 1150 Veterans = Boulevard,=20 Redwood City, California 94063. email: Yashar.Kalani@bnaneuro.net.

DOI: 10.3171/2009.10.FOCUS09207

Figures = and Tables=20

Abstract	IntroductionBiogenesisPosttranscriptional Gene = ...ConclusionsDisclosureReferences

MicroRNAs (miRNAs) are now = recognized as=20 the primary RNAs involved in the purposeful silencing = of the=20 cell's own message. In addition to the established = role of=20 miRNAs as developmental regulators of normal cellular=20 function, they have recently been shown to be = important=20 players in pathological states such as cancer. The = authors=20 review the literature on the role of miRNAs in the = formation=20 and propagation of gliomas and medulloblastomas, = highlighting=20 the potential of these molecules and their inhibitors = as=20 therapeutics.

KEYWORDS:microRNA; glioma;=20 medulloblastoma; cancer stem cell.

Introduction	AbstractIntroduction = <<BiogenesisPosttranscriptional Gene = ...ConclusionsDisclosureReferences		<= IMG=20 height=3D9 hspace=3D5=20 = src=3D"http://thejns.org/templates/jsp/images/arrow_up.gif"=20 width=3D11 border=3D0>

MicroRNAs are now=20 recognized as the primary endogenous small RNAs, about = 21=20 nucleotides in length (21=E2=80=9323 nucleotides), = involved in the=20 purposeful silencing of the cell's own message in a = variety of=20 ways, including translational repression, mRNA = cleavage, and=20 deadenylation. They were first described in = Caenorhabditis=20 elegans with the discovery that lin-4 coded = for an=20 RNA transcript capable of modulating the protein = expression of=20 lin-14.⁴² After=20 their initial discovery in 1993, miRNA research = proliferated=20 and a variety of screening efforts have led to the = discovery=20 of hundreds of miRNAs. MicoRNAs are now considered = fundamental=20 to normal cellular function in eukaryotes, and the = alteration=20 of miRNA expression and activity has been implicated = in a=20 variety of pathological processes.

Biogenesis	AbstractIntroductionBiogenesis <<ConclusionsDisclosureReferences

MicroRNAs are stem-loop structures encoded by a = cell's own=20 genome. They interact with complementary mRNA leading = to the=20 disruption of target protein expression. MicroRNAs are = generated by a multistep process (Fig. 1). = Each miRNA=20 can be transcribed separately from an individual=20 transcriptional unit, or each transcriptional unit can = encode=20 a cluster of distinct miRNAs; miRNAs can also be = processed=20 directly from other RNA species, such as introns by=20 Dicer.² = The=20 primary miRNA transcript (often abbreviated pri-miRNA) = is=20 typically transcribed from the genome by RNA = polymerase II and=20 is subsequently capped and polyadenylated.⁴⁰ The=20 primary miRNA transcript folds into a stem-loop = structure,=20 which is essential for the maturation process. In = animals, the=20 primary miRNA transcript is then cleaved in the = nucleus by=20 Drosha, an RNase III endonuclease, in association with = the=20 double-stranded RNA-binding domain protein DGCR8/Pasha = in a=20 protein complex referred to as the microprocessor=20 complex.¹⁷ Drosha=20 cleaves both strands of the stem at sites near the = base of the=20 primary stem-loop,⁴³=20 generating an intermediate known as the miRNA = precursor=20 (sometimes abbreviated premiRNA or pre-miRNA). The = miRNA=20 precursor is then exported out of the nucleus by = Exportin-5=20 into the cytosol where the RNase III = domain=E2=80=93containing=20 nuclease, Dicer, cleaves the terminal loop to generate = the ~=20 22 nucleotide mature miRNA. Dicer functions with the=20 double-stranded RNA=E2=80=93binding domain proteins = TRBP and=20 PACT.⁴⁶ Drosha=20 and Dicer cleave with great precision, generating very = exact=20 ends, and it is this feature that is likely = responsible for=20 the high specificity of miRNA with their target = mRNA.⁸

View = larger=20 version (53K)

Fig. 1. = Long primary=20 miRNAs (pri-microRNAs) are processed in = the=20 nucleus into stem-loop precursors by the = RNase III=20 endonuclease Drosha and Pasha. The = products of the=20 enzymatic reaction are transported into = the=20 cytoplasm by exportin 5 and Ran-GTP, and = these=20 precursors are further processed into = small RNA=20 duplexes of approximately 22 nucleotides = by the=20 Dicer RNase III enzyme and Loqacious = (Loqs). The=20 microRNA duplex is then loaded onto the=20 RNA-induced silencing complex (RISC). The = microRNA=20 guides the RISC to the target mRNA for=20 translational regulation.

Posttranscriptional Gene=20 Silencing	AbstractIntroductionBiogenesisPosttranscriptional Gene ...=20 <<ConclusionsDisclosureReferences

Immediately after formation of the mature miRNA, = the duplex=20 is unwound and loaded onto the RNA-induced silencing = complex,=20 which ultimately carries out the silencing of target = mRNA. The=20 RISC is a trimeric complex composed of Dicer, TRBP, = and a=20 protein of the Argonaute superfamily (Ago2 in = humans).³⁰ It=20 identifies target mRNA based on complementarity with = the=20 associated single-stranded miRNA and results in either = mRNA=20 cleavage or translational repression.² = An=20 estimated one-third of all mRNAs are thought to be = susceptible=20 to posttranscriptional gene silencing by = miRNAs.⁶⁶

MicroRNA and Neuronal = Development=20

One of the most defining moments in the development = of the=20 nervous system is when neuroprogenitors lose = multipotency at=20 mitotic exit and begin to develop their terminal cell = fate. In=20 the case of neurons, it means they commit to their = final=20 topographical position and established stable = connections that=20 will persist for the lifetime of the organism. This = transition=20 is in large part accompanied by a switch in=20 chromatin-remodeling/regulatory complexes such as in = the=20 exchange of the BAF53a and BAF45a subunits within = Swi/Snf-like=20 npBAF complexes. The subunits of the npBAF complex are = essential for neural-progenitor proliferation. The = exchange of=20 the BAF53a and BAF45a in the neuron-specific BAF = complex=20 promotes postmitotic neural development and dendritic=20 morphogenesis (Fig. 2). = MicroRNA=20 (miR-9* and miR-124) has been implicated in this = process by=20 regulating the chromatin architecture and = dynamics.⁷²=20 Interestingly, miR-9* and miR-124 are repressed by the = REST,=20 which mediates cell type- and developmental = stage=E2=80=93specific=20 gene repression, gene activation, and long-term gene = silencing=20 for protein-coding genes resulting in postmitotic = neurons. The=20 expression and function of REST are tightly regulated = by=20 context-specific transcriptional and = posttranscriptional=20 mechanisms including bidirectional feedback loops with = various=20 miRNAs. It is therefore not surprising that = deregulation of=20 REST and miRNAs are both implicated in the = pathogenesis of=20 various brain tumors.

View = larger=20 version (28K)

Fig. 2. The = subunits=20 of the npBAF complex are essential for=20 neuralprogenitor proliferation. The = exchange of=20 the BAF53a and BAF45a in the nBAF = (neuron-specific=20 BAF) complex promotes postmitotic neural=20 development and dendritic=20 morphogenesis.

MicroRNA Function and Cancer=20

Following the discovery of miRNAs and other small = RNAs, a=20 wealth of data were rapidly generated, which revealed = a novel=20 mechanism for the modulation of gene expression. As=20 gene-finding computational models have evolved from = simple=20 homology-based searches to more complex multifactorial = models,=20 the number of identified miRNAs has continued to = grow.⁴⁷ Over=20 recent years, there has been a significant push to = better=20 understand how they function in both normal and = pathological=20 states.

Though miRNAs have been demonstrated to modulate = genes=20 involved in a variety of cellular processes, a = significant=20 proportion of miRNAs regulate genes associated with = cellular=20 fate. It is now well accepted that miRNAs are = fundamental to=20 the regulation of proliferation, differentiation, and=20 apoptosis during normal development. It has been shown = that=20 miRNAs have a predilection in targeting developmental = genes.=20 Genes involved in functions common to all cells, such = as in=20 maintenance and general activities, have very few = miRNA target=20 sites, and seem to be under selection to avoid = targeting by=20 miRNAs.⁶¹=20 Furthermore, alterations in the expression of miRNAs = are seen=20 in a variety of pathological processes, including = cancer.=20 Aberrant miRNA expression has been demonstrated in = essentially=20 every cancer type studied, including breast^58,64,68,73 and=20 ovarian carcinomas,^48,74=20 pancreatic cancer,^4,27=20 non=E2=80=93small cell lung cancer,^41,56=20 leukemia,^5=E2=80=937=20 and brain tumors (Table = 1).^52,66 MicroRNA=20 expression can be altered in cancer through a variety = of=20 mechanisms including chromosomal changes, epigenetic = defects,=20 mutations, and alterations in the machinery involved = in miRNA=20 biogenesis.⁶⁶

TABLE 1: = MicroRNAs=20 implicated in glioma and medulloblastoma=20 = biology

Beyond mere biomarkers, the altered expression = profiles of=20 miRNA implicate them as key regulators of = tumorigenesis. The=20 miR-17-92 polycistron, located on chromosome = 13q32=E2=80=9333, was the=20 first example of miRNAs acting as mammalian = oncogenes.³⁵ This=20 region of the chromosome is amplified in several types = of=20 cancer and can be activated by c-Myc, a=20 well-established protooncogene overexpressed in = different=20 cancer types.^51,66 The=20 first individual miRNA assigned an oncogenic role was=20 miR-155,¹⁸ which=20 when overexpressed in a transgenic mouse model, led to = the=20 development of B-cell leukemia and high-grade = lymphoma.¹⁵

There is also evidence accumulating that miRNAs are = involved in cell-cycle checkpoint regulation. Using a = mutated=20 version of dicer-1, Hatfield and colleagues³³=20 demonstrated that miRNAs modulated stem cell division = in=20 Drosophila by bypassing the G1/S checkpoint. This work = suggested that miRNAs may make stem cells insensitive = to=20 environmental stimuli and that a similar mechanism may = be=20 implicated in tumorigenesis.¹⁶ More=20 recently, specific miRNAs capable of regulating = cellular=20 checkpoints in cancer have been described, including = miR-221,=20 miR-222, and miR-27a.^45,49,67

In addition to their involvement in = cancer=20 formation, miRNAs have also been associated with tumor = progression and metastatic potential.³ = Although=20 still not well characterized, specific = miRNAs=E2=80=94such as miR-10b,=20 miR-21, miR-30a, miR-30e, miR-125b, miR-141, miR-200b, = miR-200c, and miR-205=E2=80=94have been suggested to = play an important=20 role in tumor invasiveness and metastasis.¹ = Some of=20 the targets of these miRNAs have recently been = elucidated and=20 include tumor suppressor genes such as tropomyosin = 1=20 and other targets with metastatic potential such as=20 PDCD4 and maspin.⁷⁶

MicroRNAs and Gliomas=20

Gliomas are tumors arising from glial cells, the=20 neuroepithelial support cells of the CNS. Gliomas are = the most=20 common primary tumor of the CNS comprising over 50% of = primary=20 brain tumors.⁶²=20 Glioblastoma is the most common and deadliest glioma = with=20 approximately 10,000 new cases every year and a median = survival of only 14 months even with the most current=20 therapies.⁶² Gliomas,=20 like tumors in other parts of the body, develop = because of=20 fundamental genetic alterations that cause the = formation of a=20 tumor stem cell population that divides without regard = to=20 normal physiological biochemical signaling.^11,65 It is no=20 surprise then that the same miRNAs that are involved = in stem=20 cell regulation and differentiation are also = implicated in=20 tumor stem cell biology of gliomas.

Microarray studies of glioma tissue have implicated = a=20 number of miRNAs involved in glioma formation and = propagation.=20 In their quantitative reverse transcriptase polymerase = chain=20 reaction analysis of high-grade astrocytomas in humans = (glioblastomas and anaplastic astrocytomas), Silber et = al.⁵⁹ found 35=20 different miRNAs that were significantly deregulated = compared=20 with control brain tissue. Those miRNAs that have been = shown=20 to play a role in human glioma include miR-7, -10b, = -15b, -21,=20 -26a, -124, -128, -137, -181a, -181b, -221, -451, and = others.=20 The majority of miRNAs are underexpressed in = proliferating=20 glioma cells with the important exception of miR-10b, = -21, and=20 -221.

MicroRNAs are key regulators of tumor suppressor = genes. The=20 overexpression of miR-21, a recently described = antiapoptotic=20 factor, appears to be vitally important in not only = glioma but=20 also cancer arising from other parts of the = body.⁹ = Chen and=20 coworkers¹⁰ used=20 bioinformatics analysis to screen and identify various = genes=20 with miR-21 binding sites. Using the glioblastoma cell = line=20 T98G, they confirmed that miR-21 binds and silences = the tumor=20 suppressor gene PDCD4. Corsten et al.¹⁴=20 transfected human glioma cells with anti-miR-21=20 oligonucleotides and simultaneously implanted neural = precursor=20 cells expressing a secretable variant of the cytotoxic = agent=20 TRAIL (S-TRAIL). They found a synergistic effect of = knocking=20 down miR-21 levels and adding the apoptotic agent = STRAIL=20 evidenced by increased caspase activity and decreased = cell=20 viability in the human glioma cells in vitro. They = then found=20 complete eradication of tumor cells in a murine model=20 transfected with anti-miR-21 and then exposed to = S-TRAIL in=20 vivo.

MicroRNAs regulate oncogenes implicated in brain = tumor=20 formation. The expression level of miR-128 is = inversely=20 correlated with expression level of transcription = factor=20 E2F3a, a protein that promotes cell entry into Sphase = and=20 implicated in cancer of the bladder and = prostate.⁷⁵=20 Godlewski and colleagues²⁸ found=20 that miR-128 levels were downregulated in glioma cells = compared with normal brain tissue. They reported that=20 increasing miR-128 expression leds to a decrease in = the=20 expression of the oncogene Bmi-1. = Overexpression of=20 miR-128 in glioma neurosphere cultures specifically = blocks=20 glioma self-renewal consistent with Bmi-1=20 downregulation.

Interestingly, other than serving as key regulators = of=20 oncogenes and tumor suppressors, microRNAs may also = dictate=20 the invasiveness and aggressiveness of tumors. Conti = et=20 al.¹³ found=20 both miR-21 and miR-221 upregulated in glioma samples; = however, they noted that whereas miR-21 was elevated = in all=20 gliomas, high levels of miR-221 were only found in = high-grade=20 gliomas. MicroRNA-10b has similarly been associated = with=20 high-grade tumors. Overexpression of miR-10b may = promote=20 glioma invasion via the RhoC and urokinase plasminogen = activator receptor (uPAR), which have been implicated = in other=20 cancers as prometastatic and proinvasive = factors.⁵⁵ Huse and=20 colleagues³⁷ studied=20 the overexpression of miR-26a in a subset of = high-grade=20 glioma, most often associated with monoallelic loss of = phosphatase and tensin homolog (PTEN). Studying 3 = human=20 glioblastoma samples, they found that miR-26a directly = targets=20 PTEN and enhances the Akt pathway. By transfecting = miR-26a=20 into a murine model, they were able to show that=20 overexpression of miR-26a enhances gliomagenesis. = Because=20 miR-26a overexpression appears in only a subset of = glioma with=20 monoallelic loss of PTEN, they hypothesized that this=20 overexpression functionally substitutes loss of = heterozygosity=20 at the PTEN locus.

MicroRNAs have also been shown to be important = regulators=20 of cellular proliferation/differentiation and the cell = cycle.=20 Silber and colleagues⁵⁹ were the=20 first to discover that miR-124 and miR-137 are = downregulated=20 in high-grade gliomas compared with normal controls. = These=20 microRNAs are also upregulated during adult neural = stem cell=20 differentiation. The upregulation of miR-124 and = miR-137 in=20 tumor stem cell populations promotes neuronal = differentiation=20 of the tumor stem cells and inhibits proliferation of = the=20 tumor cells by inducing G0/G1 cell-cycle arrest. The = mechanism=20 of action is via the inhibition of CDK6 expression, a = protein=20 essential for cell-cycle progression.⁵³ Kefas et=20 al.³⁹ studied=20 miR-7, another miRNA that is downregulated in glioma. = They=20 found that miR-7 suppresses EGFR expression and = independently=20 inhibits the Akt pathway in glioblastoma. Transfection = of=20 glioblastoma with miR-7 decreased viability and = invasiveness=20 of primary glioblastoma cell lines and increased the = apoptotic=20 fraction of cells. Overexpression of miR-15b, another=20 differentially expressed microRNA, results in = cell-cycle=20 arrest while suppression of miR-15b results in more = cells in=20 S-phase.⁷⁰ Gal and=20 colleagues²³ examined=20 glioblastoma stem (CD133-positive) and nonstem=20 (CD133-negative) cells and found that miR-451, -486, = and -425=20 were significantly upregulated in CD133-negative cells = compared with CD133-positive cells. Transfection of=20 glioblastoma cells with these miRNAs inhibited = neurosphere=20 formation, and transfection with miR-451 resulted in=20 neurosphere dispersion and inhibited glioblastoma = growth. Gal=20 and colleagues found that combining miR-451 = transfection with=20 Imatinib mesylate treatment had a cooperative effect = in=20 dispersal of glioblastoma neurospheres.

MicroRNAs have recently been shown to = function=20 as bona fide tumor suppressors. Shi et al.⁵⁷ reported=20 on downregulated miR-181a and miR-181b involved in = glioma=20 formation. Their study showed that these miRNAs = functioned as=20 tumor suppressors. Transfection of these miRNAs into=20 glioblastoma cells inhibited proliferation in vitro, = resulted=20 in loss of anchorage-independent growth, induced = apoptosis in=20 glioma cell lines, and depressed the invasion of = glioma cells=20 in vitro.

MicroRNAs and Medulloblastoma=20

Medulloblastoma is the most common malignant brain = tumor in=20 children with an incidence of approximately 2 per = 100,000. It=20 appears to arise from stem cells and from granule = neuron=20 precursors in the external granule layer of the=20 cerebellum²⁶ or=20 multipotent precursors in the ventricular zone of the=20 cerebellum.⁴⁴ About=20 70% of cases occur before the age of 16 years. = Approximately=20 one-third disseminate in the CSF and up to 5% spread=20 systemically. Medulloblastoma treatment most often = involves a=20 combination of surgery and radiation therapy. = Chemotherapy is=20 usually reserved for children younger than 3 years of = age or=20 for recurrent tumors. Five-year survival rates for=20 medulloblastoma have been estimated to range from 35 = to=20 75%.¹⁹

Evidence for a role of miRNA in medulloblastoma=20 tumorigenesis has only very recently emerged. Pierson = and=20 associates⁵⁴ were the=20 first to report the involvement of miRNA in = medulloblastoma by=20 demonstrating that miR-124 modulates cell-cycle = regulation in=20 medulloblastoma cells. They showed that miR-124 = expression is=20 significantly decreased in medulloblastoma and that=20 augmentation of miR-124 levels can slow tumor cell = growth by=20 targeting CDK6.

Ferretti et al.²¹ proposed=20 a role for miRNA in modulating hedgehog signaling, a = pathway=20 recently implicated in tumorigenesis for a subset of=20 medulloblastoma.^29,32,34=20 Specifically, they showed that miR-125b, miR-326, and=20 miR-324=E2=80=935p expression was decreased in = medulloblastoma and=20 that the altered expression of these miRNAs led to = tumor cell=20 proliferation through a hedgehog-dependent mechanism.=20 Expanding on this work, Uziel et al.⁶³=20 identified 3 miRNAs overexpressed in hedgehog-active=20 medulloblastomas: miR-92, miR-19a, and miR-20. All 3 = miRNAs=20 were encoded by the miR-17/92 cluster, which has been=20 associated with a variety of cancers. Northcutt and=20 colleagues⁵⁰=20 identified a high-level, focal amplification on = chromosome=20 13q31.3, which mapped to the same miRNA cluster. The=20 expression of miR-17/92 was most elevated in = medulloblastomas=20 with activated hedgehog signaling and was also = associated with=20 elevated c-Myc and n-Myc. These studies suggest that = aberrant=20 expression of miRNAs encoded by the miR-17/92 enhance = the=20 growth potential of medulloblastoma and that = miRNA-mediated=20 modulation of hedgehog signaling may be an important=20 contributing factor to medulloblastoma = pathogenesis.

Ferretti et al.²² used=20 high-throughput screening to examine miRNA expression = profiles=20 in 34 patients with medulloblastoma. They identified = 78 miRNAs=20 with altered expression in medulloblastoma, compared = with=20 normal adult and fetal cerebellar cells. Several of = the=20 identified miRNAs have been implicated in other cancer = types=20 including glioblastoma.^12,25 The=20 majority of these miRNAs were downregulated in=20 medulloblastoma, supporting a role for miRNAs as tumor = suppressors. Additionally, they found increased = expression of=20 miR-9 and miR-125a and that increased expression of = these=20 miRNAs was capable of decreasing proliferation, = augmenting=20 apoptosis, and ultimately promoting arrest of tumor = growth.=20 The proapoptotic effect was mediated by miR-9 and = miR-125a=20 targeting of the t-TrkC receptor, which was found in = this=20 study to be upregulated in medulloblastoma cells. In = this=20 study, miRNA expression patterns were also examined in = different tumor subsets. The authors found that = miR-let7g,=20 miR-19a, miR-106b, and miR-191 were significantly = upregulated=20 in anaplastic compared with desmoplastic = medulloblastomas;=20 miR-let7g and miR-106b were differentially expressed = in=20 desmoplastic compared with classic medulloblastomas; = and=20 miR19a was upregulated in anaplastic compared with = classic=20 medulloblastomas.

Changes in expression of Her2 (ErbB2) and c-Myc = have been=20 demonstrated to impact biological activity and = clinical=20 features of medulloblastoma.^26,31,36 Ferretti=20 et al.²² examined=20 miRNA expression from medulloblastomas overexpressing = either=20 Her2 or c-Myc and identified an miRNA signature in = each group=20 of medulloblastomas. Expression of miR-10b, miR-135a,=20 miR-135b, miR-125b, miR-153, and miR-199b was altered = in=20 Her2-overexpressing tumors, whereas c-Myc = overexpressing=20 medulloblastomas had expression changes in miR-181b, = miR-128a,=20 and miR-128b. Additionally, the amount of expression = change of=20 2 miRNAs correlated with disease risk. Though miR-31 = and=20 miR-153 were downregulated in all medulloblastomas, = the group=20 found that the degree of change was directly = proportional to=20 disease severity.

It is well established that the Notch = signaling=20 pathway regulates the differentiation of granule = neuron=20 precursor cells and that Notch2 expression is = increased in=20 about 15% of medulloblastomas.^44,60=20 Expression of the transcriptional repressor HES1, a = downstream=20 effector protein of the Notch pathway, normally = declines=20 during the process of neuronal differentiation. = Conversely,=20 persistent activation of the Notch pathway and HES1 = prevents=20 the migration of granule neuron precursor cells out of = the=20 ventricular zone and inhibits neuronal = differentiation.³⁸ Based on=20 its role in differentiation, it is not surprising that = dysfunction of the Notch pathway has been associated = with a=20 subset of medulloblastoma with stem cell=E2=80=93like=20 properties.^20,69,71,77 Garzia=20 et al.²⁴ examined=20 the role of miRNAs in the regulation of Notch/HES1 = signaling=20 in medulloblastoma. They found that miR-199b-5p = targeted HES1=20 and that miR-199b-5p-mediated downregulation of HES1=20 attenuated cellular proliferation in medulloblastoma = cell=20 lines. In medulloblastoma patients, increased = expression of=20 miR-199b-5p appeared to decrease metastatic potential = and was=20 associated with increased survival.

Conclusions	AbstractIntroductionBiogenesisPosttranscriptional Gene = ...Conclusions = <<DisclosureReferences

Although the investigation of miRNAs = in brain=20 tumors is still in its infancy, there is strong = evidence=20 mounting that miRNAs are integrally involved in brain = tumor=20 development and progression. It is becoming clear that = miRNAs=20 are essential regulators of many of the key pathways=20 implicated in tumor pathogenesis. While adding another = layer=20 of complexity, the discovery of the role miRNAs in = brain=20 tumors has also revealed a new category of therapeutic = targets. As miRNA research continues to evolve, novel=20 therapeutic targets for the treatment of brain tumors = will=20 continue to emerge.

Disclosure	AbstractIntroductionBiogenesisPosttranscriptional Gene = ...ConclusionsDisclosure <<

The authors report no conflict of = interest=20 concerning the materials or methods used in this study = or the=20 findings specified in this paper.

References	AbstractIntroductionBiogenesisPosttranscriptional Gene = ...ConclusionsDisclosureReferences = <<

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